- gents cae ws “an = - 58 2 ? o ae = : ~ ‘ rs ~~ Sin See re | = : z : ee - ee Bee 3 = TI % bare reas : ts + i S eae ae s EZ : aes : ; on Na theese PEE ee eae A De RL : cf Cg ee is VVC ASE 8) A Ay Pa SUEY cate VA iM xs v o ; SPA IG AK ive ae ar) f rae eS ‘ Cn eee + 6 RG, Pes See Sete = Sc << ae | > + 3 : ny oe . 2 > a ot, - ss 5 = 7 4 re, ny ‘ Ao = See evo ot : ; Seeeee eae a, od) ral Pe ta i eae = She THE CHEMISTRY AND PHYSICS OF CLAYS AND OTHER CERAMIC MATERIALS THE CHEMISTRY & PHYSICS OF CLAYS AND OTHER CERAMIC MATERIALS BY ALFRED B. SEARLE Consulting Chemistsand Expert Adyiser,on Clays: and Clay Products; Lecturer on Brickmaking under the C: intor Bequesi, an. Ce quon‘le 1D Lecs ures PH Cojlo, ds under the C Chantrey Bequest vette of * ‘Modern Backend” a British Clays, ‘Shales, ear. g Sands” ; ‘The Natural History of Clay” ; “Clays and Clay Products” ; “ Refraetory, Materials: Then” ‘Manufasrare and Uses,” Etc. 702 B e »dyUG IEC NEW YORK VAN NOSTRAND COMPANY EIGHT WARREN STREET 1924 if y ted in Great Britain b: Neitz & Co., Lrp., EpinBu Made and a? PREFACE Tue clay-working and allied industries are exceedingly old and have reached a remarkable state of perfection in craftsmanship, aided by only very little scientific knowledge. Within the last forty years, however, and especially during the last decade, increasingly stringent demands have been made by users of electrical and chemical pottery and refractory materials, and manufacturers of paper, textiles, and other materials in which clays are used, so that inherited and artistic skill is no longer sufficient, and Science must contribute to that knowledge of causes and effects on which future developments in these industries must depend. Only those who have devoted special attention to the subject can be aware of the importance of the applications of both physics and chemistry to the industrial uses of clays and other ceramic materials. Such applications are so extensive, that it is remarkable that no volume has previously been published in which they are dealt with in a systematic manner, for there is scarcely a branch of physics or of inorganic chemistry which is not of value when applied to the treatment of clays, allied materials, and the products made from them, and the more intense such application, the greater is, and will continue to be, the benefit to all concerned. It may well be asked why those engaged in such extensive and important industries as those connected with the manufacture or use of ceramic materials have paid comparatively little attention to the fundamental, scientific principles involved, and why so many advanced students of Science have largely neglected the study of clays and other ceramic materials ? The answer to such questions is threefold: (a) physicists have chiefly concerned themselves with the properties of matter in mass; (6) students of pure chemistry, on the other hand, have chiefly dealt with the atoms and molecules of substances of a simpler character, the reactions of which can be more easily controlled, and whose properties and relation- ships can be studied in a more direct manner ; and (c) most manufacturers and users have scarcely realised, as yet, the enormous importance, to them, of chemists and physicists with a highly specialised knowledge of these materials. The difficulties experienced in obtaining perfectly pure substances, the general insolubility and apparent inertness of most ceramic materials at temperatures below a dull-red heat, and the impossibility of obtainmg many of them in some readily recognisable form—such as crystals of convenient size—have hindered research, but as these difficulties are overcome, and more and more information regarding the constitution and properties of these materials and products is obtained, great technical advances will be made. The field of research in this subject is so vast, however, that it will be many years before it is fully occupied. At present, many important fundamental principles still need to be investigated, and far too little is known of what may appear to be such simple matters as the effect of the texture of many ceramic products, the causes and control of their strength, the distribution of the water in partially dried articles, and the relief of the various strains which are produced during the drying and burning of many pieces of pottery, or of other ceramic articles used in the construction of furnaces, coke-ovens, sanitary appliances, etc. The causes and Vv Cicero = vl PREFACE prevention of distortion afford another wide field of investigation which has, as yet, scarcely been entered, and the complexity of the problems of chemical equilibrium in relation to ceramics is such that only the simplest cases are, as yet, reasonably well understood. Far too little attention has been paid to the enormous influence of the size of the particles on the progress and nature of chemical reactions, and of the physico-chemical changes which occur on heating clays and allied materials ; and the study of such phenomena is, as yet, in its infancy. Colloidal phenomena also’ appear to offer almost endless opportunities for gaining further knowledge of the nature of clays and allied materials. It will also be seen, in the following pages, that the chemical constitution of clays offers most fascinating problems, the complete solution of which has, so far, baffled the ablest chemists who have studied them, and a similar remark is equally applicable to increasing the plasticity of clays and to many of the other problems in physics and chemistry, with which those concerned with ceramic materials are in constant contact. Under these circumstances it will be obvious that the present volume is not intended to be an exhaustive treatment of the subject, but to provide, in a convenient form, such a description of the properties of various ceramic materials and articles, and of the application to them of the more important principles of chemistry and physics, as will be equally useful to students, manufacturers, and users. Several subjects of purely academic interest—such as the quantum theory and entropy—have been purposely omitted, as to have included them would have made the book unwieldy, without adding correspondingly to its usefulness. Finally, the author wishes to acknowledge the zealous and skilled assistance of several members of his staff, without which this volume could not have been written. In this respect, he is specially grateful to Mr W. L. Emmerson, who has taken a large share in collecting data, made many useful suggestions and read the proofs, and to Mr F. Stones, who has assisted in other ways, including the compilation of the Index. ALFRED B. SEARLE. 440 Giossop RoaD, SHEFFIELD, November, 1923. CONTENTS CHAPTER I. PHYSICAL STRUCTURE. Soirps.—Crystalline substances—Crystallography—Multiple forms of crystals—Pseudo- morphism—Amorphous substances—Glassy or vitreous substances—Colloidal gels— Cellular substances—Mixed materials PASTES Sires 5 : ; : : : . CoLLoips.—Colloidal state—Colloidal sols—Colloidal ore : ; ‘ : ; ‘ States or Acereaation.—Wholly crystalline—Granular—Granular fragments in a glassy matrix—Amorphous or crystalline fragments in a non-glassy matrix—Plastic materials— Amorphous or crystalline grains without cement Massive Structure.—Unstratified — Stratified — Foliated — Cellular — Capillary — Con- cretionary—Segregated—Fibrous : ALTERATION OF STRUCTURE.—Weathering—Grinding—Calcining CHAPTER II. PROPERTIES DEPENDING ON STRUCTURE. TEXTURE.—Shape of grains—Size of grains—Grading—Materials producing a fine texture— Materials producing a coarse texture—Materials producing a medium texture—Tex- ture of ceramic materials—Determination of texture—Comparison of different textures HomoGEneiry.—Treading—Wedging—Spade-mixing—Pugging—Tempering—Blunging . Porosity.—True porosity—Apparent porosity—Effect of texture on porosity—Materials increasing porosity—Materials reducing porosity—Effect of heat treatment on porosity— Effect of porosity on absorption—Effect of porosity on apparent density—Effect of porosity on spalling, etc.—Effect of porosity on thermal conductivity—Effect of porosity on resistance to corrosion—Effect of porosity on resistance to abrasion and erosion— Effect of porosity on electrical conductivity—Effect of porosity on strength—Effect of porosity on refractoriness—Effect of porosity on discoloration—Effect of porosity on the rate of drying—Porosity of ceramic materials—Determination of porosity and absorption —Penetrability PERMEABILITY.—Effect of heat on permeability—Effect of permeability on thermal con- ductivity—Effect of permeability on pie Sa a 2 of ceramic materials— Determining Permeshiliey : : : : ; : : : é CHAPTER III. e COLOUR, HARDNESS, AND MINOR PHYSICAL PROPERTIES. CoLour.—Natural sources of colour—Colours of raw clays—Colours of burned clays—Artificial colours—Colours of ceramic materials other than clay—Discoloration—Colour measure- ment. Harpness.—Hardness of raw materials—Hardness of burned ceramic materials—Determina- tion of hardness . Minor Properties.—Ring—Feel—Odour—Sectility and eee ‘ : F vii PAGES 21-25 25-26 27-59 59-61 61-86 86-93 94-123 123-137 137-138 vill CONTENTS CHAPTER IV. STRENGTH AND ALLIED PROPERTIES. Forms or StreNGTH.—Cohesion—Tensile strength—Binding power—Brittleness—Friability —Malleability — Ductility — Extensibility — Elasticity — hg oa — peice Deformability—Crushing strength—Transverse strength : ; FacToRS AFFECTING STRENGTH.—Chemical composition—Size and shape of ariilotenae —Porosity— Power of bond—Mode of preparation—Grinding—Amount of water used— Proportions of added materials—Mixing—Ageing—Shaping—Drying—Burning— Repeated changes of temperature—Repeated heating—Temperature during use— Weathering—Frost—Blows—Deposited carbon—Corrosion by slags, flue-dust, ete. STRENGTH OF CLAYS AND CLay Propucts.—Raw clay and clay pastes—Dry clays—Burned clay wares ; : ; : : : : STRENGTH OF SILICEOUS REFRACTORY MATERIALS . STRENGTH OF OTHER REFRACTORY MATERIALS STRENGTH OF GLAZES . : : : : : ; : DETERMINATION OF STRENGTH OF CERAMIC MaTERIALS.—Tensile strength" empreeeee or crushing strength — Transverse tests — Impact tests — Torsion tests — Deformability tests—Freezing tests—Binding-power tests CHAPTER V. THE SPECIFIC GRAVITY AND DENSITY OF CERAMIC MATERIALS. Densiry.—Apparent density—Volume-weight—True specific gravity—Temperature—Mode of preparation and manufacture : ; : ; : : : : : FACTORS INFLUENCING APPARENT DrEnsiry.—Texture and porosity—True specific gravity —Temperature—Mode of preparation and of manufacture . FacToRS INFLUENCING APPARENT SPECIFIC GRAVITY Factors INFLUENCING TRUE SPECIFIC GRavity.—Physical formes Uheriont constitution— Impurities—Temperature of testing—Temperature and duration of previous heating APPARENT DENSITY AND TRUE SPECIFIC GRAVITY OF VARIOUS CERAMIC MATERIALS DETERMINATION OF SPEcIFIC GRAVITY, APPARENT SPECIFIC GRAVITY, APPARENT DENSITY, AND VoLUME-WEIGHT.—Volume-weight—Apparent density—Apparent specific gravity —True specific gravity : : : : : é CHAPTER VI. CHANGES IN THE PHYSICAL STATE EFFECTED BY WATER. PuHysicaL STATES OF CERAMIC MATERIALS CoLLomaAL PHENoMENA.—Properties of colloidal inc eeeen charge—Electro-osmosis— Electrical conductivity—Electrical precipitation—Precipitation by electrolytes—Pro- tection — Brownian movement — Osmotic pressure — Diffusivity — Specific gravity — Viscosity — Reversibility — Properties of colloidal a — eget — Contraction — Action of heat—Adsorption COLLOIDAL PROPERTIES oF CLAYS. — ayes — Spee aon — Dehydratinn — Adsorption — Electrical properties — Brownian movement—Action of electrolytes— Flocculation — Deflocculation — Protection — Kataphoresie, Misa: aon bility—Measuring colloidal matter : : : : OTHER COLLOIDAL MATERIALS USED IN CERAMICS. aay colloids’ "Alumina <=" Magnesia—Organic matter—Irreversible colloids PAGES 139-146 146-1 69 170-185 185-188 189-192 192 193-202 203-205 205-207 207-208 208-212 212-221 221-226 227-228 228-236 236-251 251-253 CONTENTS PHYSICAL CHANGES EFFECTED BY WEATHERING.—Absorption of moisture—Distribution of moisture—Disintegration—Oxidation effects—Increase in plasticity—Artificial weather- ing—Influence of an excess of moisture in raw materials SLAKING Puasticiry.—Nature of plasticity—Effect of size of grains—Effect of shape and structure of grains—Hffect of aggregation of grains—EKffect of surface area and intermolecular attraction—Effect of colloidal phenomena—Separation of colloidal matter from clays— Increase and reduction of plasticity—Proportion of water required—Effect of non-plastic materials—Hffect of organic matter—Effect of adding colloidal matter—Increasing plasticity—Reducing plasticity—Plasticity and Sa Ri cat or ageing—Pseudo- plasticity—Oiliness—Mobility—Extensibility 5 MEASUREMENT OF PxiastTicr1ry—lIndirect methods—Direct aieee : Brypine PowEr.—Nature—Measurement Sires AND SLURRIES.—Slips for engobes and pee etn. process—Purification of clay— Properties of slips—Measurement of viscosity CHAPTER VII. 1x PAGES 253-257 257 257-276 276-281 281-282 282-293 CHANGES IN THE PHYSICAL STATE FOLLOWING THE REMOVAL OF WATER. Dryinc.—Removal of water from slips—Removal of water from pastes—Dried pastes— Hygroscopicity CHAPTER VIII. CHEMICAL CONSTITUTION OF CERAMIC MATERIALS. ELEMENTS.—Isotopes — Compounds — Mixtures — Solid solutions — Atoms and molecules —Atomic and molecular compounds—Laws of chemical combination—Law of multiple proportions—Atomic weight—Molecular weight—Valency . ‘ : : CuEemicaL Notation.—Formule—Structural or graphic formule—Molecular formulee— Calculation of percentage composition from molecular formule—Calculation of molecular formule—Norms—Triaxial diagrams—Chemical equations Aotps, Baszs, AND SALTS MoLecuLaR STRUCTURE OF ene are a oephian CHEMICAL CONSTITUTION OF SILICATES AND ALUMINO- ee ones _ Silica—Silicic Pi and silicates—Alumina and its hydroxides—Alumino-silicates—Raw as eae or calcined clay—Synthesis of clay—Glazes . : : ; : ‘ CHEMICAL CONSTITUTION OF OTHER CERAMIC MATERIALS. — Maptede re — Silicon carbides and oxycarbides—Lime—Iron oxides and hydroxides—Chromite—Zirconia— Carbon CHAPTER IX, 294-298 299-306 306-316 316-322 322-325 325-354 355-358 THE CHEMICAL COMPONENTS OF CERAMIC MATERIALS AND PRODUCTS. CHEMICAL ANALYSIS.—Sampling CHEMICAL COMPONENTS OF CLAYS AND CLAY PRODUCTS. PiermcaHon of analyses— Impurities in clays—Silica—Alumina—Alkaline silicates and alumino silicates—Iron compounds—Calcium compounds—Barium compounds—Magnesium compounds— Titanium compounds—Manganese compounds—Phosphorus and Vanadium compounds— Sulphur—Moisture and colloidal water—Carbonaceous matter—Water of constitution and crystallisation 359-361 361-371 x CONTENTS Composition AND Utinity.—China clays—Ball and pottery clays—Fireclays—Brick clays —Fine earthenware—Coarse earthenware—China-ware and 8 Wienke st cones— Chemical composition and refractoriness : A : n CHEMICAL COMPOSITION OF ENGOBES AND GLAzES.—Engobes or podiee Gingee anaes of constituents on engobes and glazes—Adjustment of composition—Typical glazes CHEMICAL CoMPOSITION Or OTHER CERAMIC MaTeERrIALs.—Silica bricks—Refractory silica sands—Silica glass—Spinels—Magnesite and magnesia bricks—Dolomite and dolomite bricks—Fused alumina—Zirconia, zircon, and zirconia bricks—Chromite and chrome bricks—Carbon bricks and crucibles—Carbides and carboxide bricks CHAPTER X. THE MINERALOGICAL COMPOSITION OF CERAMIC MATERIALS. Microscopic Examrnation.—Rational analysis—Recalculated analysis MINERALOGICAL COMPOSITION oF CLAYyS.—Kaolinite and minerals similar to clay—Sili- manite and similar minerals—Impurities—Silica—Silicates, aluminates and alumino- silicates—Iron-bearing) minerals—Calcium minerals—Barium minerals—Strontium minerals—Magnesium minerals—Aluminium minerals—Titanium minerals—Chromium minerals—Tin-bearing minerals—Manganese minerals—Phosphate acide and carbonaceous matter—Water MINERALOGICAL COMPOSITION OF SILICA AND SiLicEous MATERIALS. may Pe silat Crystalline silica MINERALOGICAL COMPOSITION OF OTHER CERAMIC MaTERIALS.—Alumina and aluminous minerals—Magnesic minerals—Calcic minerals—Titanic minerals—Zirconium ores— Chrome ores—Carbon and carbon compounds CHAPTER XI. PHYSICO-CHEMICAL REACTIONS BETWEEN CERAMIC MATERIALS. AVOIDANCE OF CHEMICAL ACTION.—Chemical action and physical changes—Types of chemical action Factors INFLUENCING CHEMICAL Sete —Heat — Time — Pressure — Vapour pressure —Surface tension—Viscosity—Solubility—Selective action—Catalytic action—Nascent action—HElectrical conductivity—Light—Change of state—Intimacy of association —Relative quantities of reacting substances SPEED OF REACTION ; ; : REVERSIBLE AND IRREVERSIBLE REACTIONS PHASE CONDITIONS IN CHEMICAL SySTEMS.—Phase rule—Equilibrium or phase diagrams— Solid solutions—Eutectics—Definite chemical compounds—Complex fusion curves— Time-temperature curves PuHAsE CONDITIONS IN CERAMIC PROCESSES— Binary Systems. — Lime-silica — Soda-silica — Magnesia-silica — Barium oxide-silica — Strontia-silica — Zine oxide-silica — Manganese oxide-silica — Iron oxide-silica — Zirconia-silica — Lime-alumina — Magnesia-alumina — Iron oxide-alumina — tron oxide-lime — Silica-alumina — Silica-sillimanite — Iron oxide-magnesia. Ternary Systems.—Lime-magnesia-silica — Lime-barium oxide-silica — Lime-lithia- silica — Lime-strontia-silica — Soda-lime-silica — Soda-lithia-silica — Soda- magnesia-silica — Soda-strontia-silica — Potash-lithia-silica — Barium oxide-soda- silica—Barium oxide-lithia-silica — Magnesia-lithia-silica — Lithia-strontia-silica — Iron oxide-magnesia-silica — Lime-alumina-silica — Potash-alumina-silica — Soda-alumina-silica — Barium oxide-alumina-silica — Magnesia-alumina-silica — Zinc-alumina-silica. Quarternary and other systems PAGES 371-382 382-398 398-405 406-411 4) 1-423 423-426 426-431 432-437 437-449 449-450 450-452 452-466 466-484 CONTENTS Fusion.—Constitution of fused masses—Solidification of molten masses DECOMPOSITION OXIDATION REDUCTION Corrosion.—Fireclay bricks—Silica ee eats ace payrits eS Pa aN bricks—Carbon, chromite, and carboxide bricks—Measurement of corrosion CHEMICAL REACTIONS OCCURRING AT LOWER TEMPERATURES.—Action of water—Action of acids—Action of alkalies—Weathering . CHAPTER XII. HEAT AND TEMPERATURE. Heat.—Temperature : : Heat MeasurEMENT.—Heat Rte Caberiiaters THERMAL Capaciry.—Specific heat—Atomic heat—Molecular naw : TRANSMISSION OF HEAatT.—Conduction—Thermal ee cee aon raameen GENERAL Errect or Heat on SuBSTANCES.—Changes in temperature—Changes in volume —Changes in physical state—Changes in other physical properties—Changes in chemical composition—Electrical changes—Changes in optical properties . TEMPERATURE MEASUREMENT.—Temperature units—Temperature scalese—Thermometers— Electrical pryometers—Optical pyrometers—Radiation pyrometers—Pyroscopes— Trials ; F : : A : : : ; . ; ; : : CHAPTER XIII. THE EFFECT OF HEAT ON CERAMIC MATERIALS. Errects oF Herat 1n Dryine Errects or Heat 1n Firing. Meee ihe ecules ieee fire ee encbing stage—Vitrification—Finishing temperature Errects or WirapRAWwInG HeEat.—Chemical a in ec ee changes in cooling Errrcts or EXCESSIVE Metre. Oatortion Boiling —Velneeheaion Errects oF PROLONGED HEATING Errect of REPEATED HEATING Errect oF HEAT ON THE VOLUME OF CERAMIC Mires pare ene volume- ae —Reversible volume-changes . Errects oF SUDDEN CHANGES IN TEMPERATURE Errects or HEAT ON THERMAL CONDUCTIVITY P : Errect or Hat on Speciric HEAT oF CERAMIC MATERIALS Heats or REAcTION In CERAMIC PROCESSES.—Heat of Bion Hees of tome. Heat of dissociation—Heat of transition—Latent heat of fusion . Errect of Heat ON REFRACTORINESS . xl PAGES 484-489 490-491 491-492 492-494 494-503 503-507 508-509 509-510 510-514 514-519 519-532 532-542 543 544-559 559-560 560-562 562-563 563 563-581 581-584 584-594 594-599 599-601 601-607 xi CONTENTS CHAPTER XIV. ELECTRICAL AND MAGNETIC PROPERTIES OF CERAMIC MATERIALS. ELECTRICAL CONDUCTIVITY AND RESISTIVITY OF CERAMIC MaTERIALS.—Factors influencing electrical conductivity and resistivity—Electrical conductivity of clay—Porcelain— Silica bricks—Fused silica—Magnesia bricks—Zirconia bricks—Chromite bricks—Car- borundum bricks—Clay slips—Determination of electrical conductivity, resistivity, ete. MAGNETIC PROPERTIES OF CERAMIC MATERIALS CHAPTER XV. OPTICAL: PROPERTIES OF CERAMIC MATERIALS. IDENTIFICATION OF CERAMIC MATERIALS BY OPTICAL PRoPERTIES. — Reflection — Refrac- tive index— Double refraction — Polarised light — Optical sign — paste activity— Pleochroism—Interference—Ultramicroscopic particles : : OpTIcAL PROPERTIES OF MANUFACTURED CERAMIC ARTICLES. —Transparency—Opacity— Translucency ~ ; ; PAGES 608-620 621 622-631 631-634 COABHUTPWHEE . Phase Diagram of Miscible LIST OF . Ungraded Mixture Mixture with Two Grades Mixture with Three Grades . . Schoene’s Elutriator . Lowry’s Elutriator . Boswell’s Grading Graph . Feret’s Triangular Diagram . Ludwig’s Volumeter . Seger’s Volumeter . Permeability Test . Sokoloff’s Permeability Apparatus . Tensile Test-piece . Tensile Testing Machine . Vicat Needle . . Specific Gravity Bottle . Viscosity Apparatus . Rate of Drying . . Heating Curve of Ceiba Clay . Fusion Curve of Felspar-Kaolin Mix- ture . Fusion Curve of Mica- Baoan Mixture : . Ludwig’s Chart . . Phase Diagram of Water . Albite-Anorthite Phase Diagram . . Phase Diagram of Wholly Miscible Liquids . Phase Diagram of Miscible Viquids (with maximum point) Vieuids (with minimum point) . Phase Diagram of Miscible Teqnide (with transition point) . Eutectic Phase Diagram . Solid Solution and Eutectic teins Diagram . Ternary Phase een of Drhoulass: Albite-Anorthite System . ILLUSTRATIONS . Ternary Phase Diagram of Quartz-Ortho- clase-Plagioclase System . . Time-Temperature Curve . Temperature-Composition and Time- Temperature Curves . Phase Diagram of Lime-Silica System (Day and Shepherd) . Phase Diagram of Magnesia- aioe Sys- tem (Sosman) . Phase Diagram of Lime- ateaatae System (Sosman) . Phase Diagram of Magnesia Alumina System (Sosman) . Fusion Curve of Alumina- Silica Metares (Seger) . Phase Diagram of Alanine: Ries Siyatora (Sosman) . Phase Diagram of ee Forts ‘Oxide System (Sosman) . Triangular Diagram of Magueats ‘Lime- Silica System (Ferguson and Merwin) . Phase Diagram of Lime-Magnesia-Silica System (Wallace) . Triangular Diagram of Lime- Alumina. Silica System (Little) . Triangular Diagram of Soda- AYnerna: Silica System (Wallace) . . Triangular Diagram of Magnesia- Ala: mina-Silica System (Sosman) . - Time-Temperature Curves of Crystals and Glass . Thermo-couple Pyrometer : . Resistance Pyrometer . Féry Radiation Pyrometer . . Seger Cones : : . Refraction of Light . Ultramicroscope PAGE 464 465 465 467 469 471 472 473 473 475 475 476 477 478 479 489 537 537 539 540 624 631 THE CHEMISTRY AND PHYSICS OF CLAYS AND OTHER CERAMIC MATERIALS CHAPTER I PHYSICAL STRUCTURE Ciays and allied materials—conveniently termed ceramic materials—are so complex, both in their chemical constitution and in their physical structure, and some of their properties are so difficult to investigate that, in any attempt to study them, it is well to “begin at the beginning,” and consider first the more obvious characteristics of their physical structure. Like all forms of matter, ceramic materials, and the substances into which they can be decomposed, exist in one or more of three forms or phases, namely, solid, liquid, and gaseous, and in various transition phases such as (a) pastes, which consist of a mixture of solid and liquid phases, the former predominating ; (b) slips, which are mixtures of solid and liquid phases, the liquid phase predominating ; and (c) colloids, some of which are similar to pastes and others to slips, in each case the solid phase consisting essentially of particles of ultramicroscopic fineness. Little is known of the gaseous phase of most ceramic materials, as the temperature required to convert them into this state is beyond the range of industrial furnaces. SoLips Most people have a fairly clear idea of the meaning of the term “ solid ” as applied to any substance, though it should be observed that some apparently solid materials— especially certain glasses and under-cooled fused materials—are more correctly described as liquids which are so highly viscous as to present the appearance of solids. Speaking generally, however, solid substances are of a firm, compact nature, and each piece or portion of a solid has a definitely measurable length, breadth, and thickness, which are independent of the support on which the solid is placed. As is well known, . ceramic materials occur in nature almost wholly in the solid state. All solid substances may be divided into two important classes, namely, crystalline and amorphous. CRYSTALLINE SUBSTANCES Crystalline substances consist of units of definite geometrical form, or of fragments of such units, and in this way are readily distinguished, either by the naked eye or I 2 PHYSICAL STRUCTURE by means of a microscope or other optical instrument, from amorphous substances, as the latter are not composed of such units. Crystalline substances may occur in various forms according to their environment and mode of formation. The chief forms are :—- (a) Perfect, regularly-defined Crystals.—Such crystals occur in rocks which have cooled slowly from a molten state; the least fusible minerals crystallise first and, unless their growth is impeded in some way, perfectly-shaped crystals are produced. Perfect crystals may also be produced when the liquid phase of a solution of one or more substances evaporates until the concentration of the liquid is too great for all the substances to remain in solution. If the rate of evaporation is rapid, small crystals will usually be formed, but if it is very slow—as is often the case in nature—very large and perfect crystals may be produced. Such crystallisation from solution occurs in nature in cavities in rock-formations, and it is also a result of metamorphism, in which case it is sometimes known as “ recrystallisation.” Among many refractory materials which occur in the form of perfect crystals may be mentioned quartz, dolomite, various limestones and magnesite, very large crystals being sometimes found. Many artificial refractory materials, such as artificial corundum, sillimanite, carbides, and carboxides also form perfect crystals. Crystals may occur either singly or associated in groups, when they are termed twins, trins or triplets, etc., according to their nature. Quartz, orthoclase felspar, and other minerals, frequently occur as twins, felspar having three distinct modes of twinning, whilst tridymite crystals, as their name signifies, consist of a union of three orthorhombic forms, which gives them a pseudo-hexagonal appearance. Some crystalline substances are composed of a large number of simple “ micro- liths,” or of incipient and incompletely-formed crystals, which are termed “ crystallites.” The use of ceramic materials in the form of coarse crystals is often very un- desirable, as it sometimes renders the materials unsuitable, except after a costly treatment, such as calcining, grinding, etc. (see Texture, Chapter IT). (b) Particles or aggregates of particles of irregular shape and not ex- hibiting any outward appearance of being crystals, but which have an internal crystalline structure. This type of particle or grain is most common in refractory materials which occur as compact rocks such as quartzite, ganister, sandstone, magnesite, dolomite, limestone, etc. The irregular exterior and absence of crystalline form may be due—as in the case of rocks, such as rock-quartz, which have cooled from the molten state—to a large number of crystals being formed simultaneously and impeding the growth of each other, so that few, if any, were able to develop perfect crystalline outlines, the others taking the shape of the space in which they were crystallising and forming irregularly-shaped grains with internal crystalline characteristics. Such grains are, in fact, very similar to fragments of crystals, and resemble perfect crystals which have been subjected to a cutting or abrasive action, which has altered their external shape, but not their essential nature. The irregular crystalline grains in sedimentary rocks, such as sandstones, ganisters, etc., are com- posed of grains originally derived from igneous rocks, the constituents of which CRYSTALLINE FORMS 3 crystallised irregularly as mentioned above. When an igneous rock is disintegrated, most of the loose grains are more or less irregular as a result of their mode of formation, and any perfect crystals which may have been present usually have their edges and corners removed by the attrition and corrosion which accompany prolonged exposure to weathering and other influences. The fact that a mineral is crystalline in character, even though it may possess quite irregular external outlines, may be readily detected by examining it under a microscope with polarised light. With the exception of minerals of the cubic system —which are invisible in the dark field produced by crossed Nicol prisms—all crystals and fragments of crystals are equally visible in polarised light as in ordinary light, whilst non-crystalline (isotropic) substances are invisible, or almost invisible, under such conditions. (c) ‘‘ Crystallites,’’ ‘* microliths,’’ or ‘‘ incipient crystals,’’ are generally formed by the agency of heat, and occur in nature in igneous rocks and also in some burned clay and other refractory products. They are not definitely crystalline, but possess a more or less regular shape. Globulites resemble small drops and are isotropic in character ; trichites are hair-like forms, whilst microliths take the form of minute rods or needles aggregated into groups or masses of varied shapes. Augite, horn- blende, felspars, etc., occur in rudimentary rocks in forms of this kind. Crystallites are usually colourless, but may, in certain cases, be black and opaque on account of a ferruginous coating. Some minerals, especially quartz, frequently enclose grains of other minerals. This occurs when molten quartz crystallises around the other grains during the cooling of a rock from the molten state, or the quartz may have been deposited from solution around the mineral grains. Inclusions of this kind usually occur either in the centre of the crystal or as zones around the centre. The enclosed grains are usually termed endomorphs, whilst the enclosing minerals are designated pervmorphs. The commonest inclusions in quartz crystals are rutile, hematite, limonite, pyrites, and chlorite. Calcite also frequently contains mineral inclusions. Filaments or streaks (due to various causes) are frequently found in crystals ; in orthoclase felspar, for example, they are a result of the partial kaolinisation of the material, whilst brownish patches and blotches may be due to decomposed magnetite. Tufts and vermicules of green ferruginous silicates and other minerals also occur in some clays. Inclusions of pale-green or brownish glass, often containing immobile bubbles, sometimes occur in crystals of quartz, felspar, and other minerals. In addition to solid inclusions, liquid and gaseous ones sometimes occupy cavities in crystals of quartz, felspar, etc. Cavities which are apparently empty or are filled with gas are usually spherical or elliptical in shape; sometimes they are so minute that a million of them could be contained ina volume of a cubicinch. Cavities filled with liquid and having sharply defined black borders when examined under the microscope are common in quartz crystals. They are generally spherical or elliptical, but are sometimes of a definite polygonal form; their size varies from microscopic dimensions to those sufficiently large to be readily visible to the unaided 4 PHYSICAL STRUCTURE eye. Such cavities contain water or salts of calcium, sodium, and potassium; they may also contain bubbles of gas—usually air or carbon dioxide. Cavities containing fluid are usually irregularly distributed, but they are sometimes confined to inter- secting planes, as in some specimens of quartz, felspar, topaz, and other minerals. Crystallography.—Substances which occur in crystalline form are characterised by one or more special forms, by which they may be identified. Crystals are generally bounded by a number of flat or plane surfaces, though curved ones also occur, as in some dolomite crystals. When all the faces are similar in shape, the crystal is said to be of simple form, whilst if dissimilar faces occur, the crystal is a combination of two or more simple forms, and is termed a combination. Unless distorted, all crystals have some form of symmetry, and crystals of the same mineral have the same type of symmetry. The shape of all normal crystals may be expressed in terms of lines or axes, which are the centres of planes which divide the crystals symmetrically in different directions. According to the form of the crystals these crystallographic axes may be equal or unequal in length, and either at right angles to each other or otherwise. Very careful examination has shown that six different arrangements of axes suffice to enable the shape of any crystal to be expressed in a simple manner, and as each of these arrangements of axes forms a definite “‘ system,” there are six systems of crystals. These are as follows :— 1. The Cubic System.—Crystals of this type have three equal axes at right angles, as chromate, spinel, periclase, and magnetite, all of which form octahedral crystals. 2. The Tetragonal System.—Crystals in this system have two equal lateral axes and one vertical axis at right angles. It is represented by zurcon, rutile, and anatase, which usually occur as tetragonal prisms combined with tetragonal pyramids. 3. The Hexagonal System.—Crystals in this system have three lateral axes, making angles of 120° with each other, and a vertical axis perpendicular to the plane containing the lateral ones. In this system are quartz, which forms hexagonal prisms, together with positive and negative rhombohedra, the trigonal pyramid and trapezohedron and calcite, dolomite, magnesite, corundum, hematite, and slmenite, which consist essentially of rhombohedra and scalenohedra. 4, The Orthorhombic System.—The crystals of this system have three unequal axes at right angles, and are represented by andalusite, which is a combination of the prism, basal plane, and sometimes a small macradome ; brookite, which forms thin plates ; sillimanite, which occurs as long needle-shaped crystals ; and tridymite, which consists of three orthorhombic individuals and appears to be pseudo-hexagonal. The various varieties of asbestos also belong to the orthorhombic system and form long blade-like crystals, which give aggregates of crystals a fibrous structure. 5. The Monoclinic System.—The crystals of this system have three axes, one vertical, one lateral and perpendicular to the vertical axis, and the other at an angle to both. Kaolinite, monazite, wolfram, scheelite, sphene, titanite, as well as orthoclase felspar, hornblende, mica, etc., belong to this system. 6. The Triclinic System.—The crystals of this system have three unequal axes, none of which are at right angles, e.g. cyanite, microcline, and plagioclase felspars. Multiple Forms of Crystals.—Although, under normal conditions, most sub- AMORPHOUS SUBSTANCES ‘i stances crystallise in one definite form, this is not a necessary characteristic. For instance, crystals of widely-differing substances may possess identical crystal forms. Such minerals are said to be isomorphous (Gr. isos, equal; morphe, shape or form). Typical examples of this are calcite and magnesite, which both normally form trigonal erystals with a rhombohedral cleavage. These two minerals are able to replace each other in crystals of either material without any change in the crystalline form. Well-known crystals containing a mixture of these two minerals form the mineral dolomite. Other isomorphous substances are (i) alumina and ferric oxide; (ii) sodium and potassium oxides; and (iii) the plagioclase felspars. Thus, hematite will replace calcite, dolomite, quartz, barytes, pyrites, magnetite, rock salt, fluorspar, etc. Quartz will replace calcite, aragonite, siderite, gypsum, rock salt, haematite, etc. Isomorphism is more fully dealt with in Chapter VIII. The conditions under which crystallisation occurs may also be such that the same substance may crystallise in a different form. Substances which occur in crystalline form may have several geometrical shapes and are then said to be dimor phous, trimorphous, etc., according to the number of shapes they may assume when crystallising freely. These terms are not applied to crystals whose external forms are due to their being formed in unfavourable surroundings which hinder the development of the normal outlines of the crystals (p. 2). Calcium carbonate is a.typical example of dimorphism, as it is capable of crystallising either in rhombo- hedral crystals, as in calcite, or in orthorhombic crystals, as in aragonite. Titanium oxide, which is known in three different crystalline forms (rutile, anatase, and brookite), is typically trimorphous. Where a substance has a still larger number of crystalline forms, it is termed polymorphous. Pseudomorphism.—Some minerals have a crystalline structure which is not characteristic of them, and are then said to be pseudomorphic. Pseudomorphism may be due to (a) the secondary growth of one mineral around another, the former thus appearing to take the same crystalline form as the latter ; (b) the infiltration of a solution into geometrical cavities left by the prior solution of other substances ; the fresh solution crystallising and filling the cavity may take the same shape as the latter, and thus may appear to crystallise in the same form as the mineral which had previously occupied the cavity ; (c) the gradual removal of portions of crystals by solution and a similar gradual deposition of other crystals from solution; and (d) the gradual change or metamorphism of a mineral by various agencies, so that in time the mineral has an entirely different composition, whilst still retaining its original crystalline form. AMORPHOUS SUBSTANCES Amorphous solid substances have no definite shape or geometric internal structure. They may be subdivided into (a) glassy or vitreous substances ; (b) colloidal substances or gels ; (c) substances of a cellular structure, such as kieselguhr, lava, etc. ; and (d) substances which have no definitely recognisable structure and can only be described as amorphous. The term ‘amorphous’ is the converse of ‘ crystalline’ and has reference only to the absence of a geometric internal structure, as many amorphous 6 PHYSICAL STRUCTURE substances have a definite, external form. For example, kieselguhr is seen, under a microscope, to have a perfectly-defined, cellular structure. The difference between amorphous and crystalline substances lies in the fact that whilst the latter, when crushed, still retain their definite structure, that of amorphous substances, being merely external, is destroyed when they are pulverised (see pp. 7, 8). Glassy or Vitreous Substances.—Sometimes substances which have been cooled from a molten condition do not crystallise, but form isotropic, structureless, glassy masses. These glasses, even when containing few constituents, usually have a complex chemical constitution, and those formed naturally are often of bewildering complexity, because they have been produced by the fusion of a number of minerals into one homogeneous mass. Some of these glasses contain, as inclusions, irregular grains, crystals, or crystallites of other substances. The glasses may be of various colours, according to their chemical composition, though generally natural glasses are dark green in masses, but pale brown or nearly colourless in thin sections. Colour- less, transparent glasses can only be produced in the absence of iron, copper, cobalt, and other compounds which form coloured silicates. Hence, colourless glasses are usually made of very pure lime, soda, or potash and silica. In Nature, glassy substances occur chiefly in igneous and volcanic rocks, but they seldom occur to any serious extent in the raw materials used in the ceramic industries. In articles made from clay, on the contrary, a glassy or vitreous substance is a common constituent, and is, in fact, one of the most important substances pro- duced during the firing of bricks, tiles, pottery, and other ceramic articles, the glassy material acting as a bond which unites the less fusible materials together and forms the whole into a strong, hard mass. Without this glassy material, the articles would be weak and fusible, and quite unsuitable for the purposes for which they are used. The nature and purpose of this glassy material in finished goods is more fully dealt with later, when considering the various types of structure in the raw materials and finished products. The glassy material thus formed bears a closer resemblance to glazes and slags than to the glass of commerce and usually contains more constituents than the latter. The glazes which produce a glossy appearance on pottery are also of the nature of glasses. The materials necessary for their production are finely ground, mixed in the required proportions, and then applied to the ware. The “glass” is then formed by heating the articles, so covered, in a kiln until the various constituents of the glaze react on each other and fuse to a viscous fluid which, when cooled at a suit- able rate, produces a homogeneous glassy mass. Some glazes on clay-wares are partially crystalline ; this is largely due to the composition—the fluid at a compara- tively low temperature not being able to retain some of the silicates in solution, with the result that they crystallise—but such crystallisation is also facilitated by very slow cooling. When a glaze, containing zinc or titanium oxides, is cooled slowly from 1300° C. to about 1100° C., some crystallisation usually occurs as the silicates of these metals are only slightly soluble. A “glass” which differs from nearly all others in containing only one constituent is made by fusing sand, or other fragments of quartz, AMORPHOUS SUBSTANCES 7 and is consequently known as quartz glass. It is avery valuable material for several purposes, as it is highly refractory and has an unusually low coefficient of expansion. In this respect it differs from most glasses which have a greater expansion coefficient and, consequently, are unduly sensitive to sudden changes in temperature, with the result that, when suddenly heated or cooled, they are very liable to break, on account of the internal strains which result from the irregularitiesin the expansion of the various parts of the material. Articles made of quartz glass, on the contrary, can be heated to redness and quenched in water without suffering any obvious ill-effects. Some samples of quartz glass begin to crystallise or devitrify when heated for a long period between 1200° C. and 1600° C. Such a partially crystalline structure is undesirable, as ware in which it occurs is very brittle and is not so durable as when the material has a wholly vitreous or glassy structure. Colloidal Gels.—Substances occur in clays and some allied materials which have been formed by the coagulation of colloidal sols, the coagulum being afterwards dehydrated and possibly hardened by pressure and other influences. Chalcedony and, possibly, flint and chert are of this character, but small proportions of other colloidal gels occur in many ceramic materials. Some of these colloids are quite permanent in character; others which are of a softer nature may sometimes be reconverted into the sol state by treatment with water containing a suitable salt such as sodium carbonate. Colloidal substances are so important in connection with clays that they are more fully described on p. 10 and in Chapter VI. Cellular substances are not common in clays; among other ceramic materials the most important ones with a cellular structure are: (a) those substances such as pumace, which are too fusible to be used to any great extent as refractory materials, and (6b) diatomaceous earths, which consist of the minute, siliceous skeletons of dead marine and fresh-water plants such as diatoms, radiolaria, etc. One of the most extensively used refractory materials with a cellular structure is known as kieselguhr. Its texture is readily seen when it is observed through a microscope, though to the naked eye it appears to be structureless. Moler consists chiefly of a similar material to kieselguhr, but it also contains a considerable proportion of clay and often of volcanic ash. The clay acts as a bond which unites the other particles into a fairly strong mass. Other amorphous substances are those which have no definite crystalline form and cannot be included among the foregoing groups of amorphous materials ; they usually occur as irregular granules, united to form larger masses or as stains or films on other grains. Dried or indurated clay is one of the commonest of such minerals; other amorphous clay-like substances are halloysite, allophane, collyrite, nacrite, and lithomarge. Some forms of silica, such as opal and siliceous sinter, are amorphous, but may be colloidal gels (supra), whilst many crystalline minerals, such as limestone, magnetite, and graphite often occur in the amorphous state as a result of abnormalities in their mode of formation, or of the forces to which they have afterwards been subjected. It is by no means easy to decide whether some substances are amorphous or 8 PHYSICAL STRUCTURE crystalline. If the particles of which they are composed are sufficiently small, their optical characters cannot readily be determined. The use of X-rays in this connection appears to be very promising (see Chapter VIII), and by this means W. H. Bragg has shown that china clay is largely crystalline in character, though the individual particles are far too small for this to be determined by other means. Many sub- stances which appear to the naked eye to be amorphous are found, when examined by polarised light, under a microscope, to be crystalline. Thus, cryptocrystalline magnesite is sometimes termed “‘ amorphous magnesite,” but this is incorrect, as its texture is definitely crystalline, though the particles are extremely small. Hydro- magnesite appears to be an amorphous variety of magnesite, but its ultimate structure has not been accurately determined. Graphite, which is largely used as a refractory material, consists almost wholly of apparently amorphous grains. Doubt has, however, been expressed on the precise structure of graphite, and it may possibly be largely crystalline in character. MIXED MATERIALS Most ceramic materials are not homogeneous in structure, but consist chiefly of amorphous materials with a variable proportion of crystalline substances—usually present as impurities. Silica rocks, on the contrary, are largely crystalline in character and the amorphous material in them contains the chief impurities. Other refractory materials, such as magnesite, chromite, etc., may be either crystalline or amorphous, or a mixture. Artificially prepared refractory materials, such as car- borundum, sillimanite, etc., usually contain both the crystalline and amorphous forms of the material. Effect of Heat on Amorphous and Crystalline Materials.—The general effect of heat on crystalline materials containing combined water is to convert them into an amorphous form, this change being accompanied by some decomposition of the material. Most anhydrous crystals (apart from any decomposition which may occur) do not show any change until they fuse and form a viscous fluid, but some, such as quartz, undergo marked changes. Amorphous substances, when heated, usually tend to become denser prior to fusion. Most minerals which can be fused without decomposition will, if allowed to cool under suitable conditions, enter into the crystalline state. ; The effect of heat on ceramic materials is by no means simple; it is discussed more fully in Chapters VIII, XI, and XIII. PASTES Pastes are mixtures of one or more solids and liquids and, whilst they have many characteristics of both solid and liquid substances, are merely physical mixtures and not chemical combinations, yet they have additional properties which cannot be fully predicted from those of their constituents. Thus, a sample of brick dust may appear to have a physical character very similar to that of the dry clay from which it is made, but if each material is separately mixed with about one-fifth of PASTES 9 its weight of water, the clay will produce a paste which has very definite characteristics, such as its plasticity, flow under pressure, etc., which the wet brick dust does not possess. If the clay paste is allowed to stand on a slightly-sloping board it will harden gradually as a result of loss of water by evaporation, but the brick dust will allow a considerable proportion of water to drain away, and the solid material, as it dries, will become increasingly friable and will, at a slight touch, fall to powder. In other words, in a true paste, there is much more cohesion between the solid and liquid particles. This cohesion varies with different liquids ; thus, clay readily forms a paste with water, but not with paraflin or with alcohol. The chief value of pastes in the clayworking and allied industries is the ease with which they can be moulded into convenient shapes, which they retain indefinitely. Speaking generally, a satisfactory paste can only be produced when the solid constituent is in the form of a powder. Hence, indurated clays and massive pieces of material must usually be ground before they can be made into a paste. Many clays occur in nature in a pasty form; the consistency of such clays can be varied by adding more water, the amount required depending upon the nature of the material and on the amount of water previously present in it. The larger the proportion of water the more mobile will be the resultant paste. Pastes may be regarded as very viscous fluids which require pressure to be applied to them before their “ flow”’ can be observed. This property is due to the cohesion between the solid and liquid constituents, whereby the mixture forms a uniform mass in which the liquid acts as a lubricant facilitating the movement of the solid particles and simultaneously has a restraining influence and prevents them from being separated far from each other. The extent of this cohesion depends on the affinity between the particles of solid and liquid. It is high in plastic materials like clay, which have a high capacity for being deformed by pressure without separating the individual particles from each other, whilst materials in which the cohesion is low do not form good pastes and are termed non-plastic materials. It was at one time thought that this cohesion or plasticity was due to the chemical constitution of substances which possess it, but it has since been ascertained that most insoluble substances, if sufficiently finely ground and partially converted into the colloidal gel state, become plastic. The practical difticulty—which is almost in- superable with most minerals other than clay—is to reduce them to such a fine state of division that plasticity becomes possible. Where no colloidal matter is present, the mixture does not form a true paste. Thus, a mixture of fine sand or rock dust and water does not contain sufficient colloidal material, so that the solid particles are held very loosely together, whereas a mixture of clay and water—which contains a considerable proportion of colloidal matter—forms a highly-plastic paste. The nature and effect of this colloidal matter are described more fully on p. 10, and in Chapter VI. Pastes are employed in the manufacture of most articles produced in the ceramic industries. Thus, bricks are made either by introducing the paste into a mould and compressing it so as to fill the mould completely, or by extruding the paste through an opening of a suitable size, so that the paste issues in the form of a long 10 PHYSICAL STRUCTURE column which may be cut into pieces of the required length. Pastes made of refrac- tory clays, etc., are used instead of mortar for laying bricks in furnaces, and for patching retorts, furnaces, etc. Further information on pastes will be found in Chapter VI. SLIPS A slip or slurry is a mixture of solid and liquid in which the liquid predominates, so that the mixture may be regarded as a suspension of the solid matter in the liquid. The properties of clay slips are dependent chiefly upon the colloidal state of the suspended matter in them, but whereas the colloidal matter is in the gel state in clay pastes, it is in the sol state in clay slips. Slips may, however, be made entirely from non-plastic materials and water. The properties of slips are also intermediate between those of solids and liquids, but a slip, like a liquid, takes the shape of the vessel into which it is placed. It also flows like a liquid and does not require any pressure to deform it, whilst a solid or a paste will not flow, but only changes its shape when subjected to pressure. Slips are largely used in the manufacture of clay and other ceramic products by the process known as casting, in which the slip composed of the clay or other suitable material, mixed with water, is poured into a plaster mould of the desired shape. As the plaster is porous it absorbs some of the water and the interior of the mould is thereby covered with a thin coating of paste, its thickness depending on the time the slip remains in the mould. After a suitable interval any surplus slip is poured off and the mould is set aside to dry, after which it is easy to remove the solid material in the form of an article of the desired shape. The surfaces of some wares are also often coated with clay, clayey mixtures, or glaze, which is applied in the form of a slip, the ware being sufficiently porous to absorb the water and leave a thin coating of solid matter on the surface. The use of slips in this manner is often very con- venient for applying cheaply and almost instantaneously a much thinner layer than would otherwise be practicable. Further information on slips will be found in Chapter VI. CoLLoIDs A substance in the colloidal state is sometimes regarded as intermediate between that of a solid and a liquid, but, although many pastes and slips owe some of their properties to the colloids they contain, these colloids are something different from a mere mixture of solid and liquid. Thomas Graham, in 1861, discovered one of the chief characteristics of colloids when he found that certain liquids (apparently solutions of glue, gelatin, and similar substances) behaved quite differently from solutions of crystalline substances, inasmuch as the former would not pass through a membrane having water on the other side of it, whilst crystalline substances in solution passed readily through the membrane. To these non-permeating, amorphous materials he applied the term colloid (from kolla=glue or gum) and supposed that they were a separate class of substances. Since the time of Graham, however, it has COLLOIDS LL. been found that most substances can be obtained in the colloidal state in the presence of a liquid in which they are insoluble. The colloidal state may be defined as a physical condition of matter consisting of at least two parts ! or phases, one (the disperse phase) being suspended or distributed in the other (the dispersion medium). The particles comprising the disperse phase are extremely minute, so that the force of gravity is counterbalanced by other forces which keep them in suspension. These forces are due to the electrical charge pos- sessed by each particle, which causes it to repel other particles similarly charged, and, as the particles are closely associated, these repelling influences cause the particles to be in a state of constant unordered motion, visible -under the microscope, which is termed the Brownian movement. This motion is only observable in liquids containing very minute particles in suspension, all the suspended particles having the same electric sign. If particles of opposite sign are introduced into such a liquid, the two groups of oppositely charged particles are rapidly attracted to each other and, as together they are too large to remain in suspension, they gradually settle to the bottom of the vessel and are said to forma coagulum. The coagulation or flocculation of a colloidal substance in this manner must not be confused with chemical precipita- tion, which is of quite a different character. Colloids differ greatly from solutions, in that they have only a slight influence on the vapour-pressure, freezing-point, and boiling-point of the dispersion medium. It was at first thought that colloidal particles were amorphous, and Graham proposed to distinguish them by using the terms colloid and crystalloid, but it has since been found that many crystalloid substances can be converted into the colloidal state whilst still retaining their crystalline structure. Thus, colloidal gold is almost certainly crystalline, and the X-ray spectrum of china clay recently obtained by W. H. Bragg suggests that what appears to be colloidal china clay is also crystalline ; it is also very probable that other colloidal clays have a crystalline structure. Asa general rule, however, most colloids are not crystalline, but resemble gelatin, milk, or the white of an egg. Colloidal Sols—The chief distinction between a “colloidal solution” and an ordinary suspension is that in the latter a settlement or deposition of the suspended particles occurs rapidly, whilst in a “colloidal solution” the particles remain in- definitely long in suspension as the result of a force which overcomes the natural tendency to settle as a result of the effect of gravitation. The maximum size of the suspended particles is limited by the effect of gravity ; there is no known minimum limit to their size. A suggestion by Wo. Ostwald, which has been largely adopted, is to regard particles greater than 0-0001 mm. as forming ‘“ coarse suspensions,” those between 0-0001 and 0-000001 mm. as “ colloidal solutions,’ and those smaller than 0-000001 mm. as forming molecular solutions. Colloidal sols are singularly sensitive to very minute quantities of certain substances, such as acids and salts, which cause the 1 Whilst a solution is commonly regarded as a one-phase substance and a colloidal sol as a two-phase liquid, there appears to be a regular continuity between sols and solutions, so that this view must not be taken too rigidly. 12 PHYSICAL STRUCTURE particles to adhere to each other, or flocculate, forming larger masses which rapidly settle out of suspension and form a sediment. Colloidal Gels.—When a colloidal sol is treated in such a manner that the suspended matter is coagulated, “‘ flocculated,” or so altered as to form a deposit or sediment, the product is often of a gummy or horny character and is now known as a gel. A gel appears to consist of an intensely fine network, the meshes of which can retain a very large proportion of water or other fluid. When a dry gel is placed in water it swells and becomes several times its original size. In its swollen state it is much more mobile and is almost a liquid. If the swelling can continue sufficiently, or if the mixture of swollen gel and water is well stirred, the gel may be broken up into a series of much smaller masses, which may form either a colloidal sol or a true solution according as the gel-forming substance is insoluble or soluble in water. The chief colloidal systems which are important in connection with the properties of clays are: (a) particles of solid suspended in a liquid (solutions and suspensions) ; (b) particles of liquid suspended in a liquid (emulsions). In each case the material as a whole may appear to be solid, just as a jelly may appear solid and yet contain 95 per cent. of water. Plastic clays appear to be derived from colloids of the first group, whilst clay slips and slurries appear to be crude mixtures of colloidal sols and other substances. Other colloidal systems which are of some importance in connection with ceramic materials are: (c) particles of solid suspended in another solid in the form of a solid solution. This occurs in many minerals used in ceramics and also in many manufactured products, such as glazes, enamels, glasses, and the fused mass which binds the particles of many ceramic wares together ; (d) liquid particles suspended in a solid. Much of the liquid found in minerals is in this form; (e) gas particles sus- pended in a solid, such as bubbles of gas suspended in minerals and in the finished articles produced in the manufacture of ceramic wares. When the disperse medium is water the terms hydrosol and hydrogel are used. In the clay-working industries no disperse medium other than water is used, so that the terms sol and gel in this volume invariably refer to hydrosols and hydrogels. Colloids may thus be divided into two classes : those consisting of a mobile liquid and referred to as sols, which are in turn divisible into emulsoids, in which both the disperse phase and the dispersion medium are liquids and suspensoids, in mae the disperse phase is solid. Colloids in the second class are termed gels ; they consist of a coagulated ok and they have a cellular structure which, whilst clearly indicated by some of their pro- perties, is too minute to be seen, even under a powerful microscope. Table jellies are typical gels, but many substances, such as some clay pastes, are equally in the gel state. Gels appear to be solid, but most of them, including those related to clays, con- tain a large proportion of water. Some organic gels contain as much as 95 per cent. of water and yet retain the appearance and many of the properties of a solid. By some means, which is not yet fully understood, the truly solid portion of a gel assumes a structure of a network or micellian character and can then absorb many times its weight of water without losing its characteristic properties as a solid. COLLOIDS 13 If the dispersion medium (e.g. water) is evaporated, the gel hardens and in time may form a hard, horny mass, as in the case of chalcedony and glue. Where the treatment is not too drastic, a gel may be reconverted into a sol by a sufficiently powerful grinding mill, such as the Plauson mill, but when a gel has been dried drastically it is not always possible to reconvert it to the sol state. The chief charac- teristics of a dry colloidal gel are that, when mixed with water, it swells like glue, forming a soft mass or jelly, and when the latter is dried it shrinks and forms a dense, hard, and horny mass. When a sol is dried it is usually converted into a gel, but can often be reconverted into a sol by suitable treatment. In the case of clay gels, the simplest method is to stir the material vigorously with a very dilute solution of sodium carbonate, which will rapidly convert the greater part of the clay gel into the sol state. The colloidal state of matter is of great value in the ceramic industries, as the binding of the solid particles together often depends largely upon the presence of colloidal matter. The plasticity of clays appears to be chiefly due to such material, and many non-plastic refractory materials which cannot be reduced to the colloidal state when merely mixed with water may be made plastic by the addition of colloidal matter. It appears to be quite certain that some of the most characteristic properties of clays are due to the fact that they contain colloidal matter, but it is scarcely correct to state, as some have done, that “clay is essentially a colloidal substance.” A paste made of plastic clay appears to consist essentially of particles each of which is an inert, porous core, filled with and surrounded by a relatively large, yet actually very small film of jelly-like colloidal gel, which is able to retain sufficient water to enable the particles to move freely over each other when subjected to a com- paratively light pressure, the particles retaining their shape when the pressure is removed. As such a mass dries, the jelly-like coating on each particle shrinks, drawing all the particles nearer together and forming an apparently non-colloidal, hard mass. Its plasticity and other characteristics may, however, be restored on mixing it with a suitable proportion of water. A material composed of minute spheres or “ crumbs,” each in turn consisting of a mass of interlaced lath- or plate- like insoluble crystals, could have many of the chief properties of clays, including plasticity, adsorption, shrinkage on drying, resistance to abrasion, and to various chemicals and semi-permeability. Hence, it is possible that plastic clays, contain a noteworthy proportion of matter of this nature, which would, under favourable conditions, show various colloidal phenomena. It is not necessary that the per- _centage by weight of colloidal matter should be large, as colloidal properties are all due to the nature of the surface of the particles. The presence of matter in the colloidal state also explains the behaviour of many other substances used in the clayworking industries. Thus, when colloidal gels are heated, they change more rapidly than crystals of the same composition. This may account for the greater ease with which flint and other amorphous forms of silica may be converted by heat into other forms of silica far more quickly than silica in the crystalline state. Further information on the colloidal state and its value are given in Chapter VI. 14 PHYSICAL STRUCTURE STATES OF AGGREGATION The various ways in which the components of ceramic materials are aggregated together is of great importance to the manufacturer, as it sometimes determines their usefulness. The principal forms of aggregation or structure are :— 1. Crystalline.—The mass may consist wholly of crystals more or less perfectly developed and interlocking with each other to form a compact, impermeable material. Such structures are usually the result of slowly cooling a molten material, meta- morphism, or crystallisation from solution (p. 2); typical examples are rock- and vein-quartz, quartzite, crystalline limestones, dolomite, and sometimes magnesite. Many artificial refractory materials, such as carbides and artificial alumina, are also wholly or partially crystalline. The properties of a crystalline mass are in some ways advantageous and in others detrimental; thus, when a material occurs in crystalline form it is generally fairly pure, although there is always the possibility of isomorphous replacement. Some crystalline materials have, however, the disadvantage of undergoing serious changes when heated, and this may cause difficulties when they are used. A typical example of this is quartz or quartzite, which is used in the manufacture of silica bricks. When these bricks are heated, the silica is changed from the forms in which it usually occurs in nature to allotropic forms of lower specific gravity and, therefore, occupying greater space. This change occurs very slowly in the case of crystalline quartz, but much more rapidly when amorphous silica is heated. Thus, the cost of heat treatment is much greater for crystalline than for amorphous silica, and the conversion is not usually so complete. For this reason, a crystalline structure is not so desirable in the case of siliceous materials, but as the crystalline forms of silica are abundant and relatively pure, whilst the amorphous forms are more scarce and not nearly so pure, the crystalline forms are chiefly used. Where the crystalline form is stable at high temperatures, it has the advantage that crystals are less easily attacked than amorphous material, and, therefore, bricks or other refractory articles with a suitable crystalline structure (such as sillimanite, tridymite, carborundum, corundum, periclase, etc.) are more refractory than those composed of amorphous material. . Where the material has an appreciable coefficient of expansion, crystals are more hable to crack and cause the disintegration of the ware than are amorphous grains. Wernicke has shown that quartzites of a wholly crystalline character (other than those consisting of tridymite) cannot be made into satisfactory silica bricks unless clay or other contractile bond is added to them. M‘Dowell, on the contrary, has found that the most satisfactory quartzites used in America consist wholly of interlocking crystals. In general, such quartzites are not so satisfactory as those containing a siliceous cement. From the point of view of users of refractory materials, satisfactory crystalline structures have almost negligible coefficients of expansion. Wholly crystalline structures are fairly common amongst some raw refractory STATES OF AGGREGATION 15 materials, but are seldom seen in the finished products. In the latter, there is usually some other material between the crystals ; it is generally of a glassy nature. Wholly crystalline structures may be classified according to their coarseness into (a) coarse crystalline, in which the grains are readily visible to the unaided eye, and (b) fine crystalline, micro-crystalline or crypto-crystalline, in which the particles are much finer, the last two requiring a microscope to enable the crystalline structure to be identified. A fine crystalline texture is generally preferred both in raw and finished refractory materials, as it has some of the advantages of amorphous structures, without possessing in such marked degree the disadvantages of a coarse crystalline structure already enumerated. A moderately coarse crystalline structure is, however, desirable in the case of magnesite, as such materials are more readily calcined and the carbon-dioxide escapes more easily than from a fine-grained or crypto-crystalline magnesite. Crystalline quartzites in which the quartz grains show clear cracks are not suitable for the manufacture of bricks, as they tend to be weak along these lines. 2. Granular.—Most raw materials and finished products have a granular structure and consist of grains of either crystalline or amorphous matter cemented together by some form of bond which may be glassy, amorphous, or colloidal. In the raw materials, the principal cementing materials are quartz, opal, chalcedony, iron oxides (hematite, limonite, and magnetite), carbonates of calcium, barium, mag- nesium, and iron (calcite, witherite, dolomite, chalybite, and ankerite), sulphides of iron (chiefly pyrites), hydrated silicates (zeolites, chlorites, epidote, serpentine, talc), anhydrous silicates (felspar, hornblende, mica), sulphates of calcium and barium (gypsum, barytes), and various phosphates. Of these, the commonest cement is silica, but carbonates, sulphates, silicates, and iron oxides are also important cements. ; Colloidal silica cement is usually produced by the weathering of silicate rocks ; the percolating waters containing the silica in solution descend until they reach a porous rock, in which the silica, being unstable, tends to separate from the solution and is deposited as a cement between the grains of the rock through which it is percolating. A siliceous cement may also be formed as a result of the silica being precipitated owing to an increase in the temperature of the solution. Many rocks are united by a calcareous cement derived from water containing carbon dioxide which has previously percolated through limestone or chalk and has dissolved some calcium carbonate. The solution is carried through the fissures and pores of other rocks until the carbonate is deposited and forms a calcareous cement. Ferruginous cements are common in sandstones and other rocks; they appear to have been formed by the percolation of water containing ferrous carbonate or hydroxide in solution. Gypsum acts as a cement in some shales and sands. Thus, Fontainbleau sand contains sand-calcites which consist of isolated masses composed of gypsum and sand. Some of the Northumbrian fireclays are, according to Hutchings, cemented by barytes. The precipitation of cements in rocks is usually a result of one of the following actions: (a) the mingling of solutions from different sources and their mutual 16 PHYSICAL STRUCTURE precipitation; (b) the chemical action between solutions and the rocks they traverse ; (c) a decrease in temperature causing supersaturation ; and (d) a decrease in pressure. When the cement occurs in large proportions in a brickmaking or refractory material, it is generally undesirable unless it is very highly siliceous or argillaceous and is itself refractory. In the finished goods, the cement produced by the burning process is usually of a very complex nature and is generally in the form of a homogeneous glass com- posed of various silicates, alumino-silicates, etc. The larger particles or granules in a material of granular structure may vary greatly in size from that of pebbles (asin quartzitic conglomerates, which are sometimes employed for the manufacture of silica bricks) to the fine, amorphous grains which comprise the bulk of most clays, these latter being so minute as to render identification impossible, even when they are examined under a microscope. The granules may be either amorphous or crystalline or an indefinite mixture of materials in both these forms. The principal types of granular structure are as follows :— (a) Granular Fragments in a Glassy Matrix.—These are not very common in raw refractory materials, though quite usual in igneous rocks. This structure is found in porcelain and in fired magnesia blocks or silica bricks, the two latter consisting of crystals of periclase and quartz-and other forms of silica respectively, cemented together by a glassy mass composed of various complex silicates and alumino-silicates and sometimes including minerals such as wollastonite and anorthite, in which is embedded any crystals of tridymite and cristobalite which may be present. The whole mass is usually coloured by the iron compounds present in the raw materials. The granular matter may be either crystalline or amorphous and either coarse or fine. Silica bricks may contain silica in any of the three allotropic forms of silica, so that the following structures may occur :— . (i) A mass of unaltered quartz grains united by a glassy bond. (ii) A mass of unaltered quartz crystals and tridymite needles embedded in a mass of glass. (iii) A mass composed wholly of tridymite and cristobalite united by a glassy matrix. This last texture is the most desirable, as such bricks are practically free from after-expansion and are very resistant to sudden changes in temperature. Most commercial silica bricks are of the first type and contain only a very small proportion of tridymite and cristobalite. The best, however, should correspond to types (ii) and (iii) as the larger the proportion of tridymite and cristobalite in the - articles after firing, the greater will be their value and durability. Silica bricks of type (i) are quite satisfactory if the grains are sufficiently small, but large grains make the bricks very sensitive to sudden changes of temperature, so that they tend to crack and spall when in use. Some investigators, including O. Lange, Endall, and others, do not consider the presence of tridymite to afford any special advantage and have even stated that bricks rich in tridymite are weaker and more likely to crack than “‘ unaltered calcined STATES OF AGGREGATION 17 quartz.” They agree with others, however, that bricks made from erratic boulder or “ Findlings ” quartzite and some ganisters in which a contractile bond is present, are amongst the best and most durable when in use. Gamister bricks consist of silica grains bonded together, partly with clay and partly with lime slurry. The resultant texture on burning is similar to that of ordinary silica bricks, but should preferably be of the type (ii.), p. 16. Sand-lime bricks have a similar texture to ordinary silica bricks, the grains of quartz being cemented together by means of a complex lime-silica glass. The chief difference is that in a refractory silica brick each grain of silica must be small, whereas in a sand-lime brick the grains may be relatively large. The latter structure is not satisfactory for bricks which have to be repeatedly subjected to a high temperature, because they would then be liable to crack and be of very low durability on account of the strains set up when the large grains of quartz were heated (see p. 14). The prolonged heating to which silica is subjected when in use often effects a further conversion and an increase in the amount of tridymite or cristobalite crystals present. Thus, the siliceous hearths of acid-open-hearth steel furnaces, after being in use for a considerable time, consist largely of crystalline cristobalite at the surface with tridymite a little below it, both these materials being bonded by a glassy slag con- sisting chiefly of fayalite. Silimamte bricks should consist entirely of crystals of sillimanite, bonded by a small quantity of glassy matter, but, with the kilns at present available, this structure cannot be obtained unless the sillimanite is used as the raw material, together with sufficient clay to make the required bond. The ideal structure of glass pots, accord- ing to M. W. Travers, consists of a network of sillimanite crystals interwoven so as to form a strong and sufficiently dense mass. This is also the ideal structure of all refractory clay products, but it is never attained on account of the long period of heating which would be necessary to convert all the clay into sillimanite. Bricks made of artificial corundum and, in some cases, those made of carbides, consist of crystals bonded together by a complex glass. The latter does not always fill all the spaces between the crystals, there being usually a number of cavities which render the material somewhat porous. Lime, carborundum, chromite, graphite, coke, and other refractory bricks consist of amorphous particles set in a glass composed of some fusible material which is mixed with the refractory material to give strength during the manufacture and to hold the particles together until they have been heated to a sufficiently high temperature to sinter and bind them into a hard, solid mass. Bricks and other articles made of burned clay usually have a granular structure consisting of irregular grains of “‘ burned clay,” ! united by a film of complex glass consisting chiefly of silicates and alumino-silicates. If the temperature has been sufficiently high, a few crystals of sillimanite, etc., may be formed, but these are unusual except in some porcelains and in fireclay articles of a highly refractory character, such as glass-melting pots. In most articles made of clay the proportion 1 For the present this term is preferable to one purporting to give the constitution of the material. 2 18 PHYSICAL STRUCTURE of glassy bond or matrix is very small; it is larger in vitrified ware, such as paving bricks, some stoneware, and porcelain or china ware. The texture of clay-silica bricks, or semi-silica bricks, is similar to that of other bricks, except that sand grains or crushed quartz rock are present along with the clay. Similarly, grog bricks are made of particles of burned clay (grog) as well as of raw clay. Kieselguhr bricks consist of minute grains or cells of diatomaceous silica united by a glassy clay bond. Onaccount of the nature of the kieselguhr (p. 7) the resultant bricks are extremely porous, this property being most important for this class of ware. Only just sufficient clay or other bond should be present to unite the particles together, any excess being useless and only tending to reduce the porosity of the finished articles. Obsidianite bricks, such as those made by Charles Davidson & Co., Ltd., consist of grains of silica almost wholly enclosed in a glassy matrix, so that the material has a vitreous appearance and is quite impermeable. Such a material corresponds to an artificial porphyry. (6) Amorphous or Crystalline Fragments in a Non-Glassy Matrix.— This structure is more frequent in the raw materials than in the finished products, as in the latter the strength of the articles depends almost wholly on the presence of some fused material to act as a bond. Many raw materials, however, consist of fragments which are cemented together by amorphous matter, which has been precipitated or otherwise deposited from solutions percolating through them. Thus, many sandstones consist of fragments of quartz cemented by amorphous iron, lime, barium, or similar soluble compounds, which have formed a film over the particles and produced a compact mass. The type of cement in a sandstone often gives the name to the stone. Thus, a siliceous sandstone is one which consists of grains of silica united by a siliceous cement. A calcareous stone has a bond consisting of calcium carbonate; a ferruginous sandstone, one composed of iron compounds, and so on. Sandstones with siliceous bonds are the only ones which can be used as refractory materials, as other bonds reduce the refractoriness of the stone to below the permissible limit. The ideal structure of a quartzite for the manufacture of silica bricks is one in which the grains of quartz are very minute and are bound together with an amorphous cement. This is very important, as when a quartzite of this kind is heated the elastic nature of the cement reduces the expansion of the silica to a minimum, whereas a quartzite which does not contain any cement has a very marked expansion and fre- quently cracks on account of the strains set up in the material. One of the most suitable quartzites used for making silica bricks is the amorphous form found in erratic boulders and sometimes known as “ Findling’s quartzite.” It consists of very minute grains of silica, surrounded by an amorphous siliceous cement, which is probably of a colloidal character (p. 7). Many other useful rocks are also charac- terised by a similar structure. The amorphous or possibly colloidal bond may be distinguished from the original quartz grains by examination with polarised light, though by ordinary light the rock appears to be crystalline. In some cases, silica STATES OF AGGREGATION 19 has been precipitated around irregular quartz grains and given the crystalline form of quartz. The Stiperstones quartzites near Shrewsbury are a good example of silicification, the sand grains having been enlarged by secondary growth, though crystal faces have not been produced. The ganisters of South Yorkshire and the Lickey quartzites are due to similar silicification, and many of the so-called _firestones consist of an aggregate of irregular grains of quartz bonded by an amorphous or possibly colloidal cement composed of silica and sometimes of calcareous matter. Many shales and slates are often silicified by the deposition of silica in their fissures, and some limestones may also be cemented by what appears to be colloidal silica. Laterites consist of amorphous mixtures of hydroxides of iron, aluminium, titanium, and manganese, iron hydroxide forming the principal bonding material. The bond is very weak in the freshly-obtained material, but on exposure its strength increases at the same time as the characteristic hardening of the material. Laterite may have been formed by the weathering of the basalts which lie beneath as in the Deccan of India, or from volcanic rocks, such as schists, gneiss, slate, sandstone, and granite, on which it lies, as in Africa. Holland has suggested that it may have been formed by the action of bacteria which precipitated silica in the colloidal form, the silica being removed by dilute alkaline solutions formed at the same time. Bauxite has a structure very similar to laterite, viz. grains of an apparently amorphous material united by an apparently amorphous bond. Other refractory materials, such as some magnesites, also have a structure consisting of amorphous grains cemented by an amorphous cement. Thus, crypto-crystalline magnesite con- sists of minute grains of magnesite bonded with amorphous magnesium silicate, colloidal silica, or hydromagnesite. Artificial products with a structure composed of granular pieces united with an amorphous yet not glassy cement are unusual in the industries with which the present volume is concerned. The most important are carbon or coke bricks, which consist of amorphous granules of coke or other form of carbon bonded together with an amorphous bond produced by the coking of the gas-tar or soft pitch with which the particles of coke were mixed. (c) Plastic materials cannot be accurately described as united by any definite kind of cement, and yet in a dry state they are usually hard and granular, and so, from a structural point of view, they may be considered separately. Plastic minerals—of which clays are the most important—consist largely of apparently amorphous grains, held together by extremely thin films of colloidal matter which surround each particle. In hard clays, this colloidal matter may be partially gelated, but on moistening with water it recovers its active properties and forms a plastic mass. It has long been considered by many investigators that china clay or kaolin consists chiefly of kaolinite crystals, but this was not definitely proved until recently, when W. H. Bragg obtained a characteristic X-ray spectrum. Unfortun- ately the flakes or grains of which the purest china clay is composed are so extremely minute that it is most difficult to accurately determine their shape. Some of them are aggregated together, forming fan-shaped masses termed vermicules ; others have the appearance of a pile of coins, when they are termed rouleauz. Most investigators 20 PHYSICAL STRUCTURE accept the (unproved) suggestion of Aron, that the minutest grains of clay are spherical in shape. Whether china clay is composed wholly of kaolinite crystals, most of which are too small to be seen, yet remains to be proved, but there seems no reason to doubt its essentially crystalline nature. A few definite crystals of kaolinite occur in china clay, but the bulk of the material appears to consist of minute amorphous flakes. Scattered crystals of quartz and mica and small needle-like crystals of blue tourmaline also occur. In the cheaper qualities of china clay the proportion of these minerals may be high, but in the purified product they should only occur to a negligible extent. There are several apparently amorphous materials which are related to china clay. The most important of these are halloysite, collyrite, allophane, nacrite, montmorillonite, and lithomarge. Other clays also appear to consist chiefly of amorphous grains, but the particles are much too small for their shape to be recognised. When dry the particles appear to be amorphous and porous, but when wetted they become plastic and impervious to water. They behave as though permeated and surrounded by a colloidal jelly, which acts as a binding medium, though some investigators regard this as insufficient to explain their structure, especially when a clay has been heated to redness. The colloidal gel may thereby be destroyed or it may be converted into an amorphous cement, in which case burned clays should be included in groups (a) or (6) (see pp. 16 and 18), according to the temperature attained. At present the structure of dry clays can only be described as “ granular,” that of wet clays as “ plastic,” that of slightly-baked clays as “ granular, possibly with an amorphous cement,” and that of fully-fired or vitrified clays as either “ granular with a glassy cement,” or as wholly vitreous or glassy. Further information will be found in the chapters dealing with the properties of clays dependent on their structure. (d) Amorphous or Crystalline Grains without Cement.—Some ceramic materials occur in loose incoherent beds, without any cement to bind the grains together. Most sands, many decomposed quartzites and kieselguhr, are of this type. Among the most important for refractory purposes is the Dinas sand—a pale- yellow material produced by the disintegration of the famous Dinas quartzite— occurring in the Vale of Neath, Glamorganshire. Several other refractory silica sands occur in various geological formations, especially in the Lower Greensand and Estuarine beds. Dolomite sands are deposits caused by the weathering of dolomitic limestone ; the calcareous material, being soluble in water containing carbon dioxide, is removed in solution, leaving the more magnesic material behind, together with more or less silica sand. Such sands are seldom pure enough for use as refractory materials. Siliceous sinter sometimes occurs as loose sandy deposits, though it is often com- pacted, forming a fairly stratified mass. Chromite sometimes occurs as a loose detrital sand, the beds of which tend to be uncertain in extent. Zirconia and zircon also occur in the form of sands in river beds, etc., associated with other loose incoherent 1 Ries considers halloysite to be always amorphous, the crystalline form being pholerite. MASSIVE STRUCTURE 21 materials derived from pegmatites and syenites. Monazite sands are of a similar nature. Many so-called “rare” refractory materials occur in individual grains dissemi- nated through igneous rocks or in sand deposits and placers derived from the disin- tegration of such rocks. The principal of these are beryllium oxide, ceria, didymia, lanthana, thoria, yttria, etc. Sands are usually produced by the disintegration of various rocks, either by such natural forces as wind and water, heat and cold, etc., acting on the surface or by various complex actions which take place in the interior of the rocks themselves, such as infiltrating water, hot liquids and gases from great depths, and other similar actions. Sands may be derived from all kinds of rocks, but those which are com- mercially valuable are generally derived directly from igneous or sedimentary rocks. The large class of siliceous sands used for furnace linings, etc., are generally derived from highly siliceous rocks, which may be either igneous or sedimentary. Loose detrital deposits may also be produced by precipitations from solution, as in the case of siliceous and calcareous sinters, etc., or by the deposition of the skeletons of dead organisms, as in the case of kieselguhr. b MASSIVE STRUCTURE The various components mentioned in the preceding pages may be combined in various ways, depending on the mode of formation, so as to form larger masses, the principal forms of which are as follows :— Unstratified masses are those in which the materials show no special arrange- ment or lines of stratification. This type of structure is common amongst raw ceramic materials, and is also characteristic of many of the finished products. In fact, in the case of finished articles, it is the most desirable structure. The nature of unstratified rocks comes within the province of geology, and is outside the scope of the present volume, though most primary clays, including china clay and some secondary ones, such as Boulder clay and the Pocket clays of Derbyshire, are unstratified, as are also some forms of silica rock, magnesite, and zirconia. Unstratified materials may be (i) homogeneous or uniform, or (ii) hetero- geneous or irregular in character and in the arrangement of the particles of which they are composed. - Homogeneous structures are those which are uniform throughout the whole mass, and may, therefore, be expected to have the same physical properties, no matter from which part a sample may be taken. They differ from crystalline, stratified, laminated, and other heterogeneous structures in the absence of definite planes of cleavage, or other signs of being composed of “ units ”’ of different character. In the manufacture of articles of ceramic materials, homogeneity is usually of great importance, and any definitely laminated or segregated type of structure must usually be destroyed during the process of manufacture. i As homogeneity is closely allied to ‘‘ texture,” further information on it will be found in Chapter II. 22 PHYSICAL STRUCTURE Stratified masses are those in which the materials are aggregated in the form of layers, strata, or laminations. This structure is characteristic of most of the rocks laid down by the action of water, and includes the majority of clays, most of which have been carried in suspension in water and have finally settled at the bottom of rivers, lakes, seas, etc., in beds or strata of varying thickness, often interlaid with strata of sand or other materials. For further details see the author’s British Clays, Shales, and Sands (Griffin). When a rock appears to be composed of very thin flakes, the material is said to be laminated and if the splitting occurs along the same plane as the bed in which it was deposited, the structure is termed shaly, but if the material cleaves in any other direc- tion than one parallel to the bedding plane, the term fissile is employed to distinguish such a rock from the shales. Many clays and all shales show signs of stratification; they may generally be split into thin flakes and foliations parallel to the bedding plane. When the clay has been greatly compressed in a direction at a large angle to the bedding plane and simultaneously subjected to great heat, slate may be produced. Flaky clays do not readily form a homogeneous plastic mass except after an excessive amount of grinding and are very difficult to use satisfactorily. The best clays havea perfectly homogeneous texture and are free from any laminated structure ; but, for various reasons, laminated clays and shales are often used and with care can be made into satisfactory articles. It is, however, essential to break them down to such an extent as to destroy all the laminations, otherwise the various laminz may not unite properly and the finished articles will be weak along the lines of cleavage. Lamination in the finished goods may be due to (a) the use of unsuitable materials, or (0) the use of unsuitable methods of manufacture. It is prevented, where possible, by grinding or crushing the clay or other material so finely as to destroy all visible flakiness. This is not always practicable with plastic materials, some of which must be thoroughly dried before being ground. Where such drying is regarded as being too costly, soft laminated clays cannot be properly “‘ tempered.” The destruction of the laminations may sometimes be more easily effected if the clay is mixed with some non- plastic material such as sand. Some plastic clays—especially those of a “soapy ” eharacter-—which are apparently free from laminations when first obtained, develop a laminated structure during the process of manufacture into bricks, etc. When this is the case, the method of manufacture is probably unsuitable, but the defect is some- times avoided by adding non-plastic material prior to the tempering process. The production of blisters on the surface of articles is sometimes caused by the air between the laminations in the clay, especially when the articles are shaped by extrusion, through a die or mouthpiece. Salling is also a result of lamination, though it may also occur as a result of internal strains, such as those produced when an article is subjected to sudden changes in temperature, and including those due to conversion of one mineral into another, as quartz into tridymite or cristobalite, or magnesia into periclase. A form of lamination which is sometimes developed in articles which are repeatedly MASSIVE STRUCTURE 23 heated and cooled, such as retorts and saggers, is due to the fireclay tending to re- crystallise in certain definite directions and so to develop lines of weakness. This defect cannot be wholly avoided, but may be diminished by carefully examining the broken saggers, etc., before re-using them, as pieces which have already developed this defect are naturally unsuitable. Glost saggers are particularly liable to cause this defect if they are crushed, mixed with raw clay and made into fresh saggers ; where possible, they should be avoided. Lamination also occurs in some sandstones, ganister, bauxite, laterite, etc. Siliceous materials, other than clay, which show lamination should be avoided, as their structure is not easily destroyed in the course of manufacturing them into bricks, ete. The best means of avoiding troubles due to laminations in clay products is to employ a suitable and thorough method of preparation of the clay. For some years there has been an increasing tendency to reduce the amount of treatment— especially in grinding, crushing, and tempering the clay—to a minimum and to omit weathering and “ souring,” with the result that ware made defective, as a result of lamination, is more common than formerly. Sometimes a somewhat laminated structure is desirable, particularly in the case of graphite used for making crucibles. The best graphites for this purpose consist of small flakes, which adhere and form a laminated mass. Of American graphites, those from Alabama have the smallest and thinnest flakes, the flakes in Pennsylvanian graphites being slightly larger and thicker, whilst those in the Canadian graphites are still larger and more irregular. The graphite exported from Madagascar consists of grains nearly twice as large as those in the American graphites. Ceylon graphite is largely used; it consists of very thin rectangular or triangular grains, rather than of true flakes; it is very satisfactory and gives most durable products, whilst articles made from a coarser and more obviously laminated graphite tend to spall. The excessively laminated structure of some graphites may be largely destroyed by mixing them with 15-20 per cent. of tar, briquetting the mixture, baking it quickly at 1000° C., and then crushing and screening the product so as to retain that between 18- and 100-mesh. This treatment is satisfactory for graphite for many purposes, but the product is not so satisfactory as natural Ceylon graphite. Foliated structures are produced by the recrystallisation of some of the con- stituents of a rock along lines parallel to the original bedding or along joints or cleavage planes, the rock being afterwards subject to lateral pressure. A foliated structure is characteristic of the schists, but is not common in clays and other ceramic materials. Schistose quartz, on account of its foliated nature, is not satisfactory for making silica bricks, as those made from it tend to spall badly. Cellular materials are not composed of a compact, dense mass, but are porous and apparently composed of “ cells,” as kieselguhr, moler, pumice, etc. A cellular structure may be produced by mixing combustible matter, such as sawdust, with the clay and other materials, so that when the articles are burned this combustible material burns out, leaving holes or pores in the space which it had previously occupied. 24 PHYSICAL STRUCTURE A preferable method of making articles of a cellular structure is to mix naturally cellular material, such as kieselguhr, with just sufficient clay to bind the particles together. Capillary structure, 7.e., when a material behaves as though it were composed of a multitude of minute tubes, each having the diameter of a “ hair,” is somewhat analogous to cellular structure ; it is characteristic of clays and many other ceramic materials. This structure is best understood by observing what happens when the lower end of a very narrow tube is dipped into a liquid which wets it ; the liquid rises in the tube to a greater height than that of the external liquid, because the surface film of any liquid is under a tension equal to the surface energy per unit of area of the liquid. In the case of a vertical tube, the liquid creeping up it gains potential energy and equilibrium is established when this gain is equal to the loss due to the diminution of the air-glass surface. Thus, if the tube has a radius 7, the total vertical force will be 2z7rT cos a, and this will equal the weight of the liquid raised, wr?hdq. Hence, T=4 dgrh sec a dynes per cm. But when the liquid wets the glass a=0 and sec a=:1, so that the surface tension T= 4dgrh dynes per cm. when d is the specific gravity of the liquid, g the equation of gravity=-981, r the radius of the tube, and h the height to which the liquid rises. The capillary structure of china clay may be shown by several pretty experiments devised by Liesegang and Watanabe. Thus, if a solution of a coloured salt such as ferric chloride or ammonium bichromate be added to a clay slip, and the latter is then allowed to dry, forming a thin sheet or disc, the salt will not be distributed uniformly through the mass. A narrow band at the edge of the clay will be deeper in colour than the remainder of the mass, as the salt becomes more concentrated at the edge. It has long been known that bricks and other porous substances, when saturated with a solution and then allowed to dry, segregate the greater part of the salt on their surface and form a “scum” or “ wall white.” Colloidal solutions behave in this respect in the same manner as true solutions. If to the centre of a dried disc of china clay containing ammonium bichromate a few drops of water are added, the water will drive the bichromate before it, so that there will be a central zone of water and around it a ring of deep yellow colour. If, instead of water, a few drops of silver nitrate are added in succession there will be a central red zone of silver chromate, around which is a ring of colourless silver nitrate which drives ammonium bichromate before it and forms a deep yellow or brown ring with an intervening colourless ring which is free from bichromate. Each additional drop, although placed in the centre, forms an additional ring. The appearance in this case is precisely the same as occurs in the well-known Liesegang rings, though the cause is quite different, as in the experiment with clay the periodicity of the rings is due to external causes, viz. the manner in which the drops are added, whilst in the jelly it is due to the structure of the system itself and is a phenomenon of diffusion. Diffusion rings may also be observed with clay, but they are so fine as to be almost invisible. The rings obtained with solutions of (a) ferric chloride and potassium ferro- cyanide, and (6) copper sulphate and potassium ferrocyanide respectively, on clay are ALTERATION OF STRUCTURE 25 particularly beautiful and are wavy instead of plain. It might appear that semi- permeable membranes are first produced, but the rapid movement of the second drop shows that this is not the case. Apparently, a membrane formed in the presence of capillaries is not so closed as one formed by diffusion, so that the formation of rings is not uniform but irregular. If, instead of applying the drops to the centre, the china clay disc is covered with a plate of glass and immersed in the second reagent, a different series of rings is formed, which, if the clay is saturated with copper sulphate and is immersed in a solution of soda, bear a close resemblance to malachite. These rings differ from the ones previously described in being the result of diffusion. Concretionary structures consist of a nucleus surrounded with layers or aggregations of other materials, the whole forming irregular masses and nodules. This kind of structure is often formed from leaves, shells, or the remains of plants and animals deposited in water having salts or silica in solution. Limestones and various forms of silica occur in this form. Hematite, limonite, clay ironstone, and other forms of iron oxide commonly occur in concretionary masses. Oolitic limestones are frequently, and dolomitic sandstones more rarely, composed of concretionary masses. Flint occurs in nodular masses frequently of large size associated with calcareous matter. Bauxite and laterite often occur in small round grains, varying in size from a pea to | inch in diameter, the nodules being sometimes separated from the matrix. Crypto-crystalline magnesite also forms nodular accretions. A concretionary structure is seldom produced in the course of manufacture of any clay products, as suitable conditions rarely occur. Segregated structures are those in which part of the material forms veins or segregated masses in other rocks, such as vein-quartz, magnesite veins traversing masses of serpentine and hematite, spathic and magnetic iron ores occurring as large irregular masses in other rocks. Chromite frequently occurs in massive and lenticular forms and as veins traversing serpentine and peridotite rocks and also in irregular masses in residual clays associated with serpentine. Fibrous structures in ceramic materials are rare; the only one within the scope of this volume is asbestos, which is not really a refractory material, though it is some- times used in the preparation of such articles as “ fuel ” for gas-fires and some asbestos bricks. It has a fibrous structure, the crystalline grains forming long flexible fibres which interlock and produce a strong mass. Of the several varieties of asbestos, chrysotile is specially valued on account of the strength and length of its fibres ; tremolite, crocidolite, and anthophyllite are of a similar nature, but their fibres are neither so long nor so strong. ALTERATION OF STRUCTURE During the manufacture of articles of various shapes the structure of the original material is generally destroyed, so that a rock with an unsuitable structure may not be harmful. The alteration of the structure of the raw material is effected in various ways, 26 PHYSICAL STRUCTURE such as (a) weathering, (b) grinding, and (c) calcining prior to making it into the desired shape. Weathering consists in exposing the materials to the action of rain, snow, frost, etc., so as to break down the original structure of the clay or rock and render it easy to fashion into articles of various shapes. This method is commonly adopted in the case of soft, stratified materials such as clays, shales, etc. For harder and more resistant materials, which are not readily affected by the weather, this method is of little value and one of the other methods must usually be substituted. Grinding is employed to break down compact masses into smaller grains, Gran- ular materials bonded by amorphous matter, such as sandstones, clays, etc., are readily reduced to small grains, but with masses which are wholly crystalline or consist of crystals in a glassy matrix, the grinding may be a very difficult and expensive operation. Wholly crystalline quartzites need to be ground so fine before their structure is destroyed that the cost is often prohibitive, so that it is usually preferable to use a slightly less pure material which can be more easily reduced to a suitable state. Materials of a shaly character sometimes cause difficulty by producing flat plates or flakes, which are very undesirable in the finished goods. Calcining consists in heating a material to redness or to a higher temperature, usually in order to partially decompose it. Such treatment is often useful in altering the structure of limestone, dolomite, magnesite, clay, flint, and some quartzites, sandstones, etc. In some cases, such as the first four substances just mentioned, the change in structure occurs as a result of chemical dissociation, whilst in the last two substances various changes in volume occur, with the result that strains are set up in the material, which cracks and can then be readily reduced by crushing to a structureless sand. CHAPTER II PROPERTIES DEPENDING ON STRUCTURE Some of the properties of ceramic materials are directly dependent on their structure. The principal of these are: (a) texture, (b) homogeneity, (c) porosity, and (d) permeability. TEXTURE The texture of a material is concerned with (a) the shape of-the individual grains comprising its mass, (b) the sizes of the various grains, and (c) the grading of the grains, 2.e. the proportions of particles of various sizes. The texture is said to be coarse when the particles are large or loosely spaced, and fine when the particles are small. Sometimes a material with a cellular structure (p. 23) is regarded as having a coarse texture. When it is difficult to observe the texture with the naked eye a pocket lens or low-power microscope may be used, and in some cases it is easier to examine the texture if the material is first ground smooth and then polished. The texture of a material has a very important influence on many of its properties, including shrinkage, porosity, fusibility, and, in the case of clays, plasticity. Not- withstanding its importance, it is not considered or controlled to anything like the extent it should be, and many failures in the production and use of articles from clay and other ceramic materials are due to the lack of proper attention to this property. The shapes of the grains of a material should be such as to ensure: (a) the maximum strength ; (b) the requisite porosity and permeability ; and (c) a surface of the desired smoothness. The grains of the materials under consideration may be (a) flat or flaky, (6) angular, or (c) rounded. Flat or flaky particles are usually undesirable, as they tend to cause lamination troubles (p. 22), so that materials which have a flaky texture, such as mica, should generally be treated in some way (such as grinding, calcining, etc.), which will either destroy the flakes or remove them, otherwise the material should be discarded except in the case of graphite used for crucibles (p. 23), and even then the flakes must be sufficiently small. Large flakes in any kind of ware are undesirable, as they tend to produce lamination. Angular and rounded grains are conveniently considered together rather than separately. Rounded grains which are perfect spheres and all of the same size, 27 28 PROPERTIES DEPENDING ON STRUCTURE produce a mass of maximum porosity if the grains are piled vertically on one another. The porosity is slightly lower if one grain out of each four rests equally on three others, yet is still higher than with angular grains. This is due to the fact that rounded grains of one size will roll together easily, but, on account of their shape, they cannot interlock, so that it is impossible for the interstices between the particles to be filled. When rounded grains of various sizes are present, the amount of inter- stices is less, though still larger than with angular grains which interlock to some extent, and may, therefore, fill some of the interstices by projecting portions of the grains. A strict comparison in simple terms is impossible, because rounded grains readily roll over one another until a state of minimum porosity for such grains is attained ; but angular grains require a certain amount of pressure and shaking before they will arrange themselves in a position of minimum porosity, and if this is not applied their irregularity and imperfect interlocking may cause them to produce a more porous mass than one in which rounded grains are used. No matter how much pressure, short of crushing them, is applied to rounded grains, they will always produce a porous mass, as the interstices cannot be filled. Such pressure is trans- mitted equally from grain to grain and a mass of uniform porosity is produced. When a state is reached in which every particle is resting on three others, the minimum attainable porosity is reached and no matter what further pressure (short of crushing) is applied, the porosity will remain constant. With angular grains, on the contrary, the conditions are different, an increase in the pressure causing the particles to interlock more closely and so reduce the porosity of the mass, the interlocking being dependent, to a large extent, on the pressure applied. Moreover, the pressure is not transmitted uniformly through the mass, as when rounded grains are employed, but is greatest nearest to its source and least at the greatest distance from it. Consequently, the porosity varies in various parts of the mass and, unless there is sufficient water present, a non-homo- geneous article is produced with a great tendency to spalling when it is subjected to sudden changes in temperature. Hence, when it is desired to produce the most uniform mass, rounded grains are preferable to angular ones. Round grains also give a more permeable mass on account of the smaller fractional resistance to the passage of gases and liquids. Angular grains, on the other hand, have an effect equivalent to a rougher surface and offer a greater resistance to the passage of gas and liquids through them. As rounded grains do not interlock, the resulting mass is very weak, friable, and incoherent, and unless restrained by the vessel in which they are contained, the particles roll over one another and shear when pressure is applied to them. Angular grains, on the contrary, readily interlock and form a strong, compact mass and are, therefore, preferable when great strength is desired. Materials composed of angular grains require a larger proportion of binding material to produce a compact mass than those composed of rounded grains, on account of the greater surface area of the angular grains. They have the advantage, however, of presenting a larger surface to the action of the bond and so produce a stronger mass. SHAPES AND SIZES OF GRAINS 29 From the above comparisons it will be seen that the shape of the particles must be varied according to the particular properties required. Thus, rounded grains are undesirable in the banks of open-hearth furnaces, as they cannot be placed so as to form a steeply-sloping bank, their angle of rest being very low. For this purpose angular grains are preferable, but they should not be too angular, or they will not give a mass of sufficient compactness unless tightly rammed into position. For such a purpose the best material is intermediate between angular and rounded grains, namely, sub-angular ones, though highly angular grains are often preferred in the copper furnaces of South Wales and in America, where crushed quartzite is used. The facing sands used for casting should be made of sharp angular grains, as a strong mass is required to resist the cutting action of the molten metal when it comes into contact with the sand. The sands used for “ backing,” however, consist of rounded grains, as in them strength is less important than permeability. In some cases, rounded facing sands are used so as to secure ample permeability, but it is always at the expense of the strength. For making articles which are required to have a certain amount of strength, angular grains are generally required, even though some other properties must be sacrificed. Thus, ganister is specially valuable on account of the irregular splinters which it forms when ground, interlocking readily and forming a dense and compact mass. ‘The process of grinding any material always tends to produce rounded grains. Hence, if angular grains are required, the material should be crushed with hammer- like blows, as distinct from the rubbing action of most grinding machines. For the same reason, when preparing “ grog ”’ it is best, if possible, to burn the clay in lumps rather than to grind it and make it into rough clots. The calcined clay or grog may afterwards be crushed without undue rounding of the grains. Sometimes, during the burning of ware, long, lath-like crystals are formed, which produce a very strong mass on account of the interlocking which occurs. Thus, an ideally fired clay, such as that largely attained in refractory porcelain, consists largely of a mass of interlocking crystals embedded in a glassy mass; the crystals give it great strength. Needle-shaped crystals are especially valuable for producing a strong mass, as they interlock more completely than relatively shorter and thicker ones. In the manufacture of bricks, pottery, etc., the shape of the particles is very important, especially if they are large, as the latter, if rounded, are more likely to cause trouble. The determination of the shape of the grains of any particular material is best effected by examining the material with a pocket lens, or, if necessary, with a microscope, as described in Chapter X. The possibility of producing grains of a suitable shape depends chiefly on the nature of the material, some substances being quite unsuitable. Stamps, disin- tegrators, crushing rolls, and edge-runner mills with perforated pans usually produce angular particles, whilst ball-mills, tube-mills, and edge-runner mills with solid pans generally form rounded grains. With some materials, crushing rolls tend to form flaky particles. The sizes of the grains in both raw and fired ceramic materials vary greatly 30 PROPERTIES DEPENDING ON STRUCTURE according to the purposes for which the materials or articles are to be used. Instead of the terms “large”? and “small” grains, it is more usual to refer to them as “ coarse’’ and “ fine.” Coarse grains usually produce a less porous mass than one composed of fine grains, but the average size of the pores is larger in the coarser-grained material, so that its permeability is greater. In the case of fired ware, the fine grains are less refractory than the coarser ones, and if they partially fuse when heated they will produce a less porous mass on account of the closing up of some of the pores with glassy matter. For this reason, whilst an unfired, fine-grained mass may be more porous than a coarse-grained one, the former, after it has been heated, may become much less porous. Hence, in ware fired at a high temperature, coarse grains must be used where a great porosity is required, because such pores are not so readily filled by fused matter as when a fine-grained material is used. Coarse grains are also desirable where the maximum permeability is required, as this property depends on the size of the pores as well as on the total amount of pore-space (see Permeability). Coarse particles are preferable in most cases where resistance towards sudden changes in temperature is required in any materials which have an appreciable coefficient of expansion, because, in a close-grained mass, the strains due to the © heating or cooling cannot be relieved by the rearrangement of the particles without the disruption of the mass. When the change in temperature is not too rapid, many close-textured firebricks are very durable under adverse conditions if they are made of properly-graded material. As most of the materials described in this volume have a low thermal conductivity, the coarser grains do not attain the temperature of the furnace so rapidly as the smaller ones, so that the former are able to stand much higher temperatures than the latter (see also Chapter XIII). Substances which undergo only a very small change in volume when heated or cooled may be composed of either small or large particles so far as their resistance to changes in temperature is concerned. All argillaceous, siliceous, bauxitic, magnesic, dolomitic, and similar materials have an appreciable coefficient of expansion, and this must be allowed for in considering the size of particles to be used where great or sudden temperature changes are to be withstood (see also Chapter XIII). The presence of large grains reduces the strength of materials containing them, both in the unfired and fired states, as the strength depends on the area of contact between the various particles, 7.e. on the compactness of the mass. Coarse grains are less readily attacked by corrosive substances than fine grains, as they have a smaller surface area for the same mass. For the same reason they require a smaller proportion of bonding material and of water to make a paste of the required con- sistency. Consequently, a shorter time is required for drying and the shrinkage will be less. On the other hand, though small grains require more water, they are more easily moulded to the required shape. Coarse grains usually produce a mass of more open texture, so that corrosive substances are more able to penetrate into the interior of the mass and so reach a greater surface upon which they may act than is the case with a mass composed of — EFFECT OF GRADING ON TEXTURE 31 smaller particles, which do not allow the corrosive agents to penetrate so far into the interior. Hence, whilst fine-grained materials are superficially attacked more rapidly, they are often preferable for resisting corrosive influences on account of the impermeable surface they rapidly form. Where a change of crystalline form is required, as in the case of quartz in silica bricks, it is often desirable to use fine particles which will be readily converted. When a fusible material is required, as in the production of glazes, glass, and fused quartz, the finer the particles the more rapidly will the fusion be effected. Where chemical reactions are required, the size of the grains influences the speed at which they react ; fine grains react more rapidly than larger grains because they have a large surface area and, being smaller, they have a wider field of action. This subject is more fully dealt with in Chapter XI. Materials composed of coarse grains have a rougher surface than those made of smaller grains. In some cases the roughness is not serious, but where corrosive sub- stances are present as smooth a surface as possible is desirable. Fine-grained materials, when wetted, sometimes have the serious disadvantage of entrapping air between the grains, thus causing a form of lamination (p. 22) which creates defects in the finished articles. Grading .—Besides the sizes of the individual grains it is also most important to consider the grading of the material as a whole, because in only a limited number of cases can a material consisting of particles of only one size be used satisfactorily. Thus, a material in which all the grains are uniform in size is undesirable for bricks and similar articles, as such grains, even when angular, do not bind well together. A much stronger and better product is obtained if the particles are of different sizes, so that they may interlock properly and, aided by a natural or added bond, may produce a material having the properties which it is desired it should possess. The term graded is applied to any material composed of grains of various sizes, the proportion of grains of each size being such that the mixture has certain pro- perties, such as high strength, low permeability, etc. It is sometimes misapplied to heterogeneous materials, in which the proportions of particles of various sizes are quite accidental, whereas in a properly graded material they are present in just those amounts which are the most suitable for a particular purpose. Some materials have been graded naturally by the manner in which they were deposited, but others must be subjected to a screening or other mechanical process which will remove particles of undesirable sizes. Alternatively, a graded material may be produced by mixing grains of selected sizes in suitable proportions, though such artificial mixtures are seldom as homogeneous as the natural graded materials. The use of graded material is desirable for several reasons :— (a) If only coarse material were employed, it would require a very large proportion of bond to fill the voids and cement the grains together, and the resultant mass could not be so strong as a better-graded material, on account of the small number of points of contact between the “‘ cement ”’ and the larger particles. (6) The porosity and permeability are reduced in a properly graded material, and this often has a marked effect on its resistance to corrosion, abrasion, etc. 32 PROPERTIES DEPENDING ON STRUCTURE (c) The appearance and other good qualities of a material may often be improved by removing particles of certain sizes from the remainder, as when pebbles are washed or screened out of a clay or sand. (d) The use of carefully-graded material greatly lessens the possibility of spalling, splintering, and cracking when a material or article is exposed to sudden changes in temperature. A common reason for using several sizes of grains in a material is to obtain a close texture, the finer grains being used to fill the interstices between the coarser ones and thus reducing the percentage of voids toa minimum. Figs. 1-3 show the differ- ence between the texture produced by grains of uniform and of different sizes. It will be seen that where uniform grains are employed, the voids are quite large, and that the inclusion of grains of smaller size greatly reduces the proportion of voids; if a third still smaller size is employed, still less space is left between the particles. A large number of different sizes is not necessary and may be undesirable, as they may produce a more porous mass than if only a few sizes were used. It is merely necessary to have about three different grades or sizes, one coarse, one medium, and one fine. Fig. 1.—UNGRADED Fic. 2.—MIxtTuRE WITH Fic. 3.—Mixtvur& wItH MIxtTuURE. Two GRADES. THREE GRADES. These, when combined in suitable proportions, can be made to give a mass of maximum density. The effect of grading has been most thoroughly investigated with regard to concrete and the results of these investigations may be applied with great advantage in the manufacture of articles from clay and refractory materials. In concrete, the coarse stone or aggregate forms the bulk of the solid matter ; between the pieces of coarse material are interstices or voids of considerable size. These are filled with smaller pieces of stone or similar material, but the latter still leave further voids between them. These voids are, in turn, filled with still finer material, such as sand, so that when the cement is added, the whole forms a dense and compact mass. It is possible to make concrete with stones of uniform size, but such a material requires an excessive amount of cement, and the product is weak. It is far better to start with the larger stones and to fill the voids with progressively smaller pieces, until sand, and eventually lime or Portland cement is used, the particles of which are all less than 0-003 inch in diameter. In short, a properly made concrete approaches an ideally graded material. Taylor and Thompson have found that the proportions of grains of different sizes PRODUCING A FINE TEXTURE 33 will, if plotted in the form of a graph, take the form of an ellipse for the small grains and a straight line joining the ellipse at a tangent for coarse particles. An ideal graph of this kind for dealing with crushed material and screenings may be constructed from the following data :— Intersecting tangent with vertical at zero diameter . 29-0 Height of tangent point. : pend Axes of ellipse: a : ; ; . 0-147D “ fe b+7 . ; ; ; s : . 378 To construct the graph a is found by multiplying 0°147 by the maximum size of the coarse particles and the resultant figure is marked on the horizontal scale of sizes of particles. A vertical line through this point forms axis 6 of the ellipse. The value 6 is given above, and the length of the vertical axis is laid with its centre 7 per cent. above the horizontal line of the graph. The ellipse is then complete and the straight line drawn from the point of intersection of the 100 per cent. line and the line denoting the largest-sized particles to a point representing the point of intersec- tion of the tangent with the vertical at zero diameter, the value for this being given above. The importance of grading materials for making bricks, tiles, pottery, refractory goods, etc., has not been sufficiently realised in this country and is the cause of a large proportion of the difficulties which arise. It is practised extensively on the Continent, with the result that some Continental articles are far more durable than those made in this country. Materials Producing a Fine Texture.—From the statements in the preceding pages it will be realised that for a material to have a fine texture it must be composed of very small grains, which are, in most cases, united by a bond or cement composed of still smaller grains. The texture will be found to depend upon (a) the smallness of the grains of material, whether aggregate or bond, and (b) the grading of the material, .e. the proportions of grains of different sizes, and especially the smallest ones which, under some conditions, will cover the others. Thus, a fine-grained, well-graded material will give a smooth, close texture which will be resistant to corrosion, abrasion, etc., whilst a material composed of grains of uniform size, even though they are small, will give a more open and rougher texture which will be more easily corroded and abraded, but is less liable to spall when the material is exposed to sudden changes of temperature. Fine-grained materials sometimes exist naturally as clays and some other rocks, or they may be produced artificially from materials which have been reduced to a fine powder by grinding. As such treatment is costly, the natural, fine-grained materials are preferred when they can suitably be used. The best qualities of china clay and kaolin—one of the purest forms of clay—are extremely fine and, with the aid of water, may be passed entirely through a 200-mesh sieve. A large proportion of such material is so fine that it will be carried away by a stream of water flowing at the rate of only 0-18 mm. per second or 0-43 inch per minute. Such grains do not exceed 0-01 mm. in diameter. The finer qualities of 3 34 PROPERTIES DEPENDING ON STRUCTURE ball clay consist of equally small particles, but they are more plastic than china clay or kaolin. Some ball clays are much coarser. Many other plastic clays chiefly consist of very fine clayey material practically free from coarse matter, most of the grains passing readily through a 200-mesh sieve. Most alluvial clays have an ex- tremely fine texture on account of the manner in which they have beenformed. The particles composing fireclays are usually rather coarser than those of ball and china clays, but most will pass through a 100-mesh sieve and a large proportion is carried away by a stream of water flowing at the rate of 0-18 mm. per second. Most of the clays used in the manufacture of bricks and tiles are composed of much larger grains than the best china and ball clays and usually consist of a natural mixture of sand and clay in very variable proportions as well as an indefinite proportion of organic matter. Some good brick clays only contain 50 per cent. of material which will pass through a 200-mesh sieve. Many clays contain a large proportion of silt, which may be defined as consisting of grains between 0-025 and 0-01 mm. in diameter, though natural “silt”? and “warp” also contain a considerable proportion of still finer grains of clay. It is not easy to judge the fineness of a clay by inspection, or even by mixing the material with water and then sieving it, because, whilst some clays when mixed with water may readily be disintegrated into their individual grains, others are much more compact and require boiling in water or even the addition of a small quantity of ammonia or other dispersing agent. Rock clays, shales, fireclays, and some other clays have an extremely fine texture if the individual particles can be separated, but unless special methods are adopted, each “‘ particle ”’ of clay separated by the action of water will be found, on examination, to consist of an aggregation of smaller grains. Articles made of such clays often have a coarser and more open texture than those made from clays in which the individual grains are readily separated from each other. Freshly dug clays may be classified according to their texture as (a) shales, (6) marls, (c) loams, (d) stony clays, (e) plastic clays. Shales have a laminated or stratified texture (see p. 22). They may be either fine or coarse and either rich or poor in clay, as the term “ shale’ has no connection with the size of the grains nor with the composition of the material. Marls or Malms are natural mixtures of clay ‘and chalk and may usually be recognised by their friable nature, their texture being quite different from that of other clays. The term “ marl” is often used for materials having a similar texture, but which contain very little, if any, chalk, e.g. Staffordshire marls. The greater part of most true marls can usually be washed through a 100-mesh sieve, though coarser marls are also found. Loams are mixtures of sand and clay; they are largely used for the manufacture of bricks, roofing tiles, floor tiles, terra cotta, agricultural pipes, and coarse (red) pottery. Their texture is similar to that of marls. Loams and marls are often confused, but they may be readily discriminated by adding a little hydrochloric acid, when a marl will effervesce, whilst a loam will not do so. The texture of many loams is such that they are excellent for the manufacture of the articles just mentioned. The sand in them prevents loams from shrinking excessively and enables TEXTURE OF CLAYS 35 articles made from them to be dried and burned without undue risk of cracking or warping. Some loams contain so much sand that they cannot be used unless they are mixed with plastic clay to enable them to produce a stronger mass. Any very coarse, non-plastic material such as pebbles and gravel in a loam must either be removed or crushed so fine that they will not do any harm when the loam is made into articles. Stony clays usually consist of fine clay or loam mixed with stones, pebbles, or similar non-plastic material of any size from } inch in diameter to large boulders. Much of the “ boulder clay” which forms part of the Glacial Drift is of this nature. The coarse material must be separated from the clay or loam before the latter can be used, even for brickmaking. Plastic clays, when suitably moist, possess a smooth oleaginous texture which is often so fine that no definite “ structure’ can be seen with the naked eye. If the material is rather drier it can be seen to consist of adherent granules, but many of these appear to be much larger than the particles of clay really are, for the granules consist of aggregations of the much smaller particles of clay. Most plastic clays also contain a considerable proportion of sand and some (such as boulder clay) contain large stones. The causes and nature of plasticity are considered in Chapter VI. An excessively fine texture is objectionable, because it is usually associated with a tendency to twist or warp when the articles are being dried or burned and with an undue sensitiveness to sudden changes in temperature. For the same reason, loams are often preferable to finer clays, though too much importance must not be attached to this, because earthenware and china are made from very fine-textured materials. To avoid the difficulties caused by an excessively fine texture, a suitable proportion of coarser matter must usually be present in the crude clay or other materials used. The properties allowable will, of course, be dependent on the nature of the ware to be made and on the plasticity, binding power, etc., of the clay itself. Some clays can stand the addition of quite a large proportion of coarser, non-plastic material, whilst others can only carry a very small proportion. It is not possible to judge how much coarser material may be present except by actually testing the materials. The following definitions have been suggested by Seger for distinguishing the various components of clay and these figures are usually adopted by other investigators :— Clay includes all grains with a diameter less than 0:01 mm. washed out by a stream of water with a velocity of 0-18 mm. per second under a pressure head of 20 mm. Silt includes all grains between 0-01 mm. and 0-025 mm. diameter, washed out by a stream of 0-7 mm. velocity per second. Dust sand includes all grains from 0-025 to 0-04 mm. diameter which are washed out with a stream of water with a velocity of 1-5 mm. per second. Fine sand includes all grains between 0-04 mm. and 0-33 mm. diameter separated by means of sieves. Coarse sand includes all particles with a diameter greater than 0-33 mm. On account of the small grains of which clay consists, an article made of clay > 36 PROPERTIES DEPENDING ON STRUCTURE alone will have the properties of a fine-grained material, namely, (a) low porosity and great compactness; (b) low permeability; (c) uniform texture and structure ; (d) great mechanical strength ; (e) low resistance to sudden changes of temperature ; (f) greater resistance to corrosion and abrasion than some coarser amorphous materials. As some of these properties are undesirable in certain kinds of ware, an admixture of a material composed of larger grains is often required. This must be selected with skill, as all coarse materials are not equally suitable. Some form of crushed silica rock or sand is often used, but the most generally suitable material for mixing with clay is burned clay or grog (below). When articles made of materials other than clay are too fine in texture, it is not usually difficult to obtain larger pieces of the same raw material or to avoid crushing it excessively. The texture of burned clay is much coarser than that of the raw clay, as in the burning the minute grains are aggregated together into a compact mass. Hence, by burning the clay and then crushing the resultant grog, a material composed of pieces of any desired size may be obtained without changing the composition of the material of which the articles are made. Coarse material such as grog is very valuable in making refractory articles, as where it is used as one of the ingredients, the resultant ware is much less sensitive to sudden changes of temperature than when raw clay only is employed. The addition of grog also reduces the shrinkage of the clay. Materials Producing a Coarse Texture.—When large grains or particles are united by a relatively small proportion of bond or cement, the texture of the material may be described as “‘ coarse.” Consequently, the materials used to produce a coarse texture consist chiefly of (a) relatively large grains or agglomerations of grains, and (b) the cement or bonding agent (if any) which unite them. The bond is almost invariably composed of very small particles and is fine in texture. The larger grains are usually obtained by crushing masses of suitable material and separating those which are of the desired size by means of sieves or screens. Apart from‘clay, almost any of the materials considered in this volume may be prepared in this manner, and even clay can be obtained in large pieces which will not produce a fine texture if it is first burned and converted into “ grog.” The sizes of the coarser particles naturally depends on the purpose for which the material as a whole is to be used. In bricks, it is usually permissible to have not more than about 10 per cent. of pieces 4 inch diameter, and in gas-retorts a much larger proportion of coarse material (grog) is desirable. For these and other articles the material for producing a coarser texture may be of all sizes from that of coarse sand to pieces { inch or, very occasionally, even greater diameter. Materials for producing Medium Texture.—The majority of the articles made from clay and other ceramic materials require to have a medium or rather finer texture. To produce this, they must consist of a mixture of particles of at least two sizes, namely, ‘‘ medium,” which may be between 0-08 and 0-01 inch and “fine” particles which are less than 0-01 inch in diameter. Unless the materials used provide the desired texture naturally, they must be ground and screened in such a manner that pieces of unsuitable size are separated. In the case of bricks, TEXTURE OF CERAMIC ARTICLES 37 tiles, and other clay ware, the fine material is usually clay, the medium being either sand or grog, which has previously been screened to provide pieces of the required size. When sand is mixed with clay for brickmaking, etc., it should usually consist of grains between 30- and 120-mesh. The use of much coarser grains is undesirable, as it prevents sharp corners and edges being produced on the articles. The grains of sand or other coarser material should never be sufficiently large to project from the surface of the ware. Large grains of sand and other forms of silica are also undesirable, because they usually expand when heated, even after they have been in use some time. This is due to the fact that, being large, they are not readily converted to the low-specific gravity forms of silica and, consequently, they continue to expand every time they are heated and in so doing often cause much trouble. Texture of Ceramic Articles.—In the manufacture of bricks, roofing tiles, terra- cotta, drain pipes, and coarse pottery, the “ clay,’ when prepared for use and then made into a slurry with water, should pass completely through a 24-mesh sieve. For bricks, a rather coarse material will usually suffice, but for the other articles men- tioned a coarse “clay ”’ is objectionable. For pottery, the “clay ”’ should usually be fine enough to pass completely through a 50-mesh sieve and for very fine earthen- ware and porcelain it should pass through an 80-mesh sieve after the material has been mixed with sufficient water to form a slip or slurry. For firebricks and other refractory articles, the proportion of fine grains should not be excessive or the goods may not be sufficiently resistant to heat. In the case of clay wares, there must, however, be sufficient fine particles to provide the neces- sary bond and the same is true, though in a somewhat different sense, with regard to articles such as silica bricks, magnesia bricks, etc., made of non-plastic materials. In some cases, however, in order to meet special conditions, it is necessary to sacrifice some of the refractoriness in order to obtain other necessary properties and a larger proportion of fine material may then be used. The relative fineness or coarseness thus depends on the purpose for which the articles are to be used and the conditions to which they are likely to be subjected. Particles of grog used in making clay wares vary in size from 0-25 inch to 0-025 inch diameter, according to the article required and the conditions under which it is likely to be used. Some manufacturers use all the grog which passes through a 10-mesh sieve, but this is undesirable as it includes all the dust. A better method is almost the reverse of this, namely, to exclude all the material which passes through a 16-mesh sieve. In making saggers it is convenient to use three sizes of grog, namely, (a) coarse grog, consisting of particles between 4 inch and 2 inch diameter ; (b) medium grog, consisting of particles between 4 inch and 4 inch diameter; and (c) fine grog, consisting of particles between =}, inch and ~, inch diameter. Very coarse grains are necessary in saggers, as it is specially important that they should be resistant to sudden changes of temperature, because, when in use, they are exposed to the flames as well as to eddies of hot gas and cool air and are repeatedly heated and cooled, so that they have to resist conditions much more stringent than most other wares. Under such circumstances, coarse grog will give a greater durability, 38 PROPERTIES DEPENDING ON STRUCTURE but it should not be excessively coarse or it will unduly reduce the strength of the saggers. The best mixture is a graded one, consisting of both coarse and medium particles, as this combines the necessary strength with the requisite resistance to sudden changes of temperature. The sizes of particles used by different firms vary according to the nature of the materials and the texture which has been found in practice to be most satisfactory. Thus, one well-known manufacturer of refractory goods classifies his grog into the following divisions: (a) 5-mesh to 10-mesh, and (6b) 10-mesh to 20-mesh ; whilst in another works the “‘ coarse ”’ grog consists of particles between 0-25 inch and 0-15 inch diameter, and the “fine ”’ grog consists of particles less than 0-08 inch diameter. The surface of refractory bricks should be as smooth as possible, especially where they are subject to the action of corrosive substances, which more easily attack rough-surfaced bricks than those with a close texture. This smoothness can only be obtained if the material of which the bricks are made contains sufficient fine clay. The smallest particles of clay tend to flow to the surface during the process of manu- facture, and then produce the fine surface-texture or “skin” which is so desirable when the bricks must resist corrosion and abrasion. The durability of firebricks and of other articles subjected to abrasion or corrosion depends largely on the texture. A fine-textured material will, under ordinary condi- tions, be the most durable, because the action of the corrosive material will be con- fined so much more to the surface that it will act much more slowly than it would on an article of the same material but composed of coarser particles. When a corrosive or abrasive agent attacks a coarse-textured brick or other article, it loosens and eventually separates the larger pieces, so that they fall out, leaving relatively large gaps. This is clearly impossible with a fine-textured material. Many firebrick manufacturers tend to make their bricks of too coarse a material in order to make them resistant to sudden changes of temperature, but this is always at the expense of the durability in other directions. Large blocks may usually be of coarser texture than bricks, on account of their size and the slowness with which heat passes through them. For fireclay blocks about 50 per cent. of grog between 4- and 20-mesh should usually be employed, and all fine powder other than fireclay should be scrupulously avoided. Even a small proportion of fine grog is harmful where the articles are required to resist temperature changes, but where they are only to resist compression fine grog is an advantage, as it produces a stronger mass. Careful grading is especially necessary when making the larger size of blocks, as if proper attention is not paid to this point the blocks will be too sensitive to sudden changes of temperature. Badly graded mixtures are very liable to spall and crack when repeatedly heated. Muffies must be made with as coarse-grained a texture as possible so as to resist the sudden changes of temperature to which they are subjected. They must not be too coarse, however, as they are required to have sufficient strength and durability in use, though strength is not so important as in saggers. An excessively coarse texture is undesirable as the muffle may be subjected to the corrosive influences of dust and fire gases, especially if the fuel is not of very good quality. TEXTURE OF GLASSHOUSE POTS AND BLOCKS 39 Retoris should be coarse-grained in order to secure a maximum refractoriness and resistance to temperature changes. This latter property is especially important, as retorts are often charged with cold material, whilst they themselves are at a very high temperature. An excessively coarse texture in retorts is, however, undesirable, as such a structure is not so strong as a finer one and the thermal conductivity is smaller. In some cases resistance to corrosion is also necessary, and for this a dense texture is very desirable. The best sizes of grog for retorts are between + inch and 60-mesh, though the Institution of Gas Engineers specify that none of the grog should pass through a sieve having sixteen meshes per linear inch. This specifica- tion, if followed, produces so coarse a texture that the strength of the retorts is very low. Gas retorts in Germany are usually made with two sizes of grains: (a) 0-4-0-12 inch, and (b) 0-12—0-04 inch, used in the proportion of 7-8 measures of the coarser with 2-3 measures of the finer grog. Smaller grains than those mentioned are not used, as they would unduly increase the density of the mass. The glasshouse pots used for melting glass are required to be (a) sufficiently strong to bear the pressure of the molten glass in them; (6) sufficiently resistant to the temperature changes occasioned by the heating and cooling of the pot, especially when it is hot and quickly charged with cold materials ; and (c) sufficiently dense to prevent undue corrosion by the glass. Requirements (a) and (c) are obtained by the use of a fine-textured material, but (b) requires a coarse texture, so that a com- promise must be made, a suitably graded material being quite satisfactory. For instance, they may consist of particles of grog between 0-25 inch and 0-05 inch diameter, the bulk being between 0-125 inch and 0-10 inch diameter, and the clay being sufficiently fine to pass, when dry, through a 15—16-mesh sieve. In no case should any grains larger than those mentioned be used or the strength of the pots will be unduly reduced. A small proportion of medium or moderately fine grog may be used in glasshouse pots in order to produce a sufficiently strong mass with a compact surface, but as fine grog reduces the resistance to sudden changes of temperature an excess must be avoided. Glass tank blocks require a similar texture to those of glass pots, but the strength is not so important as there is a greater thickness of material. According to the provisional specification of the Society of Glass Technology, the fireclay flux-line blocks of glass tank furnaces should consist of grains of grog of the following sizes : 1 measure of grains } inch to }y inch, and 2 measures of grains yj inch to =), inch in diameter. The grog for the tank bottom blocks should be 1 measure of grains + inch—4 inch. 4 8 1 measure of grains $ inch—,}y inch. 1 measure of grains ~, inch—;}, inch. For bottom side blocks, 1, 2, and 1 measures are used respectively. For replace- ment flux-line blocks, 2, 2, and 1 measures are used. The texture of crucibles presents a problem similar to that of glasshouse pots, the conditions of use being, in the main, very similar. Strength is very necessary in large crucibles, as some may hold as much as half a hundredweight of metal which 40 PROPERTIES DEPENDING ON STRUCTURE must be carried in the crucible, sometimes for a considerable distance. When a crucible is lifted out of the furnace it undergoes a sudden change of temperature, so that it must be made very insensitive to temperature changes. These properties are obtained by suitably grading the non-plastic materials used in the manufacture of the crucibles. The interior of crucibles should be finer than the exterior, as the inner surface must be resistant to corrosion by the contents. Earthenware and porcelain are required to have a fine texture, and the materials used in their production are, therefore, ground very fine, with the exception of the clays, which are naturally fine in texture. The texture of refractory materials other than clay varies according to their mode of formation. Thus, the siliceous materials generally used for the manufacture of silica, ganister, dinas, and similar bricks, are usually very fine in texture, though many of the pieces of which the bricks are composed may be relatively large. For instance, some of the best ganister occurs in the Sheffield district, and consists of grains between 0-1 and 0-3 mm. diameter, but the material is only ground to pass through a 24-mesh sieve. In other words, the coarser particles consist of aggrega- tions of much finer ones. Similarly, the bastard ganisters of Durham and North Yorkshire consist of grains between 0-05 and 0-15 mm. diameter as a rule, whilst the Scottish bastard ganisters are extremely fine-grained, consisting chiefly of particles less than 0-1 mm. diameter. The Chwarele ganister of North Wales also has a similar structure. All these are very suitable materials for the manufacture of ganister bricks, but ganisters consisting of grains up to 0-6 mm. diameter are not so satis- factory. It may be assumed that for most purposes a diameter of 0-4 mm. is about the maximum desirable for the individual grains of siliceous material. Silica bricks generally consist of a suitable mixture of the following grades: (a) particles between 0-125 inch and 0-25 inch diameter; (6) particles between 0-04 inch and 0-125 inch diameter; (c) fine powder. The proportions in which these materials are mixed depend on the purpose for which the bricks are to be used. As silica bricks are always sensitive to sudden changes in temperature, no matter how carefully they are made, there is no object in making them coarse-textured or in omitting fine material. On the contrary, the best silica bricks are those in which there is a minimum of voids. This is secured by making the fine grains fill the inter- stices between the larger particles and the dust should fill the interstices between the fine grains. Equal proportions of each size have proved to be quite satisfactory in some cases and where coarse grains are undesirable equal parts of the two finer grades alone may be employed. The advantage of using fine dust in silica bricks is that (a) the interstices between the large grains are partially filled and enable a stronger mass to be produced; (6) some of the dust or flour is converted into the amorphous state and so aids in binding the mass together and when heated reacts more readily with the lime also added so producing a good strong bond; and (c) the fine flour is more easily converted into one of the low specific gravity forms of silica, thus reducing the volume changes in the bricks when in use. In some cases, fine water-ground flint is added in making silica bricks to supply the required amount of fine material. It has the advantage of being very readily TEXTURE OF SILICA BRICKS Al converted into tridymite. At the same time, an excess of impalpable silica is un- desirable as a general rule, as it readily combines with fluxes and so reduces the refractoriness of the bricks. A well-graded material is specially important in making silica bricks, because only about 2 per cent. of lime is allowable for bonding purposes and if the silica is badly graded a larger proportion of lime will be necessary. The use of a sufficient proportion of fine material greatly increases the strength of silica bricks. If they are made wholly of fine natural (flowr) the maximum strength is obtained, but the bricks are unduly sensitive to sudden changes of temperature. By using a smaller amount, however (about 30 per cent.), Philipon found that a high strength could be obtained without the drawback just mentioned. According to H. Le Chatelier and B. Bogitch,t not more than 25 per cent. of grains smaller than 200-mesh should be present in silica bricks. As showing the harmful effect of excessively fine material, it should be noted that the grains produced by crushing calcined flint are generally very small and are not aggregated together as in ganister, silica rock, etc., so that bricks made from them are too sensitive to sudden changes of temperature. The texture of sand-moulds used in foundry work is very important, as on it depends very largely the quality of the castings produced in the moulds. To ensure good castings the texture much be such that the moulds will have (a) a fine surface texture, so as to produce a smooth surface on the castings ; (b) ample permeability and porosity, to enable all air and gases to escape through the pores of the mould as rapidly as possible ; (c) ample strength to withstand the pressure of the molten metal, which is often very great and is applied very suddenly ; and (d) the sand used for the moulds must contain sufficient colloidal or equivalent fine material to enable a sufficiently plastic mass to be formed by the addition of only a small proportion of water. The first and last desiderata are obtained by the use of sand composed of fine grains, including some clay, whilst the remainder require a coarse-grained sand. Consequently, in selecting a moulding sand, a compromise must be effected, the pro- portions of coarse and fine grains being varied according to the kind of castings re- quired. In a very large casting, for example, the strength and permeability of the mould are more important than a smooth surface, so that a coarse-grained sand should be employed; whilst in a very small casting, the strength of the mould is unimportant, but smoothness of finish is of the greatest importance, and to secure this a fine-grained sand must be used. To ensure ample permeability, all the grains in a moulding sand should be as nearly uniform in size as possible and should generally be between 0-25 and 0-5 mm. diameter for medium work, and between 0-1 and 0:25 mm. for fine work and facing purposes. According to R. L. Lindstrom, the moulding sands used in casting steel should not contain more than 5 per cent. of grains larger than 20-mesh (0-64 mm.), and not less than 75 per cent. of grains larger than 100-mesh (0-125 mm.). Grains of uniform size impart a less regular and smooth surface than mixtures containing grains of various sizes, and as they cannot interlock they do not produce a mass having the maximum strength, but the deficiency in smoothness may 1 Rev. de Mét., 15, 511 (1918). A2 PROPERTIES DEPENDING ON STRUCTURE usually be overcome by the use of a fine “facing coat’”’ and the strength may. be increased by various mechanical devices when designing the moulds.1 Furnace hearths made of refractory sands should have a close, dense texture so as not to absorb an unnecessary amount of metal and slag, and the individual grains should be sufficiently small to be converted as quickly as possible into the low specific gravity forms of silica (cristobalite and tridymite). The sands used for lining the hearths of open-hearth furnaces in this country usually consist of grains between 0-25 mm. and 0-5 mm. in diameter, but coarse material is sometimes used. Some of the Greensand beds which are used for this purpose consist almost wholly of grains of the above-mentioned sizes. In the Leighton Buzzard sand there is, according to Boswell, about 24 per cent. of grains larger than 0-5 mm. diameter and 74 per cent. between 0-25 mm. and 0-5 mm. diameter. The Aylesbury sand is somewhat finer, and contains more than 78 per cent. of grains between 0-25 mm. and 0-5 mm. diameter, about 15 per cent. between 0-1 mm. and 0-25 mm. diameter and 6 per cent. between 0-01 mm. and 0-1 mm. diameter. In the United States, coarser sands, consisting of grains up to 10-mesh or even to 5-mesh, are regarded as satisfactory, though they involve larger loss of metal by absorption. American steel makers rightly consider that fine-grained sands are undesirable in open-hearth furnaces, because they have only a small angle of rest, whereas the bank of these furnaces should be as steep as possible ; such sands are also undesirable on account of the ease with which the smaller grains fuse superficially, but most British steel makers who have given attention to the subject consider that in their endeavour to avoid the use of sands of too fine a texture, the Americans have gone too far towards the other extreme of excessively coarse-textured linings. When repairing furnace linings it is customary to throw the sand on to the defective portions and it is, therefore, important to use a sand or mixture which will naturally produce a material of the desired texture under such crude conditions. If the grains of sand are all uniform in size they will, when thrown into the furnace, fall or roll into the required position and will readily produce a mass of maximum density, whereas a sand composed of grains of various sizes will, in the absence of any tamping or ramming, form a mass with a more porous and irregular texture and, therefore, less satisfactory.t The texture of magnesite to be used for magnesia bricks, etc., should be that pro- duced by coarse crystals, as these produce an open or porous mass which is more readily calcined than one having a finer and denser texture, such as crypto-crystalline magnesite ; the latter requires a higher temperature as well as more prolonged heating, to ensure it being completely ‘“‘ dead-burned.” The coarse crystalline texture of some forms of magnesite also facilitates the removal of impurities such as serpentine, quartz, etc., which are difficult to remove from the finer-grained, crypto-crystalline magnesite, yet if they are not separated they reduce the refractoriness of the material. Magnesite spar and breunnerite are usually coarse-grained, whilst the 1 Further information on the use of sands for casting metals and for lining furnaces will be found in the author’s Sands and Crushed Rocks : Their Nature, Preparation, and Uses (Frowde, Hodder & Stoughton). TEXTURE OF MAGNESIA AND CORUNDUM BRICKS 48 grains of crypto-crystalline magnesite and hydromagnesite are of microscopical dimensions. The texture of magnesia bricks, blocks, etc., should be “ medium,” only just sufficient fine material being present to provide the necessary bond for the larger particles. To produce this texture, the dead-burned magnesia should be crushed and screened so as to remove all particles larger than ;}, inch diameter and all those smaller than 50-mesh. Coarser particles will require too long a heating to convert them into periclase. The fine particles constituting the bond should be as small as possible, so that they may offer a large surface to the grains of magnesia. If the fine grains consist chiefly of iron oxide, they will act as a catalyst and assist in con- verting the amorphous magnesia into periclase. An excessive proportion of fine grains should be avoided, or the bricks, etc., will crack or “ dunt’ when exposed to sudden changes in temperature. The texture of dolomite bricks is usually that due to the use of particles of dolomite less than } inch diameter, about 25 per cent. of the grains being smaller than 100-mesh. An excess of fine grains is undesirable, as it renders the bricks too sensitive to sudden changes of temperature. In order to produce a satisfactory texture in the dolomite linings of Bessemer steel converters, the ground dolomite used should not contain any particles more than 5-6 mm. diameter and about 30-40 per cent. of the material should be in the form of fine grains. The texture of bawaite bricks is usually that produced by particles between } inch and =, inch diameter, united with 15-30 per cent. of finely ground fireclay. Some manufacturers prefer to use finer bauxite. The texture of corundum articles used for refractory purposes should be as coarse as the conditions of their use will allow, as the larger the crystals the more resistant is the material to sudden cooling and shocks. Granger has recommended the use of 12-mesh grains for carborundum crucibles and a mixture of 4- to 5-mesh grains and 7-mesh grains for tubes and similar articles. It is usually advisable to make corundum bricks of 7 parts of coarse crystals of corundum, | part corundum in fine powder, together with 1 part of plastic fireclay to bind the other particles together. The texture of coke bricks is produced by the use of particles 0-04 to 0-08 inch diameter, bonded with gas-tar or soft pitch. On burning, the tar is coked, forming a strong bond. Bricks made of plumbago bonded with fireclay have very fine- grained texture on account of the nature of the materials used, but as the mixture undergoes a negligible change in volume when the bricks are in use, the close texture does not interfere with the resistance of the bricks to sudden changes in temperature, whilst it has the advantage of rendering them more resistant to corrosion and of increasing their thermal conductivity. The so-called graphite slabs used for flattening window glass require to be specially resistant to repeated changes of temperature; they are, therefore, made with a coarse texture, the materials used consisting of particles of grog and graphite up to 10-mesh and fireclay up to 28- or 30-mesh. The surface of these slabs is usually covered with (or has the upper surface to a depth of } inch made of) a material of 44, PROPERTIES DEPENDING ON STRUCTURE rather finer texture, all the particles used for this portion being capable of passing through a 25-mesh sieve. The texture of carborundum bricks is not usually of great importance, as carborun- dum is insensitive to sudden changes in temperature and very largely so to corrosion. The material should, however, be graded so as to form a sufficiently strong mass. In one of their patent specifications, the Carborundum Company state that the grains for carborundum bricks should be between 16- and 100-mesh, a suitable mixture consisting of equal parts of 16-, 24-, 36-, and 100-mesh, together with some fine powder if desired. The texture of cements should be extremely fine, so that the individual particles will have as large a surface as possible and the necessary bonding power. In refractory cements, the particles should be small enough to fuse superficially and so produce a strong binding agent and yet should be sufficiently large to have the requisite re- fractoriness. It is customary to grind refractory cements until they pass entirely through a 24-mesh sieve, but such a material is coarser than is desirable. If it is made to pass through a 60-mesh sieve it will give better results, and if ground to an impalpable powder it will be still better. The coarse particles are useless in a cement, as they have very little binding power, so that it is always desirable to use cements which have been ground to as fine a state of division as possible, having due regard to the expense of very fine grinding. For most purposes, if a refractory or similar cement does not leave a residue of more than 10 per cent. on a 200-mesh sieve, there is little to be gained by grinding it still further. Determining the Texture.—If an article or mass of material hase a coarse texture, this may be seen by the naked eye or with the aid of a small magnifying glass, but a medium or fine texture requires a microscope for its examination. The texture is more easily seen if the material is ground flat and then polished, or even if, as suggested by J. Lomas, it is ground fairly smooth and a thin sheet of clear glass is cemented to it by means of Canada balsam. A simple lens may be used for examining a comparatively rough surface, but if a microscope is to be used effectively the varia- tions in the surface must not be so great that some portions are out of focus, whilst others are clearly visible. It is a mistake to use too powerful a microscope when examining the texture of a material; a medium magnification (not exceeding 120 diameters) is best, the greater magnifications being reserved for the examination of individual particles rather than the texture as a whole. If required, the size of the particles may be measured by means of a microscope and an eyepiece micrometer. If a material is easily disintegrated into its component particles, as when dried clay is rubbed between the fingers or stirred with a larger quantity of water, useful information respecting its texture may be obtained by one or more of the four fol- lowing methods in which the particles of different sizes are separated from each other : (a) screening, riddling, or sieving (p. 45); (6) elutriation (p. 50); (c) air separation (p. 53), and (d) sedimentation (p. 53). The first method is. suitable for separating all particles larger than 0-0025 inch diameter; the others are chiefly suitable for smaller particles. A combination of method (a) with either (b) or (d) is sometimes termed mechanical analysis. All four SCREENING, RIDDLING, AND SIEVING A5 methods are equally applicable to determining the sizes of the grains of various loose materials and powders. Screening, riddling, or sieving consists in passing the material—either in its dry state or in suspension in water—over screens, riddles, or sieves having various apertures of known dimensions. The particles larger than these apertures remain behind, whilst the smaller particles pass forward and will be retained by the success- ively smaller apertures until the last sieve is reached, through which the smallest grains will eventually pass. This method is very convenient for separating all particles larger than 0-0025 inch diameter (200-mesh), but is seldom applicable to smaller ones on account of the difficulty of making accurate sieves with smaller apertures. Sieves, riddles, and screens are made with apertures of various shapes, some apertures being circular, others square, and still others being rectangular. The last- named are useless for investigating the texture or fineness of a material. Sieves with circular apertures do not give the same results as those with square ones, and the results obtained with a sieve made of perforated sheet will differ from those of a sieve made of wire-gauze and having square apertures. The latter are generally employed for testing purposes. Gauze sieves are made of various metals, including iron, steel, brass, bronze, and - occasionally copper. In some cases, gauzes made of silk are employed for the finest materials, but they are very delicate and not to be recommended. Those made of phosphor-bronze are usually the most satisfactory. All sieves used for testing should be examined to ensure the apertures all being of the desired sizes, as otherwise the results obtained may be erroneous. Such sieves should be very carefully used, especially the finer ones, as they are soon spoiled if used for material much coarser than the mesh of the sieve. For instance, no material coarser than 50-mesh should be put on to a 100- or 200-mesh sieve ; the larger particles should have been separated previously by passing the material through coarser sieves. Fine sieves which have been in use for some time may permit particles larger than the nominal mesh of the sieve to pass through them on account of the wear of sieving the material and of the stretching and sagging of the wires. For this reason the mesh of such testing sieves should be examined from time to time and, if any apertures are too large, the sieve should be repaired or discarded. In order that all results obtained when testing the texture or fineness of a material by means of sieves may be comparable, it is most desirable that “standard sieves’ should be employed, otherwise the size of grains corresponding to a given sieve may not be known, and the results, if reported, will be of little use except to the original user, and may cause confusion. The standard for sieves used in this country is that adopted by the Institute of Mining and Metallurgy ; } the relation of aperture to thickness of wire and mesh-number are shown in Table I. In other countries, different standards are used ; the one generally recognised in America is that suggested by the U.S. Bureau of Standards,? with the dimensions shown in Table IT. 1 Such sieves are made by N. Greening & Son, Warrington. 2 These sieves are made by the W. S. Tyler Co., Cleveland, Ohio, U.S.A. 46 PROPERTIES DEPENDING ON STRUCTURE On the Continent, it is usual to refer to sieves by the number of apertures per square centimetre. The relation of these to British sieves is shown in Table III. In order to convert the results obtained with a sieve expressed by the number of holes per sq. cm. into the British sieve number, the square root of the Continental figure should be multiplied by 10 and divided by 4. Thus, for a sieve having 400 holes per sq. cm., the square root of 400 is 20, which, multiplied by 10 and divided TABLE I.—/.M.M. Standard Sieves Mesh, i.e. porns Diameter of Wires. Diameter of Apertures. SCreanite eee per linear per cent. holes. inch. inch. mm. inch. mm. 5 0-1000 2-540 0-1000 2-540 25-00 8 0-0630 1-600 0-0620 1-574 24-60 10 0-0500 1-270 0-0500 1-270 25-00 12 0-0417 1-059 0-0416 1-056 24-92 16 0-0313 0-795 0-0312 0-792 24-92 20 0-0250 0-635 _ 0-0250 0-635 25-00 30 0-0167 0-424 0-0166 0-421 24-80 40 0-0125 0-317 0-0125 0-317 25-00 50 0-0100 0-254 0-0100 0-254 25-00 60 0-0083 0-211 0:0083 0-211 25-00 fi 0-0071 0-180 0-0071 0-180 25-00 80 0-0063 0-160 0-0062 0-157 24-60 90 0-0055 0-139 0-0055 9-139 24-50 100 0-0050 0-127 0-0050 0-127 25-00 120 0-0041 0-104 0-0042 0-107 25-40 140 0-0036 0-091 0-0036 0-091 25-00 150 0-0033 0-084 0-0033 0-084 24-50 160 0:0031 0-078 0-0031 0-078 25-00 180 0:0028 0-071 0-0028 0-071 25-00 200 0-0025 0-063 0-0025 0-063 25-00 by 4, gives 50. This sieve, therefore, corresponds to one having 50 meshes per linear inch, 2.e. to a No. 50, or 50-mesh sieve. Unless British, American, or other standard sieves are used the size of the aper- tures cannot be calculated from the mesh-number, as so much depends on the gauge of the wires used. For instance, two sieves may each have 10 holes per linear inch, but whilst in one the holes may be 0-05 inch in diameter and the wires 0-05 inch thick, in the other the holes may be of 0-067 inch in diameter and the wires 0-033 inch. It is, therefore, most important either to use sieves of standard design or to specify STANDARD SIEVES AT the actual dimensions of the apertures as ascertained by direct measurement, so that the maximum size of grains passing through a sieve of any particular size may be accurately known. TABLE II.—American Standard Sieves Sieve | Sieve Sieve pe ag Wire aera poence ce pee es No. | Opening.) Opening. sates Diameter.| Average a bla Maximum| P&" | Pe meter ; Diameter. F inch Opening. Opening. mm. inches. mm. inches. | per cent. | per cent. | per cent. 24 | 8-000 | 0-3150 | 1-850 | 0-0730 1 5 10 1-0 2-6 3 | 6-720 | 0-2650 | 1-650 | 0-0650 1 5 10 1-2 3:0 34 | 5-660 | 0-2230 | 1-450 | 0-0570 1 5 10 1-4 3°6 4 | 4-760 | 0-1870 | 1-270 | 0-0500 1 5 10 1-7 4-2 5 | 4:000 | 0-1570 | 1-120 | 0-0440 1 5 10 2-0 5-0 6 | 3-360 | 0-1320 | 1-020 | 0-0400 1 5 10 2:3 5:8 7 | 2-830 | 0-1110 | 0-920 | 0-0360 1 5 10 2°7 6-8 8 | 2-380 | 0-0940 | 0-840 | 0-0330 2 5 10 3:0 7-9 10 | 2-000 | 0-0790 | 0-760 | 0-0300 2 5 10 3:9 9-2 12 | 1-680 | 0-0660 | 0-690 | 0-0270 2 5 10 4:0} 10-8 14 | 1-410 | 0-0557 | 0-610 | 0-0240 2 5 10 5-0} 12-5 16 1:190 | 0-0468 | 0-540 | 0-0210 2 5 10 6-0} 14-7 18 | 1-000 | 0-0394 | 0-480 | 0-0187 2 5 10 7-0) 17-2 20 | 0-840 | 0-0331 | 0-420 | 0-0165 3 5 25 8-0 | 20-2 25 | 0-710 | 0-0278 | 0-370 | 0-0146 3 5 25 9:0 | 23-6 30 | 0-690 | 0-0234 | 0-330 | 0-0129 3 5 25 11:0 | 27-5 35 | 0-500 | 0-0197 | 0-290 | 0-0113 3 5 25 13:0 | 32-3 40 | 0-420 | 0-0166 | 0-250 | 0-0098 3 5 25 15:0 | 37-9 45 | 0-350 | 0-0139 | 0-220 | 0-0085 3 5 25 18:0 | 44-7 50 | 0-300 | 0-0117 | 0-188 | 0-0074 d. 10 40 20-0 | 52-4 60 | 0-250 | 0-0098 | 0-162 | 0-0064 4, 10 40 24:0 | 61-7 70 | 0-210 | 0-0083 | 0-140 | 0-0055 4 10 40 29-0 | 72:5 80 | 0-177 | 0-0070 | 0-119 | 0-0047 4 10 40 34:0 | 85-5 100 | 0-149 | 0-0059 | 0-102 | 0-0040 10 40 40-0 | 101-0 120 | 0-125 | 0:0049 | 0-086 | 0-0034 4 10 40 47-0 | 120-0 140 | 0-105 | 0-0041 | 0-074 | 0-0029 5 15 60 56-0 | 143-0 170 | 0-088 | 0-0035 | 0-063 | 0-0025 5 15 60 66-0 | 167-0 200 | 0-074 | 0-0029 | 0-053 | 0-0021 5 15 60 79-0 | 200-0 When the texture or fineness of a material is to be examined by means of a sieve, the material may be used either in the dry state or after it has been mixed with a large amount of water. The former is sometimes termed a “ sieving ”’ or “ screening ”’ 48 PROPERTIES DEPENDING ON STRUCTURE test, and the latter a ‘‘ washing test.”’ Whilst the results obtained are quite dif- ferent, the method of using the sieves is very similar in each case. For a (dry) sieving test, a weighed quantity of the material (z.e. 100 oz. if it is very coarse) should be rubbed gently between the finger and thumb or with a smooth wooden pestle, taking great care not to crush the individual grains but only to separate them from one another. When the material has thus been reduced to a rough “ powder ”’ it is placed on the coarsest sieve and either stirred or shaken gently until as much of it as will pass through the sieve has done so. The residue is examined to ensure the absence of aggregations of smaller grains which have escaped the rubbing, and when these (if any are present) have been broken down, and passed through the TaBLE III.—Relation of Various Sieves Meshes per Meshes per Meshes per | Meshes per Ae linear inch. square inch. cm. square cm. 30 30 900 12 144 60 60 3,600 24 576 90 90 8,100 36 1,296 100 100 10,000 40 1,600 120 120 14,400 48 2,304 150 150 22,500 60 3,600 200 200 40,000 80 6,400 sieve, the residue on it is weighed. The material which has passed through the first sieve is similarly treated on the next finer sieve in the series, and this process is con- tinued until all the sieves have been used. The weights of the various residues, together with that of the material which has passed through the finest sieve, should be equal to the weight of the original mass. If any appreciable difference occurs some of the material has been lost in the process. It is not always necessary to use all the sieves, and for many purposes it is suffi- cient if the aperture in each of the sieves used is about half that of the one preceding it. Thus, for most clays and similar materials the following sieves will suffice : Nos. 3, 6, 12, 25, 50, 100, and 200, the intervening ones only being used for special purposes.1 1 Mellor has suggested that in most cases three sieves of 5-, 50-, and 120-mesh are sufficient, whilst Boswell prefers to separate materials into the following divisions :— Greater than 2 mm. diameter. 1-2 mm. diameter. 0:05-0-1 mm. __,, 0:01-0:05 mm. ,, Less than 0-01 mm. diameter, WASHING TEST 49 Unless a material is almost or quite dry, a dry sieving test will not yield satis- factory results, and a washing test should then be employed. For a washing test a small or larger weight of material is used according to the nature of the substance. If itis fairly fine and uniform, 500 grams is ample and even 200 grams may be sufficient, but for coarse, heterogeneous material it is more con- venient as well as more accurate to use 100 oz. (64 lb.) or 3 kilograms. The material should be dried before being weighed, or a separate determination of moisture in it should be made. The weighed material is carefully mixed with about four times its weight of very soft, or perfectly distilled water in a basin of convenient size. Some clays which would be difficult to manipulate at a later stage are made easier if a few drops of ammonia are added and the mixture of clay and water boiled for about ten minutes. In any case, the object is to get all the smaller particles into suspension in the water without breaking up any coarser ones which may be present and without altering the chemical composition of the material. If the material does not readily form a fairly smooth “‘ cream ”’ or slip, the more resistant portions may be gently rubbed with a polished wooden or porcelain pestle to loosen all the particles, or with materials containing much coarse matter of a fragile nature, the fingers may be used to remove the finer particles from the coarser ones. Some “ clays ”’ which are diffi- cult to manipulate when freshly wetted can easily be tested if roughly mixed with water and then allowed to stand for twenty-four hours. The liquid containing material in suspension is next run through a sieve, the mesh of which will depend on the size of the largest particles in the sample. This may be ascertained with sufficient accuracy by looking at the portion of the material which sinks to the bottom of the liquid, and from its appearance selecting an appropriate sieve, as there is no advantage to be gained by passing the material through sieves with such large apertures that no residue will be left on them. The slip or slurry thus produced is passed through each of the sieves in turn, each residue being returned to the basin, treated with more water and again transferred to the sieves in turn. This process of treating with water is repeated until the residue on each sieve is “clean” and free from all particles which will pass through the particular sieve on which the particles are supposed to be retained. If necessary, a fine yet powerful jet of water may be directed on to the contents of a sieve so as to wash out any finer particles. During this treatment care must be taken not to break up grains of non-plastic material, or to rub the material so hard that it will be ground and so enabled to pass through the sieves on which it should be retained. The material which has passed through all the sieves is next received in a large glazed pan, in which it may be left all night, when the greater part of the solid material will have settled. The clear surplus water may be removed by means of a siphon. If, however, the suspended matter does not settle readily, the only means of removing the water is by evaporating the mixture to dryness in a large evaporating basin, heated on a water bath or over a cylinder containing steam. It cannot be boiled dry with safety, on account of its tendency to “spit.” As this operation of drying is very tedious and cannot be hurried, there is a great tendency to use as small a quantity as possible of material for the washing test, so as to reduce the time of 4 50 PROPERTIES DEPENDING ON STRUCTURE evaporation to a minimum. Alternatively, either an aliquot part, or the whole of this material may, if desired, be subjected to an elutriation (below) or other test. The residues on the various sieves are washed into evaporating dishes of convenient size and are dried by heating on water baths, or first over a very low gas flame and then on a water bath or sand tray. They are finally dried at 110° C., or as near to this temperature as is attainable, and are afterwards weighed. The results may be calculated to a percentage by multiplying each by 100 and dividing by the total weight of the material originally used. The total should add up to 100-00, apart from any losses (including any moisture present) which may have occurred. Usually a difference of 0-5 per cent. between two tests of the same sample is regarded as unavoidable, though with uniform materials a smaller error of experiment is possible. Both skiJl and care are required when using sieves in the manner described, or some fine material which ought to pass through one or more sieves may be left amongst the coarser matter. This is less likely to occur with dry material if the sieves are covered and agitated mechanically, e.g. by a small motor, for a definite time, instead of shaking them by hand. It is also an advantage to connect a series of sieves together, one above another, so that all may be agitated simultaneously. Elutriation consists in separating particles of different sizes or of different specific gravity by suspending them in a fluid, such as water, which is flowing at such a velocity as will enable it to carry off the smallest or lightest particles, whilst the largest or densest ones settle to the bottom of the elutriating vessel. When used for examin- ing the texture or fineness of a material it is usually assumed that the particles will be separated according to their sizes, but when a heterogeneous material is examined the possibility of relatively large particles of low specific gravity being removed along with the smallest particles must not be overlooked. The shape of the particles also has an important effect on their behaviour during elutriation; grains of a flaky character are carried away by a current of the same velocity as much smaller grains of a more cubical or spherical shape. Other factors which influence the results are (a) the purity of the water, as hard water has a flocculating action on some fine clays; (b) the temperature of the water, which should be kept constant at 15° C.; and (c) the amount of matter in suspension in the elutriator ; this should be kept as constant as possible to 10 grams of material actually placed in the vessel, quite apart from the weight of coarse material in the original sample. In order to avoid flocculation of the clay grains, some investigators add a little ammonia to the water used for elutriation ; this is an undesirable practice, which should be avoided wherever possible. If several elutriators are used in series, the liquid in each having a different velocity, it is possible to separate a material into as many “ grades”’ as there are vessels, but as it is difficult to secure a perfectly uniform flow, free from eddies, in very large vessels and as a very much longer time is required in order to effect the separation of larger quantities of materials, it is usually necessary to work with only a very small quantity of the material, preferably that which has previously been passed through a No. 200 sieve. As a matter of convenience, it is usually assumed that all the “ true clay ” in a material (except in the case of indurated clays and some ELUTRIATION 51 shales) will be removed by a stream of water flowing at the rate of 0-43 inch per minute or 0-18 mm. per second. Such a stream was found by Seger to remove all particles with a diameter of less than 0-0004 inch or 0-010 mm., so that it washes out all the clay. The “clay” so removed is usually con- taminated with a variable proportion of rock dust and other non-plastic material and, in some cases, it may be quite devoid of “ true clay.” A typical elutriator (fig. 4) consists of a glass vessel about 60 cm. high, the upper 10 cms. (D to #) being cylindrical and about 5 cm. internal diameter, and the lower part (EZ to F) conical and tapering from 5 cm. internal diameter to 5 mm. diameter. The lower end is connected to a tube G, 5 mm. in diameter, through which the water or other elutriating fluid enters and flows upward through the apparatus. This narrow portion is necessary to ensure a truly central flow, which is quite free from eddies and side currents. The top of the elutriator is constricted so as to hold a rubber stopper carry- ing the overflow-tube A, which also acts as a manometer and enables the velocity of the liquid to be measured at any moment. At B in this tube is a small hole, 1-5 mm. in diameter, the edges of which have been rounded by fusion so as to provide a smooth aperture. The pressure tube J should be made of barometer tubing, and should be carefully tested to see that it is really uniform. The bends at A and’ B must be very carefully made, so that no narrowing of the internal bore occurs. The upper vertical part of the tube is about 1 metre long and should be divided into em., and the lowest ten of these divisions should be sub- divided into mm. The end Z is connected to a water tank, or, preferably, to a large Marriott bottle, so as to maintain the water at a constant rate of flow, the volume of water being regulated with a small pinch-cock. A small piece of wire gauze is fitted into the bottom of the conical portion at F, by means of a perforated cork, in order to prevent any coarse particles from becoming stranded in the bend C. : The stopper and pressure-tube are removed and the vessel is Bee eS ¥ filled to the middle of the cylindrical portion with water, and the water is allowed to flow at the prearranged rate. A weighed quantity of the material to be examined (e.g. 10 grams) is then placed in the vessel. If necessary, it may be transferred by the aid of a little water. The pressure-tube is rapidly inserted and a large pan placed ready to receive the overflow. The supply of water is continued until no further particles are carried away in suspen- sion, after which the apparatus is dismantled, the residual matter is carefully removed, collected in a small basin, dried and weighed as “‘silt.” It is not 52 PROPERTIES DEPENDING ON STRUCTURE &¢ usual to attempt to dry and weigh the “clay ” portion, unless this is required for some other purpose. The elutriator should be carefully calibrated before use and the height of the water in the pressure-tube, which corresponds to the required velocity, should be ascertained. The area of the cylindrical portion D, EF is ascertained by filling the apparatus with water exactly to the level #, and then running in water from a burette or from a weighed vessel until it is exactly at the point D; the volume in c.c. or weight in grams of the water so added, divided by the height h in cm., gives the area (a) of the apparatus at the point of minimum velocity. To ascertain the rate of flow, the apparatus is set to work without any solid material and the quantity of water flowing through it in a period of exactly sixty seconds is measured or weighed, the height of the water in the pressure-tube being carefully noted. The weight or volume of the effluent water divided by the area (a), and by 60, gives the rate of flow in cm. per second. If it is either faster or slower than is required, the pinch- cock which controls it must be adjusted and another trial made. After the correct rate has been obtained, it is only necessary to note the height of water in the pressure- tube and to maintain the water at this height throughout all future tests with the same apparatus. If the same vessel is to be used for separating particles of different sizes, a record of the height of the water in the pressure-tube corresponding to such sizes should be kept. Various other patterns of elutriator are in use, but the general principle is the same in all. Krehbiel’ s elutriating apparatus consists of a number of metal containers, consisting of a hollow cylinder surmounting an inverted hollow cone. Each container has a different diameter, so that the speed of the water flowing upward through it effects the suspension of particles of different sizes. By this means a good separation can be effected, though a long time and a very large volume of water are required. A small elutriator, of the same type, originally devised by Schloesing, but modified by Lowry, to ensure more rapid working, consists of a long, conical-shaped vessel (fig. 5) mounted on a heavy foot. A loose-fitting brass cap lying on the top serves to carry a long glass tube, the lower end of which reaches to within an inch or two of the bottom of the vessel. The upper end of the tube projects into the “ constant level”? apparatus by means of which a constant pressure or head of water is maintained. The apparatus is put into operation by allowing water to flow from any convenient tap or other source of supply, 7’, into the constant level tube, A. This fills up and the water flows down the central glass tube into the elutriator, which gradually fills and overflows into a graduated glass cylinder placed alongside. The rate of flow of the water through the elutriator is controlled by the size of the orifice of the long centraltube. By means of a number of tubes, each having a definite sized orifice, it is possible to regulate the flow as required. The elutriation test is made by weighing 10 or 20 grams of the dried material, working it up with water in a mortar to a thin creamy paste, and washing it into the elutriator. ELUTRIATION AND SEDIMENTATION 53 A tube of slow rate of flow (e.g. 30 c.c. per min.) is placed in position and the water turned on. The material is agitated and a certain quantity overflows with the water into the graduated cylinder. When the water becomes clear, owing to the removal of the lightest particles by the water, the apparatus is stopped, the graduated cylinder is changed and another tube of more rapid flow is inserted. D This process is repeated with one tube after another, each a lot of overflow water being put aside so that the clay, etc., (sr in it may settle. The supernatant clear liquor is carefully poured off and the material washed out into small beakers, allowed to settle a second time, and finally it is transferred, after pouring off the clear water, to weighed watch-glasses, dried at 110° C., and the net weight of material in each portion is weighed. In this way, the original sample is graded into as many grades as there are tubes correspond- ing to various rates of flow. The apparatus must be standardised by testing materials of known fineness so as to determine the range of particles washed out by particular velocities. Of these various types of elutriators, Schoene’s (fig. 4) is usually the most accurate, Schloesing’s (fig. 5) the most rapid and Krehbiel’s the most suitable for larger quantities or for separating the finer particles into several grades. Avr separation may be described as elutriation by air instead of by water and is effected in a similar manner. Fic. 5.—Lowry’s The chief difficulty lies in securing a current of air at a pee: sufficiently uniform velocity. When this is available, air separation has the advantage of enabling much finer particles to be separated than is possible when water is used. Sedimentation.—Instead of removing some of the particles by a current of water or air as just described, it is sometimes more convenient to suspend all the particles in water and then to leave them for some prearranged time. Most of the larger or denser particles will then settle ; particles still remaining in suspension may be removed by pouring off the liquid. If the treatment with water is repeated, the periods of quiescence being increased progressively, a series of different “grades” will be produced. The rate at which the various particles will settle may be calculated by means of Stokes’ law :— __2(D—d) s brrerre or V=Cr? when part Dedly oy 54 PROPERTIES DEPENDING ON STRUCTURE where V=the velocity of the particles in cm. per sec. y=the viscosity of the liquid. r=the radius of the particles in cm. g=981. d=the specific gravity of the liquid. D=the specific gravity of the particles. Table IV shows the time taken by particles of clay, silica, and other materials of specific gravity 2-6 to fall through the distance mentioned in the last column of the table :— TaBLe IV.—Settling of Particles in Water. Size of Particles. me eee Depth from Surface. mm. inch. mm. inch. 0-1 0-004 20 sec. 140 5:6 0-05 0-002 1 min. 120 4-8 0-01 0-0004 10 min. 90 3:6 This formula assumes that (a) the particles of solid matter are much larger than the particles of liquid ; (6) the liquid is of infinite extent in comparison with the sinking particles ; (c) the particles are smooth and rigid ; (d) no slipping occurs between the particles and the liquid ; (e) the velocity is small; and (f) the particles are small, but not excessively so. E. Cunningham has shown that, to make this law applicable A ae to the smallest particles, it must be multiplied by (142), where A is “a constant depending on the collisions between the gaseous molecules and the solid particles.” It lies between 0-81 and 1-63. The finest possible colloidal particles may remain suspended indefinitely on account of Brownian movement. When the velocity of sinking is great, this formula is inapplicable, but for all grains of less than 200-mesh (other than flat flakes, see p. 50), it is sufficiently correct. Sedimentation tests may very conveniently be carried out in cylindrical vessels about 6 inches high, the most suitable diameter depending on the quantity of material used. In order to avoid errors due to irregular movement of the particles in a con- centrated suspension, the quantity of water in any vessel should never be less than 20 times the weight of the solid material. If the materials are such as correspond to Table IV, the following procedure is convenient : the material is stirred thoroughly with at least 20 times its weight of water until every particle is separated from its neighbours and is then allowed to stand! 10 minutes, after which the uppermost 1 The total depth of the water should be about 6 inches. SEDIMENTATION 55 3°6 inches of the liquid is carefully siphoned or poured off, care being taken not to disturb the material at a greater depth. The liquid so removed is assumed to contain all the particles smaller than 0-01 mm. (or 0-0004 inch), and the residue to contain all the larger particles. The vessel is refilled, its contents are again thoroughly stirred and then allowed to settle for 1 minute, after which 4-8 inches of liquid are drawn off, the residue is assumed to contain only particles more than 0-5 mm. (or 0-002 inch) in diameter and the decanted liquid to contain those between 0-01 and 0-05 mm. diameter (or 0-0004 and 0-002 inch). Some people prefer to repeat each period of settling twice or three times, with the idea of effecting a cleaner separation. This is especially necessary with some plastic clays, which are very difficult to treat satisfactorily by this process. The fact must not be overlooked that the Stokes formula, whilst quite applicable to non-plastic materials, is less satisfactory when applied to plastic ones, and par- ticularly to semi-flocculated clays. The aggregations of plastic clay particles are so voluminous and their specific gravity is so close to that of water, that they do not settle out at a normal rate and they are so highly absorbent that small particles of fine sand and silt tend to adhere to them and are carried along with the clay. Hence, this method often shows too high a proportion of “clay” and correspondingly low proportions of the coarser materials. Conversely, some of the clay also tends to adhere to the sand and silt and thus gives slightly inaccurate results. For these reasons, where much plastic clay is present, it is very important to have the material thoroughly disintegrated and to repeat each period of settlement several times, so as to ensure as sharp a separation as possible. Owing to the greater distance they have to travel, clayey materials are often more accurately and sharply separated by elutriation. An ingenious modification of the sedimentation method just described has been devised by H. G. Schurecht,1 who claims that it yields very accurate results with either plastic or non-plastic materials and is much more accurate than elutriation or ordinary sedimentation in determining the proportion of particles of various sizes. In Schurecht’s method the material is suspended in water, as before, and is trans- ferred to a glass cylinder 15-5 cm. high and 3-5 cm. internal diameter, the mixture occupying a depth of exactly 12-7 cm., when a glass plummet is suspended in it from the arm of a balance by a thin copper or gold thread. The plummet, which should just be covered by the liquid, is then weighed at intervals of 1, 2, 3, 5, 10, 20, 35, and 60 minutes, 2, 3, 5, and 7 hours, and 1, 2, 3, 5, 10, 15, 20, and 30 days, or at such other intervals as may be desired. When the liquid is to be allowed to stand for several days, the plummet may be removed and the glass cylinder tightly closed, but its contents must be disturbed as little as possible when the plummet is again immersed. From the weighings, it is possible to determine the average weight of suspended matter in that portion of the slip in which the plummet is immersed, from D(S—d) D—d specific gravity of the material, d that of the water, and S that of the slip. The 1 J. Amer. Cer. Soc., 4, 812 (1921). the equation W= where W is the average weight per c.c. of slip, D is the 56 PROPERTIES DEPENDING ON STRUCTURE resulting figures show the weight of material which settles in different periods of time and from this may be calculated the proportions of particles of different sizes. Comparison of Different Textures.—One of the most obvious means of com- paring different textures is to compare photomicrographs of the various specimens, but this method is far from accurate. It is the only one available in many instances, but where the material can be disintegrated without crushing any of the component grains, a much more accurate comparison is possible. For instance, the proportions of grains of different sizes in each material may be compared, though this is tedious and not always very enlightening and many people would prefer to use one figure to express the fineness or texture. One of the most convenient means for so doing was devised by W. Jackson and was termed by him the surface factor. Jackson accepted the figures for the average diameters of the grains in the four divisions given in Seger’s classification of the clays (p. 35) and, assuming the particles to be true spheres, he calculated the average surface area of the materials in each of the four divisions and obtained the results shown in Table V. TaBLE V.—Surface Factors Material. Clay. Silt. Dust Sand. | Fine Sand. Average diameter of grains, mm. . 0-005 0-0175 0-0325 0-080 Average surface area of equal weights of grains. ; : 3367 962 518 of The same method of calculation may be extended to larger particles which yield the following factors :— Passing through 30-mesh, average diameter 0-4 mm. Factor 45 29 99 25 9? 29 39 0-5 3? o> 36 92 2? 12 9 29 9? 1-0 29 99 23 99 2? 5 2? 99 2) 2-5 9) ? 9 but these figures are not so accurate as those for smaller grains. From these figures it is possible to obtain a “‘ fineness figure ”’ or “‘ surface factor ”’ for each material by multiplying the percentage weight of each grade of material present by the average surface area of the grains in that grade, adding the products so obtained and dividing the result by 100. This figure for the “ surface factor ” is not strictly accurate, but it is sufficiently so for comparative purposes. A modified method of calculation suggested by Purdy is intended to give a more nearly correct result. Purdy divides 1 by the various average diameters and thus SURFACE FACTORS 57 obtained the figures 200-00, 57-14, 30-97, and 12-50, which are treated in the same manner as by Jackson, but the resultant surface factor figure is smaller and Purdy regards it as rather more accurate. J. W. Mellor has shown that the true average diameter of a series of particles of different sizes is not the arithmetic mean of the largest and smallest diameters, but corresponds to th (D-+d)(D2+-d?) 4 2 where D is the diameter of the largest and d that of the smallest particles in each group. By using this formula and dividing clays and allied materials into three divisions, namely, (a) particles below 0:010 mm. diameter, (6) those between 0-01 and 0-063 mm. diameter, and (c) those from 0-063 to 0-107 mm. diameter, he obtained the following factors, 359, 53-9, and 26-0, which, when multiplied by the respective weights of the different grades, and added together, give possibly the most accurate value attainable of the average surface area of the material.1_ No surface factor is entirely accurate for the following reasons, given by Heath and Green 2 :— (i) The particles are not wholly spherical, though their sphericity has been assumed in determining the surface factor. (ui) The specific gravities of all the particles are not the same, though a constant specific gravity is assumed. (ui) The conception of an “average diameter” is an inadequate approximation. The method has, however, many conveniences and is very useful. Another method of comparing the texture or fineness of various materials is that recommended by the American Foundryman’s Association, which consists in multi- plying the percentage by weight of material passed through each sieve by the mesh number of that sieve, adding the results obtained and dividing by 100. This method is satisfactory for most purposes, though not so accurate as those mentioned above. Various attempts have been made to compare various textures by plotting the results of sieving or similar tests in the form of a graph, using the horizontal scale for the size of grains and the vertical scale for the percentage of grains of each size. This method is convenient in comparing mixtures to secure a maximum density (see also p- 33). It has been modified by Boswell, who prefers to plot the sizes of the grains on a horizontal logarithmic scale and the cumulative percentage of the various sizes on a plain vertical scale, asin fig. 6. In Boswell’s method the scale of sizes is extended in a convenient manner, and a mixture consisting of particles entirely of one size is represented by a vertical line as at A, B, instead of by asingle point. For particles of various sizes, the more nearly vertical the line, the more nearly uniform is the grading. As small variations in the sizes of the grains causes large deviations from the vertical, this method provides a very accurate means of comparison within a very small space. Another method is that of M. Feret, who recognises only three groups of particles ; 1 See also Perrot and Kinney, J. Amer. Cer. Soc., 6, 417 (1923); Bull. Amer. Cer. Soc., 2, 121 (1923). 2 Handbook of Ceramic Calculations, p. 160. 58 PROPERTIES DEPENDING ON STRUCTURE G=particles of 5 mm.—2 mm. diameter, M=particles 2 mm.—0-5 mm., and F=particles less than 0-5 mm.; he represents the proportions of material of each of these sizes 90 e Weights. Sg eas Cumulative Percenta ® A 2:0 10 Os 0:25 005 oor mm. Grade Sizes C Peaoete mm.) Fia. 6.—BoswE.Lw’s GRADING GRAPH. Lia O2F O4F 06k O8F Fic. 7.—Frret’s TRIANGULAR DIAGRAM. by a single point on a triangular diagram (fig. 7), this point being at the intersection of lines drawn from each side from the points expressing the proportion of each size HOMOGENEITY 7 59 of grains, the lines being drawn parallel to each side as shown. Each of the points, G, M, and F, represents a mixture consisting wholly of those particular sizes of grains and A one containing 47 per cent. of G, 16 per cent. of F, and 36 per cent. of M. HoMOGENEITY For most purposes for which ceramic materials are used it is not only important to have a suitable texture, but also that the texture should be uniform throughout the entire mass. Thus, if a considerable number of large particles are present they should not be segregated, but distributed uniformly through the material. In some articles, such as building bricks, homogeneity is not so important, though very desirable, but in refractory articles it is almost essential, because a material which is not homogeneous is very liable to crack or spall. Homogeneity is especially important in crucibles, retorts, glass-melting pots, pottery, porcelain, etc., and for this reason special care is required in preparing the clay to be used for such articles. Large and thick blocks must also be made as homogeneous as possible, on account of their liability to develop internal stresses. Homogeneity in raw materials is obtained by (a) constancy in the physical and chemical composition of the material; (b) thorough mixing; and (c) uniform dis- tribution of moisture. Constancy in composition is attained chiefly by the proper choice of materials, the rejection of irregular material and, to some extent, by reducing the larger particles to a sufficiently small size by means of crushing or grinding machinery. Thorough mixing is largely a matter of time and the use of a suitable form of mixer. The uniform distribution of moisture is also obtained by thorough mixing and, in the case of mixtures containing clay, by allowing them to stand in a moist condition for a considerable time, so that the moisture passes through the pores of the material and uniformly permeates the whole mass. This process, known as “ Souring,” is fully described in Chapter VI. The principal methods of producing a homogeneous mass from the materials used in the ceramic industries are (a) treading, (b) wedging, (c) mixing with spades, (d) pugging, (e) tempering, and (f) blunging. They all require the materials to have previously been crushed or ground to particles of suitable size. Treading is one of the most effective methods for thoroughly mixing fine plastic materials where a specially homogeneous structure is required, but it is by no means pleasant and is very costly. The mass is placed on a concrete floor, moistened and then roughly mixed with spades. It is then spread out and trodden by men with bare feet, the clay being squeezed between the toes and so thoroughly mixed. Several treadings, alternating with spade-mixing, are sometimes employed to secure an effectively mixed paste. Treading is chiefly employed in the manufacture of crucibles and glass-house pots. Wedging consists in cutting a plastic paste into pieces and throwing them together again, the mass being turned and the cutting and throwing actions being repeated until the mass is homogeneous. This method is very effective, but is slow and tedious, so that it is chiefly used in preparing the paste for making pottery, 60 PROPERTIES DEPENDING ON STRUCTURE china ware, porcelain, crucibles, etc. and for thoroughly mixing small quantities of material. Spade-mixing is sometimes useful as a preliminary treatment, so as roughly to mix the materials together before using one of the other methods. The first mixing of dolomite mixtures is usually done with spades and in the manufacture of crucibles and glass-house pots the materials are turned over with spades before treading or pugging them. Materials containing coke and tar are sometimes mixed wholly by means of spades. Pugging consists in passing the material and a suitable amount of water through a trough containing one or more shafts fitted with rotating blades which cut and churn the material and at the same time drive it forward to the exit end of the machine. If a cylindrical pug-mill is used the resultant paste will be extruded in the form of a long column, which may be cut up into pieces of suitable size for moulding, etc., but when an open trough mixer is used the resultant material has no definite shape. Pugging produces a fairly homogeneous structure, but it is the least satis- factory of mechanical mixers, and is only used for work where a completely homo- geneous structure is not required, or would cost too much to produce. It introduces air bubbles into the clay-paste ; these may produce too porous a mass, or they may cause other troubles at a later stage of manufacture or when the articles are in use. Pug-mills are largely used for mixing materials for the manufacture of bricks, tiles, terra-cotta, pottery, and similar articles, and in the manufacture of some refractory bricks and blocks made of fireclay, magnesia, bauxite, etc. Tempering is a term which is sometimes applied to any process of mixing in which a paste is produced, but it is often restricted to the use of an edge-runner mill which produces a much more homogeneous mass than a pug-mill, though at a greater cost. The material to be tempered is placed in a circular pan and is rubbed together and thoroughly mixed by two or more rollers which rotate in the pan. As a rule, 15-20 minutes’ treatment in a mill of this kind will produce a sufficiently homogeneous mass, but for crucibles 30 minutes’ treatment is better and for some special work several hours may be necessary. Edge-runner mills are used for tempering clay, etc., in the manufacture of paste for some of the better quality of terra-cotta, refractory cements, and sometimes for magnesia and bauxite bricks. They are almost invariably employed for preparing the materials used in making silica bricks. Blunging consists in mixing the materials with water in a tank so as to form a cream or slip. The mixing, or, more correctly, stirring, is usually effected by a number of arms on a vertical revolving shaft, and if the solid particles are not too large it produces a very homogeneous mixture. This method is specially useful for preparing the material used for fine pottery, china ware, and porcelain, as well as for casting, glazes, etc. The aim of all these methods of preparation is that a perfectly structureless material of absolutely even composition may be produced. Defective mixing, whereby complete homogeneity is not obtained, usually results in defective goods, and it has been proved repeatedly that if the mixture be imperfect no later treatment can possibly remedy defects in the earlier stages of manufacture. POROSITY 61 It is only by paying full attention to the preparation of the paste or slip that irregular behaviour can be avoided and losses prevented and, in some cases, this is far from easy. Some materials are of such a nature that it is almost impossible to grind them alone; others retain the lamellar structure in spite of their subjection to the heaviest crushing machinery, and to work such materials satisfactorily and profitably needs great skill and care. The addition of another material, such as sand, to clay will often facilitate the working of a mixture and render the composition of the mass more homogeneous. This is particularly the case with “fat” or “scurfy”’ clays, which adhere so tenaciously to the machinery when worked alone, that no progress with them is possible. Flaky or “ scurfy ’’ clays from an ancient river-bed are amongst the most troublesome of clays to render homogeneous and their treatment requires much patience and time, as well as skill. The result is that they prove very costly as a raw material and should, when possible, be abandoned in favour of a more easily worked deposit, unless only the commonest class of goods are being manufactured. In some cases the texture need not be homogeneous ; for instance, some firebricks and other articles are required to be porous internally and to have a dense surface. The former makes the articles sufficiently resistant to sudden changes in temperature, whilst the dense exterior has a great resistance to the corrosive action of flue gases, etc., and to abrasion. Porosity The porosity of a material is the total proportion of the voids or interstices to the solid particles of which it is composed. These voids are usually filled with air. It should not be confused with permeability, for although the porosity of a mass is _ directly proportional to the fineness of its particles, its permeability depends on their position and shape. Many clayworkers habitually use the term “ porosity ”? when they really mean permeability. A proof that, contrary to the usual opinion, strong clays are more porous than sandy ones is to be found in the fact that they need more water of manufacture to make them sufficiently plastic for moulding. When sand or other coarser materials than the clay-particles are added to a clay, the porosity of the latter is diminished and its permeability is increased, the porosity being least and the permeability greatest when the material is entirely composed of large irregular pieces which do not fit well together. According to E. W. Washburn,! there are six types of pores: (a) closed pores, (6) channel pores connecting separate pores, (c) blind-alley pores, (d) loop pores, (e) pocket pores with narrow necks, and (f) micropores, which are so small as not to be filled with water or other liquid in any ordinary period of soaking. These different kinds of pores produce two sets of results when the porosity of a material is determined, viz. (a) the true porosity, and (6) the apparent porosity. The true porosity is the relation between the volume of the article and the total volume of water absorbed by the pores when the article is soaked in water, plus the volume of the pores which are sealed by vitrified matter and so are not 1 J. Amer. Cer. Soc., 4, 918 (1921). 62 PROPERTIES DEPENDING ON STRUCTURE filled in the ordinary way. It is difficult to determine the total porosity accurately, but fortunately it is not usually of much importance. It is usually calculated as shown on p. 83. The apparent porosity is the relation between the volume or weight of an article and the volume or weight of the water or other liquid absorbed when the article is immersed in it. This figure is the one generally used as a measure of the porosity, and unless otherwise stated it is usually understood that figures relating to the porosity indicate the apparent and not the true porosity. The apparent porosity is determined as shown on p. 81. Both the true and the apparent porosity may be expressed as (a) percentage by weight, and (b) percentage by volume. The percentage of porosity by weight shows the weight of water absorbed by 100 units of weight, whilst the percentage of porosity by volume is the volume of water absorbed in 100 volumes of the material or article. For many years it has been customary to use the former and express the porosity in terms of the weight of the water absorbed, but the method of expressing it by volume has the advantage of indicating the volume of the pores. The volume of articles of simple shape may be calculated from direct measurement of the dimensions, but where the shape is complex the volume must be determined by finding the differ- ence between the weight in water and the weight in air. This, if gram weights are used, gives the volume in c.c., or if in ounces, when multiplied by 1-734, gives the volume in cubic inches. The expression of porosity by weight is also complicated by the fact that all materials have not the same density, e.g. clay articles may have a density of 1-4 to 2-6, so that the porosity results on different samples cannot be compared accurately. The expression of porosity wholly in terms of volume is, therefore, the most satisfactory. The porosity of various materials may be influenced by some or all of the following : (a) the shape of the particles, (6) the size of the particles, (c) the grading of the particles (if any), (d) the nature of the materials comprising the mixture, (e) the treatment to which the materials are subjected during manufacture, and (f ) the relative position of the particles, 2.e. whether they are closely compacted or lie loosely on one another. The effect of the shape, size, and grading of the particles has been considered on pp. 27-44 in connection with Texture, but it should also be borne in mind that rounded grains produce a more porous mass than angular ones when pressure is employed to shape the articles. When no pressure is applied, rounded grains may sometimes give a smaller porosity (p. 28). The porosity of mixtures consisting of perfect spheres of uniform size gives some idea of the porosities which may be obtained with round-grained materials. When one sphere rests on another the volume of the voids is 47-6 per cent. of the volume of the mass ; when each sphere rests on two others it is 39-5 per cent. ; whilst when each sphere rests on three others, 7.e. in a state of the closest possible packing, the porosity is 25-95 per cent. These results are not obtained with ordinary minerals, as they are not perfect spheres, but they have a fairly definite relation to the results obtained with clays, etc. Thus, it has been found that for all particles up to 4 inch diameter the percentage of voids is RELATION OF POROSITY TO TEXTURE 63 practically constant, provided all the particles are of the same size. Thus, a material consisting of grains of about 40-mesh will have the same porosity as one consisting entirely of grains of about 10-mesh. Mixtures of fine materials have a large surface factor and are usually more porous in the raw state than those containing coarse materials, as is shown in Table VI, TaBLE VI.—Relation of Porosity to Texture Average Fineness Average Fineness (Purdy’s Factor). Eure Space. (Purdy’s Factor). PONE ete Per cent. Per cent, 174 43-45 88 38-00 163 44-70 70 32-91 131 40-43 63 39-52 103 43-20 54 38-83 due to Ries, comparing the fineness and porosity of sands. J. 8. M‘Dowell+ found that the minimum porosity in silica bricks is obtained by using grains of medium fineness, and the maximum porosity by the use of coarse grains as shown in Table VII. Taste VII.—Tezture and Porosity of Silica Bricks Texture (per cent. passing through Sieves). Temperature Open Pore | Mean Kind of Brick. of burning, |Space, percent.| Pore Cone. of Volume. | Space. 8 16. | 20. | 30. | 40. | 40- 15 25-7 15 26-5 Coarse. . | 15-3 | 13-4 | 4-6 | 5-4 | 4-8 | 56-5 4 Brig 26-8 13 25-7 15 21:3 ; ; 15 22:3 ; Medium . | 10-2 | 13-1 | 2-2 | 5-0 | 4-7 | 64:8 14 99.7 21-2 13 19-2 15 22-7 , ; i j 15 22:3 : Fine . .| .. | 143] 4-1] 6-5] 4-1 | 71-0 14 93.9 22-5 13 21-7 1 Bull. 119, Amer. Inst. Min. Eng. 64 PROPERTIES DEPENDING ON STRUCTURE Table VIII is due to M. Phillipon,! who investigated the effect of the size of the grains on the porosity of silica bricks made of four types of quartz having the following fineness :— Per cent. of Particles between 2 and 8 mm. diam. Per cent. passing a 200-mesh Sieve. A 60 40 B 50 50 C 40 60 D 30 70 The porosities were as follows :-— Taste VIII.—Porosity of Silica Bricks Material. Porosity. Material. Porosity. Quartz 1, A 18-0 Quartz 3, A 21-0 a B 18-6 Pn B 19-8 55 C 17-7 $ C 19-0 ne D 16-2 Fe D 18-2 Quartz 2, A 22-0 Quartz 4, A 20-5 % B 19-3 ue B 19-7 a5 C 18-5 - C 17-8 Ss D 17-0 * D 15-9 Any increase of porosity produced by fine grains can only occur at temperatures below that at which vitrification commences and the pores begin to fill with fused material. Thus, in some fine-grained clays it was found that the porosity was highest with materials up to 1180° C.; between 1180° C. and 1300° C. there was little difference in the porosity, but at temperatures over 1430° C. the porosity is reduced almost to nil, as a result of vitrification. Hence, whilst fine grains are a cause of high porosity in the unfired material, in articles fired at high temperatures, fine grains are a cause of smaller porosity than coarser ones, as the latter are not so easily fused and consequently their interstices are not filled with vitrified matter. Grading (p. 31) has a very important effect in reducing the porosity of a material, and where high porosity is required, as in moulding sands used for casting molten metal, the grains should be as uniform as possible in size, so as to enable the gases evolved during the casting process ample opportunity to escape. The use of very small grains in a moulding sand is undesirable, as they occupy the interstices between the large grains and, by reducing the porosity, they prevent the gases from escaping. Sometimes, apparently, widely differing mixtures of grains of different sizes give 1 Rev. de Métal., 15, 51 (1918). EFFECT OF PRESSURE ON POROSITY 65 satisfactory results ; when this is the case it will usually be found that the porosities are very similar. Thus, Dauphin found the two following mixtures gave satisfactory results when made into silica bricks :— A. Seventy per cent. residue on 9-mesh sieve; 30 per cent. on 20-mesh sieve. Porosity, 40 per cent. B. Forty-seven per cent. residue on 9-mesh sieve ; 28 per cent. on 20-mesh sieve ; 3 per cent. residue on 50-mesh sieve ; 22 per cent. on 200-mesh sieve. Porosity, 46-4. It will be seen that, whilst the proportions of grains of various sizes are widely different, the difference in the porosity is quite small. The porosity of a dry mass depends largely on the pressure applied to it in packing, moulding, etc. Table IX, due to Foster and Emery,! shows the effect of pressure on clay grains which have passed through a 10-mesh sieve. TaBLE 1X.—Effect of Pressure on Porosity of Clay Pressure. Porosity. Pressure. Porosity. Lbs. per square inch.} Per cent. Lbs. per square inch. Per cent. A 500 38-71 J 2750 26-31 B 750 34-73 K 3000 25:58 C 1000 32-59 L 3250 25-70 D 1250 30°83 M 3000 24-94 E 1500 30-65 N 3750 24-90 F 1750 30-83 O 4000 24-13 G 2000 28-38 Pe 4000 and 4bumps 22°83 H 2250 27-47 Q 4000and6 _,, 22-41 I 2500 26-87 Other conditions being equal, hand-moulded bricks made of a soft plastic paste will be more porous than if made of a stiffer paste, as in the stiff-plastic process, on account of the larger amount of water and the smaller pressure used in making them from a soft paste. The nature of the materials comprising the mass has a very important influence on its porosity, especially in the fired state, because of the chemical reactions which take place in the firing and of the partial fusion which occurs. Similarly, when some materials, such as chalk, are in association with clay, the compound silicates which form when the mixture is heated to bright redness tend to fuse and reduce the porosity of the mass. Materials Increasing the Porosity.—If carbonaceous matter, such as sawdust, is added, as in the manufacture of light, porous bricks, the porosity before firing will be slightly reduced ; but when the mass is fired, the carbonaceous matter burns away, leaving corresponding pore-spaces, so that the porosity of the fired mass is roughly 1 Trans. Eng. Cer. Soc., 15, 143 (1915-16). 5 66 PROPERTIES DEPENDING ON STRUCTURE proportional to the volume of carbonaceous matter added and is, in some cases, very high. The chief combustible materials added to clays, etc., to increase their porosity are hardwood sawdust, cork, seeds, naphthalene and, occasionally, finely ground coke. The maximum proportion which can be added depends on the binding power of the other ingredients. The porosity of a mass may also be increased by the addition of a porous material, such as grog (p. 36) or kieselguhr (p. 7). Grog is sometimes added to fireclays to increase the porosity of the product, the increase in porosity being roughly proportional to the grog added unless the grog is very fine, when the porosity is not increased to the same extent, because some of the fine particles may fuse and close some of the pores and thus reduce the porosity. Highly porous articles, such as insulating bricks and filter ‘“‘ candles,”’ are some- times made by mixing kieselguhr with a small proportion of highly plastic clay and burning at a moderate temperature. Such articles depend for their porosity on that of the kieselguhr used. Materials Reducing Porosity.—The materials which cause a reduction in the porosity of fired articles partly composed of clay are either fusible at the temperature at which the article is fired, or they are fluxes which combine with some of the other constituents and form a fusible glassy mass, which fills the pores of the material. The most active flux in this respect is lime ; when this is added to a clay or siliceous mixture, the reduction in porosity commences at about 900° C.; magnesia is rather less powerful and felspar does not commence to reduce the porosity until a temperature of about 1200° C. is reached. The effect of felspar on the porosity of burned china clay is shown in Table X, modified from one by A. Heath. TABLE X.—Effect of Heat on Mixtures of China Clay and Felspar fired at Cone 9 (1280° C.). Felspar. Porosity. Felspar. Porosity. Felspar. Porosity. Per cent. Per cent. Per cent. Per cent. Per cent. Per cent. es 22-500 35-0 0-260 67-5 0-370 2:5 19-390 37-5 0-390 70-0 0-125 5-0 17-132 40-0 — 0-458 72:5 0-267 75 14-871 42-5 0-315 75-0 0-264 10-0 11-857 45-0 0-368 17-5 1 O 176 12:5 10-386 AT-5 0-780 80-0 0-263 15-0 9-536 50-0 0-356 82:5 0-300 17-5 6-003 52-5 0-416 85-0 0-404 20-0 4-522 55-0 0-287 87:5 0-409 22-5 2-081 57-5 0-310 90-0 0-368 25:0 0-463 60-0 0-290 92-5 0-281 27-5 0-433, 62-5 0-308 95-0 0-181 30-0 0-344 65-0 0-341 97:5 0-293 32:5 0-283 100-0 0-265 MEANS OF REDUCING POROSITY 67 According to tests made by Bleininger and Moore,! the presence of Cornish stone reduced the porosity of Florida kaolin as follows :— TaBLeE XI.—£ffect of Cornish Stone on Porosity of Kaolin Temperature of ane Cornish Stone. Porosity. Cone. Per cent. Per cent. 01 15 38 40 30 60 21 80 5 iL 15 26 30 12-5 50 I The size of the particles of any fluxes present also has a marked effect, as shown in Table XII, due to J. Keele,? which gives the effect of the fineness of felspar on the porosity of felspar-kaolin mixtures. TaBLe XII.—-Hffect of Fineness of Fluxes on Porosity Temperature of Firing in Seger Cones. Size of Particles Felspar. of Felspar. 12. Per cent. 40-mesh 40 4-960 30 9-330 20 14-21 10 18-65 100-mesh 40 1-40 30 4-92 20 8:87 10 16:37 200-mesh 40 30 Ly 20 T11 10 12-75 1 Trans. Amer. Cer. Soc., 10, 313 (1908). 2 [bid., 13, 731 (1911). 68 PROPERTIES DEPENDING ON STRUCTURE It will be seen that the finer the flux, the more the body is vitrified and the lower the porosity. According to H. G. Schurecht,! the addition of electrolytes to a clay influences its porosity when the clay is in the dry state, alkalies reducing the porosity and calcium hydroxide increasing it. Acids in small quantities increase the porosity in the dry state, but larger quantities of acid reduce it. The effect of various bonds on the porosity of silica bricks, after firing at various temperatures, is shown in Table XIII, due to Scott.? TaBLe XIII.—Effect of Bonds on Porosity (Scott) Material fired at— Bond. Cone 8. Cone 14. | Cone 16. | Cone 19. Lime i : ; : ; : 31:8 30-0 29-3 30-2 Magnesia . : : 30-8 31-4 30-2 30:8 Alumina . : : : 2 ; 33:1 33:8 J 34-0 Ferric oxide. : . : ; 31-2 30-0 30-4 29-8 2 and:carbon. . ; : 34-0 32-0 32-2 32-9 Titanium oxide ; : ; é 31-6 BS 32-5 32:7 Lime and Magnesia . : 33:3 34-5 34-2 33°7 Lime and Alumina . : 31:8 31-6 31-2 31-0 Lime and Ferric oxide : : . 28-4 28-0 27-4 26-5 3 = ,, and carbon : 28:3 27-9 29-0 28-8 Lime and titanium oxide . ; ; 29-2 29-5 29-9 ve Magnesia and alumina ; ; , 33-2 33-4 33:2 33-4 Magnesia and ferric oxide . : ; 30°8 ooo 30-4 32-4 if * ,, and carbon 31:5 31:8 oH 31-9 Ferric oxide and alumina . : : 33:0 32:5 32-4 32:2 . » andcarbon . 3 : 32:6 33:5 33°4 | 32-4 Lime, ferric oxide, and titanium oxide. 29-2 31-5 28-7 27:9 Fireclay . : : ; ; ; 31-0 31-2 ay 31-0 China clay . : : 30-5 ~ 30-8 30-8 Sulphur behaves irregularly, as sometimes it makes fireclay articles slightly more porous, but more frequently it combines with any lime and alkalies in the mixtures forming sulphates and producing a denser mass. 1 J. Amer. Cer. Soc., 1, 201 (1918). 2 Trans. Eng. Cer. Soc., 18, 487 (1918-19). EFFECT OF HEATING ON POROSITY 69 The heat treatment to which materials are subjected also influences their porosity. Burned articles usually have a greater porosity than unfired materials up to the temperature at which vitrification commences. Articles which have been fired to higher temperatures may be less porous and in some cases are impermeable. The “ marls ” used for blue bricks afford a good illustration of this; in the dry state they are moderately porous, when fired at about 900° C. they have a porosity of about 15 per cent. by weight; but after firing and “ blueing”’ at about 1300° C. they absorb less than 0-1 per cent. of water. The porosity of articles made of “clay” increases as they are heated until a temperature of about 750°-850° C. is reached. During this heating the clay shrinks and reduces the porosity, but the loss of water and carbonaceous matter, which occurs at the same time, is greater than the shrinkage, so that the porosity increases. At the moment when clay is dehydrated, the dissociation of the material and the liberation of the combined water increase the porosity to about 10 per cent. The porosity at this stage depends partly on the initial porosity of the mass and partly on the amount of carbonaceous matter present. In the manufacture of light, porous bricks, in which a large amount of carbonaceous matter has been incorporated, the porosity rapidly increases as this material burns away. The effect of the temperature of burning on the porosity and absorption of bricks is shown in Table XIV, due to J. C. Jones. b) Taste XIV.—Effect of Burning Temperature on Porosity Kind of Brick. Extent of Burning. | Pore Space. SS EAR Water Absorbed. Surface clay (plastic paste) : Soft. 33°0 449-0 Med. soft. 26-9 343-6 Med. hard. 21-2 258-9 Hard. 10-2 106-6 Shale (plastic paste) . ; : Soft. 26-2 453-1 Med. soft. 17-8 310-1 Med. hard. 11-6 130-4 Hard. 5-8 48-4 Shale (wire-cut) : : : Soft. 27-6 402-0 Med. soft. 17-1 233°3 Med. hard. 2-1 11-1 Hard. 0-9 5-6 1 Trans. Amer. Cer. Soc., 9, 578 (1907). 70 PROPERTIES DEPENDING ON STRUCTURE The influence of the temperature to which fireclays are heated on their porosity is shown in Table XV, due to E. M. Firth and W. E. 8S. Turner. Taste XV.—Effect of Burning Temperature on Porosity Temperature of Burning. Porosit: wgek Dean 1400° C. 1500° C. Mansfield ; : : : 8-0 1-4 6-6 Kilwinning. : : : 10-2 3-7 6-5 Coalbrookdale, a ] ‘ ; 12:5 5-7 6-8 - GAR, : : 12-5 6-9 5-6 Ayrshire bauxitic clay . : 25-8 18-3 7-5 Kilmarnock . . : A 18:1 6-9 11-2 Wortley . : : : 23-3 14-1 9-2 Kilwinning aluminous shale : 31-8 29-2 2-6 Huddersfield, 1 : . ; 17-6 12-0 5-6 i 2 ier 18-8 16-4 2-4 Grossalmerode . : ; : 12:8 5-4 7-4 Halifax, 1 ; : . : 24-9 24-1 - 0-8 he ee 24-0 21:8 2-2 pegernget 19-8 10-3 9-5 beeen | 5-2 20-2 —15-0 ore; 16-2 20-5 faad pao : : : : 20-2 15-9 4:3 Ruabon . ; bs : : 5-9 2:7 3-2 Armadale , s A ; 13-1 3:5 9-6 Stourbridge, A 23-3 75 15:8 .9 B 20-7 10-1 10-6 Bs C 7-2 2-1 Sra 0s D OP UR in 19-3 17-7 1-6 . EK . : d 11-6 8-6 3-0 + F 5-6 4-8 0-8 i at ee 18-3 12-1 6-2 ef Gb2., ; ‘ 16-5 12-1 4-4 Be eh ; ; P 6-0 4-3 1:7 Tie : ? : 19-7 14-2 5-5 1 J. Soc. Glass Tech., 5, 268 (1921). * Subjected to preliminary firing at 800° C. EFFECT OF HEATING ON POROSITY 71 Further heating above 1500° C. gives a further reduction in the porosity, except in the cases of clays Halifax 4 and 5, which at 1000° C. reach the maximum Beers ture at which they are of value. According to EK. M. Firth, F. W. Hodkin, and W. E. 8. Turner, china clay has the following porosities after having been fired at different temperatures :— Taste XVI.—Porosity of Kaolin Temperature, ° C. Porosity. Temperature, ° C. Porosity. 600 51-5 1200 31-7 750 52:5 1300 14:5 900 56-2 1400 8-3 1000 52-0 1500 2-4 1100 45-6 The proportion of sealed pores increases when articles partly or wholly made of clay are heated to a temperature above 1140° C., as vitrified matter then begins to form. This is due to the fact that a certain amount of gas is formed during the burning of the mass and, if vitrification has commenced, this gas is unable to escape and so forms sealed pores. The higher the temperature to which the mass is heated, the greater will be the proportion of sealed pores. The changes in porosity of fireclay articles, after they have been heated several times to a high temperature, are a good criterion as to their resistance to heat. For instance, a well-burned firebrick should not show more than 5 per cent. decrease after it has been reheated to 1400° C. for two hours. The size and shape and dis- tribution of the pores does not affect the porosity, but is important with respect to the permeability, strength, and other properties. As a homogeneous texture is usually important, the pores of an article should be as uniform as possible. Comparatively large holes scattered throughout the mass are very undesirable and their formation should be carefully avoided by properly grading and thoroughly mixing the materials used in making the articles. The size of the pores largely determines the rate at which water is absorbed. With very small pores the absorption is irregular; some burned clays absorb only a small amount for several days and then suddenly absorb rapidly a further quantity of water. Usually, about 90 per cent. of the pores in an article are filled within an hour, but the remaining 10 per cent. are filled very slowly, about 9 per cent. being absorbed in the next twenty-four hours and the remaining 1 per cent. sometimes taking a week or more. The size of the pores varies according to the texture of the unfired material and with the nature of the combustible material (if any) which has 1 J. Soc. Glass Tech., 4, 264-7 (1920). 72 PROPERTIES DEPENDING ON STRUCTURE been employed. Soft wood sawdust is undesirable, as the pores it produces are coarse and large. Hardwood sawdust is far better, as the grains are finer. Finely ground eork is excellent. Sometimes very large pores are desired and are produced by mixing seeds or grain with the clay. For most purposes, the pores should be small, not ex- ceeding 0-01 inch diameter, as when the pores are very large the strength of the mass is reduced. The influence of the size of the pores on the permeability is dealt with on p. 87. Porosity has an important influence upon other properties possessed by clays and other ceramic materials. Some of these are mentioned briefly in the following pages, but are dealt with fully later. Effect of Porosity on Absorption.—The most porous articles usually absorb the greatest proportion of water or other fluid, though porosity and absorption do not bear any definite relation to each other, as the latter also depends upon other properties than the volume of pores (p. 87). The rate of absorption varies with the texture of the material. A mass composed of grains of medium size will usually absorb water more rapidly than one which contains very small pores, even though the latter has a greater porosity. All the pores are normally filled with air and it is difficult to displace this if the pores are very narrow. Effect of Porosity on Apparent Density.—The apparent density (Chapter V) is reduced as the porosity increases, as there is a smaller volume of solid material in the total volume of the mass. Effect of Porosity on Spalling, etc.— With few exceptions, if a fired material is porous it will not be sensitive to sudden changes in temperature, but a dense mass of the same material will spall readily. This is due to the fact that the grains in a porous mass have more freedom of movement than in a dense one, so that the stresses produced by sudden changes of temperature are immediately relieved on account of the numerous pores, without any breaking up of the structure of the mass. In time, of course, larger cracks may develop, but only very slowly if the texture of the mass is sufficiently porous. A high porosity is specially important as a means of securing resistance to sudden changes in temperature in the case of refractory materials, but less so in the case of articles which are not heated to above a dull-red heat. Sensitive- ness to rapid cooling may be tested by heating a brick, or other article, to a temperature of 1350° C. for 1 hour and then subjecting it to a blast of cold air from a 3-inch nozzle, delivering 27 cubic feet of air per minute for 15 minutes (see also Chapter XIII). Under these conditions a porous fireclay brick will not lose more than about 12 per cent. by weight as a result of spalling, whilst a dense brick may lose 65 per cent. of its weight. Silica and magnesia bricks spall to a still greater extent. Effect of Porosity on Thermal Conductivity.—Ceramic materials of high porosity usually have a low thermal conductivity on account of the large volume of air included in them. For this reason, high porosity is undesirable where the interior of an article is to be heated by means of an exterior source of heat, though very often high porosity is essential in order that the articles may be sufficiently resistant to unavoidable changes in temperature, though it necessitates a slow rate of heating and a waste of fuel. EFFECT OF POROSITY ON RESISTANCE 73 Where a low thermal conductivity is desirable—as for heat-insulation purposes and to retain the heat within a furnace or kiln—a highly porous material should be used. It is not generally known that the porosity of a material appears to have no definite relation to its insulating power; the latter is more closely connected with its permeability. It is a fact that all the best heat-insulating materials are very porous, yet some very porous materials are not good insulators (see also Chapter XIII). Effect of Porosity on Resistance to Corrosion.—Any article of burned clay or other refractory material having a high porosity is seldom very resistant to corrosion, unless the pores are extremely small. Pores of medium or large size provide cavities which the corrosive materials readily enter and present a large surface to be corroded (see also p. 30). Hence, highly porous materials should not be used where there is much risk of corrosion, either at ordinary or at high temperatures. In this connection it may be noted that porous bricks and other articles made of fireclay are very liable to have carbon deposited in their pores at high tempera- tures, as a result of the dissociation of carbon monoxide and hydrocarbon gases which may come in contact with the bricks. The burned fireclay appears to have a catalytic action, the precise nature of which is not understood. The action is largely dependent on the presence of a sufficient surface of unfused fireclay, as dense fireclay bricks with a slightly vitrified surface are only very slightly affected (see also Chapter XI). Effect of Porosity on Resistance to Abrasion and Erosion.—Highly porous bricks and similar articles usually offer little resistance to abrasion and erosion, because the porous structure exposes a larger surface to the abrasive agent and also reduces the amount of solid matter in a given area or volume of the material. The effect of porosity on resistance to erosion by wind, rain, etc., appears to be irregular and no definite relation has been found between porosity and weathering. Under ordinary climatic conditions a porous material should be less able to resist the action of the weather than a dense material, but this is not always the case. When the pores in a material are small they absorb moisture which, in cold weather, may freeze and the ice in forming may, by its expansion, partially disintegrate the material. The matter has not been fully investigated, but there is evidence to show that if the pores are sufficiently large, yet not too numerous, no harm will be done. Articles which are only slightly porous do not absorb water readily, so that disin- tegration does not usually occur, but if a mass of material has a vitrified exterior with one or more cracks or other defects giving access to a porous interior, the damage by frost is likely to be serious. This is specially noticeable with glazed bricks, glazed terra-cotta, etc. Completely vitrified articles, such as blue bricks, paviours, etc., appear to be quite unaffected by frost. Building bricks are fairly porous, yet, when well made and properly burned, they are very resistant to the weather. Firebricks of all kinds, saggers, retorts, and refractory articles generally, are much less resistant, and should be stored in a warm, dry place, where they will not be exposed to frost. f 74 PROPERTIES DEPENDING ON STRUCTURE Bricks and other materials which have been “‘ steamed ”’ during the process of manufacture are not resistant to frost, probably on account of their usually having a porous interior and a dense exterior, the latter having been caused by condensation. Such a structure appears to be very sensitive to frost (see also Chapter IV). Effect of Porosity on Electrical Conductivity.—Those ceramic materials which have a high porosity usually have a low electrical conductivity. This is probably due to the fact that the decrease in the volume of the solid matter involves an in- creased resistance to the passage of an electric current, air being a very poor conductor. Many refractory materials are naturally poor conductors, but they are still poorer when they are very porous. On the other hand, some of the best electrical insulators are composed of stoneware or porcelain, which is completely vitrified and devoid of pores. This is largely due to the fact that an electric current will sometimes jump across an air-space or pore when it would not pass through a solid, vitrified mass of clay (see also Chapter XIV). Effect of Porosity on Dielectric Strength is discussed in Chapter XIV. Effect of Porosity on Strength.—Ii a series of pieces of burned ceramic material is made up in such a manner that each member of the series increases progressively in porosity, it will usually be found that the more porous pieces have much less mechanical strength than the denser ones. This must, obviously, be the case, as the greater the porosity the less is the proportion of solid material and the more imperfect the union between the grains comprising the mass (see also p. 31). For most purposes the fired ceramic materials have ample crushing strength and where it is important to do so, almost any desired porosity may be attained. In some cases, however, some of the porosity must be sacrificed in order to obtain sufficient strength. According to Bleininger,1 the porosity of clay bricks is inversely proportional 1 Pp’ constant, and P is the porosity. The constant K varies with each particular product, but the formula appears to be applicable when comparing articles made from the same material under the same conditions, but with different porosities (see also Chapter IV). Effect of Porosity on Refractoriness.—In the strictest use of the term “‘ refrac- toriness”’ there is no connection between it and the porosity, but in actual use, porous bricks, saggers, retorts, etc., often appear to be more refractory than those of lower porosity. This is due partly to the fact that their thermal conductivity is lower and, therefore, a longer heating is necessary to produce appreciable signs of fusion, and partly to the fact that porous materials of this nature are often of coarser texture and this further reduces their thermal conductivity. If the heating is sufficiently prolonged, or the test piece is sufficiently small, both porous and dense materials—if of precisely the same composition—are of equal refractoriness (see also Chapter XIII). Effect of Porosity on Discoloration.—Any porous material is much more lable than a dense one to discoloration, because the former has a greater power of 1 Trans. Amer. Cer. Soc., 12, 582 (1910). to the crushing strength, so that C=K-—, where C is the crushing strength, K is a EFFECT OF POROSITY ON USES 75 absorption. Hence bricks, tiles, and other porous architectural clay wares soon have their surface pores clogged with soot, dust, and other fine particles which dis- colour the surface and often give it an unpleasant appearance. Close-textured architectural work, such as “ terra-cotta,” on the other hand, remains clear for a much longer period. Glazed bricks, etc.—which are quite impermeable so long as the glazed surface remains undamaged—are often used in building, as no dust or dirt is absorbed, and that which merely adheres to the surface can readily be removed by washing. If the glazed surface becomes defective (e.g. by cracking or crazing), the dirt penetrates the defective portions and cannot easily be removed. Many roofing tiles are made with a rough and porous surface, so as to make algee and other vegetable growths adhere more closely and so give them an “ancient” appearance. Closely allied to discoloration is efflorescence or ‘‘ scum,’’ which is more liable to occur on porous bricks, etc., than on denser ones. Efflorescence is chiefly due to the absorption of salts in the course of manufacture or storage, or from the mortar in which the bricks are laid ; the salts are brought to the surface in solution and are left there as a scum when the water which dissolved them has evaporated (see also Chapter III). Effect of Porosity on the Rate of Drying.—The porosity of moulded articles determines, to a large extent, the speed at which they can be dried. A porous or open body can be dried quickly, because water-vapour can readily escape, whereas a close-textured body does not so easily permit its removal. For this reason it is very desirable that large blocks, etc., should be as porous as possible in the green or unfired state, though this property is often extremely difficult to control. The rate at which fired articles can be dried after they have been wetted also depends on their porosity, though still more on their permeability (p. 89 ; see also Chapter VII). Effect of Porosity on Uses.—From the preceding pages it will be seen that porosity is an extremely important factor and one which must be carefully considered if satisfactory articles are to be produced. No very definite rules respecting it can be formulated and in deciding what amount of porosity is required in an article or material for any particular purpose, it is necessary to know fairly fully the condi- tions to which the articles will be subjected. The porosity of raw ceramic materials, other than moulding sands and some other refractory materials used in the loose or powdered state, is not usually of great importance, as the positions and relative distances between the particles are changed considerably when they are made into articles. The porosity of raw clays varies greatly according to their nature. Some plastic clays appear to be devoid of porosity, some fireclays have only about 3 per cent. porosity, whilst some sandy loams have a porosity of 25 per cent. or more. Some clays which have a very low porosity in the damp state will absorb a large amount of water in excess of that previously present if they are first dried and then soaked in water. Use is made of this property when it is desired to soak clays in pits prior to converting them into a highly plastic paste of suitable consistency for moulding. The amount of water absorbed by dry clay is often considerable, in some cases 76 PROPERTIES DEPENDING ON STRUCTURE being as much as 80 per cent. by weight. This absorptive power is due, according to Rohland, to the colloids present in the clay, and is dealt with in Chapter VI. It may also be due to the capillary structure of the clay particles. In any case the total absorptive power of a plastic clay is not strictly proportional to the volume of the pores, as more water appears to be absorbed than is required to fill the pores, and the absorption is also accompanied by a swelling of the material similar to, but far less than, that of glue. Kerl divides burned clay-wares into two divisions, according to their porosity, as follows :— 1. Porous ware, including bricks, tiles, terra-cotta, refractory ware, coarse pottery, earthenware, and pipes. 2. Impermeable ware, including stoneware, china ware, and porcelain. These divisions are only general, however, as some bricks are quite impermeable to water, and some china ware is appreciably porous. Building bricks should be moderately porous, as otherwise when moisture con- denses from the air, the resultant water collects in drops on the inside of the walls, spoiling the wallpaper and giving the impression that the building is damp. If the pores are sufficiently minute, the bricks may be as highly porous as possible. The more porous they are, the better will the walls “‘ breathe.” Bricks with large, coarse pores should be avoided, as they admit rain water too easily, but do not always part with it readily, so that walls in which they are used often remain permanently damp. The porosity of ordinary building bricks should not usually exceed 20 per cent. by weight or 50 per cent. by volume. Common red bricks and facing bricks usually have a porosity of 5-10 per cent. by weight or 12-20 per cent. by volume, whilst wire-cut bricks, rubbers, and gault bricks usually have a porosity of 12-20 per cent. by weight or 30-55 per cent. by volume. In the south of England, the average amount of water absorbed by bricks during complete immersion is about 12 per cent. of the weight of the brick. North of the Trent and in Wales, somewhat denser bricks are usual and the average water absorp- tion is seldom over 8 per cent. of the weight of the brick. Bricks made by the semi-dry process—that is, by compressing the clay in the form of a damp dust—absorb only about 5 per cent. of water, though they vary greatly in this respect. For all ordinary purposes, therefore, it is not desirable to designate. bricks which absorb less than 15 per cent. of their weight of water on immersion as particularly porous. Good bricks should absorb water slowly and should part with it readily when exposed to a dry atmosphere. Bricks and other articles which are required to resist chemical action at ordinary or moderately low temperatures, or which must possess great strength, must be denser than ordinary architectural materials. Blue bricks of good quality usually have a porosity of only 0-1-2 per cent. ; it should not generally exceed 3 per cent. by weight. Obsidianite bricks have a porosity of about 0-3 per cent. The American Society for Testing Materials 1 specifies the following porosity for bricks for sewers :— 1 Vol. 21, 527-32 (1921). EFFECT OF POROSITY ON USES 77 Maximum Porosity per cent. by Weight. Mean Porosity per cent. by Weight. Class A, vitrified . . 3 or less a8 Class B, 2 , , 5 or less 6-0 Hard... ; ; ; 5-10 12-0 Medium . ; ' : 10-15 17-0 H. Burchartz 1! has suggested a porosity of 5 per cent. by weight, or 10 per cent. by volume, for clinker bricks, and 8 per cent. by weight, or 16 per cent. by volume, for hard-burned sewer bricks. The large hollow blocks used in fireproof floors, etc., are usually made of a highly porous material. This not only reduces the cost of carriage and the weight of material in the structure, but it facilitates manufacture of blocks accurate in shape and free from twists. Tiles, terra-cotta and other unglazed argillaceous building materials should have a similar porosity to building bricks, but the exposed surface should be non-porous. According to W. G. Worcester,? the best roofing tiles should have a porosity between 3 and 18 per cent., light red tiles having a porosity of 10-18 per cent., and the darkest and densest red tiles a porosity of 3-4-5 per cent. Other unglazed wares should have a porosity corresponding to the purpose for which they are to be used. Such articles as flower-pots, filters, etc., must be very porous. Refractory Articles.—The porosity of fireclay bricks is often much more important than in bricks which are not heated to a high temperature. It varies very con- siderably according to the purposes for which the bricks are to be used, but is generally between 8 and 24 per cent. by volume or 3-9 per cent. by weight. Where there is little corrosion or abrasion, a highly porous brick may be quite satisfactory and, even where these actions do occur, a porous brick may be necessary in order to give the requisite resistance to sudden changes in temperature. In the checker-work of regenerators, the conditions required demand two opposite qualities ; great resistance to sudden changes of temperature, consequent upon passing cold air amongst the hot bricks, which necessitates the use of porous bricks, whilst the high thermal con- ductivity necessary to obtain a maximum heat capacity is obtained only by using close-textured bricks. In practice, a compromise is effected by using bricks of moderate porosity. In some other cases, the difficulty may be overcome by the use of bricks having a porous interior to resist temperature changes and a dense surface to resist corrosion and abrasion (see also p. 61). Bricks of this type are largely used in blast-furnaces, 1 Mitt. Konigl. Materialpriifungsamt, 34, 79 (1916). 2 Geol. Survey of Ohio, Bull., 11, 121. 78 PROPERTIES DEPENDING ON STRUCTURE recuperators, calcining furnaces, coke ovens, crucible furnaces, chemical furnaces, retort settings, reverberatory furnaces, frit kilns, etc. Highly porous bricks are required in the roofs of coke ovens on account of the great changes in temperature to which they are subjected. The walls must be denser so as to be sufficiently resistant to corrosion and abrasion. One well-known firm of coke-oven builders specifies that firebricks for this purpose should have a porosity of not less than 12 per cent. by volume or 6 per cent. by weight. According to a Provisional Specification, published by the Society of Glass Technology, the porosities of glass-tank blocks made of fireclay should be as follows :— Flux-line blocks. : : BIE not more than 18 per cent. Replacement flux-line blocks . 4 - 23 he Bottom side blocks 5 ; a * 25 2 Tank bottom blocks : : ve ~ 30 me Porous fireclay bricks, when heated to redness, decompose carbon monoxide, liberating free carbon. ‘This is undesirable, especially in gas producers, etc., used in the production and utilization of carbon monoxide gas. In general, when the articles are to be heated to a high temperature, they should be as porous as is consistent with the other properties required. Saggers should generally have a porosity of 10-40 per cent. by volume or even greater, provided the pores are very small and do not allow the fire-gases to enter into the interior of the sagger and so discolour the goods, and that the saggers pea sufficient strength. Muffles should have as porous a structure as possible, so that they will permit sudden heating or cooling. Muffles which are too dense are not economical in fuel, as they require a much longer time to heat, as well as to fire the goods. Glass-house pots should have a porous coarse texture, but both the inner and outer surfaces should be dense so as to resist the corrosive action of the molten glass and the flames. Retorts should have as high a porosity as is consistent with the other requirements, so that they may be resistant to changes in temperature. Too great a porosity must be avoided, however, where resistance is required to fluxes, ash, etc. The Institute of Gas Engineers specifies that the porosity should not be less than 18 per cent. by volume ; in Germany many retorts have a porosity of 25 per cent. The porosity- of gas retorts which have been in use some time is reduced by the deposition of carbon in the pores of the material. The porosity of zinc retorts may be as high as 26 per cent. by volume when first used, but after a few days the porosity is rapidly dimin- ished and may be as low as 1-5 per cent. on the seventh day. According to Babcock, the bond clays for zine retorts should have a porosity of 10 per cent. by weight after being fired at 1150° C., and 5 per cent. after being fired at 1250° C.-1400° C. The porosity of silica bricks may vary very considerably, though the usual limits are 15-32 per cent. by volume. It has been found by the author that bricks with a 1 J. Amer. Cer. Soc., 2, 83 (1919). EFFECT OF POROSITY ON USES 79 porosity of about 18 per cent. usually give the most satisfactory results. One well- known firm of coke-oven builders specifies a porosity of not less than 12 per cent. by volume and 6 per cent. by weight for silica bricks to be used in coke ovens. Silica cements used for firebricks, according to R. J. Montgomery,! vary in porosity from 30-35 per cent. at Seger cone la (1100° C.) to 15-30 per cent. at Seger cone 20 (1530° C.). Kreselguhr is highly porous and can absorb as much as 24 times its weight of water without appearing to be wet. When mixed with fireclay, the resultant articles are so porous that they are very easily corroded and abraded, so that they should be carefully protected from these actions. On account of their porosity and permea- bility they are very valuable as heat insulators. In moulding sands, porosity is very important, as when molten metal is poured into the mould the gases produced must be capable of escaping readily, or they may cause blowholes and other troublesome defects in the metal itself. On the other hand, excessive porosity is undesirable, as it causes the castings to have an irregular surface and much of the sand to adhere to the metal when cooled.” Sand for lining open-hearth furnaces should have as dense a texture as possible so as to prevent the excessive absorption of slag 2 (see p. 42). Magnesia bricks should have a low porosity or a dense structure; when struck they should give a clear, ringing note. This low porosity is necessary because such bricks are largely used under conditions where there is a considerable risk of corrosion by basic slags. Where they are not subject to corrosion, bricks with a porosity up to 40 per cent. by volume may be employed. According to M‘Dowell and Howe, ordinary magnesia bricks should have a porosity of 24-30 per cent. by volume, though many Austrian and German magnesite bricks have a porosity of only 18-21 per cent. After prolonged use at a high temperature, the porosity may be reduced to as low as 10 per cent. Zirconia articles with a lime or kaolin bond usually have, according to A. Bigot,‘ a porosity of 4~7 per cent. According to O. Ruff and G. Lauschke, zirconia crucibles become less porous the higher the burning temperature, though in apparent opposition to this statement these investigators found that the most porous crucibles were produced from zirconia which had been calcined at 1500°-2200° C. Beryllium oxide, magnesia, alumina, thoria, and yttria reduce the porosity of zirconia crucibles. The first three oxides are volatilised above 2000° C., but thoria and yttria reduce the porosity at still higher temperatures. Asbestos bricks are highly porous and, therefore, withstand sudden changes of temperature, but they are not resistant to the action of fire-gases, slags, and other corrosive agencies. 1 Amer. Soc. for Testing Mils., 1918. 2 For further information see the author’s Sands and Crushed Rocks : Their Nature, Prepara- tion, and Uses (Frowde, Hodder & Stoughton). 3 J. Amer. Cer. Soc., 3, 185-246 (1920). 4 Céramique, 37, 191-3 (1919). 80 PROPERTIES DEPENDING ON STRUCTURE Stoneware should be devoid of porosity, but this is seldom attained, and the commercial articles absorb 0-5-2 per cent. of their weight of water. They should not exceed the latter amount. Porous bisque, or Biscuit ware, is an intermediate product in the manufacture of glazed ware. It is fired prior to glazing to give it greater strength in the dipping and decorating processes and to enable the glaze to adhere well to the surface. A suitable porosity is 10-15 per cent. by weight. Excessive porosity is undesirable, as too much of the glaze is then absorbed by the ware; this not only wastes glaze, but tends to cause warping and produces a dull surface. China ware and porcelain of the best quality should, when glazed, have a porosity not exceeding 2 per cent. when broken pieces are tested. Some of the best vitrified porcelains have a porosity of only 0-05 per cent. Semi-porcelain may have a porosity of 13-23 per cent. according to its nature. A good test for the porosity of vitrified ware, and for the white engobes used on sanitary ware, is to immerse the article in a 0-5 per cent. solution of eosin or ordinary red ink for 18 hours, and then rinse, dry, and examine with a lens. There should be no staining. Table XVII, due to Harvard, shows the average porosity by volume of various types of clay and ceramic materials :— TaBLeE XVII.—Porosity by Volume of Ceramic Materials Burning Temperature, Porosity per cent. by pe Volume. Fireclay bricks. ; : 1050 30-8 rs ‘; ; : : 1300 24-1 Checker bricks. ? ‘ : - 27-8 Bauxite bricks. : ; é 1300 38-4 Silica bricks. : : ; 1050 42-58 ce . : : ; : 1300 42-9 Magnesia bricks . : 1300 41-0 ¥ ms : ; : 1050 35°1 Carborundum bricks. ; : 1050 \ 35-2 t oA ‘ : : 1300 30-6 Chromite (unburned) . : ‘ * 21:3 Chromite bricks with clay bon : 1300 26-4 Kieselguhr . : ; ; es 38-0 Graphite bricks. ie 26-0 Building bricks. ; ; 1050 25-7 Light clay . : i : #8 45-7 DETERMINATION OF POROSITY 81 Determination of the Porosity and Absorption.—As explained on p. 61 there is a distinction between the true and apparent porosity, but in most cases the term “ porosity ’’ is applied to the apparent porosity as estimated from the amount of water and other suitable fluid absorbed by a given weight or volume of the sample. Hence, the absorption or apparent porosity is a measure of the unsealed pores. It is usually determined by weighing a whole brick, or a conveniently sized sample, and immersing it in water for several hours, after which it is removed, the surface moisture wiped off with a cloth, and the test piece re-weighed. The gain in weight is the water absorbed ; if this is multiplied by 100 and divided by the weight of the dry sample, the result will be the percentage porosity by weight. The percentage of apparent porosity by volume may be found by the following formula :— a 100 Ps f= 100 Pe. where p=percentage of porosity by volume, P the percentage of porosity by weight, and s the apparent specific gravity of the solid material. The apparent porosity by volume may also be found by the following method : (a) weighing the sample ; (b) suspending the sample in water from one end of the arm of the balance by a fine thread and weighing it whilst thus suspended and immersed ; (c) wiping off the surplus water and weighing the sample whilst saturated with water. Then 100(W —w) i W-1 where P is the percentage of apparent porosity by volume, W the weight of the sample when saturated with water, w the weight of the dry sample, and 7 the weight of the sample when immersed in water. When determining the apparent porosity, the sample should not be completely immersed at first or air may remain trapped in the pores and, not being able to escape, will give a low result for the porosity. It is much better to arrange that the sample is only partly immersed at first, so as to give the air an opportunity of escaping. Later, it may be completely covered and then left in this condition for several hours, so as to ensure the complete filling of the open pores with water. Where a more accurate result is required, the sample may be boiled in water for about an hour, instead of merely immersing itin cold water. This will give sufficiently accurate results for most purposes, but where the most accurate results which can be obtained are required the sample may be first boiled, then subjected to a vacuum of 29 inches of mercury for 3 hours and afterwards immersed in water at normal tem- perature for 96 hours. Table XVIII, due to R. C. Purdy and J. K. Moore,! shows the effect of using a vacuum. The increase is specially noticeable with dense materials. Purdy and Moore suggest that 15 minutes of vacuum treatment is usually sufficient after a 48-hour saturation. 1 Trans. Amer. Cer. Soc., 9, 211 (1907). 82 PROPERTIES DEPENDING ON STRUCTURE TaBLe XVIII.—EHffect of Vacuum in Porosity Determination , Percentage Gain in Porosity after Vacuum Treatment for— Porosity after 48 Sample. | hours’ Saturation without Vacuum. 5 min 10 min 15 min 20 min S82 3°22 48-10 51-80 57-90 65-00 G II 3°30 38-70 42-10 48-40 50-60 K 46 3°93 27-30 : 35°60 37°50 K 15d 4-22 13-48 14-48 18-70 20-80 Kelse 4-27 44-60 46-60 46-60 46-60 K 15e 4-5] 33°40 36-50 36-80 38-20 R4 5-12 58-20 59-40 61:70 63-70 H Il 5-29 31-20 35-40 37°60 38-90 R2 6-10 27-50 32:20 35:60 36:00 K 6d 6-46 29-90 31-60 35°30 39-30 K2 6-55 18-60 20-10 21-60 24-30 Ret 6-70 10-20 11-00 11-00 11-00 BIil 6-91 28-00 30:40 31-40 32-00 J Il TDS 11-80 13-70 15-70 16-00 rit 8-64 11-80 12-80 14-10 14-80 K 8d 9-06 22-00 23°50 24-00 24-90 Bl 9-39 13-11 20-30 23-40 K 156 19-80 6-05 6:22 6:84 7:34 H. D. Foster 1 found that the vacuum treatment has an efficiency of 97 per cent. as compared with 92 per cent. when the sample is soaked for 72 hours in cold water. In many cases, it is important that a whole brick or article should be used when making porosity tests, because the surface is usually rather more dense than the interior and, if a portion of an article is employed, a higher result will be obtained. For this reason, when a broken article or only a portion of an article 1 is used for the test, this fact should be specified. The American Society for Testing Materials specifies the following porosity test : ““ At least five half bricks shall be first thoroughly dried to constant weight at a temperature of from 200° to 250° F., weighed and then placed on their faces in water to a depth of 1 inch in a covered container. The bricks shall be weighed at the following intervals: 4 hour, 6 hours, and 48 hours. Superfluous moisture shall be removed before each weighing. The absorption shall be expressed in terms of the dry weight and the balance used must be accurate to 5 grams.” 1 J. Amer. Cer. Soc., 5, 788 (1922). DETERMINATION OF POROSITY 83 If, on the other hand, the test is made to ascertain whether the interior of an article is uniform in character or has been properly fired, it is usually necessary to measure the porosity on a portion of the article and not on the whole. In testing pipes, portions of the sample are invariably employed. The following method is adopted by various American Associations :— “The specimens shall be each approximately 2 inches square and shall extend the full thickness of the pipe wall, with the outer skins unbroken. Five individual tests shall constitute a standard test, the average of the five and the result for each specimen being given in the report of the test. Each specimen shall be dried in an oven, or by other application of artificial heat, until they cease to lose further appreciable amounts of moisture when repeatedly weighed. All surfaces of the specimens shall be brushed with a stiff brush before weighing the first time. The specimens shall be weighed immediately before immersion on a balance or scales capable of accurately indicating the weight within one-tenth of 1 per cent. The specimens shall be completely immersed in water for a period of twenty-four hours. Immediately upon being removed from the water the specimens shall be dried by pressing against them a soft cloth or a piece of blotting-paper. There shall be no rubbing or brushing of the specimen. The re-weighing shall be done with a balance or scales capable of accurately indicating the weight within one-tenth of 1 per cent. The result of each absorption test shall be calculated by taking the difference between the initial dry weight and the final weight and dividing the remainder by the initial dry weight.” Mellor ! suggests that porosity should be determined on samples roughly cubical in shape and having sides about 2 inches long, whilst Wologdine and Queneau use samples weighing only 10 grams. The figures obtained from small samples are not comparable with those obtained with whole articles. The true porosity cannot be determined directly, as it not only includes the pores which can be filled by immersing the mass in water (7.e. apparent porosity), but also any sealed pores to which no fluid can gain access without destroying the mass by grinding it to powder. The volume of the sealed pores is very difficult to determine, and can only be arrived at indirectly from measurements of the specific gravity, weight, and total volume of the material as proposed by Heath and Mellor. The true porosity may then be estimated from the formula P=100 a ), SV where P is the true porosity in terms of per cent. by volume, W the weight of the dry sample, S the specific gravity of the material when in the form of a fine dry powder, and V the volume of the sample, including the pores. According to R. C. Purdy,? the volume of sealed pores is equal to ( )noo, where T is the true specific gravity and A is the apparent specific gravity. 1 Trans. Eng. Cer. Soc., 17, 314 (1917-18). 2 Trans. Amer. Cer. Soc., 11, 60 (1909). 84 PROPERTIES DEPENDING ON STRUCTURE It has been suggested that the porosity is equal to the product of the percentage absorption and the specific gravity, but this has been shown by Purdy and Moore to be incorrect. Rough tests for comparing the absorption or apparent porosity of various materials are :— (a) If on holding the sample to the tongue a distinct suction is felt, the sample may be described as “‘ very porous.” If there is no distinct suction, but the moisture is absorbed rapidly, it may be described as ‘“‘ moderately porous,” whilst if the moisture is very slowly absorbed, it is only “ slightly porous.” (6) A rough test of the speed with which water is absorbed consists in immersing the sample to half its depth in water and noting the rapidity with which the water rises into the other half. This is a rough indication of its capillarity rather than its porosity, but the two are often closely related. It is desirable to make several tests on different articles or samples of the same nature, as many clay products and ceramic materials are not always uniform and the average of several tests is more likely to be representative than the result of any one test. This is especially necessary where only a small sample is used, as different parts of the same article may vary considerably in porosity. When the porosity by volume of an irregularly shaped sample is required, it is most convenient to use a volumeter. The two types generally employed are Ludwig’s and Seger’s respec- tively. Ludwig’s volumeter (fig. 8) consists of a flanged cylinder with a tap near the bottom, covered with a flanged conical cap, the apex of which is widened out to form a short funnel. The flanges should fit perfectly so as to give a water-tight joint, and the cover is held in place by means of a heavy metal ring placed over the flange. The cylinder is filled with water to a mark on the stem of the funnel, part of the water is then drawn off the tap into a tared vessel. The cover is removed and the sample carefully placed in the cylinder without spilling any water. The cover is replaced and sufficient water poured from the tared vessel back into the volumeter, AH so that the water is again at the mark on the stem of the funnel. The water remaining in the tared Fic. 8—Lupwie’s Votumerzr. vessel is then weighed; its weight in grams is equal to the volume of the sample in c.c. Seger’s volumeter (fig. 9) is more delicate, but is easier to use, as the water is drawn by suction into a burette and is measured instead of weighed. Otherwise, the two instruments are used in a similar manner. . In determining porosity by means of a volumeter the weight of the sample when dry and also when saturated is found, and then, whilst the sample is still saturated, its volume is determined in the volumeter. The porosity by volume is— DETERMINATION OF POROSITY 85 _(W,—W,)100 a _ Pv where Pv is the percentage of porosity by volume, W, the weight in grams of the sample when saturated with water, W, the weight in grams of the sample when dry, and V the volume of the saturated sample in c.c. as measured by the volumeter. A rather more accurate result may be obtained, as sug- gested by Heath and Mellor, by withdrawing nearly all the water from the container into the burette or into a bulb above it, the jar being tilted if necessary. After closing the stop- cock of the burette the weighed sample is placed in the con- tainer, the stopper is replaced, and a suction pump connected to the tube in the stopper so as to exhaust all the air in the container. The action of the pump is continued for fifteen minutes so as to remove the air from the sample, after which the stop-cock of the burette is opened and the water run into the jar until the sample is covered, after which the pump is disconnected and the water run in until it reaches the mark on the tube in the stopper. The difference between the readings of the burette when the jar is filled with water and when the sample is present is the volume of the solid matter plus that of any sealed pores. This figure may be designated A. The water is again drawn back into the buretite, the soaked sample is replaced, the water is returned until it reaches the mark in the stopper and the burette is re-read. The difference be- tween this reading and the one when no sample is present gives the total volume of the material and pores. If this is designated B the percentage of apparent porosity by volume B—A Fie. 9. — SuGzErR’s is 100 = ve VoLUMETER. The liquid generally used in volumeters is water, but in determining the volume or porosity of substances affected by water, paraffin or other inert liquid may be used. According to Washburn and Bunting,! vaseline is preferable to paraffin, kerosene, etc., because of its greater penetrating power and the fact that it can be heated without risk. All these media have the disadvantage that the sample must be soaked before its volume can be measured, on account of the penetration of the liquid into the pores. Where it is not desired to soak the sample it may be thinly coated with melted paraffin wax, which, when solidified, will prevent any liquid penetrating into the pores. For fine-grained materials mercury may be used instead of water, as it will not penetrate into the article and does not wet it like water and similar liquids. The use of liquids for porosity and absorption tests is not entirely accurate on 1 J. Amer. Cer. Soc., 4, 983 (1921). 86 PROPERTIES DEPENDING ON STRUCTURE account of the slowness with which they are absorbed. Washburn and E. N. Bunting} have devised a method of using gas as the pore-filling medium, and have thereby obtained results which are, in some cases, considerably higher than when a liquid is used. The gases they employed include dry air, hydrogen, and helium; the results obtained with each gas were the same in each case. The porosity of powders and other loose materials may be determined by placing a definite volume (say 600 c.c.) of the material into a graduated glass cylinder holding 1000 c.c. The mixture should be just shaken down, but not compressed at all. A definite volume of water is then poured in and the cylinder is allowed to stand a little while until the water has filled the pores in the material, after which the resultant volume is measured. The volume of the voids in the loose mixture may be found by subtracting the final volume of the mixture and water from the sum of the volumes of sample and water separately and from these figures the percentage by volume can readily be calculated. Thus, if S is the volume of the sample, W that of the added water, and M that of the mixture, the percentage porosity by volume P will be— _ (S+W—M)100 OS Ss Waa The apparent porosity of a loose material will be low if the sample is not perfectly dry, as the moisture present occupies some space which would otherwise be occupied by air. If the particles of a loose material contain no sealed pores, the apparent and true porosity will be identical and can be found from the following formula :— 100(S—A) Ss ; where § is the true specific gravity and A is the apparent density. Thus, if the mixture has a true specific gravity of 2-6 and an apparent density of 1-7, the theoretical porosity will be 34-6 per cent. by volume. Unless the particles are extremely small or devoid of sealed pores, the porosity so calculated is rather higher than that deter- mined directly from the absorption of water or other liquid. The difference is due to sealed pores and to fissures in the grains which are too small for the liquid to penetrate. Penetrability is a term which is not commonly used; it is defined by E. W. Washburn as the ease with which a liquid is drawn into the pores of a body by capillary action without attendant chemical action between the body and the liquid. It is measured in a similar manner to the apparent porosity, which it closely resembles. P Porosity per cent. by volume= PERMEABILITY Permeability may be defined as the readiness with which a substance permits a fluid to flow through it and is measured by the rate at which a standard fluid, such as water, air, or other gas, flows through a mass of unit area and unit thickness. As pressure is necessary to cause the flow, it is necessary to specify the pressure or 1 J. Amer. Cer. Soc., 5, 112 (1922). ] PERMEABILITY 87 head of the fluid. Water is usually employed, but air or other gas, being much more mobile, gives more accurate results. The velocity of flow of water through a capillary tube is expressed by the formula 2 Vv = where V is the velocity in cm. per sec., r is the radius of the tube in cm., P is the difference in pressure at the ends of the tube in cm., wu is the viscosity of the liquid, and / is the length of the tube in cm. The force which causes the water to flow is equal to 27rT cos a, where T is the surface tension and a the angle of contact between the water and the walls of the pore. When the water is rising the force which is against it is mr2hpg, where h is the height to which the water has risen at a particular moment and g is the force due to gravity, so that the resultant force will be 27T cos a—ar*hpq. The velocity is approximately proportionate to the cube of the radii of the pores and inversely to the height to which it ascends. Hence, a coarse-grained brick is more permeable to water than a fine-grained one. To be wholly impermeable, a material must be very compact and close in texture, at any rate on its exposed surface, and for this reason such surfaces are often covered with a glaze. Where a large article has to be repeatedly heated and cooled, a wholly dense material is undesirable on account of the low resistance to temperature changes of most ceramic materials of this nature. Under such conditions the “‘ body ”’ of the article is usually made of a porous, coarse-grained material, the required impermea- bility being obtained by glazing, either the interior or exterior surface. Instead of deliberately applying the glaze, it may be formed “automatically ’’ during the course of manufacture as a result of the action of fluxes, etc., on the materials of which the articles are made or of the reactions which occur between the articles and their contents. Thus, in coke-ovens, a glaze is often formed by the action of salts in the coal on the firebrick linings and an even thicker glaze is produced in zinc retorts which eventually renders them quite vitreous. In paving bricks and tiles, porcelain and some forms of stoneware, the requisite impermeability is produced by the interaction of fluxes and the material of which the goods are made, with the result that the greater part of the pores present in the early stages of firing are later filled with a glassy mass of fused silicates which renders the article impermeable. The production of impermeable articles is costly and in many cases both unnecessary and undesirable, but in others it is essential. The permeability of a mass of clay depends on its plasticity and moistness. If a clay is quite dry its plasticity is dormant and it will be readily permeable to water, yet the same clay when wetted and its plasticity made “active ” will be impermeable to water. The permeability of an article depends on (a) its thickness ; (b) the kind, number, distribution, and sizes of the pores; (c) the presence or absence of cracks, fissures, and holes ; (d) the presence or absence of a superficial glaze or slag ; (e) the difference in pressure on opposite sides; (f ) the nature of the penetrating fluid; and (g) the difference in the temperature of opposite sides of the article. In several of these respects permeability differs greatly from porosity (p. 61), 88 PROPERTIES DEPENDING ON STRUCTURE for whereas the latter is a measure of the total volume of the pores, the permeability depends on the extent to which they penetrate through the mass. A glazed brick may have a high porosity, but it should, when in use, be impermeable so long as the glaze is undamaged. In other words, permeability depends chiefly upon the number of channels or connected pores which enable the permeating liquid to pass through from one side of the article or test-piece to the other. Pores which are closed at one end, or do not penetrate far into the mass, take no part in producing a permeable mass ; a single small crack penetrating right through the mass may increase the permeability more than a thousand short or disconnected pores. In short, whilst highly porous articles are often highly permeable, there is no definite relation between the porosity and permeability of a material and a tile may have a true porosity of 25 per cent. and yet be impermeable even to gases. The absence of any relation between porosity and permeability is well shown in Table XIX, containing figures obtained by A. L. Queneau. TaBLe XIX.—Porosity and Permeability Permeability in me pate Vol. litres per hour per sq.m. . Glass pot . : ' 30-00 0-30 Glass pot . : 30-40 1-02 Firebrick ; ; 29-44 37-84 Firebrick . ‘ ‘ 30°85 14-72 Firebrick . : : 30:20 106-20 Silica brick ‘ : 42-58 3°32 Silica brick : at 42-90 192-90 The permeability of an article, especially to gases, sometimes varies in different directions, especially if the material is laminated. This subject has not received the attention it deserves. The maximum permeability in one direction is possessed by a structure composed of a large number of minute (capillary) tubes, arranged parallel to each other. Apart from the resistance caused by the surface tension of the walls of the tubes such a “honeycomb structure ’’ would be completely permeable. In the ceramic industries, no materials are known which possess this structure, so that the permeability is usually due either to fissures or cracks in the materials or to numerous pores being connected and so forming irregular channels. Effect of Heat on Permeability.— When ceramic materials are heated, allotropic and other changes which occur—especially in fireclay, silica, and magnesia—may cause large changes in the permeability and density without greatly affecting the other properties of the mass. Thus, A. L. Queneau ! has shown that a silica brick 1 Electrochem. and Metal. Ind., 7, 385 (1909). EFFECTS OF PERMEABILITY 89 burned at 1050° C. had a permeability of 3-3 litres per hour, whilst that of a brick made of the same material, but burned at 1300° C., was 193 litres, and the permeability of a brick burned at 1400° C. was 241 litres per hour. Fireclay and bauxite bricks also increase in permeability rapidly in this way. At temperatures of 1100° C. and above, articles made of “ fused silica” or quartz glass are permeable to hydrogen, oxygen, and nitrogen. This is sometimes serious, as in the case of pyrometer tubes, in which the platinum wires are slowly affected by the gases which permeate the walls of the tubes. At temperatures below 1100° C. this difficulty does not occur, as the “ fused silica ” is then quite impermeable. Effect of Permeability on Thermal Conductivity.—As the thermal con- ductivity depends on the mobility of the air in the pores, it usually increases with the permeability as explained more fully in Chapter XIII, but chromite, magnesia, carborundum, and graphite behave irregularly in this respect. The heat- insulating power at temperatures below 1200° C. is roughly proportional to the permeability. _ Effect of Permeability on Drying.—The rate at which a mass or article of clay or other ceramic material can be dried with safety depends largely on its permeability, because the water-vapour escapes more easily and quickly from a permeable material as the pores in it are directly connected with the atmosphere. Effect of Permeability on Uses.—The purpose for which an article is to be employed is often influenced by its permeability. Thus, in building reservoirs and embankments, etc., a mass of clay paste or “‘ puddle” is used to prevent the passage of water, as clay paste is almost impermeable. In this respect, it differs from dry clay, which is readily permeable to water. For this reason, when it is desired to prepare a clay paste it is desirable to dry the clay prior to soaking it in water, as the water will then penetrate it much more readily and will be more uniformly distributed through the mass. Unfortunately, the cost of drying is often so great that it is omitted. Ordinary building bricks should be permeable to air, so that they may “ breathe,” z.e. allow air to pass through them, but they should not be permeable to water, or rain will enter and make the wall “damp.” If a house is built with solid walls of impermeable bricks it will appear to be “damp,” because the impervious walls cannot absorb the moisture condensed on them during cold weather. Other argil- laceous building materials, such as terra-cotta, etc., should usually have similar properties. Paving bricks should be quite impermeable. Roofing tiles should be permeable to air, but not to water, or they will allow rain to pass through them and accumulate on or even drop from the underside of the tiles, thereby causing damage. Filters should be very permeable, so that they allow water or other liquids to pass through them at a suitably rapid rate. The pores must, however, be sufficiently small to prevent any “ dirt” or other undesirable solid matter passing through them. A filter may be tested for permeability by noting the amount of water passed through it in a given time (say twenty-four hours). Saggers and muffles should be porous but not very permeable, or they will not protect the goods from flame, smoke, etc., produced by the fire-gases. This object 90 PROPERTIES DEPENDING ON STRUCTURE is attained by having the pores as small as possible consistent with the articles being resistant to sudden changes of temperature. Retorts should be porous, but not permeable, or much of the vapour or gas pro- duced in them may be lost. This loss may be reduced by regulating the withdrawal of the vapour or gas, so as to avoid the creation of an appreciable pressure in the interior of the retort. In new gas-retorts, permeability is seldom serious, as the formation of a deposit of carbon on the interior soon prevents any loss of gas by permeation. In blast-furnaces the bricks and blocks used for the lining should not be highly permeable, or, as B. Osann? has shown, they will permit carbon monoxide in the gases to reduce the iron oxide in the bricks and cause them to fuse more rapidly than would otherwise be the case. Graphite crucibles are commonly supposed to be quite impermeable, but this is not the case, as at temperatures above 1200° C. they are permeable to furnace gases. Glazed ware, stoneware, and much chemical ware should be quite impermeable (see p. 87). Table XX, due to A. L.. Queneau, shows the permeability of various ceramic materials :— TaBLe XX.—Permeability of Ceramic Materials Permeability, c.c. Burning Litres per hour per Temperature, seein Lye ee sq. m. through slab oC. through slab 1 cm. i mahions thick. Fireclay brick ; 1050 0-0409 14-72 “ 2 ; 1300 0-0690 24-84 Checker brick. ; ae 0-0465 16-74 Bauxite brick. : : 1300 0-2120 76-39 Silica brick. : ; 1050 0-0092 3:32 S Be . ‘ 1300 0-0536 192-90 Magnesia brick 1300 0-0097 3-49 i fs : 1050 0-5170 186-10 Carborundum brick. 1050 0-0053 1-90 2 Fe’ ; 1300 0-0043 ; 1-55 Chromite (unburned) ; ) 0-0568 20-45 Chromite bricks with clay bond . : 1300 0-0075 1-70 Kieselguhr ; ; a 0-0957 34-45 Graphite bricks e ie a Building bricks 1050 0-0015 0-53 light clay. ; : a 0-0164 5-90 1 Stahl und Eisen, 1907, p. 1627. DETERMINATION OF PERMEABILITY 91 Determining Permeability—The permeability of slabs, tiles, and other articles may be determined by fastening a glass cylinder of convenient size to one surface of the article, using a rubber washer or other device to prevent any leakage between the glass and the article, filling the cylinder with water and supporting the article on thin rods, or preferably on knife-edges, over a glass vessel. After twenty- four hours, or a longer period if preferred, the volume of the water which escapes from the cylinder into the vessel below is measured. The area of the part of the article exposed to the water and the pressure or height of the water in the cylinder, should also be recorded. The result gives a relative value for the permeability. Bottles SSSSSSSSSSSSSSSSSSSSSS WN ZA Li dddddddagcccceeeeele ; . l a Sit AAA A ESSSAY Clit Eas I Fia. 10.—PERMEABILITY TEST. Fic. 11.—Soxonorr’s PERMEABILITY APPARATUS. are sometimes used instead of cylinders open at both ends, but the results are less satisfactory, as the water tends to be held in the bottle by the pressure of the atmosphere. When testing articles of considerable thickness, such as bricks, it is desirable to paint all the surface, except the parts to which the cylinder is applied and an equal area opposite to it, with a waterproof shellac or varnish. If the article is very permeable it may be kept supplied with water at a constant level by the arrangement shown in fig. 10, the water absorbed being automatically replaced by a fresh supply from the flask. In this way the time taken for the under-surface of the test-piece to become moist, dew-covered, and to liberate drops of water, can be readily observed, the time taken to pass a prearranged quantity of water through the article can be measured and valu- able information as to the relative waterproofness of different tiles, etc., obtained. A more elaborate apparatus used by Sokoloff (fig. 11) consists of a short cylinder, 92 PROPERTIES DEPENDING ON STRUCTURE C, with a flange at its lower end and a short tube, B, fitted with a rubber cork in its upper end, the joint between the two being made water-tight. Through the rubber cork a long graduated glass, A, sealed at the top with a cotton-wool plug, is inserted. A T-piece tube of metal, H, is fitted into a hole through the side of the flanged cylinder, a little way above the flange, one free end being connected by a pipe, H, to a water supply, placed well above the apparatus and provided with a tap or a pinch-cock, whilst the other end is closed with a second tap or pinch-cock. The sample R should be of the same diameter as the flange and is usually 5 cm. thick, but if water escapes through the edges of the test-piece, a thinner sample must be used. The sample is boiled until saturated with water and is then clamped on to the metal flange with a rubber washer, I, between them, and a rubber, L, and a metal, M, washer below. The upper washer should preferably have an opening about 1 inch in diameter, which should always be kept constant or the results will vary on account of the differences in the area through which the water may permeate. Water is run through the tube E into the tube A, until it reaches a height of exactly 40 inches. The tap is then turned off and the water is allowed to permeate through the sample. The test is continued until 4 inches of water have passed through, the time taken being a measure of the per- meability. If desired, pressure may be applied to the liquid in A by means of a pump, but in that case the pressure should be recorded. To ensure accurate results, the temperature of the apparatus and water should be carefully brought to and maintained at 15° C. or 60° F. The taps on the apparatus must fit accurately, and soft, filtered water or distilled water should be used, so as not to choke any of the pores in the sample and so reduce its permeability. The permeability of a material to air or other gases is determined by Wologdine and Queneau ! on a cylinder of the material, 40 mm. in diameter and about 60 mm. long, coated with paraffin wax and cemented into one end of a glass tube. The other end is closed by means of a rubber stopper fitted with a tube connected to a gas-holder and manometer so as to maintain a constant pressure. Air or gas is applied under a prearranged pressure for a definite period of time and the amount of air passed through the test-piece is measured and reported as the relative permeability of the sample. The two alternative standards suggested by Wologdine and Queneau are (a) the amount of air in c.c. under a head of 1 cm. of water which passes in 1 second through a cylinder 1 square cm. cross-section and 1 cm. in height, and (6) the number of litres passing in | hour through a surface of 1 square em. 1 metre thick. The latter is calculated from the following formula :— Der mige ~ 16-667¢PS’ where V is the permeability in litres passing in 1 hour through a cylinder of 1 square cm. area and 1 metre thick, g the quantity of air, in litres, passing in ¢ minutes under the pressure P (7.e. the height of water in cm. in the manometer in excess of that corre- sponding to atmospheric pressure) through the cylinder of cross-sectional area S and length J. As the permeability very often varies in different parts of the same article on account of holes, minute cracks, etc., it is difficult to obtain a representative figure. 1 Hlectrochem. and Metal. Ind., Oct. 1909. DETERMINATION OF PERMEABILITY 93 The permeability of a loose powder may be determined by means of the following apparatus used by the author : A circular metallic box, 3 inches diameter and 14 inches deep, with a flanged rim at the top, has the bottom cut out and replaced by a piece of wire gauze of any suitable mesh. On this gauze is laid a sheet of thin blotting-paper and the case is filled with the material to. be tested to the height of exactly 1 inch. If the material is to be com- pressed in use, the state of compression should be the same in the test sample. The sample is covered with a second sheet of blotting-paper and a flanged pipe about 4 feet in length is secured to the box and filled with water to a height of 40 inches or 1 metre. The apparatus is suspended over a glass vessel to collect the liquid passing through the sample. The time taken for 100 c.c. of water to pass through the sample is regarded as a measure of the permeability. CHAPTER III COLOUR, HARDNESS, AND MINOR PHYSICAL PROPERTIES SEVERAL of the properties of clay and other ceramic materials described in this chapter are, in many cases, of minor importance, but in some instances they are of major importance. Thus, whilst the colour of raw clays usually matters little, a suitable colour in the burned goods is often an essential property. Similarly, hardness is often of secondary consideration, but where resistance to abrasion is necessary, a hard material is usually essential. The properties dealt with in this chapter are (a) colour ; (b) hardness ; (c) “ feel” ; (d) “ring”; (e) odour; (f) taste; and (g) sectility. The last three do not usually affect the working properties of the materials in any way. COLOUR The colour of an object is due to the fact that when rays of light fall on it some are absorbed, whilst others are reflected, the nature of the reflected light giving the surface a characteristic colour. Thus, objects which reflect red vibrations appear to be red in colour and so on. When the light is completely absorbed the object is black, whilst if the light is completely reflected without change the surface will be white. Colour is exhibited in various ways, so that an object may show different colours under different conditions, viz. :— 1. The colour by reflected light. 2. The colour by transmitted light if the object is transparent. 3. The colour of the powder, as distinct from the mass (this is shown by drawing the mass along a piece of unglazed porcelain and examining the streak left on the latter). 4, The appearance of special types of colour, such as (a) play of colours, caused by the unequal bending of rays of light of different wave-lengths giving a vari-coloured effect ; (b) opalescence, which gives a pearly or milky appearance, as shown by opals ; (c) widescence, or display of colours, due to. inclusions of air or liquid in transparent crystals, or to peculiar irregularities in the surface of the material, as in some specimens of quartz, calcite, and mica; and (d) pleochroism, a quality possessed by some transparent minerals, by which they exhibit different colours when viewed by polarised light passing through them in different directions (see also Chapter XV). 94 COLOUR 95 Lustre is a property which may be classed with colour; it is due to the manner in which light is reflected from an object, rays of different wave-lengths being re- flected at different angles and so producing diffraction effects. The lustre may be (a) metallic lustre, such as is exhibited by many metals ; (6) vitreous or glassy, as in glass, quartz, etc. ; (c) resinous, as in resin; (d) pearly, as in pearls; (e) silky, as in satin spar; and (f ) adamantine, as in diamond and some forms of carborundum. Colour is a much more constant feature in opaque substances than in transparent ones; the latter often vary greatly in colour, because of the presence of minute traces of other substances as impurities. Many substances have a characteristic colour, which is of value as indicating the nature of the material, e.g. iron, chromium, copper, uranium, etc. The colours produced in transparent minerals are dealt with more fully in Chapter X on Mineralogical Constitution and Chapter XV on Optical Properties. The colour of minerals varies considerably on account of (a) the presence of other materials and (b) the prior history of the material, including the effect of the weather. The surface of a material or article may change colour on exposure. The colour of most raw ceramic materials is usually a minor property and chiefly of value as an indication of the impurities present. In finished articles, it is often a very desirable property and one which contributes greatly to their beauty. In some cases, the colour is more important than other properties, as in some ornamental ware and even in some utilitarian articles. Thus, an architect may select red bricks on account of their colour and may have to disregard the fact that the ones he chooses may not be as strong as some other bricks. It is often easy by firing bricks at a higher temperature to make them much stronger, but, if the colour is less pleasing, they will usually be difficult to sell. Fortunately, most bricks are so much stronger than is requisite for safety that the slight loss of strength which is incurred in order to produce a more pleasing tone or colour is quite negligible for most of the purposes for which such bricks are used. The colour of a raw ceramic material does not necessarily give any indication either of its purity or the colour of the fired product. Consequently, such a material should seldom be judged solely on its colour. A dark-coloured material, such as some Devonshire ball clay, may be almost white when burned and its original colour will, in such a case, be no detriment to its usefulness. The same is true of most ceramic materials whose colour is due solely to the presence of carbonaceous matter which burns off in the kiln and is, in this way, removed and can do no harm. If the dark colour of the material is due to iron compounds which are not removed by heating—it may completely spoil the material for some purposes. A light-coloured material, on the other hand, need not necessarily be of good quality, as it might contain a large proportion of colourless or light- coloured impurity, which would darken on heating or in other ways make the material unsuitable for some purposes. Where a definite colour is desirable in the finished articles, it may be produced by various means such as :— (a) By heating the material or articles at a suitable temperature, and in a suitable 96 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES atmosphere, so as to produce the desired colour. If the composition of the raw materials is suitable, no other treatment may berequired. If certain colour-producing impurities are present in the raw material they may prevent the formation of the desired colour, in which case some other method must be employed. A typical example of this method of producing colour is a ferruginous clay, which, if burned at a certain temperature (depending on the clay) in an oxidising atmosphere, will produce a pleasant red colour, but if it is burned in a reducing atmosphere a dark, dull “‘blue”’ colour is obtained, whilst when reducing and oxidising atmospheres are used alternately in the kiln, a mottled or brindled appearance is produced. (6) A coloured substance may be introduced into the raw material, so as to mask any undesirable colour and give the finished goods the desired tint. Thus, the addition of manganese dioxide will produce a dark brown or nearly black article, whilst cobalt compounds will give blue shades and chromium compounds green ones. The lighter the natural colour of the burned material, the greater will be the variety of colours which can be produced in this manner. (c) If the colour of a fired material is undesirable it may be masked by coating the article with an engobe of another material which burns to a desirable colour. The engobes used for this purpose are usually white-burning clays or a mixture of such clays with flint, Cornish stone, or felspar, together with a suitable colouring agent if required, the engobe being adjusted so as to have the same shrinkage as the article to which it is applied, as well as the desired colour when burned. The engobe is often covered with a transparent glaze, to protect it and render the article impervious to water. Sometimes, instead of a coloured engobe, a coloured glaze is used and in some faience and majolica ware a coloured glaze is used over a white engobe. Where the colour is required to be in the form of a pattern, it may be applied direct to the article or to the engobe by means of a brush, stencil, transfer, or any other convenient method. If the ware is then covered with a glaze, the method is known as underglaze decoration. As such colours must not be adversely affected by the firing of the glaze, only a limited range of colours is available. If, on the other hand, the colours are applied to the article after it has been glazed and fired, a much larger range of colours is available. This method is known as overglaze decoration. From the foregoing it will be seen that the most suitable method of obtaining any desired colour depends on the nature of the goods. NATURAL SOURCES OF COLOUR A large number of the materials with which this volume deals are, when pure, perfectly white. Thus, china clay, magnesia, lime, alumina, zirconia, and silica, when quite free from impurities, burn to a perfectly white mass. Perfect whiteness is seldom attained on account of the presence of minute proportions of impurities, which have an appreciable influence on the colour of the material in which they occur. Thus, iron in small proportions is universal in its occurrence, and almost all NATURAL SOURCES OF COLOUR 97 minerals contain at least traces of it. As iron compounds have a very pronounced colour when heated to redness (except in the presence of free lime, etc. (p. 102), iron is either a valuable accessory or a troublesome impurity, according to the condi- tions in which it occurs and the purpose for which the material is to be used. The chief forms in which iron compounds occur in the raw materials are :— (a) Free ferric oxide, Fe,Os. (b) Free ferric hydroxide, Fe,0,7H,0. (c) Magnetic iron oxide, Fe,O,. (d) Free ferrous oxide, FeO. (e) Iron oxide combined with silica or with silica and alumina. (f ) Complex iron silicates or alumino-silicates. It was suggested by the author, in 1909, that a large proportion of the iron in some clays may occur as ferrosilicic acid, possibly equivalent to nontronite (H,Fe,Si,0,), which, on heating, is decomposed into water, silica, and free ferric oxide. This would account for the great change in colour which many clays undergo when heated, but, owing to the experimental difficulties involved, the correctness or otherwise of this supposition has never been proved. (g) Iron (ferrous) carbonate, FeCOQ,. (h) Iron (ferric) sulphide, FeS,. (2) Iron (ferrous) phosphate, Fe(PO,). For the mineralogy of the iron compounds, see Chapter X. The colours produced by iron compounds are divisible into three classes : yellows, reds, and blues. In the raw state the colour of ferric compounds varies from pale yellow to reddish brown, according to the amount present in a clay or other material and the nature, size, and distribution of the individual particles. Ferrous compounds impart a bluish or greenish tinge, which is often less noticeable than the more pro- nounced colour of ferric compounds. Various modifications of the above-mentioned colours also occur, some iron compounds imparting a grey colour to the material in the raw state. Thus, most fireclays and some sandstones are grey, owing to the presence of ferrous carbonate or finely-divided iron pyrites, or, in rare cases, to iron phosphate. Ferrous sulphate is a very troublesome impurity in clay, as it cannot easily be removed by washing, and, unless converted into an insoluble material, it often pro- duces a bluish-green scum on the surface of the dry but unfired articles. If necessary, it can be avoided by adding a little barium carbonate to the clay, so as to convert the iron salt into insoluble ferrous carbonate and barium sulphate. Red and Yellows.—Some raw ceramic materials, such as bauxite, magnesite, breunnerite, laterite, and some clays, are yellowish or reddish in colour. This is commonly attributed to limonite and to hematite. The former is yellow, and the latter yellow to red, according to the state of oxidation, combination with water, and state of subdivision. They consequently impart a corresponding tint to any material in which they occur, subject to the proportion present and the influences which tend to prevent their characteristic colour being produced.t Ferrous and 1 It is said that there are some red clays whose colour is due to the presence of alge. The colour of such clays is deeper in the raw than in the fired state. 7 98 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES ferric oxide, either free or in combination with water or other substances, are the source of a large range of colours, varying from the lightest tawny yellow, through full yellow, orange, to a rich red colour, which resembles that so much desired in facing bricks. In the fired or burned ware, the number of iron compounds is usually less than in the raw materials, because the hydroxides, carbonates, sulphates, sulphides, and phosphates are usually decomposed in the kiln, yielding one or more of the oxides or complex silicates. When heated to bright redness, under oxidising conditions, most iron compounds impart a reddish colour to the clay or other material containing them. This colour is commonly regarded as due to the conversion of the iron into ferric oxide, though other complex compounds are probably present, as the material is not bleached by boiling it with hydrochloric acid, which would dissolve ferric oxide. The colour of raw clay may be partly destroyed by hydrochloric acid, but all the iron cannot be removed, either from the raw or fired clay. This would seem to support the idea that the iron is combined in some way with silica, or with silica and alumina, unless it is converted into a dense form of ferric oxide, which is soluble only with great difficulty in hydrochloric acid. The colour produced by ferric oxide in fired ware is extremely variable, as it depends upon so many factors; for this reason, the colour produced by iron com- pounds naturally present in the raw materials cannot usually be matched by adding any prepared materials. Seger considered that the colour developed by iron compounds in an oxidising atmosphere depends on :— 1. The amount of iron oxide or its equivalent present and the nature of the compound. 2. The composition of the fire-gases during the burning. 3. The temperature at which the material is burned. 4, The amount of other constituents. 5. The amount of vitrification which occurs. It will usually be found that (when other conditions are constant) the colour is, to some extent, dependent on the proportion of iron present if it is in a sufficiently finely-divided condition. Thus, the presence of the equivalent of 4 per cent. of ferric oxide? in a clay will usually impart a good deep-red colour to the burned clay. With only 3-4 per cent. the colour is more usually brown or purple, whilst with less than 3 per cent. the colour varies from deep buff to nearly white, the depth of colour diminishing with a decrease in the proportion of iron oxide present. It is, however, almost impossible to predict the colour of a burned clay from the proportion of ferric oxide, as there are so many other factors involved, such as the size of the particles and their previous history. In fact, Orton is very emphatic in expressing the opinion that the colour (when fired) of a red-burning clay bears no relation to its iron 1 It is customary to refer to the iron compounds in clays, etc., as though they were all present as ferric oxide. Actually, several compounds may be present, so that the term “‘ ferric oxide ”’ must be understood to mean the equivalent and not as necessarily implying that all the iron is in the form of ferric oxide. NATURAL SOURCES OF COLOUR 99 content, as many clays burn to the same colour, no matter whether they contain 4 or 8 per cent. of ferric oxide. He further remarks that “the distribution seems more important than the amount” and “the conditions of firing exercise a still greater influence, so that, whilst clays low in iron never burn red, it is not possible to estimate the colour of the fired ware from the proportion of iron present.” If a bufi-coloured brick is examined under a microscope it will be seen to consist of different-coloured materials, in the form of tiny patches or dots, the colours varying from white or the yellow tint of the fired clay to brown, red, or even black spots. To the naked eye these all blend together, giving a uniform buff colour, and the more abundant the dark spots, and the nearer they are together, the deeper will be the colour ; 1.e. the larger the proportion of effective iron compounds present, the deeper will bethe colour. The effect is similar to that produced by a large number of coloured dots on a white sheet of paper. When the sheet is held at a distance the whole ~ surface appears to have a uniform tint. This uniformity of colour is only attained when the particles are extremely minute and very regularly distributed, and from this comparison it will readily be understood that the colour of a piece of fired material does not depend only on the proportion of iron compounds present, but also on the size of the particles and on the manner in which they are disseminated through the mass. The effect of the size of the grains of iron compounds is well shown in the following : (a) lf a piece of clay is soaked in a solution of a soluble iron salt, dried rapidly and burned, it will have a strong red colour ; (b) if another sample of the same clay in the form of a cream or slip is mixed with a soluble iron salt, and the latter is afterwards precipitated by ammonia, and the clay is evaporated to dryness, dried and burned, it will also have a dark-red colour ; (c) if powdered hematite ore is mixed with another sample of the same clay, which is then fired as before, a brownish-red colour is pro- duced’; whilst (d) if the iron oxide were in a still coarser form, the burned mixture, as a whole, would be only slightly coloured, but it would have red blotches and spots. In all these cases, the total amount of iron is the same, the difference in colour in each case being due to its form and distribution. When a solution of an iron salt is used it permeates the whole mass and the iron ‘is disseminated throughout the clay uniformly, except in so far as some of the solution is drawn to the surface by capillary attraction. The powdered ore is less thoroughly mixed, and, therefore, gives a less intense colour, whilst the larger grains of material merely produce spots, the remainder of the clay being scarcely coloured. It will thus be seen that to produce the characteristic colour of red-burning clays the iron compounds present must be in an extremely fine state of division and uniformly disseminated throughout the whole mass. Coarse particles merely impart a colour to a comparatively narrow zone around them, whilst the rest of the mass is unaffected. For this reason, the colour of clays cannot materially be improved by the addition of artificially prepared iron oxide, as the latter is far too coarse to give a homogeneous tint to the clay. The iron compound in red-burning clays is in so impalpable a state, and is dis- seminated through the mass to such an extent, that it is almost impossible to attain a 100 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES similar material by artificial means. The nearest approach to it is to add a solution of a soluble iron salt to the clay, but unless special precautions are taken (which usually result in segregating the iron and so spoiling the effect) the solution tends to accumulate on the surface of the articles during the drying stage and is brushed or rubbed off when handling the goods. In the United States, a superficial red colour is sometimes imparted to terra-cotta by spraying the dried, but unfired, ware with a 20 per cent. solution of ferric chloride and afterwards firing the ware in the usual manner. A solution of ferrous sulphate appears to be useless for this purpose. The colour produced by iron compounds on the fired ware is also greatly influenced by the conditions under which it is burned. Comparatively small changes in the composition of the fire-gases and of the atmosphere in the kiln make a great difference in the colour of the ware. The best and most brilliant red is obtained by heating the goods slowly in an atmosphere containing a large excess of free oxygen, taking care to avoid the temperature rising so rapidly that any carbonaceous matter in the clay can reduce any ferric compounds present. To some extent, the effect of a temporary reducing atmosphere may be overcome by later careful oxidation, but the resultant colour is seldom so good as if the conditions had been entirely oxidismg throughout the whole period of firing. It is especially important that between 700° and 900° C. the atmosphere of the kiln should be highly oxidising, so that any of the lower forms of iron oxide may be oxidised to ferric oxide before the temperature at which they commence to fuse is attained. When once the fusion of some of the ferrous compounds has occurred it is almost impossible to re-oxidise them so completely as to obtain a pure red colour. The temperature to which the goods are heated is also of importance in the development of the red colour of ferric oxide ; the colour produced on firing a red- burning clay becomes brighter and lighter as the temperature increases, until a maximum brilliancy is attained, usually at a temperature equivalent to Seger Cone la (1100° C.). At higher temperatures, partial fusion takes place and the colour is gradually darkened, the red being replaced by brown. The temperature at which the red colour is perfected depends upon the clay and varies with different materials. Some clays assume a brilliant “ terra-cotta ” red colour when merely baked at 900°— 1000° C., whilst others scarcely develop any red colour at such temperatures. It has been frequently stated that ‘ such and such’ a temperature will form a red brick and that a higher temperature will form a blue one. This is only true when other conditions are satisfied. Thus, a clay practically free from iron will not produce a red colour at any temperature unless iron oxide be added to it, and many clays which will produce a good red brick will not form a satisfactory blue one, because they either do not contain sufficient iron or because they will not stand heating to, and reduction at, the necessary temperature. Hence, the principle that the tempera- ture regulates the colour is only true within limits ; beyond these the statement does not apply. As a general rule, the red colour begins to be replaced by brown as soon as an appreciable amount of fused or vitrified material is formed ; the iron oxide then appears to combine with other minerals, yet it has never been satisfactorily EFFECT OF MINERALS ON RED COLOUR 101 explained why free ferric oxide (to which the red colour is generally considered to be due) can remain uncombined with the silica of the clay for so long at temperatures above 900° C. It may be due to the clay remaining unvitrified, and, therefore, practically inert, but this explanation cannot always apply, because the red colour is retained to perfection in a few clays which have been fully vitrified. As Orton says, “It is truly hard to see how iron oxide can be wholly free and uncombined in a vitrified mass of such perfection,” though it appears to be so, and the fact that, on further heating, these clays blacken and iron silicates are then formed, makes it appear probable that the combination of the iron compounds with the silica of the clay can only result in the loss of their red colour and in the production of black (technically “blue ’’) wares due to the formation of ferrous silicates. Whether the true explana- tion of the inertness of the so-called free ferric oxide in red-burning bricks will ever be explained is a problem which only the future can solve. Effect of Minerals on Red Colour.—The presence of various materials in a clay has an important influence on the colour developed by iron compounds. If a large proportion of colourless matter, such as sand, etc., is present, especially if it is some- what coarse, the colour developed by the iron compounds is not so intense as might be expected from the percentage of iron oxide shown by analysis. The difference may be partly due to adsorption phenomena (see Chapter VI), as the colouring matter cannot so readily tint grains of sand and other impervious minerals as those of the more porous clay. The red colour produced by iron compounds is also modified by free alumina, Seger having found the following relations between the composition and colour in various clays :— Character of clay. Colour after burning. High in alumina and low in iron . White, or nearly so. High in alumina and moderate in iron . Pale yellow to pale buff. Low in alumina and high in iron Red. Low in alumina and high in iron and ee Cream or yellow. According to Seger, some clays containing iron compounds have, when burned, a yellowish colour due to the interaction between the iron compounds and the alumina, the latter decolorising the iron in the same way as lime (p. 102), though to a much -smaller extent, as lime is a more powerful base. His experiments suggest that the best red-burning clays contain two or three times as much alumina as iron oxide. This influence of alumina is not admitted by Orton,1 who denies the bleaching action of alumina upon iron oxide, because he has found buff-burning clays of practically the same colour, the composition of which fluctuates between 40 per cent. of alumina and 0-5 per cent. of iron oxide, and 15 per cent. of alumina and 2-5 per cent. of iron oxide. L. A. Keane,? on the contrary, has found that alumina sometimes aids in the distribution of the iron oxide through the mass by peptisation and also prevents the proper formation of the red colour by ferric oxide. 1 Trans. Amer. Cer. Soé., 5, 389 (1903). 2 J. Phys. Chem., 20, 724-760 (1916). 102 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES The presence of some other oxides, such as lime, magnesia, and alkalies, is also adverse to the production of a good red colour by iron compounds, as they tend to cause the clay to fuse and so darken the red colour and form an unpleasant shade of brown. This is supposed to be due to the iron oxide combining with the fused material forming iron silicates, which do not possess the red colour of the free ferric oxide, but see p. 98. Inme, when present in a red-burning clay, will combine with the iron present to form a white or cream-coloured double silicate. The white Suffolk bricks are produced by this means. Seger has found that the whitest products are obtained when the proportion of lime is equal to not less than two and preferably four times that of the iron expressed as ferric oxide. In practice, a large proportion of lime or chalk is usually necessary. The decolorising effect of lime cannot occur when sufficient sulphurous fumes are present in the kiln gases, as these combine more readily with the lime than the iron does and form calcium sulphate, which produces a scum on the surface of the goods. . The inside of the articles, however, which are not affected by the sulphurous gases, are decolorised in the normal manner. _ This effect of sulphurous gases is well shown by some tests made by Aron, who found that the red parts of bricks contained 8-49 per cent. of sulphuric acid, whilst the decolorised parts contained only 0-6 per cent. Maw has found that the presence of 5 per cent. of magnesia prevented the pro- duction of the red colour in various clays which he examined. Lime and magnesia each neutralise the colour of iron compounds to the extent of about half their weight, so that a clay containing 4 per cent. of iron oxide and 6 per cent of magnesia or lime will, when burned, have a colour such as would be produced were lime and magnesia absent and only 1| per cent. of iron oxide were present. The effect of fine fluxes in destroying the red colour produced by iron is shown by the fact that coarse, sandy clays retain their red colour when heated to a higher temperature than fine-grained clays, as the latter contain a larger proportion of fluxes which fuse at a lower temperature and so cause the loss of colour (due to the formation of iron silicates) at a lower temperature than would otherwise be the case. Thus, E. Orton found that a sandy clay retained a strong red colour up to Cone 8, but another red-burning clay containing a larger proportion of actual clay attaimed a maximum red colour at about Cone 1. Above this temperature, the colour became dark brown and was spoilt by the formation of silicates. Ries has also confirmed this effect of fluxes. The red colour is best produced in the absence of all other impurities than iron oxide and this should occur in an extremely finely divided condition, disseminated. uniformly through the clay. If necessary, the clay may be purified (Chapter VI) so as to improve the red colour produced by the iron oxide present. Blues and Blacks.—The blue or black produced in fired goods is usually due to the presence of ferric sulphide, ferrous compounds, or to magnetic iron oxide, each of which can combine with silica at a red heat to form a dark, fusible, slag-like mass, which is readily absorbed by the porous, fired clay to which it imparts its colour. a PRODUCTION OF BLUES AND BLACKS 103 Ferric sulphide (pyrites) produces isolated black spots in the fired ware. These are due to the ferric sulphide (FeS,) losing half its sulphur at a temperature of about 800° C. and to the resulting ferrous sulphide (FeS) combining with any free silica present and forming a dark fusible ferrous silicate, If the particles of sulphide are very small, they produce isolated black spots, but if they are larger or very numerous they form blotches or patches of a dark slag-like material which may be 3 inch or more in diameter and render the ware unsightly. The iron in fireclays appears to be largely in the form of pyrites, and those clays are consequently buff or light yellow when burned, with small spots or patches of dark slag disseminated through the mass in proportion to the amount present. Pyrites never occurs in such fine grains as to give an even red colour to the goods in which it occurs, but usually produces dark spots in the ware. Magnetic oxide of iron (Fe,0,) is black and is produced by the partial reduction of ferric oxide, under conditions which do not permit its complete reduction to ferrous oxide. It occurs in some raw clays and in some buff bricks, in the firing of which there has been a slight reducing action. It is similar in action to a mixture of ferrous and ferric oxides. The ferrous compound naturally occurring in raw ceramic materials is chiefly ferrous carbonate, with small proportions of ferrous hydroxide or oxide. When heated in the absence of air, all these substances form ferrous oxide (FeO), and may, without serious risk of error, be considered as though they consisted wholly of ferrous oxide. When a material containing ferric sulphide (pyrites, FeS,) is heated, it parts with half its sulphur, forming ferrous sulphide, which rapidly combines with any free silica present and this also behaves like ferrous oxide. Ferrous carbonate is often difficult to oxidise, even in a suitable atmosphere, and so does not always give a good red colour. It has a tendency to granulate and to produce black or dark brown spots, but if the clay is finely ground and the kiln is skilfully managed a fairly good red colour may be produced. Ferrous oxide is the lowest oxide of iron and has a bluish colour. It does not usually occur in the free state, but is produced in firing under reducing conditions, and then, on account of its low fusing point, it readily combines with silica, forming fusible ferrous silicates, which produce a blue or blue-black colour in the fired goods. The conditions under which ferrous oxide is produced are determined by the nature of the clay, the proportion of iron compounds present and the manner in which the clay is heated. The ferrous oxide probably exists as such for only a few moments ; it either combines almost immediately with any adjacent clay or silica, or if this is impossible it usually becomes oxidised to ferric oxide during the cooling of the material in the kiln. | Ferrous oxide is produced by the reduction of red ferric oxide by carbon monoxide and other reducing agents in the gases used for heating the material. If the reduced oxide is sufficiently abundant and properly distributed it forms a ferrous silicate and so produces the dark fusible material to which Staffordshire blue bricks owe their characteristic appearance. The reduction process is known as “ blueing”’; it may 104 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES be produced intentionally in a variety of ways, in some of which the use of free carbon, hydrocarbon, carbon monoxide, or possibly hydrogen, is required. This free carbon, carbon monoxide or hydrogen may be produced by the gases from the coal used in heating the kilns, or by the use of heavy residuum tar, juniper wood or other hydrocarbons. By whatever means either is produced, its action is to reduce the iron compounds present in the clay from the ferric to the ferrous state, in which they can combine readily with the silica and heated clay, forming complex silicates of a slag-like character. These molten slags rapidly fill the pores of the material, so that when the “ blueing”’ is complete the mass is no longer porous, but consists of a vitreous material, the pores of which have been completely filled with a tough glass or slag, the whole forming a solid mass of great strength and toughness. The manner in which the reduction takes place does not seem capable of any simple expression, but apparently the first stage consists in the production of magnetic oxide and this is later reduced to ferrous oxide or silicate, or possibly iron carbide (Fe,C) or carbonyl (FeCO) may be formed, at any rate in part, a considerable amount of free carbon being also deposited in the pores of the clay when oils, or certain special materials, are used to produce the blueing. The action of these reducing agents may be represented by four equations :— Fe.0,' 00), “=, S¥s0ee rome red gas black gas Fe,0,) - H, 2a ee red gas black water vapour FeO; 42) Vee > rere red ** smoke ” black gas 3Fe,0, + C = 2Fe,C + 9CO red ** smoke ” iron carbide gas Under the usual conditions of working, the gases which effect the burning contain about 25 per cent. of carbon monoxide, and if there is insufficient air to burn the gas to carbon dioxide, the burning gases will draw supplies of oxygen from the higher compounds of iron in the goods, thus converting them into the lower or ferrous state. In order to effect the complete blueing of bricks or tiles, the gas should be as clean as possible, as if it is charged with large quantities of soot the outside pores may be filled with deposited material which prevents the interior of the goods from being properly blued and the latter, if broken, will not be “‘ blue throughout.” The articles must also be sufficiently porous to allow the reducing gases to enter and effect the reduction. Another reaction which may aid in the blueing is the formation of magnetic oxide of iron by the partial reduction of red ferric oxide (p. 103) and the subsequent dis- sociation of this into ferric and ferrous oxides. At a sufficiently high temperature, ferric oxide can also dissociate, evolving atoms of oxygen from its molecules and forming the magnetic oxide 6Fe,0,=—4Fe,0,+ O,. The blueing is usually effected by the reduction of the ferric oxide, but if this fails BLACK AND CORED WARE 105 the kiln may be heated more intensely with as little admission of air as possible, in order to decompose any remaining red ferric oxide and convert it into the black magnetic oxide which subsequently decomposes and forms fusible silicate. Some of the magnetic oxide may also remain in the free state, in which case, as it is black, it adds to the dark colour desired. Although the process of blueing may be explained as due solely to the reduction of ferric compounds in the clay, such an explanation cannot be complete, because the colours of synthetic ferrous silicates are quite different from that produced in bricks by firing in a reducing atmosphere; the latter are probably coloured by carbonaceous matter (from the smoke produced by the fuel) as well as by the ferrous silicates, though carbonaceous matter alone, in the absence of iron, produces black, but not “blue” ware. If the temperature at which the blueing has been effected is not too high, the red colour of the ware can be restored by reburning in an oxidising atmosphere ; this reversal of colour may be repeated indefinitely if the temperature is carefully controlled and seems to suggest that the iron in blue bricks may be combined with some form or forms of alumino-silicate, in which its state may readily be changed from ferrous to ferric, or vice versa. On heating to higher temperatures, however, this compound (?) appears to be decomposed and irreversible silicates are formed. The effect of the size and distribution of the particles of ferrous iron compounds appears to be as important as in the production of a red colour by ferric compounds (p. 99). A uniform “ blue” colour is only produced when the iron compounds are in an extremely fine state of division and are uniformly disseminated through the material. When they occur in large pieces they form blotches of a blue-black material similar to those produced by coarse particles of ferric sulphide (pyrites) (p. 103). The effect of impurities in the clay (other than iron compounds) in the production of a “ blue ” colour has not been adequately studied, but so far as can be ascertained, lime, magnesia, and the alkalies do not affect the production of a blue colour unless present in such large proportions as to render the clay too fusible to enable it to keep its shape at the temperature required for blueing. Black ware and the black discoloration and cores are usually produced by ferrous and magnetic oxides, either alone or with unburnt carbon and are due to a deficiency of air when the goods are at a temperature between 500° and 900° C., particularly if the temperature rises very rapidly, so that the carbonaceous matter is not burned out before vitrification commences at the surface of the goods and prevents the proper oxidation of the carbonaceous matter in the interior. Such a state of affairs is due to unskilled management of the kiln and is considered fully in the author’s Clayworkers’ Handbook (Griffin). The colours produced by iron compounds do not always belong to one or other of the three classes mentioned on p. 97. In some cases, mixtures of different colours are obtained. Thus, if a clay containing sufficient iron oxide is burned in an atmo- sphere which is alternately oxidising and reducing, such as may be caused by the presence of an excess of air at some periods in the burning and a deficiency of air 106 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES at other periods, a variegated colour or “ brindled ” effect may be produced. In the oxidising atmosphere, the iron compounds produce a red colour, but when the atmosphere becomes reducing a part of the iron compounds may be changed into the ferrous state, this giving a blue colour. If the alterations are fairly rapid, the reduction or oxidation cannot be completed so that the resultant products contain some iron compounds in the reduced and some in the fully oxidised condition, thus producing a mottled appearance. When this effect is produced deliberately, it is known as “ flashing ’’ ; it is frequently employed in the production of certain classes of facing bricks and other goods and with small kilns worked at moderate temperatures it is not difficult to obtain very pleasing effects. The difficulties increase rapidly, however, with an increase in the temperature and size of the kilns. The great point is to secure the complete oxidation of the carbon in the clay (if any be present) at a sufficiently low temperature, so that it cannot combine and reduce the iron in the clay to such an extent that subsequent oxidation becomes impossible. Too rapid heating of the kiln when it is just below 900° C. is also the cause of thousands of facing bricks being spoiled, because the carbonaceous matter they contain is decomposed and “‘ set’ in such a way that it cannot afterwards be burnt out without spoiling the colour of the goods. Whilst this process of flashing is often intentional, to secure certain effects, it sometimes occurs when it is not wanted and is then regarded as a defect. In such cases, it is generally due to lack of skill in firing the kiln and especially to putting the fuel in the firebox in too large quantities at too long intervals, instead of smaller charges supplied more frequently. Carbonaceous Matter.—Next to iron, probably the most important colouring agent in raw materials is carbonaceous matter. The grey, bluish black, red, and other tints of unburned clay and other minerals, although usually due to mineral impurities, are, in some cases, of vegetable origin, the composition of the colouring matters being often imperfectly understood. It is not improbable that it is partly due to finely-divided coal, or to some dye-like material formed—like the brown colouring matter of peat—by the decomposition of vegetable matter. These organic colouring matters are destroyed on firing, so that a clay may be strongly coloured in the raw state and yet burn perfectly white. Ball clays are typical examples of materials containing a large amount of organic colouring matter. Some of them are dark blue or even black, but when heated to redness in an oxidising atmosphere they become almost white or pale buff, according to the other impurities present. Most natural clays, and many refractory materials, are coloured to some extent by carbonaceous matter. Further information on the colouring effect of carbonaceous matter will be found in Chapter X. Use is sometimes made of the fact that carbonaceous matter burns out in the firing, by mixing special bodies and pastes with some strong aniline dye, such as methylene blue, which will burn off in the kiln, but which imparts a distinctive colour to the material whilst in the raw or dried condition, but which does not affect the fired goods. By this means, the unfired bodies may be identified by the colour. COLOURS OF RAW CLAYS 107 CoLours oF Raw Cuays The colour of a raw material is no criterion as to its colour when burned. A clay which is grey or yellow when freshly dug may, on burning, give a good red colour, equally as well as a clay which is a red or brown when first found. This is largely due to the fact that the colours of natural clays is largely organic in character and so is destroyed when the clays are heated. It is also due to the fact that the iron com- pounds in clays are usually very pale in colour, or are almost colourless and only develop their full red or blue colour when the clays are fired. The two principal colouring materials in clays in the raw state are iron oxide and carbonaceous matter (pp. 97 and 106). Variations in colour in different strata do not necessarily indicate any appreciable difference in composition, nor is the colour of a deposit necessarily a reliable guide to the purity of the material, as the amount of impurity producing a certain colour may be very small in comparison with the colour it produces. The colour of a ceramic material may vary in different parts of the same bed on account of its proximity to other strata, or for other reasons. Thus, if a clay bed lies immediately below lignite, the portion in contact with the carbonaceous bed may be bleached by the reduction of the iron compounds, whilst the lower portion of the clay bed may be quite dark in colour. The exposed surface of light-coloured beds is sometimes brown, as a result of the oxidation of the iron compounds in the clay, and if a section is examined it may be found that the colour gradually becomes lighter and lighter, with increasing depth. The extent of coloration by oxidation is largely dependent upon the permeability of the material. Thus, an open material would be stained to a greater extent than a close-grained one, whilst the presence or absence of fissures and cracks would also affect the resultant colour. Very few clays are pure white when in the raw state, as the presence of only a very minute proportion of impurity may affect the colour to a considerable degree. Kaolin and china clay, when reasonably pure, should be white or pale cream in colour, but the tint varies irregularly with the amount of impurity present. Some less pure primary clays have a greyish tint on account of carbonaceous matter present, whilst others are slightly brown as a result of the presence of small proportions of peat or iron compounds. Some china clays are tinted pale blue on account of bright blue needle-shaped crystals of tourmaline present in them. China clay varies in whiteness to a considerable extent, according to the amount of moisture it contains. If a sample is dipped in water it may become grey or bluish in colour ; some china clays when so treated have a distinct yellow tint, though they are practically pure white when dry. _ Ball clays may be black, blue, brown, or white. The black is due to the presence of carbonaceous matter, some ball clays containing as much as 10 per cent. of carbon in the form of lignite or other organic matter. Many ball clays contain 3-4 per cent. of carbon, which is equivalent to a much larger proportion of carbonaceous matter. Some of the purest varieties of ball clays in 8. Devon are white, but the variety 108 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES known as “‘ white clay ’’ varies in colour from snow white to a pale straw tint. Other ball clays vary in colour in different beds, through grey, blue, brown, or chocolate colour to a deep black. The colours of the Dorset ball clays are equally variable. In each case, the greater part of the colour is due to carbonaceous matter, though some clays are tinted with iron oxide and hydroxide. If this is separated by a washing process, it usually produces a good quality of ochre. Many of the so-called “‘ blue”’ ball clays (including some of the best kinds of ball clays) are yellowish or grey in colour when freshly obtained, but when exposed for some time they become tinged with brown, though even after prolonged exposure they do not become much darker in colour and sometimes they turn rather paler. Some blue ball clays are spotted or mottled, but this does not affect their colour when burned. Ivory ball clays are creamy, drab or blue-grey in colour and are very similar to blue ball clays except that they contain more iron. When exposed to the weather, ivory ball clays differ from the blue variety in becoming “ rusty,” due to the oxidation of the iron salts present. Surface clays and brick earths are very variable in colour according to the impurities they contain. The commonest are yellow, brown, and blue; these colours are due to the presence of iron compounds. Thus, a yellow colour in clays may be due to the presence of ferric hydroxide (limonite, Fe,(OH),) or to the colour of the iron being rendered paler by the action of lime compounds disseminated through the clay (p. 102). The green colour of some raw clays may be due to the presence of iron silicates, such as glauconite. Many stoneware clays, when freshly dug, are grey, yellowish, or blue, the blue shade being due partly to the presence of ferrous compounds. Silt, warp, and similar beds are often of a light chocolate colour. The pocket clays of Derbyshire and Staffordshire vary very considerably in colour. Some are quite white, whilst others are yellow, red, purple, mottled, and even black. The whitest of these clays are not necessarily the purest, but owe their appearance to the presence of minute flakes of mica which make the material white and glistening. Fireclays and some shales are a grey or slightly bluish colour on account of the presence of organic matter which occurs in minute particles disseminated through them. Some fireclays are quite black and have bright cleavage faces. Near the surface, a clay may be nearly white with a slight tinge or mottling of red, grey, or yellow, but patches of strongly ochre-coloured material often occur at a slightly greater depth. Some shales, after being dug and exposed to the action of the weather, turn yellow or brown as a result of the oxidation of the iron compounds present. Some clays are naturally mottled on account of the irregular distribution of impurities in them; the “ mottling” usually disappears during the firing. CoLtours oF BuRNED CLAYS As previously explained, the colour of a burned material, and particularly of a burned clay, has no constant connection with its colour before firing, though in some cases it is possible to predict roughly what will be the colour. COLOURS OF BURNED CLAYS 109 The colour of burned clay depends chiefly, but not wholly, on the proportion of iron present. Clays which contain only a very small proportion of iron will be white or pale cream when fired, and, as the proportion of iron oxide increases, the colour will vary from primrose yellow through buff, red, brown, grey, blue, or black, whilst in some cases, it may be mottled on account of the irregular distribution of the iron compounds present. Table X XI shows the colours which may be expected from clays of various colours in the raw state, though, as already explained, it is impossible to predict the colour with certainty. Taste XXI.—Colour of Burned Clay Colour of Raw Clay. Probable Colour of Burned Clay. Red. ; ; Red. Deep yellow . Buff or red. Chocolate . Red or reddish brown. White . ; ; White or yellowish white. Grey or black : Red. buff or white. Green . ; : ; Red. Red at first, then cream yellow, buff, Red, yellow, or grey (calcareous) . or white, and then greenish yellow when becoming viscous. Kaolin and china clay, when pure, are perfectly white after burning; if they contain a very minute proportion of iron compounds, the burned clay has a pinkish or reddish tinge. For some purposes, a slight discoloration is not of great importance, as it does not appreciably affect the refractoriness. Ball clays are white or cream in colour when burned. Ivory ball clays generally become a yellowish buff when burned, as they contain a larger proportion of iron oxide than the purer varieties of ball clay. Brick earths, when burned, vary considerably in colour, according to the con- ditions mentioned on p. 96. Red-burning clays owe their colour to the presence of iron compounds and the absence of more than very small proportions of other im- purities. Some of these clays are renowned for the beautiful red colour which they assume when burned. Amongst these are the Ruabon clay, the “red marls” of Staffordshire and Leicestershire, and the red-burning clays of Shropshire, Lancashire, and North Wales. Other good red-burning clays occur in Hampshire, Berkshire, Nottinghamshire, Leicestershire, Lancashire, and Yorkshire. Good colours may be obtained with many other clays, but the ones mentioned are usually the most pleasing. Bagshot clays are well known for the excellent red colour they produce, whilst 110 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES the Oxford clays are somewhat lighter. The Midland and Western clays give varying shades, most of them burning to a red colour. The majority of surface clays are red when fired, though there are some exceptions. Many shales burn to a red colour, though others are much lighter when fired. The value of some red-burning clays is dependent on the absence of scum, specks, blisters, etc., which sometimes accompany and spoil what would otherwise be a good colour. Black, white, or cane-coloured spots also occur and spoil the appearance of some red-burning clays. The best clays give a good red of uniform character over the whole brick, but some architects and builders prefer bricks, etc., with an irregular, spotted or mottled appearance due to the presence of iron compounds in a coarser state than that which produces bricks of a uniform red tone. Thus, some of the Humber silt produces bricks of very variable colour, all shades from dark purple to dirty white, though blue, red, and yellow are sometimes found in the same brick. When these varied colours form a pleasing combination the clays may be valuable, but, as the tints cannot usually be regulated, the risk of producing unsaleable goods is often very great. Facing bricks, terra-cotta, floor tiles, roofing tiles and some coarse pottery are dependent on the production of a pleasing red colour when fired at a moderate temperature and the value of the clays used for making these articles depends on the quality of the red colour produced. If too high a temperature is required to develop the colour, the production of the goods will be costly and in most cases, the colour will not be very pleasing. K. Orton 1 has stated that a good red-burning clay should have a yellow, red, or salmon colour at 900° C. and should attain maximum brilliance at about 1100° C. Many of the bricks and tiles of the Midlands and North of England are usually burned at a higher temperature than is required to produce the most pleasing tint ; this is done to secure increased strength and a less permeable product which will remain “clean ” longer than a very porous brick. W.G. Worcester 2 has stated that a good roofing tile clay should give the following colours at the temperatures shown in Table XXII :— TaBLE XXII.—Colour of Roofing Tiles Colour. Cone. Temperature, ° C. Immature and high red colours . ’ Up to 06a Up to 980 Commercial red ; : : : ; 05a-la 1000-1100 Overmature red or brown with body still sound 2a—ta 1120-1160 Blue or black colours with failure of body . 5a and over | 1180 and over Many manufacturers, unfortunately, place the colour of their goods before any- thing else and will even sacrifice durability in order to obtain a certain “ saleable 1 Trans. Amer. Cer. Soc., 5, 413-415 (1903). 2 Geol. Survey of Ohio, Bull. 11, p. 102. COLOURS OF BURNED CLAYS 111 tint.” There is much excuse for their doing so when there is a ready demand for their goods, but it is unfortunate all the same. The result of this is that many bricks have been very imperfectly “baked”’ in the kiln, the heat being merely sufficient to develop the required colour and no more. Such bricks may last a long time under favourable circumstances, but they cannot compare for durability with those which have been heated to the point of incipient vitrification, where the particles of clay are bound together by the molten particles of the more fusible constituents of clay. Blue and black bricks and tiles are usually formed by burning clays containing a sufficient proportion of iron compounds in a reducing atmosphere (p. 102), though they may also be due to the presence of manganese dioxide occurring naturally in a clay or added thereto. Black bricks are sometimes produced by the deposition of carbon in the pores of articles, the particles being subsequently fixed by the fusible matter on the surface of the goods. Crucibles, etc., composed of a mixture of clay and graphite are also black. A purple colour on bricks and tiles is sometimes a result of the partial reduction or of the decomposition by overheating of red ferric oxide in the clay. Some clays yield this colour more readily than others, but it can sometimes be produced by adding a little manganese dioxide, coke-dust, or even ashes, to a clay, though no artificial mixture is quite reliable. It is usually found that only a small proportion of the goods fired in a kiln possess the desired purple colour ; the others may be blue, brindled, or red. A mottled or irregular colour may be due partly to the composition of the material and partly to the method of firmg. With materials containmg much combustible matter—whether in the form of cinders added under the name of “ soil,” or of material naturally present as “ organic vegetable matter”’ or “ shale oil ”—a certain amount of irregularity of colour is practically unavoidable unless the means used to mix the material are so complete and the combustible matter is so fine, that a perfect distribution can be effected. In most cases, in the parts of the goods where the combustible matter is most prevalent, it will burn without a sufficient quantity of air and will consequently take the oxygen from any iron compound in the immediate vicinity, provided that such oxygen is available. This will necessarily lead to a change of colour in certain portions of the goods, for the reduced iron compound will be bluish or even black, whilst the fully oxidised one is red. This kind of irregularity in colouring can only be avoided by so fine a grinding and so thorough a distribution of the combustible matter as is quite unattainable in commerce, and it is a fortunate thing that many of the irregular colours produced are so effective when the bricks, tiles, etc., are in use, that they are actually sought for by architects and others. In the absence of sufficient combustible matter in the goods, a mottled or irregular appearance may be produced by alternate reducing and oxidising atmospheres in the kiln. By repeatedly changing the nature of the atmosphere the iron compounds are partly reduced and partly oxidised, this giving a mottled or flashed appearance which is highly prized in goods made for some purposes (p. 106). The speckled bricks so valued in America are made of light yellow clays containing 112 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES manganese dioxide, which causes small black spots to appear on the surface of the bricks. Further information on mottling and variegated colours will be found on p. 106. Whiteware.—The term “ white” is often used very loosely in clayworking, and is used to include all shades from a true white to a distinct cream or even a pale yellow colour. White goods, when made of clay, are usually formed of white-burning clays, etc., of great purity, or of a mixture of china clay, ball clay, flint, Cornish stone, felspar, etc. When such clays or mixtures are too costly to be used to form the whole article, the latter may be made of a cheaper clay which is buff or red when burned, but is made to appear white by covering it with a white-burning mixture, such as that just mentioned. This process is known as engobing or bodying (p. 96). Some clays containing a large amount of calcium carbonate, as well as a con- siderable proportion of iron compounds, are quite white when burned. They are used for making white bricks in Cambridgeshire, Norfolk, and Suffolk and to a smaller extent in Sussex. They owe their whiteness to the combination of the iron with the lime, silica, and alumina in the clay. Gault clays (which sometimes contain as much as 35 per cent. of calcium carbonate) produce nearly white bricks. When the proportion of calcium carbonate is high, gault clays are best mixed with sufficient red clay of another formation so as to reduce the proportion of calcium carbonate in the mixture to not more than 25 per cent. Some of the white bricks in Suffolk are made by mixing a red-burning clay with a sufficiently large quantity of chalk in a wash-mill, and some of the red surface clays of some parts of Yorkshire are made to yield a whitish brick by mixing them with magnesian lime (from dolomite) in a slaked condition. These clays are naturally wet when dug and the lime is valuable, as it absorbs the moisture in the clay without the necessity of drying it before grinding. The effect of minute proportions of colouring agents naturally present as impurities in white-burning materials and also the effect of the atmosphere in the kiln, are clearly shown in Table XXIII, due to W. H. Yates and H. Ellam.t It may be explained that the bescwit ware is the unglazed material ; the glost ware is the biscuit ware which has been covered with a transparent, colourless glaze and re-fired at a lower temperature ; the results shown in the last column were obtained by reheating the ware at about 900°-1000° C. among articles which had been decorated with over- glaze colours (p. 96) which required to be fired at this temperature. When bone china ware is fired under reducing conditions it may assume a bluish shade, which is attributed by Moore and Mellor to the partial dissociation of the bone ash (calcium phosphate) and the resulting formation of ferrous phosphate. A larger proportion of ferrous phosphate is formed, according to J. W. Mellor,? when the china ware is deficient in Cornish stone or contains too large a proportion of clay. On the other hand, the higher the proportion of alkali the less is the liability to form ferrous phosphate. 1 Trans. Eng. Ceram. Soc., 17, 120 (1917-18). 2 Tbid., 18, 497 (1918-19). COLOUR OF WHITE-WARE 113 Taste XXIII.—Effect of Composition and Firing on Colour of Bone China Ware Per cent. of Ingredients. Colour after passing Pe | sa, Colour of Biscuit. Colour of Glost. through Enamel Kiln. Clay. | Ash. | Stone. 100 0 0 | Cream. Cream. Cream. 80 20 0 | Brown. Slightly paler. Brown increased. 80 0 20 | Yellow. Shade lighter. Brown. 60 40 0 | Pink. Red brown. Red. 60 20 20 | Light brown. Dirty brown, Slight, where glazed thick. 60 0 40 | Cream. Yellow. Brown. 40 60 0 | Green. Pale green. Still paler. 40 40 20 | Pale green. Green much in- | Brown in patches. creased. 40 20 40 | Yellow. Yellow. Yellow. 40 0 60 | Slight brown. Slight brown. Slight brown. 20 80 0 | Brown. Shade lighter. No change. 20 60 20 | Colour very slight. | Colour very slight. | Colour very slight. 20 40 40 4 is “ is - 20 20 60 ¥ ; 53 = 3 ms 20 0 80 | Tinge of green. Tinge of green. Tinge of green. 0 | 100 0 | White. White. White. 0 20 80 a = C 0 60 40 * Pe - 0 40 60 _ . 4 0 20 80 . S a 0 ee 00 5 > » It is extremely difficult to produce perfectly white goods, as even minute amounts of iron oxide have a strong power of discoloration. The latter may be minimised when it only occurs to a very small extent by (a) the use of a reducing fire during part of the period of burning, or (b) by adding a suitable coloring agent to neutralise the colour and so produce a pure white ware. Thus, the addition of a minute pro- portion of cobalt oxide satisfactorily corrects a faint yellow colour in the wares. This is due to the fact that yellow and blue colours are complementary and neutralise each other, producing an almost perfect white, if the total amount of colour is not too large, otherwise a green colour is produced. The action is the same as in the use of “blue” in laundry work. The prepared cobalt oxide may be added either to the 8 114 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES body or to the engobe or glaze placed upon it. The latter is the cheaper, but the former gives the most dependable results, the cobalt being added either in the form of a very fine powder or of a solution of cobalt chloride or sulphate in water. If the solid material is used, it must be ground extremely fine or it will produce minute blue spots instead of making the ware white. Instead of adding the cobalt oxide direct, it may with advantage be previously mixed with finely-powdered flint or, preferably, with china clay, calcined, and the product ground to an impalpable powder. If a solution of a cobalt salt is used, care should be taken that it is not precipitated by any free alkali in the clay prior to the colour and clay being mixed, or it will form blue spots. When the solution of the cobalt has once been thoroughly distributed through the mass, it may, with advantage, be rendered insoluble by the addition of a solution of sodium carbonate. Yellow goods (apart from those artificially coloured) may owe their colour to: (a) The presence of only a small proportion of iron compounds, as in fireclays. (6) The presence of finely-divided iron oxide simultaneously with free alumina (p. 101). (c) The presence of a considerable proportion of chalk or other form of calcium carbonate (p. 102 ) in clays burned in contact with coke or other organic matter. Thus, true marls or malms are yellowish or whitish when fired, on account of the lime present in them, the London malms burning to a rich brimstone colour. Buff-coloured Articles.—The buff colour of bricks is due usually to the presence of iron compounds in some form, but the cause of the colour in bufi-burning clays is far less clearly understood than that of either the red or white-burning clays. The iron content varies from 0-50 per cent. to 4 or 5 per cent., with an average about 1-5. Buff-burning clays do not burn buff because of the exact amount of iron they contain. So far as the iron content is concerned, they might burn either red or white, and other conditions are far more important than the exact proportion of iron present. Some- times the buff colour may be due to the effect of the iron compounds partly being reduced in the presence of lime compounds or free alumina (p. 101), though this is not the case with fireclays, unless it is correct to assume that their buff colour, when burned, is due to a compound of iron and alumina. Fireclays are usually buff colour when burned, but it has never been satisfactorily proved that this is due to the presence of iron compounds. The greater part of the iron in fireclays is in the form of pyrites (p. 103), which forms black spots or blotches or patches of a dark-brown colour. Some fireclay articles have a reddish appearance (termed “ flashing,” p. 106), due to the oxidation of iron compounds derived from the ash of the fuel used in firing. This usually occurs when an excessive supply of air is allowed to pass through the kiln during the cooling period. The colour of fireclay articles must not be regarded as a criterion of their quality, as a dark-coloured brick, which, from its colour, might be rejected as too impure, may be more durable than a light-coloured one. Many engineers, architects, and builders consider that a fireclay article which has a good uniform buff colour is the best, their idea being that it has been burned at a high temperature without showing any signs of being affected by the heat. This is an entirely erroneous idea, for ARTIFICIAL COLOURS 115 underburned firebricks are light coloured, and those which have been intensely heated are usually highly discoloured. In fact, a fireclay article of apparently poor quality, which is covered with blotches and much discoloured, may be better than one of a pleasant, pale-cream colour. In other words, discoloration is usually a sign of a high firing temperature, and when a firebrick has been heated to a tem- perature sufficient to produce blotches without the article itself being fused or warped, it is usually reasonable to suppose that it will be able to withstand that temperature when in use, whereas pale-coloured goods which have not been heated so strongly may fail in use. The presence of minute dark blotches of “slag” in firebricks are of little con- sequence and are not detrimental to their quality, unless very abundant. Even then, if they occur chiefly at the surface, and are not abundant in the interior of the article, they will do little damage, though they create a very unpleasant appearance. Some users insist that the area of-the dark spots in firebricks shall not exceed 3 per cent. of the cross-section or face of a firebrick. Clays which do not naturally produce a buff colour when burned may be made to do so by (i) destroying the colour of a red-burning clay by adding a large pro- portion of chalk or limestone (p. 102); (ii) by adding iron in the form of a silicate- mineral such as granite, schist, talc, etc., to a white-burning clay; (ii) by adding an iron solution to a clay slip and precipitating the iron by the addition of soda solution ; and (iv) by adding a stain containing 95 per cent. of alumina plus 5 per cent. of oxide of iron, derived from ferric chloride solution by precipitation, as in (iii). This last gives the best results, but is costly, and to get the best effects the body must be heated to vitrification. Brown goods are often produced by iron oxide in a clay, the conditions of firing being such that instead of the iron compounds attaining their full red, as in red bricks (p. 109), the colour is either partially developed or it is converted into a brown by overheating. The imperfect development of the red colour may be due to the presence of lime, alumina, and other substances in the clay, or to vitrification having set in before or after the red tint was fully. developed. The brown colour of some fired clays is due to the presence of manganese compounds. ARTIFICIAL COLOURS Artificial colours are produced by the use of various chemical substances which when heated to a suitable temperature assume the desired colour. The temperature to which the articles are heated must, therefore, be one which suits the colours. Thus, if the ware requires a high temperature for firing and the colour requires a low one, the ware must be burned first, then coloured and refired at a lower tempera- ture. If, however, the colour will stand the temperature required to fire the ware, the latter may be first coloured and then fired. Where it is desired to mix a colour with the material of which an article is made, that colouring agent must necessarily be able to withstand the temperature at which the article is fired. The finer colours and those produced by expensive chemicals are usually mixed 116 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES with an engobe (p. 96) or glaze, and are then applied to the ware; to mix such colours with the whole of the clay of which the articles are made would not only be excessively costly, but would be an unnecessary waste of colour. Several firms supply colours prepared ready for use by potters and others. Black articles—or the nearest approach to a true black which can be obtained in pottery manufacture—may be produced by : (a) The addition of a mixture of manganese dioxide and iron oxide to the clay or engobe. Where there is sufficient iron oxide in the clay, the addition of a suitable proportion of manganese dioxide is usually sufficient. The precise colour produced depends on the relative proportion and fineness of the two colouring agents. A red- burning clay with the addition of about 14 per cent. of manganese dioxide will usually produce a beautiful jet black. It is important to use fine precipitated manganese dioxide ; that produced from spent material used in the manufacture of chlorine, etc., is much less satisfactory. If the colour is too brown or violet, the proportion of manganese dioxide should be increased or the iron oxide reduced. (6) The addition of a mixture of iron and cobalt oxides. (c) The addition of iron, manganese, cobalt, and chromium oxides to the clay or engobe. Grey may be produced by the use of smaller proportions of the same materials as are used for blacks (supra), but the iron must not be present in very large pro- portions when light shades of grey are required. Greys may also be produced by means of iridium oxide or platinum chloride or both, but these are so costly that their use is confined to the most expensive pieces of art ware. Blue colours are produced on white wares by the addition of cobalt oxide to the clay or engobe. The shade of colour is largely dependent upon the alumina present, a highly-aluminous body giving a sky-blue tint, whilst a siliceous mass, or one containing zinc oxide, usually assumes more of an indigo shade. Blues may be modified by varying the proportion of cobalt and also by the addition of other sub- stances, such as alum or frits.1 Violet colours are extremely difficult to obtain; the best are usually produced by a mixture of chromium and cobalt oxides. Under suitable conditions, precipi- tated manganese dioxide produces an excellent violet. Green colours are generally produced by the addition of chromium or nickel oxides to bodies containing only a very small proportion of iron oxide, as otherwise the colour will not be pure green. Chromium greens are the most easily produced, and may be modified towards blue by the addition of cobalt oxide. Nickel greens are somewhat uncertain, especially at temperatures above 1100° C., when no other ingredient is used, but with cobalt oxide a fairly reliable olive-green is obtained. At temperatures above 1200° C., iron silicates impart a greenish-yellow colour 1 A frit or fritt is a partially fused mixture of two or more substances, some or all of which could not be used separately on account of their solubility in water. In the process of fritting, chemical combination occurs and insoluble substances are produced. The process of fritting is also used to distribute a small proportion of a strong colouring agent in a large proportion of white or colourless material, so as to produce a lighter tint than would otherwise be possible. ARTIFICIAL COLOURS 117 to the ware, whilst when such silicates fuse they become quite green and produce the colour of green bottles. Yellow colours are, where possible, produced almost entirely by iron oxide, the tints varying from yellow through orange to yellowish brown. For high-class pottery, titanium and antimony oxides or lead chromate may be used. For orange- yellow tints, uranium oxides may be employed. For these tints an oxidising atmosphere is essential, as a reducing atmosphere gives a greenish-grey shade. Red colours, produced at comparatively low temperatures, are usually due to iron oxide. In vitrified wares, iron oxide seldom produces a pleasing red shade. A rose or pink colour may be produced by the addition of a finely powdered frit (p. 116) of bichromate of potash and alumina, whilst a lilac tint may be obtained by the addition of a little cobalt oxide. Pink glazes may be produced by using a mixture of chromium and tin oxides. Occasionally bricks are made to appear red by dipping them in, or painting them with (just prior to sale), a slip prepared by mixing Venetian red with water into a pulp, which is pressed through a sieve to break up lumps that are formed in mixing, and then adding enough stale ale or beer to make the stain of a proper consistency. To each gallon of this mixture is added one quarter of a pound of calcined iron sulphate, previously beaten up with a portion of the stain to a thin batter. This is the mordant or fixture, without which the stain would finally wash off from the effects of the rain. A more durable and permanent stain is made with Venetian red that has been ground with linseed oil to form a stiff paste, or, if the stain is to be of a lighter shade, a mixture of Venetian red and French yellow ochre, both ground fine in linseed oil and beaten up with a small portion of a good turpentine japan to a smooth semi- paste, gradually adding in small quantities while stirring, a mixture of one part (by measure) of 90 degrees benzol or good solvent coal-tar naphtha and four parts (by measure) of turpentine, until the proper consistency of stain is secured. The liquid is strained through cheese-cloth and the coarser particles thrown away, as these would remain on the surface and be of no benefit in sealing the pores of the brick. An excess of oil in the stain is apt to produce “shiners,” but has the additional advantage of rendering the bricks waterproof. These pastes are paints rather than ceramic colours. Brown colours are obtained with iron oxide at a temperature higher than that necessary to produce a good red. This treatment is usually accompanied by a variable amount of vitrification. For the lower temperatures a good brown shade may be obtained by the addition of a little manganese dioxide to a ferruginous body. Thus, the addition of 0-5 per cent. of fine manganese dioxide will produce a beautiful chocolate tint. Browns may also be obtained by (a) mixtures of iron and chromium oxides, (6) iron chromate, (c) manganese and chromium oxides, (d) a frit composed of zinc sulphate and chromium oxide. To secure the desired colour when special colouring agents are employed, it is most important that the right atmosphere should be maintained during the firing of the ware and that the right temperature should be reached, but not exceeded. 118 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES Excessively high temperatures may cause volatilisation of the colour. Very often in producing coloured goods, different effects are obtained on account of variations in the state of the atmosphere in the kiln. Thus, articles intended to be buff may be streaked with red, red ones may be changed to chocolate colour or purplish black, whilst blue ones may be considerably deepened. These accidental shades of colour are often of great beauty, but cannot always be produced as and when desired. The effect of the atmosphere in the kiln upon various colours is shown in Table XXIV, due to Le Chatelier and Chapney. TaBLeE XXIV.—Effect of Heat on Colours Compound of | Firing | Temperature, Colour produced in Colour produced in Metal in Colour. | Cone. ibs Oxidising Atmosphere. | Reducing Atmosphere. Chromium ./ 13 1380 Violet, blue, green, gold, orange, or red. Cobalt . ; Fr ks ae Blue, green, or rose. Copper . ; Bs . Blue, green, gold, or red. Tron. ; s, as Gold, red. Blue, green. Manganese . MA Fr Violet, blue, green, < gold, red. Nickel . : is é Violet, blue, green, gold, red. Titanium. a “ oe Violet, blue, green, gold, red. The volatilisation of some colours also produces a vari-coloured effect (often of great beauty). Volatile chlorides are largely used for the production of this class of ware, lead chloride being commonly employed. By the use of a chlorinated atmo- sphere, such as is produced by this means, cobalt oxide will give a blue colour, nickel oxide a brown one, copper oxide green at low temperatures, and iron oxide a very unpleasant yellow. The production of colour effects by partial volatilisation is known as “ flowing.” Streaks of colour are often useful as a form of decoration. They are sometimes caused by firing a glaze to such an extent that it vitrifies and flows; by this means very beautiful marbled effects may be obtained. Variegated colour effects may also be obtained by the process known as “ flaming ” or “‘ flashing ”’ (p. 106). THE CoLouRS oF CERAMIC MATERIALS OTHER THAN CLAY The colour of ceramic materials other than clay is not often of great importance except as a very rough indication of their quality, though it should be remembered COLOURS OF NON-PLASTIC MATERIALS 119 that the remarks‘on p. 114 with reference to the relation between the colour and properties of fireclay bricks apply to most other ceramic articles. Siliceous Materials.—Pure quartz is colourless, but natural quartz is often rendered partially opaque by numerous “ inclusions ”’ or bubbles, and it is frequently tinted by iron oxide and other impurities which impart to it a pink, yellow, brown, or purple colour ; sometimes it is so dark as to appear almost black. Quartzites, if quite pure, would be colourless, but they are invariably tinted by traces of iron and other oxides and so vary from white to a dark brown the purer and more refractory qualities being almost colourless. Many quartzites which appear to be opaque or coloured, consist of transparent, colourless grains, the opacity, and to some extent the colour, being a mass-phenomenon. Ganister varies from grey to dark brown in colour according to the proportion of carbonaceous matter, iron oxide, and clay present. The individual grains are transparent and usually colourless. Flint varies in colour from grey to almost black, apparently on account of the carbonaceous matter present in a very fine state of division. The exterior of flints . is usually buff. Chert is similar, but much lighter in colour, as also are chalcedony and other forms of amorphous silica. Chalcedony is generally white, grey, pale blue, bluish white, or brown in colour. Kieselguhr varies greatly in colour according to its purity. The best qualities are white and are composed of colourless grains, but inferior deposits are often coloured deep red or brown by ferric oxide. In some deposits of kieselguhr the red colour of the iron oxide is masked in the raw state by carbonaceous matter, which imparts a greyish tint to the material. On calcination, the carbonaceous material burns away and the red colour is restored. The kieselguhr at Naterleuss, in Germany, is coloured green by the presence of a large proportion of carbonaceous matter. When heated to bright redness in an oxidising atmosphere, quartzite, ganister, and other siliceous materials are usually white, the less pure ones being pale yellow or buff and often contain dark-brown spots of iron compounds. Carbon.—Grraphite is greyish black with a metallic lustre. Coke has a steel-grey tint. Their colour is not appreciably altered by heat, but in the presence of air they are gradually converted into a colourless gas. Carborundum varies in colour according to its form. The crystals vary from pale yellow to grey or blue-black, and the amorphous variety, known as firesand, is white when pure, but the commercial material is usually green, grey, or nearly black with a bluish sheen. The colour is not affected by heating to redness. Silundum, which is formed in a similar manner to carborundum but at a tem- perature of 1300°-1800° C., is greenish or slate-coloured, but becomes steel-grey when heated above 1800° C. Bauxite, when pure, is white, but as some iron compounds are generally present its colour is sometimes pale grey, yellow, or even brick-red. The much rarer blue bauxite appears to owe its colour to colloidal ferrous sulphide. 120 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES When heated to bright redness in an oxidising atmosphere, bauxite varies from nearly white to a reddish or brown tint according to the iron oxide present. Crystalline magnesite is colourless, white, or yellowish grey. The crypto- crystalline magnesite is white, yellowish, or brown. Hydromagnesite is usually white and resembles chalk. Breunnerite is generally grey or yellow when freshly cut, but on exposure the iron compounds present are oxidised and impart a brownish colour to the rock. When heated to bright redness, pure magnesite remains white, but much of the natural mineral is coloured buff or reddish brown by the iron oxide and other im- purities present. When most samples of pure magnesite are heated to 1500° C. or above (2.e. when they are dead-burned), they remain white, but in most commercial samples of natural magnesite the effects of the colouring agents are accentuated and the produce is buff, red, reddish brown, chocolate brown, or black, according to the nature and proportion of impurities present. When different samples from the same deposit are compared, their colour is a useful indication of the extent to which the magnesite has been heated, though the change in colour is much less marked with the purer magnesites. Thus, a properly dead-burned and sintered magnesite containing about 4 per cent. of iron oxide is a dark chocolate-brown colour, but if the same rock has not been heated so intensely it is much lighter. The darkest colours are usually due to a partial reduction of the iron oxide or to the presence of manganese compounds. The difference in colour between pure and slightly ferruginous magnesite is clearly shown in magnesite bricks. Those made from the Styrian magnesite, which contains ferric oxide, are reddish to dark brown or black, whilst those made from the much purer Grecian magnesite are cream, buff, or other light colour, and frequently have numerous dark- brown spots. Dolomite varies in colour from pale cream to yellowish brown according to the amount of iron oxide present. The colour of calcined dolomite and of any bricks, etc., made from it, is usually yellowish brown. Lime, when almost pure, is perfectly white, but portions of commercial limes are usually coloured slightly by iron oxide or charred carbonaceous matter. Refractory bricks and blocks made of lime should be almost pure white. Zirconia.—The natural mixture of zirconia and zircon found at Sao Paulo, Brazil, varies from grey to bluish black, according to its purity. It is a curious fact that the ore richest in zirconia is almost jet black, because pure zirconia is a brilliant white. The baddeleyite found in Ceylon varies in colour from white to grey-brown, bluish black, or dark green. Natural zircon is usually tinted yellow by the iron oxide present, and some specimens are grey-green or red. The colour of pure zirconia when calcined is white if the temperature has not exceeded 1500° C., but above this tempera- ture various colours are developed as a result, according to Ruff and Lauschke, of the formation of nitrides, lower oxides, and black zirconium carbide. When titanium oxide is present in zirconia, a bluish colour is developed at a tem- perature of about 1500° C. DISCOLORATION 121 DISCOLORATION The term “ discoloration” is applied to materials or articles possessing colours of an undesirable character upon their surface or in their interior. Such a defect is due, in the main, to the same causes as the colours previously mentioned. The principal causes of discoloration are :— (a) Insoluble substances. (b) Soluble substances or ‘‘ scum.” Other discolorations may frequently be traced to :— (c) Substances derived from the fuel and present in the kiln gases. (d) Substances volatilised from articles adjacent to the ones which are discoloured. Discoloration by insoluble substances includes several colouring agents mentioned in preceding pages which produce an undesirable appearance. Blacks pots are usually due to ferrous or manganese compounds (p. 111), but some- times white ware fired in carborundum saggers is discoloured by grey, red, or black stains, due, according to H. Spurrier,! to the production of volatile ferro-carbonyl compounds. _ Blue discolorations in china are often due to ferrous phosphate (derived from bone ash containing carbon), a deficiency of alkalies in the body, or to the action of reducing gases. Brown discolorations are usually due to ferric compounds (p. 97), including absorbed vapour of ferric chloride and also the discolorations in china due to ferric phosphate. Some pieces of white china become discoloured with brown patches on prolonged exposure to air ; these discolorations are also attributed to ferric phosphate. For information on brown discolorations of firebricks, see p. 114. Ferrous phosphate is sometimes white, but on exposure to air it becomes blue or green, and finally develops a brownish crust by oxidation. See also Brown scum, p. 123. Green discolorations may be produced by copper and vanadium compounds ; thus, cupriferous pyrites produces greenish slag spots and vanadium molybdate produces irregular green patches. Green discolorations may be due to the slight deposition of soot on the goods or to the causes of black discolorations operating on a smaller scale. Pink discolorations on biscuit ware or buff terra-cotta often indicates that the ware has been heated too rapidly below 700° C., so that the combined water has not been- driven off properly. The red discoloration in some hard porcelain, known as la malade jaune or jaune de cuisson, is due, according to B. Moore and J. W. Mellor,? to the presence of ferric oxide and of oxidising conditions in the first stages of burning. Seger found that a red’ discoloration which occurs on a yellow-burning clay may usually be cured by alter- nately heating in a very smoky kiln (2.e. in a strongly reducing atmosphere) for some time and then in an oxidising atmosphere, at intervals of eight hours, so as to cause the 1 J. Amer. Cer. Soc., 4, 923 (1921). 2 Trans. Eng. Ceram. Soc., 16, 58 (1917). 122 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES sulphur to be evolved as sulphuric acid and to reduce the iron to a ferrous or to a ferro-calcite state. Yellow discolorations are often due to ferric compounds (p. 97). For means of correcting a yellowish tinge in china and white earthenware, see p. 113. Scum is a defect known by a variety of names, such as “‘ whitewash,” “ nitre,” “salt,” “mould ”’ (all of which are incorrect), and “‘ cfflorescence,”’ ‘‘ wall white,” etc., which more accurately describe it. The causes and cures of scum have been investigated in great detail by a number of people in recent years, and whilst it is by no means easy to find the cause and remedy for it in any particular case without a considerable amount of investigation, it may generally be said that scum is due to salts contained in the clay or other rock, in the water used in the course of manufacture, or to condensation products settling on the goods whilst in the kiln, especially if a continuous kiln is used. Less frequently, scum is produced on bricks, tiles, etc., a considerable time after they have been taken from the kiln, and the cause will then usually be found to be improper storage on ground saturated with soluble salts, or to the use of mortar of an unsuitable kind. Briefly expressed, the main sources and cures of scum are as follows :— 1. Soluble salts occurring naturally in the material itself. 2. Soluble salts produced from other ingredients in the material as a result of “ weathering.” 3. Soluble substances deposited on or formed in the material during the burning, either by condensation products from the kiln gases or by some chemical reaction taking place within the kiln. 4. Soluble salts contained in the mortar used in erecting a building of the bricks, etc. 5. Soluble salts developed by interaction of the mortar and bricks or tiles. 6. Soluble salts in the water used in the manufacture of the goods, or in building. 7. Soluble salts in the ground on which the goods are stacked before or after sale, or in ashes and other materials in contact with the goods. White scums are the commonest and include “ dryer white;” “‘ kiln white,” and “wall white,” all of which are formed by the accumulation of soluble salts on the surface of the goods before, during, and after firing respectively. They are chiefly due to the presence of calcium, magnesium, potassium, sodium, ferrous or aluminium sulphates, and, occasionally, to other salts such as chlorides and nitrates. These salts are of more frequent occurrence in weathered clay than in a clay or rock which has been freshly dug or quarried. Very small percentages of some of these salts— especially soda, potash, and magnesia—are sufficient to cause an objectionable amount of scum, and cases have been known where as little as 0-01 per cent. of sodium sulphate has spoiled the surface of facing bricks. White scums are more common with surface clays than with those of greater age, as the former, being nearer to the surface, more easily become impregnated with various soluble salts, whereas deposits which occur at a greater depth have usually been subjected to some natural leaching action, whereby the soluble salts have been removed. COLOUR-MEASUREMENT; HARDNESS 123 “ Kiln white ’” consists chiefly of calcium sulphate with small amounts of mag- nesium and alkaline sulphates, and occasionally a trace of alum. It is formed by the action of sulphurous acid in the kiln gases on the lime, etc., in the clay. A brown scum on some fired goods may be due to the presence of soluble iron salts formed by the oxidation of pyrites in the clay or rock to ferric sulphate which, being soluble, rises to the surface of the moist material by capillary action during the drying and is converted to ferric oxide during the firing. A grey scum is sometimes formed if calcareous water comes into contact with a clay. Calcium and magnesium sulphates, when present in a clay, also tend to impart a drab appearance. A yellowish scum may be formed by the interaction of sulphuric acid in the kiln gases with the alumina, lime, and silica in the clay at temperatures approaching to that of vitrification. Seger found a yellowish scum to be produced by the presence of soluble potassium vanadate. According to Kallauner and Hruda,! as little as 0-1 per cent. of vanadium oxide causes discoloration and as little as 0-001 per cent. may cause scumming. The discoloration and scumming may be reduced by the addition of barium carbonate, chloride, or nitrate, which form insoluble barium salts. An excess of barium chloride or nitrate must be avoided or it may increase the scum instead of reducing it. The yellow-green staining sometimes attributed to vanadium may, according to C. W. Hill,? be due to ferrous salts. CoLtouR MEASUREMENT The measurement of the colour of clays and other rocks, and of the products made from them, has not reached a stage where it can be done accurately. Fortunately, it is seldom necessary, and such comparisons as are required may usually be satis- factorily judged by the naked eye if care is taken to avoid unsuitable lighting. Transparent materials may be compared by means of a colourmeter (Chapter XV), but this is not completely satisfactory, and is quite unsuitable for opaque materials. HARDNESS A comparison of the hardness of clays and many refractory materials, both in the raw and fired states, is often a matter of great difficulty, because these materials are not strictly homogeneous and different portions of them have different degrees of hardness, so that no single figure can accurately represent the hardness of the material. For instance, the outside “‘ skin ” of a brick or tile is usually much harder than its interior; many firebricks are composed of mixtures of burned clay and quartz, which substances differ greatly in hardness and in many articles, the bulk of the material or aggregate has a hardness which is different from that of the bonding material. When the hardness of a heterogeneous substance is considered, the term usually relates to that of the material as a whole and is, therefore, only capable of a relatively rough measurement. 1 Sprech., 45, 333-5, 345-9 (1922). 2 Bull. Amer. Cer. Soc., 1, 51 (1922). 124 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES Hardness is usually measured by observing the resistance of the materials to (a) indentation and (b) abrasion by various harder substances. There is no direct relation between the resistance of a heterogeneous substance to indentation and to abrasion, as the latter is not really a measure of the hardness of the material as a whole, but of the bond, for no matter how hard the individual grains of aggregate may be, if the bond is soft and easily abraded the whole material will rapidly be worn away. Hence, the hardness of a heterogeneous substance is closely connected with the cohesion of the various particles. Resistance to indentation may be measured by a scleroscope or by a Brinel ball (p. 135), but for rough-and-ready comparisons a “‘ scratching test’ is chiefly used. A series of minerals of different hardness is used, and each of these is drawn across the material to be tested so as to make a scratch if the latter is softer than the former. When the material to be tested is scratched by one member of the series, but scratches the next softer member, it is said to have a hardness between that of the two members. The series of minerals generally used for the purpose is shown in Table XXV, where they are arranged in what is known as “ Mohs’ scale.” A series of convenient sub- stitutes is also shown in the same Table. TaBLE XXV.—Mohs’ Scale (substitutes shown in Italics) Hardness No. Material. 1 Foliated tale. 2 Rock salt or gypsum, or finger nail. 3 Transparent cale spar or copper wire. Fluor spar, scratches copper wire. 4—5 Ductile iron ; window glass. 5 Transparent apatite. 5-6 Blade of good pocket-knife. 6 Orthoclase felspar. 6-5 File. a Transparent quartz. 7-8 Will scratch a knife. 8 Transparent topaz. 9 Sapphire or corundum. 10 Diamond. When a substance has a crystalline structure its hardness will vary along dif- ferent planes and is usually lower in the direction of cleavage than perpendicular to it. The resistance of ceramic materials to abrasion is usually more important, especi- ally as many of them are subjected to a considerable amount of abrasion when in use. Thus, domestic pottery requires to resist the scratching and rubbing action of “HARDNESS OF RAW MATERIALS 125 knives, forks, etc. ; paving bricks, floor tiles, etc., require to be resistant to traffic ; and many refractory materials are required to resist the abrasive and corrosive dust contained in the hot gases in furnaces, kilns, etc., which rapidly wear away any soft portions of refractory material. The bricks which form the lining of vertical shaft furnaces, such as blast furnaces, cupolas, lime, magnesia, and other calcining kilns, etc., are subject to great abrasion by the descending charges, and it is essential that such bricks should have the neces- sary resistance to this action and to the differential movements which often occur in shaft kilns, and also exercise a considerable abrasion action. The manner in which gas retorts and some other appliances made of ceramic materials are charged and discharged also calls for the use of a material which is highly resistant to abrasion both in the hot and cold states. “‘ Rough usage” also has a great abrasive effect, as well as necessitating the use of a material which is resistant to blows and shocks. The resistance of a ceramic material to abrasion depends upon one or more of the following :— (a) The nature of the material, and especially its texture and hardness. (b) The mode of its preparation. (c) The nature of the bond (if any). (d) The amount of bond (if any). (e) The extent of vitrification. (f) The temperature of the material when it is examined. HARDNESS OF RAw MATERIALS The hardness of raw clays varies greatly, from less than 1 to more than 7 on Mohs’ scale (p. 124); some surface clays are quite soft and can be cut with a knife; others, such as shales and rock clays, are hard, because of the metamorphic changes they have undergone and the pressure to which they have been subjected. Most of the hard clays have at one time been at a great depth below the surface. The clays of most industrial importance are comparatively soft, especially when in a plastic state ; some of the harder ones, after being ground to powder and wetted, become quite soft and plastic. Clays such as loams, containing much sand, are, when dry, harder than purer clays. Marls and fireclays vary greatly in hardness, according to their mode of formation and location. Most of them, when in the dry state, can be easily cut with a knife, but some are hard enough to scratch glass. This scratching is largely confined to siliceous (quartzose) impurities in the clay and not to the clay itself. As clays are usually converted into a plastic paste, or into a slip or cream before use, their hardness is only of importance in so far as it affects the methods to be employed in grinding them to powder or otherwise preparing them for use. Clays which cannot readily be crushed to the required fineness are naturally easier and cheaper to prepare and are, therefore, more desirable than very hard materials which require much power to reduce them. The hardness of a raw clay has little or no effect on that of the finished products. 126 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES A property similar to the Brinell hardness of raw clay is the “ pressure of fluidity,” as A. 8. KE. Ackermann ! found that if a horizontal disc of metal is placed on plastic clay and loaded, each increment of load causes an increase in the penetration up to a certain critical point, at which the disc continues to sink at about ten times the previous rate without any increase of the load. This critical point is termed the pressure of fluidity and varies according to the amount of water in the clay and, therefore, to the plasticity; it is discussed in Chapter VI in connection with the mobility of clay pastes. Silica has a hardness of 7 on Mohs’ scale, but many siliceous materials in which individual grains are very hard may be easily crushed, as the bond which unites these particles is quite weak. For the same reason, the individual grains of silica in articles made of that material are much harder than the average of the articles, on account of the softness of the bond. Aluminous materials vary greatly in hardness ; bauxite usually has a hardness of 1-3 on Mohs’ scale, whilst corundum or alundum has a hardness of 9-10 and is one of the hardest substances known. Magnesite varies in hardness according to the form in which it occurs. Coarse crystalline magnesite usually has a hardness of about 4 on Mohs’ scale, whilst erypto- crystalline magnesite has a hardness of 3-5 and hydromagnesite a hardness of 3-4. Carbides and carboxides are extremely hard, corresponding to 9-10 on Mohs’ scale, being harder than crystalline alumina (9), but not so hard as diamond (10). The hardness of various minerals which are used in the ceramic industries is shown in Table XXVI. TaBLE XXVI.—Hardness of Ceramic Materials on Mohs’ Scale Material. Hardness. Material. Hardness. Baddeleyite Kaolinite . : 12:5 Bauxite Magnesite . : 3-5 Brookite Magnetite . : : 5-5-6:5 Calcite Monazite . : 4 5-5-5 Chromite : Quartz : : : 7 Common clay (dry) Rutile 6-6-5 Corundum . : Sand : : : 7 Cyanite Sillimanite ; 6-7 Dolomite Spinel : : 8 Graphite Tantalite . 6 Hematite . Titanite . 5-5-5 Hydromagnesite Tridymite . if Ilmenite Zircon : : : 75 1 Trans. Society of Engineers, 1910. HARDNESS OF BURNED CERAMIC MATERIALS 127 HARDNESS OF BuRNED CERAMIC MATERIALS Burned clay is much harder than raw clay and articles composed of it, either alone or mixed with other materials, may conveniently be arranged ! in two groups, according to their hardness :— Group I.—Soft wares, which can be scratched by iron, including fired sandy- clayey bodies, such as bricks, cooking utensils, crucibles, jars, unglazed earthen- ware, faience, roofing tiles, and most refractory materials. Group II.—Hard wares, which cannot easily be scratched by steel, including fine earthenware, ceramic stoneware, pipeclay ware, flint ware, and hard china. Building Bricks.—The hardness of building bricks is seldom of much importance, as, in the ordinary way, they are not required to be highly resistant to abrasion. Most well-burned bricks are rather harder than sandstone. Very soft bricks should not be used, except, possibly, as panels in interior work, as they are generally under- burned and deficient in strength and resistance to the weather. Bricks are not of strictly uniform hardness throughout their mass and though for many ordinary purposes they may be regarded as uniform, the surface is usually harder than the interior on account of the greater pressure applied to it. Machine- made bricks vary quite appreciably, as is shown in Table XXVII, due to H. Le Chatelier and B. Bogitch.? TaBLe XXVII.—AHardness of Building Bricks (Brinell) Portion of Brick Tested. Upper or Lower or Lifter Piston Side. Side. Brick No. 1. : F 5-4 5-6 a eNO. 2. é ; 5-6 5-6 ee O05, . : ; 5-9 6:3 These variations are due to the manner in which the bricks are manufactured and largely unavoidable. The Brinell hardness (p. 135) of various bricks, compared with metals, is shown in Table XXVIII, due to H. Le Chatelier and B. Bogitch.? 1 Brongniart, Treatise on the Ceramic Arts. 2 La Ceramique, 371, 17-18 (1919). 128 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES TaBLE XXVIII.—Brinell Hardness of Various Materials Material. Diameter of Depression. Copper . ; : ; , 4-5 Lead . : ; 3 : 10-1 Hard face brick. ; ‘ 4-5— 8-8 Soft face brick : ; : 6-8-12-0 Fireclay brick ; 6-0— 6-2 Hard silica brick. : : 5:0- 5-1 Tender silica brick . ; ; 10-1-10-7 The resistance of some building bricks to abrasion as measured by the sandblast method (p. 134), is shown in Table X XIX, due to EH. Orton.1 TaBLeE XXIX.—Hardness of Building Bricks (Sandblast) Average Loss under Material used for Bricks. Sandhinal, Grams. shale, 4 plasticclay . : 189 Shale. : . ‘ 81 Surface clay . : : : 110 Sandy shale and surface clay . 235 Shale. ; : : : 113 Vitrified or clinker bricks are, when cold, much harder than ordinary building bricks, as the vitrified or fused material in them produces a very hard bond. Accord- ing to the American standard, as adopted by the National Brick Manufacturers’ Association and the American Society for Municipal Improvements, clinker bricks should have a hardness, according to Mohs’ scale, of not less than 6-5 on an average (z.e. between that of felspar and quartz), and no individual sample should have a hardness less than 6-0. Vitrified bricks, when used for roads, etc., require to be specially resistant to abrasion, and, according to the specification of the American Society for Municipal Improvements, such bricks should not lose more than 14 per cent. of their weight 1 Trans. Amer. Cer. Soc., 14, 180 (1912). HARDNESS OF PAVING BRICKS 129 when subjected to the Standard Rattler Test, and no one brick should lose more than 18 per cent. of its weight. The ‘“ Rattler Test ” is described in Chapter IV. E. Orton ! gives the following results of the resistance of paving bricks to abrasion, the test being that described on p. 134. TaBLe XXX.—Hardness of Paving Bricks (Sandblast) Material used for Brick. Average Loss under Sandblast. Grams. Shale . 74 ee : 72 i ; t 69 er ; ; 74 Alluvial clay : 87 (laminated) Shale . , : : 65 Se : 4 98 i: : : 89 (brittle) Fireclay : : : 89 Fireclay and shale : 69 . ys : : 72 Shale . ; : 101 are ; 101 ey. : : : ; 122 Shale and fireclay : 101 Shale . : 92 e's : ’ : ; 84 ae : : ; A 72 Roofing tiles should be sufficiently hard to be handled without chipping and to prevent them from being damaged by frost. They should not be vitreous or they will “ sweat ”’ when in use. A good roofing tile should usually be difficult to scratch with a piece of steel ; roofing tiles which are harder than steel are generally too hard and vitreous. The desired hardness is largely determined by the manner in which the tiles are fired in the kiln. The temperature usually needed to secure roofing tiles of satisfactory hardness is between Cones 04 and la. Floor tiles should be sufficiently hard to resist any abrasion to which they may besubjected. F.B.O’Connor tested the resistance of various floor tiles by the method described on p. 134 with the following results :— ¥ Trans. Amer. Cer. Soc., 14, 180 (1912). 130 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES TaBLE XXXI.—Resistance of Floor Tiles to Abrasion Loss in Thickness, per cent. Type of Tile. Pressure Applied, lbs. per sq. in. 4-25 | 4 | 3°75 Buff tiles. : 12-21 14-32 3°78 Red __,, ; ; 21-24 10-16 12-00 Buff _,, : ; 21-57 21-14 Red _,, : : 35-55 6-89 Buff __,, : : 29-80 Red _,, : 18-43 Fireclay bricks vary greatly in hardness, some being sufficiently soft to crumble when cut with a steel blade, whilst others are harder than steel. The hardness depends to a large extent upon the temperature at which the bricks are fired; the higher the finishing temperature the harder will be the bricks, and soft bricks are usually underburned. The hardness of fireclay bricks by the Brinell method is shown in the Table on p. 128. The resistance to abrasion of various bricks, tested by M. L. Hartmann and J. E. Kobler, is shown in Table XXXII, the method of testing being that described on p. 134. - TaBLE XXXIJ.—Resistance of Refractory Bricks to Abrasion Kind of Brick. Depth of Cut when Cold. Inch. Carborundum brick | Zirconia brick 0-1-0:2 Bauxite brick | Grade C fireclay . Magnesia brick 0-05-0-07 Chrome brick Silica brick. 0:17 Grade A fireclay . Grade B fireclay . | ees Nesbitt and Bell, who used a similar method for determining the resistance to EFFECT OF TEMPERATURE ON HARDNESS 1381 abrasion, found that hand-made fireclay bricks were less resistant than machine- made ones, hand-made bricks being cut by abrasion to a depth of 0-04 inch in five minutes, whilst machine-made bricks, made by subjecting clay containing 7 per cent. of moisture to a pressure of 1500 Ibs. per sq. inch, were cut to a depth of only 0-02 inch. Fireclay bricks when heated to a temperature below the melting-point are more resistant to abrasion than when they are cold, but when at or above the sintering temperature the presence of molten material in them reduces their hardness and renders them much less resistant to pressure and abrasion. Sometimes, when bricks are taken out of a kiln, they appear to have softened greatly and become seriously distorted ; this is not always the result of exposure to a high temperature, but is sometimes caused by condensed steam softening the freshly-set bricks some hours before their temperature has been raised appreciably above that of the atmosphere. Renegade and Desvignes! have found that there is no relation between the hardness of fireclay bricks at high temperatures and their fusion point expressed in cones. They also found that alumina does not appreciably affect the hardness at high temperatures, but the presence of more than | per cent. of alkali has a marked detrimental effect. The resistance to abrasion of various refractory materials at a temperature of 1350° C. is shown in Table XX XIII, due to M. L. Hartmann and O. A. Hongen,? the tests being carried out as described on p. 134. Taste XXXIII.—Resistance of Refractory Materials to Abrasion at 1350° C. Material. Depth of Cut in Inches. Bonded carborundum (carbofrax A) 0-01 92 5 (carbofrax B) 0-30 a = (carbofrax C) 0-01 Grade A fireclay . , . : 0-11 Recryst. carborundum (refrax) : 0-07 Bauxite ; 0-04 Zirconia (natural) . . : 0-06 Grade B fireclay . : 0-09 Grade C fireclay . , 0-07 Chrome A ' : : ; 0-27 Silica : : : ; a Magnesia. ; " ‘ ; 2-50 1 Chaleur et Industrie, 3, 965 (1922). * Briek and Clay Record, 56, 934 (1920). 182 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES Silica bricks are composed of hard grains, but the bond is soft, so that the bricks are readily rubbed down, and they only resist abrasion to a very small extent. The hardness of the individual grains is equal to that of quartz (p. 126). The greater the proportion of lime used in making the bricks the harder will they be, as their resistance to abrasion is entirely dependent on the bond formed by the combination of lime and silica, and unless a sufficient amount of this bond is produced the particles of quartz, etc., are only feebly held together. Silica bricks which have not been burned at a sufficiently high temperature are very soft and easily abraded. Well-burned bricks are harder and emit a good “‘ ring ” when struck. W. Emery and L. Bradshaw ! found that hand-made silica bricks were less resistant to abrasion by sandblast than machine-made bricks. The average loss of weight by this treatment of six bricks of each kind was as follows :— Machine-made bricks . . 106 grams. Hand-made bricks . 143 grams. This corresponds to a difference of 26 per cent. in the respective resistances to abrasion. The hardness of silica bricks by the Brinell method is shown in Table XXVIII and by Hartmann and Kobler’s method in Table XXXII. Fused silica is not so hard as some forms of glass, though harder than others. Its hardness, as determined by Hertz and Auerbach, is shown in Table XXXIV. TaBLE XXXIV.—Hardness of Fused Silica and Glass Material. Absolute Hardness. Mohs’ Scale. Kg. per sq. mm. Quartz glass. : 223 5 Common glass . : 130-265 4-6 Magnesia bricks are fairly resistant to abrasion at atmospheric temperatures, but they are very soft at high temperatures. The resistance of magnesia bricks in the cold and at 1350° C. is shown in Tables XXXII and XXXIII. Fused magnesia has a hardness of 5-6 according to Mohs’ scale. Bauxite bricks vary in hardness according to the material of which they are made. Their hardness is increased if the bauxite contains a moderate percentage of iron oxide, as, when heated to high temperatures, such bauxites produce a material corresponding to emery, and of such intense hardness that it can scarcely be cut by steel tools. At high temperatures they appear to be very resistant to abrasion (see Table XX XIII). 1 Trans, Eng. Cer. Soc., 19, 73 (1919-20). # HARDNESS OF NON-PLASTIC MATERIALS 133 Carborundum bricks are extremely hard, and an angular fragment from them will readily cut glass. The resistance of carborundum bricks to abrasion in the cold is shown in Table XXXII. At high temperatures some carborundum bricks appear to be extremely resistant to abrasion, more so than any other form of refractory brick. This is well shown in Table XX XIII. Zirconia bricks vary in hardness according to the bond used. Zirconia itself is soft and remains so, according to H. C. Mayer, even when it has been heated to above 1427° C. The same authority has stated that a brick made of wet-ground material, containing 84 per cent. of zirconia, was flint hard ; the nature of the bond was not stated. According to R. C. Gosrow, zirconia bonded with magnesian chloride and fired at 1600° C. is extremely hard and scratches glass. This mixture retains its hardness when heated to a still higher temperature, zirconium carbide being sometimes formed. Chrome bricks are fairly resistant to abrasion in the cold, but at high tempera- tures they are rather soft (see Table XX XIII). Glazed ware should be sufficiently hard to resist the abrasion to which it is subjected in ordinary use. Domestic earthenware is scarcely hard enough for severe use, as it is too easily scratched by knives and forks. Bone china is much better in this respect, and the “hard porcelain”’ of the Continent is the hardest and most resistant of all such ware. A useful method for comparing the hardness of glazed ware is that used by G. Blumenthal, jun.,! and described on p. 136. The following figures show the hardness of various glazes tested by him :— TaBLE XXXV.—Hardness of Glazes Whiteware Glazes. Porcelain Glazes. No. Cone 4. Cone 6. No. Cone 16. wi 246 249 Bei 356 W 2 226 as Eo 370 W 3 224 284 Bee 346 W 4 214 280 P 4 334 W 5 220 261 P 5 321 W 6 218 276 P 6 306 Wi7 253 300 Pay 336 W 8 258 284 P 8 342 W 9 213 280 P 9 354 W10 224 296 P10 349 Wil 240 286 Mean 230 279 Mean 348 1 J. Amer. Cer. Soc., 4, 896 (1921). 184 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES DETERMINATION OF HARDNESS Methods of determining hardness may be divided into two groups: (a) those in which the hardness is measured by the amount of material removed from a sample by abrading or grinding it with some other substance—the so-called “ abrasive tests,” and (b) those in which the material to be tested is scratched or indented by another material, e.g. by pressure from a ball, cone, or edge-tool, to an extent depending on their relative hardness. Abrasive tests are of two kinds: (a) those in which the sample is pressed against the abrasive material, and (b) those in which the abrasive is projected on to the sample to be tested. Bauschinger’s abrasion-testing machine consists of a horizontal cast-iron plate or disc, 2 feet 6 inches diameter, revolving at 30 revs. per minute. The sample, for instance half a brick, is weighed and is then pressed on to the disc with a pressure of 75 lbs. The disc is charged with 20 grams of emery powder and revolved 22 times. The disc is then recharged with the same weight of emery and again rotated the same number of times, the process being repeated until the disc has been rotated 110 times. The loss in weight sustained by the sample is then measured and, if desired, further tests after 220, 330, or 440 revolutions may be made. The abrasion is measured by = where § is the loss in weight and A the area exposed to abrasion, or the loss in weight may be divided by the volume or weight of the sample. F. B. O’Connor ! has tested the resistance of floor tiles to abrasion by placing them in contact with a revolving table, 4 feet diameter, revolving at 1500 revs. per hour, the table being supplied at a uniform rate with 2 litres of dry crushed quartz of 20-30- mesh. The tests were continued for 1 hour, several different pressures being applied, and after the conclusion of each test the thickness of the tile was measured in eight places and the mean taken. The results obtained are given on p. 130. M. L. Hartmann and J. E. Kobler? have tested the resistance to abrasion by cutting a groove in the ends of each brick to be tested so as to expose the maximum area to cutting and then applying the bricks at a constant pressure of 25 lbs. for 5 minutes to a carborundum grinding wheel of grit 16 and grade 1, 12 inches diameter and with a 2-inch face, running at a constant speed (512 revs. per minute or 1560 feet per minute). The depths of the groove before and after the test were measured ; the difference represents in linear inches the abrasion during a 5 minutes’ test. Results obtained by this test are shown in Tables XXXII and XXXII. C. E. Nesbitt and M. L. Bell * used a carborundum wheel, of grit 16 and grade 16, 18 inches diameter and 2 inches thick, revolving at 1640 feet per minute. The sample was pressed against the wheel under a pressure of 100 lbs. per square inch for 5 minutes and the depth of cut measured. The sandblast test is typical of the methods in which the abrasive is projected on 1 Trans. Amer. Cer. Soc., 15, 233 (1913). 2 Amer. Electrochem. Soc., 37, 717-20 (1920). 3 Metall. and Chem. Eng., 15, 205-212 (1910). DETERMINATION OF HARDNESS 135 to the material to be tested. Various modifications of the test have been devised ; that suggested by the U.S. Bureau of Standards is very convenient. The sample is weighed and then mounted with its face in a vertical position and immediately behind an iron plate in which is an aperture 6 cm. diameter, so that the area exposed to the action of the sand is 28-27 square cm. Standard Ottawa sand of 20-30-mesh is then projected on to the sample through a nozzle 4 inch diameter at a pressure of 20 lbs. per square inch for 3 minutes, the distance between the exposed face of the sample and the nozzle being 12 inches. The effect of the sandblast is measured by the loss in weight during the test. Emery and Bradshaw used a similar method with Standard Leighton Buzzard sand of 20-30-mesh projected from a nozzle 0-275 inch diameter at a pressure of 7 lbs. per square inch for a period of 4 minutes, the surface to be tested being 7 inches from the nozzle. Indentation or scratching tests are much older than abrasion tests, one of the earliest scales of hardness being that devised by Mohs, which is still in general use. It is based on the ability of a mineral to scratch one mineral in the series and to be scratched by the next harder mineral in the series. Mohs examined a large number of substances and selected ten, which he numbered according to their hardness, as shown in Table XXV, in which a number of other convenient substances of equal hardness are also shown. The chief objection to Mohs’ scale is that the minerals used as standards themselves vary in hardness to an appreciable extent, but it is very useful for preliminary tests. A series of substances of similar hardness may also be placed in the wrong order if variable pressure is applied when testing them. To overcome this difficulty, Turner + makes the scratches with a diamond attached to one end of a balanced lever capable of moving vertically on a knife-edge and of being rotated. The lever is provided with a sliding weight and is graduated, so that each division of the scale corresponds to a weight of 10, 20, 30, or 40 grams at the diamond point. The sample to be tested is polished and is then tested by drawing it under the diamond point, whilst the latter is under various pressures, until a decided scratch is produced. The weight (in grams) on the point when this is effected is taken as a measure of the hardness. Martens modified this test by specifying that the scratch produced must be 0-01 mm. wide. This method was at one time used for measuring the hardness of metals, but it has not been extensively employed for ceramic materials. More accurate comparisons of hardness can be obtained by measuring the size of the indentation produced, but for this purpose a “ scratch ” is not so convenient as a circular indentation. Consequently, accurate comparisons of hardness are now chiefly made by means of the Brinell ball test, in which, as modified by Le Chatelier and B. Bogitch 2 to make it applicable to ceramic materials, a piece of thin lead foil, 0:05 mm. thick, previously blackened by the action of sulphuretted hydrogen in slightly acid solution and then dried and smeared with vaseline which is largely removed again, so as to leave a matte surface, is laid on the surface to be tested, and 1 Proc. Birm. Phil. Soc., 5, Part II. (1886). 2 La Ceramique, 371, 17-18 (1919). 136 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES on this is placed a hardened steel ball, 17-5 mm. diameter. A pressure of 500 kg. is applied to the ball for exactly 60 seconds, after which the depth or the diameter of the indentation is measured. The hardness is then calculated from the formula : P DS eee 1:5708 D(D—VD?—d?) where H is the hardness, P the load in kg., D the diameter of the sphere, and d the diameter of the indentation. It may also be calculated from the formula : P HN sae De’ where e¢ is the depth of the indentation and is equal to }(D—V/D?—d?). For hard materials, a pressure of 3000 kg. is usually required, but for softer ones 500 kg. is sufficient. The results obtained by the use of different weights are not strictly comparable, so that, as far as possible, a constant weight should be employed. When testing metals, no foil is used, but it is essential with most ceramic materials, as otherwise the indentation is not clearly defined. EK. Renegade and EH. Desvignes ! measure the hardness of refractory materials at high temperatures by supporting a cylindrical test-piece on a graphite block in an electric furnace of the Rosenhain type and pressing upon it a 90° cone by means of an amplifying lever applied through a rod of Acheson graphite, the depth of penetration being measured by a rule shaped at the end to fit the hollow formed by the cone. A modification of the Brinell test was used for testing the hardness of glazes by G. Blumenthal, jun.,? who allowed a hardened, rounded tungsten-steel point to bear on the glaze surface for 3 minutes under a pressure of 50 Ibs. and then measured the diameter of the indentation. The hardness was calculated from the formula : aa 5 ~ qtD’ where P is the load, D the diameter of the rounded point, and ¢ the depth of the indentation is D ae a? } PE where d is the diameter of the indentation. In the Shore scleroscope, a hardened steel cylindrical hammer about + inch diameter and 3 inch long, with a striking top about 0-02 inch diameter and weighing about 51, 0z., is allowed to fall through a glass tube from a height of 10 inches on to the material to be tested. The height of the rebound is measured on a scale and the hardness calculated from the figure so obtained. Although the scleroscope is regarded as measuring the hardness of a material it does not really do so, but only the elasticity ; 1 Chaleur et Industrie, 3, 965 (1922). 2 J. Amer. Cer. Soc., 4, 896 (1921). RING, FEEL, ODOUR, SECTILITY AND FISSILITY 137 it does not give very concordant results with ceramic materials and is not used to any great extent, the modified Brinell test being more suitable. MINOR PROPERTIES Ring .—The clear ringing note or “ ring’”’ which some burned ceramic materials emit when struck is often a good indication of the extent of the burning and the absence of cracks. A dull “ring” is usually due to the presence of small cracks, some of which may cause serious trouble if used under exacting conditions, whilst others are so minute as to be of little or no importance. The note emitted by silica bricks made from pre-calcined quartz is much duller than that from bricks made from the raw material; the difference is probably due to extremely minute cracks in the individual grains. Whilst of considerable use as a rough test, too much reliance must not be placed upon the sound emitted when a sample is struck, because the note depends on so many factors, some of which may affect the “ ring ” without being necessarily harmful. Feel.—Most ceramic materials and many others have a characteristic “ feel,”’ which may be (a) smooth, (6) rough, (c) meagre or harsh, (d) greasy, soapy, silky, or unctuous. Most refractory materials belong to the first three groups, whilst many clays are included in the last one. Some materials may be classed in two groups simul- taneously ; thus, china clay and dry ball clay are smooth and unctuous; many fireclays feel “rough,” but a freshly-cut surface, when rubbed with the fingers, has a slightly greasy feel. As the tongue is often more sensitive than the fingers, it has long been the custom to compare some fine clays by placing a small portion in the mouth and “ working” it with the tongue. By this means the presence of a very small proportion of grit in an otherwise impalpable material is readily recognised. Experts can readily distinguish different varieties of porcelain and other ware by the “ feel,” and this property is often very useful as a supplementary indication of the nature of a material. Odour.—Many clays, when moist, have a characteristic earthy odour, which probably is due to carbonaceous matter present, as it can be removed by treatment with a solution of iron saccharate, the odour being transferred to the latter. When heated to redness, the characteristic odour is lost. Impurities may sometimes be detected by the odour they emit, especially when heated. Thus, some clays and alum shales have a sulphurous odour on account of the pyrites present in them. This is particularly noticeable when the clay is freshly cut or heated to redness in a closed vessel. Sectility and Fissility are two closely related terms, indicating that a material to which they are applied can readily be cut, in at least one direction. Shales are of this character, though the “ cutting ”’ is possibly more in the nature of “ splitting,” or separating the existing lamine of which the material is composed. In order to be sectile a material must usually be moderately soft, but a hard material composed of thin sheets united by a soft cement will be fissile in a direction parallel to the sheet. 188 COLOUR, HARDNESS, MINOR PHYSICAL PROPERTIES The most sectile clays are ball and china clays when almost dry, and also some of the surface clays. Harder or leaner clays are friable rather than sectile, so that when a knife is applied to them they are crushed rather than cut. Most ceramic materials lose their sectility when heated to redness, but this pro- perty is retained to a large extent by certain rather soft bricks (known as cutters), and by many tiles. This property is due to the large proportion of sand present and to the heat-treatment and temperature attained in the kiln being insufficient to produce a very strong bond. It is important with such articles, as it enables the brick- or tile-layer to cut them to different sizes in order to fit them into special places. It may be noted that silica bricks are more difficult to cut than those made of fireclay, chiefly on account of their coarser texture and greater brittleness. Friability may be regarded as the converse of resistance to abrasion; it is considered more fully in Chapter IV. CHAPTER IV STRENGTH AND ALLIED PROPERTIES THE term strength is a very vague one and is used in several ways. The most frequent use of the term “strength ”’ in connection with ceramic materials is to indicate the ability of an object or mass to retain its shape when various mechanical forces are applied to it under different conditions. In this sense, the strength of a material is due to the cohesion of the particles of which it is composed and the resistance to pressure of the individual grains, especially those forming the coarser aggregate. If a material fails suddenly when subjected to a sharp blow or other sudden shock, it is said to be brittle ; if it is gradually crushed it is friable, but if it gradually yields under a succession of blows and forms a coherent mass of different shape, it is termed malleable. Ifits shape can be altered by pulling one portion of it or by passing it through a small aperture it is said to be ductile or plastic, and if it shows great resistance to such treatment or to bending it is regarded as tough. A material which regains its original shape as soon as the applied force is removed is termed elastic, and if a hard object after falling on it is caused to rebound to a considerable height the material is said to be resilient. When the shape of a mass can be altered by bending it is said to be fleasble. Whenever a force applied to a material causes alteration in its shape the material is said to be deformable, and the relation of the change in shape to the force applied is known as the deformability of the material. The term durability introduces a time element, and it is measured by the length of time the required property, such as strength or toughness, is maintained. Thus, material may have a high resistance to crushing when new, but the conditions under which it is used may be such as to reduce its strength in this respect within a short period. Such a material or article could not be regarded as “‘ durable’ under the prevalent conditions. All these properties are closely related in various ways to the “strength”’ of ceramic and other materials. As the strength of any material is due to a complicated series of qualities, different means are employed to express different kinds of strength, and no single figure can possibly represent the strength of an article in every respect. Thus, two materials may simultaneously have a high crushing strength and a great resistance to a tensile or “ pulling ”’ force, yet they may behave quite differently with respect to their malleability or ductility, etc. Consequently, each aspect of 139 ‘ 140 STRENGTH AND ALLIED PROPERTIES the strength of a material must be considered separately, after which the combination of several of these properties may be considered. Cohesion is the force which holds the particles of a mass together ; it may be regarded as the force of attraction between the atoms or molecules of a material, and, according to its intensity, a mass may be rigid like a brick, fluid like water, or it may possess various intermediate characteristics such as malleability, plasticity, etc. It is closely related to “‘ Binding Power ”’ (p. 141). Cohesion is measured by the force required to separate the particles from each other, and, according to the purpose for which the material or article is to be used, its cohesion is judged from its crushing strength, ductility, etc. As the simplest conception of cohesion is the force holding the particles together, it is measured most conveniently as the converse of the force needed to tear them apart, 7.e. the tensile strength. This is difficult to determine accurately in the case of a soft, plastic paste, but quite easy with a stiffer paste or a fairly rigid solid, such as a piece of dried or burned clay, and a knowledge of it is often of great value in investigating the properties of ceramic materials at different stages of manufacture or when they are inuse. A high tensile strength in a plastic material is very desirable as it facilitates the manufacture. In dried materials, a high tensile strength lessens the risk of damage in handling and in such irregularities of treatment as heating the material too rapidly during the drying or burning. In the production of vitreous ware, such as porcelain, where there is a considerable proportion of fluid present towards the end of the burning process, a high tensile strength at a high temperature is essential, as, other- wise, the distortion of the mass would be so great that many desirable shapes could not be produced. A high tensile strength is also important in the case of some finished articles, such as the large crucibles used for melting steel ; these are lifted out of the furnace with their contents, a pair of tongs being used, and are carried several yards before being emptied. The weight of their contents exerts a great tensile stress, especially as the materials of which the crucibles are made is somewhat soft at the temperature of molten steel. The walls of the “ pots” used for melting glass must also have a high tensile strength in order to withstand the conditions under which they are used. Equally important is the tensile strength of the glazes applied to ceramic materials. The thin film of glaze is subject to complex forces connected with its surface tension and allied phenomena, and unless the glaze is able to accommodate itself to these various stresses it will eventually crack or “ peel.” Glaze with a high tensile strength will stretch considerably before it will crack, and so will provide a permanent pro- tection under conditions where it would be unattainable with a weaker glaze. This characteristic was formerly thought to be best obtained by using glazes with a high degree of elasticity—an idea which seems to be incompatible with such materials, but R. Rieke has shown that in this connection a high tensile strength is of greater importance than elasticity. The determination of the tensile strength of ceramic materials has been greatly neglected and few published results are available. It is now being increasingly recognised that comparisons of tensile strength are often just as important as, and BINDING POWER AND BRITTLENESS 141 sometimes more so than, those of some other properties. A description of the methods of determining the tensile strength will be found near the end of the present chapter. Binding power is closely related to cohesion and to tensile strength, but differs from them in at least one important respect. The “ binding power ”’ is that which enables a material to act as a cement or binding agent in uniting particles of other materials to which it is applied or with which it is mixed. The term is used in a special sense in connection with certain ceramic materials, as being that power which enables a plastic clay to be mixed with a considerable quantity of non-plastic material, and to produce a mixture which possesses plastic properties and ample tensile strength for the purposes for which it is required. The binding power of clay is usually estimated by mixing the clay with various proportions of sand or other non-plastic material and determining the tensile compression and transverse strength of the mixture either in the plastic or dry state. The choice of a non-plastic material must depend on the purpose of the investigation ; a right selection is very important, as a material may show a higher strength with one kind of sand than with another, or different results with the same sand ground or screened to different degrees of fineness. The binding power is an extremely important property of plastic clays, as it affords a simple and excellent means of reducing their shrinkage on drying and heating to within convenient limits and enables many mixtures having very desirable pro- perties to be prepared with a facility which would otherwise be impossible. Binding power is often confused with “ plasticity,” though the two properties are quite distinct, and some plastic materials are seriously deficient in binding power. The chief connec- tion between the two is that when a material possessing binding power is mixed with a non-binding material the mixture may be plastic. Thus, a mixture of linseed oil and whiting in suitable proportions produces a plastic mass (putty), but neither the oil nor the whiting are individually plastic. The oil has, however, a considerable binding power. Unless the proportion of oil is excessive, the plastic mass of putty will differ from a clay of equal plasticity in being almost devoid of binding power (see also Chapter VI). The determination of binding power is described in Chapter VI. Brittleness is the property possessed by some materials which causes them to break or “ split’ when allowed to fall on a hard floor or when they are subjected to a sudden, single blow. It is due to a lack of sufficient cohesion of the particles and to some extent to the hardness of the material, which prevents it from yielding under sudden stress and so causes it to break. For this reason, a very soft material is never brittle. Brittleness is generally a very undesirable property. Burned ceramic materials which are brittle have usually been fired at too high a temperature, but some under-burned bricks are brittle if the vitrification has not been carried sufficiently far to bind the particles into a hard strong mass. Brittleness may also be caused by cooling articles too rapidly, this treatment producing a large number of minute cracks which render the articles weak. Brittle- ness may also be caused by using a kiln with a damp foundation. Some firebricks are said to be brittle because crystals have formed in them during the burning period and also because of improper treatment during cooling them through the “ critical 142 STRENGTH AND ALLIED PROPERTIES range,” whilst some ceramic materials are brittle when heated on account of their coefficient of expansion. Silica, magnesia, and bauxite bricks are very liable to crack and spall when heated quickly on account of the volume changes which occur during rapid heating and cooling. When heated fora long period, fused silica tends to become brittle on account of its recrystallisation or devitrification. There is no reliable means of measuring brittleness; the best available is to investigate the effect of blows, applied in various ways, on the material (see “ Impact Tests,” near the end of the present chapter). Friability is the property which enables a material to yield readily when sub- jected to a crushing or abrasive force (p. 138). Itis a typical characteristic of dry, sandy loams and of the soft, sandy bricks known as “ rubbers ” which are largely used in decorative architecture, particularly in the south of England. These bricks are so rich in sand that they have little cohesion and when two of them are rubbed together they are rapidly “ worn away.” Most dry clays are very friable, but they lose this characteristic when heated to redness. Clays which, when dry, cannot easily be crushed or “‘ rubbed away ” are known as “ indurated’ or hardened clays; many shales, fireclays, and slates are indurated clays. Malleability is the property which enables a material to change its shape without breaking or cracking when it is passed between a pair of rollers, or is subjected to a series of blows from a hammer or similar appliance. It is especially evident in such metals as gold, silver, and copper, but is equally characteristic of plastic clays and other pastes, though as these materials are soft it is not necessary to “ hammer ” them. Malleability is due to the structure of the material being such that the particles can roll over each other without losing contact. It is an important characteristic of most ceramic pastes, but is usually referred to as “ plasticity ’’ (see Chapter VI). Ductility is the property which enables a material to be drawn or pulled into any shape ; it also enables a material to be extruded through a small aperture at the end of the vessel containing it. This property enables bricks, pipes, and other articles to be extruded through a “‘ mouthpiece ” attached to the end of a pugmill, “ stupid ” or pipe-press, the extruded material being afterwards cut into pieces of suitable length by means of taut wires ona frame. Articles so produced are said to be made by the “‘ wire-cut”’ process. Although ceramic pastes are conveniently treated by this means, they are not nearly so ductile as some metals, such as gold, silver, copper, and some varieties of brass. The ductility of clays and allied materials is usually regarded as part of their “ plasticity ” (see Chapter VI). Extensibility is closely related to ductility (q.v.) and is expressed by the greatest increase in length which can be obtained in a mass without fracturing it. The extensibility is usually determined by measuring the distance between two points or fine lines marked on a test-piece made of the material, then applying a sufficient tensile force to break the material ; the two pieces are carefully fitted together and the increase in length between the two marks previously made is noted. The result may be conveniently expressed as a percentage of the original length of the test-piece. In order that the results may be comparable, all the test-pieces should be of the same shape and size. A cylindrical rod, about 9 inches long with a diameter of 1-128 ELASTICITY AND FLEXIBILITY 143 inch and, therefore, a cross-sectional area of exactly 1 square inch, is convenient. The ends may be enlarged to facilitate the test-piece being firmly held in the tensile machine. The “ dots ” or lines used as fiducial marks may be 4 inches apart. Some- times the extensibility is measured on the test-piece used for determining the tensile strength, but it is often more convenient to use a longer test-piece and to apply the tensile force more slowly than when making determinations of the tensile strength. The extensibility of a ceramic material is seldom determined, as such materials are seldom used under tension. It is chiefly of value in determining the plasticity of a paste by Zschokke’s method (see Chapter VI), as this investigator regards plasticity as measurable by the product of the extensibility by the tensile strength. Elasticity is the property which enables a material to be drawn out or bent and to assume its original shape as soon as the flexile or tensile force is removed. The extent of deformation possible under these conditions is limited with each material, and when a greater force is applied, the material may either retain the distorted shape or it will break. Elasticity is not usually of much importance in connection with clays and other ceramic materials, for, although some of them show a very slight elasticity, it is so extremely small as to be of little consequence and in most cases it cannot be measured accurately. The chief exception to this is met with in glazes, for on this property— though still more on their extensibility (p. 142) and tensile strength (p. 140)—depends the extent to which a glaze will adhere properly or will craze or peel. It is extremely difficult to adjust the contraction of a glaze and body so that both are exactly the same, yet if this is not done the glazes will not quite “ fit’ the body upon which it is placed, and if the glaze is deficient in extensibility, elasticity, and tensile strength, the results—cracking, crazing, or peeling—may be serious. According to Rieke, the importance of the elasticity of a glaze is often exaggerated, and the greater importance of its extensibility and tensile strength is often overlooked. Flexibility is that property which enables a material to be bent without breaking it. Plastic clays and ceramic pastes possess a moderate degree of flexibility (other- e “handles” could not be made by bending a roll of paste), but this property is generally regarded as part of the “‘ plasticity ’ (Chapter VI). Fired ceramic materials are seldom flexible, except to a small extent when at a high temperature near their softening point. Torsional flexibility, even to a minor extent, is desirable in some electrical insulators made of porcelain or stoneware, but under ordinary circumstances the flexibility of fired ceramic wares is neglected. Toughness is the property which enables a material to resist tensile, crushing, and other disruptive forces. It is difficult to represent it by any single figure or test, as it depends on a variety of factors, the importance of which differ on different occasions. Thus, under some conditions, toughness may be synonymous with flexibility, under others with tensile or crushing strength or, in the case of some materials at high temperatures, it may be due to the viscosity of the semi-molten binding agent. It is usually at a maximum in materials which have great cohesion, together with some flexibility or elasticity, and at a minimum in rigid materials, as the latter are 144 STRENGTH AND ALLIED PROPERTIES liable to be brittle (p. 141). Toughness is commonly, but inaccurately, measured by the “ Rattler test” (p. 200), which chiefly measures the resistance to abrasion, though a hard material of a brittle nature which is not easily abraded will give a low result in the Rattler test, thus showing that toughness, or its converse brittleness, do affect the results of that test. Toughness may be regarded as due to a combination of (a) resistance to being drawn out, and thusis the converse of ductility ; (b) resistance to flexion and torsion, and thus is the converse of flexibility ; and (c) resistance to impact, and thus is the converse of brittleness. It is also related to hardness and viscosity. A property which is related to so many others is highly complex in its nature and almost defies accurate definition or measurement. According to L. Ogden 1 the toughness of porcelain is increased by increasing the proportion of flint and decreasing that of the felspar, whilst the proportion of clay is kept constant, or alternatively, if the proportion of felspar is kept constant, an increase in the proportion of flint and a decrease in that of clay will increase the toughness. Hence, the toughness of porcelain is dependent on the proportion of flint present, the strongest porcelains containing about 35 per cent. Deformability is the property which enables the shape of a mass to be altered by the application of a force or combination of forces. It is not a simple property and cannot be determined as such, but is the converse of the transverse, impact, and torsional strengths of the material, and varies according to the means used for © deforming the mass. H. and F. Le Chatelier 2 found that substances may undergo successively three types of deformation: (a) elastic deformation, which is entirely removed when the deforming force is released; (b) a subpermanent deformation, which disappears gradually after the removal of the deforming force ; and (c) a viscous deformation, which does not disappear when the deforming force is removed. The deformability of a material is best estimated from a comparison of the effect on it of the various forces just mentioned. The crushing strength of a material is the resistance which it offers to com- pression. It is often of great importance and is the kind of strength which is chiefly — desired in most ceramic materials, especially those which in the fired state are used for constructional purposes, as they are often required to withstand a considerable pressure exerted by the materials above them. In order that a ceramic material may have a high crushing or compressive strength, (a) it must have a good binding agent which is present in sufficient amount to bind the particles of aggregate together ; (6) the grains must interlock sufficiently, especially if the proportion of bond is small ; and (c) the individual grains of aggregate must have great density and crushing strength, as porous particles are usually weaker. The crushing strength is, therefore, closely related to the texture of a material. These factors are considered more fully on . 151. : Clays and other materials in a soft, plastic state show no end point when subjected to compression between two opposing plates, the sides of the sample being quite 1 Trans. Amer. Cer. Soc., 13, 395 (1911). 2 Comptes Rendus, 171, 695-9 (1920). TRANSVERSE STRENGTH 145 free, so that such materials have no definite crushing strength. The crushing strength of dried materials appears to be very variable, but it is an advantage to know it when the materials or articles are to be piled on top of one another in a store, dryer, or kiln, as it is then possible to avoid spoiling them by piling them too high. Apart from this, a knowledge of the crushing strength of dried materials is of minor importance. The compression or crushing strength of fired materials is often an important factor as regards their suitability for their intended purpose. In many cases, the crushing strength of the cold material is of small interest, but that of the material at the highest temperature at which it is likely to be used may make all the difference as to its suitability for some purposes (see p. 163). In addition to its direct use, the crushing strength of a ceramic material is also an indication of the uniformity of the heat-treatment to which the material has been subjected in the burning or firing process. If several pieces made of the same material and in the same manner are found to vary greatly in their resistance to compression, it may usually be concluded that the temperature in various parts of the kiln is far from uniform, those parts being the hottest which produce the strongest samples. The determination of the crushing strength requires a powerful machine (p. 195); for many purposes, a determination of the modulus of rupture or transverse test (p. 197) is equally satisfactory and is easier to execute in the absence of a crushing machine. According to H. Le Chatelier and B. Bogitch, the modified Brinell test described on p. 135 may be used as a substitute for the crushing test as the results are com- parable, whilst the Brinell machine has the advantage of being more rapid, accurate, portable, and cheaper to use than a crushing press. The transverse strength or modulus of rupture of ceramic materials is often of great importance and, as it may be determined with very simple apparatus, deter- minations of it are increasing in popularity. It appears to be closely related in many ways to the crushing strength, though the two are by no means identical. The crushing strength is the resistance offered by a material to a given pressure; the transverse or cross-breaking strength is the resistance offered by a piece of the material of unit-cross-section area to a crushing or cutting force. The modulus of rupture is calculated from the transverse strength by means of the formula shown on p. 197. The transverse strength undoubtedly gives a better indication of the resistance of a material to compression than does its tensile strength, although the latter is, in many cases, closely related to it. Comparative determinations of the transverse strength of materials in various © stages of production are very useful in preventing losses during manufacture. Thus, J. E. Sproat 1 found that the loss of biscuit ware in a pottery decreased from 7 per cent. to 3-5 per cent. when the cross-breaking strength of the dry body-mixture was increased from 200 to 300 lbs. per square inch. They are also useful in comparing the strengths of slabs, tiles, thin paving bricks, pottery, saggers, etc., in order to increase their durability. According to Bleininger and Howat, the transverse test of a dry mixture of clay and sand is a better indication of its plasticity than the tensile or compression tests. 1 J. Amer. Cer. Soc., 5, 588 (1922). 10 146 STRENGTH AND ALLIED PROPERTIES It is, however, unwise to place much reliance on the assumed relationship of any “ strength test” of a dry material with its plasticity. A comparison of the transverse strength of saggers and other hollow-ware subject to irregularly distributed loads often enables articles of a better quality to be produced. In considering these various kinds of “ strength,” it will be seen that for most purposes an article on ceramic material must possess a combination of two or more of these qualities, the ones required depending on the purpose for which the article or material is to be used. Thus, the plastic raw paste used for making articles must be readily deformed or shaped and must, therefore, have a low cohesion so as to allow the paste to move relative to each other, and yet the material must possess ductility and malleability (or plasticity), together with sufficient tensile, compressive and transverse strength to prevent fracture when the shape of the mass is altered. On the other hand, a finished article must usually have a high degree of cohesion to enable it to retain its shape under the conditions to which it is likely to be subjected, and it should also possess such properties as toughness and should not be brittle. FACTORS AFFECTING STRENGTH Many factors affect the strength of ceramic materials, the most important of which may be grouped under the following heads :— (a) The chemical composition of the material. (b) The physical properties of the material. (c) The mode of preparation of the material. (d) The mode of manufacture of the article. (e) The conditions of drying. (f) The conditions of burning. (g) The temperature at which the article or material is used, or at which its strength is determined. (hk) Other conditions to which the article or material is or has ee subjected, including weathering, sudden changes of temperature, prolonged heating,etc.. The strength of raw clay will depend chiefly on factors (a), (6), and (h), and chiefly on the second ; the strength of a dried clay will depend on factors (a) to (e) ; the strength of freshly-burned products on (a) to (g); whilst the strength of articles which have been in use for some time may be further complicated. In order to ascertain the effect of these various factors upon the strength of ceramic materials they should first be studied separately, and afterwards in combination. The Chemical Composition frequently has a very important effect on the strength of ceramic articles, but care must be taken not to exaggerate its influence ; in the past there has been a tendency to attach undue importance to the chemical composition and to neglect the physical characteristics of the materials. The chemical composition of the raw or dried materials is of less importance, as in such materials chemical reactions take place very slowly, if at all, at ordinary FACTORS AFFECTING STRENGTH 147 room temperatures. When the materials or articles made of them are heated to such a temperature that chemical reactions can take place, the chemical composition of the material begins to play an important part. Most finished articles made of ceramic materials consist of solid particles of “ageregate ’’ united by a glassy bond, the nature of the latter largely determining the strength of the article at various temperatures. As the bond is usually produced by the combination of the binding agent with some of the constituents of the aggre- gate, the strength of the mass also depends on the amount of bond present and on the total proportion of fluxes which are present in the raw materials. In a cold ceramic mass, the greatest strength will usually be found in the material containing the largest proportion of fluxes, such as soda, potash, lime, magnesia, etc., provided the ware has been fired at a sufficiently high temperature to render the fluxes effective. At high temperatures, on the contrary, the larger the proportion of fluxes, the lower will be the strength of the mass, because at a high temperature the bond produced by the fluxes will be soft and mobile and, therefore, unable to impart the necessary rigidity to the material. When the temperature is reduced sufficiently for the molten glassy matter to become solid, the strength of the material as a whole will greatly increase as the solid glassy matter forms a strong bond. The effect of the chemical composition on the strength of a ceramic material is shown in the case of bricks made of lume; these are extremely weak, as no suitable bond can be found for them. Other non-plastic materials, such as bauxite, when attempts are made to shape them without an added bond, are very weak, but if a suitable bond, such as clay or lime is used, very strong bricks and other articles can be made, some bricks bonded with lime having a crushing strength of 10,000 lb. per square inch. Some magnesia bricks (especially those containing about 5 per cent. of ferric oxide or its equivalent) are very strong when cold, chiefly on account of the proportion of fusible matter present. On the other hand, some silica bricks are weak because of the poor quality and small proportion of lime used in making them. Table XXXVI, due to Phillipon,! shows the effect of varying the proportion of lime on the strength of silica bricks burned at 1300° C. Taste XXXVI.—Effect of Lime on Strength of Silica Bricks Crushing Strength, kg. per sq. cm. Per cent. Lime. Thi rtz, ; : Central Platea, | Nommendy Quartz, | Tynan. 0 50 110 a 0:5 172 208 147 1:0 278 270 220 1:5 310 290 253 2:0 304 287 263 2:5 262 252 257 1 Rev. de Métal., 15, 51 (1918). 148 STRENGTH AND ALLIED PROPERTIES Articles made of clay owe their strength when in the moist and dry states chiefly to the plastic material present, but in the fired or burned articles the plasticity is destroyed and the strength of the articles is then largely dependent on the proportion of active fluxes present. These fluxes combine with the free silica present and form glossy molten silicates. Blue bricks and other similar vitrified articles contain a larger proportion of active flux than red bricks and are, consequently, stronger. Blue bricks, tiles, etc., are particularly interesting in this respect, as the amount of glassy bonding material is not due so much to the total metallic oxides present, as to the fact that the chief of these oxides (iron) is reduced to the ferrous state by the mode of burning adopted (see p. 103) and so is converted into an active flux. Under oxidising conditions it would be largely inactive. E. C. Hill,} has found that fluorspar and magnesium carbonate, either together or separately, lower the strength of terra-cotta ware, whilst whiting and furnace slag do not appreciably affect it. This difference in behaviour has not been explained, except on the assumption that fluorspar and magnesium carbonate form viscous fluids which are largely immobile, whilst whiting and furnace slag are much more mobile and “ penetrating.” ; The effect of fluxes on the transverse strength of terra-cotta is shown in Table XXXVII, due to H. C. Hill. TaBLeE XXXVII.—-Effect of Fluxes on the Strength of Terra-cotta Modulus of Rupture. Lbs. per sq. in. Terra-cotta clay . : : ; : ; : : 1518 + 5 percent. Maine felspar .-. . 1486 +10 @ eA ’ ; : 1905 + 2:5 ,, powdered glass . ; 1489 + 50 ,, a ; ; 1687 +25 ,, white lead . d : : . : 1372 + 50 ,, ., ; 1461 + 1-25 ,, eryolite . : : , 1339 +25 ,, 2 ; ; : 1426 + 1-25 ,, whiting . : : ; : ; 1618 + 2:5 ,, . : : 4 * = 1503 +50 ,, ‘3 ' ; ; ; 1545 + 2:5 ,, Fluorspar . : : : ; : 1275 + 50 ,, S ; :; : : 1326 + 1-25 ,, magnesium carbonate . : : ; 1269 +25 ,, a > ; : 1240 +50 ,, 53 : ; 1152 +50 ,, furnace slag s : : : : 1361 +100 ,, € ; : : 1407 1 J. Amer. Cer. Soc., 5, 832 (1922). FACTORS AFFECTING STRENGTH 149 Fireclay bricks increase in strength in the cold with an increase in the proportion of fluxes, but, as previously mentioned, their strength at high temperatures is lessened by the addition of fluxes or of free silica (sand). E. Sieurin and F. Carlsson} found that 60-70 per cent. of silica gave the weakest samples. An excess of alumina is also harmful, a sudden drop in the softening point occurring with a mixture con- taining between 70 and 80 per cent. of alumina. If more than 80 per cent. of alumina is present and the mixture is carefully made, its softening point increases gradually with the alumina added. Bleininger and Brown? consider that there is a certain relation between the composition of fireclays and their crushing strength or resistance to load at high temperatures, and have stated that most ceramic materials containing more base than corresponds to the formula 0-225 RO Al,0,2S8i0, (see Chapter VIII) cannot withstand a pressure of 50 lb. per square inch at a high temperature, and that an excess of silica reduces the strength of the mixture, unless it is accompanied by a reduction in the proportion of base present. Thus, when 4-4 molecules of silica were present to 1 molecule of alumina, a material failed under a pressure of 50 lb. per square inch at 1350° C., even though only 0-17 RO were present. Lime greatly reduces the strength of fireclay bricks under load, even when only small proportions are present. According to E. Sieurin and F. Carlsson, a rapid fall is occasioned in the softening point with less than 1 per cent. of lime, beyond this proportion the fall continues, though less rapidly. Magnesia acts in a similar manner, the most rapid fall in the softening point being in silica bricks with between 0 and 0-25 per cent. With greater percentages of base, a more gradual decrease occurs. Tron oxide greatly reduces the strength of fireclay bricks at high temperatures, particularly in a reducing atmosphere. The softening point under load is rapidly decreased with less than 6 per cent. ofironoxide. This strength at a high temperature is not very much affected with percentages between 6 and 12, but beyond this it is again rapidly lowered. Other substances besides those commonly regarded as fluxes may affect the strength of clays and refractory materials. In a complex chemical substance or mixture, such as those used for making fireclay goods, the respective proportions of the constituents may greatly affect the strength. Thus, the presence of an excess of bone ash in china ware increases the brittleness of the ware. Peters * found that carborundum, up to about 45 per cent., increased the tensile strength of fireclay | rapidly, but larger proportions had the opposite effect, as the strength decreased with great rapidity. With some mixtures of ball clays and 70 per cent. of carborundum the maximum tensile strength is obtained and the compressive strength is also increased. Bricks made of a mixture of bauxite and sand are much weaker than those made wholly of bauxite, on account of the different natures of the constituents. Sand expands on heating, whilst clay and bauxite shrinks, so that the strains set up on 1 Brit. Clayworker, 30, 262 (1921-22). 2 Trans. Amer. Cer. Soc., 13, 210 (1911). 3 J. Amer. Cer. Soc., 5, 181 (1922). 150 STRENGTH AND ALLIED PROPERTIES firing reduce the strength of the finished articles. O. L. Kowalke and O. A. Hongen found that the addition of silica materially increased the crushing strength of magnesia bricks at high temperatures, whilst alumina, chrome oxide, titanium oxide, and zirconia have a similar effect, though not so marked. Chrome bricks, containing 14 per cent. of clay, are not so strong as those made entirely from chrome ore, and chrome bricks bonded with chalk are very weak. The crushing strength of fused silica is appreciably increased by the addition of 1 per cent. of zirconia. The effect of water on the strength of ceramic materials is described on p. 154, and the effect of electrolytes on p. 155. The effect of chemical composition is also considered in Chapter IX. The physical properties of ceramic materials has a very important influence on their strength. The chief physical properties to be considered are (a) the size and shape of the article; (b) the texture of the material; (c) the porosity ; (d) the coefficients of expansion or contraction ; and (e) the adhesive power of the bond. Size and Shape.—The crushing strength is to some extent dependent on the size of an article or mass, and usually the larger an article the greater will be the total power required to crush it, though it need not necessarily have a high crushing strength, as the latter is expressed in terms of unit area (lb. per sq.inch). The shape has an important effect on its strength, for if it is designed badly it may be unduly weak in certain parts and so bend or crack when subjected to a great stress. For instance, sharp corners and sudden changes in the thickness of an article are sources of weakness, and, where possible, these should be avoided. The apparently abnor- mally great strength of hollow columns as compared with solid ones is well known and it is worthy of note that perforated bricks are stronger in proportion to their area than solid ones, as shown in Table XX XVIII, due to Professor Tetmaier.1 Taste XXXVIII.—Perforated v. Solid Bricks Crushing Strength, tons per sq. ft. No. of Holes. Diameter of Holes, in. Solid Bricks. Perforated Bricks. 276 411 14 0-6 161 178 12 0-7 147 149 12 1-0 130 154 10 0-7 202 238 14 0:8 329 383 14 0-8 Seger also found that a solid and a perforated brick made from the same material had crushing strengths of 315 and 504 tons per sq. foot respectively. 1 Brit. Clayworker, 29, 22 (1920-21). re a i Bn EFFECT OF TEXTURE ON STRENGTH 151 Texture is concerned with (a) the shapes and sizes of the individual particles, and (>) the arrangement and size of the pores or interstices between the grains. In ceramic materials, the maximum strength is obtained by the use of irregular angular grains of numerous sizes, which interlock freely ; rounded grains produce a weak mass as they cannot interlock properly. Thus, bricks made of powdered calcined flint or of sea-sand are usually weak, because the grains are rounded and, consequently, do not hold well together. The strength of a ceramic mass, both in the dried and fired state, usually increases with the fineness of the grains, but an excessive pro- portion of fine grains is undesirable, as H. Ries 1 has shown (Table XX XIX) that it decreases the tensile strength of dried clays in a similar manner to a large proportion of large grains, such as coarse sand. TaBLE XXXIX.—Effect of Grain-Size on Crushing Strength (Ries) Percentage of Grains of each Size. Diameter of Grains. Clay 1. 2. 3. 4, 5. mm. 0-005-0-0001_. ; 87-96 30-645 22-00 44-00 59-00 0-01 -0-005 6-95 14-210 5-66 TAL 11-00 0-25 -0-01 : 3:00 5-585 26-55 24-35 14-70 0-5 -0-25 : : 1-00 6-400 11-45 7-80 3:50 10 -0:5 : : 2 42-950 33°44 16-35 11-40 Tensile strength (lbs. per sq.in.) . 20 105 289 297 453 On the other hand, for articles requiring to possess great resistance to compression at high temperatures, H. J. Knollman? considers the presence of a considerable proportion of fine material to be an advantage, and various investigators have shown this to be the case with silica bricks. Thus, the effect of the grain-size on the strength of silica bricks, burned at 1300° C., is well shown in Table XL, due to Phillipon.® The size of the grains of grog in fireclay mixtures has an important influence on ~ the resistance of the fired material to transverse loads at high temperatures. Large ceramic slabs or other articles which are required to carry heavy loads should be made of comparatively coarse material. Various investigators are agreed that about 50 per cent. of grog of sizes corresponding to 4-20-mesh should be used and all fine materials should previously have been removed from it. This is also recognised in the Standard Specification for Retort Material, etc. 1 Trans. Amer. Cer. Soc., 6, 79 (1904). 2 J. Amer. Cer. Soc., 4, 759 (1921). 3 Rev. de Métal., 15, 51 (1918). 152 STRENGTH AND ALLIED PROPERTIES Taste XL.—Effect of Grain-Size on Crushing Strength Compressive Strength, kg. per sq. cm. Diameter of Grains. i ‘ Allier Amorphous eNGentral Platoon, | Normandy Quartz Silica. 0:05 250 300 210 0:06 140 230 80 0-07 50 150 25 0-08 23 80 5 0-09 14 5) 0:10 8 oe 0-12 5 20 0-14 3) 10 0-16 A 5 0:18 me 4 The strongest ceramic mass consists of a suitably graded mixture of angular grains of various sizes selected so as to produce as compact a mass as possible and provided with a sufficient amount of binding material to cover each particle of ageregate and to unite them together. The production of graded aggregates of this kind is described on p. 32. The proper interlocking of the particles of aggregate is of very great importance, and the low transverse strength of many fireclay and other slabs is due to their being made of badly-graded mixtures. Similarly, the strongest burned material consists of a mass of interlocking crystals formed in situ and united by a glassy cement. Such a mass is stronger and more compact than can be obtained by any artificial means of assembling the particles. It should be observed that the texture of a mass is not always uniform in every direction on account of the method used in shaping it. Thus, if a tile is made by applying pressure to its upper and lower surfaces, its structure when laid flat will not be quite the same as that when it is on edge. Hence, in all machine-made products, and to some extent with hand-made ones, the structure varies in different directions, and this should be taken into consideration when investigating the strength of an article. Further information on texture will be found on p. 27. Porosity.—The size and arrangement of the pores in a ceramic mass aflect its strength, inasmuch as the larger and more numerous are the pores, the thinner must be the enclosing “ walls ” of solid material. Hence, a highly porous material—if the pores are large—must be very weak. When the pores are extremely small, their effect on the strength of the material is less noticeable. In many fired ceramic materials, the tensile and crushing strengths are roughly EFFECT OF BOND ON STRENGTH 153 inversely proportional to the porosity. Thus, if certain bricks having a porosity of 6-12 per cent. have a crushing strength of 7000-15,000 lbs. per square inch, other bricks of a similar character, but with a porosity of 14-25 per cent., will probably have little more than half the strength of the former ones. This proportionality does not always hold, however, and A. V. Bleininger + has found that when the porosity and the crushing strength of numerous clay and shale bricks examined by him were plotted as a graph, the latter consisted of two portions, namely, an almost straight line indicating a gradual increase in strength with a decrease in porosity down to 3—4 per cent., and then a steep curve indicating a rapid increase in strength for lower porosities. The relation must clearly depend to a great extent on the structure of the bricks. The relation between the porosity and crushing strength of ceramic materials at high temperatures has not been sufficiently fully examined to permit very definite conclusions being drawn. Such conclusions, when based on a small number of samples, are often misleading, and when ceramic materials of different composition are examined no general conclusions seem possible; thus M. F. Beecher ? has found that some fireclay bricks made by him, with a porosity of 51-8 per cent., suffered less deformation when heated under pressure at high temperatures than several commercial fireclay bricks having a porosity of less than 20 per cent. In ceramic materials of a non-argillaceous character, it appears that great strength is incompatible with high porosity, though there is only a very indefinite relation between the porosity and strength of such materials (see also p. 74). Power of the Bond.—In ceramic materials, four types of bond occur: (a) bonds of a plastic nature, such as clays; (6) bonds of an adhesive nature, such as glue, dextrin, molasses, etc. ; (c) bonds of a hydraulic nature, such as mortar and Portland cement ; and (d) glassy bonds such as occur in vitrified ware. Each of these bonds increases the strength of the mass as a whole, though to very different extents and in very different ways. A plastic agent, such as clay, possesses a peculiar binding power (p. 141), whereby it is able to extend its plastic properties to limited proportions of non-plastic materials which may be mixed with it, and when the mixture is dried and afterwards burned, the mass becomes stronger than before, although it has lost its plasticity. Shales and indurated clays produce masses which are weaker in the raw and dried state, but develop great strength on firing, so that the strength finally attained may not be greatly different from that reached by more plastic clays and non-plastic material. The chief difference in this respect is that whilst plastic clays will develop © great strength when heated to a relatively low temperature (900°-1000° C.), lean clays must be heated to a higher temperature (1200°-1600° C.), so that they undergo considerable vitrification in order to attain the same strength. An adhesive, such as glue, tar, viscous mucilage, dextrin, flour or starch paste, heavy mineral oils, cellulose or molasses, forms a coating around each of the particles and so unites them together. The resultant mass will be moderately strong when dry, but as such adhesives are destroyed by heat they can only be used to a limited 1 Trans. Amer. Cer. Soc., 12, 564 (1910). 2 J. Amer. Cer. Soc., 2, 336 (1919). 154 STRENGTH AND ALLIED PROPERTIES extent in connection with ceramic materials. The addition of 1 per cent. of dextrin, according to H. W. Douda,! increases the crushing strength of dry clay from 16-52 per cent., depending on the nature of the clay. A hydraulic bond, such as Portland cement, lime, etc., behaves like an adhesive, so far as the dried materials are concerned. On heating the mixture, the hydraulic bond is usually converted into a glassy bond. A glassy or vitrified bond consists of a glass- or slag-like mass of molten material which, when the ceramic material containing it is at a sufficiently high temperature, melts and flows into the interstices between the solid particles. At that stage the mass as a whole is weak, but when it has cooled and the bond is solidified, an extremely strong bond is obtained. A glassy bond can seldom be used when the material is in the moist or dried state—unless water-glass (a sodium silicate) is included in this type of bond—as it only becomes adherent when fused and afterwards allowed to solidify. In most ceramic materials, the glassy bond is produced when the articles are in the kiln and, consequently, such materials only attain their maximum strength after they have been “‘ burned ” or “ fired.”” The formation of a vitrified bond is described on p. 160 and in Chapter XIII. The mode of preparation of ceramic materials often has a very important influence on the strength of articles made from them. The grinding of the materials determines the sizes and shapes of the various particles and so directly affects the strength of the mass (p. 151). The amount of water present or added to the material largely influences the strength of the mass, both directly in the damp material and indirectly in the dried and fired product. When less water is present in the ground raw material than will form a thin film around each particle, the addition of a further quantity of water will increase the strength of the material. When each particle is completely surrounded by a film of water of the required thickness, the addition of further water will effect a reduction in the strength and an increase in the fluidity of the mass. A deficiency of water is usually preferable to an excess in the preparation of ceramic materials. The effect of using a variable proportion of water in the manufacture of silica bricks containing 1} per cent. of lime, the whole material being ground to 200-mesh, is shown in Table XLI, due to M. Phillipon.? Further information on the effect of water will be found in Chapter VI in the section on “ Water of Plasticity,” and in Chapter VII. The proportion of added materials, such as the bonding agent, etc. (p. 153), may seriously affect the strength of the prepared material. It is considered later with respect to various substances. Sometimes a very small proportion of an added material will have a very marked effect on the strength of the product. This is particularly noticeable in the case of 1 J. Amer. Cer. Soc., 3, 885 (1920). 2 Rev. de Métal., 15, 51 (1918). EFFECT OF BOND ON STRENGTH 155 electrolytes, which, when added to clay in the form of pastes and slips, considerably increase the strength of some dry and fired goods made from them, though few figures have been published which show the actual increase. Taste XLI.—Effect of Amount of Water on Strength Compressive Strength Compressive Strength mii oe aeons Water, per cent. after drying, 1300° C., kg. per kg. per sq. cm. a aay 15 12 180 16 13 195 ali 15 190 18 15 220 19 17 250 20 ry 265 21 20 270 H. G. Schurecht + has found that the presence of some electrolytes had an important effect on the strength of dried clay, the following substances increasing the strength, in the order shown :— (a) Sodium hydroxide, which may quadruple the strength of a clay paste. The addition of 0-8-2-5 per cent. of alkali was the most effective. (b) Sodium silicate. (c) Sodium carbonate. (d) Tannic acid. (e) Calcium hydroxide. H. W. Douda? has found that the addition of 1 per cent. of sodium hydroxide increased the dry strength of a stoneware clay by 79-22 per cent. on the unground material, and nearly trebled it after wet grinding. A flint clay which showed a modulus of rupture of 305 lbs. per square inch, after wet grinding with plain water had its modulus of rupture increased to 439 Ibs. per square inch when the material was ground with a | per cent. solution of sodium hydroxide. Further information on the effects of electrolytes will be found in Chapter VI. The method of mixing ceramic materials has a very great influence on the strength of the products. If, as is often the case, the mixing is incomplete, the strength of the articles will vary in different parts of their structure. The process of “ pugging ”’ is a far less effective method of mixing than “ tempering ”’ the material in an edge-runner mill with revolving pan (the so-called wet-pan mill), though pugging is much cheaper than tempering. H. W. Douda? has found that by tempering a 1 J, Amer. Cer. Soc., 1, 201 (1918). 2 Ibid., 3, 885 (1920). 156 STRENGTH AND ALLIED PROPERTIES stoneware clay for 2 hours in a wet-pan mill, he increased its strength by 74 per cent. when dry and doubled its strength when burned at Cone 2. Similarly, the modulus of rupture of a dried flint clay was raised from 35 lbs. per square inch to 305 lbs. per square inch after tempering the wet material for two hours in a wet-pan mill. Similar increases in the strength of the dried goods were obtained with several different materials. The time occupied by the mixing or tempering process also has an important effect on the strength of the product. Not only is a sufficient period of mixing or tempering very necessary to secure a uniform mixture of the materials, but it will also be found that imperfectly mixed ceramic materials are a constant source of annoyance and loss. On this subject, also, little information has been published respecting clay wares, but Table XLII, due to R. M. Howe and W. R. Kerr,! shows the effect of the time of tempering on the strength of the silica bricks. TasLeE XLII.—EHffect of Tempering on Strength of Silica Bricks Time of Tempering, Modulus of Rupture, mins. Ibs. per sq. in. 10 440 15 446 20 499 The difference is much greater with clays than with non-plastic materials. Ageing.—When a plastic material has been mixed, its preparation is not finished ; it should, if possible, be set aside for several days in order to “ age”? it. This “ ageing” of clay pastes has an important effect, as it appreciably increases the strength of the product, especially in the dry state, by securing a more even distribution of the water present and facilitating the retention of the colloidal matter in the mass. The period of rest during which the material is “‘ ageing” may vary from a few hours, which has an appreciable effect for some clays, to the period of a century or more, which is reputed to be the length of time which the ancient Chinese kept their pastes prior to using them for the manufacture of china and porcelain. It is seldom practicable to allow more than a fortnight for ageing brick and tile clay, but for much coarse pottery a shorter period may suffice. An adequate period for “ageing ” is almost essential to the production of retorts, glass-pots, and crucibles of best quality, as all these articles are required to stand severe conditions when in use. The effects of ageing are not so marked with non-argillaceous materials, though still apparent. For further information on “ ageing” see Chapter VI. 1 J. Amer. Cer. Soc., 5, 164 (1922). EFFECT OF METHODS OF .SHAPING 157 Effect of Shaping on Strength.—The method of shaping articles has an im- portant influence on their strength. Hand-moulded articles are seldom as strong as machine-made ones, provided the machine is of a suitable character for the material. It is not always easy to ensure the suitability of the machinery, as so much depends on apparently trivial properties in the material. Thus, in making articles by extrusion or expression, a defective die or mouthpiece puts an excessive strain on the column of clay as it issues from the opening and so the material may crack. The cracks may not be serious and sometimes are not visible when the articles are in the freshly-made state, but during drying and firing, cracks or other defects may develop and seriously weaken the goods. A laminated structure is sometimes imparted to a column of clay made by extrusion ; it is usually due to lack of sufficient space between the bridge which carries the knives of the machine and the end of the mouthpiece, but it may also be due to the lamellar structure of the original clay (p. 22). The various methods of shaping ceramic articles afford many other opportunities for reducing the strength of the material. Thus, if they are shaped by compression in a press which does not apply a uniform pressure over the whole surface, the resulting article will be subject to internal strains and will have a low strength, no matter how great a pressure may be applied to some parts of it. When the pressure is applied uniformly, its amount affects the strength of the product. Under normal conditions, the greater the pressure applied the greater will be the strength of the article produced. Thus, Table XLIII, due to Watkin,! shows the effect of the pressure applied in producing tiles by compression of an almost dry clay-dust on their tensile strength. TaBLte XLIITI.—Effect of Pressure on Strength Tensile Strength, Ibs. per sq. in. Tensile strength, Pressure, lbs. per sq. in. Ibe. per eq) ine Pressure, lbs. per sq. in. 4000 1400 2000 850 3750 1750 850 3500 980 1500 960 3250 1250 960 3000 1150 1000 940 2750 “fe 750 810 2500 1060 500 790 2250 1060 The effect of the method of manufacture upon the strength of bricks is well shown by the following figures due to Emery and Bradshaw,” who found that lime-bonded, 1 Trans. Eng. Cer. Soc., 17, 111 (1917-18). 2 Loe. cit., p. 132. 158 STRENGTH AND ALLIED PROPERTIES hand-made silica bricks had’a crushing strength of 1630 lbs. per square inch, whilst bricks made of the same material by power-driven presses had a crushing strength of 2270 lbs. per square inch. Bricks of each type were heated slowly to 600° C., maintained there for one hour and then withdrawn from the furnace and allowed to cool in air. Their respective average crushing strengths before and after reheating were— Decrease in crush- Before. After. ing Strength, per cent. Machine-made bricks : : 2270 2080 8-5 Hand-made bricks. : 1630 1050 35:5 Bricks, tiles, etc., which are pressed and afterwards repressed are improved in appearance, but reduced in strength, unless exceptional care is taken to ensure their being exactly the right size, so as just to fill the box of the repress. If an article is too small, its shape will be altered in the repress and its structure will be disturbed by the second pressing. If the article were not quite homo- geneous—and mechanically pressed ones tend to laminate—it would be less homogeneous after repressing it and, therefore, more liable to crack or break when subjected to any sudden stress. Effect of Drying on Strength.—The manner in which articles made of clay and allied materials are dried has a great influence on their strength when in the dry and also in the fired state. To avoid rupture, all materials having plastic or kindred properties must be dried under conditions which will permit the water to be removed at a uniform rate throughout the mass and without the formation of an impervious skin or crust through which water from the interior of the mass cannot penetrate. The necessity of drying under suitable conditions (see Chapter VII) is particularly great with articles containing a large proportion of plastic clay, as these are specially liable to form a hard surface-skin which, at a later stage in the drying, is very liable to crack under the pressure of the water vapour which cannot escape through it, as the partly-dried material is very weak and cannot withstand the strains produced in it by the pressure of water vapour in the interior and by the uneven contraction. Joints in the articles are usually very weak at this stage, and so need special care in | drying, and also when drying articles which vary greatly in thickness in different parts. When articles are dried before being placed in the kiln it is often desirable not to remove the whole of the water, as this causes them to become very friable and difficult to handle without rubbing the edges and fine mouldings, and so causing irreparable damage. A very small proportion of water is sufficient to overcome this difficulty. Materials consisting largely of plastic clay increase in strength and rigidity when EFFECT OF DRYING ON STRENGTH 159 dried slowly and, under favourable conditions, the more thorough the drying the greater is the crushing strength. For this reason, an article which has been air-dried at ordinary room temperature may have only about half the strength of a similar one which has been dried at 110° C. The difference in strength is due to the small proportion of moisture still left in the air-dried product, which enables the material to retain some of its original mobility. The difference in the strength of clay wares when dried at different tem- peratures was investigated by C. W. Saxe and O. 8. Buckner,’ and is shown in Table XLIV. Taste XLIV.—Variations in Tensile Strength of Clays according to Mode of Drying (Tensile Strength in Ibs. per sq. in.) Clay. Air Dried. qe re ee eoaae e 4 or 24 hours. | for 24 hours. : Desiccator. Ball clay ; 57-5 59-4 63-4 130-5 Ball clay ; 32-9 34-9 38-7 69-8 Ball clay , 30-1 32:5 35-4 65-8 Plastic clay . ; 65:3 69-9 77-1 107-4 Ballclay 49-7 52-1 54-5 85-5 Plastic clay . ; ; 58-9 59-8 54-4 83-3 German crucible clay. 64-1 70-6 76-1 87-9 American crucible clay . 52-6 61-2 62-0 107-2 Slip clay ; : : 46-1 51-2 57-3 79-2 The effect of cooling the completely dried material in a desiccator so that it cannot absorb moisture from the air during cooling is appreciable. It is shown in the last column of the Table. j The strength of wholly non-argillaceous materials increases to some extent during the drying, but when fully dry they are not usually so strong as articles containing clay, because they do not contain either so much or so stronga bond. In most cases, such materials are much more porous than those containing clay, so that the moisture escapes more readily and the shrinkage is much less. There is, consequently, little liability of damage by rapid drying, as the stains which may occur in clay ware do not arise. For this reason, non-argillaceous materials may usually be dried quite satisfactorily at a fairly rapid rate. Effect of Burning on Strength.—The conditions to which ceramic articles are subjected in the kiln forms one of the most important factors in determining their final strength. If they have been heated at a sufficiently slow rate, so as not to crack 1 J, Amer. Cer. Soc., 1, 113 (1918). 160 STRENGTH AND ALLIED PROPERTIES them, they will be much stronger after firing than in the plastic or dry state, the final strength depending chiefly on (i) the nature and amount of the bonding material produced during the heating; (ii) the particles of aggregate united by the bond ; and (ili) the better consolidation of the grains of material which result from the shrinkage of the material during the firing. The bond affects the strength of the fired material after the latter has been allowed to cool, because the bond is, as its name implies, the agent which unites the other particles together. Consequently, the stronger the bond and the larger the pro- portion of it present, the stronger will be the articles. For this reason, vitrified masses —in which all interstices between the particles of aggregate are filled with the bond —are stronger than porous ones; the vitrified material not only contains a larger proportion of bond, which unites the other particles together more securely, but the bond itself also possesses great intrinsic strength. Clays containing at least 6 per cent. of ferric oxide, or its equivalent, when fired in a reducing atmosphere, form a strong product, because the reduced iron oxide acts as a flux, combines with the clay and so produces a mobile fluid of fused matter which penetrates and fills the pores (see Chapter XIII). When the same clays are fired in an oxidising atmosphere, the oxidised iron oxide does not fuse, and the small proportion of fused material formed does not fill the pores, so that a weaker and less vitrified article is produced. Porcelain, china, stoneware, and other articles: of a vitrified nature are all much stronger than porous materials. Magnesia bricks, when cold, are very strong, their strength being probably due to the proportion of fluxes present and the high temperature at which they are fired. Most refractory materials cannot have their strength developed to the maximum mentioned above, because sufficient bond cannot be formed at the highest temperatures commercially attainable at present. The aggregate affects the strength of an article according to the strength of the individual grains of which it is composed (see p. 151), unless these grains are porous and are completely saturated with the bond, in which case their strength may be increased. ; The shrinkage which a material undergoes in firing affects its strength as described on p. 158, but if the shrinkage is excessive it may reduce the strength of the finished article on account of the large amount of rearrangement which the particles undergo during the heating and shrinking ; this is particularly the case with mixtures which are heterogeneous in character and are deficient in vitrified bond. When a large amount of vitrification occurs, greater changes in the volume of the material may take place without decreasing the strength, though an excessive amount of vitrification must be avoided or deformation or cracks may result. The maximum strength in a fired mass is usually obtained when the amount of vitrified material or bond is just sufficient to fill the pores and to unite all the individual unfused particles, but there is not enough to separate these particles from each other to an extent which allows them to slide or slip apart. For further information on shrinkage see Chapter XIII. The temperature attained in the firing affects the strength of the finished and cold FIRING TEMPERATURE AND CRUSHING STRENGTH 161 articles, because it determines, to a large extent, the amount of fusible matter or bond produced, and this, in turn, controls the strength of the mass. The effect on the crushing strength of firing materials at various temperatures is shown in Table XLV, due to Saxe and Buckner ! :— Taste XLV.—Effect of Firing Temperature on Crushing Strength of Clay Room Clay. Dry. | 55°C. | 110°C. | 200° C. | 325° C. | 575° C. | 825° C. 20° C. Ballclay. a3! 89 125 110 111 195 221 Ball clay. . | 38 60 62 76 84 126 163 Ball clay. - | 36 59 66 73 78 129 158 Plastic clay ae. eo 114 134 129 133 151 468 Ball clay. : a Ot 80 94 98 103 151 266 Plastic clay Bn Oo 107 125 121 119 184 322 German crucible clay 86 91 114 127 141 253 331 American crucible clay . | 73 104 145 145 153 255 294 Slipclay . ; bie 14 19 24 17 19 9 24 Shp clay. als De. 75 88 80 87 92 274 It will be observed that the strength of clays from 110° C. up to about 325° C. is practically constant. During the period of decomposition of the clay (500° C.) the strength of the materials mentioned increases quite rapidly except in the case of the two slip clays, where the decomposition has little effect on the strength. It is unfortunate that these investigators did not extend their tests to include clays fired at all temperatures up to those at which loss of shape occurs. The lack of strength in many bricks, saggers, etc., is due to the fact that insufficient care is taken in firing them. Some saggers, for example, are merely placed in a kiln with other goods and burned at the same rate as the latter, with the result that the temperature to which they are heated is not sufficiently high to produce the maximum attainable strength. If such saggers were fired independently to a higher temperature their strength would be considerably increased, though the cost of this additional firing must not be overlooked. Table XLVI, due to R. M. Howe and W. R. Kerr,? shows the effect of the tem- perature in the burning on the modulus of rupture of silica bricks. The rate of firing in the kiln also affects the strength ; if too rapid it may cause cracks or “‘bloating’’ in the articles, both of which are a distinct source of weakness. 1 Loc. cit., p. 159. 2 Loc. cit., p. 156. 11 162 STRENGTH AND ALLIED PROPERTIES TaBLeE XLVI.—Effect of Firing Temperature on Modulus of Rupture of Silica Bricks. Temperature of Burning. Average Modulus of Rupture, lbs. per Seger Cone. o¢. sq. in. 11 1320 303 14-15 1410-14385 368 16-17 1460-1480 444 17 1480 572 18 1500 533 19 1520 514 The duration of the firing also affects the strength of the articles when cold, because prolonged heating at a sufficient temperature produces an increase in the proportion of fusible bonding material equal to that formed by a shorter heating at a higher temperature (see Chapter XIII). Hence, it is not only necessary for the final tem- perature attained in the burning to be sufficiently high, but in many cases it is equally necessary to maintain the kiln at that temperature for a sufficient time to enable the maximum strength of the contents to be developed. This is especially the case with porcelain (also with silica and magnesia bricks) in which special reactions must occur before the maximum strength is obtained, but it is also important with many other ceramic materials. The atmosphere in which the goods are fired may influence the strength ; some, such as red bricks, firebricks, and most refractory materials are fired, as far as possible, in an oxidising atmosphere, as this ensures a material which will have the greatest strength at high temperatures, but if some of the same materials (which contain ferric oxide or its equivalent) were fired in a reducing atmosphere, their strength, when cold, would be greatly increased, because of the larger proportion of bond formed (see p. 148). A reducing atmosphere greatly reduces the resistance of refrac- tory materials, and of most ceramic materials containing ferric oxide, to load at high temperatures. The cooling of the kiln or oven also affects the strength of the contents, as too rapid a cooling may produce fine cracks or “ dunts”’ in the ware. Articles made of silica, magnesia, and vitrified ware are particularly sensitive in this respect, and articles made of them, therefore, should be very carefully cooled after firing. The cracks produced are frequently so small as to escape notice, when the articles are immediately withdrawn from the kiln, but they greatly reduce the strength of the ware. With some materials, such as most clay products, the actual rate of cooling need not be detrimental if it is effected by passing a large volume of air, at a temperature CHANGE OF TEMPERATURE AND STRENGTH 163 only slightly below that of the contents, through the kiln. The cracks and other defects attributed to rapid cooling are chiefly due to the great difference between the temperature of the articles and that of the air admitted to the kiln. In other materials, however, the rate of cooling is important, especially where a critical range of cooling has to be passed through, as in the case of silica and magnesia; the rate of cooling must then be regulated so as to effect all the necessary physical or chemical changes desired and yet prevent any undesirable ones. Such regulation can only be learned by constant inquiry. An investigation of the critical range of cooling of various porcelains and other ceramic materials would probably result in a decrease of the total time required for cooling the kilns and would also reduce the proportion of cracked and dunted ware. Repeated changes of temperature, especially in the case of refractory materials, gradually reduces their strength to an extent depending on (a) the coefficient of ex- pansion or contraction of the material, and (b) the chemical and physical changes which may take place during the repeated heating and cooling. Materials such as silica and magnesia, which are very sensitive to sudden changes in temperature, soon lose so much of their strength as to become useless unless carefully treated (see also Chapter XIII). Repeated Heating.—When fireclay and grog bricks are repeatedly heated to a high temperature, their resistance to blows is reduced ; this reduction is attributed by Mellor and Austin to— (a) The volatilisation of alkalies and silica, which results in a reduction of the mechanical strength. (b) The irregular contraction of the mass as a result of the presence of irregular _ patches of crystals formed by chemical reactions, which occur on repeatedly heating the material. (c) Crystallisation which occurs when clays are repeatedly heated at temperatures above 800° C. for long periods of time. The ordinary burning period is not sufficiently long to effect any appreciable amount of crystallisation in firebricks, but when the heating at high temperatures is prolonged for long periods, crystallisation, with formation of sillimanite, frequently occurs. The recrystallisation of quartz as cristobalite and tridymite, and of magnesite as periclase are described in Chapter VIII. The effect of repeated heating and cooling on quartz glass or “ fused silica ”’ is very slight, provided the temperature does not exceed 1120° C., but, according to a report by the National Physical Laboratory, prolonged heating at 1188° C. causes a slight, though not serious, diminution of strength after eight hours. About 45 per cent. of the strength is lost on heating for only four hours at 1350° C. For this reason, fused silica ware should not be heated for any long period to a temperature above 1200° C. The temperature during use has an important influence on the strength of ceramic materials, because their strength is usually much lower at high temperatures. They retain their original (cold) strength until a temperature is reached at which the bonding material begins to soften and yield under pressure, or at which, as 164 STRENGTH AND ALLIED PROPERTIES previously mentioned (p. 147), the fluxes present in the material begin to form com- pounds which soften or fuse and so decrease the strength of the mass at the high temperature to which it is exposed. In this way, fluxes play a double part, as they reduce the strength of a ceramic material at high temperatures and increase it at lower ones. Sometimes the reverse is the case and the addition of what appears to be a flux actually increases the strength of the fired material. Thus, O. L. Kowalke and O. A. Hongen have found that the addition of 74 per cent. of silica to magnesia bricks increases the temperature at which they fail under a load of 66-5 lbs. per sq. inch from 1680° to 1870° C. Such a case is quite unusual and is difficult to explain satisfactorily. The strength of refractory materials when hot is often more important than the ~ strength when cold, as the latter is usually ample, whilst the former is not merely lower, but is often an unknown factor. Moreover, as the strength when cold is usually much greater than the strength when heated, any material which is sufficiently strong at the highest temperature attained during its use, is almost certain to be perfectly satisfactory at ordinary temperatures. As most ceramic materials are not quite pure, they do not usually possess a sharply- defined softening or yielding point when heated, and consequently their crushing strength diminishes gradually over a long range of temperature; this is still more noticeable if the material be subjected to a considerable pressure or load which is not intense enough to crush the cold material, but causes it to rapidly lose its shape when any softening of the bond or other fusible constituent occurs. If the material under examination consists of a single pure substance, such as pure silica, however, the strength remains fairly constant until the fusion-point is reached and loss of shape then occurs suddenly, especially if the material is under pressure. That the long range of temperature through which some materials lose strength progressively is due solely to the gradual production of fused or partially fused material is clearly shown by varying the pressure applied to the material, when it will be found that loss of shape occurs at a lower temperature when the pressure is greater and vice versa. When only a very small amount of mobile material has been formed a much greater pressure is required to deform the mass and to cause the unfused particles to move through or in the fused portion. As the temperature increases, or the time of heating is prolonged, more and more fused and mobile material is formed and less pressure is needed to move the remaining particles. The fact that most ceramic materials have a much lower strength at high tem- peratures than when in the cold state, is very important in connection with their use in the construction of furnaces, retorts, crucibles, and other refractory articles. It clearly shows that at temperatures and pressures approaching the critical point it is much safer to use a single substance of a comparatively simple nature, like silica, rather than a complex material such as fireclay. In commercial use, however, attention must not be unduly concentrated on this one factor, for others, such as the great sensitiveness of silica to sudden changes in temperature, are also important. Moreover, it is often preferable to use a material like fireclay, which, as a result of its long-softening range, gives ample warning of its gradually diminishing strength, STRENGTH AT HIGH TEMPERATURES 165 rather than a material such as very fine silica, which collapses somewhat suddenly and with little or no warning. The loss of strength of fired ceramic materials with a gradually increasing tem- perature has been investigated by V. Bodin, who found that some refractory materials, such as fireclay, bauxite, carborundum, corundum, silica, and zirconia, decrease in strength when heated up to about 800° C., but increase in strength rapidly at temperatures between 800° and 1000° C. At temperatures above 1000° C., their strength, in most cases, decreases gradually. These observations are in agreement with the fact, long known to users of the large crucibles employed in the manufacture of “crucible steel,” that the strength of such crucibles when they are at a bright- red heat is greater than at ordinary temperatures, though at still higher temperatures the strength is gradually reduced on account of the softening of the fusible matter present in the materials of which the crucibles are made or absorbed by the crucibles during use. Bodin found that the strength of bricks made of magnesite and chromite decreases gradually up to the highest temperatures and does not show any increase at 1000° C. This distinguishes these two materials from the others examined by him. Loss of strength at high temperatures may also be due to physical or chemical rearrangements of the constituents of the mass. Thus, A. 8S. Watts ? has suggested that the softening of fireclay bricks at about 1300° C. when under pressure sometimes may be due to a period of weakness, coinciding with the formation of sillimanite, this being analogous to the period of weakness in silica bricks when the quartz changes to cristobalite or tridymite. This appears to be confirmed by the fact that some defective fireclay saggers show an important amount of crystal development. When sufficient sillimanite is formed, the strength of the material is increased. Articles made of carborundum decrease in strength at a temperature of about 1200° C. on account of the partial oxidation of the carborundum with the formation of free silica and carbon dioxide. The decomposition is arrested when a film of silica is formed over the surface, thus preventing further oxidation. The loss of strength at high temperatures may also be due to structural defects developed during the heating of the material, either (a) as a result of original defects, or (b) as a result of changes effected by the conditions of heating. The ratio of strength at low temperatures to that at high ones may be calculated, in the case of fireclay products, from the following formula :— Bending temperature=Ce*” proposed by J. W. Mellor,’ where C denotes the bending temperature (in Seger cones) without any load, w the pressure applied in lbs. per square inch, e is the exponential constant, and & a numerical constant depending on the nature of the clay, the method employed in making the test pieces, etc. ; it varies from 0-003 to about 0-02. 1 La Céramique, 23, 177-184 (1920). 2 J. Amer. Cer. Soc., 3, 450 (1920). 3 Trans. Eng. Cer. Soc., 15, 117 (1916). 166 STRENGTH AND ALLIED PROPERTIES G. A. Loomis,! on the contrary, considers that there is no definite relationship between the strength and refractoriness of fireclay bricks at high temperatures, though he found that bricks having a softening point lower than Cone 28 would not withstand a pressure of 40 lbs. per square inch at 1350° C. It appears impossible that any simple formula can be used to express the relation between the strength of fireclay bricks, etc., at high temperatures to that at ordinary temperatures, as such a relation must be dependent, to a large extent, on the texture of the articles. Thus, very fine-grained bricks usually have a smaller relative strength at high temperatures than coarse-grained, open bricks, because the latter are not so readily affected by the fluxes present. Fine-grained bricks, on the other hand, are more dependent on the films of fused matter between the grains, because at high temperatures the materials forming these films, whilst not sufficiently softened to flow freely, are viscous enough to permit the deformation of the mass. Effect of Weathering on Strength.—Exposure to weather greatly decreases the strength of most ceramic articles, those composed of clay which has merely been dried being the most affected. Table XLVII, due to Gebauer,? shows the compressive strength of air-dried clay bricks and the effect on them of constant contact with a moist atmosphere. Taste XLVII.—E£ffect of Moisture on Strength of Dry Clay. Compressive Strength, kg. per cm.? 2 : Nature of Bricks. Ntiee a re Air Dried. Storage in Contact with Moist Air. Machine-made . : 42:3 » 242 * A 34-9 21-1 Hand-made . 30-9 21:3 - 26-1 20-1 Most building bricks and roofing tiles are very resistant to weathering, but this is not the case with firebricks. Table XLVIII, due to Howe, Phelps, and Ferguson,? shows the effect of exposure on the strength of refractory bricks. It will be seen that the cold fireclay bricks lost from 1-21 per cent. of their crushing strength after six months’, and 11-28 per cent. after twelve months’ exposure. One brand of specially strong machine-made bricks was found to withstand six months’ 1 U.S. Bur. Stand. Tech. Paper, 159 (1920). 2 Tonind. Ztg., 44, 301-3 (1920). 8 J. Amer. Cer. Soc., 5, 109 (1922). WEATHERING AND STRENGTH 167 exposure to the weather practically without loss in strength. Some very porous hand-made fireclay bricks are greatly reduced in strength on exposure and may be made practically worthless after six months’ exposure. Taste XLVITI.—Effect of Exposure on Crushing Strength of Bricks Type. Strength. a anaved. After 6 Months’| After 12 Months’ Exposure. Exposure. Fireclay brick A. Maximum 1418 1404 1109 Minimum 782 855 891 Average 1127 1114 1005 Per cent. deviation 22-8 13-4 9:3 Fireclay brick B . Maximum 1355 909 755 Minimum 545 545 545 Average 885 693 640 Per cent. deviation 26-9 20-5 10-1 Silica brick . ‘ Maximum 2727 2245 1418 Minimum 1373 1327 909 Average 1830 1680 VE Per cent. deviation 23-1 17-7 17:5 Magnesia brick. Maximum 5582 4255 3914 Minimum 3273 2726 2082 Average 4464 3780 2978 Per cent. deviation 14:5 12-7 17-4 Silica bricks may lose as much as 39 per cent. of their crushing strength when cold if they are exposed to the weather for twelve months, whilst magnesia bricks decrease in crushing strength by 15 per cent. after six months’ exposure and 33 per cent. after twelve months’ exposure. The strength of magnesia bricks when heated under load is also affected, some bricks, after being weathered for twelve months, failing, under a pressure of 25 Ibs. per square inch, at a temperature of 40° C. lower than new bricks. Magnesia bricks are more seriously affected by the weather than almost any other kind of refractory brick and much greater care is therefore required in their storage. Effect of Frost on Crushing Strength.—The crushing strength of articles is reduced by subjecting them to the action of frost, as shown in Table XLIX, due to J.C. Jones.4 1 Trans. Amer. Cer. Soc., 9, 567 (1907). 168 STRENGTH AND ALLIED PROPERTIES The effect of freezing does not appear to be related to the original strength of the brick, but to depend upon other factors such as texture, porosity, temperature, and duration of burning, etc. Some curiously contradictory results are sometimes obtained when the crushing strengths of bricks, etc., which have been repeatedly subjected to freezing, e.g. in a refrigerator, are determined. TaBLeE XLIX.—EHffect of Freezing on Crushing Strength Crushing Strength. . : Per cent. Loss Per cent. Kin : Hard eee a dooee aaa in Strength. | Pores Space. Frozen. Unfrozen. Plastic surface clay Soft 1194 1374 13-1 33-0 Med. Soft 3567 3400 4-61 26-9 Med. hard 4289 5315 19-9 212 Hard 7377 7260 1-61 10-2 Plastic shale . : Soft 2671 2913 8-6 26-2 Med. soft 4625 5793 20-2 17-8 Med. hard 8522 10143 16-5 11-6 Hard 7606 11470 33°8 5-8 Wire-cut shale : Soft 3729 4637 19-6 27-6 : Med. soft 6965 8117. 14-2 17-1 Med. hard 9165 11315 19-4 2-1 Hard 11500 11997 4-] 0-9 Articles composed of uniform grains appear to be more resistant to frost than those composed of various-sized grains, as in the former the water is more easily distributed when expansion occurs as a result of the water freezing. Thus, a uniform- grained brick may have a porosity of 20 per cent. and not burst when frozen, whereas one with only 5 per cent. porosity may be destroyed if its texture is irregular. This statement has been contradicted by Purdy and Moore,? who consider that an article composed of particles having the greatest range of sizes of grains, and consequently of pores, has also the greatest resistance to frost. According to J. C. Jones,® a hard, brittle brick requires a greater force to dis- integrate it than a softer brick, but the distance through which the force must act ig smaller, as the former cannot be strained to the same extent as the latter with- out failure. For this reason, vitrified bricks require a greater force to commence 1 Increase. 2 Trans. Amer. Cer. Soc., 9, 704 (1907). 3 Tbid., 536. EFFECT OF SLAGS, ETC., ON STRENGTH 169 disintegration, but when once the destruction is started it proceeds more rapidly than with softer and more porous bricks. Hard bricks are often less resistant to frost than softer ones on account of the pores, though small, being rigid and having a small elastic limit, so that they are unable to stand much strain when a force is applied. There is, however, no definite relation between the hardness of bricks, etc., and their resistance to frost. J.C. Jones obtained the following results :— TaBLe L.—Effect of Freezing on Strength of Bricks Per cent. Loss of Strength. Extent of Burning. Shale in Plastic Surface Clay. Wire-cut Process. | Average. State. Soft . ‘ : 13-1 8-6 19-6 13-8 Med. soft . 5 4-6 1 20-2 14-2 9-9 Med. hard . : 19-9 16:5 19-4 18-6 Hard . ‘ : 1-61 33:8 4-] 12-1 It will be seen that the surface clay has least resistance to frost in the medium, hard-burned condition ; the plastic shale suffered most in the hard-burned condition ; whilst the shale shaped by the wire-cut process was almost equally affected in the soft and medium hard-burned conditions. Effect of Blows in Reducing Strength.—The crushing strength of most ceramic materials is greatly reduced by subjecting them to repeated blows. Effect of Deposited Carbon on the Strength.—-The deposition of carbon on and in the pores of fireclay articles is very detrimental to their strength, both at low and high temperatures, on account of the decomposition of the clay which appears to occur as a result of the catalytic action of the carbon. The diminution in strength is most serious in retorts which are used for the distillation of carbonaceous materials. Carbon does not appear to have the same effect on highly siliceous materials. Effect of Slags and Flue-dust on Strength.—The prolonged subjection of any ceramic articles to the action of molten slags greatly reduces the strength of the articles, especially at high temperatures. If the articles are porous they absorb the molten slag, and Nesbitt and Bell 2 found that, after such penetration, the crushing strength of some red-hot silica bricks was reduced from 1451 lbs. per square inch to 775 Ibs. per square inch. Denser bricks would have a less penetration and, conse- quently, the crushing strength would be less affected. 1 Gain. 2 Proc. Amer. Soc. Test. Mat., 19, 619-39 (1919). 170 STRENGTH AND ALLIED PROPERTIES STRENGTH OF CLAYS AND CLAY-PRODUCTS The strength of various ceramic materials depends on the factors mentioned on pp. 146-169. In some cases, the strength is a very important characteristic, but in others it is not so important and may be sacrificed to secure other properties. Strength of Raw Clay and Clay Pastes.—The strength of raw clays and clay pastes, 2.e. of clays in the plastic state, varies very greatly. Some lean clays will break under a tension of a few ounces, whilst other highly plastic clays may withstand a tension of several pounds per square inch. The duration of the application of the force also affects the result, as a small force acting throughout a long period will have an effect similar to that of a much larger force acting for a much shorter time. As the strength varies with the plasticity of the clay and on the proportion of clay and water present, no definite figures can be usefully published. Plastic clays are usually much stronger in the raw state than shales and indurated clays. Strength of Dry Clays.—The strength of ceramic materials in the dried con- dition is often important, for, obviously, the dried articles must be sufficiently strong to withstand the amount of handling to which they are likely to be subjected and also the weight of any articles placed above them when they are set in the kiln. Such articles as tiles must be particularly strong when in the dry state. Some pieces of pottery also require to be made of material of considerable strength when dry. The strength of dry clay is probably largely due to the interlocking of the grains. This appears to be confirmed by the fact that a well-graded material (p. 152) produces a stronger mass when dried than an ungraded mixture, though others consider that the strength is due rather to the cementing power of the plastic or colloidal matter in the clay. Like clays in the plastic state, dried clays vary greatly in tensile strength; in some, it is as low as a few pounds per square inch, whilst in others it may be as high as 400 lbs. per square inch. Brick clays in the dry state have usually a tensile strength of about 100 lbs. per square inch. Clays which have the greatest plasticity when in the form of a paste usually have the greatest strength when dry. According to H. H. Sortwell,! the average modulus of rupture of various ball clays when dried is as follows :— TaBLe LI.—Transverse Strength of Ball Clays Lbs. per sq. in. Tennessee. , ; 366 Kentucky . : : 282 Devonshire . : : 443 Dorset . : : : 405 Other English clays 419 1 New Jersey Ceramist, Mar. 1922, p. 5. STRENGTH OF DRY CLAYS 171 A. S. Watts? classifies bond clays with reference to their strength as bonds as follows :— TasLe LII.—Transverse Strength of Bond Clays Strength of Dry Clays, Ibs. per sq. in. Low. ; ; 0-100 Medium low . : 100-200 Medium 2 : 200-400 Medium high : 400-800 High . A : over 800 Bleininger and Howat ? found that the tensile strength of various American clays, when dry, varies from an average of 210 lbs. per square inch with some highly plastic ball clays to 34 lbs. per square inch with china clay. When such clays were mixed with an equal weight of standard Ottawa sand their tensile strength varied from 250 Ibs. per square inch to 26 lbs. persquareinch. Their results of tensile and crushing tests are shown in Table LIII. Taste LITI.—Strength of Dry Clays (Ibs. per sq. in.) Without Sand. With Sand. Crushing Tensile Crushing Tensile Strength. Strength. Strength. Strength. Ball clays ‘ ; 565-1148 135-210 464-777 124-190 Plastic fireclays ; 631- 954 155-172 476-553 113-150 Shales. ; , 636— 806 126-187 403-449 77-111 Plastic kaolins. : 455-— 539 104-147 286-559 54-110 Primary kaolins. 205— 349 34— 69 164-172 29- 35 1 J. Amer. Cer. Soc., 3, 247 (1920). P 2 Trans. Amer. Cer. Soc., 14, 274 (1914). 172 STRENGTH AND ALLIED PROPERTIES The transverse strengths of the American clays, examined by Bleininger and Howat,! varied from 558 lbs. per square inch with some ball clays to 74 Ibs. per square inch with some china clays, and when mixed with half their weight of standard sand the transverse strengths were reduced to 330 and 53 lbs. per square inch respectively. Table LIV shows the average limits of strength :— TaBLe LIV.—Transverse Strength of Dried Clays (Results in lbs. per sq. 1m.) Without Sand. With Sand. Ball clays . . : 375-558 242-330 Plastic fireclays . ‘ 484-520 216-280 Shales : : : 311-403 178-237 Plastic kaolins ; 239-325 122-210 Primary kaolins . 74-166 53- 82 The dry strength required in any one case will depend on the nature of the articles tobe made. Large articles, such as bricks, do not need so strong a material, but tiles, pottery, and other thin articles require a much stronger material when in the dry state so as to avoid damaging them in handling. When tiles or other articles are made by compressing the finely ground material in the form of a powder, the dry product is very weak and has to be handled very carefully. As the cohesive power of most materials in the raw and dried conditions is com- paratively low compared with that in the burned state, most articles are very weak, and must, therefore, be very carefully handled so as to avoid damage. Strength of Burned Clay Wares.—The strength of articles, etc., made of burned clay varies according to the materials used and the mode of manufacture, very great variations sometimes occurring in articles made under apparently identical conditions. The need for careful control is, therefore, very evident. The strength of building bricks is not of such great importance as is sometimes supposed, because even a poor brick (if sound) is stronger than well-laid brickwork, because of the weakness of the mortar. Usually the bricks have at least ten times the crushing strength of the brickwork ‘when ordinary mortar is used, but if the bricks are laid in Portland cement the strength of the brickwork is increased so that the crushing strength of the individual bricks is only 24 to 5 times that of the brickwork. Table LV shows the average results of a very large number of tests made by the author on various kinds of building bricks. 4i Sox (Tschermak) 12a pp Si,0,J=K OK (2) aZtstotaal (Clarke) (4) Ee yalCo (Wartha) \[si,Ogl=Al we ) y, OK O/¢ OK (3) Si——O——Al (5) \Q SN Al 0 070 LO fe) ee (Groth) One 0) ' i 0 ) 1= TO : Se ae at CD OREM (Vernadsky) i xe 1 \o_K T OK (6) O=Si—O O—Si=0 Sad De 0=Si—0° 6 \, 0—si=0 basi HzO ox JK (Mellor and Holdcroft) Similarly, the graphic formula of clay has been expressed in various ways, Mellor preferring a combined ring and chain formula, whilst W. and D. Asch use a series of closed rings (see section on The Constitution of Clays, p. 343). Molecular Formulz.—In ceramic processes many of the chemical compounds used and the reactions which take place are not fully understood, so that it is impossible to apply definite chemical formule. For the sake of convenience, however, and to facilitate comparison, a modified type of formula (termed a molecular formula) is 310 CHEMICAL CONSTITUTION often used, not necessarily to denote a chemical compound, the molecules of which contain a definite number of atoms, but simply to show the molecular proportions of each constituent. Thus, whilst in pure chemistry Al,0,8i0, would represent a definite aluminium silicate (sillimanite), in the ceramic use of this formula it could equally well represent either sillimanite or a mere mixture of silica and alumina without in any sense implying any chemical combination. A molecular formula, as used in the ceramic industries, is, therefore, a grouping of the various constituents present in a special manner, to facilitate comparison, but not necessarily to denote chemical combination. The method of classification adopted is based on the amount of oxygen present, the elements (other than oxygen) being always reckoned as oxides. Any entirely volatile constituents are disregarded, calcium carbonate being taken as calcium oxide, as the carbon dioxide takes no part in the composition of the fired material. Partially volatile constituents may or may not be included according to the extent to which they persist in the final product. In molecular formule the constituents are grouped as follows :— (a) Metallic oxides of the R,O and RO type (p. 307). (b) Metallic oxides of the R,O, type (p. 307). (c) Non-metallic and sometimes metallic acid oxides of the RO, type (p. 307). It is customary (following a suggestion of Seger) to take the total number of molecules in group (a) as unity. This involves the use of fractions of atoms, which is theoretically objectionable, but is a great practical convenience. An ideal ceramic molecular formula is represented by RO.zR,0,.yRO,. Thus, the following is the molecular formula of a pottery body :— 0-81 CaO 0-06 K,O0 joo Al,03.4°68i10,. 0-13 Na,O It will be seen that the basic R,O and RO oxides are classed together at the beginning of the formula, whilst the acid, silica, is at the end. In the centre is the R,O, group, which may act either as an acid or as a base according to circumstances. It is sometimes difficult to decide in which group to place certain constituents. Thus, borax is reckoned as Na,O and B,O, and would in the ordinary way be classed with the other R,O, compounds, and yet it is suggested by some investigators that the behaviour of RO, in glazes is such that it should be placed with the RO, group. Fluorine is generally expressed as if it were free fluorine and is placed with the silica in the RO, group. J. HE. Hansen, however, suggests that it would be preferable to introduce the fluorine as a compound in the correct position in the formula according to whether it is basic, amphoteric, or acid. Thus, CaF, would be placed in the RO group, Al,F, in the centre of the formula, and SiF, in the RO, group. He suggests that other compounds containing fluorine should be reduced to their simplest com- pounds and inserted in the proper place ; thus, cryolite should be represented partly as Al,F, and partly as Na,F,. Sodium silico-fluoride should be represented partly as Na,F, and partly as Sif,. This would give an indication of the mineralogical constituents. COMPOSITION FROM MOLECULAR FORMUL . 3all The opacifying and, therefore, feebly soluble constituents in glazes, such as TiO,, SnO,, SbO,, and ZrO, are usually grouped separately. It will be noticed that in a ceramic molecular formula the whole of the molecules corresponding to the chemical composition of a material are not always given. Thus, as explained on p. 310, the volatile constituents are purposely omitted. Apart from these variations the general principles employed in building up a ceramic molecular formula are that given on p. 310. Calculation of Percentage Composition from Molecular Formulz.—To calculate the parts by weight or the percentage composition of a compound or mixture,! from the formula of the type described in the foregoing pages, the following method is used: Multiply the number of molecules of each constituent or group in the formula by the molecular weight of that constituent. Add the results so obtained together, then multiply each result by 100 and divide each in turn by the total of the results. If the arithmetical work has been correctly done the total of the results finally obtained will be exactly 100. Example.—To find the percentage composition of pure china clay corresponding to the formula Al,032Si0,.2H,O, multiply 1 mol. of Al,O3 by 102=102 2 mols. of SiO, by 60 —120 2 mols. of H,O by 18 = 36 258 102 x 100=10,200+258= 39-53 120 x 100=12,000+258= 46-52 36x 100= 3,600+258= 13-95 100-00 The required percentage composition is, therefore, 39-53 per cent. of alumina, 46-52 per cent. of silica, and 13-95 per cent. of water. When it is desired to produce a substance having the same composition as that corresponding to a given formula, it may be desirable to use materials which lose some of their constituents when heated. In that case, it is desirable to ascertain the molecular formula corresponding to the various materials available and to substitute a number of molecules of each material corresponding to the number of molecules in the formula. Example.—To find the proportions of felspar (K,0A1,0,6Si0,),* china clay (Al,032Si0,2H,O),? and flint (SiO,),? required to produce a hard porcelain having the formula K,05A1,0321Si0,. It is obvious that all the K,O in the porcelain formula must be derived from the felspar, so 1 Strictly, a mixture cannot have a chemical formula, as its composition is not wholly homo- geneous, but in order to simplify various calculations such formule are applied i in ceramics to mixtures as well as to chemical compounds (see p. 310). * Simple formule are assumed for convenience. The formule exactly corresponding to the materials available should be calculated from their analysis. 312 CHEMICAL CONSTITUTION that as there is one molecule of K,O, one molecule of felspar (K,0A1,0;68i0,) must be used. But this felspar also introduces one molecule of alumina and six of silica. Deduct from the formula of the porcelain that of one molecule of felspar, thus : Porcelain formula K,O 5Al1,0, 21Si0, (a) 1 mol. felspar K,0O Al,O, 6S8i0O, First difference 4A1,03, 15Si0O, The difference must be made up of clay and flint in a similar manner. Thus, it will be seen that the necessary alumina can be supplied by molecules of the clay which also supply some of the silica. On deducting 4 molecules of clay from the result previously obtained First difference (above) 4A1,0, 15S8i0, (6) 4 mol. of clay? 4A1,0, 8SiO, Second difference 78i0, there is a surplus of 7 molecules of silica ; this would be supplied by 7 molecules of flint Second difference (above) 7Si0, (c) 7 mol. of flint 7Si0, Final difference Nil. Adding together (a), (6), and (c), it will be found that the total contains the potash, alumina, and silica in the same proportions as in the original formula, but in addition are the other in- gredients present in the materials used. (In this instance, the only additional material is the ‘“‘ water ’’ in the clay, but-in practice various “impurities? would be introduced.) The new formula, corresponding to the original one, is, therefore, 1 mol. of felspar K,O Al,O3 6Si0, 4 mol. of clay 4A1,03, 8Si0, 7 mol. of flint 7Si0, The percentage of felspar, clay, and flint required is then found as described on p. 311. Mol. Wt. Per cent. 100 1 mol. f = 556 xX ——= 27-69 mol. felspar x 556 6X so08 4mol.clay x258=1032 ,, = 51-40 7 mol. flint x 60= 420 5) ee 2001 2008 100-00 Calculation of Molecular Formulz.—The formula of a compound or mixture may be calculated from the percentage composition in the following manner :— (a) Find, from the percentage chemical composition of each substance used in the mixture (usually by analysis), the corresponding weight of each constituent. (5) In the case of a mixture, add together all the identical constituents. (c) Divide the total amount of each constituent by its molecular weight. (d) Rearrange the results in the order RO, R,O3, RO». (e) Divide the result by the total number of molecules of RO so as to make RO=1. Example.—To find the molecular formula of a ball clay composed of 49 per cent. silica, 36 per cent. alumina, 1 per cent. of lime, 2 per cent. potash, and 12 per cent. of water. (For simplicity, all constituents of less than 1 per cent. are omitted in this example, but they should be included in actual practice.) The percentage composition required by (a) is given above. As this is not a mixture, section (bd) 1 The 8H,O in 4 molecules of clay, being volatile, is excluded from the calculation. CALCULATION OF MOLECULAR FORMULA 313 is not required, so proceed at once to (c) and divide the constituents of the material by their respective molecular weights. Silica : ¢ : : . 49—~— 60=0-817 Alumina . : , : . 386+102=0-353 Potash . ; A : . 2+ 94=0-210 Lime : : ; 2 . 1+ 56=0-018 Water. r : : . 12— 18=0-670 The figures in the last column show the molecular ratios in which the various substances are present. In accordance with (d) these are arranged as follows :— 0-018 CaO 0-021 K,0 and in accordance with (e) the figures are divided by 0-040 the total of RO molecules present, the final result being : i 0-353 Al,O3 0-817S8i0, 0-670H,0, 0-46 CaO 0-54 K,O which is the required molecular formula. It will be observed that it corresponds fairly closely, but not exactly, with the formula of kaolinite, Al,03.2Si0,.2H,O, to which the purest clays bear a close resemblance. Example.—Find the molecular formula corresponding to a glaze composed of } 9-05 Al,O3 20-95S8i0, 17-2H,0, Cornish stone. . - ; - o4¢ Felspar . - : ; : . 34 Whiting. : : : ; ee Flint. 3 : : ‘ 0 (a) From the chemical composition, the weights of the various constituents are :— Potash, Soda, Lime, Alumina, Silica, Other K,0. Na,O. CaO. Al,O3. SiO,. Ingredients.* Cornish stone . : 1-94 1-02 0:44 5-64 23-94 1-02 Felspar . = A 3:78 0:75 0-10 6-49 22-37 0-51 Whiting 5 4 aie = 12-32 ee A 9-68 Bint . : = ae ae Sih Ee 9-84 0-16 Totals . 4 5-72 1-77 12-86 12-13 56-15 11-37 (b) The result of adding the various constituents common to two or more materials is shown in the line marked “ Totals.” (c) Dividing each of these “ totals’ by its molecular weight gives :— Mol. Wt. 5-72 Potash — 94=0-061 1-77 Soda ~~ 62=0-029 12-86 Lime = 56=0-230 12-13 Alumina—102=0-119 56-15 Silica + 60=0-936 1 For the sake of simplicity, these are not considered in working out this example, though where the results are important they should be included. 314 CHEMICAL CONSTITUTION (d) Rearranging the figures in the last column gives :— 0-061 K,0 0-029 xo boa Al,O; 0-936 SiO, . 0-230 CaO (e) Dividing these figures by 0-32, the total of the RO group, gives :— 0-19 K,O } 0-09 Na,O 0-37 Al,O; 2-93 SiO,. 0:72 CaO As noted on p. 311, it is convenient, though not strictly correct, to represent fractions of molecules ; this is done in ceramics in order to facilitate comparisons of various materials. For some purposes, e.g. in comparing clays, it is more convenient to take the alumina in the formule as unity, and it is sometimes desirable to multiply the formule by 100 so as to clear them of fractions. The chief disadvantage of the use of molecular formule is that small proportions of some constituents may be excluded and yet may have an important influence on the material. Thus, the position of iron compounds is anomalous, for whilst ferrous oxide should be placed in the RO group, ferric oxide does not behave like alumina and cannot conveniently be placed with it. Moreover, if a material containing ferric oxide is heated in a reducing atmosphere, some ferrous oxide will be formed, though the amount is difficult to ascertain. Titanium oxide is another substance it is difficult to place properly, though it should apparently be in the RO, group. Colouring agents are not generally included in the formula, so that a colourless glaze may appear to have the same formula as a deep blue glaze, the amount of cobalt oxide required to give this colour being quite small. A still more serious objection to the use of molecular formule is that the behaviour of a substance is not dependent entirely upon its chemical composition, and, though it may seem a comparatively minor matter, yet, as the physical characteristics— which are so important—are not considered in a formula, serious errors may arise. Thus, felspar and flint may be substituted in a glaze for Cornish stone without altering the formula, but the two glazes would behave very differently and one might be quite satisfactory whilst the other is useless. The physical properties of the constituents must, therefore, be taken into consideration as well as their chemical composition. In the examples given on pp. 311-314 a means of simplification (namely, the omission of small percentages of constituents) is used, which may, in practice, lead to serious errors. Even more objectionable is the practice, often found in technical classes, of using typical analyses for the calculations. So far as the imstructor of the students is concerned there is no objection to such typical analyses being used ; but when, as is sometimes the case, such analyses are used in investigations or in works practice instead of direct analyses of the actual materials available, serious errors may arise. A further error may be due to “ rounding ”’ off the figures in a formula in order to make them into whole numbers or to eliminate troublesome fractions. In spite of the disadvantages mentioned, however, the use of molecular formule NORMS 315 is very convenient in facilitating comparison as well as in making complicated reactions more easy to understand. Norms.—A method originally devised for studying the nature of rocks,! but applied by H. F. Staley 2 to ceramic materials, which overcomes some of the objections to molecular formule, consists in the use of norms. This mode of calculation is based on the assumed satisfaction of the bases in the order of their activity with alumina and silica and reckoning any excess as free silica or alumina. According to Cross, Iddings, Pierson, and Washington, the relative affinity of various substances for silica, beginning with the strongest, is: K,O, Na,O, CaO, MgO, FeO, Al,O,, Fe,Os. Similarly, the relative affinity of oxides for alumina is: K,0, Na,O, CaO, MgO, FeO. Ferric oxide does not combine with alumina. These investigators also claim that, in fused masses, each base first unites with as much alumina as possible and a corre- sponding amount of silica; any surplus base then forms the highest silicate possible under the conditions obtaining, some or all of the following compounds being assumed to be produced :— Low Silica. Higher Silica. K,O Al,O, 4810,, leucite. K,O AI,O, 6810,, orthoclase. Na,O Al,O, 2810,, nephelite. Na,O Al1,0, 6Si0., albite. CaO Al,O, 2Si0,, anorthite. af K,0 S10,, potassium metasilicate. Na,O Si0,, sodium metasilicate. 2CaO Si0,, calcium orthosilicate. CaO Si0,, calcium metasilicate. 2ZnO SiO,, zine orthosilicate. ZnO Si0,, zinc metasilicate. 2PbO Si0,, lead orthosilicate. . PbO SiO, lead metasilicate. The following examples of classification by “‘ norms ”’ are due to H. F. Staley. f 05 ae pees 0-5 ean rer Molecular formula . 10:5 CaO 1:5 Si0, 0-5 CaO 0-2 Al,0,2-4 S810, 0-5Na,O 0-5Si0, | 0-2 Na,O 0-2 Al,O, 1-2 SiO, ea 05CaO 0:5 810, | 0-3 Na,O 0-3 Si0, ; 0-5 SiO, | 0-5 CaO 0-5 SiO, 0-4 SiO, The use of norms in the study of molten or partially molten materials is often convenient, though not necessarily exact ; it often suggests what might occur under favourable conditions, and very often the substances predicted by a “ norm ”’ calcula- tion are very similar to those actually found ; in many cases, however, the results are quite hypothetical. Triaxial Diagrams.—When a large number of similar substances are to be compared, as in ascertaining the effect of certain constituents on a glaze or pottery body, a particularly useful means of simplifying the work is to employ a Stokes’ triaxial diagram (see Chapter X1). 1 Quantitive Classification of Igneous Rocks (Cross, Iddings, Pierson, and Washington). 2 Trans. Amer. Cer. Soc., 13, 130 (1911). 316 CHEMICAL CONSTITUTION Chemical Equations.—The initial and final products of a chemical reaction may be expressed briefly and clearly by arranging their formule in the form of an equation, the reacting substances being placed on the left-hand side of the sign of equality and the resultant products on the right-hand side of it. The total number of atoms on each side of an equation must be equal. Thus, the reaction between silica and carbon in the manufacture of carborundum may be expressed as 2810, + 4C = 28iC + 200, Silica. Carbon. Carborundum. Carbon dioxide gas. A single chemical equation shows only the beginning and end of a reaction and gives no indication of any intermediate ones which may occur. Sometimes a chemical reaction is reversible if the conditions of temperature or pressure are changed ; this reversibility may be indicated by substituting the sign == for the usual sign of equality, the half arrows indicating that the reaction may proceed in either direction. Thus, the equation CaCO, —= (ad “ CO, Calcium carbonate. Lime. Carbon dioxide. indicates that under favourable conditions of heat and pressure (i) calctum carbonate may be decomposed into lime and carbon dioxide ; (ii) lime and carbon dioxide can combine to form calcium carbonate ; and (iii) if the conditions are first favourable to reaction (i) this will occur, but if in the course of time the conditions are altered (for instance, if the pressure is increased because the carbon dioxide gas cannot escape) reaction (ii) may commence and all further progress with reaction (i) will cease. The importance of these reversed reactions is often overlooked (see also Chapter XI). Acids, Bases, and Salts.—Many chemical compounds are classed, according to the manner in which they react with each other, as acids, bases, and salts, the last- named term being frequently extended to include all compounds which are neither acids nor bases. An acid cannot be defined satisfactorily, because there is no single criterion of acidity. When the matter is carefully investigated, all the usual definitions of acids are found to be faulty. Thus, the old definition that an acid is a substance which combines with a base to form a salt is very unsatisfactory, because it attempts to define an acid in two unknown terms—base and salt, and, whilst such a definition is often useful, it is far from being ideal. On the other hand, the usual modern definition of an acid as a substance which liberates free hydrogen ions when it is dissolved in an ionising solvent, excludes all insoluble acids and some soluble ones, including silicic acid. Similarly, the definition that an acid is a hydrogen salt is often convenient, but in the absence of a reliable definition of a salt it is unsatisfactory. The subject is further complicated by the fact that silica and some other substances act, at high temperatures, in a manner precisely similar to acids at lower temperatures, except that they do not contain hydrogen. Thus, silicic acid, H,Si0,, is soluble in water and reacts with caustic soda to form a soluble sodium silicate and free water. Silica ACIDS, BASES, AND SALTS 317 (Si0,) is insoluble in water and has only one obviously acid character, viz. it combines with caustic soda in the same manner as silicic acid, except that only half the water is liberated, thus : (i) 2NaOH+H,8i0,—Na,Si0,+2H,0 ; (ii) 2NaOH+Si0, =Na,Si0,+H,0. In such cases, silica behaves as an anhydride (v.e. an acid which has parted with one or more molecules of water), but it is commonly referred to as an acid, though this is not strictly correct. On the other hand, silicic acid, H,SiO,, is not ionised when dissolved in water and so cannot be included among the acids, which are defined in terms of their ionisa- tion products. Silicic acid can only be regarded as an acid in this sense by a process of analogy, inasmuch as its sodium salt Na,SiO, is soluble in water and on electrolysis is readily ionised. Many acids may be regarded as consisting of two parts: (i) the hydrogen which can be liberated on ionisation, and (ii) the remaining portion which is frequently termed the acid radicle.1 This term is misleading, as soluble acids owe their acid properties to the free H-ions and are inert when the conditions are such that no free H-ions exist. If, however, the term “acid radicle”’ is regarded as that part of an acid which enters into the resultant salt and it is also understood that it is not, in itself, able to confer acid properties, it may be used with great convenience and without much risk of serious error. Ina molecule of a soluble acid, the “‘ acid radicle”’ and the negative ion are identical, but, in insoluble acids, the existence of ions cannot be proved and the term “ radicle ’’ may then be preferable. Both a radicle and anion may consist of several atoms, and these may retain their grouping when the substance of which they form a part undergoes certain chemical changes, such as the reaction of an acid and a base to form a salt. Thus, sulphuric acid consists of two radicles—the hydrogen ion (H,) and the negative ion (SQ,). The term “acid radicle”’ is also largely used for a purely imaginary grouping of atoms, such as SO,, SiO,, etc., these groups being really the anhydrides of the corre- sponding acids. Such a grouping is convenient when salts are regarded as molecular rather than ionic compounds. Thus, sulphuric acid may be regarded in three ways as composed of (i) H, ions and SO, ions. (ii) H ions and HSO, ions. (iii) H,O ions and SO, ions (radicles). Both of the first two conceptions agree with the facts. The third conception does not agree with the facts respecting soluble acids, but it is a convenient method of expressing reactions between bases and anhydrides and other apparently acid substances which are devoid of hydrogen. The names of acids give some idea of their composition and properties. Thus, acids 1 A radicle is a group of atoms which can enter or leave a molecule without the elements in it being separated. Typical radicles are K,0, Al,O3, SiO,, SOs, etc. 318 CHEMICAL CONSTITUTION may have the suffixes 7 or ous, according to the ratio of oxygen atoms to hydrogen atoms in them. Thus, sulphuric acid (H,SO,) has more oxygen in its molecule than sulphurous acid (H,SO,), nitree acid (HNO,) has more than nitrous acid (HNO,). Where several similar compounds are known, for those which contain less oxygen than in the ones designated by the suffix ous, the prefix hypo is used, e.g. hyposulphurous acid, whilst for compounds which contain more oxygen than in those designated by the suffix zc, the prefix per is used, e.g. perchloric acid. Whilst the term “acid” is usually defined so as to be applicable only to those compounds from which hydrogen can be liberated when they are ionised, the term is frequently applied to elements or groups of elements which, in combination with hydrogen or water, will produce an acid. Thus, silica, Si0,, is frequently regarded as an acid because, in combination with water, it produces a true (silicic) acid (H,0+8i10,=H,SiO,). Similarly, carbon dioxide is often regarded as an acid, because when dissolved in water it forms carbonic acid (H,0+CO,=H,CO,). Alumina sometimes acts as an acid (H,0+Al,0,—H,AI,0,), and sometimes as a base (as in the alums). These uses of the term “‘acid”’ are based on the erroneous idea that the Si0,, CO., or other corresponding radicle conferred the acid properties, and such radicles were, in consequence, frequently termed “ acid-radicles.” It is now known that the acidic property of a substance can only be produced if it contains hydrogen in a certain form. Nevertheless, it is often convenient—provided its limitations are understood—to regard the portion of an acid combined with the hydrogen as an acid-radicle, and the assumption that any substance which combines with a base to form a salt may be regarded as an acid is largely used in the ceramic industries. The study of a book on elementary inorganic chemistry sometimes leads to the conclusion that acids are of a comparatively simple composition, whereas some of the acids in ceramic materials are very complex. Thus, whilst silicic and aluminic acids are fairly simple, such acids as various alumino-silicic acids, in which the acid radicle contains the elements of silicon, alumina, and oxygen are often extremely complex. One of the simplest alumino-silicic acids may be represented by H,A1,8i,0,, or 2H,OAI,0,2810., the respective ions being H, and AI,Si,O,, but in the ceramic usage of the term “acid” (supra), the oxygen connected with the hydrogen ions is omitted from the acid radicle and Al,Si,0, or Al,0,2Si0, is regarded as the acid radicle of alumino-silicic acid. A still further abbreviation (though an inaccurate one) is obtained as described above, by regarding the acid radicle as though it were itself an acid. In this sense, the chief acid materials used in the ceramic industries are silica and clay, the former representing silicic acid the latter a group of alumino-silicic acids. Alumina—which sometimes behaves like silica—is not commonly regarded as an acid, though in some compounds it behaves as an acid radicle and forms aluminates. The characteristic properties of most acids are (i) their sour taste; (ii) their power of corrosion; and (ii) their power of uniting with bases to form salts. All these properties are usually evident at ordinary temperatures. The acid substances used in the ceramic industries do not possess these properties at ordinary temperatures, CONSTITUTION OF BASES 319 and the second and third only become evident at temperatures exceeding about 700° C.; at ordinary temperatures, such acids appear to be inert solids. Thus, whilst clay is of an acid character and lime is basic, they do not react on each other at ordinary temperatures, but must be heated to a temperature of 700° C. or more before any reaction occurs. Consequently, the truly “acid”’ nature of the alumino-silicic acids has been largely overlooked—often with serious results. A base is equally as difficult to define as an acid. The usual definition of a base as a compound of a metal (or a group of elements equivalent to a metal), capable of replacing the hydrogen ions in an acid when the two are placed in contact under suitable conditions, is open to the objection that it is impossible to define an acid satisfactorily. A base may also be defined as a compound which can be resolved into two ions, one of which is the hydroxyl (OH) ion; but this definition is unduly restrictive, as many substances which act as bases do not contain the OH-group, though most of them are readily converted into hydroxides when in contact with water. A third definition of a base is that it is a substance which will react with an acid to form a salt. This is the definition most widely used in the ceramic industries, not because it is the most accurate, but because it groups in the most convenient form a number of substances, all of which behave towards silica and other “‘ acid ’’ substances in a similar manner. When so broad a definition is used, the composition of a base becomes a matter of secondary importance ; consequently, the term “ base ”’ is often applied to carbonates and other compounds to which the other definitions are inapplicable. These various definitions of the term “ base”’ are liable to be misleading unless sufficient care is taken when the termis used. Most bases are the oxides or hydroxides of a metal, or compounds of these with a weak acid (such as the carbonates), an oxide being a compound of a metal with oxygen alone, and a hydroxide or hydrate being a compound of a metal or its equivalent with oxygen and hydrogen, the two latter being united to form a hydroxyl group (OH), which may retain its identity when it is trans- ferred from one substance to another by chemical reaction. The constitution of a base is often peculiar ; under suitable conditions all oxides and hydroxides can be split up into a metal and an oxygen or hydroxide ion, but many oxides, when acting as bases, combine 7m toto with acid radicles. This dual means of expressing the behaviour of bases is liable to confusion, though, when rightly under- stood, it is often convenient. For example, when the base sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to form common salt, the reaction may be represented by . NaOH + HCl = NaCl + HOH Base. Acid. Salt. Water. In this case, it is clear that the H-ion of the acid combines with the OH-group in the base, forming water, and enabling the remaining materials to form the salt. It is equally correct to represent the reaction of sodium hydrate and silicic acid in the same manner. 2NaOH + H,.Si0O,; = Na,Si0, + 2HOH Base. Acid. Salt. Water. 320 CHEMICAL CONSTITUTION If the grouping of the atoms is rearranged in molecular formule the equation becomes Na,OH,O + SiO,H,O = Na,OSiO, + 2H,0 Base. Acid. Salt. Water. If the reaction takes place at a high temperature, both the base and acid will undergo partial decomposition prior to the reaction and will lose water. The equation representing what occurs at a high temperature is then Basic radicle. Acid radicle. Salt. This method of representation is often so convenient that its inaccuracies are commonly regarded as negligible, and various oxides, and even some carbonates, are regarded as bases simply because of their behaviour with acid radicles at a high temperature. No serious error need arise if the purpose and limitations of this mode of expression are borne in mind and if improper deductions are not drawn from it. An alkali is a base of specially active character, e.g. sodium and potassium oxides and hydroxides. The chief bases used in the ceramic industries are bauxite and other forms of alumina, lime, magnesia, calcined dolomite, and zirconia. On account of the ease with which they are converted by heat into bases, various carbonates, such as sodium carbonate, potassium carbonate, calcium carbonate (whiting, limestone, marble), magnesium carbonate (magnesite), calcium-magnesium carbonate (dolomite, magnesian limestone), etc., are commonly referred to as “ bases,”’ though they are actual by the salts of the corresponding bases. Oxides have special terminations according to the proportion of oxygen they contain, those ending in 2c having more oxygen than those ending in ous. Sesqui- oxides have a proportion of oxygen intermediate between oxides whose names end in 2c and those ending in ows ; sesquioxides often exert both acid and basic functions, according to the conditions to which they are exposed. Thus, alumina may act as an acid and form aluminates, or it may act as a base and form aluminium salts. Ferric oxide, Fe,Og,, is sometimes regarded as a sesquioxide because it behaves in a similar manner. A salt is commonly defined as the product of the interaction of an acid and a base, during which reaction the hydrogen of the acid is replaced by a corresponding part of the base. Such a definition is open to the objection that neither an acid nor a base can be defined satisfactorily, and therefore any definition of a salt based on these terms must be unsatisfactory. Nor does this definition include the product formed by the reactions of an anhydride or a so-called “ acid oxide ”’ such as silica with a base. In both these cases the same salts are formed as when the corresponding acids are used, so that any complete definition of a salt should include the product of a base with either an acid-oxide, an anhydride, or an acid. A normal salt is one in which the valencies of both the acid and base are mutually satisfied. When only part of the replaceable hydrogen is removed, the resulting salt is termed an acid salt, as it possesses some acid properties, whilst, if there is an excess of base over that required to replace the hydrogen, the salt is termed a basic salt. A NOMENCLATURE OF SALTS 321 neutral salt is one which does not combine with a further quantity of either the acidic or basic radicles, of which it is composed. The use of the term “ neutral salt ’’ to indicate that it does not affect the colour of an indicator such as litmus is meaningless unless the name of the indicator is stated, because some salts which are neutral to litmus and to methyl orange are strongly acid to phenolphthalein. The names given to salts bear a relation to the constituents from which they have been formed. Under normal conditions the name of a simple salt will combine that of the metal and the acid radicle composing the salt, as sodium silicate, which is a self-explanatory title. The salts produced by the interaction of bases with complex acids form correspondingly complex salts. Thus, alumino-silicic acid forms alumino-silicates, silico-tungstic acid forms silico-tungstates, and so on. In the absence of oxygen the termination 7de is generally employed, as in chlorides, carbides, silicides, etc. When oxygen is present the names of the salts terminate according to the acid from which they are derived ; thus, acids containing oxygen and whose names end in 7c form salts whose names end in ate, sulphuric acid forming sulphates. Other examples are : Perchloric acid forms perchlorates. Nitric acid forms nitrates. Nitrous acid forms nitrdfes. Hypochlorous acid forms hypochlorites. The name of the base used in forming a salt may be utilised to indicate some of the characteristics of the salt. A salt formed from a base whose name ends in ic may have the basic portion of its name ending in 7c, whilst if the name of the base ended in ous the salt may have the basic part of its name ending in ous. Thus— Base. Salt. ferric silicate. Ferric oxide yields {fen sulphate. ferric chloride. ferrous sulphate. ferrous chloride. ferrous silicate. Ferrous oxide yields 1 It is sometimes convenient if the name of a substance (either a salt or other com- pound) indicates the number of atoms of any element in a molecule of a compound ; in that case, the number is sometimes prefixed to the name of the element, as in triman- - ganic tetroxide, in which there are three atoms of manganese to each four of oxygen, and in tricalcium silicate, which contains three atoms of calcium to each atom of silicon. Again, chromium sesquioxide has two atoms of chromium to each three of oxygen, whilst chromium trioxide has one of chromium to three of oxygen. The effect of acid and basic substances upon each other is further dealt with in Chapter XI. Neutral Substances.—Substances which react with neither bases nor acids are termed neutral. Normal salts—which are the products of the reaction of acids and bases—belong to this class, as also do various oxides, though the latter sometimes 21 322 CHEMICAL CONSTITUTION act as bases or as acids according to the nature of the substances with which they are brought into contact. Alumina and ferric oxide are typical examples, as they may (i) remain inert; (ii) act as bases forming aluminium or ferric salts, such as sulphates ; or (ii) take the part of an acid radicle and form aluminates or ferrates with soda, potash, or lime as the base. The activity of many substances used in ceramics depends largely on their tem- perature ; they may be inert or neutral when cold, but actively basic or acid when at a red heat. Carbon, in various forms, chromic oxide, and even silica and clay may be included among such substances. In considering whether a ceramic material is basic, acid, or neutral, it is not sufficient to investigate its behaviour at low tem- peratures, because, as previously explained, many such substances do not commence to react below a dull-red heat. Failure to realise this has led to much misconception as to the nature of clays and other ceramic materials. The investigation of these substances is still further complicated by the fact that their constitution is so complex that they do not behave in a normal manner, but break down into a corresponding number of simpler substances. For instance, a complex alumino-silicic acid such as pure china clay when fused with sodium car- bonate appears to be decomposed into its constituent oxides, which combine separately with the base, water and carbon dioxide being expelled in the gaseous form. This may be represented by the equation Al,0,2Si0,2H,0+3Na,CO,=Na,0Al,0,+2Na,02Si0,+2H,0+3C0,, or as H,Al,Si,0,+3Na,CO,=Na,Al,0,+2Na,Si0,+2H,0+3C0,. Under some natural conditions the alumino-silicates appear to retain their complex structure, as in orthoclase (K,OAI,0,6810,), ete. Molecular Structure of Solids.—Although all compound gases are composed of molecules, the ordinary conception of a molecular structure of solid substances and of some liquids does not fit some of the known properties of matter in these states. Even when solid or liquid particles are so minute as to be in the colloidal (sol) state, the ordinary conception that they have a molecular structure is scarcely applicable. Investigations of the molecular or atomic structure of amorphous substances is particularly difficult, but recent investigations of the structure of crystals has shed some light on the constitution of certain ceramic materials, and particularly on that of clay. Thus, if X-rays are passed through a crystal or through a small quantity of powdered crystals and allowed to fall upon a photographic plate, a line will be produced on the film corresponding to each important plane of atoms in the specimen used. As each substance has a definite X-ray spectrum consisting of a series of lines at definite distances apart, a mixture of two or more substances will produce an X-ray spectrum having lines characteristic of each constituent, and if the spectrum of an unknown crystalline substance is compared with those of various other substances its composition can be determined. Bragg } has examined photographically the interference figures produced by the 1 X-rays and Crystal Structure, G. Bell & Sons, London (1916). MOLECULAR STRUCTURE OF SOLIDS 323 reflection of X-rays by the atoms of the various elements in a crystal. By this method it is possible to calculate (a) how the atoms are arranged, and (b) the distance between the centres of the atoms. The distance between the centres of two neighbouring atoms may be expressed as the sum of two constants represented by the radii of the corresponding spheres. The diameter of a sphere representing an atom is termed the diameter of the atom, and is usually expressed in Angstrom units (A=10-8cm.). According to modern ideas of the structure of crystalline chemical compounds, the mass is regarded as being partitioned off with space units or s~pace-lattices. Analogous portions of matter are supposed to be distributed in each space unit. Each space-lattice is thus considered as being made up of units or points which represent the centres of gravity of the constituent molecules. From this point of view a crystal is an aggregate of ellipsoids or spheres so piled on one another that the corresponding axes are in accord with a definite geometrical plan. Each atom then occupies a space equal to one space unit, and the molecule may be regarded as the mass formed by the atoms arranged in a network or space-lattice. Sometimes the space-lattice may include a large number of atoms—far larger than those which form a single molecule of the substance when in a gaseous form. Considering the elementary atoms composing crystals as points, all crystals may be described as a homogeneous arrangement of points. There are 230 modes of arranging points in a homogeneous structure such as is possible to crystals, and fundamental to these 230 modes are 14 space-lattices, defined by Frankenheim and. Bravais. Hence the structure of a crystal is fundamentally that of a space-lattice or three-dimensional trellis-work, in which the particles are situated at the corners of each section or mesh of the net, so that the strings represent the lines of intersection of the planes and the knots their points of intersection. Three of the fourteen space-lattices are of cubic symmetry, the points being arranged as a simple cube, a centred cube, and a face-centred cube respectively. Two others are tetragonal, four are rhombic, two monoclinic, one triclinic, one hexa- gonal, and another is rhombohedral-trigonal. In the simplest crystals the space-lattice is directly formed by the chemical atoms, but in more complex crystals the space-lattice points may be surrounded or replaced by groups of atoms. The crystal faces are parallel to the various planes of atoms of the space-lattice, and any three adjacent points of the lattice will determine the position of a crystal face. If all the points of a space-lattice are joined by straight lines the resulting parallelopipeda will correspond to the “ bricks” of which Haiiy considered crystals to be built. In the simple cube-lattice the atoms are at the corners of a cube. In the body- centred cube-lattice the atoms are at each corner, and one other is at the centre, making nine in all. In the face-centred cube-lattice there is an atom at each corner and one at the centre of each face, making fourteen in all. In more complex groups a corre- spondingly larger number of atoms may be involved. Most of the atoms in metals are arranged in the form of face-centred cubic space- 324 CHEMICAL CONSTITUTION lattices, though some have their atoms in the form of cubes with an atom at each corner and one in the centre of each cube. A third type consists of closely packed right-triangular prisms, the bases of which are equilateral triangles and the altitudes 1-633 times the side of the triangles. The atoms in this structure are arranged at each of the prism corners and at half of the prism centres. The simple cubic arrangement of atoms is also found in single salts having equal numbers of positive and negative atoms. Sodium chloride has a cubical space-lattice structure with atomic centres 2-81 x 10cm. apart. Carborundum has a face-centred cubic-lattice, the atoms forming cubes with an extra atom in the centre of each face, the distance between the atomic planes being 2-179 x 10-8 cms. Crystals of fluorspar, magnetite, and zinc-blende are of the double face-centred space-lattice type. W. L. Bragg found that the space-lattice structure of pyrites was rather complex, the structure being fundamentally of the face-centred cube type, but the sulphur atoms were not placed exactly equidistant from the iron atoms. He found hauerite (Mn8,), ullmanite (NiSbS), cobaltite (COAsS), and cuprite (Cu,O) to have somewhat similar structures. W.H. and W. L. Bragg found that calcite has carbon and oxygen atoms on triangular planes perpendicular to the crystal axis. The calcium atoms lie — in planes just above and just below the carbon and oxygen planes. Rhodochrosite — (MnCO,), siderite (FeCO;), sodium nitrate (NaNO ;), dolomite (MgCO,CaCO;), and hematite (Fe,O,;) have somewhat similar structures. Zircon, according to L. Vegard, is of the face-centred tetragonal type, as also are rutile and cassiterite. P. Landrieu } states that the points of the space-lattice of double compounds may be considered as occupied by the ions of the molecules which are uniformly dis- tributed through the lattice, giving electrostatic equilibrium. If this is the case it would account for the abundance of inorganic double compounds and their rarity amongst organic compounds. In mixed crystals, however, the points of the lattice are occupied by molecules or ions which are isoteric in the sense used by Langmuir. Thus, mixed crystals appear to be due to the replacement of ions or molecules with others with which they are isoteric. The X-ray spectra of various substances seem to show that in a crystal there is no definite molecular structure, but that crystals consist of atoms arranged in regular order, the unit of which is what is known as a space-lattice, the atoms in which, though closely packed, are unconnected. In the various space-lattices which appear to be the units of crystal structure, the atoms at the surface act as though they unsaturated, and have a zone of definite chemical attraction of a thickness of at least 10°° cms. This zone of attraction appears to cause the growth of crystals in a saturated solution by the attraction of atoms in the solution to the surface of the crystal. These atoms when in position attract others, the process continuing and the crystal growing until the solution will not allow any further withdrawal of atoms. If this conception of the structure of solids is correct, it may account for the phenomena of adsorption or the attraction of atoms by solid surfaces; the forces holding adsorbed molecules are the same as those holding together the atoms of crystals, the adsorption depending on the residual valency of the adsorbed molecules. 1 Bull. Soc. Chim., 31, 1217-41 (1921). > POLYMORPHISM 325 From the foregoing it appears that the atoms in crystals are not combined to form molecules, which in turn are aggregated to form crystals, but that a large number of atoms is aggregated in definite order to form one large structure, or “ giant molecule,” arranged to form a “ space-lattice,” and that each separate space-lattice forms a unit from which large crystals can grow by the mutual attraction of the individual atoms. The examination of materials by their X-ray spectra has one great drawback. Numerous researches have proved that the quality and quantity of the characteristic X-rays emitted are independent of the state of the chemical combination and valency of the elements concerned. Thus, the rays emitted by FeSO,, Fe,03, Fe,O,, (NH,),FeC,N,, give identical adsorption coefficients ;1 the amount of X-rays emitted by a given weight of tin is unchanged when the tin is converted into oxide,? and finally the characteristic rays of Br and I are emitted by C,H;Br and CH,I respec- tively. Hence the scattering of X-rays appears to be a purely atomic effect, and whilst they lead to a knowledge of the mean positions of the atoms they cannot, from the nature of the case, throw any direct light on the question of the existence or non- existence of molecules in the crystalline state. Polymorphism is a modification of crystal structure (p. 5), and may be due to different causes in different substances. The chief explanations of its origin are as follows :— 1. It may be the result of a molecular change as suggested by Smits.? 2. It may be the result of a structural change in the arrangement of the individual components of the crystal lattices, these components being molecules, atom groups, or atoms, as suggested by Bravais. In some cases, according to Bridgeman,’ the geometrical centres may remain constant, but the orientation of the pots may vary. 3. It may be due to a change in the form of some of the atoms without there being any change in the arrangement or orientation. R. B. Sosman® suggests that a change may take place in the atomic nucleus or in the planetary electrons. Thus, he considers the inversion of pure iron at 770° C. to be an atomic inversion, whilst he attributes the allotropic transformation of silica partly to structural and partly to atomic changes (see also p. 328). THE CHEMICAL CONSTITUTION OF SILICATES AND ALUMINO-SILICATES A large number of ceramic materials are of an acid character and contain silica as an important constituent; some of them also contain alumina. These two types of substances are respectively known as silicates and alumino-silicates. The silicates form simple salts consisting of a number of atoms of the basic radicle 1 J. L. Glasson, Proc. Camb. Phil. Soc., 15, 437 (1910). 2 Chapman and Guest, ibid., 16, 136 (1911). 3 Die Theorie der Allotropie, Leipzig (1921). 4 Proc. Am. Acad., 52, 91-187 (1916). 5 J. Wash. Acad. Sci., 7, 62-67 (1917). 326 CHEMICAL CONSTITUTION and a number of groups of one of the silicic acid radicles, or (what amounts to the same thing and is probably stated more correctly) they consist of positive (metallic) ions combined with negative silicic ions. THE CONSTITUTION OF SILICA The silicic acid derivable from these salts is a compound of hydrogen, oxygen, and silicon, but, in many instances, in the reactions which occur with ceramic materials the anhydride replaces the acid. This anhydride is silicon oxide—well known as silica— which is usually expressed by the formula SiO, ; it is highly probable, however, that its molecule is much more complex than this formula suggests, though the silicon and oxygen atoms are in the ratio of 1:2. Thus, the depression of the freezing-point of lithium metasilicate caused by the addition of ignited amorphous silica was found by Schwarz and Sturm ! to indicate the molecular formula of such silica to be Si,Q,. This agrees with the independent investigations of J. Beckenkamp,? who considers silica to have the formula in which two oxygen atoms are more strongly linked to one silicon atom than the two silicon atoms are linked to each other. Curie ? gives the formula of silica as Si,0, or but G. Martin,‘ after examining the oxidation products of different organic silicon compounds, considers that a molecule of silica should be represented by the formula Si,0,,, the atoms being arranged in the form of a hexagonal ring compound, which may be represented by the formula 1 Ber., 47, 1735 (1914). 2 Z. anorg. Chem., 110, 290-310 (1920). 3 Encyl. Chim., 2, 152 (1884). 4 Chem. News, 61 (1915). CONSTITUTION OF SILICA 327 W. and D. Asch ! have made an elaborate survey of the literature on the silicates, and have concluded that silica at ordinary temperatures is a ring compound, each ring containing either five or six atoms of silicon, according to its origin. These rings they term respectively pentites and hexites. The graphic formule are shown on p. 308. W. H. and W. L. Bragg consider that a-quartz consists of three inter-penetrating hexagonal space-lattices, derived from each other by equal translations parallel to the axis ¢, together with rotations of 120 degrees about that axis; Beckenkamp 2 considers that the silicon atoms in silica more probably form a rhombohedral lattice, whilst R. B. Sosman ’ considers that the silica molecule has a chain structure consisting of definite SiO, triplets 4 (7.e. groups consisting of one atom of silicon and two of oxygen), which persist in the liquid, glassy, and crystalline states, and that the allo- tropic forms are built up by three different combinations of the silica chains. He also considers that the triplets retain their individuality in silicates and in more complex compounds, and that the oxygen and silicon atoms in the quartz crystal occur on a helix, there being three silicon atoms to each turn of the helix and six oxygen atoms attached to them, thus forming three silica triplets. The position of the oxygen atoms will determine whether the crystal is right handed or left handed. This structure would place the oxygen atoms upon the edges of an imaginary hexagonal prism containing silicon atoms of corresponding planes. He considers each silica triplet to be constructed as follows: there are three atoms, the two oxygen atoms each consisting of two electrons surrounded by six others. The silicon atom consists of two electrons surrounded by twelve others. The silicon atom shares one pair of electrons with each oxygen atom, and the oxygens share a pair of electrons between them. The silica also shares a pair of electrons with each of two neighbouring silicon atoms, thus forming a thread or chain structure having the form the silica triplets forming threads or wires which are in rapid motion whilst the silica is in the liquid or fused state, but which, on cooling, form an intimately tangled mass which, if drawn out, as in glass tubes, would cause the wires to be parallel to the rod or tube. This would explain the difference in the expansion in a direction parallel to and that in one perpendicular to the axis of the rods, as observed by Callendar.® Rayleigh ® also noticed the ribbon-like crystallisation of silica glass rods. Silica occurs in various allotropic forms which are stable at different temperatures. These are shown in Table CV. 1 The Silicates in Chemistry and Commerce (Constable & Co., Ltd., London). 2 Loc. cit., p. 326. 8 J. Franklin Inst., 194, 741-64 (1922). 4 The term “triplet”? is used by him instead of ‘“ molecule,” as the latter term has other connotations which are not applicable in this instance. . 5 Phil. Mag., 23, 998-1000 (1912). * Proc. Roy. Soc., London, A 98, 284-96 (1920). 328 CHEMICAL CONSTITUTION TaBLE CV.—Forms of Silica Amorphous. —. | Amorphous silica feat silica. f a-quartz. uae LB-quartz. ; fa-tridymite. Crystalline . . | 4 Tridymite \ aanayite : . a-cristobalite. WE {ee The nature and stability ranges of these various forms of silica have been very fully investigated during recent years, but they are not yet fully understood. This is partly due to the low thermal conductivity of silica and silicates generally, which renders the rate of any changes which may occur very slow. R. B. Sosman ! considers the allotropic changes in silica are partly structural and partly atomic, as described on p. 325. He regards the sluggish changes as due to the rearrangement of the silicon and oxygen atoms, and the rapidly reversible changes as due to changes in the atom and possibly in the relative positions of the atoms. He thus concludes that the a—B change in quartz is due to a change in the position of the oxygen atoms of adjacent pairs, which are pushed farther apart than they are in a-quartz and form vertical pairs,? and that tridymite and cristobalite are formed by the different orientation of the oxygen pairs which are linked with other atoms. Cristobalite has a variable inversion temperature. Fenner has shown that the higher the temperature at which it is formed, the higher is its B-a inversion point. Sosman explains this by assuming that some of the oxygen atoms are in relatively fixed positions, whilst others rotate about the axis of the silica thread, this structure being maintained by rapid cooling to 200°-300° C. R. B. Sosman considers that in f-cristobalite the threads of silica triplets lie parallel to the cube diagonal of an imaginary octahedron or perpendicular to the octahedral face, the oxygen atoms occurring in octahedral planes perpendicular to the axes of the threads, whilst in tridymite the threads lie parallel to the hexagonal axis, and each pair of oxygen atoms lies in a plane perpendicular to the thread-direction. Both this and the structure suggested by Beckenkamp probably necessitate the assumption of twinning to explain the dihexagonal symmetry of f-tridymite. There 1 Loc. cit., p. 327. 2 As the pairs are close together in a-quartz, a fairly high temperature is necessary to alter their positions; whilst in the tridymite structure the oxygen atoms are farther apart, and con- sequently a lower temperature suffices to alter their positions. In a-cristobalite the distance between the oxygen atoms is intermediate between that of quartz and tridymite, as also is its inversion temperature. CONSTITUTION OF SILICA 329 is, however, at present no X-ray data regarding the structure of tridymite and cristobalite, so that until this is obtained no definite proof exists. Sosman suggests that his theory of the constitution of silica might be extended to the constitution of silicates, as many of these show similar changes of density and stability. Amorphous silica, when heated under suitable conditions, is changed into one of the crystalline forms. It may be converted into crystalline quartz by direct heating in the presence of a catalytic agent such as sodium tungstate, alkali-phosphate, lithium chloride, or sodium molybdate. It is possible that cristobalite and tridymite may also be formed during the heating. Thus, Fenner+ found that chalcedony is first converted at 800° C., in the presence of a flux, into quartz and tridymite, the latter on further heating disappearing ; but Washburn and Navias 2 consider that chalcedony is converted on heating partly into isotropic silica glass which later crystallises, forming cristobalite. In the absence of a catalytic agent it is difficult to convert amorphous silica into the crystalline state, though the possibility of doing so has been shown by Kyropoulos and Braesco, and also by Fenner. Fenner ! found that when amorphous silica is heated without any flux to a tem- perature of 1030° C., it is changed completely in sixty-nine hours into cristobalite, but Houldsworth and Cobb ? found that amorphous silica in the presence of 5 per cent. of soda is converted into cristobalite at 700° C. Another method by which amorphous precipitated silica may be converted into quartz, is to heat it to a temperature of 300°-400° C. with water under pressure and in the presence of a catalytic agent such as carbon dioxide, sodium carbonate, boron fluoride, or sodium silicate. Quartz, when heated, is usually converted into a B-form at 575° C.; this form, when heated to a still higher temperature, changes into tridymite or cristobalite, according to the conditions of the experiment. The conditions under which these two modifications are formed are not clearly understood, but it is probable that tridymite is the stable form at the lower temperatures and EOD AS at a temperature nearer to the melting- point. The conversion of quartz into other allotropic forms of silica may, according to Le Chatelier and B. Bogitch, be effected in three ways. (a) By solution and subsequent crystallisation. (6) By the action of heat and of any impurities present which can act as a catalyst. (c) By the action of gases or vapours. Several molten silicates are able to dissolve quartz at high temperatures, and recrystallisation takes place to a greater or less extent, with the formation of tridymite or cristobalite, according to the temperature and to the conditions of crystallisation. Thus, at 1200° C. tridymite may crystallise out, whilst by heating the material to 1500° C. some of the tridymite may be dissolved, and, if the melt is cooled rapidly, cristobalite may crystallise out. The crystallisation which occurs on cooling to only a small extent is due, according to Grum Grzmailo, to the fact that the molten fluxes 1 Amer. J. Sci., 36, 380 (1913). 2 J. Amer. Cer. Soc., 5, 565 (1922). 3’ Trans. Eng. Cer. Soc., 21, 258 (1921-22). 330 CHEMICAL CONSTITUTION will dissolve quartz to a greater extent than tridymite and cristobalite, and the latter are, therefore, thrown out of solution to give place to the quartz. The more mobile the molten silicate in contact with the quartz grains, the more rapid is the solution and recrystallisation into cristobalite or tridymite; if the molten material consists of quartz dissolved in fused calcium silicates, the crystallisation on cooling slightly is so rapid that the quartz may be converted wholly into cristobalite, no tridymite being produced. The presence of almost any base, but particularly of lime, is very useful in aiding the transformation of quartz into cristobalite or tridymite, the value of such a base depending on the fluidity of the molten silicate produced. Thus, Seaver found that 77 per cent. of a quartzite heated in the presence of lime was converted into cristo- balite and tridymite, whilst only 48-95 per cent. was transformed when the same material was heated under the same conditions, but in the absence of lime. O. Rebufiat found that phosphoric acid was very effective in transforming quartz into cristobalite and tridymite. Scott gives the following catalytic agents in order of their effectiveness in aiding the transformation of quartz: iron oxide, lime, magnesia, titanic oxide, alumina. The action of flue dust, according to Mellor and Emery, is also very noticeable in facilitating the conversion of quartz into low specific gravity forms of silica. Gaseous vapours are often very effective in aiding the transformation of quartz into tridymite and cristobalite, their effect being often noticeable in the interior of articles where no solid matter could penetrate. The rate at which the allotropic changes take place when quartz is heated depends upon (a) the size of the grains of silica; (b) the nature of any impurities which may be present, and their amount; (c) the temperature attained during the heating; and (d) the duration of heating. The effect of the size of the particles in the conversion of quartz into cristobalite and tridymite is shown in Table CVI, due to Seaver. TaBLeE CVI.—Effect of Grain-Size on Inversion of Silica Percentage of Cristobalite formed by burning at 1450° C. for 40 hours. Silica Bricks. Coarsely Ground Quartz. 1 Firing ‘ : 77-35 48-95 2 Firings. 82-87 68-62 3 Firings. ‘ 85-98 Ferguson and Merwin state that the lowest temperature at which quartz is changed into tridymite is 870°C. That the rate of conversion of quartz into tridymite depends upon the temperature, is shown by the fact that at a high temperature the conversion is far more rapid than at a lower temperature. At 1300° C. the transition is fairly CONSTITUTION OF QUARTZ 331 rapid, but below 1000° C. it is very slow. The rate of change is most rapid between 1410° and 1435° C. The conditions governing the conversion of tridymite into cristobalite are by no means properly understood. It is generally considered that tridymite tends to form cristobalite at temperatures above 1470° C., though in steel-melting furnaces in which bricks are heated to a temperature considerably over 1470° C., cristobalite is frequently absent, even though it is supposed to form above this temperature. Bleininger and Ross found that the production of cristobalite occurs chiefly at the highest tempera- tures, and give the following figures in support of this statement :— TaBLe CVII.—Formation of Cristobalite Temperature, ° C. Cristobalite, per cent. 1350 57-13 1450 81-26 1500 90-00 Seaver found that quartz is converted almost wholly into cristobalite at 1630° C., very little tridymite being produced. He also found that repeatedly heating silica bricks at 1450° C. with a soaking period of forty hours caused the conversion of most of the quartz into cristobalite, but no tridymite was formed. Fenner considers that in the absence of a flux cristobalite always forms at a temperature of 1470° C., but if a flux is present tridymite may form. Day and Lacroix, on the other hand, have independently stated that cristobalite may be formed even in the presence of fluxes, such as lime, alumina, and iron oxides, which are supposed to favour the formation of tridymite, whilst Le Chatelier has found cristobalite in furnace linings containing as much as 10 per cent. of metallic oxides, and it is also found just below the metallic superficial coating of Bessemer converters. H. Le Chatelier considers cristobalite to be metastable at all temperatures below its fusing-point, and that it will revert to tridymite if conditions permit. He suggests that the conditions which favour the formation of cristobalite will, if continued sufficiently long, cause tridymite to form. All forms of silica when heated to a sufficiently high temperature fuse and form amorphous silica glass, which is the final product of the effect of heat on silica. Thus, when heated under suitable conditions, quartz will pass through its various allotropic modifications and finally fuse, forming silica glass, but if the heating is very rapid the quartz may fuse without appearing to pass through the intermediate stages. Quartz fuses into silica glass at about 1500° C. Tridymite and cristobalite fuse respectively at 1670° C. and 1625° C. When fused silica is cooled very slowly it will pass through the various allotropic modifications in the reverse order and finally form a-quartz, but under ordinary conditions the molten glass solidifies without crystallisation and forms amorphous silica glass. Cristobalite may, however, be produced by cooling fused silica to 1500° C., and maintaining it at that temperature for several hours. Similarly, 332 CHEMICAL CONSTITUTION tridymite may be produced from fused silica by maintaining the material at 800° C. for twenty days, but in the presence of a catalytic agent the transformation is much more readily effected. Fenner states that, on cooling, cristobalite reverts to tridymite between 1470° C. and 870° C., though Foxwell states that the change may be so slow as not to occur under the usual conditions of cooling. Fused silica has never been converted into quartz merely by cooling slowly, but it has been effected by maintaining at a temperature beteeen 300° C. and 750° C. (in no case rising above 800°-870° C.) for a long period in the presence of a catalyst. The relation of these various allotropic modifications of silica to each other has not yet been discovered. W. & D. Asch have suggested that the difference between them is due to the fact that they contain different numbers of atoms in their molecules, the number being in proportion to their specific gravities. Thus, as tridymite and cristobalite have lower specific gravities than quartz, they should have fewer atoms in their molecules. This has not been proved ; if it is correct the molecules or space- lattices must contain a very large number of atoms. Further light on the constitution of the various allotropic forms of silica may be obtained by investigation with X-rays by Bragg’s method. The structure of quartz is described on p. 327, but up to the present the relations between quartz and tridy- mite and cristobalite have not definitely been determined. THE CONSTITUTION OF SiLicic AcIDS AND SILICATES Silicates are classed according to the ratio between the oxygen combined with the base and that combined in the acid radicles of the silicate, as shown in Table CVIII. Taste CVIII.—Nomenclature of Silicates F . Acid Oxygen: Basic ee Metallurgical Mineralogical Oxygen. ———_———_---- Name. Name. RO Base. R,O3 Base. Sf ee | Ne eee Subsilicate Lessthan1 . ..| 3RO SiO, | B,024Si0, {ice pee Sires 2RO Sid, | 2R,0, 38i0, {ei ae orthosilicate. Monosilicate - 15 .| 4RO 3810, | 4R,0, 9810, | Sesquisilicate a - 2 : RO SiO, | R,O;3810,| Bisilicate metasilicate. “# 3 2RO 3810, | 2R,0, 9810, | Trisilicate Pe bs 4 RO 2810, a Quadrisilicate dimetasilicate. Orthosilicates have a ratio of acid oxygen : basic oxygen of less than 1-5-1-7. Metasilicates have a ratio greater than 1-7. In the absence of alumina the dividing line between ortho- and metasilicates is always 1-7, but where alumina is present a CONSTITUTION OF SILICATES 333 range of intermediate ratios occurs, as mentioned above, due to the alumina acting both as an acid and as a base. The principal theoretical silicic acids from which silicates are formed, according to Clarke,! are: Oxygen Ratio. Dimetasilicic acid H,Si,0; or H,O 28i0,_ ne SAA! Trisilicic acid H,8i1,0, or 2H,O 3810, . eaters Ba | Metasilicic acid H,SiO, or H,O Si0, pares Diorthosilicic acid H,Si,0, or 3H,O 2S8i0, . 1:33: 1 Orthosilicic acid H,SiO, or 2H,O SiO, og Some of these acids are unknown in nature as isolated substances, but there are many compounds which appear to be salts of them. Orthosilicic acid has been prepared by passing silicon tetrafluoride into water, filtering the precipitate, and drying it by washing with benzene and then with ether. If a solution of an alkali silicate is acidified with hydrochloric acid part of the silicic acid separates out as a gelatinous precipitate, but if the solution is very dilute the whole of the silicic acid remains in solution. The excess of acid and the sodium chloride may be separated by dialysis. If colloidal silicic acid is evaporated im vacuo at 15° C. over sulphuric acid, a trans- parent glass is obtained corresponding to the metasilicate H,Si0;. An acid of this composition is also obtained by dehydrating precipitated silicic acid with 90 per cent. alcohol. The following examples will show the molecular composition of various natural silicates :— Subsilicates (oxygen ratio, less than 1 : 1)— Silimanite . ; : ell. Oy oi), Tricalcium silicate . : . 38CaO Sid, Orthosilicates (oxygen ratio, 1 : 1)— Forsterite . : : . 2Mg0O SiO, Fayalite : . 2FeO Si0, Calcium orthosilicate . : . 2CaO Sid, Willemite .. : : . 2Zn0 Sid, Phenakite . : . 2BeO SiO, Zircon ; : . ZrO, SiO, Many other igitig tian also occur in nature. Diorthosilicates (oxygen ratio, 1-33 : 1)— Barysilite . : . 8PbO 2810, or Pb,Si,0, Metasilicates (oxygen fee 2: 1)— Sodium metasilicate : : . Na,O SiO, or Na,Si0, Calcium metasilicate (Wollastonite). CaO SiO, or CaSi0, Magnesium metasilicate (Enstatite). MgO SiO, or MgSiO, Trisilicates (oxygen ratio, 3 : 1)— Meerschaum . : , . 2MgO 38i0,2H,0 1 Bull. U. 8. Geol. Survey, 125 (1895). 334 CHEMICAL CONSTITUTION Mixed silicates apparently composed of two or more single silicates, are fairly common. Such mixtures are termed (a) crystalline 1somor phous nuiatures ; (b) mixed crystals ; or (c) solid solutions ; though the last term may include amorphous as well as crystalline substances (see also p. 300 and Chapter X1). Isomorphous mixtures consist of crystals which contain more than one base (or of crystals in which the original base has been completely replaced by another) without altering the form of the crystal. Isomorphous mixtures are possible because the shape of a crystal does not depend wholly on its chemical composition, but on the number and mode of combination of the atoms. Hence, as found by Mitscherlich, the same number of atoms combined in the same manner, produce the same crystalline form, and, consequently, one atom or group of atoms may be replaced by another atom or group without altering the crystal form. Elements which replace each other in isomorphous crystals are characterised by equal valency or combining capacity. This enables different elements to enter into combination in the same crystal, or one element may be replaced by another without altering the form of the crystal, provided suitable elements are available. These isomorphous changes are sometimes important in the case of ceramic materials, as the crystals of the latter may be alike though the chemical composition is different. Thus, albite and anorthite felspars form apparently identical crystals when mixed in any proportion, and the melting-point of the various mixtures lies in a straight line connecting the melting-points of the extreme members of the series. EK. T. Wherry ! considers that isomorphism is dependent more upon approximate equal volumes than upon equal valencies and chemical relationship. If correct, his suggestion would explain (a) the limited replaceability of potassium and sodium and the more complete replaceability of potassium and barium in the felspar group ; (6) the significance of water in analcite; (c) the scarcity of potassium pyroxenes ; (d) the presence of alkalies in the beryl group ; (e) the excess of silica in the nephelite group; (f ) the extensive isomorphism in the garnet group; (g) the replacement of lithium for iron and aluminium for magnesium in tourmaline; (h) the absence of (Ca, Na) and (Si0,, $i;0,) replacements in the zeolite group; and (2) the absence of (SiO,, Si;0,) replacements in micas. Isomorphous crystals of different substances cannot be distinguished by inspection ; consequently, ceramic materials which may be isomorphous must be carefully tested before use, or they may contain substances due to secondary replacement which may wholly unfit them for the purpose for which it is desired to use them. As a result of isomorphism it is extremely difficult to obtain some ceramic materials in a perfectly pure state, because most of the methods of purification only partially separate the undesirable substances. Thus, orthoclase—which is a potash-felspar—usually con- tains some sodium, whilst albite—which is a soda-felspar—usually contains some potassium. In Table CIX (due to Arzuni) an element in any series can usually replace or take the place of any other element in the same series. 1 Am. Mineral., 8, 1-8 (1923). CONSTITUTION OF MIXED SILICATES 335 TaBLE CIX.—Isomorphous Elements Series I. Hydrogen, potassium, rubidium, cesium, ammonium, sodium, lithium, and silver. Series II. Beryllium, zinc, cadmium, magnesium, manganese, “‘ ferrous ”’ iron, nickel, cobalt, platinum, calcium, copper, strontium, barium, lead. Series III. Lanthanum, cerium, yttrium, erbium. Series IV. Aluminium, “ ferric” iron, chromium, cobalt, manganese. Series V. Manganese, copper, mercury, lead, silver, gold. Series VI. Silica, titanium, zirconium, tin, lead, molybdenium, uranium, platinum, rhodium, iridium. Series VII. | Nitrogen, phosphorus, vanadium, arsenic, antimony, bismuth. Series VIII. | Niobium, tantalum. Series IX. Sulphur, selenium, chromium, manganese, nitrogen, arsenic, antimony. Series X. Fluorine, chlorine, bromine, iodine, manganese. Note.—The same element may appear in two or more series. Thus, to take an example from Series II, gehlenite, 3CaO Al,0, 25i0., is a definite crystalline mineral, yet the calcium in it can be replaced by “ferrous” iron, magnesium, strontium, barium, or copper, and apparently by any other element in the series without altering its crystalline form. The same substance illustrates Series IV, as the alumina in it may be replaced by ferric iron, chromium, cobalt, or manganese without altering the physical properties of the crystals. If the replacement or substitution involves several elements, it is easy to under- stand from Table CIX why many natural silicates have extremely complex and variable compositions, yet, in spite of this, each constituent atom occupies a definite position or portion of the space occupied by the molecule. It is not necessary that the crystalline form should be completely identical in order that isomorphous replacement may occur. Federov has shown that each sub- stance has its individual crystalline form, the angles of which differ—often only to a trifling extent—from those of other substances. Provided the shape of the crystals does not differ greatly, isomorphism is possible. The formula of an isomorphous mixture or of a mass of “ mixed”’ crystals is sometimes written by collecting in brackets all the elements which can replace each other without altering the crystalline form. Thus, what may appear on analysis to be a very impure iron carbonate, containing calcium, manganese, and magnesium, carbonates may also be regarded as a complex isomorphous mixture with the formula (Fe, Ca, Mn, Mg) CO;. The elements in brackets may be in any proportions. This use of a formula is not strictly correct (especially if it involves the use of Tinie but it is very convenient and often instructive (see p. 310). Mixed Crystals—Substances of different crystal form can only combine in fixed 336 CHEMICAL CONSTITUTION proportions, but those of the same or sufficiently similar crystalline form can be mixed in any proportions. Mixed crystals only differ from isomorphous mixtures in their origin, the term mixed crystals being usually confined to isomorphous crystals produced from a mixture of different substances, whilst the term “isomorphous crystals ”’ is also used to include substances in which one element has been completely replaced by another or to relate to two or more ‘different substances, all of which have the same crystalline form. The structure of mixed crystals appears to consist of separate layers of each kind of crystal, and their properties are largely additive and characteristic of mixtures rather than of compounds. For instance, when reacting with a base, one replaceable element appears to be more seriously attacked than another, though some silicates which are regarded as mixed crystals appear to act as definite chemical compounds. Solid solutions are substances which appear to be solid and homogeneous, but are really mixtures. They may be regarded as two-phase systems, one being the solute, or substance in solution, and the other the solvent. If such substances are crystalline, they are known as “‘ mixed crystals,” but if amorphous—as glazes, glasses, and various vitrified materials—the term “ solid solution ”’ is usually employed. Many silicates appear to be “ solid solutions ” ; when their constituents are melted together they do not form mixed crystals on cooling, but usually form a non-crystalline vitreous mass or glass. Most single silicates, if fused, readily crystallise when cooled slowly, but mixtures of two or more silicates only crystallise with difficulty. Complex mixtures of silicates seldom crystallise, but remain in the vitreous state. Consequently, when it is desired to prepare a glaze or other vitreous substance, it is always desirable to make its composition somewhat complex, by mixing several silicates or several sub- stances which will form a mixture of silicates. Mixtures corresponding to exactly whole numbers in chemical formule are avoided for the same reason—complex glasses and glazes being less likely to crystallise or “‘ devitrify ’’ than those which are much simpler in composition. Hydrous Silicates.—Many crystalline silicates lose water when heated, and are, therefore, known as hydrous or hydrated silicates. The elements (hydrogen and oxygen) which produce this water may be present in silicates in one or more of three forms, known as (a) colloidal water ; (b) water of constitution ; and (c) water of hydration or water of crystallisation, the first and last being the most weakly com- bined and most readily removable. The colloidal water in a substance is present in indefinite proportions. It is removed continuously and fairly steadily when the substance is dried and gives no indication of being definitely combined. This is a characteristic of many colloidal gels, including colloidal silica and colloidal alumina ; it is also a well-known character- istic of plastic clay pastes. Some of the iron hydroxides in clay also appear to contain water of this type. When only the “colloidal water’ (sometimes termed the ‘‘ water of plasticity ’’) is removed, the plasticity of the material may be restored by mixing it with a suitable quantity of water and allowing it to stand or “ sour ” (p. 274) until that water is uniformly distributed. If the clay has been dried at too high a temperature, so that some of the “ water of constitution’ is also driven off, the WATER OF CRYSTALLISATION AND HYDRATION § 337 plasticity of the clay cannot be fully restored. HE. Lowenstein found that, when dried at 25° C. over sulphuric acid, clay lost up to one-quarter of its ‘‘ water of constitution,” as well as the whole of its “ colloidal water’ and moisture. This has been confirmed by Mellor, Sinclair, and Devereaux,! who found that the water in halloysite was most easily removed, that in ball clay being much less readily affected, and that in china clay being only slightly affected. When exposed to a moist atmosphere the loss of colloidal water is restored by absorption. The water of constitution appears to be a definite part of the molecule, though it does not exist therein as “‘ water,”’ but usually in the form of hydrogen, attached to one or more parts of the molecule and of hydroxyl (OH) groups attached to the mole- cule, but not directly to the hydrogen. In such cases, water is only formed when the molecule is partially decomposed. For instance, when an almost ‘“‘ pure” clay is heated to 600° C. it evolves about 13 per cent. of water, which appears to have been formed by the partial (or possibly complete) decomposition of the clay- molecule. According to W. and D. Asch, who employed very elaborate formule, the elements of the water of constitution in clays and some alumino-silicates may be attached either to the basic or to the acid radicles of their equivalents in the formula, or to both of these simultaneously. What they term the “acid water” may also be attached to either the alumina or to the silica group or “ ring,” in a manner analogous to the distribution of the same elements in complex aromatic organic compounds. When the water of constitution is removed, the original substance cannot be reproduced merely by the addition of water under favourable conditions, as is often the case when colloidal water or water of crystallisation is removed. Mellor and Holdcroft found that dehydrated china clay, when heated to 300° C., under a pressure of 200 atmospheres, only absorbed 2-5 per cent. of water, whilst Rieke was only able to effect the recombination of 1-1 per cent. of water in a calcined Bohemian kaolin. Laird and Geller,? on the contrary, found that if the clay had not been heated to a temperature higher than 700° C., it could be rehydrated to some extent by heating for 8-48 hours at a temperature of 200—250° C. in an autoclave. If the clay has been heated higher than 700° C., the rehydration is correspondingly more difficult. The water of crystallisation or water of hydration is that which is evolved when substances of a crystalline nature are heated to a low temperature. It differs from “water of constitution’ inasmuch as the original substances can be reformed by dissolving the heated substance in water and then allowing it to recrystallise, whereas “‘ water of constitution ” cannot be so restored. The precise manner in which the atoms comprising the water of crystallisation are united to those forming the remainder of the substance has never been definitely ascertained, but the bond appears to be a very loose one and quite different in character from that of the water of constitution. Sometimes a silicate will form different kinds of crystals with different amounts of water of crystallisation, e.g. sodium silicate occurs in three crystalline forms :— 1 Trans. Eng. Cer. Soc., 21, 104 (1921-22). 2 J. Amer. Cer. Soc., 2, 828 (1919). 22 338 CHEMICAL CONSTITUTION Formula. Crystalline Form. Na,Si0,9H,O . ; . rhombic. Na,8i0,6H,O- . : - monoclinic. Na,Si0,4H,O : . hexagonal. W. and D. Asch have devised the following illustration to show the various ways in which the water may be present :— | (y) OH-Ca, Lars. BESS Pa Ca-OH (7) A | | | : OH | Si | Al | Si pone - 6H,0(8) (y) OH-Ca ONY OE cer (y) (OH), (OH) (OH), (8) (a) (8) where a represents hydroxyl groups associated with the alumina ring, B hydroxy] groups associated with the silica ring, y hydroxyl groups associated with the base, and 8 is the “ water of crystallisation.” If the above formula truly represents the constitution of the material so represented it is obvious that each of these four hydroxyl groups would possess different properties. Unfortunately, such highly complex alumino-silicates are so difficult to deal with— chiefly on account of their inertness and insolubility—that it has not, hitherto, been possible to make much progress in this direction. J. Thugutt + has found one-third of the alumina in sodium nepheline hydroxide behaves differently from the remainder. He also observed the same difference in the case of kaolin. P. Silber 2 found that the sodium attached to the aluminium in some alumino-silicates is separated by gaseous hydrochloric acid, but that associated with the silica is not affected. In both cases, if a structural formula similar to that on p. 346 is used, it will be seen that two-thirds of the soda will be combined with the silica and one-third with the alumina, so that it is quite natural that one-third should behave differently from the rest. Clays and some alumino-silicates behave curiously with regard to parting or recombining with water. When the water is that described on p. 336 as “ colloidal water,” or that described on p. 337 as “ water of crystallisation,” its removal and recombination are usually fairly simple, but the matter is quite different as regards the “‘ water of constitution.”’ The latter can only be replaced (if at all) by elaborate methods of synthesis, in which the original constitution of the molecule is rebuilt by indirect means. Hence, if a clay is heated to a temperature at which it loses part of its water of constitution, the essential properties of the clay cannot be restored by any simple treatment with water, no matter how severe such treatment may be. The power possessed by such a material of recombining with water depends on: (a) The manner in which the atoms previously associated with the lost hydroxyl groups can be attacked without completely destroying the structure of the molecule. 1 N. Jahrb., 9, 557 (1894-5). 2 Ber. d. Deutsch. Chem. Ges., 14, 941 (1881). CONSTITUTION OF ALUMINA 339 (6) The solubility of the product and the ease or difficulty with which it will react with other substances. Unfortunately, most ceramic materials are extremely difficult to deal with in this respect. THE CONSTITUTION oF ALUMINA AND ITS HYDROXIDES Alumina is usually represented by the formula Al,O;, but it is most probable that its formula is a multiple of this, especially when in combination with other materials, though its molecular weight in the solid state has never been determined. The graphic formule suggested by J. W. Mellor and by W. and D. Asch respectively, are as follows :— Sed K (Mellor) (W. and D, Asch) Rankin and Merwin! claim to have identified two allotropic forms of alumina : the a-form which is found in Nature as corundum and a f-form produced at high temperatures whose precise relation to the a-form is not, as yet, known. The B-form is regarded as a polymerised form containing a much larger number of atoms in the molecule than are present in the a-form. The temperature at which this polymerisa- tion or change in constitution occurs does not appear to have been definitely ascer- tained. Houldsworth and Cobb,? and Wholin, suggest a temperature of 1060°- 1130° C., but Keppeler* has reported its occurrence at about 750° C., viz., at the temperature at which the maximum shrinkage of dried precipitated alumina occurs. Mellor * suggests a temperature of just over 800° C. There appear to be three distinct types of aluminium hydroxides :— (i) The Diaspore type, which occurs near Var and Hérault, in the south of France. This type contains 12-14 per cent. of water, and has a formula approximating to H,A1,0, or Al,O,H,0. (u) The True Bauxite type, which is found at Les Baux, in France ; it contains 20-24 per cent. of water, and appears to correspond to H,A1,0, or Al,O,2H,O. (ui) The Hydrargillite or Gubbsite type, which contains 27-35 per cent. of water, and corresponds to H,A1,0, or Al,O,3H,0. Cornu and Redlich * consider that there are only two hydroxides, and that the dihydrate bauxite is a mixture of these. 1 J. Amer. Chem. Soc., 38, 568-88 (1916). 2 Trans. Eng. Cer. Soc., (Oct. 1922). - 8 Keram. Rund., 21, 307 (1913). 4 Trans. Eng. Cer. Soc., 10, 94 (1910-11). > Zeit. f. Chem. u. Ind. der Kol., 4, 90 (1908). 340 CHEMICAL CONSTITUTION Aluminium hydroxides containing other proportions of water are also found, but their composition appears to be very variable. They are probably mixtures of dia- spore (Al,0,H,O) and bauxite (Al,0,2H,O), especially as R. Wohlin found that some of these materials, when heated, lose their “‘combined water”’ at two different tempera- tures. Bigot has found that the aluminium hydroxides which contain more than 14 per cent. of combined water lose it in two stages, but those which contain less than 14 per cent. of such water lose it almost entirely at some temperature above 500° C. Houldsworth and Cobb? have found that some grey bauxite has four critical ranges of temperature :— (a) Below 180°C. ; (b) 300°-365° C. ; (c) 510°-620° C. ; and (d) 940°-965° C. In the last stage heat is evolved, the change being exothermal in character. The two ranges (b) and (c) are those at which the water of constitution is evolved. The arrangement of the atoms in these hydroxides has never been accurately determined. The constitutional formule generally accepted are : OH OH OH yes a x O=Al > “AL ALS No” Now oH” So” SoH Diaspore Bauxite No satisfactory simple formula for hydrargillite has been found, but Mellor has OH suggested the formula AlZOH for gibbsite. &8 caren 8 ALUMINO-SILICATES A very large and important class of ceramic materials—including the clays and felspars—contain both alumina and silica as well as water, and many theories have been advanced regarding their constitution. It is comparatively easy to obtain clean and pure crystals of felspar and some other compounds of silica and alumina with a metallic oxide, but the physical properties of clays are such as to make it a matter of extreme difficulty to obtain specimens which are sufficiently pure to be used in the determination of their chemical constitution. This is partly due to (i) the complexity of the materials; (1) their general inertness at temperatures below 600° C.; (ii) the ease with which they appear to be completely broken down into simple substances (such as silica, alumina, and water) at high temperatures; and (iv) the apparent impossibility of producing readily soluble salts corresponding to the acids. In order to make clear some of the difficulties involved in investigating the chemical constitution of clays, felspars, and allied materials, some of the more important theories on the subject may be briefly described.” 1. The theory that alumino-silicates are essentially salts of a silicon hydroxide, in which the hydrogen is partly replaced by alununivum and partly by other metals, cannot be correct, because (i) the percentage of silica varies, whilst the ratio of alumina : base 1 Loc. cit., p. 339. * The subject is discussed at much greater length in The Silicates in Chemistry and Commerce, by W. & D. Asch (Constable). CONSTITUTION OF ALUMINO-SILICATES 341 remains constant, and (ii) the hydrogen in the supposed hydroxide cannot be entirely replaced by a single metal, neither can the requisite complex hydroxides be prepared and alumino-silicates reproduced from them. 2. The theory that alwmino-silicates are double salts of aluminium and other metals and are also isomorphous mixtures of these double salts, as suggested by Berzelius and later by Smithson, cannot be correct, because reactions may occur in which the pro- portion of silica varies, whilst the ratio of the alumina: base remains constant. Moreover, it does not appear possible to produce any such alumino-silicates by the interaction of the hypothetical constituent salts as can be done in the case of other (true) double salts. Thus, the action of Na,Si0; upon Na,Al,O, does not produce a double salt of these two substances, but analcime, Na,OA1,0,48i0,2H,O, in which there is a larger proportion of silica than would occur in a true double salt. 3. The theory that alumino-silicates are molecular compounds, 1.e. composed of various chemical compounds which have nothing in common as regards their chemical nature, but are loosely held by some unknown bond, is highly improbable, because such compounds must be very easily decomposed, whereas the felspars and analogous alumino-silicates are only decomposed with great difficulty. Some alumino-silicates are highly stable substances and may be dissolved and recrystallised without change, and when decomposed they split up into other complex substances and not into the individual molecules of which this theory assumes they consist. 4. The theory that aluwmino-silicates are isomorphous mixtures of silicic and aluminie acids, and 5. The theory that clays are mixtures of mutually precipitated colloids, are not - capable of definite proof on account of the experimental difficulties involved in the artificial preparation of such materials. The second of these theories appears to be improbable, because such precipitated colloids ought to be readily decomposed by liquid chemical reagents, whereas clays are stable at temperatures below 600° C. Moreover, mutually precipitated colloidal alumina and silica does not possess some of the essential properties of clay. 6. The theory that alumino-silicates are double salts of silicic and alumime acids or isomorphous mixtures of these double salts is a modification of the fourth theory and is equally unsatisfactory, as it does not account for the invariable presence of both alumina and silica in the decomposition and reaction products. Moreover, unless the term “ double salts” is extended in an undesirable manner to include “ double acids ” free from bases, this theory makes an unnecessary and improper distinction between compounds consisting only of alumina and silica and the alumino-silicates, whereas it is generally agreed that there is a very intimate relationship between them. 7. The theory that alwmino-silicates are composed partly of complex alumino- silicic acids and partly of salts of these acids is more nearly correct than the preceding ones, but it is unnecessarily complicated and does not fit the essential facts. The behaviour of artificially prepared mixtures of other complex acids and their salts does not correspond with the behaviour of clays and of the more clearly defined alumino- silicates. 8. The theory that clays are alumino-silicic acids and that felspars and allied 342 CHEMICAL CONSTITUTION minerals are the normal salts of these acids is the only theory so far propounded which meets most of the facts, and it appears to do so in a simple and natural manner. Accepting this theory, the various minerals containing silica and alumina may be arranged as suggested by J. W. Mellor! on p. 345. In all these compounds the alumina and silica act together as a complex radicle and not as two independent groups. The alumino-silicic acids and their derivatives may be classified in three groups. (a) Alumino-silicic anhydrides, which may be regarded as derived from the corres- ponding acids by loss of water. The typical formula for this group is Al,,Si,03,,4 oy or Al,0,ySi0.,, to which correspond such minerals as andalusite, chiastolite, kyanite, and sillimanite. (b) Alumano-silicic acids, including clays and clay-like materials, the compositions of which are shown on pp. 348-352. (c) Salts of alumino-silicic acids, which are sub-divided according to the pro- portion of base present and the ratio of alumina : silica. The following minerals are typical of the groups in which they are placed :— Alumino-monosilicates—- Augite MgOAI1,0,8i0,. Chlorite 2Mg0Al,0,810,2H,0. Alumino-distlicates— Anorthite felspar CaOAl,0,2810,. Celsian felspar . BaOAl1,032S810. Muscovite mica K,03A1,0,6810,2H,0. Zoisite 4Ca03A1,0,6810,H,0. Scapolite . 4Ca03Al,0,6810,. Alumino-trisilicates— Natrolite . Na,OA1,0,3810,2H,0. Alumina garnets ROAI,0,3810,. Lepidolite KLiOAI,0,3810,. Biotite KHO2MgOAl,0,3810,. Alumino-tetrasilicates— Spodumene Li,OA1,0,48i0,. Laumonite Ca0Al,0,4810,4H,0. Leucite K,0OA1,0,4810,. Analcime Na,OAI,0,4810,2H,0. Glaucophane Na,OA1,0,4810,. Alumino-pentasilicates— Chabazite Ca0Al1,035810,7H,0. Harmotime BaOA1,0;,5810,6H,0. Alumino-hexasilicates— Orthoclase felspar K,OA1,0,6810,. Albite felspar Na,OAI,0,6S10,. 1 Loc. cit., p. 339. CONSTITUTION OF CLAYS 343 The groupings just mentioned do not indicate the arrangement of the atoms relative to one another in the alumino-silicic acid. This is a matter of great difficulty, and no agreement upon it has yet been reached. Two widely differing sets of theories have been proposed. (a) The theories based on the idea that alumino-silicic acids are ‘‘ chain com- pounds ” analogous to some organic acids. (6) The theories based on the idea that alumino-silicic acids are ‘“‘ ring compounds ”’ analogous to benzene and its derivatives. The objection to the first group of theories is that they do not explain the partial replacements of hydrogen and of aluminium by other metals, nor do they account for the remarkable stability and general inertness of the alumino-silicic acid radicle. Those who maintain the correctness of a theory belonging to the first group, object that the conception of alumino-silicic acids as ring compounds is “ fantastic’ and “a mere juggling with formule.” Yet no “ chain formule ” yet devised has enabled the prediction of so many properties as have been made by the use of the “ ring formule” proposed by W. and D. Asch, nor do the chain formule agree with so many of the facts. Unfortunately, W. and D. Asch failed to realise the importance of using pure substances when calculating some of the formule on which their theory of ring compounds is based, and in several instances they have been so unwise as to propound explanations without adequate knowledge of the facts. The result has been that the importance of the essentials of their theory has been overlooked and the theory itself has been needlessly encumbered with complexities. The whole subject requires a further large amount of investigational work before any single theory can be accepted, as showing the true constitution of the alumino- silicic acids. THE CONSTITUTION OF CLAYS The chemical constitution of clay has not been satisfactorily determined, though it has been the subject of many investigations and theories. It is comparatively easy to separate a large proportion of sand and silt from most samples of clay merely _ by mixing them with water and allowing the sand and silt to settle whilst the clay remains in suspension. Other methods may also be used for separating the finest particles of felspar, mica, and other materials, but whatever method of purification is used the final product—commonly known as clay-substance and sometimes as true clay—is seldom sufficiently definite in its characteristics to be regarded as a pure chemical compound. Common clays, and to a smaller extent china clay, are simply indefinite mixtures of some material conveniently termed “ clay-substance ” of un- known composition with other minerals such as quartz, felspar, mica, etc., so that any consideration of the chemical constitution of clay must be based on that of the “ clay-substance ” obtained by separating all the materials which are “not of the nature of clay.” In some natural clays, the proportion of “‘ clay-substance ” appears to be very large, e.g. in the fireclays, and as far back as 1863, C. Mene suggested that the Alsatian fireclays were definite chemical compounds and not merely indefinite mixtures, but later investigations have shown that they are by no means free from 344 CHEMICAL CONSTITUTION admixtures. Probably the purest clays obtainable commercially are some of the finest qualities of Cornish china clay, some of which appear to contain 95 per cent. or more of “ clay-substance.”’ It is now generally agreed that whilst the “ clay-substance ”’ obtained from various kinds of clay is by no means constant in composition, yet in all cases it is chiefly composed of one or more substances which may best be described as alumino-silicic acids, though, as explained previously, their acid properties are not appreciable at ordinary temperatures. Hence, a convenient—and on the whole fairly correct— definition of clay is that it is “a naturally occurring material whose composition corresponds to that of an alumino-silicic acid mixed with an indefinite amount of sand and other minerals, the whole producing a mass which becomes plastic when mixed with a suitable quantity of water.” In considering the constitution of clay, the fact must not be overlooked that many clays are chiefly valued because of their plasticity, but as this is a physical and not a chemical property, it may be possessed by materials of very different chemical com- position. For the same reason, the colloidal nature of some clays does not affect a consideration of the chemical constitution of the clay molecule (assuming one to exist), as colloidal properties are almost wholly of a physical nature. Consequently, the commercial uses of many clays are largely independent of their chemical com- position and are chiefly due to the physical characters of the material. This fact, and the probability that clays have been derived from many minerals (and not merely from felspar), explains the differences in the chemical composition of “ clay- substance” obtained from different sources. Yet when obvious impurities in the clay-substance have been eliminated or allowed for, the constancy of composition of the final product is truly remarkable. Physically, the purest specimens of clay-substance yet obtained appear to consist of a mass of crystals—each so small as to be invisible, so that their crystalline nature can only be recognised by means of their X-ray spectrum—these crystals being inter- locked with each other to form extremely minute “crumbs” which, when suitably moistened, have the structure of a colloidal gel, which is saturated with and surrounded by water held by intermolecular attraction and surface tension. The chemical properties of such a structure are almost wholly dependent upon the nature of the interlocked crystals which form the bulk of the dry mass, and it is these crumbs of interlaced, yet invisible crystals, which are the “true clay” or “ clay-substance.” As the purest form of clay-substance appears to be that derived from carefully prepared china clay or kaolin, it is important to observe that an analysis of this substance corresponds almost exactly to H,A1,8i1,0,, which is the formula of the crystalline mineral kaolinite, and it is by no means impossible that this is identical with pure clay-substance, the only difference being of a physical nature due to the larger size of the crystals of kaolinite. It is sometimes assumed, though without sufficient evidence, that the clay-substance or essential constituent of all clays is identical with kaolinite and has the formula Al,0;2Si0,2H,O or H,AlI,Si,0,, but this cannot be correct, as the analyses of the most carefully purified clay-substance obtained from different clays show that a substance of this composition is not always CONSTITUTION OF CLAYS 345 the chief constituent. It must suffice at present to state that whilst all clays appear to consist essentially of one or more alumino-silicic acids, they do not all consist of one such acid. On the contrary, several distinct alumino-silicic acids appear to be present in some clays. This is confirmed by some investigations started by the author in 1909 and still not completed, in which clay-substance of various origins was heated with concentrated solutions of caustic soda and other hydroxides and oxides to various temperatures at different pressures up to 100 lb. per square inch for a considerable time and then cooled very slowly. ‘The resulting crystalline products had all the characteristics of definite salts of different alumino-silicic acids, some of which have been separated and identified. W. Pukall has also obtained a complex alumino-silicate (8Na,0.6A1,03.12810,.12H,O) by heating kaolin with sodium chloride, thus showing the existence of an alumino-silicic acid corresponding to the formula 6A1,03.12Si0,.20H,O in the kaolin he examined. Unfortunately, “ clay-substance ”’ reacts so feebly at low temperatures, and is so easily decomposed at temperatures below which it becomes active, that it is extremely difficult to obtain definite alumino-silicates which can only have been produced by the simple combination of the acid with a base. Arguments founded on the products obtained by fusion are of little use unless it can be proved that the clay has not been decomposed prior to fusion. The alumino-silicic acids, including those which occur in clays, have been classified by Mellor ? as follows :— Alumino-monosilicic acid (allophanic type) . Al,0,S10,.7H,0. Alumino-disilicic acid (kaolinitic type) . So NAO AER SOF Alumino-trisilicic acid (natrolitic type) pe ALO ,5010,7H,0. Alumino-tetrasilicic acid (pyrophyllitic type). Al,034810,.7H,0. Alumino-pentasilicic acid (chabazitic type) . Al,0,58i10,7H,0. Alumino-hexasilicic acid (felspathic type) . Al,O;6810,.7H,0. The more important minerals corresponding to these types are : Allophane . : : . ; : . Al,O,S8i0,H,0. Kaolin. ; ; : ; . Al,0,2810,2H,0. Halloysite . : : : : : . Al,0,2810,2H,0. Rectorite . : : : : . Al,0O;28i0,H,0. Newtonite t 5 ; : : . Al,0,28i0,5H,0. Natrolite : : . Al,0,38i0,H,0. Pyrophyllite . : , ; : . Al,0,4810,2H,0. Montmorillonite ; ; : : . Al,0,48i0,H,0. Among other alumino-silicic acids not included in Mellor’s list are : Cimolite . : ; : ; : . 2AI1,0,9810,3H,O Collyrite . : : : : . 2Al1,0,810,9H,0. Schrotterite : é ; . 8AI1,0,3810,30H,0. 1 Loc. cit., p. 339. 346 CHEMICAL CONSTITUTION If attention is confined to the kaolinitic group, which appears to be the most important, several possible formule are devisable if the relative arrangement of the atoms are to be shown. The generally accepted formule H,AI,Si,0, and Al,0,2810,2H,O do not show this arrangement. The subject has been examined very fully by W. and D. Asch,! who maintain that the minimum formula is H,,Al,,81,.054, the silica, alumina, and oxygen atoms being arranged in four adjacent “rings,” with hydrogen atoms attached. As ex- plained on p. 348, the published objections to this theory of the constitution of kaolin have no scientific value, and as W. and D. Asch have succeeded in showing that no other formula hitherto published will fit all the known facts, their formula can scarcely be abandoned until a more satisfactory one is forthcoming. Aschs’ graphic formula for kaolinite is as follows :— (OH), OH vg (OH), I | i Si Al Al Si oe Piha Fate: € Oe O O O O os O O O =Si Si—O— Al Al—O—AlI Al—O—Si Si=(OH), \ | | | 0O O O O O 00 O +2H,0 | ee | N\ (OH),=Si Si—O—Al] Al|—O—AI Al—O—ANi Si=(OH), O O O O O O O EN a Ds ine ee Nona Si Al fe Si I | | (OH), OH OH (OH), 1 Oe eer | Si | Al | Alea Gees | 4+2H,0 pe | soi U7 NZ a 2 Yo a ae or even to Si) oeiAl hoe A ema A where Si represents the silicon-oxygen hexagonal “ring” shown in the kaolinite formula and Al the alumina-oxygen “ring.” Si and Al represent corresponding pentagonal rings which appear to be necessary in calculating the formule of some clays. 1 The Silicates in Chemistry and Commerce (Constable). CONSTITUTION OF CLAYS 347 According to W. and D. Asch, the chief types of clay formule are as follows :— (a) Si A R Si which corresponds to 3A],0,12Si0,7H,0., as Fg (b) Si R Si “A 3A1,03,10Si0,7H,O. A Si ie (c) R—Si ri 3.A1,0,18Si0,7H,0. A Si Si nee (d) R—Si Y 8A],0,15Si0,7H,0. NS fe ae a apr 9 (e) Si R R Si . 6A],0,12Si0,7H,0. ER ay A eRe (f) Si BR Si .. 5A1,0,12Si0,cH,0. Sm Oks Arey aA is ZN (g) Si R Si R Si . 6A],0,18Si0,7H,0. eA ya Are AL (h) Si R Si R Si 5 6A1,0,16Si0,7H,0. AP eee Ae (t) Si R Si R Si ‘3 5.A1,0,18Si0,7H,0. Unfortunately, these formule are based on the chemical analyses of crude clays, some of which contain free silica in the form of quartz, and to that extent the formule are incorrect. When special efforts have been made to remove all impurities from natural clays the chemical composition of the residual ‘“‘ clay-substance”’ does not, in all cases, correspond to kaolinite, but more closely resembles some of the other formule just mentioned. The chief objection to most of the simpler formule is that they do not explain the action of various metallic oxides on clay or the behaviour of clay on heating. There is still some uncertainty as to the effect of heat on the china clay molecule, but, assuming for the moment that it is decomposed with the formation of free silica, free alumina, and free water (steam), any graphic formula ought to show the atoms arranged in such a manner that these three substances would be the probable product of dissociation. This is not the case with F. W. Clark’s formula : /0-Si(OH)s HO—AC ‘ No—8i0>a1 which is unsymmetrical and typical of a readily decomposable substance, whereas clay is largely inert and stable. Pukall’s formula : OH HO-Si-0-O-: Al-OH HO .$i-0-0- Al- OH OH is symmetrical, but, as pointed out by Singer,! the double silica bond, like the double carbon bond in organic compounds, would be a source of weakness, so that this formula 1 Sprechsaal, 44, 52-4; Chem. Centrabl., 1, 967 (1911). 348 CHEMICAL CONSTITUTION : suggests a readily decomposable compound with H,AISi0, as the most likely product, though no such compound is known and there are several reasons why it is not likely to be formed. The complete dissociation of a substance with Pukall’s formula into silica, alumina, and water is difficult to understand, and the same remark applies to all chain formule in which the two atoms of silica are shown united by a double bond. If a chain formula must be adopted, that of K. Haushofer,} fe) OH HO—AIC Ssi€ re) ) 0 HO—AIC Ysi¢ 0 OH or the very similar one suggested by Mellor and Holdcroft ? as a modification of Groth’s formula, SALT aes a eos aa Yo OH owt? $i 07 appear to be the least objectionable, though both these formule suggest that the substances produced when the clay is dissociated would be H,AISiO, and H,AISi0, rather than silica, alumina, and water. With Asch’s formula, on the contrary, the general stability and inertness of clay may be expected, and its decomposition first into two simple hydroxides and then into three simple oxides is what would be anticipated.. There is a further objection to Groth’s and similar formule, masmuch as they represent the alumina as having a basic nature, whereas clays do not behave as aluminium salts, but as complex (alumino-silicic) acids. The arrangement of those atoms of hydrogen and oxygen in clays which are evolved when a clay is heated (1.e. the so-called water of constitution) has been the subject of much speculation. Their positions in the simpler formule are shown on pp. 336-337, but the complex ring formule devised by W. and D. Asch permit a more symmetrical distribution of these elements among the alumina and silica atoms (see p. 338). Tort CHEMICAL CONSTITUTION OF BURNED OR CALCINED CLAY The difficulties experienced in finding a satisfactory formula to express the chemical constitution of raw clay are no greater than those experienced in investigating the nature of calcined or “burned” clay. Indeed, there is justification for the remark that the chemical constitution of clay which has been heated to redness is quite indeterminable. The effect of heat on clay is by no means clearly understood, and though there is evidence that the products obtained by heating the purest procurable china clay to 600°-1000° C. are free silica, free alumina and water, the last- named escaping as steam, it is by no means certain that this is the case, and the 1 Die Constitution der Naturlichen Silicate, 1874. 2 Loc. cit., p. 339. CONSTITUTION OF HEATED CLAYS 349 products when other clays are heated are so complex as to be beyond present-day methods of investigation.? G. Shearer ? found, on making an examination of the X-ray spectrum, that— (i) When clay is heated and loses its “combined water,” it loses its internal crystalline structure and gives no spectrum, but the nature of the amorphous matter is not revealed. (u) A new crystalline substance (not sillimanite) appears to be formed at about 1000° C. el 11700 _; Saas - pp ttt i =neueeer de am Pa 600 aa _ Razee _ eee 300 200 100 O oe 4 6 8 (OI Ame ae Ore ION 2ZOne 2aoe 260 ce) 530 ; Time in Minutes. Fic. 18.—HEatina CURVE oP CHINA CLAY. (i) Sillimanite appears to be formed at a higher temperature. J. W. Mellor® suggests that the crystalline substance formed at 1000° C. is alumina, but this has not yet been proved. At present, the X-ray spectrum analysis of calcined china clay does not help very much as regards the elucidation of its chemical constitution. A method which has proved useful in the case of china clay and of some fireclays consists in heating the material in an electric furnace, the temperature of which rises quite steadily, and observing the rise of temperature of the clay during a number of successive intervals of time. If an inert substance, such as firebrick dust, is heated in 1 The fact that.the percentage of water absorbed by calcined clay is considerably lower than that absorbed by a similarly calcined mixture of alumina and silica in the same proportions appears to show that the products of calcination of clay are not free silica and free alumina, but a compound containing both these oxides. 2 Trans. Eng. Cer. Soc., 22, 106 (1922-23). 3 Ibid., p. 105. 350 CHEMICAL CONSTITUTION this way, the time-temperature graph of the material is the same shape as the graph of the furnace, which should be quite regular. If a sample of raw fireclay or china clay is examined in this manner, the time- temperature curve will be similar to that shown in fig. 18. Instead of being a simple — curve, this shows a “‘ halt ”’ in the rise of temperature at about 200°-300° C.,1 a second “ halt ”’ at 500°-600° C., and a sudden rise in temperature due to the material becoming hotter than the furnace at about 925° C. Mellor and Holdcroft found a further kink in the curve at 1100°-1200° C., but they consider that this is not a critical point, but is merely due to the cooling of the clay to the temperature of the furnace. P. Satoh ? found a further exothermal reaction at 1200°-1300° C. . The first two “halts” are due to heat being absorbed by some endothermal chemical changes taking place in the clay ; the third change of temperature is due to an exothermal or heat-producing reaction which, according to Mellor and Holdcroft, is due to the commencement of the condensation or polymerisation of any free alumina present, but Knote and Ashley have independently indicated that it may possibly be due to the formation of sillimanite (Al,0,Si0,), and W. and D. Asch consider this point to mark the polymerisation of anhydride formed at 500°-600° C, The change at 1200°-1300° C. appears to be due to the formation of sillimanite. The first halt (at 200°-300° C.) appears to be associated with the dehydration of colloidal matter, but this has not been fully investigated; it is comparatively unimportant. The second change (usually between 500°-600° C., but 2m vacuo at 300°-400° C.) coincides with the evolution of the water of constitution of the clay and clearly indicates the decomposition of the clay. The nature of this decomposition is not known; Sokoloff, like Mellor and Holdcroft, maintains that the clay is dissociated into free alumina, free silica, and steam, but it is possible that various compounds of silica and alumina (corresponding to one or more anhydrides, 7Al,0,y8i0,, with or without free silica) may be formed.* That the decomposition of the clay accompanies the loss of water is shown in Table CX, due to A. M. Sokoloff.4 TaBLE CX.—Effect of Heat on Gluckov Kaolinite Temperatures ec Loss of Water, Soluble Alumina, Molecular Ratio, per cent. per cent. Al,03H,0. 300 0-72 2°12 1:2-94 400 0-67 2-08 1:3-10 600 10-49 28-46 132-71 700 11-92 32°30 Lenore 800 12-99 34°66 1: 2°67 1 This does not occur with pure china clay. 2 J. Amer. Cer. Soc., 4, 182 (1921). 8 Knote, T'rans. Amer. Cer. Soc., 12, 226 (1910); W. and D. Asch, The Silicates in Chemistry and Commerce. 4 Zeit. Kryst. Min., 55, 195 (1915) ; Ber. Tech. Inst. K. Nikolaus I., 22, 1 (1913). EFFECT OF HEAT ON CLAY 351 Corresponding results with china clay, alumina, and silica, obtained by Mellor and Holdcroft, are shown in Table CXI. TaBLE CXI.—Decomposition of Clay by Heat Kaolin. Alumina. Silica. ac or Soluble Matter. Loss on Loss on Soluble Loss on Soluble Heatme. |. «| i.» | Heating. Matter. Heating. Matter. Alumina. Silica. Per cent. | Per cent. | Percent. | Per cent. | Per cent. | Per cent. | Per cent. 110 12-64 0-08 0-12 se ae? | 16-00 2-60 600 1:37 0-16 0:16 2°45 42-96 a 1:36 700 0-62 0-12 0-98 2°41 20:40 ee 1-36 800 0-56 0°12 0-68 1:58 7:84 1-24 1-12 900 0:23 0-12 0-20 1-65 5:92 0-43 0-76 1000 0-25 0:06 0-16 0:05 = 0-05 0-68 (at 1200° C.) This Table shows the increased insolubility of the alumina with a rise in tem- perature above 600° C. Mellor and Holdcroft suggest that this is due to some change (polymerisation ?) in the alumina. They state that alumina from different sources behaves differently, but point out that the alumina from aluminium nitrate behaves very similarly to that in china clay. On the other hand, the low solubility of the “alumina ”’ in china clay suggests that a silica-alumina compound is formed as stated by J. M. Knote,! and separately by W. and D. Asch. The former suggests that clay, when decomposed by heat, forms a mono- and a tri-silicate (Al,O,Si0, and Al,0,3S8i0,) ; he has also made the curious suggestion that when heated above 950° C. these two silicates may recombine to form a disilicate. He bases his sugges- tions on his discovery that raw clays and clays heated above 950° C. are not appreciably attacked by sodium carbonate and are only slightly soluble in hydrochloric acid, whilst dehydrated clays heated to temperatures below 900° C. are not appreciably affected by sodium carbonate, but are strongly attacked by hydrochloric acid. W. and D. Asch, on the contrary, consider that no dissociation of the silica and alumina complex occurs, and that the effect of heat below 1000° C. is only to remove water and form a single anhydride. They claim that Mellor and Holdcroft’s experi- ments do not indicate the formation of free silica and free alumina, but confirm their theory as to the formation of anhydrides without any change other than the loss of water. It is possible that two changes occur, viz. the formation of one or more anhydrides at 500° C., and the polymerisation of these at a higher temperature. The fact that calcined clay is more readily attacked by acid seems to confirm this. 1 Loe. cit., p. 350. 352 CHEMICAL CONSTITUTION The anhydride suggested by W. and D. Asch has not yet been separated, but this does not disprove its existence. The thermal curves of other alumino-silicates are interesting in comparison with those of clay. Houldsworth and Cobb ! give the following results :— Pyrophyllite (Al,0,48i0,H,O)— (a) Slight endothermal reaction at 480° C. (possibly due to clayey impurity). (6) Marked irreversible endothermal reaction between 720° and 830° ©., quite distinct from those in the clay curves. Allophane (A1,0;810,5H,0)— (a) Endothermal reaction, 50°-140° C. (6) Endothermal reaction, 270°-350° C. (c) Endothermal reaction, 860°-905° C. (d) Slight exothermic reaction, 950°-970° C. Halloysite (H,A1,S8i,0,.Aq. )— (a) Endothermal reaction, 50°-130° C. (6) Endothermal reaction, 490°-560° C. (c) Exothermal reaction, 880°-930° C, Cyanite (Al,0,S10,)— (a) Irreversible endothermic reaction, 775°-850° C. Andalusite (Al,0,S10,)— (a) Very slight exothermic reaction, 940° C. Sillomanite (India) (Al,0,Si0,)— (a) Slight endothermal reaction, 480° C. (6) Exothermal reaction, 950° C. Possibly due today ae THE SYNTHESIS OF CLAY When any soluble acid is converted into a salt by the addition of an alkali or base, the acid may be liberated by treating the salt with an acid which is stronger under the conditions of the experiment. So far it has not been possible to obtain clay by the similar treatment of any minerals which could be regarded as “ salts” of clay, though some investigators claim to have prepared materials which are almost identical in chemical composition, though their physical properties are not in all respects the same. Thus, W. Pukall? found that when common salt (sodium chloride) and clay are heated together at 950° C. a compound is formed corresponding to the formula 8Na,O 6A1,0, 12810, 12H,O.2 When this is treated with a weak acid, 1 Trans. Eng. Cer. Soc., 22, 111 (1922-23). 2 Berichte d. Deutsch d. Chem. Gesellsch., 43, 2107 (1910); Sprechsaal, 43, 440, 452 (1910) ; Chem. Centralb., 2, 1100 (1910). 3 Other investigators, when examining the action of salt on fireclay in the process of “ salt- glazing,”’ have reached the conclusion that a different compound isformed. Thus, Knett regards the following equation as expressing the formation of salt glaze: (AlFe),0,Si0,+6NaCl= SYNTHESIS OF CLAY 3538 such as carbonic acid, however, the soda is only partly removed and some silica is also withdrawn, the final residue having a composition corresponding to 2Na,.0 4H,0 6A1,0; 10810, 12H,0. With a strong acid, such as hydrochloric acid, Pukall’s salt is dissolved, and when ammonia is added an acid or highly acidic salt is precipitated which contains more hydrogen and oxygen than the original clay molecule. This precipitate also differs from the original clay inasmuch as it is com- pletely decomposed at 350° C. with the evolution of water, whereas clay is not dehydrated rapidly below a temperature of about 500° C. Pukall’s precipitate has been shown by W. and D. Asch?! to be an alumino-silicic acid similar to, but not identical with, clay. J. H. Collins also claimed that a material identical with china clay is produced by heating felspar with water under a high pressure. The author has repeated this experiment under varying conditions, and whilst the product is undoubtedly an alumino-silicic acid it does not appear to be identical with china clay. S. R. Scholes 2 has been granted a patent for the production of a material very similar to clay in composition and properties by fusing a mixture of felspar or other alkaline alumino-silicate with potassium carbonate, and then boiling the fused mass with water through which carbon dioxide is passed so as to remove all the soluble salts in solution. Scholes claims that this material has been used successfully for making articles similar to those produced with natural clay, but the author’s tests of a material prepared by him in the prescribed manner show that it is an alumino-silicate (not an alumino-silicic acid), and, therefore, not “ clay.” It is, of course, possible that the conditions under which clay can be produced are not obtainable in a chemical laboratory, either on account of the pressures or quantities of material involved being too small or the time available being insufficient. It is sometimes suggested that an examination of some granitic rocks—especially in Cornwall—will reveal the production of china clay from felspar by a process of slow hydrolysation as a result of which the potassium in orthoclase felspar is replaced by hydrogen and the alumino-silicate converted into alumino-silicic acid (clay), the potash being removed in solution. Subterranean saline fluids may act similarly. (AlNa3),0;Si0,+Fe,Cl,. The ferric chloride is further decomposed by ey forming red ferric oxide, which dissolves in the glaze and colours the latter brown. Fe,Cl, +H,O=Fe,0; +6HCI. F. H. Clews and H. V. Thompson (J. Chem. Soc., 121, 1442 (1922)) disregard the alumina, as they found the following reactions to occur between salt and silica under various conditions at temperatures between 569° and 1045° C. :— (a) 4aNaCl+ySi0,+20, =2aNa,0 ySi0,+22Cl, (b) 2aNaCl+ySi0, +2H,O=aNa,0 ySiO,+2xHCl. (c) 4HC1+0,=2H,0+20,. In the absence of moisture only action (a) occurs, but in moist air all three occur, (6) predominat- ing, and the reactions occurring most rapidly at 1000° C. For other formule, see p. 391. 1 Loc. cit., p. 346. 2 Eng. Pat., 117, 755. 23 354 CHEMICAL CONSTITUTION According to Fiebelhorn, their action may be represented by— K,0 Al,0, 68i0,+2H,0=A],0, 2810, 2H,0+K,0 4810, Felspar Water Clay Potassium silicate. Other investigators consider that the potassium silicate formed has the formula K,O 388i0,, and that the one molecule of silica in the free state is either carried away with the water or remains mixed with the clay. Van Hise considers that a more correct equation is one involving the use of carbon dioxide— 2K AlSi,0; +2H,0+CO,—H,Al,Si,0, +4810, 4K CO . Unfortunately, there is no evidence that clay is produced solely from orthoclase; on the contrary, it may be formed, as suggested by Van Hise, from andalusite, anortho- clase, cyanite, epidote, leucite, microcline, nephelite, orthoclase, plagioclase, scapolite, sillimanite, sodalite, topaz, zoisite, or garnet, or, as suggested by Rosler, by the decomposition of hauyne or analcite, or, as claimed by A. B. Jameson, from rhyolite, pumice, and various natural glasses. Under these circumstances, it is not surprising that the precise chemical constitution of clay is still uncertain. THE CHEMICAL CONSTITUTION OF GLAZES The chemical constitution of glazes and glasses is extremely complicated, and, like that of clay, is not yet properly understood. Glazes and glasses consist of sub- stances which are amorphous and, when molten, form homogeneous fluids. When cooled under suitable conditions they do not crystallise, but retain many of the properties of a fluid of exceedingly high viscosity. When heated to a temperature much below their melting-point they become plastic and mobile, but regain their rigidity on cooling. They may, in short, be regarded as “ under-cooled liquids ” or “ solid solutions.” Two distinct views are held as to the chemical constitution of glasses and glazes, viz. : (a) That they are solid solutions of various silicates mixed together indiscrimi- nately and not in definite proportions. (b) That they are definite chemical compounds. In the first conception of their constitution they are regarded as similar to any other mixtures of liquids, the physical properties of which must be confined within wide limits of composition as distinct from definite chemical compounds, the com- position of which cannot vary. This view of their constitution seems all the more probable when it is remembered that the composition of many glasses may vary within wide limits without their physical properties being impaired. On the con- trary, this permissible variation makes their use possible under comparatively rough conditions. The view that a glass or glaze is a homogeneous mixture of several substances forming a solid solution does not exclude the possibility of its containing a large CONSTITUTION OF MAGNESIA 355 percentage of one constituent which may be a complex chemical compound. It is well known that many definite organic compounds which are not quite pure will only crystallise with difficulty, and in some of these cases the proportion of impurity which prevents crystallisation is very small. A similar characteristic may explain why glasses and glazes possess their valuable property of remaining amorphous when solid. This analogy seems all the more reasonable when it is remembered that most glasses and glazes will crystallise if kept for a sufficiently long time at a suitable temperature, and that, on slightly altering the composition of some glasses and glazes, crystallisation readily occurs. As a general rule, the more closely the composition of a glass or glaze corresponds to that of a definite silicate, the more readily will it crystallise. On the other hand, a glass or glaze which crystallises too easily may be improved by adding rather more of one of the constituents (usually, but not always, silica) so as to remove its composition somewhat from that of a definite chemical compound. The nature of the definite chemical compounds in glasses and glazes is not definitely known ; investigations which have been made to ascertain what eutectics are present have not been wholly successful. They may be compared to metallic alloys, but the latter appear to be composed of much simpler compounds and often contain a larger proportion of metal in the elementary state, whereas glasses and glazes appear to consist of highly complex compounds. The chemical composition of most glasses and glazes does not correspond exactly to that of definite chemical compounds when the ordinary methods of formulation are employed. If they are assumed to have a very high molecular weight, however, it is much easier to regard them as definite compounds containing an excess of one or more ingredients, just as many commercial materials contain a variable percentage of other substances as “impurities.” For instance, no one would regard a silica brick as a solid solution ; it would be considered to be composed of somewhat impure quartz—a definite chemical compound. As the vitrified nature of glasses and glazes is one of their most valuable properties, it is generally useless to adjust their composition so that each consists of one definite chemical compound ; such a product would seldom be satisfactory, as it would have a great tendency to devitrify or crystallise, and so spoil the transparency and uniformity of the glass or glaze and prevent its being used for its intended purposes. When a matte or crystalline glaze is required, however, the glaze should approach more nearly in composition to a single definite compound in order to facilitate the production of a mass of minute crystals and so produce the required decorative effect. Many coloured glazes owe their colour to the formation of definite chemical com- pounds. Thus, some blue glazes are due to the formation of copper and cobalt zeolites (see also p. 395). THE CHEMICAL CONSTITUTION OF MAGNESIA Magnesium oxide—like silica—occurs in several allotropic forms which may be due to the different number of atoms in the molecules of the various substances. Usually the oxide is expressed by the formula MgO, though it is probable that the 356 CHEMICAL CONSTITUTION true formula is a multiple of this, and that further heating causes a modification (such as polymerisation) in the complexity of the molecule. The two most important allotropic forms are readily distinguished, as shown by J. W. Mellor, by their specific gravity, and the conversion of one form to another is shown by the change in the specific gravity. If the light-burned magnesia be described as the a-form, then the B-form—which is identical with periclase—may be formed by heating the a-form to 1300° C. Although both forms have the same chemical composition they differ in several important respects. Thus, the a-variety is soluble in acids, whilst the B-variety is very resistant to their action. The f-form (periclase) is crystalline, whilst the a-form is amorphous. The former is the more desirable as a refractory material on account of its constancy of volume at different temperatures. Although the conversion commences at about 1300° C., it is necessary to attain a higher temperature, 1530°- 1790° C., if the rate of conversion is to be reasonably rapid. The best method of effecting this conversion is to heat the magnesia in an electric furnace. N. Parravano and C. Mazzetti+ state that the change commences at 800° C., but proceeds very slowly at this temperature. This figure agrees closely with the results obtained by Ditte ? in his measurements of the density of the oxide after it has been heated at different temperatures. Other inversion temperatures suggested include Le Chatelier’s? figure of 1600° C., and that of Campbell, who gives 1100° C. as the temperature of transformation. The difference in chemical constitution between the a- and B-forms of magnesia has never been determined. It is probably analogous to the difference between charcoal and diamond. THE CHEMICAL CONSTITUTION OF SPINELS Spinels are compounds of aluminium, oxygen, and various metals, in which the two former constituents play the part of an acid radicle, so that spinels may be regarded as aluminates. The simplest contain only one metal besides the aluminium, whilst others are more complex and have three or more metals. The mineral spinel is a magnesium aluminate having, according to Mellor, the following formula :— O—Al=O Me O—Al=0 Other minerals belonging to the spinel class include O—AIl=O O—Al=0 Gahnite, Zn€ Chrysoberyl, Be€ O—Al1=O &e. O—Al=0 They are characterised by a high degree of refractoriness, and some of them form brilliant gem stones. 1 Annali Chim. Appl., 7, 3-12 (1923). * Comptes Rend., 73, 111, 191, 270 (1871) ; 76, 108 (1878). 8 Le Chauffage, 339 (1912). CONSTITUTION OF LIME, IRON OXIDES, ETC. 357 ’ Tue ConstTITUTION oF SILICON CARBIDES AND OXYCARBIDES Various substances—chiefly known by their trade names—are commonly regarded as silicon carbide, the typical formula for which is SiC. Of these materials carbor- undum and crystolon are true silicon carbides, silundum and siloxicon are carboxides, whilst the chemical nature of silfrax, silit, and some other materials made from carbon and silica are not clearly understood. It is probable that they are mixtures of carbides and carboxides in indefinite proportions, and, in some cases, they also appear to contain some silicon nitride. Silundum and siloxicon appear to be best represented by formule between Si,C,O and 81,0,0, or multiples of these. The arrangement of the atoms forming these carbides and allied materials has not yet been satisfactorily determined, but some information is given on p. 324. THE CHEMICAL CONSTITUTION OF LIME Lime occurs in two forms, amorphous and crystalline, the crystalline variety being formed when amorphous lime is heated at a high temperature for a long time. The crystalline variety occurs in two allotropic forms, the inversion point being at about 420° C. THE CHEMICAL CONSTITUTION OF IRON OxIDES AND HyDROXIDES Tron oxides occur in three forms: ferrous oxide (FeO), ferric oxide (Fe,0;), and magnetic oxide (Fe,O,). R.B.Sosman and J. C. Hostetter + consider that there is a continuous series of mixed crystals containing iron and oxygen varying in com- position from about Fe,0, at one end to Fe,O, at the other. The magnetic per- meability diminishes as the oxygen content increases. The decomposition-pressure also rises with increased oxygen-content. As the centre of the curve representing this is nearly horizontal, A. Smits and J. M. Bijvoet ? suggest that there may be two series of mixed crystals, and that the centre of the curve represents a mechanical mixture of both these series. Other investigators regard the various mixtures obtained as solid solutions of magnetic oxide in ferric oxide or vice versa. Sosman states that ferric oxide dissociates on heating, forming free oxygen and a solid solution of magnetic oxide in ferric oxide, the dissociation pressure falling as the percentage of FeO in the solid increases until the dissociation pressure of Fe,O, is reached, when the latter dissociates into oxygen and a mixture of oxides whose character has not yet been determined. Ferric oxide appears to have an inversion point at 678° C., which is sharp and reversible and affects its magnetic susceptibility, whilst another change, according to Honda, occurs at -40°C. Magnetic oxide is also said to have an inversion point at about 530° C., when its magnetic properties also change. 1 J. Amer. Chem. Soc., 38, 807 (1916) ; Trans. Amer. Inst. Min. Eng., 58, 409, 439 (1917). 2 Proc. Amst. Acad., 21, 389 (1919). 358 CHEMICAL CONSTITUTION The chemical constitution of the iron hydroxides is largely uncertain. Limonite —a ferric hydroxide to which is usually given the formula Fe,(OH),—appears to be very variable in composition, and there are good reasons for supposing that in the iron hydroxides much of the ‘‘ water”’ present is in the form of “ colloidal water ” (p. 336) and not as true “‘ water of constitution.” If that is the case, the various colours of these different hydroxides are really due to the colloidal matter present and not to the existence of innumerable ferric hydroxides as is sometimes supposed. Tue CHEMICAL CONSTITUTION OF OTHER REFRACTORY MATERIALS Comparatively little work has been done on the constitution of the more unusual refractory materials. There are many evidences, however, which show that their constitution is by no means simple. Thus, chromite shrinks considerably at about 500° C., probably on account of polymerisation. Zirconia appears to polymerise similarly to magnesia, and carbon has a very complex allotropy, graphite being the stable form at temperatures above 500° C. According to G. Asahara,! amorphous carbon, when examined by X-rays, shows interference rings, but no distinct maxima; the absence of the latter may be due to distributed intensity. Carbons produced by the decomposition of gases, such as carbon monoxide, acetylene, or carbon disulphide, or from Fe,C and coal, also show interference figures, thus indicating that “‘ amorphous ”’ carbon is really composed of extremely minute crystals. W. H. Bragg ? considers the difference between diamond, graphite, and other forms of carbon to lie in the bonding of the layers of electrons. Thus, he considers that, as in a diamond, the carbon atom lacks four electrons to complete its second layer, it shares these in common with each of four of its neighbours. He regards the layers of atoms in graphite as being joined by weak forces, though the atoms in each layer are united as firmly as in the diamond. He suggests that in organic crystals there is no sharing of electrons and no electrical separation into ions, one molecule being attached to the next in a very weak manner. The constitution of the complex mixtures, solid solutions, etc., which occur in the various raw materials and final products in the ceramic industries, cannot be dealt with in this section, as the varieties are far too numerous. In Chapter XI, however, the various reactions which may take place under different conditions are described, from which the constitution of any particular mixture may be predicted approximately, though the reactions are so complex, and our knowledge is at present so complete, that no definite rules can be laid down with respect to many of the changes which may occur. 1 Sci. Papers Inst. Phys. Chem. Research, 1, 23-9 (1922); Jap. J. Chem., 1, 35-41 (1922). 2 Min. Mag., 19, 316-8 (1922). CHAPTER IX THE CHEMICAL COMPONENTS OF CLAY AND CERAMIC MATERIALS AND PRODUCTS THE chemical constitution of “ pure clay ”’ is largely a matter of academic interest, though, when greater knowledge concerning it is available, it will probably be of great practical value in extending the uses for which clay may be employed, and in improving the quality of articles and materials made from clay. Chemically pure clay is almost a scientific curiosity, but more crude and impure clays and other ceramic materials are of such great commercial importance, that a knowledge of their other constituents is essential if progress is to be made in the ceramic industries. The presence and amount of the components or constituents of clays and allied materials are ascertained by chemical analysis, so that this subject may conveniently be considered prior to dealing with the individual components. Chemical Analysis.—The results of a chemical analysis of any mineral substance are usually expressed in terms of various oxides, such as silica, alumina, ferric oxide, titanic oxide, lime, magnesia, soda, potash, carbon dioxide, sulphur trioxide, ete. Elements such as chlorine, which are not found in combination with oxygen in most minerals, are expressed as elements, and, if necessary, a correction must be made for any oxygen reported as combined with sodium (in the form of soda) when in reality some or all of the sodium is present as sodium chloride. Another matter which requires special attention is the custom of reporting the iron present as ferric oxide (Fe,03). This is incorrect, as iron only occurs very infrequently in this form in clays. Its usual form of occurrence is in the ferrous state, as pyrites or marcasite, and to a small extent in the ferric state as limonite. It is often preferable to report the iron as ferric oxide and ferrous oxide, and a determination of the amount of ferric sulphide present is of value in some cases. As 160 parts of ferric oxide correspond to 120 parts of ferric sulphide and to 144 parts of ferrous oxide, the actual difference in the percentage of each of these substances, when any or all of them are converted into ferric oxide and weighed in this form, is not often serious, but, where the matter is important, a correction must be made. A similar correction may sometimes have to be made for other substances. A chemical analysis, as usually made, does not indicate in any way how the various elements (or oxides) are combined, but merely gives the total amount of 359 360 CHEMICAL COMPONENTS OF CLAYS, ETC. each. Hence, when it is desired to know what substances are present, special methods of analysis must be employed. It is possible to assume that certain elements will combine together in certain ways, and thus to obtain a very fair idea of the probable components of a mixture, but unless such assumptions are exceptionally well founded, they may lead to serious error. The chief value of a chemical analysis is to be found in the information it provides as to the proportion of each element (or oxide) present. From this information, any predictions may be in the nature of fairly safe guesses or they may be little better than speculations. Thus, if a chemical analysis shows that a clay contains a high pro- portion of fluxes, such as lime, soda, or potash, it will usually be safe to assume that the clay will not be highly refractory. It would not usually be safe to predict, as is sometimes done, the temperature at which the material would fuse or vitrify, as this will depend largely on the nature of the chemical compounds present. Thus, soda and potash, in the form of mica, are more detrimental to the refractoriness of a clay than when they are in the form of felspar, as the former fuses more readily than the latter, and soda, in the form of sodium silicate, is still more readily fusible. A chemical analysis is also valuable in showing the presence of some deleterious ingredients, such as (i) lime, which may destroy bricks and tiles made of clay containing it; or (ii) ferric oxide, which may either discolour a clay or enhance the beauty of the product, according as a white or red material is desired. Very great care is needed in drawing conclusions from the results of a chemical analysis, as a high proportion of calcium carbonate is a valuable constituent in some “clays,” because of its bleaching and vitrifying power, but in Beck used for other purposes its presence may be highly objectionable. It should scarcely need to be pointed out that unless an analysis is complete its value may be very small as the omission of a small percentage of some constituents may make an adequate interpretation of the results of the analysis very difficult. For this reason, no constituents should be estimated “‘ by difference” in order to make the analysis appear to be complete, though this is frequently done. It is especially important that the alkalies should not be estimated “ by difference,” as a small error in the proportion of them may have an important effect on the properties of the material. It is sometimes tempting to do this, as the work required to deter- mine the alkalies is as great as that of determining all the other constituents put together. Space does not permit a detailed description of the methods employed in the ultimate chemical analysis of ceramic materials, and the reader who desires informa- tion in this direction should refer to other volumes dealing more particularly with this subject, especially Mellor’s Quantitative Inorganic Analysis (Griffin, London). __ Sampling .—It is obviously essential that a chemical analysis, to be of value, must be truly representative of the material with which it deals. If it relates to a large mass of material, the analysis must be typical of the whole and not of one abnormal portion. Yet, unless special care is taken in procuring the sample to be analysed, such a typical product or “ fair average sample” will not be obtained. A single CHEMICAL COMPONENTS OF CLAYS 361 lump selected haphazard from the mass will be most unlikely to be fairly representative, and even a large number of such casually selected samples may not be satisfactory. Very frequently large variations occur, so that it is most important that proper care should be taken to obtain a truly representative sample. Failure to do this may result in heavy financial loss. When a large, natural deposit of clay or other material is to be analysed, the samples should be taken from many different parts of the bed, both horizontally and vertically. It is often desirable to have several distinct analyses made of different parts of the deposit, so as to determine whether the material is constant in composition throughout the whole bed. Where the material appears to be fairly uniform, about 1 per cent. of the whole of the material may suffice for the preliminary material from which the final sample is to be taken, but with a very heterogeneous material, 2 or even 5 per cent. may be necessary to obtain a reasonably typical sample. The preliminary sample should be composed of numerous smaller samples selected systematically from every part of the deposit and not merely from those which are the most readily accessible. When obtained, this preliminary sample should be treated as described later. If the material to be examined is contained in trucks or in a heap, it is equally necessary to take an ample quantity in the form of small samples to produce a large preliminary sample, though sometimes the whole mass is treated as a preliminary sample. Samples from large quantities of material contained in trucks or shiploads are often taken by means of mechanical appliances. The preliminary sample obtained as just described is next treated as an original material from which a representative sample is to be obtained by the process known as “ quartering,’ which is usually effected as follows :— The material should be thoroughly mixed and placed in a symmetrical and rather flat pile and divided into four equal parts. One of these parts is removed, the material in it is again mixed and then quartered, this being continued until a sample weighing about 28 lb. is obtained. This is coarsely crushed and again quartered, one quarter (about 7 lb.) being sent to the analyst, who should then obtain a repre- sentative sample by still further crushing and quartering until a suitable quantity _ is obtained. When very small samples are required for analysis, fine grinding and thorough mixing are very essential. The samples sent to the analyst should not be ground too finely before transit, or any oxidisable compounds present may be oxidised and so render the results inaccurate. Thus, ferrous oxide may be oxidised to ferric oxide. It is also desir- able to avoid, as far as possible, the use of iron or steel instruments in sampling, grinding, and sifting, so as not to include any adventitious iron in the sample. Sieves made of phosphor-bronze may be used instead of iron or steel ones for the same reason. CHEMICAL COMPONENTS OF CLAYS AND CLAY PRODUCTS Commercial clays consist primarily of one or more alumino-silicic acids, as described on p. 344, together with a varying proportion of other substances which are commonly termed “ impurities.” 362 CHEMICAL COMPONENTS OF CLAYS As most of the “ impurities” in ceramic materials reduce their resistance to heat they are often termed fluxes, though their ability to effect the fusion of the material depends largely on circumstances, and at low temperatures they have no fluxing action at all. The nature of the alumino-silicic acids has already been dealt with in Chapter VIII and need not be further considered. Attention may, therefore, be concentrated in this section on other materials present in clays, and their effect on the behaviour of the clay under different conditions. Although any component of a clay other than the alumino-silicic acid may be regarded as an “impurity,” the fact must not be overlooked that many clays owe their technical importance to the presence of some “impurities.” For instance, the pleasing red colour of some terra-cotta, the imper- viousness and resistance to corrosion of stoneware, the delicate translucency of china-ware, and the enormous resistance to crushing of some engineering bricks, are all due to the presence of a suitable proportion of certain “‘ impurities ”’ which impart to these materials their valuable characteristics. A pure alumino-silicic acid may be highly refractory, but it will usually be very weak when burned, owing to the absence of ‘“‘ impurities ” in the form of bases with which to form a vitrifiable bond which will unite the clay particles into a mass of great strength. The proportion of impurities allowable in a clay will, of course, depend on the purpose for which it is to be used. Thus, where it is to be employed in the manu- facture of whiteware, the clay must not, of course, contain a large proportion of colouring impurities, such as iron oxides, etc. Similarly, where a clay is to be used as a refractory material it must be as free as possible from fluxes, as these would reduce its resistance to heat. As a chemical analysis does not reveal the state in which the various substances are combined, it is very important, in analysis of clays, silica rocks, and other materials containing alumino-silicates, to remember that the proportion of fluxes shown by such analysis does not represent the total amount of impurity present, as the silica and alumina with which the fluxes are combined (or with which they will combine when the material is heated) will be included in the total amounts of silica and alumina, so that the presence of 1-6 per cent. of potash may represent 10-0 per cent. of impurity in the form of felspar, the 1-8 per cent. of alumina and the 6-6 per cent. of silica being included in the total silica and alumina present. Hence, a clay which may appear to contain a very small percentage of lime, magnesia, potash, and soda may actually contain 20 per cent. of minerals other than clay. For this reason it is often more important to know the mineralogical composition of a clay or other ceramic material than its composition as shown by chemical analysis (see Chapter X). As the percentage of moisture and loss on ignition are very variable in most ceramic materials, it is often convenient to calculate the analysis on the dried or burned material if a large number of comparisons is to be made. Such a calculation may involve small errors as the nature of this material removed by drying or burning may not be exactly known ; these errors are negligible in most cases, though occasion- ally they are important. SILICA IN CLAYS 363 IMPURITIES IN CLAYS The effect of impurities in a clay depend upon : (a) Their nature. (b) The proportion in which they occur. (c) The size and shape of the grains of clay and of the impurities. (d) The conditions under which interaction takes place, including (i) the temperature reached, (ii) the duration of the heating, (iii) the atmosphere of the furnace or kiln, and (iv) the effect of any other substances which may be present. The principal impurities in clays may be classed as follows : (a) silica, (b) alumina, (c) alkaline silicates and alumino-silicates, (d) iron compounds, (e) calcium com- pounds, (f) barium compounds, (g) magnesium compounds, (fh) titanium com- pounds, (7) manganese and other compounds which occur in very small proportions in some clays, (7) moisture and colloidal water, (k) carbonaceous matter and combined water (7.e. water of constitution and of crystallisation). Various other impurities which may occur in certain clays are usually of minor importance. Silica occurs in clays and allied minerals (a) in the free state, as quartz or other form of crystalline silica and as amorphous, hydrated, or colloidal silica, and (6) in combination (i) with alumina in the form of clay and other alumino-silicic acids, or with fluxes and alumina in the form of felspar, mica, or other alumino- silicates, or (1) with various bases forming soluble silicates, such as wollastonite (CaOSi0,), ete. The effects of free silica in clay are as follows :— 1. It reduces the plasticity (p. 270). 2. It lessens the shrinkage on drying (p. 96) and firing (Chapter XIII). 3. It reduces the tensile and crushing strengths (p. 151). 4. It may increase the resistance to sudden changes in temperature (see Chapter XIII). 5. It reduces the refractoriness in many cases, though not in all (see also Chapter XIII). Combined silica does not affect the melting-point in the same way as free silica. The size of the grains is also important, as very small particles of silica will often act on a flux under conditions where larger particles of silica increase the refractori- ness of the mass. In some cases silica will increase the refractoriness of a clay, quite apart from the size of the particles, especially if the “clay ” is very impure ; any improvement which may be effected in this manner by the addition of silica is of very limited extent. Seger found that, with pure clay and pure silica, the melting-point is lowered, with an increase in the proportion of silica until a molecular ratio corresponding to Al,O,:17S8i0, is reached, after which further additions of silica raise the melting-point. Sieurin and Carlsson! found that silica reduced the 1 Loe. cit., p. 149. 364 CHEMICAL COMPONENTS OF CLAYS refractoriness of a clay when under load, the minimum resistance being reached with a mixture containing 60-70 per cent. of silica. Fig. 38, due to Seger, shows the refractoriness of mixtures of pure silica and alumina. If a clay contains more than two molecules of silica to each molecule of alumina, and also a high proportion of ' fluxes, it will usually have a low refractoriness ; Bleininger and Brown have also suggested that no clay should be used for refractory purposes which contain more than 0-225 molecules of fluxes or 2 molecules of silica to each molecule of alumina. The effect of silica on the refractoriness of a clay is also clearly shown by Ludwig’s chart (p. 382). Alumina occurs in clays in the form of felspars, mica, hornblende, tourmaline, and other similar alumino-silicates, all of which are moderately fusible. Free alumina is seldom found in clays, but is abundant in bauxites and laterites, and is therefore present in some clays derived from these materials. Aluminous compounds, apart from the clay, have the following effects on clays if the temperature is sufficiently high :— 1. They reduce the plasticity of the clay, as they are non-plastic (p. 270). 2. They increase the strength of the fired clay (p. 147). 3. They increase the density and impermeability of the burned ware (p. 66). 4, They reduce the refractoriness of the clay if they are in the form of alumino- silicates, but free alumina increases the refractoriness of most clays (below). Sieurin and Carlsson! found that alumina up to 70 per cent. increases the refractoriness under load (p. 149). With 70-80 per cent. of alumina, however, the refractoriness suddenly falls, but increases slowly with still higher percentages. L. Bertrand 2 found that raw clays containing more than 29 per cent. of alumina, or fired clays containing more that 32 per cent., had softening points over 1650° C. Those containing 20-29 per cent. of alumina in the raw clay, or 21-32 per cent. in the fired material, had softening points often above but sometimes lower than 1650° C. Clays with less than 20 per cent. of alumina (or 21-5 per cent. with the fired material) generally soften at temperatures below 1650° C., though occasionally such clays are found which soften above this temperature. Alkaline Silicates and Alumino-silicates.—The chief alkalies in clay oceur— (1) Silicates and alumino-silicates (incl. felspar and mica) being so typical it is often assumed that all other insoluble alkali compounds in clays behave like either felspar or mica, though others may also occur. The effect of felspar and mica on the refractoriness of clay is shown in figs. 19 and 20, due to Simonis and Rieke respectively. (ii) As soluble salts, such as potassium sulphate, sodium sulphate, and sodium chloride. These salts also reduce the refractoriness of the material; they may also, if present in sufficiently large proportion, form a white scum on the surface of the articles either before or after firing. Soluble salts also affect the plasticity of clay (p. 272), some tending to increase it, though most of them reduce it. According to Dorfner, in stoneware, porcelain, and other vitrified ware, the higher the molecular 1 Loe. cit., p. 149. : 2 La Céramique, 25, 153-157 (1922). 365 O 0 20 70 7 30 50 Per cent Mica 50 40 Per cent Kaolin 60 3 70 _| ASRS _ J SESRR RRR ae 80 Fie. 20.—Fusion Curve or Mica-Kaouin MIxtTurss. ee PT ; <2 8 ~~ pay nae 2) Ses +5 p 2 o..9 oa Ss = 4 ° S& 9 85 ; 4 Ros fs S & : Sages § eM SS el So oO 8 oi s< 8 yy Oy, & Se 9 aha is VOL & VE WE, Sve s ° hed Sere Zi g 8 8 Pe Sere aac g Seer tes = 366 CHEMICAL COMPONENTS OF CLAYS proportion of potash, the more fusible is the mass, whilst the higher it is in lime the less fusible it becomes. On the other hand, the lower the proportion of potash, the more plastic will be the material. In incompletely sintered ware, such as faience and earthenware, the larger the proportion of potash, the more solid will be the body with a given firing temperature. Iron Compounds.—The various iron compounds which may occur in clays have been mentioned on p. 97. As regards their chemical effect, they may be classed as (a) ferric oxide (Fe,03), (b) ferrous oxide (FeO), (c) magnetic iron oxide (Fe;0,), (d) iron sulphides (FeS and FeS,), (e) iron carbonates (FeCO;), (f) ferrous and ferric hydroxides which behave like the respective oxides, (g) ferro-silicates and ferro-alumino-silicates, (h) soluble iron salts (chiefly ferrous sulphate). The chief effects of iron compounds in clays are: (1) They effect an alteration in the colour (see Chapter II). (ii) They may reduce the refractoriness of the clay. (iii) Soluble iron compounds may form a scum on the ware. A very small quantity of iron oxide is undesirable where white ware is required, unless a comparatively large proportion of calcium carbonate in the clay is not objectionable, when a correspondingly large proportion of iron may be present, as its colour will then be neutralised on heating, and a white product formed instead of the usual red colour due to ferric oxide (see also p. 102). | Ferric oxide does not greatly reduce the refractoriness of a burned clay, provided it is always maintained in an oxidising atmosphere, as ferric oxide is highly refractory. In a reducing atmosphere, on the contrary, it acts as a powerful flux. Sieurin and Carlsson 1 found that when a clay containing ferric oxide is heated under pressure the iron oxide reduced the refractoriness rapidly if less than 6 per cent. were present, slightly if between 6 and 12 per cent. were present, and rapidly if more than 12 per cent. of iron oxide were present. Magnetic and ferrous oxides are very undesirable, as they are powerful fluxes, and combine with clay to form mobile, fusible silicates, ferro-silicates, and alumino- silicates, the most fusible silicate being fayalite (melting-point, 1050-1075° C.) and the most fusible alumino-silicate, according to Rieke, corresponding to the formula 2Fe,0A1,0;28i0,, which melts at Cone 3a (1140° C.). Ferrous carbonate, when present in a clay, may either be reduced in the burning process to ferrous oxide with the evolution of carbon dioxide (the oxide then acts as a flux, and produces black, slaggy masses of fusible silicates which are very undesirable), or it may be oxidised to ferric oxide, and be comparatively harmless unless white ware is required. Ferric sulphide (pyrites), when heated, loses half its sulphur at 400°-600° C. and the rest at a higher temperature. In a reducing atmosphere it produces ferrous oxide which acts as a powerful flux, but if the burning is carried out entirely in an oxidising atmosphere, the iron may be completely oxidised to form ferric oxide, which does not greatly affect the refractoriness of the clay. 1 Loe. cit., p. 149. CALCIUM COMPOUNDS IN CLAYS 367 Ferro-silicates and ferro-alumino-silicates behave in a manner similar to felspar, 2.e. they are moderately fusible and increase the amount of vitrified matter or ‘‘ bond”’ in the fired ware and so slightly increase the strength of the ware. According to their colour they may improve or spoil the appearance of the ware. Some ferro-silicates, such as nontronite (p. 97), appear to be decomposed into their constituent oxides when heated. Soluble iron compounds usually produce a light-coloured scum on the surface of the dried ware containing them. On burning, this is usually converted into unpleasant brown or black patches. Calcium compounds occur in clays as (a) calcvwm carbonate, in the form of erystals of calcite or aragonite, or as chalk or other form of limestone. Occasionally it occurs in the form of fragments of shells; (b) calevwm sulphate, in the form of gypsum and selenite; (c) calciwm phosphate, in the form of coprolites and other fossilised animal excreta; (d) lume felspars, chiefly oligoclase and anorthite ; and (e) other calcium silicates and alumino-silicates. When calcium carbonate or calcium sulphate is heated, it evolves carbon dioxide or sulphur trioxide, and forms lime, which is a powerful flux and readily combines with silica and with alumino-silicates to form a mobile fluid of great corrosive power. When cooled, this fluid solidifies to a glassy mass, which forms a strong bond and produces impervious and acid-resisting ware. Calcium phosphate, when heated to redness, exchanges phosphorus pentoxide for silica, and forms a calcium silicate having the same properties as that produced by the action of lime. Calcium silicates and other stable calcium compounds melt at a comparatively low temperature, and then act as fluxes in a manner similar to lime, but much more slowly. The calcium alumino-silicates produce a tougher and more viscous bond than the simple silicates, and are, therefore, preferable. The lime produced by heating some calcium compounds (as explained above), if present in an amount equivalent to less than 10 per cent. of lime, reduces the refractoriness of kaolin from Cone 35 (1770° C.) to Cone 15 (1435° C.), but if 10-20 per cent. of lime is present the refractoriness increases to Cone 19 (1520° C.) and then falls with 34-5 per cent. of lime to Cone 7 (1230° C.). With 50 per cent. of lime the refractoriness rises again to Cone 19 (1520° C.), but with 59-7 per cent. of lime it falls to Cone 13 (1380° C.), after which, with 70 per cent. of lime, it rises sharply to Cone 27 (1610° C.). Fig. 34 shows the phase diagram of the lime-silica system according to Day and Shepherd. Clays containing a larger proportion of silica than is present in kaolin (2.e. more than two molecules of silica to each molecule of alumina) are more rapidly reduced in refractoriness by small amounts of lime than those of a composition similar to kaolin. Thus, according to Rieke, the addition of 7-6 per cent. of lime reduces the refractori- ness of a mixture corresponding to Al,0,48i0, to Cone 12 (1350° C.), whilst mixtures corresponding to Al,0,2Si0, or to Al,O,38i0,, with about 6 per cent. of lime re- spectively, fuse at about Cone 28 (1630° C.). The addition of about 20-40 per cent. of lime reduces the refractoriness of a mixture corresponding to Al,0,48i0, to below Cone 6 (1200° C.), but the addition of 62 per cent. of lime to such mixture increases its 368 CHEMICAL COMPONENTS OF CLAYS refractoriness to about Cone 26 (1580° C.). Mixtures corresponding to Al,0,38i0, are the most fusible when about 48 per cent. of lime is added to them. The slag produced by the combination of lime with silica and other minerals is very mobile when fused, and, consequently, a mass containing it loses shape very shortly after the lime compound begins to fuse. According to Pukall, a high proportion of lime in a clay causes fritting and fusion rather more slowly than an equally high proportion of potash. Thus, with same total molecular ratio of bases (RO) to alumina and silica, a high molecular proportion of potash will form a stoneware at Cone 7 (1230° C.), whilst a high molecular proportion of lime will produce a good earthenware, except where the proportions of silica or alumina are also high. At very high temperatures, both these mixtures will produce porcelains unless the alumina or silica is excessively high. According to Dorfner, the higher the molecular proportion of lime in the bases, the total RO being constant, the less fusible is the product if a porcelain, stoneware, or other vitrified ware is being produced ; but in a porous ware, the higher the molecular proportion of lime in relation to the total bases (RO), the more friable will be the ware and the less the liability of the glaze to craze. Barium compounds do not occur to any great extent in clay, but when present they act in a manner similar to the corresponding lime compounds. Magnesium compounds occur in clay chiefly as magnesite (MgCO,) and as complex silicates and alumino-silicates. They act as fluxes and reduce the refractori- ness of clays in which they occur, though they are less powerful than the corresponding lime compounds and act more slowly, so that they are usually less harmful and less likely to cause the ware to lose its shape when heated. The slag produced by the action of magnesium compounds on clay is also more viscous, thus further preventing the occurrence of a rapid loss of shape. Rieke has found that magnesite gradually reduces the refractoriness of kaolin from Cone 34 (1750° C.), with less than 1 per cent. of magnesite, to Cone 10 (1300° C.) with 40 per cent. of magnesite. With more than 45 per cent. of magnesite the refractoriness is increased, and with 64 per cent. the refractoriness of the mixture is the same as that of Cone 29 (1650° C.). When an excess of silica is present, the refractoriness is reduced more rapidly, 30 per cent. of magnesite sufficing to reduce the refractoriness of a mixture containing 133-3 parts of free silica to 100 parts of kaolin to Cone 10-11 (1300-1320° C.). Titanium compounds act as fluxes in clay, but, as they usually occur in only small proportions—seldom exceeding 2 per cent.—their effect is frequently negligible. The presence of 10 per cent. of titanic oxide in silica reduces the refractoriness of the latter from Cone 36 to Cone 33-34; the eutectic (which melts at Cone 20) contains 40 per cent. of titanic oxide. Titanic oxide has a similar effect on alumina, 20 per cent. of it reducing the refractoriness of alumina from Cone 42 to Cone 37 ; larger proportions are progressively more active. Manganese compounds when present in clays and clay products act as fluxes, and in many respects resemble the corresponding ferrous compounds. Rieke has found that the addition of 7-38 per cent. of manganese monoxide to kaolin reduces PHOSPHORUS, VANADIUM, AND SULPHUR IN CLAYS 369 its refractoriness to Cone 30 (1670° C.), 13-73 per cent. reduces it to Cone 12 (1350° C.), and 24-15 per cent. reduces it to about Cone 1 (1100° C.). Larger percentages still further increase the fusibility of the kaolin. Phosphorus and vanadium compounds occur in very small proportions in some clays, but these constituents are usually negligible. If in sufficient proportion to have any appreciable effect, they reduce the refractoriness of the material (see calcium phosphate (p. 367). Vanadium sometimes produces a greenish colour in burned clays (p. 123). The effect of vanadium oxide on the refractoriness of kaolin is shown in Table CXII, due to O. Kallauner and T. Hruda.? TaBLe CXII.—Effect of Vanadium on Kaolin Kaolin. Vanadium Pentoxide. Cone. Refractoriness, ° C. 100 0 35 1770 99 i 34 1750 95 5 39 1730 90 10 32 1710 80 20 30 1670 60 40 15 1435 40 60 5a 1180 20 80 08a 940 0 100 020 675 Sulphur occurs in clays chiefly in the form of iron sulphides (pyrites), or as gypsum (calcium sulphate), though more soluble sulphates may occur to some extent either from the oxidation of sulphides or by contact with water containing sulphates insolution. When the clay is heated the sulphur compounds are usually decomposed, the sulphur being evolved as sulphur trioxide. The temperature of decomposition varies according to the nature of the compound. Thus, ferric sulphide (FeS,.) gives off half its sulphur at 400° C. and the remainder at a higher temperature ; sulphates are decomposed between 800° and 1000° C. At 700° C., in the presence of reducing gases, they are usually reduced to sulphides. Insoluble sulphur compounds are not usually objectionable unless the material is heated until vitrification begins, when they evolve gases which may produce a spongy or “ bloated ’”’ ware of greatly distorted shape. This is especially noticeable if brick clays containing iron pyrites are heated too rapidly from 500°-900° C. Soluble sulphates are chiefly objectionable on account of the white scum they produce on the surface of the ware. This may be made less objectionable by heating the ware to such a temperature that the scum-forming compound combines with 1 Sprechsaal, 45, 333-5, 345-9 (1922). 24 370 CHEMICAL COMPONENTS OF CLAYS the clay and forms a transparent colourless glaze, most of which is absorbed by the ware. Moisture and colloidal water (p. 336) are present in natural clays in widely varying proportions. To them, the clays largely owe their plasticity and other physical properties, so that these forms of water can scarcely be regarded as impurities. A knowledge of the proportions present is very important, however, if definite quantities of different materials are to be mixed together, as in the preparation of earthenware, stoneware, or porcelain bodies and engobes, for any variation in the proportion of moisture from that which is assumed to be present will affect the amount of clay used and so may alter the properties of the materials. For this reason, a knowledge of the proportion of free water present is sometimes of great importance. Carbonaceous matter occurs in most clays, but the proportion varies greatly, some clays containing less than 0-5 per cent., whilst others contain 5 per cent. or more. Bricks are sometimes made from “ colliery refuse ’’ containing as much as 12 per cent. of carbonaceous matter, but such materials are mixtures of clay and coal and cannot be regarded as clays. Some indurated clays or shales contain 25 per cent. of carbonaceous matter; they are chiefly used as a source of oil, which is obtained by heating them to about 500° C. and condensing the vapours evolved from them. Carbonaceous matter in clay—(a) affects the colour of the raw and dry clays, but not that of the burned material unless the latter is deprived of sufficient air in the heating process ; (b) may increase the plasticity of the clay if the carbonaceous matter is in a colloidal state, otherwise it may act as a non-plastic material and reduce the plasticity ; (c) increases the porosity of the fired ware ; (d) increases the permeability of the fired ware by producing relatively large pores ; (e) may increase the amount of water absorbed by the raw clay; (f) may increase the shrinkage of the clay; (g) reduces the amount of fuel required to burn the goods; (h) may cause trouble in firing, as, unless special precautions are taken, black cores of charred carbonaceous matter may be left in the fired ware ; these cannot be “ burned out ”’ at a temperature above 950° C., owing to the sealing of the pores at and above this temperature ; («) may cause the reduction of iron compounds to the ferrous state, in which case they will act as fluxes and form dark fusible silicates. The extent of the action of carbonaceous matter depends largely on the conditions during the burning and on the texture of the clay or clay mixture. In a material which is open in texture and is heated slowly with an ample supply of air, the car- bonaceous matter burns away quietly and usually does little or no harm. If the heating is effected too rapidly, so that too high a temperature is reached before all the carbonaceous matter has been burned out of the ware, the latter may be spoiled by (a) discoloration due to the charred carbonaceous matter which is covered by a film of fused siliceous material and so cannot be burned away, or (b) bloating or swelling of the ware, due to the exterior pores being sealed by fused siliceous material and so preventing the escape of the gases produced in the interior of the ware by the continued action of the heat on the carbonaceous material. The only means of preventing either of these defects consists in controlling the heating and air supply WATER OF CONSTITUTION AND CRYSTALLISATION 371 in the kilns during the early stages of firing (7.e. before a temperature of 950° is reached) so as to burn off the carbonaceous matter so slowly that the temperature does not reach that at which partial fusion can occur. Although it is usually undesirable for clays to contain more than about 5 per cent. of carbon in the form of carbonaceous matter, free carbon is sometimes added for special purposes. Thus, the presence of 15-25 per cent. of plumbago or graphite increases the resistance of firebricks or crucibles to corrosion by slags, fluxes, etc., prevents undue oxidation of the contents of crucibles, and makes such articles less sensitive to sudden changes in temperature. Coke is sometimes used instead of graphite ; though much cheaper, it is far less effective. Sawdust is sometimes added to clays to produce ware of high porosity, the pores being produced when the saw- dust or other carbonaceous powder burns away. (See also Chapter X.) The water of constitution and crystallisation of pure clay can scarcely be regarded as impurities, but when they form a part of other minerals or carbonaceous matter in a clay or clay mixture their effect may have to be taken into considera- tion. Water in both these forms is only evolved when the material is heated and, if the temperature rises sufficiently slowly and the kiln or furnace is well ventilated, little or no harm will be done. With too rapid heating, on the contrary, the various effects known technically as “ steaming” may spoil the ware. From the foregoing pages it will be seen that the beneficial or harmful effect of an impurity in a clay must be judged by the purpose for which the clay is to be used ; some impurities may be allowable under some conditions, but must not be present when the same clay is used for other purposes. Consequently, in selecting clays or other ceramic materials, the purposes for which they or the finished articles are to be used must always be taken into consideration. COMPOSITION AND UTILITY The effect of the composition, as of the physical properties, of a clay or clay mixture on the utility of articles made from it is a matter of great technical importance. It usually determines the commercial value of the raw material as well as limiting the purposes for which it may be profitable employed. Various investigators have endeavoured to find a simple means of correlating the composition of clays with their utility, but, so far, with little success. This is only to be expected, as most natural clays are such complex mixtures of different minerals that it is unlikely that any simple ratio of two ingredients (such as silica and alumina) can accurately express their utility. All that such a ratio can indicate is the general fact that, for some articles made of mixtures of clay and silica, the proportion of each of these materials should be within certain broadly defined limits. As it is difficult to ascertain the proportion of true clay present, the assumption is usually made that the alumina is strictly proportional to the true clay and, therefore, that the ratio of silica : alumina may be taken as a rough basis of classification. When it is desired to include the other constituents (e.g. the bases) in such a ratio, the validity of the assumption is very questionable ; nevertheless, W. Pukall has stated that the molecular ratio 372 CHEMICAL COMPONENTS OF CLAYS of silica : bases-+-alumina in ceramic bodies varies from 0:5: 1 to 3:1; kaolins and fireclays poor in silica have ratios from 0-5: 1 to 1: 1, whilst many clays suitable for bricks, tiles, and terra-cotta have a ratio from 2: 1 to 3: 1 and the clays used for fine ceramic wares generally have a ratio between 1:1 and 2: 1. China clay and kaolins, when pure, contain about 46 per cent. of silica, 40 per cent. of alumina, and 14 per cent. of water. The best commercial samples contain about 45 per cent. of silica, 39 per cent. of alumina, 13 per cent. of water, and 3 per cent. of other oxides, notably lime, magnesia, soda, potash, iron, and titanium oxides. It must be remembered, however, that 3 per cent. of impurities in the form of fluxing oxides may be equivalent to 10 per cent. of the total impurity in the material, the other 7 per cent. being included in the figures for silica and alumina, because many impurities do not occur as simple oxides, but as silicates and alumino-silicates, such as felspar, mica, etc. Kaolins containing more than 4 per cent. of alkalies are termed alkaline kaolins. The greater part of the alkali-bearing minerals may, in some cases, be removed by careful preparation and washing. Ferruginous kaolins contam a large proportion of iron oxide and, consequently, are not so white when burned as are pure varieties. For pottery manufacture, china-clay should not usually contain more than 0-5 per cent. of lime, though some foreign kaolins contain up to 3 per cent. ; for paper- making, the composition is of minor importance if the clay is sufficiently white and plastic. Magnesia may occur in proportions from 0-3 per cent., but in the best qualities not more than 0-2 per cent. should be present. Free silica should not be present in the best china clay or kaolin to a greater extent than 10 per cent., though some commercial samples and some siliceous kaolins contain as much as 25 per cent. . of fine grains of silica.} Ball clays and pottery clays are similar in chemical composition to china clays, but contain more alkalies and iron oxide. The following composition is typical of high-class ball clays :— Silica : ; : : : 40-48 per cent. Alumina . : : ; : é 32-36 per cent. Tron oxide ; ; : Less than 2 per cent. Lime aa : é . ; Less than 1 per cent. Potash eas Less than 3 per cent. Pottery clays usually contain 45-86 per cent. of silica, the average being about 48 per cent.; 7 per cent. of alkalies is sometimes present, but the proportion is not generally more than about 2 per cent. Not more than 2 per cent. of lime and about. 1 per cent. of magnesia should be present, less than these amounts being usually desirable. The iron oxide should be as low as possible so as to prevent the discolora- tion of the burned clay.+ 1 Further analyses of china clays and ball clays will be found in the author’s Refractory Materials; Their Manufacture and Uses (Griffin). COMPOSITION OF FIRECLAYS 373 Fireclays vary greatly in composition, and may contain 34-981 per cent. of silica, the average being about 54 per cent., but the best qualities are those in which the percentage of silica is not much greater than that of the alumina, as these contain the largest proportion of true clay. The Institution of Gas Engineers has specified that fireclays should not usually contain more than 75 per cent. of silica, though in some cases 80 per cent. may be present, provided the material fulfils all the other requirements of the Institution’s specification. The American Gas Institute has specified 75 per cent. of silica as the maximum amount allowable in articles to be sold as “ fireclay bricks”; those containing a larger percentage of silica are termed “ siliceous bricks.” Fireclays should not usually contain more than about 2 per cent. of potash and soda, though in some cases 5 per cent. may be present without having any serious effect on the quality. Bleininger and Brown? have suggested that for each one molecule of alumina the proportion of fluxes should not exceed 0-225 molecular equivalents in a refractory clay. Fireclays should not contain a large proportion of lime, as this is a very powerful flux. Some contain as much as 10 per cent., but this is much too high for a refractory clay, the average being about less than 2 per cent. The proportion of magnesia seldom exceeds 6 per cent., equivalent to about 12 per cent. of magnesite, and averages about 0-5 per cent. Several good fireclays in this country contain 3 per cent. of magnesium carbonate. The proportion of iron compounds in the best fireclays should not exceed 2 per cent., expressed as ferric oxide: some of the less pure ones contain 5 per cent. In America, some fireclays are used containing as much as 7 per cent. of iron oxide. The so-called Windsor firebricks, which are made from a soft red sandstone, contain about 4°5 per cent. of iron oxide, but are sufficiently refractory for gas-retort settings, etc., provided they are used in an oxidising atmosphere, so that the iron remains in the ferric state. When heated under reducing conditions the iron combines with the silica and forms a fusible slag. The Ewell bricks—made at Ewell, in Surrey—and similar bricks made at Chalfont St Peter, are used for the same purposes as bricks made of Windsor loam; they consist of— Silica , ; . 84-65 per cent. Alumina . : creel’, ig Lime ; ‘ sy 1.00 - Magnesia é pee etlsao 2 Iron oxide ; SN es: Fe Riley ? appears to have been one of the first chemists to find titanium oxide in English fireclays and in many more recent analyses this constituent is not mentioned, being included in the figures for silica and alumina. The proportion of titanium 1 The application of the term “ fireclay ” to a material containing 90 per cent. or more of silica 1s a misnomer. 2 Loe. cit., p. 149. 3 Quart. J. Chem. Soc., 15, 311 (1862). B74 CHEMICAL COMPONENTS OF CLAYS oxide in English fireclays is generally less than 24 per cent., whilst some American clays contain nearly 5 per cent., though these are abnormal, the usual proportion being less than 2 per cent. A few clays have been found with 10 per cent. of titanium oxide. Further analyses of various fireclays will be found in the author’s Refractory Materials: Their Manufacture and Uses (Griffin). Brick clays depend for their value on their physical properties rather than on their chemical composition and, consequently, the permissible range of composition of brick clays is very wide. The clays used for building bricks may have from 35-90 per cent. of silica, the average being between 60 and 70 per cent. of silica. If the true clay present is sufficiently plastic, good bricks can usually be made from a material consisting essentially of about 50 per cent. of such true clay and 50 per cent. of silica; this would correspond to about 73 per cent. of silica in the mixture. So small a proportion of clay can only be satisfactory when the true clay is exception- ally plastic; otherwise, the bricks would be too weak to be satisfactory. Brick clays (with the exception of those used for firebricks) may usually contain a comparatively large proportion of metallic oxides (fluxes), as they are not heated to a sufficiently high temperature to cause distortion due to excessive fusion. In the absence of a moderate proportion of these oxides, the brick will be deficient in vitrifiable bonding material, and’so will be relatively weak. With a suitable proportion of fluxes bricks of enormous strength can be produced, each particle of unfused material being held in place by the crude glass formed when the fluxes combine with the silica in the clay. Clays suitable for building bricks may contain up to 15 per cent. of alkalies (the average being about 3 per cent.), up to 15 per cent. of calcium compounds expressed as lime (the average being below 2 per cent.), and up to 7 per cent. of magnesia (the average being about 1 per cent.). Iron compounds (expressed as ferric oxide) occur in proportions up to 32 per cent., the average, being 3-8 per cent. ; the red colour of many bricks is due to the fully-oxidised iron compounds present and the colour of “‘ blue ” bricks to the reduced iron compounds present. Further analyses of brick clays will be found in the author’s Modern Brickmaking (Scott, Greenwood & Son). Some clays which shrink excessively, and others which have a poor colour when burned, are improved by the addition of sand, especially if this contains a considerable proportion of iron compounds other than pyrites. The metallic compounds most likely to cause trouble in the manufacture of building bricks are (a) ime compounds and (6) “ soluble salts.” The lime compounds produce lime which slakes and may crack the bricks containing it. This may be largely avoided by grinding the limestones to a fine powder, as particles of limestone less than 0-04 inch in diameter seldom crack bricks containing them. In some works, washed chalk (calcium carbonate) is purposely added to bricks to reduce the shrinkage of the clay and to act as a binding agent. Soluble salis are chiefly objectionable because they form a white efflorescence or “scum”; consequently, brick clays containing an appreciable proportion of soluble salts can only be used where the appearance of the fired bricks is of no importance. Fine earthenware necessarily requires clays which are sufficiently white when COMPOSITION OF EARTHENWARE AND PORCELAIN 375 burned, though by using a sufficient proportion of flint or other suitable non-plastic material it is often possible to make good use of a clay which is not sufficiently white when used alone. Various blue colours may also be used to destroy the yellow colour of some burned clays. In England, the clays used for fine earthenware are chiefly ball clay and china clay, but in some Continental works red-burning clays mixed with chalk or finely-ground limestone are used. The product is white, as calcium carbonate has a powerful bleaching action on red-burning clays. EKarthenware—whether fine or coarse—is porous except for the glaze and it is, in this way, distinguished from porcelain and other dense ware. Coarse earthenware may be made of almost any clay which can be made into articles of the desired shape. The colour of the burned clay may be disguised, if required, by covering the ware with a suitable engobe or an opaque glaze. China ware and porcelain are made of mixtures of various white-burning clays and fluxes of such a nature and in such proportions as will produce a dense, vitreous, and translucent body. China ware is made of china clay, ball clay, flint, bone ash, and Cornish stone; it owes its translucency largely to the formation of a glassy binding material formed by the combination of bone ash and silica. Such ware usually varies in composition between the following limits :— 1-15-8-33RO Al,0, 1-97-9-088i0, 0-35-2-67P,0,. Ideal porcelain should consist of a mixture corresponding to the formula Al,O,Si0, (sillimanite) and a fusible glass which binds the other particles firmly together. When materials capable of producing such a mixture are heated to a sufficiently high temperature they will form a felted mass of sillimanite needles bonded with a glassy cement. A perfect porcelain is seldom obtained, though Marquardt’s porcelain approaches very closely to it. There are at least five distinct types of porcelain and the clays used for one type are not necessarily suitable for another. 1. Sévres porcelain is made essentially of a mixture of clays and fluxes which are intended to form a special kind of glass, the chief materials being white-burning kaolin, felspar, and quartz. Sévres porcelain corresponds approximately to the formula, 0-30—-0-35RO R,O, 2:8-3:5S8i0,. The Meissen and Viennese porcelains are similar in composition to that of Sévres. 2. Hard-paste porcelain, including porcelaine dure, Halle porcelain, and Berlin porcelain, corresponding approximately to the formula, 0-2-0-3RO R,O, 4:2-4-88i0,. The varieties of hard-paste porcelain consist essentially of a “skeleton ”’ of unfused material saturated with glassy matter, some of the latter forming a felted mass of crystals of sillimanite on cooling. Such porcelains are peculiarly resistant to sudden changes in temperature and in this way are quite distinct from others. 3. Soft porcelain or porcelaine tendre, corresponding to the formula,0-4—0-45RO R,O; 4-8-5-38i0,. This is really a complex glass and the chief difficulty attending its manufacture lies in the facility with which it loses shape in the kiln owing to a deficiency of sillimanite, or of the materials from which this substance can be formed, during the conditions of manufacture. 376 CHEMICAL COMPONENTS OF PORCELAIN 4. Chinese and Japanese porcelains vary so greatly in composition and are so complex in character owing to the very impure materials used, that they can scarcely be said to correspond to any formula, though they are chiefly within the following limits :— Japanese. . s ; . 0-3-0-4RO Al,O, 6-2-7-4810,. Chinese : ; ; : : . 0-40-45R0 Al,O, 5-5-6810) . Sévres imitation of Oriental porcelain . 0-37RO Al,O; 5-15Si0,. Seger’s _,, iH 9 . 0-36RO Al,O, 8-558i0.. 5. Dental porcelains vary greatly in composition, some being highly refractory and others very fusible. The following typical formule are due to A. 8. Watts } :— 0-74K,0 7-8308i0, kas Wi ey CO eENE \1-1441,0, { 0-0027%0;, 0-638R,0 > 0:565A1,0, (5-90S8i0, (b) {orixao} 0-023B,0, 0-221CaO J} 0:090Fe,0, 0-011TiO,. 0-295K.,0 (c) {oxo} 0-086A1,0, Mee 0-055Ca0 28" Many attempts have been made to produce hard-paste porcelain at lower tem- peratures than that required for the best qualities of this material. Thus, Pukall gives the following typical composition for porcelains maturing at Cone 7 (1230° C.) :— 0-5K,0 050a0 pAlsO 5SI0,. Dorfner prefers a material containing more alumina and silica and corresponding to 0-65K,01 0-35CKO JU 7AlaOs 11-688i0, and Hertwig-Méhrenbach * has suggested the following limits of composition for porcelains maturing at Cone 9 :— RO 3A1,0, 14-458i0, to RO 4-8A1,0, 278i0,. The ware corresponding to the lower limit is not very translucent, as most of the vitrified material is only formed at a much higher temperature, but that corresponding to the upper limit is highly translucent. The former, if fired at Cone 14, produces translucent ware. The materials used for the production of all these porcelains are china clay or kaolin, felspar (or some equivalent mineral such as china stone or pegmatite), and free silica in the form of crushing quartz or calcined flint. These materials must be 1 Trans. Amer. Cer. Soc., 17 (1915). 2 Sprechsaal, 53, 363-5 (1920). FLUXES IN PORCELAIN 377 very finely divided and extremely well mixed so as to ensure the steady and regular progress of the chemical reactions between the constituents and the uniform progress of the physical changes which take place when they are heated. Otherwise, distortion of the ware will occur. The materials must be sufficiently uniform in composition to enable successive batches to produce ware having the same characteristics, though some adjustment is possible by varying the proportions of each material in the mixture. The materials must be sufficiently free from iron and other discolouring impurities to enable white ware to be produced. Slight differences in the materials from various sources, as well as slight variations in the proportions in which they are used, largely account for the differences between specimens of porcelain and china ware from different factories. One of the remarkable features of the older Chinese porcelains is the manner in which very crude and impure materials have been used to produce a ware of extra- ordinary technical and artistic quality. The very serious complexities thus introduced add greatly to the esthetic value of such ware. The chief purpose of the clay and some of the free silica in all porcelain is to produce the felted mass of sillimanite crystals which form the “skeleton ”’ of the finished ware and to provide a “ reinforcement ” which will prevent undue distortion of the ware. The other ingredients have two purposes: (a) part of them commence to fuse at a comparatively early stage in the firing and so produce a solvent for the remainder, and (5) these ingredients, together with the clay and free silica, combine to form a glassy fluid—which must be produced slowly and progressively to avoid loss of shape in the ware—from which the sillimanite may crystallise, the remaining material forming the vitrified magna to which—with the sillimanite—porcelain owes its peculiar characteristics. The nature of the fluxes in porcelains, etc., is very important, as upon them the nature of the finished products largely depends. H. Hope? has given the following summary of the effect of various fluxes upon white porcelain mixtures :— Inme, or its equivalent, gives a strong porcelain with moderate shrinkage and only a slight tendency to blister, but the colour of the ware is rather poor. Magnesia tends to cause blisters, but if less than 0-1 equivalents are present, this is not serious ; the colour of the ware is usually very good. Strontia has the least tendency to blister and gives a strong ware with high porosity and low shrinkage ; the colour of the ware is fairly good. Barium oxide gives a weak body, with excessive shrinkage and blistering. The translucency of the ware is better than other fluxes, but the colour is poor. Zine oxide produces ware of a very good colour, even in small quantities, but with more than 0-05 equivalents there is sometimes a bluish or greenish tinge. With 20 per cent. of felspar, zinc oxide gives a good strong body, but with more than 0-02-0-03 equivalents of zinc oxide the ware tends to opacity and there is a tendency to “ shivering.” Soda and potash—preferably in the form of felspar—are the best fluxes for porcelain. 1 Trans, Amer. Cer. Soc., 11, 494 (1909). 378 CHEMICAL COMPONENTS OF PORCELAINS Table CXIII shows the chemical composition, and Table CXIV the mixtures used for various porcelains. Taste CXIII.—Chemical Composition of Porcelains Porcelain. | Silica.|/Alumina. sha Potash.| Soda. | Lime. | Magnesia, Tne Borax.} Loss. Oxide. Oxide. Sévres(1848)| 59-2 | 35-2 Se 374 te “3 As Dental! A. | 68:2 | 16-7 | trace 3g 0:23 ~. | 25 , B. | 68-1 2-2 e 0:8 trace |10°6 | 1-2 ee C .| 696] 11:3 0-3 2°4 0-2 0:3 Meissen .| 58:5 | 35-1 0:8 0:3 Vienna . | 59°6 | 34-2 0-8 ple Berlin . | 64:3 | 29-0 0-6 China ST 10-Ds ore, 1:3 Refractory | 61-6| 30-0 | 1-56 3°56 TaBLE CXIV.—Miztures Used for Various Porcelains Bone China. | Dental.! ie Porcelain. Sévres.| Berlin. |Chinese.|Seger. Chemical. : A.) Bal G:| A. eBay A.|B.| C Kaolin . a . 38 Vi 47 13-0 | 33-35] 26] 30} 4]..].. Ball olay. .0é.ir 24] oveseh de le Be ee Py “|b 80 te Felspar . A A 38 23 15 30-0 | 15-19] ..| .. | 81] 61] 12 10 2) 2) 58 Cornish stone. A ae ae oe zh eee OO MPO ne aalleereuhiere aA 3 ot aeeine Fimt ": 4 ; eS Se ae .. |10-14] ..|..| 15] 29| 60 “es Quartz . é ? 24 aie 38 UAB: ph ee A oo Pere Nero leeten hers 10 3 Bone ash ; : Me a Pee? fer [o2—A2 aa sites tee lee re Sodium carbonate . . as KA stealtecs 21 38 Borax . ; 1| 11 Calcium carbonate . 5; 1 Potassium ,, PAip ee Magnesic porcelains usually consist of magnesia, alumina, and some silica. The two latter being in the form of clay, though Heinecke’s magnesic porcelain consists of powdered alumina and magnesia bonded with dextrin, no silica or clay being used. Steatite porcelain consists of a mixture of natural magnesium silicate (steatite), 1 Formule on p. 376. COMPOSITION OF SEGER CONES 379 together with clay, felspar, and flint, though in some cases steatite is used alone in the form of a fine dust, which is moulded under great pressure. Fused steatite has also been used by the British Thompson-Houston Company. Seger cones are made of artificial mixtures similar to porcelains. Table CXV shows their chemical composition as originally proposed by Seger, but the estimated temperature which they indicate is derived from later determinations in 1908 by Simonis. TaBLeE CXV.—Composition of Seger Cones . Estimated Potash. | Soda. | Lime. sae Alumina. a an Silica. Rey. Ree i 6S 0-5 0-5 2 1 022 600 0-5 0-5 0-1 2:2 1 021 650 0-5 0-5 0-2 2°4 1 020 670 0:5 0-5 0-3 2-6 1 019 690 0-5 0:5 0-4 2°8 1 018 710 0-5 0-5 0-5 3 1 017 730 0:5 0-5 0-55 3 1 016 750 0-5 0:5 0-6 3:2 1 015 790 0-5 0-5 0-65 3°3 a 014 815 0-5 0:5 0-7 374 1 013 835 0-5 0-5 0-75 3°5 1 012 855 % 0-5 2 0-5 0-8 ou 3°6 1 O11 880 0-3 ag 0-7 es 0-3 0-2 3:50 | 0:45 | 010 900. 0-3 aut 0-7 a 0:3 0-2 3°55 | 0:50 09 920 0-3 eas 0-7 ie 0-3 0-2 3:60 | 0:40 08 940 0-3 sae 0-7 ts 0:3 0-2 3°65 | 0:35 O7 960 0-3 2 0-7 i 0-3 0-2 3°70 | 0-30 06 980 0-3 - 0-7 ie 0-3 0-2 3°75 | 0-25 05 1000 0-3 Si 0-7 Cit 0-3 0-2 3°80 | 0-20 04 1020 0-3 oe 0-7 ss 0:3 0-2 3°85 | 0-15 03 1040 0-3 ar 0-7 as 0-3 0-2 3°90 | 0-10 02 1060 0-3 AG 0:7 “e 0-3 0-2 3°95 | 0-05 Ol 1080 0:3 ae 0-7 a 0-3 0-2 4 # 1 1100 0-3 se 0-7 Pi 0-4 0-1 4 2 1120 0-3 7 0-7 ie 0-45 0-05 4 3 1140 0-3 0-7 0-5 4 4 1160 0-3 0:7 0-5 5 5 1180 0-3 0-7 0-6 6 6 1200 0:3 0:7 0-7 7 ix 1230 380 CHEMICAL COMPONENTS OF SEGER CONES TaBLe CXV.—Composition of Seger Cones (continued) ; Estimated, Potash. | Soda. | Lime. poy Alumina. ns om Silica. mee, re mages 0-3 AS 0-7 7 0:8 ae 8 _ 8 1250 0-3 ae 0-7 on 0-9 oe 9 a 9 1280 0-3 ae 0-7 oe 1-0 ba 10 ah 10 1300 0-3 ne 0-7 es 1-2 = 12 ae 1] 1320 0:3 ifs 0-7 ms 1-4 wee 14 A 12 1350 0:3 mS 0-7 bp 1:6 ae 16 es 13 1380 0-3 ie 0-7 oe 18 te 18 as 14 1410 0:3 ee 0-7 che 21 oh 21 at 15 1435 0-3 we 0-7 ‘3 2:4 ar 24 a 16 1460 0:3 a 0:7 wi 2°7 ae 27 a ue 1480 0:3 e 0-7 ot 3-1 a 31 i 18 1500 0:3 - 0-7 BS 3°5 on 3D ae 19 1520 0:3 a 0-7 7 3°9 Ms 39 ak 20 1530 0:3 = 0-7 bp 4-4 as 44 eo 21 0:3 Ae 0:7 ck 49 a 49 a}, 22 Not 0-3 oa 0-7 ahs 54 or 54 -5 Wade { manu- 0:3 if 0:7 is 6:0 rs 60 4 24 factured. 0-3 at 0:7 A 6:6 be 66 ac, 25 0-3 es 0-7 ) and R=>_k4+ 60 (where : >k). The re- fractory index expressed in terms of Seger cones is shown in Table CXVI. TaBLE CXVI.—Refractory Indices and Seger Cones Cone. Refractory Index. Cone. Refractory Index. 14 17-5 27 65 15 22-6 28 72 16 28:0 29 80 17 33°7 30 89 18 39-2 31 102 19 44-6 32 114 20 50:0 33 127 26 57-6 34 141 Bischof’s “ refractory coefficient ” is found by dividing the alumina: silica ratio by the fluxes : alumina ratio, 2.e. where a is the amount of oxygen in the alumina, 6 the amount of oxygen in the silica, and c that in the fluxes x3. Refract ficient =~ - efractory coefficient => += or 5. This coefficient does not take into account various physical factors which are of great importance in deciding the refractoriness and so is seldom correct, except when limited to highly refractory clays; it is not applicable to second-grade fireclays and clays used for building purposes. Seger has suggested a modification of Bischof’s formula, viz. : Q= (a+); where Q is the “ refractory coefficient,” and a and b bear the same significance as in 382 CHEMICAL COMPOSITION OF ENGOBES Bischof’s formula. The fluxes ¢ are not considered, as the formula applies only to very pure clays. The formule suggested by Bischof and Seger are not satisfactory as they can only be employed within very small ranges of composition ; the chart prepared by Ludwig (fig. 21) is surprisingly accurate within much wider limits. Ludwig based his chart on the assumption that clays are solid solutions of the minerals constituting the ‘impurities ” in the clay and that they must, therefore, reduce the melting-point of the clay in proportion to their molecular concentration. He determined the refrac- toriness of many clays of different composition and then calculated their formula, assuming the alumina to be unity, 7.e. cROAI,O,ySiO,. He then marked off the 0-5 7-0 1 Key Zi eZ: 30 35 40 45 50 55 6-0 Sz O2 Fic. 21.—Lupwia’s CHART. refractoriness of the clays on a graph having the molecular ratios of RO as ordinates and those of SiO, as abscissee. When the iso-refractory lines corresponding to the various Seger cones are plotted on the graph, they enable the refractoriness of any material of a composition lying on or between any of these lines to be ascertained with a fair degree of accuracy. This chart is very useful for clays containing up to 6 per cent. of RO bases, but for less pure clays it is useless, on account of the heterogeneous nature of the materials. THE CHEMICAL COMPOSITION OF ENGOBES AND GLAZES The subject of engobes and glazes cannot be fully dealt with in this volume, as it would require a volume to itself, so diverse are the manifold possible compositions. It is only possible, therefore, in the following pages to give a general idea of their chemical composition, their adaptation for special purposes, and some of the methods of avoiding defects. ~ Engobes or bodies are of extremely variable composition and it is impossible CHEMICAL COMPOSITION OF GLAZES 383 to give figures of general application, as each engobe must be modified so as to adhere well to the body upon which it is to be placed. Engobes usually consist of a mixture of white-burning clay and flint, together with at least two fluxes, the proportions of these ingredients being modified so as to secure a covering which has a contraction identical with that of the ware to which it is applied. In some cases, lime is used as the only flux, but two fluxes are generally desirable, as they produce a more stable “glass ’’ which binds the particles of clay more firmly together than when only one flux is present. Hence, many engobes contain a considerable proportion of felspar or Cornish stone, which incidentally provides potash—an excellent flux. Two typical engobes with one flux correspond to the formule : (i) CaO 0-5A1,0; 48i0, ; (ii) 0-15K,0 Al,0, 58i0,. A widely used engobe, with two fluxes maturing at Cone 4, corresponds to the formula : 0-7CaO 0.3K,0 POPALOs 48505, The chemical composition of engobes is of minor importance compared with that of glazes, as the former are essentially mixtures of clay and vitrifiable matter, and their most important characteristics are physical, viz. (1) a suitable appearance (usually a perfect white) when fired, and (ii) complete adhesion to the ware to which they are attached. Glazes may be Rrdedatuc far as their chemical composition is concerned —into (a) Alkaline glazes, consisting chiefly of silicates of potash and soda; the most important of these are salt glaze and some of the unstable glazes occasionally used for ornamental ware and enamels. (6) Felspathic glazes, consisting of silica and alumina, with alkaline or alkali- earthy bases. (c) Lead glazes, which may consist of (i) a simple lead silicate, or (ii) a glaze similar to felspathic glazes, but having their melting-point reduced by the addition of lead compounds. (d) Enamels, which are ordinary lead or ieeratie glazes rendered opaque by the addition of tin oxide or other opacifying agent. (Unfortunately, the term enamel is also used for transparent glazes applied to metals, but in the pottery industry its use is largely confined to opaque glazes.) In order that it may be satisfactory, a glaze must possess three characteristics : (a) it must be of the right character as regards transparency or opacity and colour ; (b) it must be perfectly adapted to the body or ware to which it is applied so as to avoid crazing or peeling ; (c) it must possess a suitable fusibility in order that it may mature at a convenient temperature and one suitable to the ware to which the glaze is applied. These characteristics are secured by suitably modifying the chemical composition 384 CHEMICAL COMPOSITION OF GLAZES of the glaze-mixture. All these characteristics are equally important and yet, in some cases, they are incompatible, so that various compromises must be effected. An increase in transparency is usually obtained by making the glaze more fusible or by reducing the proportion of non-fusible matter in it. The adhesion of a glaze to the ware may sometimes be increased by adding clay (though this may make it less transparent), by making it more fusible (though this may cause “ crazing ’’), or by varying the proportions of the various minerals in such ways as previous experience may suggest. The effect of composition upon the fusing-point of glazes is a subject of very great complexity and whilst an increase in the proportion of bases will usually make a glaze more fusible, and an increase in the proportion of flint or clay will usually make the glaze less fusible, no general rule is equally accurate in all cases, as so much depends on the mutual reactions of the different materials. Thus, if a glaze is of such a composition that all the bases are suitably combined with silica and alumina, the addition of an acid material, such as flint, would make the glaze less fusible, roughly, in proportion to the amount added. If, however, the glaze contained some uncombined base, such as whiting, on account of the lack of acidic matter to combine with it, the addition of such a material as flint would enable further combination to occur and the glaze would be made more fusible, thereby appearing to contradict the general rule that the addition of a base increases the fusibility of a glaze. There can be no definite relationship between the fusibility and composition of a glaze, because the fusing-point of a glaze depends upon— (a) The molecular ratio of fluxing bases (RO) to silica. (b) The nature of the fluxes. (c) The molecular ratio of the alumina to the fluxes and to the silica. (d) The ratio of the silica to boric oxide if present. The only certain method is first to study the molecular proportions of the various RO bases and silica in a glaze (the Al,O, being taken as unity) and then, having obtained a good general idea by this means, to carry out a series of experiments. It frequently happens that unexpected eutectic compounds are produced and these upset any predictions from the chemical formule. Various investigators have suggested that the behaviour of glazes is dependent to a large extent on the ratio between the oxygen in the bases and that in the acid (chiefly silica) portions of the glazes. Thus, Seger pointed out that crazing may some- oxygen in silica times be prevented by increasing the value of the ratio but this does oxygen in bases’ not adequately allow for the effect of alumina. Hopkins * has pointed out the importance of considering the atomic volumes of the bases present in glazes and suggests that crazing is most likely to occur when the ratio atomic volume of oxygen in the basic oxide atomic volume of basic element in the oxide 1 Trans. Eng. Cer. Soc., 9, 120 (1909-10). CONSTITUENTS OF ENGOBES AND GLAZES _ 385 is low. This figure he terms the oxygen strain. The atomic volumes of the principal elements considered by Hopkins are as follows :— Zn : . atomic volume, 9:2 Ba : . atomic volume, 36-7 Na : , 4 ie cor K : : zt, » 47:0 Ca : 2 ier ek O ’ 14:3 9 > J. W. Mellor has pointed out that Hopkin’s theory does not apply to glazes containing copper, which has an atomic volume of 7-1, but is not superior to zinc as an anti-craze. This theory also does not agree with the work of Damour and Hovestadt, who give, respectively, the following orders :— ZnO, PbO, CaO, CuO, BaO, Na,O ZnO, PbO, BaO, CaO, K,0, Na,O for the influence of bases on the coefficient of expansion of glass. INFLUENCE OF CONSTITUENTS ON ENGOBES AND GLAZES The following information on the influence of various constituents of engobes and glazes is sometimes useful in determining the probable effect of altering the com- position of a given engobe or glaze. Silica is used in glazes to supply the chief acid radicle and also to regulate the temperature at which a glaze will mature. An excess of silica, which remains un- altered in the glaze, decreases its transparency and prevents complete fusion except at a higher temperature. On the other hand, a deficiency of silica reduces the ductility of a glaze and imparts a tendency for it to boil and blister. Purdy + has found that a high proportion of flint causes “ shivering,” the effect being greatest as the temperature rises from Cones 5-11. Ware containing 30-50 per cent. of flint, when fired at a temperature above Cone 5, will not shiver or craze from this cause. A low proportion of flint and a high proportion of clay will cause crazing irrespective of the proportion of felspar present. Alumina often acts as a base in glazes, but it does not influence the fusing-point to any great extent. Its chief value is to permit considerable variations in the composition and firing of the glaze without greatly altering its physical properties ; it is particularly useful in preventing crystallisation. It also acts as a clarifier and is especially necessary in lime glazes, where it is required in considerable proportion to prevent turbidity. In lead glazes, it has the opposite effect and causes turbidity, so that the minimum amount possible should then be allowed. Alumina tends to reduce crazing in fritted glazes and acts similarly to silica and boric acid together, though more powerful than either of these two when used alone. Alumina cannot, according to Purdy and Fox,? wholly prevent crazing, unless the ratio of bases to acid is at least 1 : 2-5 and, according to these observers, when less than 0-25 equivalents of alumina are present, crazing occurs quite apart from the oxygen ratio. W. Scheffler has stated that more than 0-4 equivalents of alumina are undesirable 1 Trans. Amer. Cer. Soc., 13, 157 (1911). 2 Ibid., 9, 95 (1907). 25 386 CHEMICAL COMPONENTS OF GLAZES in ordinary glazes and that an excess tends to increase the viscosity of the glaze and prevents the elimination of air bubbles. According to Purdy and Fox, alumina stiffens fritted glazes and raises the fusing temperature when 0-25-0-3 equivalents of alumina are present. With a smaller proportion, the opposite effect is noticed. Keeler 1 has observed the following effects of varying the ratio of alumina : silica in terra-cotta glazes :— Alumina : Silica Ratio. Product. High alumina, low silica . Immature glazes in most cases. If they do fuse, they tend to flaw and craze. Low alumina, low silica. . . Crazing, pinholes, immaturity. High alumina, high silica . Beading, immaturity, waviness, but no crazing. Low alumina, high silica . Fair bright glazes, with tendency to waviness. Clay is used in glazes to supply alumina and to give the requisite adhesion to the ware. On account of its alumina content it is often effective in preventing crazing. Its chief value is that, being composed of very small particles, it combines more rapidly with some of the bases. An excess must be avoided, as it makes glazes opaque. Fluxes are employed in glazes to produce a material having the required fusing- point and transparency. The choice of the fluxes is very important ; two or more being preferable to one flux, as explained on p. 383. In most leadless glazes, the fluxes are soda and potash, with the addition, in some cases, of magnesia, baryta, or zinc oxide. Seger gives the following as the order of strength of various fluxes in glazes :— TasBLeE CXVII.—Fluxing Power of Glaze Fluxes Fluxing Power of Colour- ing Oxides (Order of Fluxing Power of Colour- ing Oxides (Order of Fluxes in Glazes (Order of Activity). Fluxes in Glazes (Order of Activity). Activity). Activity). Lead. Manganese oxide. Zinc. Chromium oxide. Baryta. Cobalt oxide. Lime. Nickel oxide. Soda. Tron oxide. Magnesia. . Potash. Uranium oxide. Alumina. It is desirable to have at least two bases in a glaze, and preferably three. Lead glazes are the only exceptions to this rule, for, owing to the peculiar behaviour of lead compounds with silica one flux is sufficient, as it is easily possible to make glazes 1 Trans. Amer. Cer. Soc., 18, 282 (1916). CONSTITUENTS OF ENGOBES AND GLAZES — 387 of a mixture of lead oxide and silica without any other additions. Yet even in lead glazes it is often convenient to use a second base or flux. The fact that several fluxes produce a more fusible mixture than an equivalent proportion of one flux is due to the well-known law that one gram-molecule of a substance dissolved in any solvent causes a constant depression of the fusion-point, so that if several bases are present, each acts independently and the fusion-point is depressed to a correspondingly greater extent than that caused by an equivalent proportion of only one flux. Soluble bases should, as far as possible, be avoided, as they necessitate the fusion of the base with silica, known as “ fritting,” which is both troublesome and costly, in order to produce an insoluble material. Soluble salts do not remain distributed uniformly through a glaze or engobe, but rise to the surface by capillary attraction and so are unable to react properly with the ingredients below the surface. Potash is usually added in the form of felspar or Cornish stone, but potassium nitrate (nitre) is sometimes used. Felspar adds the desired amount of flux in a more concentrated form than does Cornish stone, but if in excess it tends to cause peeling and may also cause crazing if the other constituents are not in the right proportions. Both felspar and Cornish stone also add alumina and silica to a glaze, and this must, if necessary, be corrected by reducing the proportion of clay or flint used. Purdy ! has found that a high proportion of felspar causes crazing in ware fired at Cones 1-3, but not in that fired at or above Cone 5. This is contradictory to Seger’s observation. Purdy considers that with a clay : flint ratio of 3: 3 to 3: 7, felspar will not cause crazing unless more than 70 per cent. of felspar is present. I{ more than 35 per cent. of clay is present and the felspar is increased greatly, crazing may occur. As many pottery clays contain more than 35 per cent. clay, a high proportion of felspar generally causes crazing. Where difficulties arise, due to the use of felspar, Cornish stone or a mixture of this stone and felspar may sometimes be satisfactorily substituted with advantage. Soda is added in the form of a soda felspar or as a soluble salt, such as sodium carbonate, in which case fritting with some of the other constituents of the glaze is necessary. The action of soda is almost identical with potash and the two are often replaceable. Sometimes, soda produces a slightly more “ fusible” glaze and potash a rather more glossy one. The substitution of soda for lime, or the addition of soda to a glaze sufficiently rich in RO base, directly increases the tendency to crazing ; it also increases the gloss and the fusibility of the earthenware glazes, but reduces their durability. Glazes high in alkalies are generally suitable for bodies rich in quartz, whilst glazes low in alkalies are more suitable for bodies rich in alumina. Lime is usually added as whiting or other form of a calcium carbonate, though calcium sulphate (plaster of Paris) is sometimes used. The addition of any lime compound increases the glossiness and fusibility of earthenware glazes and slightly reduces the tendency to crazing, but in terra-cotta glazes it tends to increase the amount of crazing; magnesia and barium oxide were even more deleterious in this 1 Trans. Amer. Cer. Soc., 13, 157 (1911). 388 CHEMICAL COMPONENTS OF GLAZES respect. Lime glazes usually require 0-5 equivalents of boric oxide to give the required fusibility to enable them to be used without other bases, but more than one equivalent tends to cause turbidity. Mixtures of soda (or potash) and lime in glazes are very important and give better results than when either of these substances is used separately. The usual permissible range of composition of these two substances (expressed as oxides) is 0-6K,07 ,, s0-2K,0. 0-4CaO f 0-8CaO. Soda may be partially or completely substituted for potash in the above proportions. Lime glazes usually require at least 4 equivalents of silica and at least 0-5 equivalents of alumina to give a satisfactory gloss. Lead compounds are used in plumbiferous glazes and do not require any other flux, though others are often used to ensure greater stability. Insoluble lead compounds produce glazes which can be used at low temperatures without requiring to be fritted. They give great mobility and a high refractivity to glazes, and consequently impart a great lustre and brilliance unobtainable by any other element, although barium compounds sometimes give similar results in leadless glazes containing soda and boric acid. Small variations in the composition of leadless glazes are less important, and less likely to cause trouble than when no lead is present. The chief disadvantage of lead compounds, and the reason for their restricted use, is the poisonous nature of soluble lead compounds. If lead compounds are fritted with silica, prior to use, their solubility is so greatly reduced as to render them prac- tically harmless. The limit of solubility generally accepted is that suggested by T. H. Thorpe in evidence before the Lead Commission, viz. that in lead glazes the sum of the bases (including the alumina) in molecular parts, when calculated as lead monoxide, multiplied by 223 and divided by 60 times the sum of the acids, calculated as silica, should give a quotient which is less than 2, that is, Sum of bases (including alumina) x 223 2 Sum of acids x 60. wi J. W. Mellor has pointed out that the multiplicands are unnecessary and that the same result is obtained more simply by requiring that the molecular ratio : Sum of bases (including alumina) Sum of acids should be less than 0-5. An alternative limitation, also suggested by Thorpe, is that not more than 1 per cent. of lead oxide should be capable of removal from a glaze by solution in dilute hydrochloric acid. When fritting glaze materials containing lead, care should be taken not to frit any alkali or borax along with the lead compounds, or soluble substances may be formed which may cause poisoning. This objection may be overcome by using two kinds of frit—one containing all the lead and the other all the borax and soluble alkali compounds. BORIC COMPOUNDS IN GLAZES 389 Borve acid and boric oxide are used in some glazes in place of part of the silica to lower the fusing-point without materially altering the constitution of the glaze. Its action is not thoroughly understood, some investigators regarding it as behaving like an acid, whilst others claim that it is basic in glazes. Thus, Seger, dealing with glazes in use in Germany, found that boric acid acts as an acid like silica. He states that it decreases the coefficient of expansion of glazes and decreases crazing, but Purdy and Fox,} and also Burt,? found that in glazes of the type used in the United States, boric acid increased the coefficient of expansion and also the crazing. Purdy and Fox have stated that boric acid and alumina both counteract devitrification, the best results being obtained with at least 0-13B,0, to 1810,, irrespective of the oxygen ratio and alumina content, though the alumina content should not be less than 0-2 equivalents. C. F. Binns ? considers that boric oxide acts as a base in glazes, as he has found that glazes containing boron sesquioxide do not obey the bisilicate law if boron be regarded as acid, but do if the boron is regarded as basic. He considers that boric oxide should be classed with alumina in the molecular formula and that (apart from its effect on the fusibility) it may be used to replace alumina. This is confirmed by Singer,* who has also found that boric oxide and alumina may replace each other without altering any property of the glaze except the melting-point. He also found that either may be substituted by cobalt oxide in producing cobalt blue glazes, and that both change the colour of copper glazes in the same manner, though the intensity of boric oxide is nearly twice as great as that of alumina. Singer also found that boric oxide and other sesquioxides such as manganese, iron, cobalt, and vanadium can replace the alumina in zeolites, the series being amorphous. The dual behaviour of boric oxide—sometimes as an acid and sometimes as a base—appears to be closely related to the general composition of the glaze. Thus, Stull and Radcliffe * found that when the base oxygen : acid oxygen ratio is less than 1:2, boric oxide behaves as an acid, but if this ratio is greater the boric oxide behaves as a base, whilst if the ratio is exactly 1 : 2, the boric oxide behaves as a free acid. When boric oxide behaves as an acid, it increases the tendency to crazing and when it behaves as a base it decreases the crazing. Colouring oxides are added to glazes in order to produce certain desired colours. Such oxides may usually be included in the RO portion of a glaze formula and gener- ally act as bases, though some sesquioxides, such as those of manganese, iron, cobalt, chromium, and vanadium are more appropriately included in the Al,O; portion of a glaze formula. THE ADJUSTMENT OF THE COMPOSITION OF GLAZES AND ENGOBES It is impossible in this volume to deal exhaustively with the adjustment of glazes and engobes to enable them to fit any given ware or to overcome certain defects ; all 1 Loc. cit. p. 385. 2 Trans. Amer. Cer. Soc., 7, 131 (1905). 3 Tbid., 10, 158 (1908). 4 Jbid., 12, 676 (1910). 5 Ibid., 12, 127 (1910). 390 CHEMICAL COMPONENTS OF GLAZES that can be done here is to indicate some of the general principles involved. Further technical information on the subject will be found in the chapter on “ Defects ” in the author’s Clayworker’s Handbook (Griffin, London). The chief purposes in altering the composition of an engobe are : (i) To make it more fusible and thereby increase its adhesion and impermeability. (ii) To increase its porosity. (iii) To increase its shrinkage, so that it will better accommodate itself to the ware to which it is to be attached. (iv) To reduce its shrinkage for the same reason. (v) To alter its colour or to make it white. The fusibility of an engobe may be increased by reducing the proportion of silica or clay or by increasing the proportion of potash, soda, or lime according to the composition of the engobe (p. 383). The porosity of an engobe may be increased by partly replacing a highly plastic clay by a less plastic one, by increasing the pro- portion of silica or grog, by substituting a coarser form of silica or grog for a finer one, or by adding a finely powdered organic substance, such as naphthalene, which will burn away inthe kiln. The shrinkage of an engobe may be increased by adding more clay or by replacing a lean clay by a more plastic one ; it may be reduced by the reverse means or by adding silica, grog, or other non-plastic refractory material. The colour of an engobe is altered by the addition of an appropriate metallic oxide (see p. 115), and an engobe may be made white by the use of purer material free from colouring impurities or by the addition of cobalt oxide (p. 113). The chief purposes in altering the composition of a glaze are : (i) To increase the fusibility and, therefore, its glossiness and transparency. (ii) To increase the proportion of crystalline matter present and so produce a matte glaze. (ii) To increase the shrinkage of the glaze and so prevent it from off the ware. (iv) To reduce the shrinkage of the glaze and so prevent “ crazing ”’ or the forma- tion of hair-like cracks. (v) To alter the colour of the glaze or to increase its opacity. The fusibility of a glaze may be increased by (a) Decreasing the proportion of silica and increasing the fluxes (bases), but preserving, as far as possible, a composition between a bisilicate and a trisilicate. (b) Using a more powerful flux (base), but retaming the same molecular proportions. (c) Increasing the number of fluxes, but retaining the same molecular proportions of base to silica. (d) Decreasing the percentage of alumina and silica, but keeping the base : acid ratio within suitable limits. (e) Increasing the proportion of boric acid with or without a reduction of the silica. The proportion of crystalline matter in a glaze may be increased by replacing part of the present RO bases by zinc oxide, or, to a less extent, by reducing the 4 ‘ peeling ”’ TYPICAL GLAZES 391 proportion of alumina in the glaze. Simple silicates crystallise far more readily than alumino-silicates. The shrinkage of a glaze may be increased so as to prevent peeling and cracking by— (a) Lowering the proportion of silica and increasing the fluxes without reducing the ratio of base to silica to less than that in a bisilicate. (6) Increasing the proportion of silica at the expense of the boric acid (if present). (c) Substituting a flux or base of high molecular weight for one of lower molecular weight. (d) Adopting any of the methods used for increasing the fusibility of the glaze. The shrinkage of a glaze may be reduced so as to prevent crazing by— (a) Increasing the proportion of silica and decreasing the fluxes so as to form a bi- or trisilicate. (6) Increasing the proportion of boric acid and decreasing the silica. (c) Substituting a flux with a low molecular weight instead of one with a high molecular weight. The colour of a glaze may be altered by the addition of a suitable metallic oxide (see Colours, p. 115), or it may be made whiter and more opaque by the addition of arsenic oxide, tin oxide, or cryolite, and by substituting materials which burn white for those which are coloured when burned. TYPICAL GLAZES The following are typical formule for glazes suitable for various purposes; they are by no means exhaustive, as the number of different compositions which can be used for each purpose is enormous :— Salt glaze is produced by throwing damp common salt into the kiln when its contents are at a sufficiently high temperature. The salt is decomposed by the heat and moisture forming soda (Na,O) and hydrochloric acid (HCl). The former com- bines with the silica and alumina, of which the articles are made, forming complex silicates, the composition of which appears to vary considerably. According to Barringer, the limits of the alumina: silica ratio in the ware within which it is commercially possible to produce a good salt glaze are 1 : 4-6 to 1 : 12-5, and Mackler —who analysed pieces of glaze carefully chipped from the ware—found that salt glazes vary in composition from 1-2Na,0 Al,O, 3:3-7-58i10., the average being 1-5Na,0 Al,0, 5810.. Ii an article is made of a highly aluminous clay deficient in free silica, it is usually very difficult to produce a good salt glaze on it, but the assumption that salt glazes are silicates and not alumino-silicates cannot be correct, as bricks made of pure silica do not form a good salt glaze when treated in the ordinary manner. 392 CHEMICAL COMPONENTS OF GLAZES Terra-cotta glazes vary greatly in composition, as the clays used in the manu- facture of terra-cotta vary so greatly. A glaze which has long been used with great success corresponds to the formula : 0-1-0-7CaO 0-5-0-2K,0 }ossato, 3°38105. 0-4—0-1ZnO Stoneware consists of a vitrifiable body and can, therefore, be glazed at a higher temperature than most terra-cotta and faience ware. The composition of the glazes used naturally varies with the clays of which the articles are made. Thus, Seger has stated that the best stoneware glazes correspond to the formula : RO 0-1A1,0; 2-5810., whilst R. Purdy considers that a typical formula is : 0-3-0-15CaO 0-3-0-45K,0O }oaoato, 3-0810,, 0-4ZnO and a Bristol stoneware glaze largely used by the author corresponds to: 0-18K,0 0-38Ca0 oassato, 1-588i0,. 0-44Zn0 W. Scheffler has found that in transparent stoneware glazes for use under Cone 9, the silica should not exceed ten times the equivalent of the alumina or six times the actual weight of alumina and that at least 0-3 equivalents of potash should always be present, whatever the remaining 0-7 equivalents of bases may be. Sanitary ware is largely made of fireclay covered with a felspathic engobe and a leadless glaze, which are fired at a temperature corresponding to Seger Cones 8 or 9 (1250° or 1280°C.). According to Parmelee and Williams,! the following requirements are important for fritted leadless glazes applied to sanitary ware :— (a) The silica should be present to the extent of at least five equivalents. (b) The alumina should be 0-5-0-6 equivalents in the absence of boric acid, but may sometimes be less if 0-5 equivalents of boric acid is present. (c) The presence of 0-5 equivalents of boric acid is advantageous. They found that a high proportion of lime caused dulness, a high proportion of zinc oxide caused blistering in the glaze, and a high proportion of alkali caused opalescence and crazing. In the best glazes they examined, the composition of the RO part of the glaze formula was: 0-4-0-6K,0 0-0-3ZnO 0-4-0-6Ca0. An engobe and a glaze which may be regarded as the bases of most of those used 1 Trans. Amer. Cer. Soc., 18, 812 (1916). EARTHENWARE AND PORCELAIN GLAZES 893 in this country for sanitary ware—the variations being largely due to local circum- stances—correspond to the formule : Engobe . : : 1-0K,0 22Al1,0, 658104. 0-16K,0 Giewe {oicso bomato, 2-510». 0-20ZnO It will be noted that this glaze does not conform to the conditions prescribed by Parmelee and Williams, yet it is extensively and satisfactorily used in this country, _ whilst glazes containing boric acid are seldom applied to sanitary ware. Earthenware and Faience.—Seger gives the following relations between the RO, silica, and alumina in various glazes :— 1RO to 1-5S8i0, 1RO to 3S8i0,. 1RO 0-01A1,0, 2-5810, 1RO 0-4A1,0, 4:5810,. Common earthenware and fine French faience English and German white earthenware According to E. Berdel1 earthenware glazes free from lead and boric acid for temperatures between Cones 1 and 6 should have compositions within the following limits :-— K,0 : 34 2.0 0-0-15ZnO }0-14-0-18A1,0, 1-4-1-8S8i0,. 0-1-0-2Mg0O 0-5BaO A typical glaze for English earthenware (tableware) corresponds to— 0-29PbO 0-39CaO | 0-10K,0 021N2,0| 2-628i0, 0-37AL,0.q wank ? by As the terms “ earthenware ’”’ and “ faience”’ include many kinds of ware from those made of crude red-burning clay covered with a roughly-made glaze composed of galena and crushed quartz, flint, or fine sand, to some of the finest examples of the potter’s art, no single formula can possibly prescribe the limits of composition of glazes and engobes for such a variety of wares. Porcelains, like earthenware, vary greatly in composition and the glazes vary correspondingly. Seger has stated that the following are the maximum and minimum compositions for hard porcelain glazes :— Maximum . ; » RO1-25A1,0, 12810, Minimum . : . ROO0-5Al,0, 5810, but he recommended that such glazes should be based on the composition of some 1 Ker. Rund., 25, 88 (1917). 394 CHEMICAL COMPONENTS OF GLAZES Seger cones, and Mellor! has suggested that the compositions shown below (which approximate to Cones 4-11) may be used as glazes for felspathic porcelain. RO 0-4A1,0, 3-5810, Used as a glaze at Cone 7 Cones Nos. 4-6 {Ro OD er 0, fe fh _ 9 ROO6 ,, 55 ,, 4 ig bette RO 07 Ao , . eee » 78 oie roe eae 9-1{ Ro Oi aes . ie ee 8: zs BO 12 elie 3 a iv Le Yaichiro Kitamura ? gives the following formule for porcelain glazes to be fired at temperatures between 1380° C. and 1500° C. :— 1-0RO(0-5KNaO 0-5Ca0) 0-9A1,0, 8Si0, 1-0RO(0-1KNaO 0-9Ca0) 0-7A1,0, 5S8i0, yeaah dee 0-5KNaO 0-5Ca0) 1-05A1,0, 108i0, 1-0RO(0-1KNaO 0-9CaO) 0-95A1,0, 9-580, Noor {4 -ORO(0-5KNaO 0-5CaO) 1-141,0, 11-5810, i: ae -1KNaO 0-90a0) 1-1A1,0, 14-58i0, Sortwell * has stated that the best porcelain glazes have an alumina-content of (0-3++75 silica), the silica varying from 3-13 equivalents, the permissible variation being greatest at the higher temperature. The proportion“of alumina he recommends differs considerably from that used by European manufacturers of porcelain. Table CXVIII shows the percentage composition of various well-known porcelain glazes. At Cone u¢ ee ee ae Taste CXVITI.—Chenuical Composition of Glazes for Porcelain Ferric |, . Mag- Loss on An : g Silica.| Alumina. Oxide. Lime. La Potash.|Soda. Traian Formula. Authority. Pegmatite used at Sévres .|70-64| 16:87 | 0-73 | 1:31|0:20] 4:22 | 4-97] 0:34 |RO1-07R,Og 7-45Si0, | M. Vogt. Pegmatite used at Limoges . | 76-11] 14-61 | 0-66 | 1-44/0-42| 2-99 | 3-03] 1-23 |ROR,Og 8-91Si0, Seger Pegmatite used at Limoges . | 75:99} 14-80 | 0-37 | 1:09] 0:36 | 4:31 | 3-49] 0-65 | RO1:24R,0., 10-84Si0, Berlin glaze . | 73-24| 13:97 | 0-31 | 2-57) 0-51 | 4:81 |1:71| 3-83 |RO1-12R,0, 9-58Si0, Japanese glaze* | 61:97} 12:92 | 0:39 | 9-57| tr. | 4:17 | 1:12] 10-21 | RO0-55R,0, 4-42Si0, Japanese glaze} | 64-96] 12-74 | 0-80 | &78| tr. | 1:95 | 2:30] 9-35 |RO0-59R,Og 5-04Si0, Chinese _sea- green glaze { | 64:80) 14-33 | 1-39 |10-:09| 1-55 | 5-61 |0-81| 1:39 |RO0-52R,0s3 3-82Si0, ” ” vy ” * +0-30P,0, + +0-16P,0, included in Loss on Ignition. tf + Titanic acid 1 Trans. Eng. Cer. Soc., 14, 176 (1914-15). 2 J. Chem. Ind. (Japan), 24, 89-105 (1921). 3 J. Amer. Cer. Soc., 4, 718 (1921). COLOURED ENGOBES AND GLAZES 395 Coloured engobes and glazes are produced by (a) the suspension of a colouring oxide in the engobe or glaze, or (b) the formation of coloured compounds when the engobe or glaze is fired. Thus, copper oxide and silica in the presence of an alkali form a bright blue glaze, whilst the substitution of alumina or boric oxide for some of the silica produces a green colour. Cobalt-blue glazes are due to the production of cobalt zeolites, according to Singer ;! the replacement of half the cobalt by boric oxide or alumina intensifies the colour, boric oxide being most effective. Coloured glazes and engobes are chiefly produced by adding a suitable proportion of one or more of the following oxides to the glaze prior to applying it to the ware (see p. 115). It is sometimes preferable to use specially prepared materials instead of the simple oxides ; for instance, what are known as chrome-tin pinks are prepared by calcining a mixture of a chromium compound, tin oxide, and a calcium compound so as to produce a colouring agent, which is added in suitable proportion to a clear glaze. Potassium bichromate is the most suitable chromium salt, a typical stain suggested by Seger consisting of— Stannic oxide. ; . 50 parts Calcium carbonate ee Flint ; : s Vr bia Borax ; ; , ee: Oe Potassium bichromate ey Me Prepared cobalt is made by heating hydrated black cobalt oxide to a white heat, after which it is cooled and finely ground. It should contain about 80 per cent. of cobalt and 20 per cent. of oxygen, and should be free from glassy or fused material, as this is an indication of the presence of impurities. The compositions of various other prepared colours also sold by firms dealing in potters’ materials are regarded as trade secrets. Opalescent glazes are produced by (a) using an excess of silica and a high oxygen ratio (at least 1:4). Purdy and Fox? found the best results in this case are obtained with 0-20 equivalent of alumina, an oxygen ratio of 4-5 and a SiO, : B,O; ratio of 1: 0-25. Whitmore,? however, prefers an oxygen ration of 1: 5-5-6-5. He also suggests a B,O, : SiO, ratio of not more than 1 : 3 or 1: 4, 0-4-0-5 equivalents of lime and a firing temperature between Cones 1-5. Purdy has recommended an opalescent glaze corresponding to— (a) 0-126Na,0 0-124K,0 2-61828i0, 0-50Ca0 0-2A1,0s19.65468,0, 0-250PbO (b) The precipitation of a silicate of boric oxide as suggested by Stull and Radcliffe.‘ (c) Adding an opacifying agent which remains in suspension in the glaze, the most popular of these being tin oxide, arsenic oxide, magnesia, zinc oxide, and zirconia 1 Loc. cit., p. 389. 2 Loc. cit., p. 385. 3 Trans. Amer. Cer. Soc., 11, 262 (1909). 4 Ibid., 12, 129 (1910). 396 CHEMICAL COMPONENTS OF GLAZES The proportion to be added depends largely on the nature of the glaze and the tem- perature at which it is fired. Five per cent. of tin oxide is usually ample. Matte glazes are produced by (a) the precipitation or crystallisation of compounds from the molten glaze, e.g. calcium aluminate, calcium-aluminium silicates, or zinc silicates, the last being the easiest to produce (see Crystalline Glazes, p. 397); (b) the glaze being imperfectly fused during the firing process; (c) the formation of a crinkled skin due to the bulk of the glaze contracting more than the surface ; (d) the elevation of the skin of the glaze by innumerable vesicles of gas; or (e) the addition of vanadium oxide, zinc oxide, or other crystallising agent to a glaze containing at least 0-15 equivalent of alumina. C. F. Binns! considers that matte glazes fired at a tempera- ture corresponding to Cone 01 are best produced by using a mixture corresponding to— RO 0-35A1,0, 1-68i0,. He suggests that the best composition for the RO bases is 0-09K,0, 0:20Ca0, 0-575PbO, 0-135Zn0O, but the following may also be used :— (a). (6). (c). 0-225 K,O 0-135 K,O 0-200 CaO 0-575 PbO 0-170 CaO 0-575 PbO 0-200 ZnO 0-575 PbO 0-225 ZnO 0-120 ZnO For higher temperatures (e.g. Cone 9), he suggests a mixture corresponding to— 0-3K,0 ee }0.64.1,0, 2-48i0>. According to E. Orton ? matte glazes of type (a) may be produced at Cones 2-11 within the following range of composition :— 0-5 —0-1 PbO 0-5 -0-1 ZnO 0-1 -0-2 K,O $0-2-0-4A1,0, 1-2-2-48i0,. 0-45-0-35CaO 0-40-0:30BaO When these glazes are overheated they produce bright, glossy glazes. Orton found that an excess of lead or of alkalies above the proportion mentioned produces a glassy structure, as also does an increase of silica, especially at high temperatures. According to Orton, variations in the proportion of alumina do not appear to affect 1 Trans. Amer. Cer. Soc., 5, 50 (1903) ; 7, 115 (1905). 2 Tboid., 10, 547 (1908). CRYSTALLINE GLAZES 397 the mattness, but Sortwell+ has found that aluminous matte glazes, maturing at Cones 12-16 and having an alumina: silica ratio of 1 : 4 for 3-6 equivalents of silica rising to 1 : 6 for 10 equivalents of silica, do not clear at higher temperatures, but are true matte glazes. Crystalline glazes are most. satisfactorily produced from glazes which are free from alumina, as this substance tends to prevent crystallisation, but if only 0-15 equivalent of alumina is present it may not interfere. Lime or zinc oxide appears to be essential for the production of good crystals. The chief oxides which give good crystals and may be regarded as crystallising agents are: zinc oxide,? titanium oxide, tungstic oxide, molybdic oxide, vanadic oxide, and bismuth oxide. Oxides of manganese, uranium, cobalt, iron, copper, and nickel may also be used for coloured glazes. According to Koerner,* bismuth oxide and uranium oxide produce crystalline glazes at as low a temperature as Cone 010-09, and tungstic oxide produces beautiful fernlike and star-shaped formations, showing partly iridescent reflexes quite unlike the effects obtained with zinc and titanium oxides. Purdy and Krehbiel 4 found that manganese dioxide had the greatest tendency to cause crystallisation ; zinc oxide is next, but with it the crystals tend to segregate in local areas, leaving others devoid of crystals. If a fusible silicate (glaze) is coloured with a coloured silicate (as by adding cobalt) and is then saturated with zinc silicate whilst in the molten state, the zinc silicate will crystallise out as willemite and will be coloured by the coloured silicate, the background remaining colourless if there is no excess of colouring agent. Titanic oxide produces very small crystals uniformly distributed through the glaze. Although glazes with a 1:4 oxygen ratio are commonly used in the production of crystalline glazes, Purdy and Krehbiel consider an oxygen ratio of 1 : 2-8 to produce better and more uniform crystalline glazes. The lower oxygen ratio is confirmed by the researches of Schott on Jena glass. Pukall * considers that the best crystalline glazes are obtained when copper and manganese oxides or mixtures of both of them are used, the best firing temperatures being at Cones 4-7. He obtained irregular results when using vanadic, molybdic and tungstic acids. The composition of the “ foundation glaze ” from which crystalline glazes may be prepared by suitable additions is very important ; the following are typical formule for this kind of glaze :— 0-2CaO To mature at Cone 09-2, 0-1Na,0 2:1810, }oonao.f 0:-7PbO 0-2B,03. 1 Loe. cit., p. 394. 2 Zinc oxide differs from some other crystallising agents in producing silicate crystals instead of oxides. 3 Trans. Amer. Cer. Soc., 10, 61 (1908). 4 [bid., 9, 319 (1907). 5 [bid., 10, 185 (1908). 398 CHEMICAL COMPONENTS OF GLAZES To this is added 18-28 per cent. of a crystallising oxide (p. 397). To mature at Cone 08-4, 0:25Na,0 2-0Si0, 0-40CaO 0-5B,03. To this is added 2-25 per cent. of rutile or 0-25-2 per cent. of oxides of chromium, cobalt, or copper, or 5-10 per cent. of the oxides of iron, manganese, uranium, or nickel. Purdy and Krehbiel + have found that the presence of soda is more conducive to the development of crystals than potash, the crystals in soda glazes being larger and grouped most pleasantly. They suggest that the best proportions of zinc to alkali are : 0-3ZnO to £0-6ZnO 07K Na0 0-4KNaO. Aventurine Glazes.—The presence of finely-divided metallic particles in a glaze produces a spangled effect, resembling the mineral aventurine. Mackler * has used a glaze corresponding to— 0-25K,0 > 2-258i0, oaaso 0-50CaO J 0-75B,0, together with 20 per cent. of finely-divided metallic iron, and H. G. Schurecht ® has found that red to black aventurine glazes may be produced with 0:41-0:81 equivalents of iron oxide in a glaze of the following composition :— 0-4Na,0 (0-05A1,0, {0208.0 }oesasio, 0-6PbO \0-41-0-81Fe,0, Glazes with 0-41-0-73Fe,0, give a red colour under oxidising conditions and a black one under reducing conditions. The ratio of iron oxide to silica is important, glazes containing 2-48i10, requiring at least 0-41¥e,0,, and glazes containing 4-2S8i0, requiring at least 0-58Fe,0;. Increases in the amount of iron oxide increased (a) the size of the crystals, (b) the number of crystals, (c) the refractoriness of the glaze. Other metals, particularly copper and gold, may be used instead of iron. It is not necessary to employ the metals, as if the conditions of firing are suitable so that compounds are reduced to the metallic state, an aventurine effect will be produced. If the particles of metal are too small they will dissolve in the glaze and will merely colour it, producing a “ stained glaze.” THE CHEMICAL COMPOSITION OF OTHER CERAMIC MATERIALS Silica bricks should usually be made from rocks containing at least 97 per cent. of silica, as the best silica bricks have a chemical composition within the following limits :— 1 Loc. cit., p. 397. 2 Tonind. Zeit., 207 (1890). 3 J. Amer. Cer. Soc., 3, 971 (1920). COMPOSITION OF SILICA BRICKS 399 Silica. ; . 95-98 per cent. Alumina ' ; : : . 0-5-2:3 ,, Tron oxide. : : ; ro gprs! bat hanes Lime . : : : i . 02-24 ,, Soda and potash . ; ; . O2-15 ,, If the impurity in silica rocks is in the form of clay, as in a ganister, a silica per- centage of only 90 may not be harmful, but where the impurities are fluxing oxides, such a large proportion would be very detrimental to the refractoriness of the goods made from it. There is no serious difficulty in obtaining silica rocks which contain less than 4 per cent. of clayey matter and 1 per cent. of fluxes ; some silica rocks contain as much as 99 per cent. of silica and less than 1 per cent. of impurities. The chemical composition is not, in itself, a reliable guide, as some of the purest quartzites are often unsuitable for physical reasons, as shown on p. 14. At the same time, it is desirable to use a material which is as pure as possible, consistent with the requisite physical structure. The relative values of wholly crystalline and bonded quartzites have been considered on p. 18. A good quartzite should contain about 97-5 per cent. of silica, 1-5 per cent. of alumina, and not more than 0-5 per cent. of iron oxide. The proportion of alumina and iron oxide together should not exceed 2 per cent. The proportion of alkalies should not exceed 2 per cent., nor the total fluxes more than 3 per cent. Alumina in the form of clay may be present up to about 2 per cent., but the total lime and alumina should be not more than about 4 per cent. The alumina in quartzites is generally combined in the form of felspars, micas, or alumino-silicates such as kaolinite. The presence of a lower percentage of combined water, together with a high percentage of alumina and potash, indicates the presence of potash felspar, whilst larger proportions of water and potash indicate mica, and low potash and high alumina and water indicate kaolinite or clay. Potash occurs in silica rocks in the form of felspar or mica, an excess of water indicating the presence of the latter compound. The lime in silica rocks may be combined either as calcite, dolomite, soda-lime, felspar, or as calcium phosphate. Lime is also added artificially to give the necessary bonding strength to the bricks. Asa rule, about 2 per cent. is used. Other basic fluxes used as bonds are (a) Portland cement; (b) soda compounds, including water-glass; (c) ammonium alum; (d) calcium chloride; (e) magnesia ; (f) felspar or Cornish stone; (g) complex silicates; (h) barium sulphate; and (2) calcium sulphate. Carbon dioxide is sometimes present in silica rocks as a result of the presence of lime or magnesia or both, in the form of carbonates or in the form of chalybite, the latter being very important. Combined water in silica rocks may indicate the presence of several minerals, including mica, clay, and limonite. Usually it varies from 0-1—1-0 per cent., though in exceptional cases it may he as high as 3-5 per cent. As a general rule, not more than 5 per cent. of metallic oxides other than alumina should be present in silica rocks, which are to be bonded with lime, or they will be too impure to be used as first-class refractory materials. 400 CHEMICAL COMPONENTS OF REFRACTORIES The ganister employed in the manufacture of ganister bricks usually contains 87-96 per cent. of silica, 4-5 per cent. of alumina, 0-1-5 per cent. of ferric oxide, 0-25-0-75 per cent. of lime and magnesia, and 0-1 per cent. of alkalies. The best qualities of Yorkshire ganister contain about 95 per cent. of silica. The bricks termed in America “ quartzite”’ bricks are not the same as those so named in this country, but consist of natural mixtures of fireclay and silica containing 70-80 per cent. of silica ; they correspond more nearly to what, in this country, are known as “ semi-silica ”’ bricks. The Institution of Gas Engineers terms materials containing over 92 per cent. of silica, silica material, and those containing 80-92 per cent. of silica, siliceous materval. The American Gas Institute specifies that silica bricks and blocks should be made from material containing over 94 per cent. of silica, not more than 1-7 per cent. of iron oxide, and between 1-7 and 3 per cent. of lime. Further analyses of siliceous rocks will be found in the author’s Refractory Materials : Their Manufacture and Uses (Griffin), and Sands and Crushed Rocks (Frowde, Hodder & Stoughton). Refractory silica sands to be used for lining furnaces and for furnace hearths should contain 95-99 per cent. of silica when used in steel furnaces, whilst for lower temperatures, such as are attained in copper furnaces, a lower percentage of silica may be permitted. In America, copper converters have been used satisfactorily when lined with a mixture containing only 60-70 per cent. of silica. For the highest tem- peratures not more than 0-5 per cent. each of iron oxide, lime, magnesia, and alkalies should be ‘present in the sand, whilst for lower temperatures rather less pure sands may be employed. For further information see the author’s Sands and Crushed Rocks (Frowde, Hodder & Stoughton, London). Silica glass (also known as fused quartz, fused silica, vitreosil, etc.) should ~ consist, as nearly as possible, of pure silica, the presence of impurities reducing the refractoriness and resistance to corrosion of the articles and spoiling their appearance. For transparent silica ware, not more than 0-3 per cent. of impurities is allowable, and opaque silica ware is made from sands containing at least 99 per cent. of silica and as little iron oxide as possible. See also the author’s Sands and Crushed Rocks (Frowde, Hodder & Stoughton, London). Spinels correspond to metallic aluminates (p. 356), the mineral spinel (MgO.A1,0,) showing on analysis 28 per cent. of magnesia and 72 per cent. of alumina. Various artificially prepared spinels, corresponding approximately to the formula ROAI,0, have been proposed as refractory materials, but have not been used extensively— chiefly on account of their cost. The chief metallic oxides forming the RO in the above general formula are those of iron, magnesium, sodium, chromium, zinc and copper. Engelhorn ! has stated that spinels containing magnesia and zine are the most resistant to alkalies, whilst those containing magnesia and chromium oxide are the most resistant to lime, cement, alkali-fluxes, glasses, etc. 1 Eng. Pat., 16,714 (1906). MAGNESITE AND MAGNESIA BRICKS 401 Magnesite and Magnesia Bricks.—The best magnesites contain over 45 per cent. and often 47-5 per cent. of magnesium oxide. The proportion of lime varies from 0-5 per cent., and the iron oxide and alumina together sometimes reach 12 per cent. in Styrian magnesite ; the Grecian and Indian magnesites are much purer. Tron oxide is not regarded as an objectionable impurity, as it appears to act as a catalytic agent which facilitates the dissociation of the magnesite and effects the formation of periclase at a lower temperature than is possible with a pure magnesite ; for this reason 2-4 per cent. of it is regarded as a useful constituent. Magnesites containing a small proportion of iron oxide are preferred by manufacturers of magnesite bricks to the purest varieties, as the former can more readily be converted into dead-burned magnesia. _ The effect of silica on magnesite is to lower its refractoriness, as the magnesian silicate which is formed when the magnesite is heated is more fusible than either the magnesia or the silica. A small proportion of silica is not very objectionable as, when fused, it forms a good bond. In fact, S. G. Thomas considered the presence of at least 5 per cent. of silica to be necessary for this purpose, and patented the addition of clay to make up this amount if it was lacking. He found it was undesirable, however, to increase the amount of alumina present by using too much clay. For refractory purposes the following are the maximum proportions of fluxing oxides usually regarded as allowable in magnesite :— Lime . ; . 95 per cent. Alumina. : . : mand Silica . ‘ ; : ne ts - Tron oxide . : ; : ne Fs Alkalies 3 Dead-burned magnesia of the best quality should not contain more than half these proportions of impurities, with the possible exception of the iron oxide. Magnesia bricks vary considerably in chemical composition according to the nature of the materials used in their manufacture. Bricks made from Styrian magnesite usually contain about 85 per cent. of magnesia, 1 per cent. of alumina, 8 per cent. of iron oxide, 5 per cent. of silica, and 1 per cent. of other impurities. Bricks made from Greek magnesite contain usually 92 per cent. or more of magnesia, about 2 per cent. of alumina, practically no iron oxide, 1-5 per cent. of silica, and up to 1 per cent. of lime. For the reasons mentioned above, however, many bricks made from Grecian magnesite have about 6 per cent. of iron oxide purposely added. See also the author’s Refractory Materials: Their Manufacture and Uses (Griffin). According to Kowalke and Hongen! the addition of aluminium, chromium, titanrum, and zirconium oxides, and especially silica, increases the strength of magnesia bricks at high temperatures. By the addition of 7-5 per cent. of silica the refractoriness under load was, in one case, increased from 1680°-1870° C. 1 Trans. Amer. Electrochem. Soc., 33, 215 (1918). 26 402 CHEMICAL COMPONENTS OF REFRACTORIES Dolomite and Dolomite Bricks.—Pure dolomite has a composition correspond- ing to the formula CaCO;MgCO, or CaMgC,0,, but natural dolomites and magnesian limestones usually contain varying proportions of calcium and magnesian carbonates on account of the presence of an excess of one or other of these materials ; they are, in fact, mixtures rather than compounds, though rocks consisting almost wholly of pure dolomite are abundant in some parts of the world. The pure double carbonate of lime and magnesia contains 21-75 per cent. of magnesia, 30-5 per cent. of lime, and 47-75 per cent. of carbon dioxides, but the dolomites generally used contain about 28-35 per cent. of lime, 14-20 per cent. of magnesia, 1-7 per cent. of silica, 0-14 per cent. of alumina, 0-5-5 per cent. of ferric oxide, and 43-46 per cent. of carbon dioxide. S. G. Thomas gives the following average composition of suitable dolomites for refractory purposes :— Lime . : ; . about 33 per cent. Magnesia. : ; 18-20 re Carbon dioxide . : 42-46 ¥ Silica . 2 ; . lessthan 7 7 Iron oxide and alumina = 5 Be Dolomite containing a small amount of alumina and ferric oxide is often preferred to the purer varieties, as it enables the dolomite to be “‘ dead-burned ” more readily. In some cases, iron oxide or “iron scale” is added artificially for the same purpose. As much as 20-30 per cent. of iron oxide may be added without reducing the refractoriness of the mixture to below Cone 31. Such large additions are, however, unnecessary and undesirable. Dolomite bricks have a composition similar to that of the calcined materials, with the addition of a small quantity of clay added for bonding purposes. For further information see the author’s Refractory Materials: Thew Manufacture and Uses (Griffin). Pure calcareous limestone is scarcely used as a refractory material, but lime is an important base in the manufacture of glass and whiting (finely ground calcium carbonate) is an important ingredient in some glazes. Chalk is used in the manu- facture of some bricks and pottery. For further information see the author’s Refractory Materials : Their Manufacture and Uses (Griffin). Bauxite and Bauxite Bricks.—Pure alumina is seldom used for the manufacture of bauxite bricks and other refractory articles. The bauxite usually employed for this purpose contain about 90 per cent. of alumina, though much less pure bauxites are used, some containing as little as 40 per cent. of alumina. Diaspore (which shrinks less than bauxite when heated) has recently been used instead of bauxite for aluminous bricks. The principal impurities in bauxites, as shown by analysis, are : silica, iron oxide, titanic oxide, manganese oxide, vanadic oxide, lime, magnesia, alkalies, sulphur trioxide, and phosphoric acid. Very small proportions of impurities seriously affect the refractoriness of bauxites, ZIRCONIA, ZIRCON AND ZIRCONIA BRICKS = 403 so that for refractory purposes the finest materials should be employed, the following being regarded as the permissible limits of composition :— Alumina 50-90 per cent. Silica. ; ; 3-25 ms Tron oxide. 0-5-12 Re Water . : : : 10-30 = K,ab, and that of the back reaction is == Kya xaxbxbxb == K,a*b? 29 450 PHYSICO-CHEMICAL REACTIONS Hence, abt me ord is the most general expression of the Law of Mass Action. The velocity of a chemical reaction may also be expressed as a ratio of chemical affinity : resistance to chemical action, 1.e. ake neue affinity — resistance. The resistance will depend upon the extent to which the conditions are favourable. These have been described in the preceding pages (pp. 437-449). The speed at which a given reaction will take place can seldom be predicted in the case of ceramic materials, as the conditions are usually too complex. The speed must, therefore, be found by actual measurement. The velocity of reaction is usually expressed as the ratio between the amount of transformed substances and the time required for the transformation, the amounts of substances being calculated in molecular units, the results being stated in “ molecules per minute.” The velocity of reaction between two ceramic substances is sometimes of con- siderable importance—(a) when it is desired to effect a reaction in the quickest possible manner, as in preparing various chemical compounds, glazes, frits, etc., and (6) when it is desired to delay the completion of a reaction because such completion would involve loss of shape or other undesirable consequences. The velocity of reaction is usually increased by raising the temperature at which the reaction occurs and is diminished by cooling. It is also increased by using finely ground materials in the case of solids and by any mechanical means, such as stirring, which will bring the particles into more intimate contact with one another. If means. can be employed for removing the products of the reaction as soon as they are formed the reaction will proceed to completion more rapidly than if no such removal occurs. Hence, if it is desired to produce a glass or definite silicate, the velocity of reaction will be greatest if the materials are fused and stirred together, but if it is essential that the pieces of material shall retain their shape (as in the manufacture of porcelain ware) the temperature must not be allowed to rise too rapidly, but must be maintained constant under such conditions that the desired reactions will occur more slowly and under better control. REVERSIBLE AND IRREVERSIBLE REACTIONS The terms reversible and irreversible reactions are used to denote chemical reactions which will or will not proceed in opposite directions when the relative masses. of the reacting constituents are changed. In an irreversible reaction, the products cannot, by any simple change in the conditions, such as by varying the temperature or adding an excess of one of the products, be reconverted into the original materials which existed prior to the reaction taking place. Thus, when nontronite is heated REVERSIBLE AND IRREVERSIBLE REACTIONS 451 it is decomposed with evolution of water Fe,0,2810,2H,O = Fe,0, + 28i0, + 2H,0, . but under no circumstances yet discovered can a mixture of ferric oxide, silica, and water form nontronite. The decomposition of clay is another example of irreversible reactions. In a reversible reaction, on the contrary, the original substances may be reformed by a simple change in the conditions or in the proportions of the product. Thus, when a chemical action occurs and the products of the reaction are not removed, a certain point of equilibrium may be reached at which the action may proceed indifferently in either direction, though no equilibrium-point may be apparent at ordinary temperatures. For instance, at 100° C., the reaction between barium carbonate and sodium sulphate is reversible : BaSO, + Na,CO, == BaCO, + Na,SO,. The system is in equilibrium when there are 5 parts of sodium carbonate to 1 part of barium sulphate. With greater concentrations of barium sulphate the latter is decomposed, and with smaller concentrations barium carbonate is decomposed. Thus, in a solution of these salts, either heavy spar or witherite may be deposited, according to the relative proportions of each and the temperature of the solution. The action of heat upon calcium carbonate also involves a reversible reaction : CaCO, —= Ca0+CO,. The dissociation of the calcium carbonate occurs only when its dissociation tension exceeds the vapour pressure of the carbon dioxide present in the system. As soon as the latter becomes greater than the former, lime and carbon dioxide recombine to form calcium carbonate (p. 434). Among the numerous reversible reactions which occur in Nature, the following are of special interest in connection with ceramic materials :— Na,OAI,0,(Si0,),+-2H,0 —~ Na,OA1,0,(Si0,),2H,O+28i0, Albite. Analcime. 2H,03Mz0(Si0,), —= MgSi0, +Mg,Si0,+2H,0 Serpentine. Enstatite. Olivine. Ca0Al,0,(Si0,),+-Mg,8i0, —~ (Mg0Al,0,)(CaOMg0(Si0,),). Anorthite. Olivine. Augite. The proportions of these substances which exist in any mineral depend on the conditions necessary to produce equilibrium at the temperature and pressure of the system. Whenever a reversible reaction can occur and the products of the reaction cannot escape, the condition of equilibrium persists and such a reaction may, therefore, be termed a balanced reaction. The distribution of the reacting masses, when in equilibrium, is determined by the relative concentration of the changing substances, as previously explained. 452 PHYSICO-CHEMICAL REACTIONS Until comparatively recent times the existence of equilibrium reactions was unknown ; at present the tendency is to regard all reactions as equilibrium reactions, and that even where they appear to be complete, to consider them as only in apparent completion, i.e. in reality, the equilibrium merely lies so far to one side as not easily to be disturbed. If this view is adopted, it will be seen that balanced or equilibrium reactions are especially common in the case of ceramic and other materials which may exist in two or more allotropic forms such as silica, magnesia, etc., the particular form present being then largely dependent on the temperature and pressure. Balanced reactions are equally important in the case of many ceramic products, especially those in which a considerable amount of vitrification has occurred, as in porcelain, stoneware, etc., and in glazes, because the composition of such material depends very largely on the nature of the mixture and its state of equilibrium. This subject is dealt with more fully in the sections on Phase Conditions (below) and Equilibrium Diagrams (p. 454). PuasE CONDITIONS IN CHEMICAL SYSTEMS The reactions familiar to students of elementary chemistry are, for the most part, of a simple nature, and some of those which are, in reality, complex are treated as though they were simple. When studying ceramic materials, on the contrary, con- siderable complexity is unavoidable on account of the impurities present in the materials increasing indefinitely the number of reacting substances and of the factors which influence the results. Thus, whilst the reaction between two pure substances may be as simple as shown by the types of chemical action on p. 435, when one or more slightly impure substances are heated-—especially if much fused material in the fluid state is produced-—-many complex phenomena may arise, these being due partly to physical and partly to chemical phenomena. They may all be included under the term “ phase conditions,” and in this connection may be studied in greater detail and with greater facility than if each reaction is taken separately. Matter may exist in three forms or states (namely, as a solid, liquid, or gas), which are conveniently termed phases ; thus, a substance may exist as a solid phase, liquid phase, or gaseous phase. For example, the compound expressed by the formula H,O is known to exist as ice (solid phase), water (liquid phase), and steam (gaseous phase). The particular phase in which any substance exists at any given moment depends upon the conditions then prevailing, and when a substance is in the phase conformable to any particular set of conditions it is said to be in equilibrium. Thus, if a drop of water is allowed to fall on a red-hot plate at a temperature of about 900° C. the greater part of the water may remain in the liquid state for a short time, because the steam produced from that part of the drop which is in immediate contact with the hot plate may act as a “ buffer.” The rapid movement of the drop and the comparatively short time it retains its liquid form show clearly that the liquid state is not the one in which an equilibrium can be maintained. On the contrary, the water is rapidly converted into steam, as that is the gaseous phase which is stable at temperatures above 100° C. PHASE CONDITIONS 453 Students of ceramic materials have received invaluable assistance from a formula devised by Willard Gibbs and known as the phase rule. In accordance with this rule, the conditions under which any given series of phases can exist is expressed by the equation P+V=C0+2, where P is the number of phases in the system, V is the number of external conditions which may be stipulated, and C is the minimum number of components. The number of phases may be defined as the number of different homogeneous parts which may exist under the prevailing conditions. The number of external conditions (represented by the symbol V) is usually either 1 or 2, the fixed conditions being generally temperature and pressure, though other conditions may be fixed, if required. In most cases they play a minor part and so need only be considered when the subject is being investigated exhaustively. A component may be either an element or a compound, as its composition does not enter into the definition of a component. Each different substance which may be present, as a separate entity, in the system is a component. A system is the whole, composed of all the components in a state of equilibrium. A eutectic is not a phase, because it contains two phases. In a homogeneous system there can only be one phase; in a heterogeneous system there may be two or more phases. The degree of freedom or variation of a system is the number of independent variables which must be fixed before the state of the system can be clearly defined. In the condition of equilibrium of three two-phase systems—solid-vapour, vapour- liquid, and solid-liquid—which meet at a point (the so-called trzple point) the system is invariant. As a simple illustration, water may be considered. This substance can exist in three phases, namely, ice, water, and steam. These three phases together form a system in which there is one component, namely, the compound dihydrogen oxide (H,O). Substituting in Gibbs’ rule the respective values for P and C the equation becomes 3+V=1-+42. Hence, V=0, so that under no conditions can all three phases exist simultaneously in equilibrium. If itis assumed that P=2 then V=1, or conversely, when one condi- tion (e.g. either temperature or pressure) is specified as fixed, two phases may exist in equilibrium. If V=2, as when a definite temperature and a definite pressure are specified, P=1, so that only one phase could exist under the specific conditions. In the ice-water-steam system there is only one component, but Gibbs’ rule is equally applicable where there are several components. Thus, the conditions of equilibrium of calcium carbonate involve three phases, namely, solid (calcium carbonate), solid (lime), and gaseous (carbon dioxide), so that calcium carbonate forms a three-phase system. Calcium carbonate may be resolved into two com- 1 For a detailed explanation of this rule and some of its applications to other branches of chemistry, see The Phase Rule, by Findlay (Longmans, Green & Co.). 454 PHYSICO-CHEMICAL REACTIONS ponents—calcium oxide and carbon dioxide—so that C2, and on substituting this in Gibbs’ rule, the equation becomes P+V=2-42, 1.€. P+V=4. If only one condition is fixed, 7.e. V=1, then P=3, the three phases can exist only at one definite temperature or pressure. For example, at 600° C. and at all pressures, all three phases can exist simultaneously, but if both the temperature and the pressure are fixed, V=2 and P=2, so that only two phases could exist under those conditions. Granite, composed of quartz, felspar, and mica, has three components (alumina, silica, potash) and three solid phases (mica, quartz, and felspar). It is, therefore, univariant. It is also in equilibrium because, not being at a transition-point, it can survive small variations in temperature without changing the state of the system. The phase rule is very valuable because 1. It permits the classification of systems of similar behaviour. 2. It shows whether the phases of a heterogeneous system are those necessary for equilibrium. 3. It assists in identifying chemical individuals among a series of basic salts or solid solutions. Gibbs’ rule is capable of very wide application and is not limited by the com- plexity of the factors represented by the symbols P, C, and V. It is, however, qualitative rather than quantitative in its nature, because it does not show which phases will occur, where only a limited number are possible, and it only states the maximum number of phases which could possibly occur. Its value is, therefore, limited to showing the maximum value for each of the symbols P, C, and V in any system. When the particular nature of the factors represented by the symbols is required to be known, another method must be adopted, namely, an equilibrium diagram or phase diagram. The chief value of the phase rule consists in its power of indicating the maximum value of one or more of the symbols in a given system. In some cases, where this value is unity no further investigation is necessary, but if, for instance, the rule is employed to determine how many phases can occur in a given system and that number is greater than unity, the use of a phase diagram may be essential if it is desired to ascertain the nature of the possible phases. The chief difficulty experienced in using the phase rule lies in ascertaining what are the precise components, as in complex cases their nature is very difficult to determine. EQUILIBRIUM OR PHASE DIAGRAMS An equilibrium or phase diagram is one which shows what phases can exist under given conditions, the latter being usually one of the following pairs :— EQUILIBRIUM DIAGRAMS 455 (a) Temperature and pressure, (6) Temperature and composition, and (c) Temperature and time (p. 464), but any other conditions may be considered if desired. It is usually best to limit the conditions to two or at most three, as otherwise the diagrams become so complex as to be almost illegible. When only two variables are considered a plane diagram suffices, one variable being represented by the ordinates or horizontal lines proceeding from a vertical scale at the side of the diagram, and the other by the abscisse or vertical lines rising from a scale at the base of the diagram. If, for example, there is only one component (7.e. C=1), and the temperature and pressure are regarded as fixed (i.e. V=2), then, according to Gibbs’ phase rule, P+V=C04+2 P+2=1+42 Pex so that only one phase can exist for any given temperature and pressure, though a different phase may exist at dif- ferent temperatures and pressures. Hence, the phase diagram will show which phase is stable under any particular conditions of tem- perature and pressure. This is clearly shown by fig. 22, which is the equilibrium diagram of water and indicates the state of water ‘at any given temperature and pressure. When there are three phases and two components in a system 6 ¥—2-19 or Vi, Termperature. Fressure. Gaseous : Fic. 22.—PHAsre D1AGRaAmM OF WATER. the phase diagram can only show one stipulated condition. When this is the temperature it may be plotted as ordinates, whilst the composition is plotted as abscisse. The temperature-composition diagrams of reactions in ceramic processes are generally concerned only with the solid and liquid phases, as such materials are seldom volatilised. They may also be termed “ melting- or fusing-point diagrams,” as they show the dividing line between the solid and liquid phases of the constituents present. Temperature-composition diagrams are often much more complicated than those showing temperature and pressure in systems having a single component, because various kinds of chemical action, solution, precipitation, etc., may occur between the components under the particular conditions of temperature indicated, Conse- 456 PHYSICO-CHEMICAL REACTIONS quently, various different types of graphs may be obtained. These may be divided into three classes according to the changes which occur, as follows :— (a) The substances may dissolve in each other in all or some proportions forming a solid solution. (6) The substances may form an eutectic. (c) The substances may combine to form a chemical compound. A solid solution is a substance which appears to be solid and has many of the characteristics of a solid and yet behaves, in some respects, as a solution. The con- ception of a solid substance dissolved in a liquid is familiar ; that of one solid sub- stance dissolved in another is less familiar but is wholly analogous. Just as a liquid solution appears to be homogeneous so that it is impossible by inspection to distinguish between the solvent and the solute or dissolved substance, it is equally impossible in the case of a solid solution, as the latter is equally homogeneous like glass, which is a typical solid solution. The analogy may be carried still further by comparing the behaviour of an ordinary hot saturated solution with that of a molten mass which if left to itself would form a solid solution. Both these liquids, if cooled under favourable conditions, will produce crystals. In the former, it is agreed that the compound of which the crystals are composed existed in the solution. In the latter, it is not so generally accepted that the crystallising compound is actually present in solution, though by analogy it should be so. Solid solutions may be (a) vitreous, or (b) crystalline, depending on the nature of the components and the manner in which the solution has been formed. The general term solid solution is applied both to amorphous and crystalline substances, but especially to amorphous ones; the crystalline forms are often termed mixed crystals. Just as some liquids can be mixed in all proportions, so some substances form solid solutions in all proportions, whilst others only form such solutions in definite proportions and in addition may form an eutectic (p. 459) or definite chemical compounds (p. 462). Although there are no general rules regulating the solubility of liquids in each other, it is generally true that those of the same chemical type are so uble in each other, whilst liquids of different types have only a slight attraction for each other. This applies both to ordinary liquids and to fused solids and explains why minerals of the same chemical type are often soluble in each other in all proportions. Thus, some silicates form solid solutions or mixed crystals in whatever proportions they are mixed. When a temperature-composition diagram of two substances which are wholly miscible in each other is drawn, either of two types of graph may be obtained, namely, (a) a plain continuous curve, or (6) a curve with a transition-point, showing that an abrupt change in composition occurs at the critical temperature. Solid solutions differ from definite chemical compounds inasmuch as the latter have a definite melting-point, whereas the former have a range of fusion. This distinction must not be pressed unduly, especially with refractory materials. The simplest type of plain curve (without a transition-point) is shown in fig. 23; in EQUILIBRIUM DIAGRAMS 457 this, the graph consists of a straight line joining the melting-points of the two substances forming the components of the mixture. Such a graph is typical of 1600 % C Ro Q 7300 1200 1000 Anorthite. 100 90 80 70 60 50 40 30 20 70 oO Albite, Fie. 23.—AtBrrn-ANORTHITE PHAsE DIAGRAM. mixtures of albite and anorthite felspars (p. 416) and of other ceramic materials which do not react on each other. On examining such a graph, it will be seen that any increase in the proportion of the substance melting at A° will increase the melting-point of the mixture, whilst an increase of the material melting at B° will decrease the melting-point of the mixture. More often, however, a _ definite curve is obtained when the fusing- points of various mixtures are plotted; the form of the graph is that of a continuous curve, such as that shown in fig. 24, in which the line ACHB is the temperature curve of apparent solidification. When a molten mixture of any composition or a solution of several substances commences to freeze it deposits, at first, a solid substance which is poorer in one substance than the mixture as a whole. As the temperature falls a richer mixture is deposited, until Temperature. Termperature. = Cormposition. Fig. 24.—-PHasz D1aGRAmM or WHOLLY MISCIBLE LIQuips. 458 PHYSICO-CHEMICAL REACTIONS at a certain critical point the mixture or solution is entirely solid and consists of a homogeneous solution of the two constituents. Thus, in the case illustrated in fig. 24, a mixture of composition J, when cooled, begins to solidify at the tempera- ture E, the first solid to be deposited having a composition shown by DI. The Temperature. Composition. Fig. 25,—Puasr Diagram or Miscrste Liguips (WITH MAXIMUM POINT). portion represented by I is solid, and the portion from I to J is liquid. As it Temperature. cools further, the material becomes more solid and less liquid, the solid portion having a composition which approaches nearer and nearer to that of J, until, at the tem- perature L, its composition is shown by the left of the line ADKFB give a solid phase and those to the right of ACHB give a liquid phase, whilst between the two lines is a mixture of solid and liquid. In some cases, two substances dissolve in each other in all proportions, but at one point the temperature and com- position curves of each substance coin- corresponding to this point will show a Fie. 26.—Puase Diagram oF MiscrBiE ; : : LigvuIDs (WITH MINIMUM POINT). sharp melting-point like a pure sub- stance. The fusing-point of this substance may be higher or lower than that of either of the two pure substances forming the mixture. In fig. 25 there is a maximum melting-point at some point between the two extremes of composi- tion, and at this point the solid solution phase has the same composition as the liquid phase. This type of graph is, however, extremely rare and it is doubtful whether it ever occurs in ceramic processes. Fig. 26 is similar, but instead of K and it is entirely solid. Conditions to cide and a substance with a composition Composition. a maximum point it shows a minimum point which must not be confused with EQUILIBRIUM DIAGRAMS 459 an eutectic (below). Naturally, zoned crystals are probably associated by a curve of this type. It is very difficult in some cases to decide to which of the last three types any binary mixture belongs, especially when the melting-point of the individual sub- stances are very close to each other. The form of continuous curve in which there is a transition-point is shown in fig. 27; it indicates the existence of two types of solid solutions or mixed crystals having an equilibrium point at K, compositions of the two types being shown by the points D and E. A natural example of this kind of diagram is shown by the pyroxenes (p. 415). Thus, if a mixture of silica, lime, and magnesia is heated, when the magnesia is in excess, enstatite (MgOSi0,), an orthorhombic pyroxene, separates with very little calcium silicate (CaOSiO,), but on cooling from the tem- perature K to B, the magnesia and lime unite with the silica, forming a monoclinic pyroxene, augite (CaOMgO28i0,). In this system, CaOSiO, is slightly soluble in MgOSi0,, whilst MgOSiO, is very soluble in the augite series. The pre- sence of other substances, such as Composition. iron oxide, may alter the order of Fic. 27.—PuHasre Diacram or MiscrsLe LIQuips Cae eric (WITH TRANSITION POINT). crystallisation. Still more complex graphs are considered on p. 466 e seg., but, in order to understand them better, it is necessary to consider the nature of eutectics. An Eutectic is a substance of definite melting-point which may be higher or lower than that of the two substances from which the eutectic is produced. The existence of an eutectic is usually indicated by a sharp transition point in the equilibrium diagram graph of two or more substances. If the product corresponding to this transition-point is separated from the mixture it will usually be found to have a definite composition and a sharp melting-point—two important characteristics of a chemical compound. Some eutectics do not appear to consist of stable chemical compounds, but resemble a mechanical mixture of the substances concerned. The difference between a solid solution and an eutectic is that the former is a simple mixture, whilst the latter has a lattice-structure containing two or more sets of atoms and approaching a chemical compound in its properties. Thus, an eutectic is not a single-phase substance but a di- or poly-phase mixture. Fig. 28 shows the temperature-composition curve of two substances, A and B, which form an eutectic, ©. It will be seen that on cooling the fused mixture, the substance with the melting-point A, separates out along the line AC, and the substance B separates out along the line BC until the composition Cis reached, when the mixture solidifies en masse as an eutectic. The transition-point C is sometimes termed the eutectic-point. In some cases, it is not merely a point but a straight line. Thus, when Temperature. 460 PHYSICO-CHEMICAL REACTIONS water is cooled by freezing, the temperature remains at 0° C. until all the water is converted into ice. If the time during which the cooling of a solution or fused mass is arrested by the formation of an eutectic is plotted against the composition of the mixture as a whole, a triangle will be obtained with its apex at the composition of the eutectic. From this it is possible to find the composition of any mixture of the same components at any temperature. The formation of eutectics is very common in the preparation of metallic alloys ; it also occurs frequently in various ceramic processes, as will be described in detail later (p. 466). In some cases, the graph or curve is complicated by two actions taking place Temperature Temperature. Composition. Composition. Fig. 28.—Eutnctic PHASE DIAGRAM. Fic. 29.—Soitip SoLution AND EvTEoTIC PuHaseE DIAGRAM. simultaneously : (a) the formation, to a limited extent, of solid solutions, and (6) the formation of an eutectic. Thus, when two molten substances do not dissolve in each other in all proportions, the lines forming the graph corresponding to the composition of the solid deposited on cooling the fused mixture and the freezing-point do not meet, but appear as shown in fig. 29. In this diagram, a solid whose composition is represented by the line AC gradually separates out at temperatures AB, and at the temperature B an eutectic of composition B separates out. Kutectics are not confined to two constituents, and in some cases ternary, quater- nary, or even more complex eutectics are formed. They are much more difficult to investigate than the binary eutectics, and are sometimes very difficult to recognise, because in a highly complex liquid with a high viscosity no sharp eutectic-point is found. Under such conditions it is never easy to establish complete equilibrium, and, consequently, a liquid of high viscosity may cause erroneous conclusions to be reached. An eutectic is usually recognised by a dip or depression in the graph of the equili- brium diagram, though this is not always the case. Hence, it is not necessarily correct to define an eutectic as a substance having the lowest melting-point in a series. EQUILIBRIUM DIAGRAMS 461 According to Vogt, the eutectic-point of two minerals is a mathematical function of their melting-points, latent heats, molecular weights, and electrolytic dissociation, but Doelter and some other investigators do not agree with this. Even if it is applic- able to some mixtures, the number of cases where it is inapplicable make this theory of limited value. The following summary of the work of Vogt and others on eutectics shows their normal behaviour :— (a) Minerals having a high melting-point and a relatively high latent heat of fusion have an eutectic-point nearer to the more infusible mineral. (6) Minerals which have a considerable difference in melting-points have an eutectic nearer to the more fusible mineral. (c) Where the minerals have about equal melting-points the eutectic is approxi- mately in the middle of the graph. (d) Where the more fusible mineral has a higher molecular weight than the less fusible mineral the eutectic is nearer the more fusible mineral. These statements must be modified as regards mixtures containing isomorphous crystals (p. 334) and where supersaturation occurs (p. 462), as in these cases and in some others, crystals sometimes separate in a different order from that which might be expected from a study of the phase diagram. Vogt has endeavoured to explain such irregular separation of minerals by means of a “common ion” theory by suggesting that the presence of a common ion in two or more components of a fused mass may cause the separation of a mineral containing it even when only a small amount is present. Thus, in a fused mixture containing felspar and spinel, the introduction of a ferro-magnesian mineral will cause the separation of spinel even though only 2 per cent. of the latter be present. Similarly, Harker has found that whilst in the ultrabasic rocks of the Isle of Rum, the olivine and anorthite have usually crystallised as anticipated forming an eutectic, yet when pyroxene was present the olivine always crystallised first. When a common ion is present it appears to alter the eutectic composition and to displace the eutectic-point in a direction away from the mineral which has the higher fusing-point ; by this means it accentuates the separation of minerals in inverse order of their fusibility. Such explanations based on the existence of a common ion only hold when the solutions are so dilute that their dissociation is com- plete and in the absence of complex reactions. Apart from the abnormal cases, the composition of an eutectic may be calculated with considerable exactness from van’t Hoff’s law, which states that T2 dT =0 0198—-, where dT is the depression at the absolute temperature T and L is the latent heat of fusion per gram of solvent. The depression of the melting-point of a substance by the presence of another dissolved substance may also be expressed by the ratio MA=4, where M is the molecular weight of the solution and A is the depression of the freezing- or solidifying-point caused by a 1 per cent. solution. The reason that, when 462 PHYSICO-CHEMICAL REACTIONS several fluxes are present, their total effect is more powerful than that of an equal weight of any one of them, is because each flux causes a separate depression of the fusion-point, and, consequently, several fluxes reduce the fusion-point more than the presence of an equivalent amount of only one flux. When some fused materials are cooled, no separation of the crystals of the eutectic occurs. This is attributed to the supersaturation of the molten material with some constituent which commences (and continues) to separate until the mixture has been cooled below the eutectic temperature; the extent to which the cooling can be carried beyond this point depends on the degree of supersaturation. Eventually a point is reached at which another constituent begins to separate rapidly, thus causing a rise in temperature, due to latent heat (which may amount to about 100 calories per gram). The separation of the second constituent continues until the remaining material has the same composition as the eutectic, after which the whole mass then crystallises in the form of an eutectic. The phenomena of supersaturation may produce abnormal effects in the order in which the crystallisation of different substances occur. Definite Chemical Compounds.—lIn some cases, definite chemical reactions occur when molten liquids come into contact with each other and the resulting compounds may separate out in the form of almost pure crystals. Similar compounds are also formed when some mixtures are maintained at definite temperatures for a long time. The appearance of the phase diagram is somewhat different when com- pounds are formed from that produced when only mixtures are formed, as the solution of a pure substance in a compound or vice versa lowers the melting-point. Thus, a phase diagram in which compounds are formed shows a series of humps, according to the number of compounds formed, the summits of the humps representing the com- position and melting-points of the compounds formed. The curve between any two humps resembles the graph in a simple phase diagram of two mixtures which do not form compounds, but have an eutectic composition or form solid solutions. A definite chemical compound differs from a solid solution in that the former has a sharp melting-point, whilst solid solutions have a fusion range. Some highly refractory substances have so low a thermal conductivity, however, that unless the fusion of a few minute grains is observed under the microscope, a definite compound, such as quartz, may appear to have a fusion range and so be mistaken for a solid solution. On the other hand, Ludwig’s hypothesis, that fireclay bricks are solid solutions of “impurities”? in the pure clay, enables the melting-pomts of many clays to be calculated from their chemical composition (see p. 382). The formation of compounds prior to fusion often has a very important effect on the temperature at which the fusion occurs. Thus, litharge attacks silica at a tempera- ture of about 700° C., although its melting-point is 200° C. higher, and that of silica is about 1600° C. Similarly, the surface of crystalline potash felspar becomes “ soft ” about 200° C. below the fusing-point of that mineral. Cobb has shown that lime and silica can combine very extensively, forming calcium silicate at about 800° C.—a temperature much below the fusion-point of the product. This formation of compounds—some of which may fuse at a higher temperature COMPLEX FUSION CURVES 463 than that at which they are produced—plays a very important part in the production of vitrified ware (see Chapter XIII.). Complex Fusion Curves.—The fusion curves of many binary mixtures are not so simple as those which have been described in the preceding pages, but involve many other phenomena. Thus, the fused mass after cooling slowly may contain chemical compounds, eutectics, and solid solutions. In a ternary system, in which more than two substances are present in the molten mass, the phase diagram cannot be plotted in a single plane, but takes the form of a A Orthoclase Albite Anorthite B C Fic. 30.—TrErRNary PHAst DiaGRAM OF ORTHOCLASE-ALBITE-ANORTHITE SYSTEM. triangular diagram, as shown in fig. 30, where A, B, and C represent 100 parts of each of their constituents. If E is the eutectic-point of A—B, and F the eutectic-point of A—C, whilst the substances corresponding to B—C are completely miscible, the composition of any melt and the crystals separating from it may be plotted. Thus, if the composition changes from G—H and the mixed crystals from I—J, when the curve GH touches the curve EF, a tangent to the curve GH from H cutting the curve IJ will give a point which corresponds to the limit of mixed crystals of the system BC in A. By examining the compositions of various mixtures, the curves KL and MN may be developed. The former is the limit of solubility of BC in A, whilst the latter is the limit of solubility of Ain BC. The simplest form of ternary system is one in which no compounds or solid solutions are formed, but only one eutectic point. Where the three substances, taken in pairs, form eutectics with each other, the 464 PHYSICO-CHEMICAL REACTIONS corresponding phase diagram is similar to that shown in fig. 31. Such a system has four eutectic-points, namely, the binary eutectic points, because A—B has a eutectic D ten 9 ” K B—C ” ” F and I is the ternary eutectic-point, which is the eutectic characteristic of the system as a whole. A quarternary system would require a phase diagram of four dimensions, so that Quartz _A. Orthoclase ok Plagroclase B Cc Fig. 31.—Trrnary PHASE DIAGRAM QUARTZ-ORTHOCLASE-PLAGIOCLASE SystTEM. it is almost impossible to express graphically on paper the reactions which occur in such a system; a model in the form of a solid tetrahedron may be prepared which will show these changes. Time-TEMPERATURE CURVES In accordance with Gibbs’ phase rule (p. 453), if both time and temperature are stipulated and there are two components in the system, V=2 and C=2, so that P+2=2+42, i.e. P=2, so that two phases are possible. In dealing with ceramic materials these will usually be the liquid and solid phases. Whether any liquid material is produced depends on whether the temperature is sufficiently great. This must be ascertained by actual investigation of the products at different temperatures or after different EQUILIBRIUM DIAGRAMS 465 intervals of time. On the other hand, if there is only one component and two con- ditions V=2, C=1, and P42=142, so that P=1 and there can only be one phase. In the former case, the tempera- ture-composition diagram can be made to show under which of these conditions the solid and liquid phases can exist either separately or simultaneously, whilst in the latter case only one phase is possible. For some purposes, however, there is no need to ascertain the nature and number of the phases, and a simple time-temperature graph will then suffice. Such a diagram is often very useful for showing the rate at which a substance can be heated when heat is supplied to it externally at a constant rate. Where no change occurs in the rate of heating, the graph would take the form of a straight line or a regular and unbroken curve. If, however, any physical or chemical change occurs in the material this will be shown by some uregularity or change in the direction of the curve indicating a transition-point. Hence, a time-temperature curve of the heating or cooling of some materials is of great value and shows when changes begin to take place in them. ‘Thus, the cooling curve shown in fig. 32 shows that a change begins to occur at a temperature G° and continues to EK° when Fic. 33.—TEMPERATURE-COMPOSITION AND TIME-TEMPERATURE another change occurs, as a Cone result of which the tem- perature remains constant for a time represented by the horizontal portion of the curve, after which the cooling continues uniformly with no further change in state. A comparison of temperature-composition and time-temperature curves is very interesting and instructive. Thus, fig. 33 shows these two curves, the right-hand portion showing the time-temperature curve of a material of a composition indicated by the vertical line. From these two curves it will be seen that the range of temperature during which solidification occurs, as shown by the temperature-composition graph, coincides with the irregularity in the time-temperature graph. Ternperature. Time. Fic. 32.—Time-TEMPERATURE CURVE. Temperature. Temperature. Composition. Time. 30 466 PHYSICO-CHEMICAL REACTIONS Time-temperature curves are also useful in determining the quantitative results of any reaction. Thus, in the formation of eutectics, the time taken in a given apparatus to form the eutectic is directly proportional to the amount of eutectic formed, so that by estimating the time during which no cooling occurs, the amount of eutectic which can be formed may be found. Time-temperature curves are also very useful in connection with the study of crystallisation (p. 489). These applications of graphs are dealt with more fully in the next section. In considering the cooling-curves of substances, it must be remembered that the effect of supercooling (p. 487) may conceal any small critical poimt or range on the curve. A heating curve is often more reliable than a cooling one, as the effects are not so pronounced and minor critical points are less liable to escape detection. PHASE CONDITIONS IN CERAMIC PROCESSES 39 The reactions occurring during the heating or “‘ burning”’ of various materials used in the ceramic industries are very complex, and in order to obtain even a partial conception of them it is necessary to understand what occurs in the case of some of the simpler systems ; from a comparison of these, a considerable amount of informa- tion respecting the more complex systems may be obtained. Binary Systems Binary systems of interest to students of ceramics chiefly consist of a base (usually a metallic oxide) and an acid (usually silica), but other binary systems in which both components are metallic oxides must also be considered. The binary systems vary greatly in stability, some of them being readily decomposed, whilst others are highly stable. This characteristic cannot be predicted with certainty from their composi- tion, though the more stable binary compounds are usually those in which the molecular ratio most nearly approaches unity. Thus 3CaO SiO,, with a ratio of 3:1, is much less stable than CaO SiO,, with a ratio of 1: 1. Binary systems consisting of a base and silica may form definite chemical com- pounds, solid solutions or eutectics according to their components. Binary silicates are usually formed at a temperature about 200° C. above their fusing-point, though this should not be taken as a fixed rule, as the difference between the temperature of formation and the fusing-point is not always constant and Cobb has shown that lime can combine with silica at a temperature much below the fusing-point of either of these oxides and also below that of their product. Singulo-silicates usually form at lower temperatures than those having a higher oxygen ratio. Sub-silicates, which are more basic, and bi- and tri-silicates, which are more siliceous, are usually formed at higher temperatures. Thus, if a bisilicate consisting of RO SiO, comes into contact with a basic material, it tends to produce a substance having the formula 2RO SiO,, but if it originally corresponded to 4RO SiO, it would tend to combine with silica and to form 2ROS8i0,. Silicates with only one base have a lower reltaeones than the materials from Sats they are formed, thus : Fusing-point BINARY SYSTEMS 467 CaO. SiO,. CaOSi0,. 1900° C. 1850° C. 1512° 0. MgO Si0,. MgOSiO,. 2000° C. 1850° C. 1524° C. Fusing-point Silicates with more than one base fuse at lower temperatures than those with only one, in accordance with van’t Hofi’s Law (p. 461). Similarly, mixtures of sili- cates have a lower fusing- point than the same silicates when heated separately ; thus, 70 parts of CaOSiO,, fusing at 1512° C., and 30 parts of MgOSi0,, fusing at 1524° C., form an eutectic which fuses at 1350° C. The lime-silica system has been examined in detail by Day and Shepherd,! who found three eutectic-points as follows :— 1. At a temperature of 1417° C. an eutectic mix- ture consisting of tridymite and pseudo - wollastonite is formed. 2. At 1480° C. an eutec- tic consisting of pseudo- wollastonite and a _ lime- olivine is formed. 3. At 2015° C. an eutectic of lime olivine and lime is formed. Two definite compounds are shown in the equili- brium diagram (fig. 34), namely, calcium orthosilicate (2CaO SiO.) which melts at 2130° C., and calcium meta- 2400 2200 \) Ca0 + Melt Eutectic 2015°C, 2000 1800 Inversion of } az BCa2S/04 1410°C, 1480° wCla 5103 1400 Eutectic 1417°C, Tridymite + a Casi 03 Inversion of a -CaSi03 1200°C. 1200 1000 Tridymite + (3 CaS103 Inversion of Tridymite 800 to Quartz: 800°C, Quartz +BCaSi03 | CaO%O> NIO™ 20 60 70 80 90 SiOz 100 90 80 70 60 50 40 30 20 10 O Fic. 34.—PuHase Diacram or Limn-Siica SystEm. (Day AND SHEPHERD. ) silicate (CaO Si0,) which melts at 1540° C. J. Cobb found that the orthosilicate is formed unless there is a large excess of silica present, in which case the metasilicate may be formed. 1 Amer, J. Sci., 22, 265 (1906). 468 PHYSICO-CHEMICAL REACTIONS Calcium orthosilicate exists in three forms : a. Stable above 1410° C.—monoclinic ; specific gravity 3-27; hardness, 5-6. B. Stable 1410°-675° C.—orthorhombic ; specific gravity 3-28. y. Stable below 675° C.; monoclinic specific gravity 2-97. Calcium metasilicate (wollastonite) exists in two forms : a. Stable between 1512° C. and the melting-point. B. Stable up to 1512° C. According to Boudouard, the metasilicate is the most fusible silicate of lime. It must be clearly understood that an equilibrium diagram obtained from observations on the cooling of a fused material will not necessarily show the lowest temperatures at which the compounds indicated can be formed. When such compounds are formed by gradually heating their component oxides, the temperature at which the latter interact depends on the intimacy of association (p. 445) and the allotropic form of the substances, as well as on the other factors mentioned on pp. 437-449. An equilibrium diagram for cooling may lead to erroneous conclusions if it is assumed to be applicable in the reverse direction to that on which the observations were based. Calcium silicates may, according to Cobb, be formed at as low a temperature as 800° C., if they are in intimate contact, the action taking place below the melting- point of the eutectic without any signs of fusion. Hedvall found that quartz and cristobalite begin to react with lime at about 1400° C., the latter being more resistant than the former. Quartz-glass, at 1000° C., reacts rather feebly with lime, but pre- cipitated silica is rapidly attacked. The soda-silica system includes two compounds, sodium metasilicate (Na,OSi0,) and sodium tetrasilicate (Na,048i10,) ; the latter appears to be the chief constituent of water-glass. When silica is heated with common salt (sodium chloride) in the presence of water-vapour, as in salt glazing, combination occurs as shown in the equation :— SiO, + 2NaC] + H,O--Na,OSi0, +2HCI. The magnesia-silica system includes two compounds, which are, according to Allen and Wright: (a) MgOSiO, (m.-pt. 1554° C.), (6) 2MgOSiO, (m.-pt. over 1750° C.), which is identical with the mineral forsterite. It is also produced when magnesia bricks containing silica as an impurity are maintained for some time at a temperature above 1500° C. In some cases, as has been noted by Kowalke and Hongen, forsterite crystals may completely surround periclase (MgO) crystals, thus producing a very strong bond, and, for this reason, the presence of 5 per cent. of silica is considered desirable by many Continental users of magnesia bricks. The magnesia-silica system is particularly difficult to investigate as the products are so highly viscous that there is always some uncertainty as to whether a state BINARY SYSTEMS 469 ' of equilibrium has been reached. Fig. 35 shows the equilibrium diagram of the MgO-Si0, system, according to Sosman.! > S % $ | § - RK w | 2 | i ‘< Si St Oz 1:1 Bit MgO Fic. 35.—PuHasr Diagram oF Maenesia-Srtica System. (Sosmay.) The barium oxide-silica system includes, according to P. Eskola,? the following compounds and eutectics :— Compounds :— (a) 2BaOSi0,. (6) BaOSiO, (m.-pt. 1604° C.). (c) 2BaO 3Si0, (m.-pt. 1450° C.). (d) BaO 28i0, (m.-pt. 1420° C.). Eutectics :— (a) 2BaOSiO, and BaOSiO, (m.-pt. 1557° C.). (b) BaOSiO, and 2BaO03Si0, (m.-pt. 1437° C.). (c) BaO 2810, and SiO, (m.-pt. 1374° C.). In addition to the foregoing, mixtures of 2BaO3Si0, and BaO2SiO, form a continuous series of solid solids containing these components in any proportions. The strontia-silica system includes, according to P. Eskola,? the following compounds and eutectics :— Compounds :— (a) 28rOSi0, (m.-pt. above range of electric furnace).3 (6) SrOSiO, (m.-pt. 1578° C.). Eutectics :— (a) 28rOSiO, and SrOSiO, (m.-pt. 1545° C.). (6) SrOSiO, and SiO, (m.-pt. 1358° C.). 1 Trans. Faraday Soc., 12, 170 (1917). 2 Amer. J. Sct., 4, 331-75 (1922). 8 According to Jaeger and Van Klooster [Sprechsaal, 52, 256 (1919)] it is above 1750° C. 470 PHYSICO-CHEMICAL REACTIONS The zinc oxide-silica system includes the following definite compounds :— ZnOSiO, (m.-pt. 1437°-L1° C.),} 27nOSiO, (m.-pt. 1509-5°-L0-5° C.) ; 2 the latter is identical with the mineral willemite. The manganese oxide-silica system.—Manganese oxide and silica produce orthosilicates and metasilicates : 2MnOSi0, (m.-pt. 1290°-1300° C.)? is identical with the mineral tephroite. Artificial tephroite has no definite melting-point, but darkens on heating and decom- poses before melting completely. MnOS8i0, (m.-pt. 1273°+1° C.) 1 is identical with the mineral rhodonite, which melts between 1221° and 1270° C.).4 Tephroite forms solid solutions with fayalite (2FeOSiO,), but whilst rhodonite may take some iron into solution, it does not appear to form a definite metasilicate. The iron oxide-silica systems are complicated by the existence of three iron oxides (p. 418) and by the comparative ease with which the red ferric oxide and the black magnetic oxide can be reduced and form the dark ferrous oxide. Ferrous oxide reacts with silica, forming fayalite (2FeOSiO,), and griinerite (FeOSi0,). The former forms solid solutions with tephroite (2MnOSi0,), and also with forsterite (2MgO8i0,). Very little is known of the iron oxide-silica systems, although they play a highly important part in the production of vitrified materials, especially in the manufacture of blue bricks. They are also important in the manufacture of silica bricks, but the latter are simpler, because in them most of the iron is in the form of fayalite, though free magnetite occurs in some bricks. The zirconia-silica system includes only one eutectic, which is identical with the mineral zircon ZrSiO,, with a fusing-point, according to Washburn and Libman,? of 2300° C., which is intermediate between that of zirconia (ZrO,), viz. 2700° C.? and silica (SiO,), viz. 1470° C. If zirconia and zircon are heated they appear to form a eutectic having a fusing- point of about 2300° C., which is the same as that of the zircon and is probably identical with it. In that case there is no true eutectic, as this term cannot be applied to either of the components of the system, but only to an intermediate substance containing both components. ‘The lime-alumina system includes various calcium aluminates which are much more fusible than either lime or alumina. Free lime and alumina begin to interact between 800° C. and 900° C.; the reaction becomes very rapid as 1100° C. is approached and, according to Cobb, is practically complete at 1300° C. The chief calcium aluminates which are found in burned clays are : 5CaO 3A1,0, (m.-pt. 1386° C.), 3CaOAl,0, (found at 1530° C.). 1 Jaeger and Van Klooster. 2 J. Amer. Cer. Soc., 3, 634 (1920). BINARY SYSTEMS 471 Rankin and Wright state that the following compounds may also be formed :— CaOAl,O, 3Ca05Al1,0, ; they are not found in burned clays. .CaOQ2Al,0, may be formed at temperatures below 1100° C., but above this temperature a compound richer in alumina is found which is insoluble in hydrochloric acid in the cold, whilst the more calcareous com- pound is completely soluble. The phase diagram of alumina and lime is shown in fig. 36, due to Sosman.! : Temperature, °C. Al,0; 5 as Sees 7 Cao Fie. 36.—Puasze Diagram oF Lims-ALumina System. (SOSMAN.) The magnesia -alumina system yields one compound—spinel (MgOAI1,0,), with a melting-point of 2135° C. The phase diagram is shown in fig. 37. The iron oxide-alumina system like the corresponding silica system is com- plicated by the existence of three iron oxides (p. 418). So far as the ferrous oxide- alumina system is concerned the chief compound is hercynite (FeOAI,0;). The iron-alumina systems have not, however, been fully investigated. The iron oxide-lime system is complicated like the corresponding ones contain- ing alumina and silica respectively. When lime and iron oxide are heated together ferrates may be formed. The two chief compounds are calcium metaferrate (CaOFe,0,), which melts at 1205° C., and, according to Sosman and Merwin, dis- sociates at the same temperature, forming long, black, needle-shaped crystals, whilst the second compound calcium orthoferrate (2CaOFe,0,) melts at 1400° C. and almost immediately dissociates, forming black crystals having a yellowish brown tinge by reflected light. The phase diagram, due to Sosman,? is shown in fig. 40. 1 Loe. cit., p. 469. 472 PHYSICO-CHEMICAL REACTIONS Very little calcium meta- or ortho-ferrate is formed when clays are burned as the proportion of iron oxide is not usually sufficient. The silica-alumina system has been investigated by Shepherd and Rankin,! who found two eutectics, one (m.-pt. 1800° C.) containing just over 50 per cent. of silica and the other (m.-pt. about 1580° C.) containing about 6 per cent. of silica. When clays are heated to about 1300° C., sillimanite (Al,0,Si0,) is gradually formed to an extent depending on the duration of the heating. As the temperature increases, the rate at which sillimanite forms also increases, a rise of temperature acting in the same way as a prolongation of the time of heating. Seger in 1893 published data (fig. 38) showing the refractoriness of a series of Temperature, °C. --Spinels--—> Alz03 7-7 MgO Fic. 37.—PuHast Diagram or Maanesta-ALuUMINA System. (SOSMAN.) mixtures of silica and alumina. The conditions of his experiment were not such as to produce a sharply defined eutectic, but the transition-point at or near a composition corresponding to Al,0;17Si0, is very noticeable. It is also interesting to note that such an artificial mixture of alumina and silica has the same composition as some of the best ganisters, because ganister has long been prized commercially for its resistance to heat, whilst, according to Seger, it is more fusible than any other mixture of alumina and silica. The difference may be partly, though not wholly, explained by the fact that Seger used extremely finely-ground alumina and silica, whereas ganister is com- posed of coarser grains. Fig. 38 also shows the importance of using either pure silica or pure alumina where a heat-resisting material is required as the loss in refractoriness with even a small proportion of impurity is very great. Fig. 39 shows the phase diagram of the alumina-silica system, according to Sosman.? The silica-sillimanite system may be regarded as a portion of the silica-alumina 1 Amer. J. Sct., 27, 302 (1909). 2 Loe. cit., p. 469. BINARY SYSTEMS A73 system, but in view of the importance of sillimanite in the ceramic industries it is convenient to consider it separately. Cristobalite and sillimanite and also sillimanite and alumina form eutectics, but do not dissolve each other to any appreciable extent 100 90 80 70 60 50 40 30. 20 10 0 Per certt Al203 0 1020 a 00. 40 50 160 470 8055 90, 100 Per cent SiOz Fic. 38.—Fusion Curve or Atumina-Sitica Mixturss. (SEGER.) so as to form solid solutions. According to Rankin and Wright,1 the eutectic composition is 87 parts of silica and 13 parts of alumina, which is equivalent to Temperature, °C, 8 & % & S GB SiO; 1:1 Alz0, Fie. 39.—Puase Diagram or Atumina-Sinica System. (SOsMAN.) approximately 79 parts of silica and 21 parts of sillimanite. This fuses at 1610° C. and corresponds very closely in composition to Seger Cone 28, which has a refractori- ness of 1630° C. 1 Amer. J. Sct., 39, 9 (1915). AIT 4, PHYSICO-CHEMICAL REACTIONS The iron oxide-magnesia system has not been fully investigated. It is known that ferrous oxide enters into solid solution in magnesia to a limited extent and that ferric oxide may combine with magnesia to form magnesio-ferrite (MgOFe,0;). This substance also forms mixed crystals with magnesia. It has been detected in magnesia bricks by Cornu and Cronshaw independently. Both ferric and ferrous oxide rapidly react with magnesia. For this reason magnesia bricks should not usually be heated in contact with very hot iron oxide. In what are known as “ Metalkase bricks,’ which are iron cylinders packed with dead- burned magnesia, use is made of the reaction of these two substances to form a magnesic mass with a very strong bond. TERNARY SYSTEMS When three substances are heated together the number of compounds which can be produced is greatly increased. Such systems are of considerable importance in the ceramic industries, though few of them have been investigated. The ternary systems may conveniently be divided into two groups : (a) Base-base-silica systems. (b) Base-alumina-silica systems. This subdivision is desirable because alumina behaves abnormally, acting some- times as a base (in which case systems containing it may be included in the first of the above groups) and sometimes acting with the silica to form a complex acid (alumino- silicic acid), and so causing the systems in which it so acts to behave almost as binary systems. BASE-BASE-SILICA SYSTEMS The lime-magnesia-silica system is very complex ; according to Ferguson and Merwin, there are fourteen crystalline phases in this ternary system, the triangular equilibrium diagram showing fourteen invariant points at which three crystalline phases and a liquid phase can coexist, and, of these, six are eutectics. Besides these, there are five series of solid solutions. Ferguson and Merwin found that the most fusible (eutectic) mixture contains 32 per cent. of lime, 7 per cent. of magnesia, and 61 per cent. of silica, which does not correspond to any definite chemical formula ; it melts at 1320° C. Two well-known substances found in this system occur in Nature as diopside (CaOMgO2Si0, (m.-pt. 1391° C.)) and monticellite (2CaO2MgO28i0,), a material similar to which has been found by A. Scott im some British magnesia bricks. Fig. 41 shows a triangular diagram of the lime-magnesia-silica system prepared by Ferguson and Merwin. The lime-barium oxide-silica system has only been partly investigated ; 1 Amer. J. Sci., 48, 6 (Fourth Series) (1919). TERNARY SYSTEMS Temperature, °C. CaO eg, 71:7 Fez03 Fic. 40.—Puase Diagram oF Lime-Frrric OxipE System. (SosMAN.) S/02 1710 Tridymite Cristobalite Bea Oel Cs 1543 0,Ca OS/0z Lr ; . : { 025102-\M§0 Siz 7. §Ca0 2Mg0 6510, 1340 1320 i9o5 25; SO SiO, 1557 2CaOMg0 28/02 Mg0 CaO 2300 MgO 2570 ise Fie. 41.—TRIANGULAR DiaacRamM OF MaGnesiA-Lime-SiticA SYSTEM. (FERGUSON AND MERWIN.) 475 476 PHYSICO-CHEMICAL REACTIONS according to P. Eskola,1 fused calcium and barium silicates cannot remain mixed in indefinite proportions ; on cooling, 2CaOBaO38i0,, which is infusible, forms but dis- sociates, yielding a-CaOSiO, and a liquid whose composition has not been determined. In the lime-lithia-silica system, when prepared by fusing lithium and calcium silicates, are two series of mixed crystals and an eutectic (melting-poimt 979° C.) containing 50 per cent. of calcium silicate. The lime-strontia-silica system, according to P. Eskola,! forms a continuous series of solid solutions with a minimum melting-point at 1474° C.+3°. An analysis o 1600 1500 1400 1300 1200 1100 1000 (4) 10 20 30 40 50 '-460> ~707> 480 90 100 Per cent Ca Si03 100 90 80 70 60 50 40 380 20 10 O Per cent Naz St03 Fig. 42.—PuHasE Diacram oF Sopa-Lims--Sinica System. (WALLACE.) of this substance shows that it contains 44 per cent. calcium silicate (CaOSiO,) and 56 per cent. strontium silicate (SrOSi0,). The soda-lime-silica system (fig. 42), when prepared from sodium and calcium silicates, includes, according to Wallace,? two series of mixed crystals with an eutectic (melting-point 1140° C.) containing 70 per cent. CaSiO3;. The curve between 0 and 70 per cent. CaSiO; shows a maximum at 58-8 CaSiO, corresponding to the formula 2Na,8i0,3CaSi0;, and a minimum at 20 per cent. CaSi0O; which does not correspond to any definite formula. The soda-lithia-silica system, according to Wallace,* forms an unbroken series of mixed crystals, with a minimum fusing-point at 40 per cent. Li,Si0,. The parts of the soda-magnesia-silica system, which contained 0-10 per cent. and 90-100 per cent. of magnesium silicate, were found by Wallace 2 to consist of mixed 1 Loe. cit., p. 469. 2 Trans. Eng. Cer. Soc., 9, 175 (1909-10). TERNARY SYSTEMS ATT crystals of sodium and magnesium silicates ; the material containing between 20 and 80 per cent. of magnesium silicate is a glass. The soda-strontia-silica system, prepared from sodium and strontium silicates, consists, according to Wallace,’ of an unbroken series of mixed crystals, those with a minimum fusing-point corresponding to a mixture containing 20 per cent. by weight of strontium silicate and 80 per cent. of sodium silicate, corresponding approxi- mately to SrSi0,5:36Na,SiOs. In the potash-lithia-silica system Wallace! found that mixtures of potassium silicate and lithium silicate containing more than 60 per cent. of lithium silicate S/O2 CaO 3Ca0Al, 0, Fig. 43.—Triane@uLar Diagram oF Limg-ALumina-Sitica System.? (LItTTLe.) produce an unbroken series of mixed crystals. Mixtures of these two silicates containing less than 50 per cent. of lithium silicate are glasses which exhibit no signs of any crystallisation. The barium oxide-soda-silica system, when prepared from barium and sodium silicates, forms, according to Wallace,! an unbroken series of mixed crystals. The portion with a minimum fusing-point contains 40 per cent. of barium silicate and corresponds approximately to BaSi0,2-64Na,Si0s. The barium oxide-lithia-silica system, obtained by fusing lithium and barium silicates, according to Wallace,! shows two series of mixed crystals and one eutectic (melting-point 880° C.) containing 78 per cent. of barium silicate (BaSiOs). The magnesia-lithia-silica system obtained by fusing lithium and magnesium silicates, according to Wallace,! shows two series of mixed crystals and one eutectic (melting-point 876° C.) containing 55 per cent. of magnesium silicate (MgSiOs). 1 Loc. cit., p. 477. 2 Textbook of Inorganic Chemistry, 4, 78 (Griffin). 478 PHYSICO-CHEMICAL REACTIONS The lithia-strontia-silica system, prepared from lithium and strontium silicates, forms two series of mixed crystals, according to Wallace, with one eutectic (melting-point 1000° C.) containing 60 per cent. of strontium silicate. In the iron oxide-magnesia-silica system the binary compounds, forsterite (2MgOSi0,) and fayalite (2FeOSiO,), form solid solutions in which each of these silicates are miscible in all proportions. BASE-ALUMINA-SILICA SYSTEMS A very important class of reactions which take place in ceramic processes includes NazO NaAl Siz 0, Na Leucite NaAl Siz 03 Albite Alz0; 105 AlzSi05 Sil/limanite. Fig. 44.—TrianeuLtar Dracram or Sopa-ALumrNa-Sitica System. (WALLACE.) those in which alumina and silica are present together with one or more bases. The abnormal behaviour of alumina in such substances has been explained on p. 474. As a result of this behaviour, the effect of fluxes on such systems depends upon various factors, including the nature and amount of base, and to some extent on the relative proportions of alumina and silica. In the latter connection it should be observed that Montgomery and Fulton ? found that the maximum effect of the fluxes is obtained with a silica : alumina ratio between 7:1 and 5: 1. The lime-alumina-silica system (fig. 43), prepared by fusing calcium meta- silicate (CaSi0;) and sillimanite (Al,Si0;), shows two eutectic points—one at 1300° C., consisting of calcium silicate and anorthite, and one at just above 1500° C., consisting of anorthite and sillimanite. The system contains one ternary compound (anorthite), 1 Loc. cit., p. 476. 2 Trans. Amer. Cer. Soc., 19, 303 (1917). TERNARY SYSTEMS 479 which is formed at about 1540° C. and corresponds to equal parts of calcium silicate and sillimanite. The mineral gehlenite (2Ca0A1,0,Si0,) does not appear in the phase diagram of this system, so that it is not, apparently, a ternary compound, but a solid solution. Owing to the abnormal behaviour of alumina in ternary systems, the ratio of the other metallic oxide (base) present is often of great importance and often seems to determine the behaviour of the alumina. This appears to particularly be the case in a ternary system composed of lime, alumina, and silica. Thus, according to G. Rigg, when a slag composed almost wholly of 2CaOSiO, dissolves 19-6 per cent. of alumina, the fusing-point rises from 1382° C. to 1453° C., but the solution of a S/O. 2 MgO Al203 Fie. 45.—TriancutarR Diagram or Maqnesia-ALUMINA-Siuica System. (SosMAN.) similar proportion of alumina by a slag consisting chiefly of CaOSiO, reduces the fusing-point from 1460° C. to 1272° C. Again, G. A. Rankin has stated that when clay and an excess of lime are heated, the product first formed corresponds to 5Ca03Al,0, and the succeeding product to 2CaOSiO,, both these compounds ab- sorbing lime as the temperature increases. He found that, at 1335° C., the fluid mass consists of 2CaOSi0,, 5CaO3Al,0,, and 3CaOAl,05, whilst at a still higher temperature these appear to dissociate and form 3CaQAl,0; and 3CaOSi0,, the latter finally dissociating into 2CaOSiO, and free CaO. Cobb’s experiments seem to indicate that the simple calcium silicates and aluminates are formed at lower temperatures and ternary compounds at higher ones. EK. Selch 1 found that at Cone 04a two molecules of lime are required to decompose one molecule of clay, whilst at Cone 9 only one molecule is required. 1 Sprechsaal, 173 (1916); 55, 1, 2 (1922). PHYSICO-CHEMICAL REACTIONS The potash-alumina-silica system, according to B. A. Rice, has three eutectics, namely :— 480 TaBLe CXXII.—Eutectics of Potash-Alumina-Silica System Percentage Composition. Molecular Composition. Approximate Temperature Potash. | Alumina. | _ Silica. K,O. Al,O3. sio,. {Si Relea tion, ° C. 1. 55-0 45-0 1-0 1-291 780 2. 17°5 82-5 1-0 " 7-430 880 3. 17-4 5:2 T7T-4 1-0 0-276 6-978 870 Two of these eutectics are devoid of alumina, but are not simple silicates; the ternary eutectic contains much more silica and much less alumina than potash felspar. The system had two areas of low fusibility, one with high alkali, low silica, and low alumina, and the other with high silica, low alkali, and low alumina. The soda-alumina-silica system (fig. 44), according to Wallace,? includes the following :— I. Compounds : (a) triple oxides—soda leucite (NaAISi,0,), nepheline (NaAISiO,). In Nature, albite (Na,OA1,0,6S10,) also occurs, but its position with reference to this system 1s not known. (b) Double oxides—sillimanite, andalusite, and kyanite (Al,Si0;). (c) Single oxides—corundum (A1l,0;), quartz, and tridymite (Si0,). II. Mixed Crystals : (i) Sillimanite with corundum. (i) Sillimanite with silica. (iii) Corundum with soda. (iv) Nepheline with corundum. (v) Nepheline with silica and sodium silicate. vi) Sodium silicate with silica. ( B. A. Rice ! has found that there are three eutectics as follows :— TaBLeE CX XIII.—Eutectics of Soda-Alumina-Silica System. Percentage Composition. Molecular Composition. Approximate Temperature Soda. | Alumina. | Silica Na,O Al,Os. SiO.. ee nei ie ion, ° C. 1 51-5 48-5 1-0 0-972 830 2 18-4 ach 81-6 1-0 4-579 860 3 17-5 5-4 tied 1-0 0-185 4-550 800 1 J. Amer. Cer. Soc., 6, 525 (1923). 2 Loc. cit., p. 476. TERNARY SYSTEMS 481 As in the corresponding potash system, two of the eutectics are devoid of alumina; the ternary eutectic has less alumina than any natural sodium alumino-silicate. The system contains two areas of low fusibility, one with high alkali, low silica, and low alumina, and the other with high silica, low alkali, and low alumina. Soda and its simpler compounds are among the most powerful fluxes which occur in ceramic materials and are the cause of much corrosion of refractory materials and the formation of fused silicates, aluminates, and alumino-silicates. Many of the members of the soda-alumina-silica system are readily fusible and in the molten state form very mobile liquids which readily penetrate the pores of firebricks, etc. Some of them absorb silica, readily forming mixed solutions, and these, when molten, are the cause of the loss of shape which usually occurs when clays, etc., are heated in the presence of soda, either as an impurity or otherwise (Chapter XIII.). In some cases, the sodium may be derived from sources which are easily over- looked, such as the small proportion of salt which occurs in some coals. This salt volatilises, decrepitates, and decomposes in the presence of moisture, and, at a temperature of about 800° C., combines with the clay or other siliceous material, forming compounds which belong to this ternary system (see Salt glaze, p. 391). In the barium-oxide-alumina-silica system the most fusible mixture, according to A. S. Watts,! is intermediate in composition between (a) 35 per cent. barium oxide, 10 per cent. alumina, and 55 per cent. of silica, and (b) 40 per cent. of barium oxide, 10 per cent. of alumina, and 50 per cent. of silica, both of which mixtures show signs of fusion at Cone 6 (1200° C.), though are not completely molten at that temperature. The magnesia-alumina-silica system (fig. 45) may, according to G. A. Rankin and H. EK. Merwin,? contain all the compounds found in the corresponding binary systems, namely, forsterite (2MgOSi0,), clincenstatite (MgOSi0,), sillimanite (Al,0,Si0,), and spinel (MgOAI,0,), besides simple oxides and also a ternary com- pound 2M¢g02A1,0,5Si0., which occurs in two forms with a variable inversion-point ranging from 925°-1150° C. N. L. Bowen? found several eutectics between clay and magnesia at different temperatures, one between periclase and forsterite melts at 1850° C. (40° C. below that of forsterite), and one between clincenstatite and silica melts at 1543° C. He did not find any eutectic between forsterite and clincenstatite. According to A. 8. Watts,4 the most fusible mixture of this system contains 20 per cent. each of magnesia and alumina and 60 per cent. of silica, correspond- ing to MgO 0:392A1,0; 2S8i0,, which fuses at Cone 12 (1350° C.), but, according to Rieke, the eutectic of the magnesia and china clay system (m.-pt. 1300° C.) consists of ten parts of clay to nine parts of magnesia, corresponding to the formula 5MgOA1,0,2810,. 1 Trans. Amer. Cer. Soc., 19, 457 (1917). 2 Amer. J. Sci., 45, 301 (1918). 8 [bid., (4), 38, 307 (1914). 4 Trans. Amer. Cer. Soc., 19, 453 (1917). 31 482 PHYSICO-CHEMICAL REACTIONS The zinc-alumina-silica system, which includes the substance produced by the action of zinc oxide upon clay, gives rise to various compounds, the chief of which are: (a) Zinc orthosilicate (willemite) (2ZnOSi0,). (6) Zinc metasilicate (ZnOSi0,). (c) Zinc spinel (ZnAl,O, gahnite). When iron oxide is present as an impurity, an isomorphous mixture of zine and ferrous silicates may be formed. Stelzner + states that a mixture corresponding to 4Fe,Si0,Zn,SiO, forms homogeneous orthorhombic crystals, whilst some natural fayalites (p. 416) contain as much as 5 per cent. of zinc oxide, and stirlingite contains 10 per cent. Zinc and ferrous silicates are, therefore, isomorphous or at least partially so. QUARTERNARY AND OTHER SYSTEMS Quarternary systems containing silica are often very complex on account of the number of compounds, solid solutions, and eutectics which are theoretically possible. Such systems may not only have the complexities of the ternary systems (including those arising from the dual behaviour of alumina), but the presence of an additional base or metallic oxide greatly increases the number of possible substances. There are further complications due to the different behaviour of systems obtained by heating four separate oxides and those resulting from the fusion of two binary systems or a ternary system with a single oxide. Thus, according to Schurecht,? excluding carbon dioxide, eutectics, and solid solutions, at least sixteen definite compounds may be formed during the burning of crude dolomite. They are produced by the reaction of five substances : lime, magnesia, ferric oxide, alumina, and silica. CaSiO, 3Ca0Al,0, CaOFe,0; 3Ca028i0, 5CaQ3Al,0, Al,0,8i0, 2CaOSi0, .Ca04Al,0, Ca0Al,0,28i0, 3Ca0Si0, 3Ca05Al,0, 2Ca0Al,0,8i0, CaOMgOSi0, 2Ca0Fe,0, 3Ca0Al,0,8i0.. Similarly, felspar (which forms part of a ternary system) produces, according to Bleininger,? eutectics with iron oxide and lime, respectively, in the proportions shown below :— 91 parts of felspar to 9 parts of iron oxide (m.-pt. 1100° C.). 97 parts of felspar to 3 parts of whiting (m.-pt. 1100° C.). Very complex eutectics are sometimes formed. Thus, according to Ferguson and Buddington,* akermanite (2CaOMgO2Si0,) and gehlenite (2CaOA1,0,Si0,) form an 1 Neues Jahrb. Min., 1, 170 (1882). * J. Amer. Cer. Soc., 4, 128 (1921). * Trans. Amer. Cer. Soc., 10, 259-263 (1908). * Amer. J. Sci., (4) 50, 131-140 (1920). QUARTERNARY SYSTEMS 483 eutectic containing 74 per cent. of akermanite and 26 per cent. of gehlenite. The crystals of this eutectic have the same character as the minerals from which they are obtained, and mixtures of akermanite and gehlenite in various proportions form an isomorphous series. Orthoclase and albite felspars form a non-continuous series of solid solutions or mixed crystals, but albite and anorthite dissolve in each other in all proportions. Felspar and mica dissolve clay, but the products do not show any eutectic composi- tion. Felspar and flint show a pronounced eutectic. Felspar dissolves clay more readily than flint. According to Mellor,! at the tem- perature of a potter’s oven, felspar will dissolve about 20 per cent. of china clay, but only about 15 per cent. of flint. On the other hand, R. C. Purdy 2 and, independently, Zoellner consider that flint is more soluble in felspar than clay. Zoellner found that at the temperature of a porcelain kiln (Cone 15) felspar will dissolve 14 per cent. of clay substance and 60-70 per cent. of quartz. Bunzli found that felspar could dissolve 70 per cent. of its weight of flint in a crystalline form, and as much as 100 per cent. of amorphous flint, but the conditions under which these great solubilities were obtained were not precisely stated. Muscovite and alumina, according to Rieke, form an eutectic consisting of 90 per cent. of mica and 10 per cent. of alumina, which fuses at Cone 12 (1375° C.). The number of mathematically possible combinations of any four substances taken in groups of two, three, or four at a time can be determined by regarding them as permutations, in which case the numbers are: 6 binary combinations, 4 ternary combinations, 1 quaternary combination, but in ceramic materials the number of original substances is not limited to four ; a much larger number may be present as (if those present in very small proportions are included) there may be ten or more metallic oxides in addition to alumina, silica, and various “ acid oxides.” With such complex mixtures and compounds capable of existence, it is almost impossible to predict exactly what will be formed in any given case, because the presence of a small amount of impurity may greatly increase the number of sub- stances which may be produced. In an attempt to simplify the subject and enable general comparisons to be made, the method of calculation by norms (p. 315) has been applied with considerable success both to igneous rocks and to ceramic materials. The weakness of ‘“‘ norm”? calculations lies in the uncertainty as to what compounds are likely to be present, especially in view of the multiplicity of compounds and solid solutions which are possibly contained in a material, and by the fact that when more than one of these or any other fluxing agent is present, their combined action is greater _than that of an equal weight of any flux when taken separately. 1 Trans. Eng. Cer. Soc., '7, 97 (1907-08). 2 Prans. Amer. Cer. Soc., 13, 479 (1911). 48 4 PHYSICO-CHEMICAL REACTIONS Great caution is necessary in applying the ideas of chemical equilibrium, the phase rule and solid solutions to ceramic materials. In few, if any, such materials is a state of true equilibrium ever reached, and comparatively small changes may, under such conditions, have remarkable results. In most clay products, fusion is far from complete, so that the data relating to various “systems” are only partially applicable and may prove misleading. The effect of Wenzel’s Jaw that “reactions proceed in proportion to the contact-area of the reacting surfaces ”’ must never be overlooked, and, consequently, the sizes of the various particles is of great importance. Consequently, the data given in the foregoing pages must be used with discretion and skill, or the consequences may be serious. FUSION The term “fusion” in connection with ceramic and allied materials is applied to any treatment (usually the application of heat) which results in the conversion of a substance from the solid to the liquid or molten state. As explained on p. 438, where small quantities of a substance are heated to the melting-point, the change from the solid to the liquid state occurs rapidly and without any rise in the temperature of the substance until the substance is completely liquefied and fusion is complete. When large quantities are heated, especially if the substance has a low thermal conductivity, or is not quite pure, signs of fusion may appear at a temperature much below that at which liquefaction is complete. If the original substance is porous, a stage may be reached in which the pores are filled with liquid produced by partial fusion of the materials ; this liquid solidifies on cooling, and the cold mass is then said to be vitrified. If the heating is continued at a suitable temperature, the whole of the material will become liquid and the mass is then said to be “ completely fused.” 1 The “ melting-, fusing-, or solidifying-point’’’ of a substance is defined as the temperature at which its solid and liquid phases are in equilibrium at atmospheric pressure, but the temperature at which a silicate melts and that at which it solidifies. may differ considerably owing to the effects of supersaturation and supercooling which are often extremely difficult to avoid. The nature of the physico-chemical reactions which take place during the fusion of one or more ceramic materials can best be learned from a careful study of the appropriate equilibrium diagrams (pp. 454-484), but the fact should be borne in mind. that there is very little evidence as to the nature of the substances which are produced in the earlier stages of fusion, as most of the information which has been obtained relates to substances which have been either partially or completely heated and afterwards cooled prior to investigation. It is certain, however, that the change of state from solid to liquid or vice versa of any substance is limited by certain critical 1 The term fused is also applied to substances which have undergone fusion even though. they have afterwards been allowed to cool and have become solid. ‘“‘ Fused quartz,” for example,. is a solid, glass-like material which owes its name to the fact that it is produced by fusing quartz.. Unless care is taken, confusion may arise from this dual use of the word “‘ fused.” FUSION A85 conditions which determine which substances may exist and in what forms. These hmiting factors which separate the solid from the liquid state 1 are :— (a) The critical temperature, above which the substance cannot exist as a solid. (b) The ecritecal presswre at which the solid phase is formed when the substance is at the critical temperature. (c) The critical density, which is the density of the solid substance when at the critical temperature and pressure. At the critical temperature the density of a solid is the same as that of its saturated vapour. (d) The critical volume, which is the volume of 1 gramme of the substance when at the critical temperature and pressure. The combined effect of these various factors is often complicated, especially as the presence of even a small proportion of impurity alters the melting-point of a substance. A change of pressure (unless it is very great) does not greatly affect the melting-point. When a clay or other ceramic material is heated very slowly, as under industrial conditions, the general order in which the changes leading to fusion occur is as follows : the smaller particles on the exterior of the mass tend to fuse first ; they are followed by gradually increasing particles on the exterior. Later the particles nearer the interior behave in a similar manner. If the mass contains several substances (e.g. if it is impure) they will melt in approximately the order of the fusibility, though reactions which take place between some of them may alter this order considerably. This is particularly the case where a ceramic “‘ body ”’ or glaze contains a considerable proportion of fluxes, added so as to form a vitrifiable or glassy material which may form either a small, medium, or large proportion of the final product according as the latter is intended to be porous, vitrified, or completely fused. In any given mixture—provided the proportion of bases to silica is sufficient—the amount of fused material produced will depend on the temperature and duration of the heating ; the higher the temperature or the longer the heating the greater will be the amount of fusion. In most ceramic materials, little glass is produced below a temperature of 1000° C. unless a considerable proportion of soda is present. The glassy matter usually begins to develop at about 1140° C., and increases fairly rapidly up to about 1250° C., the felspar fusing readily about this temperature. The partial fusion continues rapidly up to about 1300° C., the fused material dissolving the smaller particles and thus increasing the volume of the fused product. After this, the fusion proceeds more slowly, with a steady rise in temperature, because the alumino-silicates are more viscous and flow less readily than the simple silicates and so require a longer time to effect an equal amount of corrosion. — Quartz particles begin to show signs of fusion or solution on their edges at about 1280° C., the action becoming more rapid with an increase of temperature, and (on 1 In some cases, such as iodine, ammonium chloride, etc., solids may be converted directly from the solid into the gaseous state without previous liquefaction, in which case the critical point will be between the solid and gaseous states. Such substances are said to sublime. 486 PHYSICO-CHEMICAL REACTIONS prolonged heating) at 1320° C. most of the quartz is dissolved, the remaining particles being rounded as a result of their edges having been dissolved. If sufficient fluxes are present, as in a glaze, complete solution of the quartz and fusion of the whole material may occur at as low a temperature as 900° C. Felspar particles remain unchanged until a temperature of about 1100° c. is reached, but above this temperature they begin to fuse, and on prolonged heating at 1180° C. they are usually fused and completely mixed with the glassy product, not- withstanding the fact that the fusion-point of felspar, when heated alone, is above this temperature, pure orthoclase melting at about 1200° C. and commercial felspar in comparatively large pieces melting at about 1280° C. In a mixture of hard felspar, clay, and flint, such as is used in the manufacture of porcelain, the felspar begins to exercise a noticeable solvent action at about 1280° C., attacking first the clay particles and then the flint. At about 1300° C., sillimanite begins to crystallise out, and at about 1380° C. the felspar is completely fused. According to A. 8. Watts,1 the same amount of fusion or solution occurs with soda felspar at 1300° C. as occurs at 1340° C. with potash felspar. It is impossible to devise simple formule which will indicate the rate of fusion in terms of time and temperature, because it also depends very largely on the size of the grains of material, on the surface exposed to chemical action, and on the viscosity of fused products; the last-named depends on the nature of these products, some substances producing much more viscous fluids than others. Thus, magnesia pro- duces a very viscous slag or glass, and, consequently, the range of temperature through which fusion occurs (sometimes termed the vitrification range) is much longer where magnesia is the chief flux present than when lime is present in considerable proportion. Thus, articles containing magnesia as a flux are less likely to soften and lose their shape than those in which lime is used. The rate of fusion also depends on the solubility of the less fusible materials in the liquid portion. This, in turn, depends partly on their mutual reactability, a fusible mobile silicate rich in bases having a much greater solvent action than one—such as felspar—of a more neutral character. The action of felspar is very slow, so that it does not cause the.ware to lose shape very quickly as is the case when calcium silicate is the chief constituent of the fused portion of the material. The constitution of fused masses is by no means fully understood and there is much difference of opinion regarding the chemical constitution of completely fused salts. Some investigators maintain that such masses are composed of definitely molecular compounds, whilst others regard them as dissociated into free oxides. According to Sosman,? the regularity of the curve of the relation between the specific volume and the molecular composition of molten substances does not favour the existence of compounds in the molten mass, and the idea now generally favoured regards completely fused materials as similar to some extent to dilute solutions which are, to a varying extent, dissociated into ions. This view is supported by the experiments of Barus and Iddings, from which it appears that silicates are more strongly dissociated than basic materials, which would account for the fact, pointed 1 Trans. Amer. Cer. Soc., 11, 185 (1909). * Loc. cit., p. 469. SOLIDIFICATION OF MOLTEN MASSES 487 out by Doelter, that basic minerals separate more easily from slags than do silicates. On the other hand, pure silica, when fused, is not appreciably dissociated. If it is correct that some fused masses are in a dissociated condition, the effect of a common ion should be similar to its effect in aqueous solutions. This “common ion theory ” has been used to explain the behaviour of eutectics (p. 461). It is very probable that some fused substances which produce mixed crystals on cooling do not contain such compounds when in the molten state. It is much more likely that the separate components of the mixed crystals existed independently in the molten mass. For instance, fayalite may exist as forsterite and iron olivine, whilst labradorite may be in the form of separate molecules of albite and anorthite. The Solidification of Molten Masses.—When a molten mass cools it eventually becomes solid, but the physical and chemical nature of the solid produced may vary greatly, according to the nature of the materials present and the rate of cooling. The passage from the liquid to the solid state consists essentially in the limitation of movement of the molecules. This freedom is greatest at a high temperature, and if the temperature is reduced sufficiently the molecules must be arrested and the substance then enters the solid state. If the arrest is instantaneous, the molecules may be unable to arrange themselves according to their polarity, and the solid mass then consists of a heterogeneous arrangement of molecules. Thus, the fully cooled solid may consist of : (a4) a homogeneous glass, (b) a wholly crystalline mass, or (c) a heterogeneous mixture of both crystals and glass. On cooling sufficiently, a homogeneous liquid acquires, in general, the property of crystallising, but this requires both time and freedom of movement.! If either or both of these factors are lacking— as when the liquid is cooled too quickly and too intensely—it may solidify without crystallising and is then said to be “‘ undercooled.”’ Such undercooled liquids may form relatively stable solids, which are then termed amorphous (in opposition to crystalline) solids. An amorphous substance has the external appearance of a solid with great viscosity and rigidity, but it differs from a crystal in its complete isotropy and in the absence of a definite melting-point; on heating it passes gradually into a liquid state and its properties change steadily with a rise in temperature. Quartz-glass, glasses (silicates), and many metallic oxides are typical examples of amorphous substances. Substances in the vitreous state are in a state of elastic strain, but their elasticity is restored by heating to a temperature within the range of crystallisation. In the vitreous state, energy is stored in the molecules as in a collection of coiled springs ; it becomes kinetic when the molecules are set free by the action of heat upon the aggregate. The energy then supplies heat to the system which would, otherwise, have to be drawn from an outside source. The stability of a vitreous solid depends on the temperature. If the solid is heated it may liquefy, or the heat may impart just sufficient freedom to the 1 The degree of freedom necessary for crystallisation may occur at a temperature much below that of fusion. Alternatively, the vitreous state may be produced by a gradual increase in the viscosity of the liquid phase. 488 PHYSICO-CHEMICAL REACTIONS constituent molecules for their polarity to come into play so that the mass crystallises. The stability of a crystalline solid persists from the transition temperature to the fusion-point. Liebisch ! has shown that the regular heating of an amorphous mass usually causes it to crystallise, e.g. gadolinite, devitrifiable glasses, etc., but, in some cases, the presence of a crystal of the same or an amorphous substance appears to be necessary for the complete crystallisation of a liquid. The physical conditions which influence crystallisation include the following :— (a) The temperature attained during heating and the duration of the heating. Thus, silica glass tends to crystallise if it is maintained at a temperature between 1200° and 1600° C. for a long period, the amount of devitrification depending on the duration of the heating. R. Sosman has found devitrification to take place when silica glass is heated between 200° and 275° C. for a long time. This is known as the a-f8 transition range. A. A. Klein? found that the microstructure of porcelains depends more on the attainment of a high temperature than on prolonged heating at a lower temperature. Thus, porcelains heated to Cones 13-14 and maintained at that temperature for 12-108 hours consisted chiefly of amorphous sillimanite with a few crystals of silli- manite, but a porcelain heated to Cone 15 and maintained at that temperature for only one hour consisted chiefly of sillimanite crystals with practically no amorphous silimanite. It is unusual, however, to find so great an increase in the velocity of crystallisation to occur with so small an increase of temperature. (b) The viscosity of the molten fluid, as excessive viscosity hinders the crystallisation of fused glasses, etc., by hindering the motion of the particles. Substances like ee and borates with a high velocity and a high crystallising temperature are easily cooled at a rapid rate through the whole range of crystallisation and thus, most naturally, pass into the vitreous state on cooling. Tamman 8 has shown that the viscosity of molten substances increases during the cooling, the corresponding graph being in the form of a parabolic curve. Doelter suggests that the feeble mobility of the ions in a fluid having a high viscosity when at the freezing-point hinders the production of ionic equilibrium and, consequently, hinders crystallisation, the substance remaining dissociated. This agrees with Roozeboom’s theory that in substances with a sharp melting-point the ionisation equilibrium is readily established. The viscosity also affects the size of the crystals, the most viscous fluids producing the finest-grained masses. Conversely, to produce large crystals a very mobile fluid must be employed. (c) The rate of cooling influences both the amount of crystallisation and the com- position of the crystals. Slow cooling favours crystallisation, whilst rapid cooling retards it. If the cooling is very rapid no crystals may be formed at all, the whole mass then consisting of a homogeneous glass. Hence, where crystallisation is to be avoided, rapid cooling through the crystallising range (p. 489) is essential, but where 1 Berl. Akad. Ber., 1, 350 (1910). * J. Amer. Cer. Soc., 3, 978 (1920). 3 Sprechsaal, 35 (1904). COOLING CURVES 489 a crystalline mass is desired, slow cooling is necessary. Some glasses and glazes have a great tendency to crystallise if allowed to cool very slowly between 860° and 790° C., but the precise range varies according to the constituents of the glass or glaze. When a fused mass, on cooling, forms mixed crystals, the composition of the crystals varies according to the rate of cooling. Thus, if the cooling is completed before the system has reached a state of equilibrium, the composition of the crystals will be different from what would be the case if the cooling had been much slower. These variations in composition are due to the fact that the solidification of mixed crystals takes place through a range of temperatures and not at a fixed point, the composition of the solid and liquid por- tions changing progressively, provided the rate of heating or cooling is suffi- ciently slow to permit it todo so. It is probable that the dual materials present in mixed crystals are combined during the cooling and that in the fused mass they exist as separate substances. A study of the time-temperature curves of a fused substance during slow cooling is very interesting in connection with the phenomena of crystallisation. Thus, if a thermo- couple is inserted in the fused mass and the temperature of the latter is noted at regular intervals of time, it Time. will be found that if the cold product Fig. 46.—TiImE-TEMPERATURE CURVES OF CRYSTALS : d AND GLASS, is a homogeneous glass, there will be no critical point, the cooling curve being perfectly regular (fig. 46 a). At the temperature when crystallisation occurs, however, there is a definite kink in the cooling curve, as shown in fig. 46 6. The “flat”? in the second curve indi- cates that the temperature ceases to fall until all the matter which can crystallise at that temperature has done so, after which the fall in temperature continues. Precisely the same curve is produced when the crystalline substance is melted (see Tiume-Temperature Curves, p. 464). When several substances, capable of crystallising at different temperatures, are present, a series of “steps”’ is produced in the cooling curve. (d) The number of centres of crystallisation formed in a definite area per second. According to Tamman, the presence of a mineraliser increases the crystallisation capacity, but does not affect the crystallisation velocity. In the complete absence of “‘ dust ” or of any substance—preferably a minute crystal of the substance in the fused material or of a compound isomorphous with it—crystallisation may be delayed indefinitely, but will occur rapidly in the presence of one of these “‘ mineralisers ”’ or “ nuclei.” Temperature, 490 PHYSICO-CHEMICAL REACTIONS OTHER CHEMICAL REACTIONS OCCURRING AT HiagH TEMPERATURES Among other important chemical reactions which occur between or in connection with ceramic materials at high temperatures are those best grouped under the four captions: (a) decomposition, (b) oxidation, (c) reduction, and (d) corrosion. Some of these reactions have already been mentioned, but they are so important in connection with the group-terms used to distinguish them, that a brief mention of these is desirable. DECOMPOSITION Decomposition is the splitting up of compounds into simpler substances (p. 436). It may be of two kinds : (a) Dissociation, in which the substance is divided into two or more substances, no others taking part in the reaction. (b) Decomposition by reaction with other substances (see Double Decomposition, p. 486). A typical example of dissociation is observed when clay is heated and the water which is chemically combined with it is evolved as a result of the decomposition of the material. It is probable that the decomposition begins at a comparatively low temperature, probably 200°-250° C., but the water does not escape in large propor- tions until a temperature of about 450° C. is reached. For this reason, it is often considered that the decomposition of clay with evolution of water commences at about 450° C. Bauxites are also decomposed at 600° C. with the loss of their combined water, but as the reaction is very slow, except at high temperatures, the calcination of bauxite is generally carried out at temperatures above 1200° C. Various carbonates, such as calcite, magnesite, and dolomite, are decomposed by heating (burning) them in kilns so as to produce the corresponding oxides, which are highly refractory. The action in each case is typically represented by the following equations :— CaCO,(-+-heat)==Ca0+CO, CaMg(CO,).(-+-heat)==Ca0 +Mg0+2C0,. The evolution of carbon dioxide from calcium carbonate commences between 600° C. and 725° C. and continues up to 850°-900° C. When carbon dioxide is driven off from a carbonate the residual oxide rapidly combines with any other suitable material if present. Alternatively, the presence of such a material (e.g. an acid, silica, or clay) may cause the carbon dioxide to be evolved at a lower temperature than when the carbonate is heated alone. This is due to the mutual affinity of the oxide (lime) and the acid being greater than that of the oxide (lime) and carbon dioxide. Magnesium carbonate is decomposed at 800°-900° C. with the evolution of carbon dioxide, the residue being caustic magnesium oxide or magnesia. At higher tem- peratures, the magnesia is crystallised and forms periclase. Dolomite is decomposed in two stages when heated, the carbon dioxide in the DECOMPOSITION ;s OXIDATION 491 magnesium carbonate being evolved first and that in the calcium carbonate at a higher temperature. Some impurities may be decomposed and volatilised during the heating ; thus, sulphates are decomposed, forming sulphur dioxide or sulphur trioxide ; carbonates present may be decomposed as previously described ; organic matter may be oxidised, forming water and carbon dioxide; various iron compounds may either be dis- sociated or they may react with clay, etc., and so be decomposed. The decomposition of sulphates takes place at a higher temperature than that of carbonates, usually about 800°-1000° C., evolving sulphur trioxide in an oxidising atmosphere. If an excess of silica is present, the dissociation is replaced by a double decomposition which occurs at a slightly lower temperature and silicates are formed. Unintentional, and sometimes undesirable, dissociations and decompositions some- times take place when clays and other ceramic materials are heated to too high a temperature in use. Thus, carborundum when heated to about 1250° C., decomposes slowly, the silica being volatilised and the carbon burning to carbon dioxide and escaping. This decomposition takes place without any general fusion of the mass. _ If the carborundum is not very pure or the atmosphere in which it is heated is dusty (as is usually the case with flue- or kiln-gases), a siliceous glaze sometimes forms on the surface of the carborundum and prevents, or at any rate hinders, the decom- position of the mass under ordinary conditions of use. Other carbides and carboxides are similarly decomposed when heated to too high a temperature, but siloxicon is more easily oxidised than carborundum, though if heated in a neutral or reducing atmosphere it is not affected until it reaches 1840° C., when it commences to decom- pose, forming carborundum, free silica, and carbon monoxide. The interaction of furnace linings, walls of retorts, crucibles, and other refractory articles with their contents is typical of a series of undesirable decompositions which are of great technical importance. They are described under the caption Corrosion on pp. 494-503. OXIDATION Oxidation plays an important part in the various reactions involved in the pro- duction of ceramic articles. The chief technical objects of oxidation are : (a) The conversion of impurities by weathering or during the burning process (t.e. carbon, sulphur, etc.) into a form which can easily be removed, as by their con- version into gases or soluble substances. (6) The oxidation of iron compounds either to render them more refractory or for the production of a desirable colour, as in the manufacture of red. bricks, terra- cotta, etc. The removal of carbonaceous matter is most readily effected by oxidation, i.e. by heating it in a suitable atmosphere in which it combines with oxygen, forming carbon dioxide and water, both of which escape as gases. The decomposition occurs most rapidly between 800° and 900° C., though it commences at a much lower tem- perature. In many cases, it is a matter of great technical importance that all the A92 PHYSICO-CHEMICAL REACTIONS carbonaceous matter should be oxidised in the early stages of the burning process, as otherwise it will be impossible to oxidise the iron compounds. If the ceramic material is heated too rapidly superficial fusion may occur, sealing the pores, preventing all access of air to the interior, and so making it impossible to burn off all the carbon (p. 423). If, on the contrary, the temperature rises slowly and is not allowed to exceed about 900° C., the removal of the carbonaceous matter by oxidation is comparatively simple. Sulphides are decomposed when heated, if the atmosphere is oxidising, sulphates or sulphur dioxide and a sulphate being formed. Pyrites (FeS,) loses 60-75 per cent. of its sulphur at a temperature below 800° C. and the remainder above this tem- perature. The latter is removed very slowly, the evolution decreasing in rapidity as the temperature rises; usually some combination occurs and a corresponding proportion of sulphur is never expelled. Ferrous compounds are oxidised to the corresponding ferric compounds or to red ferric oxide when they are heated in a current of air. A temperature of 900° C. is usually required for rapid oxidation, but if the temperature is very high the red ferric oxide (Fe,0,) may be decomposed and magnetic iron oxide (Fe,0,) produced. A rather peculiar case of oxidation of metallic iron sometimes occurs in furnaces, because in the presence of free iron or metals of the iron group, carbon dioxide dis- sociates into carbon, carbon monoxide, and oxygen, the carbon being deposited on the bricks and some of the free iron is oxidised to FeO. The action appears to occur at different temperatures, according to the pressure, as shown in Table CXXIV (due to Harvard). TaBLE CXXIV.—Ouxrdation of Ferrous Oxide Pressure, mm. Temperature, ° C. Pressure, mm. Temperature, ° C. 15 500 305 700 3D 550 535 750 70 600 800 800 145 650 In most commercial furnaces the action does not take place below 690° C. It may be observed that the oxidation of the impurities in clays does not usually take place until a temperature of 800° C. or above is reached, though peaty matter and limonite are decomposed much below this temperature, and Purdy and Moore + have pointed out that the oxidation of the impurities in some clays occurs before the clay is completely dehydrated. REDUCTION Reduction plays both a useful and a harmful part in the chemical reactions connected with ceramic processes. The chief reducing agents used in connection 1 Trans. Amer. Cer. Soc., 9, 204 (1907). REDUCTION 493 with ceramic materials are carbon monoxide and hydrogen (generally produced by burning coal with an insufficient amount of air), but finely divided carbon and several metals have a powerful reducing action when at a sufficiently high temperature. The presence of sulphur dioxide in the kiln-gases, as occurs when a sulphurous coal is used, has a reducing effect upon any iron oxide present and may produce ferrous compounds. Sulphur trioxide, if present in the kiln-gases, may attack the bases present in the clay and form sulphates. If an oxidising atmosphere is maintained these sulphates will persist and may produce a scum on the goods, but if reducing conditions arise they will be decomposed again. Silica is reduced by carbon at temperatures above 1200° C., forming the element silicon and carbon monoxide. The reaction is more rapid in the presence of iron and some other metallic substances. In some cases, silicon carbide may be produced (below). Zirconia, when heated to very high temperatures, tends to be reduced by its combination with carbon, forming zirconium carbide. As an instance of a desirable reducing action may be mentioned the production of a considerable amount of fluxing material from the iron compounds present, by the formation of fusible silicates as in the manufacture of blue bricks, in order to produce a vitrified mass of requisite strength and to produce the blue colour which is characteristic of ferrous silicates. The slightly bluish sheen of porcelain is due to the same cause. Some colours used in glazes require to be fired in a reducing atmosphere in order to produce the desired effect. Reduction processes are sometimes used in the manufacture of refractory materials, such as silicon carbide, by the reduction of silica :— Si0,+3C0=Si0-++2C0. The reaction begins, according to Lampen, and also to Tucker, Gillet, and Saunders, at a temperature of about 1820°-1920° C.; other carbides and carboxides are pre- pared in a similar manner. A reducing action may be harmful when it is produced accidentally, as it may result in the decomposition of a useful material and the production of undesirable fusible compounds. Thus, if fully oxidised iron compounds, such as ferric oxide, are reduced to the ferrous state, in contact with clay or silica, they immediately lower the refractoriness of the bodies in which they occur, because ferrous silicate is readily fusible and may, therefore, cause much trouble and loss. Carbon has a stronger affinity for oxygen than iron, and, consequently, no oxidation of the iron compounds can occur in the presence of carbonaceous matter. It is, therefore, necessary to use a highly oxidising atmosphere in the earlier stages of firing clays, etc., containing carbonaceous matter, in order to oxidise and remove the carbon. At a later stage, when all the carbonaceous matter has been burned away, some or all of the iron com- pounds may be reduced to the ferrous state without damaging the ware, but great care is needed in the control of the furnace or kiln, as in a reducing atmosphere at high temperatures the ferrous compounds will attack the silica present and form slag-like masses of fusible iron silicate. Carbon has a greater affinity for oxygen than sulphur; consequently, it may retard ° 494 PHYSICO-CHEMICAL REACTIONS the oxidation of the latter, and, conversely, sulphates may, if required, be converted into sulphides by heating them in an atmosphere containing carbon monoxide or hydrogen at temperatures above 700° C. Reduction therefore forms a useful means of preventing the formation of ‘‘ scum ”’ caused by soluble sulphates. CoRROSION Corrosion is a term which is applied to the result of a large number of chemical reactions of an undesired character. These reactions are the same as the desirable ones, but they occur at an unsuitable time. Thus, the action of iron oxide on magnesia or of lime on silica may be useful for the production of a suitable bond, but when these substances attack refractory materials whilst the latter are in use their effect is undesirable and may be included under the term “ corrosion.” The factors which influence corrosion are described on pp. 437-449. In addition to these, the situation of the material is often very important as regards the action of corrosive substances. Thus, the action of a flux is greater if it is in contact with the cooler side of the material than if it is in contact with the hottest surface. Thus, if the corrosive substance is gradually cooled as it penetrates the material, its action will be correspondingly lessened, but if the temperature increases as the corrosive substance penetrates the action will be intensified. In some cases, the continued action of a corroding agent may, in time, prevent or hinder further action taking place. Thus, when calcium compounds attack fireclay a calcium alumino-silicate is first formed which is high in calcium and has a low fusing-point. As more clay is dissolved, the material formed becomes richer in alumina and silica and at the same time more refractory, so that in time a protective coating is formed and further corrosion is prevented or at least proceeds very slowly. There are three chief groups of substances which have a corrosive action on ceramic . materials : (a) Free bases which combine directly with clay, silica, etc. (6) Silica, clay, and other materials which react like acids with basic refractory materials such as magnesite, etc. (c) Salts which, on heating, are decomposed, liberating a base and an acid, the base uniting with the fireclay, silica, or analogous refractory material and producing a fusible compound. To this class belong substances such as calcium sulphate, sodium chloride, etc. Thus, the soluble salts present in some coals used for making gas or coke cause much trouble by corroding the material of which the retort or coke oven is made. Sodium chloride is usually the most important of these salts, though others may also be present. (d) Direct-acting salts which act like free bases, though to a less degree. Felspar, mica, Cornish stone, etc., are of this class. The chief corrosive agents to which clay, silica, and analogous materials are likely to be subjected are molten metals, slags, such as those produced in open- hearth, reverberatory, converting, and cupelling furnaces, consisting chiefly of basic, subsilicates or sesquisilicates (such as the slags produced in assaying), CORROSION 495 and metallic oxides.1 The action of volatilised substances may also be serious in some Cases. Lime should not be brought into contact with clay or silica at high temperatures or partial fusion may take place. Whiting (CaCO,) attacks fireclay bricks in the same manner as lime, and so does Portland cement; the last-named, according to Hirsch, forms a calcium alumino-silicate, corresponding to the formula 2-5CaOAl,0,38i0,. The action is not so intense as that of lime or calcium carbonate. Iron compounds form ferrous or ferric alumino-silicates, the former being more fusible and, therefore, more corrosive than the ferric compounds. Iron oxide corrodes fireclay bricks more severely than silica bricks. Sulphides are also very corrosive and rapidly penetrate clay products and silica. Iron sulphide combines with silica to form fayalite (FeOSiO,) and sulphur dioxide which escapes as a gas. Any sulphur which may be present in the fuel used in furnaces may exert a corrosive action upon the brickwork at high temperatures, the sulphur being converted first into sulphur dioxide and then into sulphuric acid which attacks the clay. The presence of steam often facilitates the corrosive action of other substances ; thus, its presence is necessary for the decomposition of some of the soluble salts in coal, and, for this reason, the preliminary drying of a coal may reduce the amount of corrosion of any firebricks with which it may be heated. Fireclay bricks which are high in alumina are usually less easily corroded by fluxes, etc., than those rich in silica, but the amount or rate of corrosion is not always proportional to the total amount of alumina present. The solution of alumina by a slag containing a metasilicate may increase the fluidity of the slag and, therefore, increase its solvent action on bricks. With silicates richer in flux than metasilicates, however, the solution of alumina decreases the viscosity and increases the refractori- ness of the bricks. Howe, Phelps, and Ferguson * have found that the resistance of some fireclay bricks to corrosion by slag is increased by a high alumina-content, but some coal ashes (clinker) and some metallurgical slags attack highly aluminous bricks more readily than highly siliceous bricks. Table CX XV shows the refractoriness of various mixtures of slag and refractory material, obtained in the slag-test devised by R. M. Howe, S. M. Phelps, and R. F. Ferguson (p. 503), in which the lowering of the refractoriness of a powdered mixture of slag and refractory material is regarded as the index of the intensity of the reaction. Metallic vapours sometimes cause the destruction of firebricks. Thus, when vapours of metallic zinc penetrate the bricks of the blast furnaces in which the ores are smelted, they are oxidised by the CO, present and are solidified in the pores. This may cause disintegration if the bricks are not very strong. A similar corrosion occurs when zinc is smelted in retorts. J. W. Cobb considers that the penetration of vapours of sodium chloride into fireclay retorts may cause the formation of ferrous chloride which will, in turn, volatilise and act as a corrosive agent. 1 Very few crucibles will withstand prolonged heating when containing a mixture of red lead and copper oxide. 2 J. Amer. Cer. Soc., 6, 589 (1923). 496 PHYSICO-CHEMICAL REACTIONS TaBLe CXXV.—Refractoriness of Mixtures of Slags and Fireclay Bricks (The Results are expressed in Seger Cones) Percentage of Slag in Mixture. peat! 0. |. 44.8) 42 |.36. | 20. | 980.0) oars Acid open-hearth . | O24 o2-7 32 | 30 | 28 1° 26) oe 8 Blast furnace . : 382-4 30-1 29) 2071 17 2138 |) oe 9 Basic open-hearth . oats) pote Coa LO eT ale 9 8 5 Heating furnace. | 32) ol) 300%" 26.) 19 [71s ap 9 8 Coal ash (clinker) . . | 82 | 82.) 32°) 381 1 31°) 31. | 30 O ae Flue-dust has a complex action upon firebricks producing a fusible mixture of various silicates and alumino-silicates, according to the composition of the dust. Mellor and Emery ! determined the action of various corrosive flue-dusts upon clays and siliceous refractory materials with the following results :—- TaBLE CXXVI.—Corrosive Action of Various Flue-Dusts Firebrick. Silica Bricks. ste 2 inee Boiler flue-dust high in lime and ferric oxide . : dP mC Ferruginous boiler flue- dust : sP sC - Ochreous dust from retort brick. kP sC dP gC Basic slag ; dP sC dP mC Ferruginous dust ote in lime kP mC dP mC Bull dog . : ‘ a dP sC = Hematite (reducing ciate : = dP sC sP sC Tap cinder. ; dP mC dP mC dP sC Lime . : ' : : sP mC sP mC Lime and salt . sP mC sP mC Salt : : dP sC sP mC Sodium leks sP sC sP mC Salt and felspar ; sP sC sP mC Soda lime-glass : : sP sC sP mC P= penetration. k=complete. s=slight. m=medium. d=deep. C=corrosion. gC=great corrosion or slagging. 1 Trans. Amer. Cer. Soc., 18, 230 (1918-19). SILICA BRICKS 497 Mellor and Emery found that the oxides of copper, zinc, and iron in a finely divided condition have a very high penetrative power under reducing conditions at very low temperatures. Copper oxide will penetrate at 600° C. and iron at 1100° C. They found tap cinder to be the most corrosive dust amongst those they examined, though the dust from the slag chamber of a steel furnace is extremely corrosive. Carbon monoxide and some hydrocarbon gases are decomposed by hot fireclay with the deposition of carbon in the pores. The latter may rupture the bricks. Blasberg has stated that the presence of less than 4 per cent. of carbon in a fireclay brick may cause its disintegration. When no disintegration occurs, a superficial deposition of carbon on refractory articles is sometimes an advantage, as it may protect them and prevent wear and corrosion. Silica bricks have a more intense acid reaction at high temperatures than fireclay bricks, and consequently, if both kinds of bricks have a similar texture, the silica bricks are more rapidly attacked by basic fluxes such as slags, metallic ores, ashes, etc. As silica bricks are usually composed of coarser particles than clay bricks, the former are often the more resistant. Silica in silica bricks will combine with a greater proportion of lime than will an equal weight of fireclay. Silica bricks may increase slightly in refractoriness on prolonged heating, because the calcium silicates and alumino-silicates, formed at first, melt at a comparatively low temperature (about 1170° C.), but on prolonged heating they combine with more silica, and their fusion- point is raised proportionately. Silica bricks are less affected by iron oxide than are fireclay bricks when in an oxidising atmosphere, but they are rapidly attacked in a reducing atmosphere, because the ferrous oxide formed combines with the silica, forming fayalite (2FeOSiO,). The action of slags upon silica bricks is greatest with those containing the highest proportion of alumina, so that silica bricks should be as low in alumina as possible. According to the slag test devised by Howe, Phelps, and Ferguson (p. 503), silica bricks are highly resistant to acid open-hearth and heating-furnace slags which are chiefly composed of silica and iron oxide, but such bricks are not resistant to basic open-hearth and blast-furnace slags which are rich in lime. Table CXXVII shows results obtained by these investigators on the refractoriness of powdered mixtures of silica brick with different slags in various proportions. Taste CXXVII.—Hffect of Slags on Silica Bricks (The Results are expressed in Seger Cones) Percentage of Slag. Type of Slag. Acid open-hearth . cok OL OP aL Buk al ak ee etn nae nt aesan) ats Blast furnace . ; ieee |POb aM oko) OL feu rato Pe 6 3 Basic open-hearth . Welt ol Nook 7 plein. trou hwto nko 4 Heating furnace. wh OL” {28001 29-428 28 4 28 280 2r 26 498 PHYSICO-CHEMICAL REACTIONS Mortars and cements for silica bricks should be as low in alumina as possible, as highly aluminous cements react strongly with silica bricks. Mellor and Emery ! found the siliceous materials were affected by various fluxes in the manner shown in Table CXXVI. The bond of silica bricks is most easily corroded, so that a brick with interlocking grains and very little cement is most resistant to penetration and corrosion. In coarse-grained silica bricks, only the bond is attacked, but in fine-grained bricks both the bond and the silica grains are attacked. Silica glass is attacked by various metallic oxides, such as copper oxide, lead oxide, and others, and also by many metals, including calcium, magnesium, aluminium, nickel, sodium, potassium, and cerium. These metals reduce silica at red heat to silicon. At 900°-1000° C., nickel-chrome wire exerts a strong corrosive action upon silica glass. Silica glass is not so rapidly attacked by lime as finely ground silica, the action not being very great below 1400° C. Below this temperature, according to Hedvall, calcium monosilicate is formed, but above it a bisilicate is produced, the temperature of formation of the latter being 1390°-1420° C. with amorphous silica, and 1412°-1444° C. with crystalline silica. Silica is volatilised in the presence of carbon on account of the latter reducing the silica to silicon, which is volatilised slowly at 1750° C. and more rapidly at higher temperatures. The volatilised element is reoxidised and re-forms silica. The British Thomson-Houston Co. found that the best conditions for volatilisation were obtained with a mixture consisting of 4 parts of silica, 2 parts of carbon, and 4 parts of fireclay, with or without 1 part of manganese oxide. In an oxidising atmosphere in the absence of carbon no volatilisation occurs. Magnesia bricks are basic in character, yet they may be heated in contact with silica without much interaction occurring provided the bricks are sufficiently hard and dense and the temperature is not too high. For this reason, silica and magnesia bricks are sometimes used in the same furnace without any intermediate zone, though, as a matter of precaution, it is generally desirable to use an intermediate zone of neutral bricks. Fireclay and magnesia bricks begin to interact seriously, according to Bischof, at about 1600° C., but M‘Dowell and Howe found them to interact at about 1530° C. if lime is present to the extent of 3 per cent. Silica and magnesia bricks react strongly at 1610° C. Iron oxide alone has little effect upon magnesia, but in the presence of phosphorus much corrosion occurs. When subjected to the slag-test devised by Howe, Phelps, and Ferguson (p. 503), magnesia bricks show a high resistance to basic open-hearth slag. Table CXXVIII shows results obtained by these investigators on the refractoriness of various powdered mixtures of slag and magnesia brick. Carbon rapidly attacks magnesia bricks, and many carbides, including those of nickel and chromium, are even more destructive, the magnesia being practically reduced to magnesium. The action begins, according to Northrup, about 1450° C. 1 Loc. cit., p. 496. BAUXITE BRICKS 499 and increases rapidly as the temperature rises ; M‘Dowell and Howe found a loss of 27 per cent. at 2200° C. The action is intensified by the presence of lime. When magnesia bricks are heated in contact with carbon electrodes in an electric furnace they are greatly corroded ; in some cases, half the face of the bricks is removed. Taste CXXVIII.—Effect of Slags on Magnesia Bricks (The Results are expressed in Seger Cones) Percentage of Slag. Type of Slag. 16. 20. 30. 40. 50. Acid open-hearth 36 33 32 31 27 Blast furnace. ' 36 33 26 17 14 Magnesia bricks are also rapidly disintegrated by the action of steam, especially when such bricks are laid in a wet cement and then heated so as to dry them rapidly. Bauxite bricks are generally regarded as basic, but they are probably almost neutral and do not behave as either truly acid or truly basic materials. Bauxite bricks are not appreciably affected by basic slags, but they are much more strongly attacked by lime. If the surface of the bricks is dense, however, the rate of corrosion is very much reduced. Table CX XIX, due to Howe, Phelps, and Ferguson (p. 503), shows the re- fractoriness of various powdered mixtures of slag and diaspore brick, and that bricks made of diaspore are very resistant to slags. TaBLE CXXIX.—Hffect of Slags on Diaspore Bricks (The Results are expressed in Seger Cones) Percentage of Slag. Type of Slag. 0. 4, 8. 12. 16. 20. 30. 40. 50. Acid open-hearth . « b 36 Bb.) B81 B35) 83.) 80126.) 2071 138 Blast furnace . mypoo8 4 Bo-ce 3251.29.49 20 1d wie dad s)he LO Basic open-hearth . vue BO. A530) de O2Ed DON 14. welde) GL) lO a0 Heating furnace. oo O08 O4 OSA hor ao) 20 elo eM. 9 500 PHYSICO-CHEMICAL REACTIONS The resistance of bauxite bricks to the action of slags is probably due in some measure to the action of alumina in increasing the viscosity of any molten mass or slag containing it, and therefore rendering such slag less active than would otherwise be the case. Zirconia bricks are very resistant to the action of slags, both acid and basic, and consequently are valuable as a refractory material. According to Mineral Foote- Notes, zirconia bricks withstand the corrosive action of (a) acid slags containing manganous and ferrous oxides, such as bessemer converter slags, puddling furnace slags; (b) glass of various kinds ; (c) cobalt nickel speiss ; (d) ladle slags which are not basic. The following substances attack zirconia moderately : (a) basic ladle slags, (b) iron oxide, (c) copper oxide, (d) portland cement, (e) litharge. The following substances attack it rapidly: (a) iron sulphide, (b) sodium carbonate, (c) sodium hydroxide, (d) fluorspar or cryolite. Carbon, chromite, and carboxide bricks are very inert to chemical action, and are highly resistant to slags and fluxing agents. Table CX XX shows the effect of various proportions of different slags on the refractoriness of chromite bricks as determined by Howe, Phelps, and Ferguson. TaBLE CXXX.—EHffect of Slags on Chromite Bricks (The Results are expressed in Seger Cones) Percentage of Slag. Type of Slag. 4 8 12 16 20. 30 40 50 Acid open-hearth SS PBS 33 31 30 20 15 13 12 Blast furnace. : A ts 35 34 32 16 9 8 Heating furnace . ne Fi $3 36 35 36 36 Carborundum is attacked slowly at a bright-red heat by sodium carbonate, caustic soda, and sodium peroxide, whilst red lead attacks it rapidly. Carborundum bricks are not attacked by either silica or molten iron separately, but when together a corrosive action takes place. The bricks are attacked by slags to a greater extent in an oxidising atmosphere than under reducing conditions. Measurement of Corrosion.—The American Society for Testing Materials has suggested that the resistance of refractory materials to corrosion may be tested by drilling a hole 24 inches diameter and 4 inch deep into the sample, which is then heated to a temperature of 1350° C. in not less than five hours, and 35 grams of powdered slag are introduced, the temperature being maintained for two hours. The furnace is then cooled and the brick cut vertically so as to show the depth of the penetration. Another method for a slagging test on refractory materials adopted by the MEASUREMENT OF CORROSION BY SLAGS 501 American Society for Testing Materials,! suggests that 35 grams of slag should be ground to 30-mesh and placed in a refractory ring in contact with the brick, which is heated to 1350° C. and maintained at that temperature for two hours. The brick is then cooled, cut, and the area of penetration measured. The slag suggested has the following analysis :— Per cent. Silica : : ; . 19-00 Alumina . , : . 12:89 Ferric oxide. : . 15°73 Lime : : ; abo OO Magnesia ay 0-93 Manganese oxide. so) gobo Soda 3 - : TSO The melting-point of the slag is about 1270° C. The corrosion test suggested by the committee of the English Ceramic Society 2 is very similar to the last-mentioned method. G. H. Brown ® objects to the customary method of placing the slag in a depression in a firebrick or in a cell cemented on to the brick, and prefers to place the bricks on end in fireclay boxes each 9 inches x8 inches x3 inches internally, and burned at Cone 12 before use. The bricks are packed on two sides with finely ground slag, and the boxes with their contents are burned in a down-draught kiln reaching 1400° C. in thirty-six hours, reducing conditions being maintained during the last twelve hours. In this manner, a considerable portion of the brick is subjected to the action of the slag and structural defects are readily detected. The use of so large an amount of slag reduces errors due to changes in its composition and facilitates a study of the time effect. He found that the slag prepared from coal ash and also a synthetic mixture of similar composition vigorously attacked a silica brick and a bauxite one, penetrating to the centre and producing a honeycomb structure. A magnesite brick showed penetration to the centre and considerable solution at the surface, whilst a carborundum brick showed no penetration, but excessive surface solution. Some brands of clay bricks were not attacked at all, but others, which were under- burned or containing coarse grog, were irregularly penetrated. The depth of penetration is not always an important measure of the harmfulness of a slag, as basic slag may penetrate deeply and yet not corrode seriously, whilst lime, though it may not penetrate far, forms a very corrosive slag and is, therefore, much more harmful. The results of a test in which a small quantity of slag is placed in a cavity in a brick and heated do not always agree with those experienced in working on a large scale, because in the test the penetration is complete in about two hours, but in a large furnace, owing to the much larger quantity of slag present, the penetration may continue until the brickwork is destroyed. The amount of slag used in the test 1 Amer. Soc. for Testing Mils., 20, I., 620-23 (1920). 2 Trans. Eng. Cer. Soc., 18, 516 (1918-19). 3 Trans. Amer. Cer. Soc., 18, 277-81 (1916). 502 PHYSICO-CHEMICAL REACTIONS also affects the results. R.M. Howe ! having found that by using 105 grams instead of 35 grams of slag, the penetration was increased by 19 per cent. The atmosphere of the furnace during the slag test also has an important influence on the penetration, some slags being more active under oxidising than under reducing conditions, and vice versa. It is also desirable to make slag-tests at a temperature approximating to that of the furnace at which the bricks are to be used, as a small difference in temperature may cause a very large difference in the amount of corrosion or penetration. Thus, R. M. Howe found that a slag melting at 1050° C. only penetrated a firebrick to a depth of 0-02 inch at 1150° C., but at 1450° C. the penetration was 0-79 inch. Similarly, a zinc-slag penetrated a firebrick 0-04 inch at 1150° C., and 0-32 inch at 1450° C., the melting-point of the slag being 1025° C. A further source of error pointed out by R. M. Howe is that the difference between. the penetration due to the porosity of the brick and that due to chemical action is not shown, yet it is very important, as penetration due to porosity does not reduce the strength, whilst corrosion rapidly weakens and may eventually destroy the brick- work. For this reason, a large penetration due to porosity may do far less harm than a much smaller penetration due to chemical action in a denser brick. The results obtained by R. M. Howe and shown in Table CXXXI, show that there is no definite relation between the slag-penetration and the durability of the bricks in actual practice, as the hand-made bricks, though showing a large penetration due to- its porosity, gave better service than a denser brick showing no penetration in the test. TaBLE CXXXI.—Comparison of Slag Test and Durability ‘ : Process of Refractoriness. ; Durability Texture of Bricks. Ma hacen pate: Slag Penetration. ee, Fine . | Steam press 31 0-29 Fair. Medium . . | Hand-made 32 0-52 Very good. Medium . . | Steam press 32 0-00 Very good. A further difference between slag-tests, in which the slag is placed in a depression or cavity in the bricks and the effect of slag in actual furnace-working, which prevents any accurate comparison, is that, in actual use, the slag only attacks one surface of the brick, and that portion of the brickwork is only heated to a very small depth, whilst in a slag-test the brick is heated throughout to the same temperature. Con- sequently, in the test the slag will penetrate the brick to a much greater extent in a given time than it would do in actual use, because in the latter it would soon reach a cooler part of the brickwork and would then become viscous or even solid, and so 1 J. Amer. Cer. Soc., 6, 406 (1923). LOW TEMPERATURE REACTIONS 503 would cease to corrode the brickwork until the surface of the brick was worn away by the solution or abrasion of its hottest face. R. M. Howe, S. M. Phelps, and R. F. Ferguson ! suggest that the best method of testing the resistance of refractory materials to the action of slag is to powder the bricks and slag to be tested so that they will pass completely through an 80-mesh sieve, mix them in various proportions, and determine the refractoriness of the mixtures. They claim that this method more nearly agrees with the conditions observed in works’ practice, the chief of these being (a) increased intensity of action with increased slag concentration ; (b) the different action of different slags; and (c) the effect of any slag differs with different refractory materials. CHEMICAL REACTIONS OCCURRING AT LOWER TEMPERATURES The chemical reactions to which ceramic materials are subjected at ordinary temperatures are chiefly due to (a) water, (6) acids, and (c) alkalies. Water has no rapid effect upon most ceramic materials at ordinary temperatures, with the exception of certain metallic oxides such as lime, magnesia, and calcined dolomite, which are hydrolysed more or less rapidly by it. Lime is the most rapidly attacked and magnesia least, dolomite being intermediate, as might be anticipated from its composition, the lime in the dolomite being most rapidly converted. The effect of water may be reduced to a great extent by adding a small proportion of clay, iron oxide, or other fluxes, calcining the materials at a temperature sufficiently high to fill all the pores and coat all the particles with a glassy film of silicate or spinel. Some bricks composed of calcium and magnesium silicates and aluminates develop hydraulic properties, and if present in burned dolomite may absorb water and cause disintegration of the calcined mass. Campbell? found that two calcium ferrates (5CaO3Fe,0, and 6CaO3Fe,0,) also develop hydraulic properties. Caustic magnesia is very soluble in water, and still more so in slightly acidified water, such as that containing carbonic acid, but magnesia bricks made of a much more intensely calcined magnesia are practically insoluble in water or dilute acids, except on very prolonged exposure. On the other hand, they are very rapidly attacked by steam, so that they must not be brought into contact with this vapour or they will be rapidly disintegrated (see also Corrosion, p. 494). Most silicates, when subjected to the action of water for a long period of time, become more or less hydrolysed, but the action is very slow. This is described under Weathering (p. 506). Hydrochloric acid attacks many calcareous and ferruginous substances, in- cluding dolomite, magnesite, calcite, iron oxides, etc., olivine, serpentine, chlorite, nepheline, epidote, leucite, and apatite ; some other phosphates, monazite, etc., are attacked to some extent. Some sulphides are also dissolved. Calcined china clay and calcined halloysite differ from one another in that the latter can be completely dissolved in hydrochloric acid. Raw bauxite is readily 1 Loc. cit., p. 495. 2 J. Ind. Eng. Chem., 11, 116-20 (1919). 504 PHYSICO-CHEMICAL REACTIONS attacked and completely dissolved by hydrochloric and sulphuric acids. When intensely calcined, however, it is practically insoluble. This is due probably to polymerisation, which appears to occur at about 900° C. Sulphuric acid when hot and concentrated attacks clays and other hydrated alumino-silicates and also most basic materials. When clay is heated for a long time with concentrated sulphuric acid the clay is decomposed, possibly with the separation of silica and the formation of aluminium sulphate. Nitric acid behaves in a similar manner to hydrochloric acid. In addition, it decomposes sulphides forming nitrates. Phosphoric acid attacks silica glass and many silicates at temperatures above 400° C., forming the corresponding phosphates and liberating silica. Calcined and crystalline silica is not attacked by phosphoric acid below 300° C., but precipitated silica is attacked at lower temperatures. Phosphoric acid also combines with silica to form a crystalline substance having the formula Si0,P,0,, this substance occurring in four allotropic forms, two of the low-temperature forms being attacked by water rapidly, whilst the other two, which are stable at high temperatures, are insensitive to water or acids, even hydrofluoric. The resistance of siliceous glasses containing phosphoric acid to hydrofluoric acid is due to the formation of Si0,P,0,, which is not affected by any acids. Hydrofluoric acid decomposes many siliceous minerals which are unaffected by other acids, silicon fluoride being volatilised. Schwarz! found the following amounts-of various kinds of silica to be dissolved by 1 per cent. solution of hydrofluoric acid in one hour at 100° C. :— Quartz . : ‘ : . 5-2 per cent. Tridymite ; : ' 20S 1 Cristobalite . ; : . 25:8 . Gelatinous silica. : .. 529 3 According to Gautier and Clausmann, silica glass is only one-tenth as soluble in hydrofluoric acid as ordinary glass. The action of carbonated water (which is dilute carbonic acid) is, in some cases, comparatively rapid, as the action of rain water upon limestone, basalts, and olivine (see Weathering, p. 506). Silica is very resistant to all acids, except hydrofluoric acid and phosphoric acid at high temperatures. W. Ostwald considers the difference in solubility of different forms of silica to be due to their different degrees of dispersion (see p. 11), quartz being the least dispersed and, therefore, least soluble. Carborundum is slightly attacked by hydrofluoric acid, but not to any great extent. Crystolon is only attacked by hydrofluoric acid, but it is decomposed readily on heating with alkalies and alkaline carbonates, and it is also attacked at red heat by most metallic oxides. Siloxicon is decomposed by hydrofluoric acid. Silundum, however, is quite unaffected by it. 1 Z. Anorg. Chem., 76, 422 (1912). ACTION OF ALKALIES 505 Alkalies usually combine with acid substances, but their action in the cold upon silicates and alumino-silicates is slight, and it is very feeble in the case of crystalline silicates. Freshly precipitated hydrated silica is readily soluble in alkaline solutions. Flint is readily soluble in alkaline solutions at 200° C., this fact being used in one method of making sodium silicate. Mylius and Meusser found the following amounts of silica glass to be dissolved from surface of 14 square inches by various hot solutions of alkaline hydrates and carbonates :— TaBLE CXXXII.—Solution of Silica Glass by Alkalies A lean Time of Contact. Reagent. Concentration. eee ; m.gms, Ammonium hydrate 10 per cent. 0-8 18 2days | Sodium hydrate 10 Ht 0-4 | Potassium hydrate 30 “ 1-2 Sodium hydrate 1/N 2-0 Sodium carbonate 1/N 0:6 a a | Barium carbonate rane sol. 0-0 Acid sodium phosphate z 0-0 | Sodium hydrate 2/N 33-0 100 3 hours Potassium hydrate 2/N 31-0 | Sodium carbonate 2/N 10-0 Silica reacts with some bases at a moderate temperature in the presence of steam. Use is made of this fact in the hardening of sand-lime bricks, the calcium silicate produced acting as a bond for the particles of sand. The following actions may occur :— Ca(OH),+S8i0,=—CaO8i0,H,0, CaOSi0,H,O+Ca(OH),=2Ca0Si10,+H,0, Ca(OH),.+2Si0,=Ca028i0.H.0. The briquetting of ores with lime, soda, and water-glass are similar instances of the action of alkalies upon silica in the presence of water. Alkalies do not greatly affect silica in the absence of water below a temperature of 1000° C., but if heated to a higher temperature it is attacked slowly and converted into fusible silicates. The effect of reagents on most ceramic materials is not usually very distinct on account of the heterogeneous nature of the latter and the limited extent to which they may be attacked. No one substance can usually be separated from the rest by chemical reagents, as before one is completely dissolved another will have begun to decompose. 506 PHYSICO-CHEMICAL REACTIONS WEATHERING Exposure to weather has both chemical and physical effects on ceramic materials. The chief chemical actions which occur are : (a) The solution and replacement of some substances after oxidation and hydrolysation. (b) The oxidation of carbonaceous matter, sulphides, and some ferrous compounds. (c) The hydration or hydrolysis of substances which are ordinarily insoluble. Solution and replacement by water has an important effect in changing the chemical nature of rocks through which it percolates, especially when it contains dissolved substances in solution. Thus, the percolation of water containing carbon dioxide through clay beds maycause the solution and removal of anysoluble substances present. For instance, calcium carbonate may be removed in this way from a highly calcareous clay or clayey limestone, and it is generally thought that the pocket clays of Derbyshire are composed of the insoluble residuum from a clayey limestone, the greater part of which has been removed by the solution of the calcium carbonate. Some clay beds containing calcium carbonate are richer in this impurity near the bottom of the bed than at the top, this latter portion having been partially purified by carbonated water descending from the surface and dissolving the calcium carbonate in its passage. Even silica—though normally regarded as insoluble in water—is not wholly so. On the contrary, there are many rocks which owe their great strength to the precipitation in them of silica which was previously dissolved in the water percolat- ing through them. The disintegration of rocks and clay beds by weathering is largely, though not wholly, due to the removal, in solution, of soluble substances originally present as the bonding material.t Conversely, water containing silica in solution, when in contact with siliceous rocks, tends to permit the precipitation of the silica in the pores of the rock, with the result that the particles of aggregate are still more firmly united with a siliceous cement. As water can contain both silica and calcium bicarbonate simultaneously in solution, a limestone may have part of its calcium carbonate removed in solution by percolating waters and replaced by silica—chiefly in the form of chert. Thus, near Carlow, in Ireland, a bed of chert 30 to 40 feet thick has replaced the original Carboni- ferous Limestone. The flints and chert in some English chalk and limestone beds are due to metasomatic replacement of this character. Sometimes quartz is replaced by calcite brought in solution by percolating waters, but this type of replacement is not common. Another typical rock formed by partial replacement is dolomite, which has been formed by the replacement of some of the calcium carbonate in limestones by magnesium carbonate. Some dolomites, however, appear to have been produced by simultaneous crystallisation of both magnesium and calcium carbonates. ; Weather has a bleaching action on clays and some other minerals containing 1 The physical actions which occur during weathering are described on p. 253. OXIDATION AND HYDROLYSIS 507 suitable carbonaceous matter, as the latter, when decomposing as a result of exposure to air and water, forms humic and other acids which dissolve some of the iron and other compounds (p. 107) and remove them in solution. Prolonged exposure to the weather also results in the solution and removal of selenite and gypsum. Any other soluble salts in a clay or other ceramic material may also be dissolved and carried away by water percolating through the mass. Cementation, or the binding of the particles of aggregate into a strong mass of rock by means of a binding agent or ‘“‘ cement,” is caused, according to Hatch and Rastall, by (a) the mingling of solutions from different sources, as when a solution containing oxygen comes into contact with one containing iron compounds in solution, the result being the precipitation of iron oxide and the cementation of the rock in which the precipitation occurs ; (b) the chemical reactions which occur between rocks and solutions percolating through them. Thus, the action of water upon anhydrous rocks causes the formation of hydrates or hydroxides, which fill up the pores and cement the rock-particles. Thus, felspar may be replaced by zeolites or hematite by limonite. Temperature and pressure play an important part in modifying the effect of chemical reactions in cementation. Oxidation.—Exposure to weather (7.e. to water and air) converts ferrous com- pounds into the ferric state, limonite being the chief product. Iron sulphides are, to some extent, converted into sulphates, marcasite being more readily oxidised than pyzites. Copper sulphides are oxidised in a similar manner, erubescite being readily broken down and chalcopyrite with more difficulty. Sunlight appears to favour oxidation and has, therefore, an important influence on the weathering of clays. Remarkable differences in the character of some clays occur after even a few hours to sunlight and air, whilst other clays are scarcely affected. The difference is, apparently, due to some oxidation processes, possibly associated with colloidal changes accompanying the hydration of the material ; but the precise nature of these changes can only be surmised, as they have not been fully investigated. Hydration or hydrolysis, which consists in the addition of one or more molecules of water to a substance, is an important result of weathering. It is really the reverse of the neutralisation of an acid by a base, and may be expressed by the equation : salt-++water= (acid radical+ H)-+ (basic radical+OH) acid — base. Thus, in hydrolysis, a salt splits up, forming an acid and a base or, occasionally, a basic salt. The duration of the exposure to water and the enormous masses involved enable changes to take place which cannot be accomplished in the chemical laboratory, with the result that most silicates and oxides can be hydrolysed and compounds, such as clays, formed, which cannot be produced by any artificial means. In the same way, the hydrolysis of iron compounds in clays and other rocks sometimes results in the formation of a particularly strong ferruginous cement. The yellow films or stains widely distributed among rocks are due to the production of limonite (p. 418) by the oxidation and hydrolysis of other iron compounds. CHAPTER XII HEAT AND TEMPERATURE Tue application of heat is the chief means by which bricks and other articles made of clay and other ceramic materials are converted from the friable or pasty state into hard, strong solids possessing other valuable properties. A knowledge of the principal effects of heat on such materials is, therefore, of great importance to those concerned in the manufacture and use of clay and allied substances, as well as of various refractory materials. In dealing with heat and its effects it is necessary to avoid all confusion of thought with respect to the terms “ heat ”’ and “‘ temperature.” Heat is the property possessed by all matter of creating a certain well-known sensation in the nerves, by means of which the substance is recognised as hot, warm, cool, or cold. Heat is caused by the motion of the molecules of which all matter is com- posed, and it may be transferred from one body to another, either by direct contact or through a third body. The true conception of heat was not understood until com- paratively recently, and many expressions are employed which suggest that heat is a form of matter instead of a result of molecular motion. Hence, heat is said to be “‘ ab- sorbed,” “‘ evolved,” ‘‘ conducted,” “‘ passed,” “‘ held,”’ “‘ contained,” and though these terms are not strictly accurate, they are accepted and incommon use. Care should, however, be taken that their use does not lead to an erroneous idea of the nature of heat. The measurement of heat is termed calorimetry. Temperature is a term denoting the thermal condition of a body as regards its power of transmitting heat to or receiving heat from other bodies. Thus, whilst one body which is twice the volume of another similar body under identical conditions may be said to contain twice as much heat, the temperature of both bodies is the same. If, however, the smaller body contains the same amount of heat as the larger one, the temperature of the small body would be higher. If, for example, the heat released on burning 1 Ib. of coal (about 14,000 heat units) could be transferred to 40 lb. of silica, the temperature of the latter would be raised to 1000° C., 2.e. to a bright- red heat. The idea of temperature is allied to that of intensity and not of quantity, so that when a substance is at a higher temperature than another, it appears to be hotter, though the total quantity of heat may be the same. Similarly, the quantity of water in two tanks of equal size may be the same, but if one tank is 100 feet above the other the water will be delivered from the higher tank at a much greater pressure. Hence, temperature bears a similar relation to the quantity of heat in a substance as the ‘‘head’”’ or pressure of water or steam bears to its volume. 508 MEASUREMENT OF HEAT 509 If two masses are at the same temperature there will be no passage of heat from one to the other, because they are in a state of thermal equilibrium. When the state of equilibrium is disturbed, however, heat will pass from one to the other until equi- librium is again restored. Here, again, the analogy with the pressure and volume of water is applicable ; if the water is at the same level in two vessels no flow of water will occur, but if the water in one vessel is at a higher level than that in the other, water will flow into the lower vessel until the level in both is the same. If heat is conceived in terms of the motion of the molecules of a substance, temper- ature may be conceived as due to the intensity or rapidity of the molecular movement. The temperature of a substance is dependent upon (a) the amount of heat ; (b) the mass and volume of the substance ; and (c) the nature of the substance. The measurement of temperature is termed thermometry or pyrometry ; the former term is used for temperatures between about —30° C. and 300° C. (2.e. for temperatures for which a thermometer can be used), and the latter for these and also for other temperatures for which a pyrometerisemployed. The two chief scales of temperature are the Fahrenheit and Celsius or Centigrade scales respectively ; they are described in the section on Temperature Measurement. Heat Measurement.—In order to compare the quantities of heat in different bodies it is necessary to have a unit of heat. Unfortunately, there are three units in use, two being employed mainly for accurate scientific work and one in industry. The two former are based on the metric system and are termed the major and minor calorie respectively, whilst the third is in English measure and is termed a British Thermal Unit (commonly abbreviated to B.T.U.). A minor calorie 1 is the amount of heat required to raise the temperature of 1 gram of water 1° C. (strictly from 0°-1° C.) at atmospheric pressure. A large or mayor Calorie is the amount of heat required to raise 1 kilogram of water 1° C. at atmospheric pressure, and it is, therefore, one thousand times as large as the calorie. A British Thermal Unit is the amount of heat required to raise 1 lb. of water 1° F. (strictly from 32°-33° F.) at atmospheric pressure. Another unit sometimes used is termed the Centograde unit, often erroneously termed a calorie; it is the amount of heat required to raise 1 lb. of water 1° C. at atmospheric pressure. Ostwald has pointed out that, in the case of chemical reactions, the calorie is not wholly satisfactory as a unit of heat in dealing with the heat of reaction, and he prefers to use the mechanical equivalent, which represents the energy expended or “‘ work’ done in bringing about the reaction. The mechanical equivalent of heat was first investigated by Joule, who found that one calorie is equivalent to 41,800 gm.-cms., or 41,800,000 ergs ? or 4:18 joules. Thus, the conversion of ice into liquid 1 Usually spelt with a small c to distinguish it from the major Calorie which is spelt with a capital C. 2 An erg is equivalent to the work done when a force of 1 dyne is overcome through a distance of 1 cm., a dyne being the force which, acting on 1 gm. for 1 second, gives it a velocity of 1 cm. per sec. 3 A joule is equivalent to 10,000,000 ergs, and a kilo joule (=J), to 1000 joules. 510 HEAT AND TEMPERATURE water at the same temperature requires 6-0 J or 1434 calories, whilst the condensation of steam to liquid water at the same temperature evolves 40-5 J or 11,745 calories. The following factors are useful in converting heat values into energy values :— B.T.U. to calories. . multiply by 0-252 B.T.U. to joules 3 : : : j aj) sya B.T.U. to foot-pounds : : 7 ‘ eC B.T.U. to watt-hours : ‘ : 53g eee B.T.U. per cubic foot to elec DEE CCL... mee cs | B.T.U. per lb. to calories per kilo. : ; 2 3, OPBBG Calories to B.T.U... . : ; : oy, ay Calories to joules : 2 sone ears Calories per c.c. to B.T.U. nee pale foot , » 9s O26 Calories per alo. to B.T.U. per lb. 3 3 se ee Kilo joules to Calories 3 : : ae Se, The quantity of heat in an article or in a given weight of material is measured by means of a calorimeter, the method usually adopted being that known as the “method of mixtures.”” This depends on the fact that if two substances are at different temperatures, the hotter one will transfer heat to the cooler one until both attain the same temperature. The calorimeter used primarily consists of a vessel containing a known quantity of water into which the hot sample is placed, and to which it imparts heat until both the sample and the water are at the same temperature, the distribution of heat being aided by stirring the water. By measuring the tem- perature of the water before and after the immersion of the sample, the amount of heat in it can be calculated as described in the following section. Calorimeters of various special designs are used for accurate work, as it is very important to reduce all loss of heat by radiation, etc., to a minimum. The measurement of conducted heat is dealt with on p. 515. THERMAL CAPACITY In accordance with the conception of heat as a form of matter (p. 508), various materials appear to have different capacities for heat, 1.e. equal masses of different substances which are all at the same temperature will require to absorb different quantities of heat before they are all raised to a given higher temperature. This apparent ability to “ contain ”’ heat is due to an incorrect understanding of its nature, but such a conception is so convenient that it isin common use. Hence, the apparent ability of a substance to absorb and retain heat is termed the thermal capacity of that substance, and in order to compare the thermal capacities of different substances a standard of measurement is used, termed the specific heat, which may be defined as the amount of heat required to effect an increase of one degree in the temperature of a unit mass of the substance under a definite pressure (usually one atmosphere). For convenience, the specific heat of water is assumed to be unity, and other substances are expressed in relation to this. The specific heat depends on whether the substance is in the crystalline or glassy THERMAL CAPACITY 511 state, or, if the former, what is its crystalline form and internal molecular arrangement. Thus, crystalline quartz has a specific heat of 0-185 between 12° and 100° C., whilst the specific heat of amorphous silica (opal) between 12° and 100° C. is 0-2375. The effect of heat or temperature on the specific heat of various substances is considered in Chapter XIII. The specific heat is not constant at all temperatures, so that it is necessary to specify the particular rise in temperature employed in the definition; this may be from 0°-1° C., 32°-33° F., 60°-61° F., or any other convenient range near atmospheric temperature. For special purposes, the specific heat at other temperatures may require to be determined, in which case the temperature should be specified in any statement of the results. The specific heat is usually fairly constant between 10° C. and 200°-300° C., but boron, carbon, and silicon are exceptions, and vary very greatly between these temperatures. At higher temperatures the specific heat of ceramic materials increases somewhat more rapidly than the corresponding rise in temperature (Chapter XIII). Vogt has stated that the specific heat at different temperatures may be calculated from the formula : C,=C,(1+0-0000782), where C, is the specific heat at temperature ¢° C., and C, is the specific heat at 0° C. The specific heat of substances is generally greater when they are in the liquid than in the solid and gaseous states. Table CX XXIII shows the specific heat of various substances :— TaBLeE CXXXIII.—Specific Heats i f Specific Heat of ape ale ener Hiement. Equal Masses. Sulphur : 0-1776 Iodine . : ; 0-0541 Magnesium . 0-2499 Bromine (solid). 0-0843 Zinc. , 0-0955 Potassium. : 0-1655 Aluminium . 0-2143 Sodium ; 0-2934 irons : 0-1138 Arsenic . 0-0830 Nickel . 0-1070 Antimony . 0-0523 Cobalt ‘ 0-1067 Bismuth 0-0305 Manganese . 0-1217 Silver . 0-0570 7 eae 0-0548 Gold . , 0-0324 Copper 0-0952 Carbon at 980°. 0-4580 Lead . : ; 0-0314 Boron at 600° , 0-5000 Mercury (solid). 0-0319 Silicon at 232° 0-2020 . Platinum. 0-0324 512 HEAT AND TEMPERATURE A determination of the specific heat of a substance is usually made by means of the method of mixtures (p. 510), by heating a weighed quantity of the substance to the required temperature for some time until it is uniformly heated. It is then quickly transferred to a suitable calorimeter containing a weighed quantity of cold water at a known temperature, and the latter is stirred so as to distribute the heat. The temperature of the water is noted at intervals of half a minute or so, until a constant temperature is reached. The specific heat is then calculated from the following formula :— : W,x(T;—T,) S ae fal Se ak Specific heat W,x(T,—T,) where W, is the weight of the substance in grams. W, is the weight of the water in grams. T, is the maximum temperature of the substance. T, is the original temperature of the water. T; is the final temperature of the water. The above formula is based on the assumption that the quantity of heat in the substance at the given temperature is W,T,s, where W, and T, have the same significance as in the formula and s is the specific heat at the temperature T,. The heat in the water prior to the determination is W,T,, and after the determina- tion it is W,Ts, and as the whole of the heat lost by the substance is assumed to be transferred to the water W,(T,—T;)s=W,(T;—T,). For the same reason, the total heat lost by the substance in passing from the temperature T, to T, is W, (T;—T,). Where accurate results are required, a special calorimeter must be used and numerous precautions taken to avoid the loss of heat by radiation. This is especially necessary in the case of substances having a low thermal conductivity, which take a considerable time before they are cooled to a constant temperature. The atomic heat of a solid substance is the product of its specific heat and atomic weight, and it has been found by Dulong and Petit that for elementary sub- stances the atomic heat is nearly constant, viz. 6-2-6-3, though this does not appear to apply to some elements with atomic weights of less than 40. According to more recent investigations, 5-9 is a more reliable figure for the atomic heat at constant volume, as shown in Table CX XXIV, due to E. B. Millard. Boltzmann 2 has shown that the atomic heat can be directly deduced from the classical Kinetic theory, and that the constant of Dulong and Petit should be 3R= 5-97 calories per degree. Lewis ° states that the atomic heat of extremely electro- positive metals (with atomic weight below 40) is irregular, because certain electrons in them are held so feebly that they acquire thermal energy apart from that of the atoms. 1 Physical Chemistry for Colleges (M‘Graw-Hill Book Co., 1921). 2 Sitzb. Kgl. Akad. Wiss. Wien., 63 (2), 679 (1871). 3 J. Amer. Chem. Soc., 29, 1165, 1516 (1907). ATOMIC HEATS 513 TaBLE CXXXIV.—Atomic Heats of Various Elements at Constant Volume and Element. Sodium Magnesium Aluminium Potassium Iron Nickel Copper Zine ‘ Palladium Silver Atomic Heat|/Atomic Heat at Constant | at Constant Volume. 6-4 5:8 5:7 0:5 5:9 5:9 5-6 5-6 po 5:8 Pressure Element. Pressure. 6-4 Cadmium 6:0 in. 5:8 Antimony 71 Iodine 6:0 Platinum 6:1 Gold . 5:8 Thallium 6-0 Lead 6:1 Bismuth . 6:1 Average Volume. 5°9 6-1 5:9 6-0 5:9 5:9 6-1 5-9 6-2 Atomic Heat|Atomic Heat at Constant | at Constant Pressure. 6-2 6-4 6-0 6-9 6-1 6-2 6-4 6:3 6:3 - The atomic heats at constant volume of the principal elements having a low atomic weight are shown in Table CXXXV, which is also due to EH. B. Millard. Substance. Sulphur Phosphorus TaBLE CXXXV.—Irregular Atonuic Heats Atomic Heat at Constant Volume. Carbon (diamond) . Carbon (graphite) Boron 5: 5:6 1-6 1-9 2°5 Substance. Silicon . Aluminium Oxygen Hydrogen 4-8 5:7 4-0 2°3 Atomic Heat at Constant Volume. The atomic heat of elements decreases rapidly at low temperatures and increases slightly at higher temperatures. According to M. Born and E. Brody,! the atomic heat at different temperatures (at constant volume) is 5-8546-+-0-000649¢. The molecular heat has been studied by Regnault, who found that the specific 1 Z, Physik, 6, 132-9 (1921). 33 514 HEAT AND TEMPERATURE heats of compound bodies possessing similar chemical formule are inversely pro- portional to their equivalents ; conversely, equivalent quantities of compound bodies possessing similar atomic composition possess also the same specific heat. According to Kopp and Woestyn, the molecular heat of compounds is n x6-4, where is the number of molecules. Thu., a solid compound consisting of one atom of one solid plus two atoms of another solid will have a molecular heat of about 19-2 calories per formula weight. The heat capacity of solids which consist of elements which at ordinary atmospheric temperatures are gases or form combinations which do not conform to Kopp’s law, have molecular heats less than that of compounds formed from elements which are solid at ordinary temperatures. This law is not reliable. THE TRANSMISSION OF HEAT The transmission of heat from one body to another may be effected in various ways :— (a) By conduction, in which the heat is transferred by molecules of the hotter substance bombarding those of the cooler substance and so setting up a corresponding motion amongst the molecules in the latter, which were moving less rapidly, the acceleration of motion progressing at a definite rate through the second substance. The rate at which the heat passes through a material—as measured by the rise in temperature of the latter—is termed thermal conductwity. ; As a result of their different molecular structure, materials have different powers of conducting heat. Most metals have a higher thermal conductivity than non- metals or compounds, 7.e. they conduct heat rapidly through their mass. Ceramic materials are, on the whole, very poor conductors of heat, though some, such as carborundum, have a higher thermal conductivity than others. Factors Influencing Thermal Conductivity —The chief factors to be considered in connection with the rate of passage of heat through ceramic materials are :—- (a) The chemical composition of the material used. (b) Its previous heat treatment. (c) The texture or physical condition of the material. (d) The porosity of the material. (e) The temperature at which the material is used or tested. Materials differ with regard to their thermal conductivity, which also changes with the heat-treatment to which they have been subjected, just as the temperature and duration of firing during manufacture modifies other properties of such materials. It is also important to know the condition of the material during use, as the thermal conductivity of a brick or block may be quite different from that of the same sub- stance when in the powdered state. Thus, magnesia when in the form of a refractory brick is a moderately good conductor of heat, whilst in the form of a powder it is extremely resistant to heat and has a high insulatory value, as will be seen in Table CLXX VIII. TRANSMISSION OF HEAT 515 The influence of texture and porosity may be considered together, as the principal effect on the thermal conductivity is due to the relation between the amount of solid and of air which the heat has to traverse in passing through the material. As air is a much better insulator than any solid material, the larger the proportion of air the greater will be the thermal insulating power of the material. Hence, a fine-grained, close-textured material has a much greater thermal conductivity than one with a coarser open texture. The relation between insulating power and texture or porosity cannot, however, be expressed in very simple terms, as it is modified by (a) tem- perature, (b) the size and (c) the shape of the pores or interstices, (d) the position of the interstices relative to each other and to the solid matter. If the rate of radiation increases until it equals the rate of conduction through a solid the pore-spaces will cease to act as insulators. With pores 0-01 cm. diameter this equality of heat-transfer occurs, according to Dougill, Hodsman, and Cobb,}! at 3600° C. At lower temperatures, or with wider pores, the rate of radiation is lower than that at which the heat passes through the solid material, so that the presence of pores in materials used at any temperature ordinarily attainable decreases the thermal conductivity. A. T. Green? has, however, stated that at much lower temperatures (e.g. 1400°-1500° C., and even at 1150° C.) some of the pore-spaces lose their insulating properties and transmit heat at nearly the same rate as the solid matter. If this statement is correct, pore-spaces in ceramic materials do not have so great an insulating power at high temperatures as is usually assumed. In comparing the thermal conductivity of different substances, the unit is the amount of heat which passes in one second through a mass of the material of unit thickness and area when the difference in temperature of opposite faces is one degree. If the C.G.S. unit is adopted the thermal conductivity will be expressed as gram-calories per second per centimetre-cube for a difference of 1° C. This can be converted to the British Unit, viz. B.T.U. per second per 1 inch cube, by multiplying the conductivity in C.G.8. units by 8-672. The reciprocal of thermal conductivity or resistance offered by unit mass of a substance to the passage of heat is termed its resistwity. The thermal conductivity of a solid substance, such as a ceramic material, is usually determined by exposing one face to a constant source of heat and measuring the temperature at opposite faces, or at the hot face and at a point in the material a convenient distance from it. The greatest source of error occurs in applying the heat uniformly to the hot face and ensuring its uniform distribution through the material to be tested, so the various methods which have been devised for deter- mining the thermal conductivity differ chiefly in the means used to avoid this source of error. In Wologdine’s method, the test-pieces are in the form of round flat plates, 160 mm. (6-4 inch) diameter and 50 mm. (2 inch) thick. Holes are pierced to depths of 5 mm. (0-2 inch), 45 mm. (1-8 inch), and 50 mm. (2 inch) from the upper surface and thermo-couples attached to pyrometers inserted in them. The lower surface of the 1 J, Soc. Chem. Ind., 34, 465 (1915). 2 Trans. Eng. Cer. Soc., 21, 394 (1921-22). 516 HEAT AND TEMPERATURE test-piece is heated in a gas-furnace and the heat passing through it is measured by a water-calorimeter, whilst the temperature at each of the levels above mentioned is read at intervals. — The thermal conductivity may be calculated from the formula :— _ Pt —4) Q 60 where Q is the quantity of heat traversing per second an area equal to that of the base of the calorimeter, ¢, and ¢, the temperatures of the water entering and leaving the calorimeter, and P the quantity of water passed in grams per minute. The S(T; — To) QL in square cm., L the thickness of the plate in cm., and T, and T, the temperatures of the upper and lower surfaces. Various modifications of Wologdine’s method have been devised in order to lessen the error possible. Goerens! used a “ guard ring” consisting of an outer jacket through which water flows continuously, and his thermo-couples were arranged to run parallel to the hot face. Dougill, Hodsman, and Cobb 2 used a second calorimeter to surround the first one, but the value of this is doubtful, according to Griffiths, who measures thermal conductivity by heating one surface of the material to be tested by placing it in contact with molten tin. R. A. Hornung? uses a hollow copper plate 20 inches by 20 inches by 1 inch (through which water flows backwards and forwards, entering and leaving at diagonally opposite corners) bedded upon a 2-inch layer of loose kieselguhr and covered with a layer of the material to be tested, upon which rests three coils of resistance wire, which is heated electrically. The coils are covered by another sample and a second plate identical with the lower one, the whole apparatus being packed loosely with kieselguhr into a box. The temperature of the cold side of the slab is the average of four readings of the temperatures of water entering and leaving the plates, whilst the temperature of the hot side is indicated by a thermo- couple placed between the two plates. The average amperage of the centre coil is the total amperage divided by 3, as the three coils are in parallel and the voltage is measured across the : entral third of the middle coil. The heat transmitted in B.T.U. per inch thickness may be calculated from the formula : coefficient of thermal conductivity is , where § is the area of the bottom voltage x amperage x 3-416 x 24 x thickness in inches area in square feet x diffusion temperature in ° F. R. A. Hornung also measures the thermal conductivity by a hot-air method in which the samples to be tested are made into a cubical box of 3 to 4 feet side, in which is placed a heating coil and fan to cause the circulation of the air. The coil is heated 1 Ber. Ver. Deut. Fab. Feuerfester Produkte, 34, 92 (1914). * Loc. cit., p. 516. 3 Trans. Faraday Soc., 12, 193 (1916-17). ‘ Trans. Amer. Cer. Soc., 18, 192 (1916). CONDUCTIVITY AND DIFFUSIVITY 517 to the desired temperature, which is maintained for 48 hours to ensure uniform heating, and the temperatures of the interior and exterior are then taken every 10 minutes for 2-3 hours, the average being calculated and the transmission of heat per 24 hours in B.T.U. per square foot calculated from the formula : (FA x FV) + (CA x CV) 3-416 x 24 DxA : where FA and CA are the amperages, and FV and CV are the voltages of the fan and coil respectively ; D is the difference in temperature, and A is the mean area in square feet. For temperatures above 300° F., a heating coil may be used in a cylinder 8 to 10 inches diameter and 24 inches long, no fan being employed. In A. T. Green’s? determinations of the thermal conductivity the samples were heated through a hot plate by means of a graphite resistance furnace. The tempera- ture was measured at three places: (a) on the hot face of the samples, (b) at a distance of 4 cm., and (c) at a distance of 5-4 cm. The thermal conductivity was then calculated from the formula : > (2vi Ghee Use as "iB ( Va Ue 9 where ee sar re = e'dB, QW kt Ay Var where 9, is the temperature of the hot face, x is the distance, ¢ is the time, @ 1s the temperature after the time ¢ at the distance , and k is the diffusivity. The Diffusivity is the rise in temperature produced in | c.c. of the substance by 1 calorie acting during | second through 1 square cm. of a layer 1 em. thick, having a temperature difference of 1° C. between its faces. The coefficient of diffusion is represented by : K dh’ where K is the amount of heat in gm.-cals. which is transmitted in 1 second through a plate 1 cm. thick per square cm. of its surface when the difference in temperature between the two sides is 1° C., d is the specific gravity, and h the specific heat of the material. Fourier’s law for the linear flow of heat is : d*@. dé dx? dt" where 6 denotes temperature, 7 the distance from a hot surface, ¢ the time, and & the 1 Loe. cit., p. 515. 518 HEAT AND TEMPERATURE diffusivity. Calculations based on this formula, made by Heyn, Bauer, and Wetzel, give much lower results for conductivity than those obtained by means of calorimeters. The loss of heat through a furnace wall is given by the expression : S I > where H represents the flow of heat, 7', the temperature of the cold or outer surface, T, that of the hot or inner surface of the wall, S the area, | the thickness, and K the mean thermal conductivity between the temperatures 7, and Tj. If s is the inner area of the wall and S the outer area, the geometric mean is +/s§ H = K(T; — To) and H=k(T, — ye : In electrical furnaces the term thermal mho is generally used to express in watts the heat radiated in gm.-cals. per second per cm. cube for a difference of 1° C., a watt being equivalent to 0-2388 calories per second. The reciprocal value of this, the thermal ohm, represents the difference of temperature divided by the flow of heat in watts per cm. cube. The thermal conductivity expressed in C.G.S. units may be converted into thermal ohms by multiplying the reciprocal of the conductivity by 0-2388. To reduce gm.-cals. to watts, the reciprocal of the conductivity is multiplied by 4-186. The watts may be resolved into power as follows :— Watts x 0-00134111 = hozse-power. Watts x 00568776 = B.T.U. per minute. Watts x 0-0143329 = calories per minute. Contact conductivity, or rather its reciprocal, contact resistance, which is denoted by R, represents the difference in temperature in ° C. between the hot body and the surrounding medium, divided by the number of watts or gram-calories per second which flows from each square cm. of surface. 36,000 24 when v denotes the velocity of air (which results from the temperature difference) in cm. per second, and the loss of heat per square cm. surface per second for a difference in temperature of ¢° C. will be t z cal. per second. The thermal conductivity is a very important property in ceramic materials. Thus, the walls of a furnace which is internally heated are required to have a low thermal conductivity so that as little heat as possible will be lost through them. The walls of a muffle, retort, or similar appliance, on the contrary, must have a high thermal conductivity in order that the heat may pass through them in order to heat CONVECTION AND RADIATION 519 their contents. The application of the thermal conductivity of ceramic materials is further dealt with in Chapter XIII. (6) Convection, in which the heated particles move away from the hotter to the cooler parts of the mass and carry “heat” with them. Convection can only occur in fluids, as the particles in a solid are not sufficiently mobile. (c) Radiation, in which the heat is carried neither by conduction nor convection, but in a manner comparable to the transmission of light. Heat may be radiated instantaneously through space as well as through air and other gases, and when so radiated it scarcely affects the temperature of the medium through which the “ rays of heat”? are passed. There is a very close relationship between radiated heat and light. Both can be reflected by mirrors and deflected or refracted by prisms and ~ lenses ; in fact, the chief difference between them appears to lie in the difference in wave length. Hence, when a body is sufficiently heated, the heat-rays emit a form of light and the body is said to be “ incandescent.”” When heat-rays are absorbed by any substance the latter is heated and its temperature increased in proportion to the amount of heat absorbed, but the air through which the heat-rays pass may remain at a much lower temperature. This is due to the fact that radiated heat is absorbed more by some surfaces than others, the nature of the surface being of greater importance, in this respect, than the composition of the heat-absorbing material. Similarly, the amount of heat radiated from a body varies according to the colour and nature of the radiating surface, being low for polished metal and high for rough black surfaces. In the latter, it is proportional to the fourth power of the absolute temperature— = k(T,* = fire where EF denotes the radiation from a body at 7, to one at T, and & is a constant. The radiation loss, apart from convection, is usually (for a temperature difference of 100° C.) 0-015 gram-cal. per second for each cm. of heat-radiating surface. THE GENERAL EFFECT oF HEAT ON SUBSTANCES ! Heat effects various changes upon any body subject to its influence, the principal ones being : (a) A change in the temperature of the mass. (6) A change in the volume of the material (expansion and contraction). (c) A change in the physical state of the material. (d) A change in other physical properties of the material. (e) A change in the chemical composition of the mass. (f) A change in the electrical condition of the mass. (g) A change in the optical properties of the mass. Changes in temperature may be purely physical in character and a simple result of what is termed the absorption of heat (this may simultaneously effect a change in 1 The effects of heat on ceramic materials as distinct from other substances are considered in Chapter XIII. 520 HEAT AND TEMPERATURE the physical state of the substance, p. 523), or they may be the result of chemical changes which evolve or absorb heat. Exothermal and Endothermal Changes.—The absorption of heat by a substance causes a change of state first by sensible heat which raises its temperature to a certain point and then by additional heat which becomes latent and effects the change of state from solid to liquid or from liquid to gas. By reconverting the changed sub- stance into its original state the heat previously rendered latent is evolved as sensible heat and most of it can be recovered. When heat is absorbed and rendered latent by a change in the state of a substance, that change is termed endothermal. When the reverse change occurs and the latent heat is evolved, the change is termed exo- thermal. Neutralisation of acids by bases or alkalies is an exothermal reaction, but hydrolysis is an endothermal reaction. Thus, the neutralisation of caustic potash by hydrochloric acid is accompanied by the liberation of energy equivalent to 57:3 J or 136,947 calories, which may be expressed thus : KOH + HCl = KCl + H,0 + 57:3 J. Some endothermal reactions become exothermal at higher temperatures and vice versa. Consequently, a compound may be unstable at a low temperature, stable at higher temperatures and conversely. The reversal of the direction of a reaction with a change of temperature shows how necessary it is to ascertain the conditions of a reaction when investigating the character of the change. A compound formed with the evolution of heat requires the addition of more heat to decompose it (see also Changes in Chemical Composition, p. 528). Changes in volume effected by heat result in either an increase or a decrease in the volume of the substance; the former is termed expansion and the latter contraction or shrinkage. These changes are often of great technical importance, especially when substances which undergo such changes are made into articles which are required to be of definite size within very narrow limits. Thus, first-class firebricks are not expected to vary in length by more than 1 per cent., whilst for some electrical fittings made of stoneware or porcelain a much greater degree of accuracy in size is required. The changes in volume caused by the action of heat may be divided into two groups : (a) reversible, and (b) irreversible changes. (a) Reversible volume-changes are those in which the volume of a substance changes on the application of heat, but the original volume is regained when the heat is withdrawn, 7.e. when the substance is cooled to the pat temperature. Such changes are usually of a wholly physical nature. (b) Irreversible changes are those in which any increase or decrease in volume which may occur is not reversed on cooling. Such a change may be the result of the formation of a substance of different specific gravity, such as the conversion of quartz to cristobalite, or some change of a chemical character whereby new compounds are formed. The conversion of a substance into another allotropic form is usually classed as a physical change, but it is also chemical in character, as when the chemical VOLUME CHANGES ON HEATING 521 constitution of the substance is examined it will be found to be different with each allotropic form. When a chemical change occurs a change in volume is almost inevitable, though, for various reasons, it is not always easy to recognise. The term “irreversible” is not strictly correct, as theoretically all changes are reversible under suitable conditions. This term, therefore, should be understood in a comparative sense as a change which is so extremely slow that it may be considered under ordinary conditions to be irreversible as distinct from the more readily reversible changes which occur at an appreciable speed. The unit of measurement of the reversible change in volume on heating or cooling is termed the coefficient of expansion or contraction ; it is expressed as a fraction of the length, cross-sectional area, or volume of the substance (the latter being taken as unity) when heated so that its temperature rises 1° C., though any suitable tempera- ture may be used, provided it is specified. The coefficient of expansion may be expressed in terms of length, 7.e. the coefficient of linear expansion, or in terms of volume, 1.e. the coefficient of cubical expansion. If the coefficient of linear expansion is represented by a, the original length of the piece being |, at 0° C. and 1, at ¢° C., then t, =L,(t + at). Hence, if the length prior to expansion be unity, that after expansion will be (1+qa) and the volume after expansion will be (1-+a@)* or 1+-3a+3a?+a3. As a is a small fraction, a? and a® will often be so small as to be negligible, and it is then sufficient to regard the volume of the expanded material as (1+3a). Hence, the coefficient of cubic expansion is almost exactly three times the linear coefficient. If the linear expansion or contraction is as high as 10 per cent., omitting the fractions as suggested will produce an error of about 10 per cent. of the total linear contraction. Most pure substances expand suddenly when melted, but a few (notably ice and bismuth) contract. Thus, 1000 c.c. of ice yield only 910 ¢.c. of water at 0° C. This expansion or contraction relates to the individual particles and not to the mass as a whole, as in a porous material the air-spaces may be filled by the molten material. The expansion of materials on heating may be determined in various ways. Where it is large, as in the case of some metals, a bar is heated with one end fixed and the other in contact with an indicating lever and the amount of deflection noted. The chief difficulty lies in avoiding or neutralising the expansion of the furnace in which the test-piece is heated. P. A. Boeck 1 endeavoured to eliminate this error by heating a cylindrical test-piece in an electric tube furnace, closed at one end and supported upon two brass pillars, the tube being fastened securely to the support opposite the closed end, but allowed to slide freely on the other support. By this means the tube of the furnace can only expand in one direction. The test-piece is placed between two smaller quartz tubes into the main tube of the furnace and is supported to prevent contact with the walls of the tube by platinum rings. As the end of the main tube is closed, the test-piece and the inner quartz tubes are only able to expand in a direction opposite to that of 1 Trans. Amer. Cer. Soc., 14, 470 (1912). 522 HEAT AND TEMPERATURE the main tube, so that the expansion of the inner quartz tubes neutralises that of the outer tube; any residual change in volume is due entirely to the test-piece. The expansion of the test-piece is measured by means of a cross-hair fitted on to the inner tube at the open end of the furnace and viewed through a microscope having a micrometer scale. The temperature of the test-piece is measured at the same time by means of a thermo-couple. Hodsman and Cobb ! use a similar method, but they arranged so that the outer tube and the distance-pieces both expanded in the same direction and both had fiduciary marks on them, so that their relative positions at different temperatures could be measured, the movement being due to the expansion or contraction of the test-piece as the two pieces of silica were assumed to expand equally. Mellor uses a much simpler direct method, by having two fiduciary marks (such as fine saw cuts in which fine platinum wires are fixed) and measuring their distance apart by two cathetometers placed outside the furnace in which the test-piece is heated. The permanent volume changes in ceramic materials may readily be measured by determining the length or cubical contents of the test-piece before and after treatment. The provisional standard specification of the English Ceramic Society ? for testing the contraction of clay consists in drying the clay at a temperature not exceeding 70° C., crushing it so that it will pass through a 28-mesh sieve, mixing it with water and shaping it into a suitable form, by means of a mould. The test-piece is marked with vertical lines 9 cm. apart and allowed to dry, first at the ordinary temperature, then for four to five hours at 70°-80° C., and finally at 110°C. The drying shrinkage is then measured. The test-piece is then heated in a furnace at the rate of not more than 100° C. per hour to 900° C., and afterwards at the rate of not more than 4 cones (about 80° C.) per hour to the end of the firing. The test-piece is then allowed to cool and is measured. The difference between the vertical lines (a) before and after drying gives the drying shrinkage, (b) after firing and after drying gives the kiln shrinkage, and (c) before drying and after firing gives the total shrinkage. These results are usually expressed as a percentage of the distance between the fiduciary marks before drying the test-piece. The after-expansion or after-contraction of refractory materials should be determined, according to the provisional speci- fication of the English Ceramic Society,? by means of a test-piece 3 inches long and 1-2 inches wide and deep, the opposite ends being ground parallel on an abrasive wheel. The test-piece is fired to Cone 14, if it contains less than 80 per cent. silica or Cone 12 if it contains more than 80 per cent., maintained at the correct temperature for two hours and then cooled, the size before and after heating being accurately measured. The American Ceramic Society * suggest the use of test-pieces 1} x14 x1 inch for shrinkage tests. 1 J. Soc. Glass Tech., 3, 201 (1919). * Trans. Eng. Cer. Soc., 17, 300 (1918). 3 J. Amer. Cer. Soc. Year Book, 1921-22, II, 39. LIQUEFACTION ON HEATING 523 When the shrinkage by volume is to be determined, a volumeter such as is used for porosity determinations (p. 84) must be used, the volume of the sample being determined before and after the test and the percentage of volume change calculated from these figures. It is sometimes desired to determine the number of times a sample can be quickly heated and cooled, before it cracks. This is termed its resistance to spalling and is found by weighing the test-pieces and heating them to a temperature of 1350° C. for one hour until the temperature of the test-pieces is uniform throughout. Each test- piece is exposed for fifteen minutes to a blast of cold air from a ?-inch nozzle, supplied with air at the rate of 27 cubic feet per minute. The test is repeated ten times and the bricks are then allowed to cool and are reweighed. The loss in weight expressed as a percentage of the original weight is regarded as due to spalling. A tentative specification of a test of resistance to spalling issued by the American Society for Testing Materials ! consists in heating the brick to 1400° C. for five hours, exposing one end in the furnace to 1350° C. for one hour, then removing and placing in cold running water for three minutes. The treatment is repeated until the end of the brick spalls off, the number of treatments required being regarded as a measure of the resistance of the article to spalling. Changes of Physical State.—When sufficient heat is applied to a solid substance it will, in time, cause the solid to change to a liquid and finally to a gas, though in some cases the heat required to effect these changes is so great that they are almost unattainable. When a substance is placed in a furnace, the temperature of which rises at a uniform rate, the temperature of the substance will also rise uniformly until a point is reached at which either a change in state or decomposition occurs. In the former case, the substance will commence to liquefy, 7.e. to turn into the liquid state. During this change, or liquefaction, the temperature of the substance will remain constant, and if the substance is pure the temperature will remain constant until liquefaction is complete. When the substance is entirely liquid the temperature again commences to rise. If the substance can be converted into gas without decom- position, its temperature will continue to rise on prolonged heating until the gasifica- tion or volatilisation of the substance occurs, when the rise in temperature will again cease until all the liquid has been converted into gas. After this, the temperature will rise indefinitely. The temperature at which the arrest, due to liquefaction, takes place is termed the melting-point ; that at which the arrest due to vaporisation occurs is termed the boiling-point. Thus, water remains at 0° C. until wholly converted into ice and at 100° C. until wholly converted into steam. The heat absorbed by any material without the latter showing any rise in tem- perature is said to be rendered latent or inactive and is termed latent heat, in order to distinguish it from the “sensible heat’ which causes an immediate rise in tem- perature. Probably the best conception of latent heat is that which regards it as the power required to overcome the mutual attraction of the molecules and by separating them converts the substance first into a liquid and afterwards into a gas. Hence, the latent heat of fusion is the amount of heat required to convert unit weight 1 Tent, Stand., 297 (1922). 524 HEAT AND TEMPERATURE of a solid at the temperature at which fusion can occur wholly into a liquid. It may be determined in the same manner as the specific heat (p. 512), by means of a calori- meter into which a weighed quantity of the molten material is placed. The heat evolved is determined and from it is subtracted the amount of heat evolved by treating an equal weight of the same substance in the solid state at the same temperature in a precisely similar manner ; the difference is the latent heat of fusion. The latent heat of fusion may be roughly estimated by a consideration of the heating and cooling curves of the material together with the heat capacity of the crystalline and the liquid material near its melting-point, but this method is only accurate to within about 15 per cent. Similarly, the latent heat of vaporisation or of gasification is that which is required to convert a liquid, previously heated to its boiling-point, wholly into vapour or gas. The absorption of this heat does not cause any rise in the temperature of the liquid, but is used (a) to overcome the forces of attraction between the molecules of liquid, and (b) to push back a suitable volume of air to make room for the liberated vapour. In a vacuum (b) does not enter into the problem, and consequently, less heat is required to vaporise or evaporate a liquid so that its boiling-point is correspondingly reduced. The quantity of heat required to produce the molecular weight (in grams) of vapour formed from a liquid is termed the molecular heat of vaporisation or the molal latent heat; it varies with the boiling-point of the liquid. According to Trouton, the molal heat of vaporisation of a liquid at atmospheric pressure in calories=20-3 x the boiling-point on the absolute scale. As the pressure increases, the latent heat decreases until at the critical pressure no heat is required. The melting-point of a substance is the lowest temperature at which the complete conversion from the solid to the liquid state can occur. Although it is sufficient, for most purposes, to regard the melting-point of a substance as the lowest temperature at which it becomes fluid, the conditions under which refractory materials melt are so complex that this definition does not apply. A much better definition is that which regards the melting-point as the temperature at which the crystalline and amorphous states of the substances are in equilibrium, and this temperature is not affected by adding a small amount of heat to or with- drawing it from the material. In accordance with this definition, the melting-point of a substance may be identified in three different ways :— (i) By a change in state, from solid to liquid. (u) By a change in structure, from crystalline to amorphous. (i111) By a change in energy, due to absorption or liberation of heat as shown by a time-temperature graph (p. 464). If the substance is a single, pure element or a compound which does not decompose on fusion, the melting-point will be sharply defined as the point at which a rise in temperature ceases until the substance has been completely liquefied. In the presence of impurities, or when a mixture instead of a single substance is heated, fusion takes place gradually over a range of temperature and the material softens and may lose its shape at a temperature much below that at which it can be completely MELTING- AND SOFTENING-POINTS 525 fused. Some single compounds which have a very low thermal conductivity, such as silica, do not show a sharply defined melting-point, but melt gradually over a range of about 110° C. Thus, whilst the true melting-point of silica, according to Day and Shepherd, is about 1600° C., its viscosity is so great that it does not flow or change its shape until a temperature of about 1750° C. is reached. Such sub- stances also show a curious difference in melting-point according to the size of the particles and the rate at which the temperature rises. As their conductivity is so low, an impractical length of time would be required to fuse a quantity completely at the same temperature as a few grains can be fused, and consequently, bricks and other articles made of such materials may sometimes be fused at temperatures which are actually above their true melting-point if the duration of the heating is not too long. In consequence of the great time required to effect complete fusion at the true melting-point, this point is seldom determined. The so-called “ fusion-point”’ of a ceramic material usually refers to the lowest temperature at which any appreciable. signs of fusion are visible, such as the rounding of the sharp edges of the test-piece. For many purposes, the temperature at which a test-piece the shape of a Seger cone (fig. 50) loses its shape by bending—termed the softening-point—is substituted for the melting-point in determining the fusibility or, conversely, the refractoriness ! (see p. 526). There is, however, no direct relationship between the softening-point and the true melting-point of a ceramic material. At the same time—assuming the true melting-point to be indeterminable—the softening-point, which is higher, serves at least as an indication of a temperature at which an appreciable change of state occurs. The apparent melting- or softening-point of a substance depends upon :— (a) The chemical composition of the material. (6) The amount and nature of any impurities present. (c) The pressure applied. (d) The rate at which the temperature of the furnace rises. (e) The thermal conductivity of the material. (f ) The shape and size of the piece to be heated. The chemical composition of the material is an inherent property and the only alteration in the conditions of heating which can affect it is the nature of the atmosphere in the furnace. If the material contains reducible compounds, such as ferric oxide, and the material is heated in a reducing atmosphere, the formation of ferrous silicate and other ‘“‘reduced”’ compounds may seriously affect the melting-point. For this reason, in determining the softening-point or melting-point of a ceramic material the heating should always be under oxidising conditions. Raoult discovered in 1882 that when any substance dissolves in another (molten) substance the melting-point of the latter is reduced in proportion to the amount of material dissolved. If the 1 If the term “‘ refractoriness’ is understood to include resistance to furnace conditions, the attack of slags, gas, etc., the abrasive action of flue dust and similar deteriorating influences, it becomes almost impossible to define it, so that its use should be confined to the resistance of a material to heat under oxidising conditions in a “‘ clean ’’ atmosphere. 526 HEAT AND TEMPERATURE added substances are in proportion to their molecular weights, they will reduce the melting-point of the ‘‘ solvent’ by the same amount, 7.e. equimolecular solutions in a given solvent have the same melting-point. Ludwig has made use of this fact in calculating the refractoriness of fireclays from their composition (p. 382). If a substance contracts on melting, its melting-point will be reduced by the application of external pressure, and as ceramic materials occupy less space when molten than in the solid state, if pressure is applied to any ceramic article or test- piece, it will reduce its melting-pomt. In addition, the softening-point will be reduced because the pressure enables the particles to move readily and thereby show distortion when a much smaller percentage of molten material is present than would be the case if no pressure were applied. The application of pressure does not affect the true melting-point of the material ; it merely indicates at an earlier stage than would otherwise be noticeable the extent of liquefaction which has occurred. The rate at which the temperature of the furnace rises affects the apparent softening-point or melting-point merely as a result of the thermal conductivity of the material. Where the conductivity is low the heat penetrates the material slowly, and if the external temperature is rising rapidly the temperature at which an appreciable amount of fusion occurs is higher than would be required if the temperature were rising more slowly. Thus, Cone 5 has a nominal fusing-point of 1180° C., but it will fuse at 1130° C. if kept at this temperature for 24 hours, and at 1100° C. if heated for five days at that temperature. If only a very small quantity of material—say a few grains—is heated, the heat penetrates these separate particles much more readily than a larger mass. Conse- quently, the separate particles will melt at a lower temperature than the larger mass, although both samples are composed of the same material. Add to these considerations the fact that most ceramic materials contain variable amounts of impurities which, by forming solid solutions and in other ways, reduce the melting-point of the chief constituent and it is easy to understand that such materials do not exhibit a sharply defined melting-point, but show signs of fusion over a relatively long range of temperature before the fusion is complete. In comparing the melting- or softening-points of different materials, one of the most serious sources of error lies in heating the furnace too rapidly or at different rates for different samples. Thus, if a fireclay has a softening-point corresponding to Cone 28 when heated slowly, it may easily be made to appear to have a softening- point equal to Cone 32 or even higher if it is heated very rapidly. This serious difference is wholly due to the low thermal conductivity of the material which makes it necessary to heat the furnace slowly when making tests of this nature. A rise of 10° C. per minute is usually satisfactory and is widely accepted as a standard rate of heating. : Two methods are available for the determination of the melting- or softening-pornts of ceramic materials : (a) The sample is heated at a definite rate (usually 10° C. per minute) in a suitable furnace, under oxidising conditions, and the temperature of the sample is measured at frequent intervals or continuously by means of a thermometer or pyrometer, MELTING- AND SOFTENING-POINTS 527 and the temperature at which fusion occurs is noted. This method is quite satis- factory for pure substances with a high thermal conductivity, because such substances show a marked arrest in the rising temperature of the sample during the fusion. This method may also be used for determining the fusion-point of individual particles, as their change of shape when fusing can be observed under the microscope. In a modification of this method, the sample is heated to a definite temperature, withdrawn from the furnace and examined ; if no signs of fusion are observable, another sample of the same material is heated to a still higher temperature prior to examination. By this means, the point at which “first signs of fusion” occur can be ascertained after a number of trials. In order that the results may be comparable, the size and shape of the test-piece and the rate of heating must be the same in each case. (b) With substances having a low thermal conductivity, it is often more convenient to determine the softening-point by heating it in the manner just described with similar-sized pieces of other (standard) materials also of low thermal conductivity, the softening-points of which have been accurately determined. That of the standard material which behaves like the sample is taken as the temperature corresponding to the softening-point of the latter. The shape of test-piece and standards most generally favoured is that of the Seger cones (p. 540), the sample being cut or finely powdered, mixed with a little dextrin if necessary, and moulded to the desired shape, dried, and placed with suitable Seger cones in an electric, gas-blast, or Deville furnace, and heated at a suitable rate (about 10° C. per minute). Seger recommended that the critical point (softening-point) should be taken as that at which the apex of the cone or sample bends over and just touches the base upon which it stands, but some other investigators prefer to withdraw the sample from the furnace and examine it for “ the first signs of fusion ”’ as indicated by a rounding of its edges. This method is the one specified by the Institution of Gas Engineers in its Standard Specification. It is, of course, necessary that the temperature should be uniform throughout the furnace ; otherwise, some of the cones will be at different temperatures from the test-piece. They should, therefore, be placed as close together as possible, and it is often convenient to cover them over with an inverted crucible so as to protect them from currents and eddies and ensure a more uniform heating. Kanolt 1 determines the melting-point of refractory materials by placing the sample on a bed of alundum in the bottom of a refractory tube, approximating to the composition Al,0,Si0, (sillimanite) in the case of materials melting below 1800° C. For higher temperatures, graphite crucibles are used. Observations are made through a window in the top of the furnace. In determining the melting- or fusion-point of a substance, it is important to use a container which will not contaminate the material being tested. Magnesia may be conveniently heated in graphite crucibles, but oxides which form carbides or are reduced to the metallic state should preferably be heated in a bauxite or tungsten container. ; If the material to be tested is liable to shrink or crack during the heating, it is often 1 Trans, Amer. Cer. Soc., 15, 167 (1913). 528 HEAT AND TEMPERATURE convenient to make the test-piece partly with calcined material, as this lessens the contraction and liability to cracking without altering its refractoriness. The boiling-point of a substance is the temperature at which it is converted into vapour at such a rate that its vapour pressure is that of a column of mercury 760 mm. in length or 1 atmosphere. Most ceramic materials have boiling-points which are so high as to be outside the range of attainable temperatures, so that their boiling-points are not of great importance. The chief exception to this is water which, when pure, has a boiling-point of 100° C. or 212° F. The boiling-point of any substance is reduced if the pressure of the atmosphere in which it is reduced is also reduced, so that water can be boiled rapidly im vacuo at about 60° C., and slowly at lower temperatures. If a liquid is heated above its boiling-point in a closed vessel so that the vapour of the liquid cannot escape, the vapour-pressure will increase in proportion to the heat absorbed. A typical example of this is the pressure of steam in an ordinary boiler. Water heated under pressure has a more solvent action than that at the ordinary boiling-point, and is probably one cause of the decomposition of felspar and other minerals from which clay is derived. Any substances which dissolve in a liquid will increase the boiling-point of the latter in proportion to the concentration of the solution. Changes in other physical properties which are effected by heat include: (i) Changes in specific gravity (Chapter V). (ii) Changes in porosity (Chapter II). (11) Changes in hardness (Chapter III). | (iv) Changes in strength, including those in toughness, elasticity, etc. (Chapter IV). (v) Changes in crystalline form (Chapter I). (vi) Changes in texture (Chapter I). (vii) Changes in viscosity (Chapter XI). These various changes are also dealt with in the next chapter. Changes in Chemical Composition.—Heat is an important factor in chemical reactions as it facilitates such changes as decomposition, oxidation, reduction, etc. Indeed, most chemical changes take place much more rapidly at a high temperature than at a lower one. In some cases, the velocity of a reaction is doubled if the temperature of the reacting substances is raised a few degrees. When a reaction occurs slowly it may usually be completed far more quickly by the aid of heat. Some substances evolve heat at the moment of crystallisation, plaster of Paris and Portland cement being good examples. When either substance is mixed with water to form a paste, the temperature rapidly rises, the mass “ sets ’’ solid, and if examined later will be found to have crystallised. Quicklime combines with water, forming calcium hydroxide, and also evolves heat during the process. In the former case, the heat is termed heat of crystallisation, in the latter, heat of combination. Conversely, most substances containing water of crystallisation or HEAT OF SOLUTION 529 “combined water ” (p. 337) part with it when heated. The effect of heat in producing endothermal and exothermal physical changes has been described on p. 520. The same terms are applied when heat is absorbed or evolved as the result of chemical reactions. The chemical changes accompanied by heat-phenomena may be investi- gated quantitatively and the heats of reaction may be accurately determined. The principal groups of reactions accompanied by a change in temperature may be grouped according to the— (a) heat of solution ; (6) heat of combination (including that of neutralisation and combustion) ; (c) heat of dissociation. The heat of solution of a substance appears to be due to a combination of the sub- stance and the solvent and to depend upon (i) the nature of the substance and solvent, and (ii) the concentration. The greater the concentration, the more rapidly is heat evolved. Thus, the addition of 1 mole of HCl to 250 c.c. of water evolves 16,950 calories. The effect of the addition of further quantities of water is as shown in Table CXX XVI, due to E. B. Millard :— Taste CXXXVI.—Heat of Solution of Hydrochloric Acid Amount of Water } : Total Calories Evolved Haied clo. Total Volume, c.c. Calories Evolved. Peeler aad. 250 500 450 16,950 500 1000 350 17,250 1000 2000 150 17,400 The great amount of heat evolved when sulphuric acid is dissolved in water is wellknown. A corresponding amount of heat is evolved when many other substances are dissolved, e.g. phosphoric pentoxide, anhydrous copper sulphate, metallic sodium, lime, etc. Conversely, the solution of saltpetre, or ammonium nitrate in water or of common salt in ice, is accompanied by a reduction in the temperature of the mixture. On slaking quicklime, CaO0+H,0=Ca(OH),, 15 Calories are evolved, whilst if the hydrate is dissolved in water, 3 more Calories are liberated per mole of the hydrate. The nature of the reactions between the solute and solvent in “ simple ” solution is not completely understood, though it seems fairly clear that some form of combination must occur. Heat of Combination.—When two solutions capable of reacting with each other are 34 530 HEAT AND TEMPERATURE mixed, the heat-changes which occur are dependent on (a) the extent of the ionisation of the dissolved substances, and (b) whether the products of the reaction are soluble or insoluble in the fluid. Solutions which are highly ionised and produce soluble compounds usually have very low heats of reaction, but if a precipitate is formed the heat-effect may be considerable. The neutralisation of an acid by a base when both are in solution is a type of exothermal reaction whose thermal properties are easily investigated, but a similar investigation of the reactions of ceramic materials is much more difficult on account of their insoluble character. The heat of neutralisation between an ionised acid and ionised base is about 13,800 calories per molecule of water formed. The effect is very different between slightly ionised solutions, because the heat effect is due to the following reaction :— Ht+OH-=H,0 +13,800 calories at 20° C. This value is increased at lower temperatures and decreased at higher ones. In more general terms, the heat of reaction, 7.e. the amount of heat absorbed or evolved during a chemical reaction at any temperature, is expressed by the formula : Q2=Q,—ACp(T,—T)), where Q, is the quantity of heat in the system at the higher temperature, Q, is that at the lower temperature, A Cp is the increase in heat capacity of the system during the reaction, and T, and T, are the higher and lower temperatures respectively. According to Berthelot’s law, “every reaction which takes place independently of the addition of energy from outside the system tends to form the combination which is accompanied by the greatest evolution of heat.” Hence, when carbon burns, CO, is formed in preference to CO. The final product in any reaction may usually be predicted by means of this law. Thermo-chemical changes and reactions may be expressed by ordinary chemical equations to which are added the thermal effects noted. Thus, CaCO,=CaO0+CO,+175J (or 41,825 calories). solid. solid. gas. This equation represents an absorption of heat which is necessary to decompose the calcium carbonate. C+0,=CO,+394 J (or 94,166 cals.) C+0O =CO +284 J (or 67,876 cals.) CO+0 =CO,+110 J (or 26,290 cals.) If the heats of formation of two compounds are known, the net effect of a reaction may be calculated. Thus, the heat of formation of magnesium chloride is 632 J, that of the sodium chloride is 408 J, whence, in the reaction MgCl,+Na, =2NaCl+Mg —632+2 x 0=2 x —408-+0-+-2 xc=184 J. HEAT OF FORMATION 531 The reaction is, therefore, exothermic, the heat equivalent to 184 J or 44,000 calories being liberated. According to Hess’s law, the quantity of heat evolved or absorbed by a reaction which takes place in two or more stages is the same as would have been evolved or absorbed if the reaction had occurred in one stage. Thus, the formation of carbon monoxide from carbon and oxygen is accompanied by the evolution of 67,876 calories, the formation of carbon dioxide from carbon monoxide by the evolution of 26,290 calories, and in the formation of carbon dioxide from carbon the same amount of heat (viz. 94,166 calories) is evolved as if the formation had taken place in the two stages just mentioned. Hence— (i) The heat of formation of a compound is independent of its mode of formation. (u) The thermal value of a reaction is independent of the time occupied by the process. (ii) The thermal value of a reaction is the sum of the heats of formation of the final products of the reaction less the heats of formation of the initial products of the reaction. The heat of formation of various oxides from their elements is shown in Table CXXXVII. Taste CXXXVII.—Heat of Formation Oxides, Calories. Oxides. Calories. H,0. . A ; 58,060 Na,O . s Z 100,900 CO . 2 : : 29,000 K,0O . ‘ ; ; 98,200 CO, . . : ; 97,000 FeO . z ‘ . 65,700 SiO, . F ; ? 196,000 Fe,0, : : : 195,600 MgO : ; A : 143,000 Fe,Q, : ; : 270,800 BaO . : : : 133,400 CaCO, : 2 : 273,800 CaO . : ; : 131,500 MgCO, ; . : 269,900 Al,O; : : " 392,600 CaSO, ; : : 321,800 The heat of dissociation of a substance is the amount of heat evolved when the substance is dissociated into its constituents. It is the same as the amount of heat absorbed when the compound is produced by the union of its constituents. The amount of heat rendered latent when a compound forms or evolved when the com- pound is dissociated is often termed its heat-content. Thus, a compound which when formed absorbs x calories will have a heat-content of +z calories, but if z calories are evolved when it is formed its heat-content will be —z calories. The heats of chemical reactions are often obscured by unobserved latent heats, by heat spent in doing work of different kinds, also by the dissipation of heat in the act of solution, by the effects of allotropism and isomerism, by a preliminary 582 HEAT AND TEMPERATURE dissociation of the reacting molecules and by differences in the specific heats of the initial and final products of the reaction. From the foregoing it will be seen that the study of thermal changes which occur during the heating and cooling of substances is very important (p. 464); it is dealt with more fully on p. 599. In dealing with complex materials, like those used in ceramics, an investigation of the various points at which endothermal and exothermal changes occur is often a valuable aid to an understanding of the changes which occur when such materials are heated either in the course of manufacture or in use. It reveals the nature of the substances formed under certain conditions of heating and cooling, and thereby greatly facilitates the control of the production of refractory and other ceramic materials in which certain properties are required to be constant within somewhat narrow limits. Powillet Effect—In 1822, Pouillet showed that sand, alumina, glass, etc., become heated when wetted with water ; this is generally known as the Pouillet effect. The heat evolved is proportional to the mass of the powder and to the exposed area of the solid. The heat evolved at 7° C. is nearly 0-00105 cal. per sq. cm. Below 4° C. there is a cooling effect instead of a heating one. Electrical Changes.—Heat may cause two kinds of electrical changes: (a) it may generate a current of electricity, and (b) it may increase the power of the substance for conducting electricity. Conversely, the generation and conduction of electricity produces heat. The generation of electricity as the result of heating ceramic materials is not of great significance, but the changes effected by heat in the electrical conductivity of such material is often very important, especially in the case of electrical insulating materials. The passage of a powerful electric current through an insulator generates heat, and as the electrical conductivity increases rapidly with an increase in temperature, the insulating properties rapidly diminish with an increase in the voltage of the applied current when once a passage of the current through the insulator has been established. The electrical properties of ceramic materials are further dealt with in Chapter XIV. Changes in Optical Properties.—Heat effects various changes in the optical properties of substances, these changes being usually a result of some change in the physical state of the substance. Thus, the change from one allotropic form of a substance to another will usually cause a change in the optical properties, and a change from the crystalline to the glassy or amorphous state will also effect a change in the optical properties of a substance. The crystalline form and, in some cases, the colour may alter as a result of heating. For further information on the optical properties of ceramic materials see Chapter XV. TEMPERATURE MEASUREMENT In order to compare the temperature of various substances or that of the same substance at different times it is necessary to have a unit of temperature, which is TEMPERATURE MEASUREMENT 533 commonly termed a degree. Unfortunately, there are two entirely different units in common use, each kind being the basis of a different scale. The Fahrenheit scale of temperature consists of 212°, the difference between the two points on a thermometer indicating the temperature of melting ice (32° F.) and that of boiling water (212° F.) being divided into 180 equal parts or degrees. In the Centigrade or Celsius scale of temperature the position on the thermometer indicated by the melting-point of ice is taken as 0° and the boiling-point of water as 100°. Consequently, yo OC. = (2° F. — 32), 2° F = (y C. x 3 439, The Centigrade scale is the one chiefly used for scientific work, but for some purposes the relation between the temperature and some of the other properties of a substance or the changes in the conditions to which it is subjected are more easily recognised if the Absolute scale of temperature is used. This is simply the Centigrade scale, modified so that its zero is --273° C., and 0° C.=273° A. At the zero-point on the Absolute scale all gases cease to exist as such, and various other phenomena occur which show that this temperature is a natural critical point. This scale is based on what is known as the Mariotte-Gay Lussac law :— Pv = Podo(1 + at), where #, v, and ¢ are the pressure, volume, and temperature (in °C.) of a perfect gas, P, and v, being the corresponding pressure and volume at zero on the Absolute scale, and a==1/273. Hence, t p= pol + a) and by substituting T (the temperature on the Absolute scale corresponding to ¢° C.) for 273-4, Poo = —— xT Se OTS a whence the well-known equation, 7 po = RT. To convert any temperature on the Centigrade scale to °A., it is merely necessary to add 273 and vice versa, 1.e. ge A. =y° C. + 273 yo C.=«@° A. — 273 Table CX XXVIII shows a comparison of the various temperature scales. 534 HEAT AND TEMPERATURE TaBLE CX XXVIII. —Centigrade and Fahrenheit Scales ° Cent. | ° Fah. ° Cent. | ° Fah. ° Cent. | ° Fah. ° Cent. | ° Fah. ° Cent. | ° Fah. a _ || | —— ———_- || —_ O— ——_ | ——_ —_ —— ie) Ho CO bo Or oO Reel bo bo =) Ne) — — eo} CU ee) — [sy) bo bo for) ie} lop) punt -~j Ww oo ws oo > TEMPERATURE SCALES 535 TABLE CXXXVIII— Continued Sa@ertten| ee Hah: ° Cent. | ° Fah. © Cent. | ° Fah. © Cent. } * Fah. ° Cent. | ° Fah. 205 401-0 || 246 474-8 || 287 548-6 || 328 622-4 || 430 806 206 402-8 || 247 476-6 || 288 550-4 || 329 624-2 || 440 824 207 404-6 || 248 478-4 || 289 552-2 |) 330 626-0 || 450 842 208 406-4 || 249 480-2 || 290 554-0 || 331 627-8 || 460 860 209 408-2 || 250 482-0 || 291 555°8 || 332 629-6 || 470 878 210 410-0 || 251 483-8 || 292 557-6 || 333 631-4 || 480 896 211 411°8 || 252 485:6 || 293 559-4 |) 334 633-2 || 482 900 212 413-6 || 253 487-4 || 294 561-2 || 335 635-0 || 490 914 213 415-4 || 254 489-2 || 295 563-0 || 336 636-8 || 500 932 214 417-2 || 255 491-0 |} 296 564-8 || 337 638-6 || 538 | 1000 215 419-0 || 256 492°8 || 297 566°6 || 338 640-4 || 593 | 1100 216 420°8 || 257 494-6 || 298 568-4 || 339 642-2 || 600 | 1112 217 422-6 || 258 496-4 |) 299 570-2 || 340 644-0 || 649 | 1200 218 424-4 || 259 498-2 |; 300 572-0 || 341 645-8 || 700 | 1292 219 426-2 || 260 500-0 || 301 573°8s|| 342 6476 || 704 | 1300 220 428-0 || 261 501-8 || 302 5756 || 343 649-4 || 760 | 1400 221 429-8 || 262 503-6 || 303 5IT-4 || 344 651-2 |} 800 | 1472 222 431-6 || 263 505-4 || 304 579-2 || 345 653-0 |} 815 | 1500 223 433-4 || 264 507-2 || 305 581-0 || 346 654-8 || 871 | 1600 224 435-2 || 265 509-0 || 306 582°8 || 347 656-6 || 900 | 1652 225 437-0 || 266 510-8 || 307 584-6 || 348 658-4 || 926 | 1700 226 438-8 || 267 512-6 || 308 586-4 || 349 660-2 || 982 | 1800 227 440-6 || 268 514-4 || 309 588-2 || 350 662-0 || 1000 | 1832 228 442-4 || 269 516-2 || 310 590-0 || 351 663°8 || 1037 | 1900 229 444-2 || 270 518-0 |} 311 591°8 || 352 665-6 || 1093 | 2000 230 446-0 || 271 519-8 || 312 593-6 || 353 667-4 || 1100 | 2012 231 447-8 || 272 521-6 || 313 595-4 || 354 669-2 || 1148 | 2100 232 449-6 || 273 523-4 || 314 597-2 || 355 671-0 || 1200 | 2192 233 451-4 || 274 525-2 || 315 599-0 || 356 672-8 || 1300 | 2372 234 453-2 || 275 527-0 || 316 600-8 || 357 674-6 || 1400 | 2552 235 455-0 || 276 528-8 || 317 602-6 || 358 676-4 || 1500 | 2732 236 456°8 || 277 530-6 || 318 604-4 || 359 678-2 || 1600 | 2912 237 458-6 || 278 532-4 || 319 606-2 || 360 680-0 || 1700 | 3092 238 460-4 || 279 534-2 || 320 608-0 || 370 698 1800 | 3272 239 462-2 || 280 536-0 || 321 609-8 || 371 700 1900 | 3452 240 464-0 || 281 537-8 || 322 611-6 || 380 716 2000 | 3632 241 465-8 || 282 539-6 || 323 613-4 || 390 734 2100 | 3812 242 467-6 || 283 541-4 |) 324 615-2 || 400 752 2200 | 3992 243 469-4 || 284 543-2 || 325 617-0 || 410 770 2300 | 4172 244 471-2 || 285 545-0 || 326 618-8 || 420 788 2400 | 4352 245 473-0 || 286 546-8 || 327 620-6 || 426 800 2500 | 4532 536 HEAT AND TEMPERATURE The methods used for the measurement of temperature may conveniently be divided into two groups, termed respectively (a) thermometry and (b) pyrometry. There are no definite limits to these two groups, as the ranges of pyrometers and thermometers overlap to a considerable extent, but, as a rule, the term thermometry is applied to the measurement of temperatures up to about 300° C. and pyrometry to the measurement of higher temperatures. The principal means of measuring temperature are as follows :— 1. By measuring the increase in the volume of a solid, liquid, or gas and calcu- lating the temperature from the result. This method is chiefly used in thermometers. 2. By comparing the optical characters of the light emitted from several sub- stances at known temperatures and from the results preparing a scale which can be extrapolated for higher temperatures. This method is used in optical pyrometers. 3. By measuring the changes in the electrical potential or the resistance of an electric circuit when part of the circuit is maintained successively at various known temperatures, and afterwards extrapolating for other temperatures. This method is used for thermo-couples and electrical resistance pyrometers. 4. By measuring the radiation from the hot substance by means of a calorimeter, thermometer, or pyrometer. In the ceramic industries, a combination of a mirror and a thermo-couple pyrometer is usually employed and is known as a radiation pyrometer. 5. By observing the effect of heat in changing the shape of cones, bars, or other “pyroscopes, the temperature corresponding to such behaviour having previously been ascertained. 6. By the method of mixtures, a ball of metal or other suitable material being heated to the temperature it is desired to measure and then dropped into a calorimeter containing water. The specific heat and weight of the test- piece and the weight and temperature of the water before and after the test being known, the temperature of the test-piece can easily be calculated from the formula on p. 512. The thermometers used in determining the temperature of ceramic materials usually consist of a sealed glass tube containing mercury. If sufficiently long, they may be used to measure temperatures up to about 350° C., or up to 450° C. if the tube is filled with nitrogen, or to 600° C. if an alloy of sodium and potassium is substituted for the mercury. For some purposes, a thermometer may be used more conveniently if it is provided with a maximum temperature indicator. | Gas thermometers are now seldom used, as they have been replaced by pyrometers which are more convenient. A gas thermometer consists of a bulb of metal, porcelain, or other impervious material, provided with an outlet pipe connected to a sensitive pressure-gauge. The bulb is filled with air or gas and the reading of the pressure- gauge is noted. The bulb is then placed in the furnace, the temperature of which PYROMETERS 537 is to be determined and the pressure-gauge read at intervals until a maximum reading is obtained. The expansion of the gas in the bulb can then be used to calculate the temperature of the bulb. The pyrometers most suitable for high temperatures are of three chief types, namely, electrical, optical, and radiation pyrometers. ew NN YEN Y es lly 4 N\ ‘f/ S| a WS nome Rcd en e I ee ee 3 3 sae = Fic. 47.—THERMO-COUPLE Fia. 48.—RESISTANCE PYROMETER.! PYROMETER.? Electrical pyrometers are of two distinct kinds :— (a) Those in which the electricity generated in a thermo-couple is measured at different temperatures (fig. 47). (6) Those in which the variations in the electrical resistance of a wire is measured at different temperatures (fig. 48). Thermocouple pyrometers are based on the fact that when the junction of two * Type of pyrometer supplied by the Cambridge & Paul Instrument Co. Ltd. 538 HEAT AND TEMPERATURE dissimilar metals is heated a current of electricity is generated, the voltage depending on the temperature of the junction. Resistance pyrometers are based on the fact that the electrical resistance of a wire varies with its temperature. Thus, if a coil of wire, which forms part of an electric circuit, is placed in a hot furnace, the difference in resistance is proportional to the temperature of the coil. The wire resistance coil is usually protected by a refractory sheath, and a counterbalancing wire of the same length as the leads is placed in the tube to neutralise their resistance. The resistance of the coil is measured by means of a wheatstone bridge and a galvanometer, the resistance in two arms of the bridge being adjusted until the galvanometer shows no deflection. Pyrometers of both the thermo-couple and resistance types, in which the scales are graduated so as to read directly in Centigrade degrees, can be obtained from scientific-instrument makers, who should be informed of the purpose for which the instrument is to be used, so that the couple may be made of suitable metals. Such pyrometers can be used to measure all temperatures up to about 1400° C., but above the latter temperature their readings, after prolonged use of the instrument, are uncertain, so that frequent standardisation is necessary. It is often convenient to attach a continuous recorder to the pyrometer so that a continuous record of the temperature may be obtained. Optical pyrometers have become very popular during recent years. They are based on the principle that the light emitted by a heated body is proportional to its temperature. The chief types of optical pyrometer are :— - 1. Those in which the light from the hot body is varied until it matches a fixed standard. Thus, in a pyrometer devised by Féry, the light is reduced by inserting a standard wedge of dark glass, the position of the wedge being read on an arbitrary scale. In the Le Chatelier optical pyrometer, the intensity of the source of light is reduced by means of an iris diaphragm. In the Wanner pyrometer, the standard light and that from the hot body are viewed through a polariser, a ray from each source being compared and the temperature calculated from the angle through which the analyser must be rotated in order to make both rays match. 2. Those in which the standard light is varied to match the light from the hot body. Thus, if an electric light is used as the standard, its intensity can be varied until it coincides with that of the hot body ; the temperature of the latter is assumed to be proportional to the current flowing in the standard lamp circuit as indicated by amilliammeter. Thus, in the Holborn-Kurlbaum pyrometer, the current supplied to the lamp is varied until the filament is just invisible when viewed against the hot body as a background. 3. Those in which the light from the hot body is just extinguished by interposing a series of standard coloured glasses of different thicknesses as in Lovibond’s tintometer or. the Wedge pyrometer, in which the hot object is viewed through a wedge of dark-red glass which is slid through a tubular eyepiece so as to insert different thicknesses of glass between the object and the eye until the object is just invisible. The temperature is read off on the scale attached to the instrument. Optical pyrometers are often useful, though not very accurate, except PYROMETERS 539 for strictly comparable conditions and frequent standardisation is necessary. They cannot be used to give a permanent record, and require skilled attention and very careful usage. Radiation pyrometers may include the optical pyrometers just described, as the latter are based on the light radiated by the hot body. The term radiation pyrometer is, however, chiefly confined to those in which the radiated heat is measured by its effect on the junction of a thermo-electric pyrometer, the radiated heat being focussed either by means of a lens or a concave mirror, and the voltage of the current generated measured by means of a galvanometer the scale of which reads directly in Centigrade degrees as in other pyrometers. In Féry’s radiation pyrometer (fig. 49), a gilt-surfaced concave mirror is mounted on a rackwork focussing arrangement so as to focus accurately the heat rays on to the junction. The Forster radiation pyrometer is so constructed that the focus is “fixed” as in some hand cameras, no adjustment being required if the pyrometer is placed at a suffi- cient distance from the hot object. In Féry’s spiral pyrometer, the heat is focussed on to a coil of flat metal ribbon to which a pointer is attached. When heated by the rays falling upon = it, the coil unrolls and causes the pointer to move over a scale which indicates the temperature of the source of heat. Whilst very convenient it is not so accurate as the type with a thermo-electric junction. Radiation pyrometers are accurate for temperatures over 700° C., but require to be skilfully used, and if roughly handled give erroneous results. The mirrors must be specially cared for, as they will not reflect properly if they become damaged or dirty. They do not deteriorate - on prolonged heating in use in the same thermo-electric and resistance pyrometers, and for this reason are very convenient. Pyroscopes are devices for indicating the temperature of a furnace by the change of shape which they undergo when heated. Strictly, they do not measure the tem- perature, but the amount of heat which they have absorbed; but whilst this may appear to be objectionable it has certain advantages. In heating ceramic materials, the precise temperature attained by the gases in the furnace is of minor importance ; what really matters is the effect of heat on the contents. As most pyroscopes are themselves ceramic materials, their behaviour indicates the effect of the heating and is, therefore, in some respects more satisfactory than that of a pyrometer. (In many cases, both a pyrometer and a pyroscope should be used as the latter will not show a decrease in temperature. ) 1 Type of pyrometer supplied by the Cambridge & Paul Instrument Co., Ltd. Zaz eee ESS Saat Zi Z CMe tesa eAeseeQee eee Ss) LALA AAA AAAS Cael iy WANN STI N KM N N N G Fic. 49.—Firy RADIATION PyROMETER.! 540 HEAT AND TEMPERATURE One of the best forms of pyroscopes is the Seger cone, invented by H. Seger, of Berlin, who made various mixtures of finely powdered (100-mesh) silicates in such proportions that they would fuse at different temperatures. The materials were shaped into three-sided pyramids, 5 cm. high and 1-5 cm. on each side at the base,* as shown in fig. 50, and the fusion-point (or more correctly the softening-point) was taken to be that at which the apex of the cone bends over and just touches the base. Table CXV shows the series of Seger cones and the temperatures at which they fuse.2 It will be seen that the difference between the cones is generally 20°-30° C. It is important that the temperature of the cones should rise at a suitable rate, as if heated too rapidly they will reach a higher temperature than that indicated in the table before bending on account of their being poor conductors of heat. This is no objection in practice, because the ceramic materials with which they are employed have the same low thermal conductivity. The maximum rate of heating is usually taken as 2° C. per minute. This type of heat recorder is very largely used in the ceramic industries on account of its cheapness, simplicity, and reliability. In use, three or more cones of different values are placed near to the object whose tem- perature is to be estimated and heated up with it. The temperature of the object at different times is then indicated by the bending of cones of different fusing-points. Thus, if it is desired to heat a material to a certain temperature three different Seger cones are used-—one to indicate a temperature about 20° C. below what is required and to act as a “ warner,” the second cone shows when the desired temperature has been reached, and the third cone shows whether it has been exceeded. Cones are sometimes adversely affected by the gases in a furnace, and especially by carbon monoxide. They should, therefore, only be used under oxidising conditions. Other pyroscopes of a similar nature are also made. Thus, calorites consist of long square bars, one end of which is painted to show the temperature at which it fuses. Holdcroft’s thermoscopes are similar bars which are placed in a horizontal position in the furnace, their critical point being indicated by their sagging in the middle. A number of suitable thermoscopes may be placed on a fireclay holder in a kiln, and the temperature will be indicated at intervals by the sagging of the bars. Opinions differ considerably as to whether the cones or thermoscopes are the more convenient. Watkin’s pyroscopes are small tablets which melt at specified temperatures. They are very small and often difficult to see when in use. Brearley’s Sentinel pyrometers are similar but rather larger. Their critical point is reached when they are completely fused. . Fie. 50.—SErGER Conzss. 1 For the highest temperatures, a smaller series of cones 1 in. high is used, 2 The composition of the various cones is given in the same Table. PYROSCOPES AND ‘“ TRIALS ” 541 Fusible metals and salts are also of some value for lower temperatures, but they are not much used in the ceramic industries.. They are convenient, however, for standardising pyrometers, etc. The chief salts used and their melting-points are : (i) a mixture of equi-molecular parts of sodium and potassium chlorides (650° C.), (11) sodium chloride (800° C.), (iii) anhydrous sodium carbonate (850° C.), (iv) anhydrous sodium sulphate (900° C.), (v) sodium plumbate (1000° C.); (vi) anhydrous potassium sulphate (1070° C.), and (vii) anhydrous magnesium sulphate (1150° C.). The fusion-points of sheets or strips of various metals is shown in Table CXXXIX, due to Seger. TaBLE CXXXIX.—Melting-Points of Metals and Alloys Metal or Alloy. Melting-Point, ° C. 800 parts of silver, 200 parts of copper. : ; : 850 DOO Ls OE an 900 Pure silver . ; ; 954 400 parts of silver, 600 orth of gold 1020 - Pure gold. : 1074 950 parts of gold, 50 parte of pistiniin : : 1100 2007) | ra 100 - 3 ! : : 1130 850 Co, LOO =; is : : 1160 800 __,, pa 200" 6, A : 1190 HOOe 5; POUL 5 me 1220 700s, ,, 900 < . ; ; 1225 600 __s,, ,, 400 : rn : 1320 500 » 900 : ; 1385 Platinum : ; : ; 1775 Trials made of clay and dipped in a glaze-mixture are often used to ascertain the progress of the heating of pottery ovens. The glossiness and colour of the glazed pieces are a sufficient indication to a skilled burner of the progress of the firing. This apparently crude and uncertain method has enabled potters’ firemen for many years to control the firing of kilns with remarkable success. A simple means of ascertaining whether the heating of a kiln filled with bricks, tiles, or some other ceramic ware has been effective, is to measure the distance between the top of the contents of the kiln and the top of the kiln itself. As most bricks will shrink 4 inch per linear foot in burning, it is clear that in a kiln filled with bricks to a oe of 8 feet, the loss in height after the firmg is completed will be 4x8=4 inches, and that by measuring the shrinkage at intervals the progress of the heating 542 HEAT AND TEMPERATURE may be ascertained. Though crude, this method is in regular use and proves to be satisfactory. Wedgwood—the great potter—used to measure the contraction of small test-pieces which were drawn from the kiln at intervals and their diameter measured in a V-shaped gauge. By substituting calipers for the gauge used by Wedgwood the contraction of larger and more conveniently shaped pieces can be accurately measured. Strict accuracy of size is very important where the articles are to be used in conjunction with others, as in the ceramic portions of switches and other electrical fittings, though it is seldom practicable to measure the contraction of individual pieces whilst they are in the oven or kiln; when the heating is finished, it is obviously too late for such measurements to be used in controlling the firing. There is great need for a means of determining the changes in volume which take place in ceramic articles whilst they are in the kiln, so that the heating may be regulated accordingly. Unfortunately, there seems to be no suitable means of obtaining this information at the time when it is most needed. Pyrometers and other instruments for ascertaining the temperature of the kiln are exceedingly useful, but they give only indirect information and are, therefore, of less value than is desirable. The difficulty is greatly increased when articles are burned in saggers, as it is then almost impossible to gain access to them, or to know precisely what is occurring to them at any given moment. In this respect, so much has to be left to chance, and to the subconscious judgment of the firemen, that the art of kiln-control is far ahead of the science. CHAPTER XIII THE EFFECT OF HEAT ON CERAMIC MATERIALS In the previous chapter, the general effect of heat upon various substances has been considered ; this chapter is devoted to the effect of heat upon ceramic materials—a matter of special importance as they are all subjected to heat at some stage in their manufacture, and some ceramic materials owe their value to their resistance to heat when in use. The effects of heat upon ceramic materials may be conveniently considered in telation to— 1. Drying clays and other ceramic materials, both in the natural state and after they have been made into articles. 2. Burning or firing ceramic materials and articles made from them, this subject being sub-divided into (a) smoking, (b) decomposition, (c) the “ full fire” stage of heating, and (d) the finishing stage, including “ soaking ” and “‘ vitrification.” 3. The effect of withdrawing heat, 1.e. cooling the fired products in such a manner as not to damage them. 4, The effects of excessive heating, as shown by “squatting,” fusion, boiling, volatilisation, etc. In addition, the effects of heat on ceramic materials may also be considered in tespect of the resulting changes in the volume of the material as a whole or of its component grains, in the thermal conductivity, specific heat, refractoriness, and resistance to sudden changes in temperature. EFFects oF Heat In DryiIne The effects of heat during drying are partly physical and partly chemical in character. The resulting changes are practically the same as occur when the drying is effected at atmospheric temperature, the water being removed by a current of dry air. The drying is, however, greatly facilitated and accelerated by the application of a gentle heat. The changes which occur in drying have been fully described in Chapter VII. In actual practice, the drying of ceramic articles is not usually completed in the drying appliances, even when the articles appear to be quite dry; the remaining water is removed when they are in the kiln or oven. 543 544 EFFECT OF HEAT. EFrects oF HEat IN FrRine The effects of heat in burning or firing ceramic articles are most conveniently studied by considering separately each stage in the process, commencing with the effect of a very gentle heat—technically termed “ smoking ’’—and proceeding until the final or “ finishing stage ”’ of the firing is reached. It must, of course, be under- stood that there is no sharp line of demarcation between these various stages, the effects of heat being continuous, so that the division of the firing into several “‘ stages ” is wholly empirical. Smoking.—When a piece of clay or an article made of clay and allied materials which has been dried to the extent customary in manufacture is placed in a kiln or oven, the first effect of the heat is to remove any moisture still remaining in the material. It is by no means unusual for 5 per cent. of water, equivalent to 1 cwt. or 3000 cubic feet of steam per ton of goods, to be present in the “ dried ” articles taken to the kiln and to be evolved as steam during the first stage of heating. For the steady evolution of this enormous volume of vapour without any damage to the contents of the kiln, it is very important to provide ample ventilation and to take precautions that the temperature shall not rise too rapidly, as, otherwise, the goods may be cracked by being unable to withstand the pressure of the steam produced in their interior and unable to escape with sufficient rapidity through the small pores in the articles. The first or “ smoking ” stage of the burning of ceramic materials is finished at a temperature of about 120° C. When this temperature is attained throughout the kiln and its contents, the whole of the water will have been vaporised and the second stage of heating may then be commenced. At so low a temperature as 120° C. very few chemical changes occur, other than those produced when the heating is effected by waste gases bearing products of combustion from another part of the kiln. Such gases often contain sulphuric trioxide which can combine with any lime present in the clay and, in the presence of moisture, may form calcium sulphate in the form of a white deposit or ‘“‘ scum ” on the surface of the goods. The removal of the water is the chief change which occurs during the “ smoking ” ; other changes which may be produced simultaneously include the following :— (1) Shrinkage occurs as a result of the particles approaching each other on the removal of the water which previously separated them. The extent of this shrinkage on drying is described in Chapter VII. (ii) The porosity is increased, because the pores of the material at the end of the smoking stage are filled with air instead of water, as described on p. 75. The change in porosity also causes a change in the apparent density (p. 205). (iii) The permeability is also increased for the same reason (p. 89). A mass of clay when saturated with water is practically impervious, but, when dry, it may allow gas or water to pass through readily. (iv) The colour may change slightly (p. 107), most clays becoming rather lighter when perfectly dry. Other materials change to a greater or less extent. - DECOMPOSITION DURING BURNING 545 (v) The hardness of clays is increased (see Chapter III), but non-plastic materials usually become more friable. (vi) The compressive strength of clay wares increases on drying, but the strength of non-plastic materials usually decreases. Hence, non-plastic materials must be very carefully handled in the dry state as they are liable to be very friable (see Chapter IV). Other minor changes in thermal and electrical conductivity, etc., may also occur. At the end of the “ smoking ”’ stage, ceramic materials are either (a) harder than before, if made largely of clay or containing some other colloidal gel, or (6) more friable, if made of non-plastic materials. The latter may, however, be hard if water-glass has been used as a bond. In either case, they have the same properties as the thoroughly dried material, the other effects of heat at this stage being usually negligible. : The decomposition stage is the second stage in the burning or firing of ceramic materials ; its range of temperature extends from about 120° C. to about 900° C., and in this interval many important chemical and physical actions occur. It is usually essential at this stage that the temperature shall rise very steadily, though the rate of heating may vary within wide limits with different materials. Slowly at 120° C. and much more rapidly at 400°-600° C., minerals containing chemically combined water are dissociated, the water being evolved as steam. The most important of these ceramic materials is clay, which is completely decomposed between 500° and 700° C., and usually below 600° C. as described on p. 350. Various other hydrous silicates and alumino-silicates which may be present are also decomposed. Limonite (p. 418) loses its combined water at about 500° C., haematite (p. 418) being formed. Some forms of hydrous alumina lose their combined water at one temperature ; others, including bawaite, retain a portion of it at a much higher temperature (p. 340). The numerous hydrous minerals which occur in very small quantities also lose their combined water during this stage of burning. Allotropic changes also occur in some of the minerals present. Thus, at about 575° C. a-quartz is converted into the B-variety, and at a higher temperature quartz is converted into other forms having a lower specific gravity (p. 216). As the temperature continues to rise to above 700° C., other minerals are dissoci- ated ; some of the carbonates (such as limestone, chalk, magnesite, and dolomite) evolve carbon dioxide and so are converted into oxides (p. 490). Any sulphides present, such as pyrites, muscovite, etc., are partially decomposed, a portion of the sulphur being oxidised and evolved as a gas, whilst the remainder is only evolved at a higher temperature. At about 900° C., and sometimes at a lower temperature, any carbonaceous matter present will be decomposed, usually producing a black charred mass, which is con- verted into carbon-dioxide gas if the atmosphere in the kiln or oven is sufficiently oxidising. Any shale oil and volatile hydrocarbons are evolved as hydrocarbon gases which may burn and produce carbon-dioxide gas and steam. Gypsum and other sulphates are partially decomposed at a temperature of about 800° C., the sulphur being completely expelled at temperatures above 900° C. 35 546 EFFECT OF HEAT Any ferrous iron compounds will, if the atmosphere in the kiln or oven is sufficiently oxidising, and the contents are sufficiently porous for that atmosphere to penetrate properly, be converted into ferric compounds. Many other minor reactions will also occur in which minerals present in propor- tions too small for them to be considered separately are decomposed in a manner similar to some of those previously mentioned. At a temperature between 120° and 900° C., many chemical changes commence which are not completed until a higher temperature is attained, but having once started they proceed gradually and are completed (as far as this is permitted) in the next stage of the burning. Thus, the bases present as impurities in the material or produced by the action of heat on carbonates, etc., begin to attack the silica and alumina also present and, conversely, the acidic materials present in basic ones react with the bases. By this means, fusible silicates and alumino-silicates are produced and if they are sufficiently numerous and have a sufficiently low melting- point, an appreciable amount of glassy or vitreous matter will be formed to surround many of the individual particles and partially to fill the pores or interstices between them. This is more fully considered on p. 551. Apart from the reactions which result in the production of a fused glassy material, the chief changes which occur at this stage are oxidation processes which depend for their satisfactory completion on the presence of an ample supply of air. As these changes occur in the order of their affinity for oxygen, the heating must be properly controlled or undesirable reactions may occur whilst desirable ones remain incomplete. Thus, carbon has the greatest affinity for oxygen, so that in a highly porous and heat-resistant body the oxidation of compounds of iron, sulphur, etc., will not be completed until all the carbonaceous matter has been fully oxidised, and where the latter is present in large quantities it is important that the temperature should rise sufficiently slowly to remove the carbonaceous matter entirely before too high a temperature is reached. Otherwise, as the heat is nearly always applied externally to the materials, the exterior may be raised to such a temperature that partial fusion occurs and the exterior pores are thereby sealed before all the carbonaceous gases can escape. The result of such sealing will be shown at a later stage by the articles being swollen or bloated by the pressure of gases in their interior. The appearance of the articles may also be spoiled by the charred material remaining behind and being incapable of oxidation because the air in the kiln cannot gain access to it on account of the sealed pores. It is out of place in this volume to discuss the various methods employed to burn different goods properly at this stage; further information will be found in the author’s works, The Clayworkers’ Handbook; British Clays, Shales, and Sands ; Refractory Materials : Their Manufacture and Uses; Modern Brickmaking, etc. The nature of these oxidation processes need not be described in detail as they have been discussed on pp. 491-492. Various physical changes also occur during the decomposition stage of firing, these 1 The only exceptions being clamp bricks and some articles in which sawdust or other combustible matter forms an essential part of the material of which they are made. CHANGES DURING BURNING 547 being partly as a result of the chemical changes mentioned. The chief of the physical changes are as follows :— Changes in volume occur as a result of the following causes: (a) the water which previously separated the particles having been removed, the remaining particles draw closer together. The expulsion of “ combined water,” carbon dioxide, sulphur oxides, etc., has a similar effect and produces a further shrinkage which will be more or less pronounced according to the extent to which they occur; (b) a reduction in volume, or shrinkage, also occurs in some materials as a result of allotropic changes brought about by the heat, as when magnesia: is slowly converted into periclase, though this change is usually much more pronounced at higher temperatures (p. 551) ; (c) an expansion or increase in volume occurs in some materials containing, or largely composed of silica, due to allotropic changes caused by heat (p. 568). Changes in porosity occur to a varying extent during this stage. The porosity usually increases until near the end of this stage of the firing, though when the pro- portion of readily fusible matter is sufficiently large, the porosity may be reduced at temperatures above 800° C. Further information on changes in porosity of ceramic materials when heated will be found in the section on Porosity in Chapter II. The permeability generally increases during this stage (see the section on Permeability in Chapter II). Colour changes frequently occur in this stage of firing as a result of the chemical reactions which occur. Clays generally become lighter in colour unless they contain much iron—in which case the colour is intensified by the oxidation of the iron with the production of a bright red colour. Iron pyrites is so difficult to convert into red ferric oxide in the presence of clay that it usually forms black spots of fused ferrous silicate. The colour changes are described in the section on Colour in Chapter III. In some articles made of ceramic materials the changes in colour produced by heating is not important. In other cases, on the contrary, the value of articles such as red facing bricks, roofing and floor tiles, terra-cotta, etc., is largely determined by their colour and it is, therefore, important that, with them, this stage of heating should be controlled with special care and skill. Such articles are fired almost solely with reference to their colour, the other properties required receiving but little attention and the burning may be completed in this stage when the colour is spoiled by heating the articles to higher temperatures. Hence, red bricks and similar goods are finished at a temperature of 900°-1000° C. The hardness of ceramic materials usually increases at temperatures between 120° and 900° C. Clay is changed from a soft plastic state in which it is used when shaping the articles into a hard stony material which has quite different properties. Colloidal gels and other binding agents are usually hardened and to some extent are fused, thus cementing together the non-plastic particles, though this cementation may not be completed until a later stage of firing. The changes in hardness which occur are discussed at length in the section on Hardness in Chapter III. The strength (when cold) of ceramic materials is increased during this stage of the burning because the hardness increases and the bonding material becomes firmer. The chief increase in strength—except when the more readily fusible substances are 548 EFFECT OF HEAT present—occurs in the next stage when vitrification proceeds rapidly, but a notable increase often occurs during this stage as a result of the commencement of vitrifica- tion. The strength of the materials at the high temperature may be less than that at lower temperatures, V. Bodin having found that many ceramic materials, when tested whilst hot, decrease in strength up to 800° C. and then increase again as the temperature rises to 1000° C. The strength of materials at different temperatures is discussed in detail in Chapter IV. Changes in the specific gravity and apparent density of ceramic materials occur at this stage as a result of various chemical changes and the formation of various allotropic modifications of the original substances. Thus, the specific gravity of silica begins to decrease, whilst that of magnesia rises. Clay which has been heated to expel all the combined water does not change much in specific gravity when heated to 900° C., unless it is very impure and contains a large quantity of free silica, in which case the latter causes a decrease in specific gravity. The changes in specific gravity are discussed in detail in Chapter V. As the porosity changes, the apparent density is also modified according to the extent of the change (see also Chapter V). The thermal conductivity and specific heat of ceramic materials generally rise as the temperature increases (pp. 584 and 594). The electrical conductivity of ceramic materials increases with the temperature (Chapter XIV), whilst their resistivity decreases. Consequently, ceramic materials. become weaker insulators when heated than in the cold state. Summarising, it may be said that the changes which take place in the “‘ decom- position stage ” of firing are the results of oxidation and chemical reactions between the various bases and acids, though where the proportion of soda and potash is small the latter reactions do not make much progress. In some of the very impure clays. used for brickmaking, etc., sufficient soda, potash, and lime are present to ensure a considerable amount of glassy compounds (due to the combination of these bases. with silica) being formed and the resulting products are then hard, dense, and sonorous. when struck. When a brilliant red colour is desired, the firing must usually be finished before much vitrifiable material has been formed as it tends to produce an unpleasant brown instead of the brilliant terra-cotta red which is so much desired in some clay products. In the case of some pottery ware made of natural or artificial mixtures of clay and calcium carbonate, the firing is finished at about 900° C., as a much higher temperature would cause the formation of fused calcium-iron silicates. The firing of ceramic articles made of basic or highly aluminous materials is not completed in this stage, so must be considered later. Plumbiferous glazes and those rich in borax are completely fused in this stage of burning, but do not call for any special comment except in so far as they give some indication (by analogy) of the general nature of the fusible compounds in other ceramic materials. The full-fire stage in burning ceramic ware commences at that temperature at. which the kiln or oven may be heated as rapidly as is possible without serious risk. CHANGES DURING BURNING 549 of damage to its contents. In the case of the most impure clays, it probably com- mences at about 800° C., but it is usually safer to regard it as commencing at 900° C. with some reservation with respect to the more fusible clays, glazes, etc. The “ full-fire stage ’’ is in every respect an intermediate one and in it the various chemical and physical changes which commenced in the prior, or decomposition stage continue at a more rapid rate, some of them reaching completion. In the “ full-fire ” stage, the production of fused material increases rapidly and the more mobile and fusible glasses produced readily penetrate the pores in the more refractory material, dissolving the latter and so producing a still further quantity of the fused glass, slag, or “ vitrified matter.” The chemical changes which occur in this stage are not essentially different from those in the latter part of the previous stage. They consist chiefly in the combination of the bases or basic silicates with a further amount of silica. The higher the tempera- ture attained in this stage, the more complete will be the chemical reactions between the basic and acid materials present and the greater the amount of silica entering into solution in the fused portion of the material. These reactions are more conveniently considered separately (see Votrification, p. 551). The porosity of the materials continues to increase and reaches a maximum in this stage, but in the following one it begins to decrease as a result of vitrification, so that the end of the full-fire stage is sometimes considered to be that at which the - porosity reaches a maximum. It is, however, very difficult to prescribe any definite upper limit to the full-fire stage, because it differs so greatly with different materials and is, in itself, somewhat indefinite in character. The permeability also reaches a maximum in this stage, but is subject to the same limitations. The colour of any ferric oxide or allied compounds assumes a maximum brilliancy in this stage, but as the amount of vitrified matter increases, the colour is considerably modified by other chemical changes which take place. The strength of ceramic materials (after cooling) increases as the temperature of firing rises and continues to do so during this and especially during the next stage, when a considerable increase occurs on account of the vitrification of the bond (see Chapter IV). The strength at high temperatures, which, according to Bodin, decreases with many materials up to 800° C., rises during this stage and attains a maximum about 1000° C. The maximum strength at high temperatures generally corresponds with the point of maximum porosity ; this must not be confused with the maximum strength at ordinary temperatures which is often greatest when the porosity is least (p. 152). The specific gravity of some of the materials changes in the same manner as in the decomposition stage, but usually at a more rapid rate as the temperature is greater. The changes which occur in the thermal (p. 584) and electrical conductwities (p. 608) also continue in the same general manner, but more rapidly, the latter always and the former generally increasing during this stage of burning. The finishing stage of the firing of ceramic ware is that in which the desired reactions and other changes are completed or have progressed to such an extent as 550 EFFECT OF HEAT to produce articles or materials having the requisite properties. In this stage the temperature of the kiln or oven does not rise appreciably above that of the full-fire stage. On the contrary, it should usually be maintained as constant as possible. For this reason, this period is often known as the soaking stage, the materials or articles being regarded as “ being soaked in the heat ’”’ until it has penetrated completely through them in much the same manner as water will penetrate a porous material immersed in it for some time. The purpose of this prolonged heating or soaking at an almost constant temperature is to enable the various changes which have commenced in previous stages to be completed or at any rate to progress to an extent which will produce the desired properties in the product. If the temperature is allowed to rise rapidly until the firing is finished there is always a danger—except with the most refractory materials —that the articles will be distorted or their colour or other desirable properties will be spoiled by overheating. This serious defect is largely avoided by maintaining the temperature at an almost constant level during the last stage of firing, as this procedure enables the process to be more satisfactorily and readily controlled. This prolonged heating or “ soaking” at a suitable high temperature may have the following effects :— (a) It may increase the amount of fused matter and, consequently, the amount of chemical action between the fluxes and the more refractory constituents, thus producing a larger amount of vitrified or glassy material which will gradually fill the interstices between the other particles, rendering the whole impermeable, as in the case of stoneware, or even translucent, as in porcelain. By allowing ample time for the various chemical reactions to occur, prolonged heating of a ceramic material at the close of the firing imparts great stability to the material as a whole and enables a state of stable equilibrium to be attained. This is very important, when the product is to be reheated in use, as an unstable product —due to insufficient “ soaking ’’—will continue to undergo various physical and chemical changes which may have very serious consequences. The heating must not, of course, be prolonged to such an extent that undue distortion or loss of shape (see Squatting, p. 560) occurs. It is in the avoidance of this defect that the skill of the fireman is revealed, for the chemical reactions which are facilitated by the burning process must be stopped before they have proceeded too far. In other words, in the production of ceramic ware, most of the chemical reactions possible between the various constituents can never be completed, as that would render the material quite useless for the purpose for which it is intended. In this sense, as J. W. Mellor has pointed out, the chemistry of the firing of ceramic substances is a chemistry of arrested reactions. Unless the arrest takes place within a very narrow range of time or tem- perature, the product will be spoiled, hence, the enormous importance of as complete a control as possible over the kilns or ovens in which the burning is effected. In most of the chemical processes employed in other industries the principal object is to complete the reaction as rapidly as possible consistent with obtaining the maximum yield of the desired product, but this is not the case in firing ceramic wares, as the possible reactions cannot be allowed to proceed to completion, but must VITRIFICATION 551 be stopped at a point which enables the product to possess the complex series of properties which are essential to its use to the best advantage. Anyone with even a small knowledge of chemical reactions knows how difficult many of them are to arrest when they take place at the temperature of boiling water, and can realise how much more difficult it is to arrest such reactions when occurring at 1000° C. or above without damage to the product. Yet such a stoppage must be effected promptly and at precisely the right time, especially in the manufacture of some of the most delicate wares. (b) It may permit the crystallisation of some of the constituents. Thus, silli- manite (p. 413) may be formed in clay goods, cristobalite or tridymite (p. 426) in siliceous materials, periclase (p. 428) in magnesia bricks, etc. In some cases, the production of a mass of felted crystals gives the mass an added strength and a greater resistance to sudden changes in temperature. In others, the crystals are important because they are the most stable form of material under the conditions which it will be used. In glazes and sometimes in binding materials, the production of crystals is undesirable and is regarded as a defect. (c) It may cause the volatilisation of alkalies (p. 561) and thus increase the refractoriness of the residual material, though this can only occur to a very small extent in the soaking stage of burning ; it is more important when ceramic products are subjected to prolonged heating at a high temperature during use. Other effects of prolonged heating are described on p. 562. The length of the soaking period will, of course, depend on the desired qualities in the finished product, and owing to the complexity of most ceramic materials it must usually be found by trial based on experience. Vitrification.—One of the chief purposes of a prolonged finishing stage in the burning of ceramic materials and articles is the production of a suitable amount of vitrified or glassy material which will surround the remaining particles and fill the interstices between them to a suitable extent, dependent on the properties desired in the finished product. In the case of building bricks and earthenware, it is sufficient if the more refractory particles are united sufficiently firmly to produce a mass of ample strength. In stoneware, engineering bricks, and acid-resisting materials, a larger proportion of vitrified material is required so as to fill the pores and prevent the permeation of liquid into the article, and in porcelain and china-ware a still larger proportion of vitrified material is needed so as to produce a translucent material without loss of shape. The temperature at which a sufficient amount of vitrification is reached will depend on the nature and proportion of the fluxes and of the most fusible materials present. It will obviously be reached earlier with a fusible clay or with one rich in fluxes than with a highly refractory material such as kaolin, magnesite, or bauxite. Vitrification commences when the fusion-point of the least refractory constituents (or of the most fusible product of any reactions which may have occurred) is reached, but owing to the complex nature of ceramic materials no single temperature can be stated as that of the commencement of vitrification. With some very fusible clays and glazes it is as low as 450° C., whilst some highly refractory materials, such as 552 | EFFECT OF HEAT magnesia and alumina, show no signs of fusion below 2000° C. With most of the crude clays used in the ceramic industries the commencement of vitrification appears to occur at about 750°-800° C. A readily fusible substance present in the material may be the first to fuse and so start the vitrification, but it is more usual for a sodium or potassium salt to decompose and combine with the silica, forming a fusible silicate. This fused basic silicate will, as the temperature rises, act as a solvent for some of the other silicates and bases present, and the molten material thus forms a liquid in which the various substances can react far more rapidly than when all are in the solid state. By this means, fused complex compounds, eutectics, and solid solutions are formed. These changes, which have been described in Chapter XI, increase in extent and velocity as the amount of vitrified material increases and the temperature rises, until—unless its progress is previously arrested—a point is reached when there is so much fused material present that the mass is unable to retain its shape and loss of shape occurs (see Squatting, p. 560), and.if the heating is still further continued at a suitable temperature, the whole of the material is reduced to a molten or liquid state and forms, when cooled, a glass or slag. The rate of vitrification is very slow at low temperatures, but increases rapidly at temperatures above 1200° C. (sometimes at a much lower temperature), except with the most refractory materials, some of which can only be melted in the intense heat of the electric arc. The chemical changes which occur during the production of vitrified material cause various physical changes to take place simultaneously. Thus, the volume of the material changes as the interstices are filled and the solid material is dissolved, and also as products of different specific gravity are formed. The porosity and permeability decrease as the pores are filled with the molten material. The strength of the hot material decreases as the amount of fused material increases, the mass becoming more mobile, but when cold the strength is increased as a result of the larger amount of fused glassy matter produced acting as a bond uniting the other particles firmly together. The multitudinous changes which occur simultaneously with the vitrification are too numerous to be described in detail ; much information relating to them will be found in the earlier chapters in this volume. The term vitrification range is applied to the range of temperature between the commencement of fusion and the loss of shape due to the production of vitrified material. It varies according to the nature of the substances present. Some clays, such as those containing a large quantity of lime or soda compounds, have a very short vitrification range on account of the fluidity of calcium and sodium silicates and alumino-silicates ; in some such clays the range may be as low as 30° C., whilst in some refractory clays the range may be 300° C. or more. Table CXL, due to Wheeler, shows the vitrification range of different clays. In clays and in other materials, the vitrification range depends chiefly on the nature of the fluxes present. In siliceous materials bonded by lime or in calcareous or magnesic materials bonded with clay the range will obviously be short, whilst if a FINISHING TEMPERATURE 553 moderately refractory clay is used as a bond for a siliceous material, a longer vitri- fication range will be obtained. Magnesia is usually regarded as producing the longest range of vitrification obtainable with siliceous materials, especially if clay is present, as the product is more viscous than that of the corresponding lime- or alkali- compounds, and as it does not penetrate the pores so readily its rate of attack is much slower. The property does not appear to be present in magnesia-soda glasses which are not more viscous than those in which the magnesia is replaced by lime. TaBLeE CXL.—Vitrification Range of Clays Nature of Clay. Vitrification Range. ee aie Very calcareous clay . : 75 24 Very impure clays and shales ; 300 149 Less impure clays and shales . ae a ; 350 176 Fireclays, potter clays, kaolins : : : 400 204 Some china clays and pure fireclays 500 260 Finishing Temperature.—The temperature at which the firmg of ceramic materials or articles are finished depends on the nature of the contents, but in any case it will be below that at which the vitrification of the product is so far advanced that serious loss of shape occurs. Different materials and articles require different finishing temperatures for reasons already stated, and even articles of one class, such as building- and fire-bricks, cannot all be finished at the same temperature on account of variations in the composition of the materials of which they are composed. The following notes are intended to give some idea of the temperatures used for different articles :— Building bricks, roofing tiles, and terra-cotta are burned at various temperatures according to their composition and the type of the product required. Bath-bricks, “rubbers,” and similar soft articles are merely baked at a moderate red heat (about 800° C.). Most building bricks, roofing tiles, and terra-cotta are fired at about 900°-1000° C., at which temperature only just sufficient vitrified matter is present to surround the particles and unite them strongly together. Engineering and vitrified bricks, which contain a much larger proportion of vitrified matter, are fired at 1000°- 1300° C. (see Table CXLI). Salt-glazed articles must be fired at a higher temperature than building bricks, because the salt and clay will not combine satisfactorily at temperatures below 1180° C. (Cone 5a). Earthenware is a term applied to articles made of so many kinds of clay and mixtures of clay with other materials that no satisfactory figures can be given for its finishing temperature. Some of the most crude earthenwares and some majolica wares are finished at 790° C. (Cone 015a), whilst white ware is finished at 1140°- 1300° C. when in the biscuit state and at 1080°-1300° C. when glazed. Between these wide limits it is impracticable to give closer figures unless all the required 554 EFFECT OF HEAT properties are known with great exactitude, and as much of this information is rightly regarded as private property, it cannot be published at present. Karthenware is required to have a porous body to which the glaze will adhere readily. The ware should also be strong enough to withstand ordinary usage, but it need not be vitrified. It is, therefore, burned at a lower temperature than porcelain ware, though the temperature reached in some kilns in which the best qualities of earthenware are burned approaches very closely to the finishing temperature of porcelain. In the manufacture of earthenware and porcelain, the ware is first fired in the unglazed state, producing biscuzt, which is then glazed and refired. Stoneware is required to contain more vitreous material than once and must, therefore, be fired under conditions which ensure the requisite amount of vitrification. The best qualities of stoneware are quite devoid of porosity even apart from the glaze. The nature of the glaze sometimes affects the temperature at which stoneware is finished as it is customary to apply the glaze to the dried ware, whilst earthenware is made into biscuit and then glazed. The term “ stoneware ”’ includes many varieties of ware; that to which it is chiefly applied in this country is usually finished at a temperature of about 1200°-1250° C. (Cones 6a-8). China and other kinds of porcelain are heated so as to secure the maximum amount of vitrification without loss of shape. The finishing temperature is naturally largely influenced by the nature of the materials used and this accounts for the great variations shown in Table CXLII. Some “soft ’’ porcelains formerly made were fired at still other temperatures. Like the better qualities of earthenware, it is usually necessary to fire porcelain articles twice—once in the unglazed state and afterwards when glazed. The latter temperature may be lower than the former if the purposes for which the ware is to be used permit this. Glazed ware may be fired in either one or two periods ; in the so-called “ single- firing ’’ process, the glaze is applied to the unfired ware and the burning must then be controlled in a manner similar to that described for earthenware, but continued to such a higher temperature as may be necessary to fuse the glaze completely and cause it to spread uniformly over the glazed surface. In the so-called “ glost-firing,” the glaze is applied to the fired or biscuit ware and the glost-firing can, therefore, be effected much more rapidly as the only changes of importance which are required to take place are those which occur in the glaze. Glazes are required to be completely fused and to become sufficiently mobile that they distribute themselves uniformly except in those cases where an irregular distribution is required as a means of decoration. As the various chemical reactions should proceed to completion in the fused glaze, the temperature may usually be raised fairly rapidly, but care must be taken not to spoil the glaze by overheating it, especially if it has been applied to unfired ware. The retention of a suitable oxidising or reducing atmosphere during the firing is very important in glazed ware and especially in the case of coloured glazes, for many substances used as glaze-stains are profoundly modified in colour by the nature of the atmosphere in which they are heated. Thus, chromium compounds are green in a FINISHING TEMPERATURE 555 reducing atmosphere, but red or buff in an oxidising one ; lead glazes may be blackened in a reducing atmosphere, and manganese and cobalt compounds form bubbles of oxygen if the glaze is heated too long in an oxidising one. Hence, it is impossible to fire all kinds of colours at one time in a kiln. Steady firing is essential for glazes, or the ware may be blistered or “feathered.” Very slow firing is detrimental to glaze, so that the firing should be as rapid as possible without damaging the ware. Tables CXLI and CXLII show the finishing temperatures for various classes of goods :— Taste CXLI.—Finishing Temperature for Building Bricks, Tiles, etc. Type of Ware. Finishing Temperature. rey Seger Cone. sakes Red bricks, and tiles rich in iron and lime : 015a-0la 790-1080 Red bricks, and tiles free from iron and lime . la-10 1100-1300 Glazed bricks ; : , : : : 6a-9 1200-1280 Staffordshire blue bricks ; : : ; 10-14 1300-1410 Clinkers and paviours . : : : la—-10 1100-1300 Vitreous tiles ; : : : } : 5a-6a 1180-1200 Salt-glazed bricks, drain pipes, etc. 5a-10 1180-1300 TasBLe CXLII.—Finishing Temperatures of Earthenware, ete. Type of Ware. Finishing Temperature. Seger Cone. =C, Whiteware, biscuit . ’ : : 3a-10 1140-1300 Whiteware, glost . ; 01la-10 1080-1300 Hasy earthenware, biscuit 4a—5a 1160-1180 Hard earthenware, biscuit : : Baba 1180-1200 Easy earthenware, glost . : : 03a—la 1040-1100 Hard earthenware, glost . ; ; 2a-3a 1120-1140 White stoneware (soft glaze) . : 09a-03a 920-1040 White stoneware (hard glaze) . la-10 1100-1300 Stoneware with salt glaze : : 5a—10 1180-1300 Majolica ware . : : : 015a-05a 790-1000 China biscuit ware . : : 9-10 1280-1300 Hard porcelain ware : : : 7-20 1230-1530 Hard porcelain glaze : : : 13-16 1380-1460 Glass colours . ; ‘ i é 022-021 600- 650 Easy enamel kiln. ‘ : 020-018 670— 710 Medium enamel kiln : ; 017-016 730-— 750 Hard enamel kiln 016-015a 750— 790 | Porcelain colours and lustres 022-010a 600— 900 556 EFFECT OF HEAT Refractory articles such as firebricks, crucibles, retorts, etc., should usually be fired at a temperature higher than that at which they are to be used. If this is done, any further heating to a lower temperature cannot have very much adverse effect. In practice, however, it is customary to burn the goods till they are sufficiently hard and have a good “ring,” and it is only within the last few years that the necessity for finishing these articles at a sufficiently high temperature has been properly appreciated. It is not always necessary to burn firebricks at the maximum temperature reached by the materials in connection with which they are to be used and in some cases it is impossible to do so, as the bricks, if heated to that temperature, would soften and lose their shape, whereas they are quite satisfactory when only one face or side is raised to that temperature as in actual use. Nevertheless, it is always desirable to burn firebricks at the highest practicable temperature, and it is also important to maintain that temperature for a sufficiently long period (p. 550) in order that the various desirable changes may take place. A mistake which is commonly made is to reach a high temperature, but not to maintain it for a sufficiently long time, the firing being stopped before the heat has had time to penetrate the articles sufficiently. Fireclay bricks are frequently finished at about Cone 5a (1180° C.), but this is too low. The better qualities should be fired to Cones 7-12 (1230°-1350° C.), whilst still better ones would be obtained if fired to Cone 14 (1410° C.) or still higher temperatures, though these are seldom reached. In each case, the kiln or oven in which the bricks are burned should be maintained at a temperature within about 50 degrees of the maximum for at least 24 hours so as to ensure a sufficient “‘ soaking.” Silica bricks of the best qualities ought to be fired at Cones 14-18 (1410°-1500° C.), including a soaking at or near the maximum temperature for at least 8to 10 hours. A still higher temperature (up to Cone 26 or 1580° C.) is desirable with a soaking period of 24 hours, but this is seldom done on account of the great cost of reaching and maintaining so high a temperature, and many commercial silica bricks are only fired to Cones 9-12 (1280°-1350° C.). Baucite bricks should also be fired at a very high temperature, preferably between Cones 14-18 (1410°-1500° C.), though these temperatures are not always reached, some bauxite bricks being heated to only Cone 8 (1250° C.), which may be regarded as the minimum permissible temperature and a higher one is very desirable. Articles made of crystalline or artificially prepared alumina, such as alundum pyrometer tubes and electric furnace cores, are preferably burned at about Cone 14 (1410° C.). Magnesia bricks of the best quality are generally made of magnesia which has been fired at Cones 17-18 (1480°-1500° C.), whilst those containing a considerable proportion of iron oxide are burned at Cones 11-12 (1320°-1350° C.). Still higher temperatures would be preferable for calcining or ‘“‘ dead-burning ” the magnesia as it is practically impossible to overheat it, but owing to the great cost of using very high temperatures the magnesia and the bricks are seldom heated higher than the above-named temperatures. At Eubcea, however, the Societé des Travaux Publics FINISHING TEMPERATURE 557 et Communaux has stated that the magnesia bricks made by it are always fired at Cone 35 (1770° C.) in Mendheim gas-fired kilns, whilst in Austria some firms fire the bricks to Cones 26-30 (1580°-1670° C.). Chromate bricks should preferably be burned at Cone 16 (1460° C.) or at a higher temperature. Carbon bricks do not require such a high-burning temperature as some refractory bricks and very often they are finished at Cone 05a—la (1000°-1100° C.), but it is desirable to heat them to Cone 10 (1300° C.), because when they are fired at too low a temperature they are rather weak. Carbide bricks are burned at various temperatures according to the properties which are desired. Siloxicon bricks are usually burned at about Cone 14 (1410° C.), whilst carborundum bricks are burned at various temperatures up to Cone 35 (1770° C.). The higher the burning temperature, the more pronounced is the cellular crystalline structure of the bricks, this latter being highly desirable. The Carborundum Co., in one of its patents (1902), specifies that the bricks should be fired until this structure is developed. Saggers are not usually burned to a very high temperature, but are generally placed in the same kiln as the ware which will later be fired in them, the new saggers being burned on top of those in use. When possible, it is much better to heat them in a separate kiln and to burn them to a higher temperature than that at which they are to be used, as this ensures maximum durability. Glass-melting pots are seldom properly fired before use, but are generally baked or annealed at about Cone 05a (1000° C.) so as to make them just strong enough to be transported to the furnace in which they are to be used. It would appear to be better, though the cost would be great, to burn them at a high temperature before use, as this would result in a much stronger and more durable pot. Glass manufacturers contend, however, that the lightly burned pots withstand the sudden rise in temperature when they are placed in the furnace, better than those which have been more intensely heated in the course of manufacture, and as the pots eventually reach the tempera- ture of the glass-melting furnace the importance of this contention must not be overlooked. Retorts are generally burned at about Cone 12 (1350° C.) as specified by the Institution of Gas Engineers, but retorts of much better quality would be obtained if a temperature of Cone 14 (1410° C.) were reached. The firmg temperature should, in any case, be higher than that at which the retorts are to be used so as to reduce the contraction, when in use, to a minimum. Crucibles are not usually burned at a high temperature, but are merely “ baked ” at a bright-red heat which enables them to be handled easily. It is considered unnecessary to fire them at a greater temperature, as they attain this when in use. Whilst this practice is cheap, it is not really satisfactory, as the crucibles are not so strong as if they had been properly burned before use. Crucibles containing a large proportion of grog are generally burned at Cones 7-12 (1230°-1350° C.) before use, as their strength would be very low if they were merely baked at a lower temperature. 558 EFFECT OF HEAT Plumbago or graphite crucibles are heated to Cones 018-010a (710°-900° C.). Table CXLIII shows the burning temperatures of various refractory materials :— TaBLE CXLIII.—Firing Temperatures of Refractory Materials and Articles Material. Finishing Temperature. nah WE Seger Cone. Bauxite bricks ‘ ; : : 1500 18 Chrome bricks. : ; : 1450 16 Crucibles : ; 5 ; . 1000 05a Fireclay bricks ; : : 1250-1500 8-18 Glass-melting pots . ; : : 1100 la Grog. : : : 1500 18 Lime. : : : ; 900-1200 010a—6a Magnesia bricks : : 1500 18 Porcelain ware ; : : : 1300-1500 10-18 Retorts . : ; ; < : 1500 18 Saggers . ; : ; ; 1350 12 Silica bricks. : : : 1200-1500 6a-18 From the foregoing pages it will be seen that in burning all articles made of ceramic materials there is formed a quantity of vitreous or glassy material, the first portion of which surrounds the more refractory grains and later portions gradually fill the interstices between them and also dissolve the grains so that, eventually, if the heating is sufficiently prolonged, at a suitable temperature, the whole of the material would form a molten liquid and all the reactions which could take place in order to produce a state of equilibrium at that temperature would be completed. As this would involve complete destruction of the article, the reactions which occur must be arrested at a point at which sufficient vitreous material has been formed to impart to the article (when cold) the desired properties of which the chief are usually the strength and porosity, though other properties dependent thereon, such as resistance to acids as well as other independent properties, may require to be considered in determining the finishing temperature and the duration (if any) of the soaking period. Thus, if colour is of great importance, as in some pottery and terra-cotta, the finishing tempera- ture must be low and the proportion of vitreous material small ; if the strength of the product is to be the predominant property, the finishing temperature must be higher and the soaking period prolonged so as to produce a larger proportion of vitrified material ; whilst if complete impermeability, resistance to acids, hardness, or trans- lucency, or any of these are to be predominant, the finishing temperature must usually be very high and the duration of the soaking very prolonged in order that the reactions which produce the fused vitreous material may proceed as far as possible without causing loss of shape in the product. CHANGES DURING COOLING 559 To anyone acquainted, even to a minor degree, with the nature of the reactions involved and the necessity of arresting them at precisely the right time, the firing of porcelain and some other forms of ceramic ware, under the conditions and with the restrictions imposed in commercial work, must be a continual source of wonder. Were we less accustomed to it we should find it difficult to believe that such complex reactions should be controlled so effectively, with such crude and imperfect means as are used regularly in the manufacture of earthenware, china-ware, and other pottery of the highest quality. In probably no other industries does the art of controlling chemical reactions, whose very nature is largely unknown, rise so high as in the firing of ceramic wares. EFFECTS OF WITHDRAWING HEat When the supply of heat to an oven or kiln containing ceramic articles (or to a furnace or other structure of which they form a part) ceases the articles gradually cooldown. The heat will be lost slowly if the kiln or oven is tightly sealed, but it may be withdrawn rapidly by passing a sufficient quantity of cold air through the chamber. The latter will usually destroy the goods by causing them to crack and disintegrate, as most ceramic materials, when fired, are sensitive to sudden changes in temperature. To avoid damage, it is necessary to ascertain what are the best conditions for cooling. These naturally differ with the nature of the product, but, as a general rule, the more porous the material, the more rapidly can it be cooled with safety. Glass, or ware containing a large proportion of vitreous matter, on the contrary, must be cooled skilfully, and it is usually necessary with such materials to vary the rate of cooling at different temperatures; this variation or control of the cooling is known as annealing. It is usually possible and desirable to allow the ware to cool rapidly from the finishing temperature to about 900° C., but below the latter temperature the cooling must usually be much slower, and even above it an excessive rate of cooling may be dangerous with large, thick pieces of highly vitrified ware. Many fused and vitreous materials tend to crystallise if maintained too long at a temperature above 900° C., so that rapid cooling through the “ crystallisation zone ”’ is essential where a vitreous structure is desired as in glazes and in most ceramic ware. Cooling the kilns improperly is a fruitful source of many defects, especially “ crazing,” ‘“‘ cracks,” “dunts,” and “ feathering” or crystallisation, and great skill and care are needed to avoid them. The few investigations which have been made on the effect of cooling ware at different rates during various stages of cooling show that there is scope for a very careful study of this subject. At present there is not sufficient information for any precise statement to be made on the various rates which should be adopted ; they appear to differ with each class of ware. Chemical Changes in Cooling.—The temperature at which cooling commences and the rate at which the cooling proceeds is very important, as it controls the con- ditions under which the reactions which eventually lead to the fusion of the material 560 EFFECT OF HEAT are arrested. If the rate of cooling is very slow, the reactions will proceed much further than would be the case if it were rapid. Apart from this no chemical changes occur which would not take place during the soaking stage of firing (p. 550). The physical changes which occur on cooling are chiefly those connected with the solidification of any fused matter present. This usually sets to an amorphous solid glass or slag, but if the cooling is very slow crystallisation may occur, the nature of the crystals produced depending on the composition of the molten material (see Chapter XI). The change of silica, magnesia, etc., from their high temperature forms to allotropic forms corresponding to lower temperatures also occurs if the rate of cooling permits it, but this is seldom the case. The changes in specific gravity, volume, and conductivity are usually the reverse of those which occur in the firing, but they proceed at a far slower rate and, therefore, to a much smaller extent. In most cases the strength of ceramic materials increases as the temperature falls and is much greater in the cold than when they are at a high temperature. This is due to the cold vitrified material forming a strong bond uniting the remaining particles firmly together, whereas when the materials are at a high temperature the vitrified material is in a soft or even in a fluid state and cannot resist so great a pressure as when it is cold and solid. EFFECTS OF EXcEssIVE HEATING When any ceramic material or article is heated under such conditions that it loses its shape, swells undesirably, or undergoes any other changes which reduce its usefulness for the purposes for which it is intended, it is said to be over-burned. Over- burning is also said to occur when a material which should be highly porous becomes less so, even though it does not loseits shape. If an undesirable colour is produced as a result of excessive heating, the article or material is also said to be over-burned. Over-heating may, therefore, be described as any result of heating which enables the reactions which lead to fusion to progress further than is desired in the particular ~ case under consideration. Hence, what may be a sign of over-burning when a material is required for some purposes may indicate the opposite (under-burning) of the material for other purposes. Over-heating may be due either to the material or article being raised to too high a temperature, to the heating being unduly prolonged or to the heating being repeated too frequently. Distortion or squatting occurs when the amount of fused material present in a mass is sufficient to cause the latter to change its shape. The cause of this distortion is the mobility acquired by the solid particles in the presence of the molten material, whereby the mass as a whole cannot resist the pressure due to its own weight and, therefore, becomes reduced in height and increased in width until a shape is reached at which the whole mass remains in a state of equilibrium. If the temperature rises still further, or if the heating is prolonged at the critical temperature, more molten material will be formed and still further changes of shape will occur, until eventually EFFECTS OF EXCESSIVE HEATING 561 the whole mass becomes fluid. The increase in mobility may be due either to the material being heated to too high a temperature, to the heating being too prolonged or the pressure too great. The difference between the temperature at which squatting begins and that at which the whole mass is fused and becomes fluid is termed the fusion range (p. 524), whilst the refractoriness (defined on p. 525) is intermediate between the two extremes of this range. As particles immersed in or floating on a fluid move more readily when pressure is applied to them, the amount of squatting is greater when a ceramic material is heated under pressure, or, alternatively, the change in shape occurs at a lower temperature because of the increased mobility of the solid particles when under pressure. Hence, the resistance to heat or refractoriness of a material is lower when it is under pressure or supports a load (p. 164), than when it is quite free. The shape of a mass also affects the temperature at which squatting first appears, a tall mass changing its shape at a lower temperature than a short one, as the pressure on unit area in the lower part of the latter is much less. The shape of the mass before heating also determines that after squatting has commenced ; thus, Seger cones bend over (fig. 50) before becoming wholly liquid. Boiling, or the conversion of a liquid into vapour or gas, will occur if the liquid is raised to a sufficiently high temperature without it undergoing decomposition. This change seldom occurs in ceramic materials as their boiling-points are usually unattainable in commercial furnaces. Glazes sometimes present the appearance of _ having been suddenly solidified whilst in a state of ebullition, but this is due to a wholly different cause and is quite distinct from true boiling, being caused by the partial escape of moisture or other gases from the ware after the glaze has been fused. A spurious boiling also occurs when clays rich in carbonaceous matter are heated too rapidly, the surface of the clay being sealed with fused material before the gases formed by the burning carbonaceous matter has escaped from the mass (p. 546). The volatilisation of some of the constituents of ceramic materials may occur at very high temperatures. Thus, the alkalies and some of the silica in clay may be partially volatilised so that when clays are heated repeatedly at high temperatures they become slightly more refractory (see also p. 551). Thus, Mellor 1 found that nearly 20 per cent. of the alkalies in an earthenware body were volatilised by overfiring it at 1400° C. Under reducing conditions, the loss is still greater as also in the presence of water vapour. Mellor? also found that a sagger lost 22 per cent. of its alkalies after firing it seven times at 1200° C. The volatilisation of alkali from some glazes and the resultant dulling of the glaze is mentioned on p. 562. Silica is appreciably volatile at high temperatures, especially in the presence of carbon, as the latter appears to reduce it to silicon which volatilises and is again oxidised and re-forms silica. This change of composition does not occur to any very great extent below 1750° C., but at higher temperatures quartz crystals, large enough to be readily seen with the naked eye may be completely volatilised in a reducing 1 Trans. Eng. Cer. Soc., 5, 75 (1906). 2 [bid., 6, 130 (1906-7). a 562 EFFECT OF HEAT atmosphere. In the absence of carbon, however, silica does not appear volatile at any attainable temperature. EFFECT oF PROLONGED HEATING The effect of a prolonged “soaking ”’ during the firing of ceramic materials has been described on p. 550. A moderately long “soaking” is often beneficial, but if it is unduly prolonged, the effect may be deleterious instead of useful, as the pro- duction of an excessive amount of fused matter may endanger the stability of the mass or crystallisation may cause a decrease in strength and may render the material brittle or, as in the case of devitrification of silica glass, it may spoil the material for the purpose for which it is intended. R. Sosman found that devitrification occurs when fused silica is continuously heated at 200°-275° C. for a long period of time, on account of the a-B-inversion range (p. 329) which occurs at or near this temperature. As a prolonged heating at a lower temperature has the same effect in producing fused material as a much shorter heating at a higher temperature, an excessively prolonged heating may cause distortion or squatting (p. 560). It may also cause excessive shrinkage in clay wares or excessive expansion in siliceous materials, if the finishing temperature during the burning of the ware was not sufficiently high, and these changes in volume may endanger the stability of the mass. Swelling, bloating, and similar defects may occur as a result of prolonged heating at high temperatures, and if any fluxes are present which can combine with silica or bases, an increased amount of fused material may be produced, so that the effect of excessively prolonged heating is similar to that of heating the material to too high a temperature and so over-burning it (p. 560). As volatilisation of part of the soda and potash may occur if the heating of the glaze is prolonged and this may lead to the glaze becoming dull instead of glossy, undue prolongation of the heating at the finishing temperature of glazes should be avoided. Excessive heating of a molten glaze in contact with clay will usually enable reactions to occur between them with the result that an unfused product is formed. This destroys the glossy appearance of the glaze and so makes the ware defective. Such reactions are precisely the same as those which occur when a crude clay or pottery body is heated excessively, but the effect being confined more closely to the surface of the ware is more easily observed. Prolonged heating at a sufficiently high temperature enables the reactions which lead to complete fusion to progress further than if the period of heating were shorter so that its general effect (that of over-burning) is the same as that when these reactions are not arrested at the proper time (p. 560). Hence, prolonged heating, whether during the firing in the course of manufacture of the articles or whilst the articles are in use, may be either useful or harmful. If it forms part of the firing process it is generally useful as it increases the strength and resistance to acids and abrasion and reduces the porosity (p. 69), but if the prolonged heating occurs when the articles are in use, its general effect is harmful, as it tends to increase the amount of fused matter present. For this reason, ceramic materials REPEATED HEATING 563 which require to be heated to high temperatures during long periods should be of a highly refractory nature, or they will eventually collapse through squatting or analogous distortion (p. 560). EFFrect oF REPEATED HEATING When ceramic materials are repeatedly heated and cooled the effect is similar in many respects to that of prolonged heating (p. 562), the same kind of changes taking place in both cases. The effects of repeated heating and cooling also differ according as the treatment is (a) at a rapid rate, and (b) at a slow rate. The effect of rapid heating and cooling when repeated many times is discussed in the section on The Effect of Sudden Changes of Temperature (p. 581). The chief materials which are subjected to repeated heating and cooling are the walls of some kilns and furnaces, saggers, glass-melting pots, continuous retorts, crucibles, etc. The effect of repeatedly heating saggers to temperatures between Cones 8 and 16 (1250°-1460° C.) is to reduce the strength on account of the strains set up in the material and also increase the tendency to crystallisation. For this reason, most English saggers can only be used fourteen to fifteen times, though in some works on the Continent where saggers of better quality are employed, they will stand fifty or more heatings. The difference is largely due to the care taken in the selection and preparation of the materials of which the saggers are made. Crucobles, when repeatedly heated, become hard and vitreous as a result of the action of the contents upon the crucible. The resistance of the crucibles to sudden changes of temperature is correspondingly decreased so that after a time crucibles “perish ” and can no longer be used. When fused silica is repeatedly heated at about 1200° C. it shrinks and devitrifies forming tridymite or cristobalite, especially in the presence of basic or alkaline dust or ash. In all these instances, the effect of repeated heating is to permit the various reactions which lead to ultimate fusion to make further progress to completion and to overcome the arrest of these reactions which occurs when the article or material is allowed to cool for the first time. Repeated heating at a moderately high tem- perature also facilitates the production of a state of equilibrium by the formation of crystals from a molten or vitreous mass, and, consequently, effects partial devitrification. EFrFrects oF HEAT ON THE VOLUME OF CERAMIC MATERIALS The changes in volume which occur when ceramic materials are heated or cooled may be divided into two groups : (a) Permanent expansion or contraction. (b) Reversible expansion or contraction. 564 EFFECT OF HEAT The permanent expansion or contraction is due to chemical or physical changes in the materials, and, as is characteristic of them, such changes take an appreciable time to occur, whilst a reversible expansion is common to all materials and is almost instantaneous in its occurrence so that it cannot be arrested, however quickly the substance may be heated or cooled. Permanent changes in volume can, on the contrary, be arrested by rapid heating or rapid cooling. The difference between these two kinds of volume change is discussed further on p. 520. The effect of heat on the total volume of a material may differ greatly from that on the individual particles composing the mass as a whole. Thus, in a mixture of silica and clay, the individual grains of silica will expand and those of the clay will shrink when heated and the result may be that the total volume of the brick or other article made of such a mixture may be unaffected. Semi-silica bricks of this character are largely used where constancy in the volume of a structure, such as a retort or coke oven, is important. In consequence of the difference between the behaviour of the mass or article as a whole, and that of its constituent parts, care must be taken in investigating or studying the effects of expansion and contraction to distinguish clearly between these changes. PERMANENT VOLUME-CHANGES Permanent changes in volume differ with the materials in which they occur. The chief change which occurs when ceramic materials are made into various articles and fired is the contraction or shrinkage which occurs when they are heated. Some materials, such as suitable mixtures of clay and silica, are practically constant in volume, whilst others, such as silica, expand when heated and do not regain their original volume on cooling. The extent to which these permanent changes in volume are completed in any given material depends chiefly upon the temperature to which it is heated, so that when firing ceramic materials which are required to have a constant volume when in use, they should be finished at such a temperature as will ensure these permanent changes taking place to the fullest extent. . The principal causes of the permanent changes in volume are :— (a) The nature and composition of the material. (b) Its previous treatment (if any). _ (c) The sizes and grading of the grains. (d) The pressure applied in shaping the articles. (e) The proportion of water used in mixing the materials. (f) The porosity of the material. (g) The temperature at which the material has been fired or reheated. From the above it will be seen that the permanent changes in volume may be divided into two groups : (i) Changes which are dependent upon the inherent qualities of the materials. (11) Changes which are dependent on the method of manufacture. VOLUME CHANGES ON HEATING 565 The changes in the first group have been described in the previous chapters on Chemical Constitution, Specific Gravity, Chemical Reactions, etc., and are chiefly due to—(a) decomposition and the removal of some constituent which causes a reduction in volume, as in the case of clay ; (b) to the formation of other allotropic forms of the same material which cause a change in the specific gravity and, consequently, either an expansion or contraction ; or (c) a chemical reaction which produces a new material of greater specific gravity and with consequent volume change. The physical causes of change in volume, such as texture, porosity, etc., are considered in the respective chapters dealing with these subjects. Whilst it is very desirable it is not always possible to complete the various changes in volume which occur during the firing of ceramic materials, so that a compromise must usually be made, and—where it is important—the maximum permissible permanent expansion or contraction in use should be specified. This is particularly the case with refractory materials, as it is in these that after-contraction or expansion is most harmful. Clays, when properly burned, shrink permanently to a varying extent, depending on their composition. This contraction or shrinkage takes place in two stages: (a) shrinkage during drying, and (0) shrinkage during burning. The shrinkage in drying is due to the removal of water from the surfaces of the grains and has been described in Chapter VII. The kiln- or fire-shrinkage of clays is due to their decom- position, with the consequent loss of water (p. 350) and the gradual drawing together of the resultant anhydrous grains. In the case of an earthenware or porcelain, the shrinkage due to the loss of water is partly counterbalanced by an expansion due to the fusion of crystalline silicates and the transformation of flint and quartz into the low specific gravity forms of silica which have a greater volume than the original materials. The amount of shrinkage is to some extent dependent on the fineness of the particles, fine-grained materials shrinking more than coarse-grained ones. There appears to be little or no fire-shrinkage between 600° C. and 900° C., but above 900° C. the shrinkage increases considerably in amount up to 1100° C. It thus appears that during the burning of the carbonaceous matter scarcely any shrinkage takes place, so that the temperature may safely be raised fairly rapidly between 600° and 900° C. if little carbonaceous matter is present. Shrinkage continues up to the highest temperature at which the clayware is fired and would still continue, if the heating were prolonged, up to the point at which fusion occurs, unless the conditions are such as to cause bloating, when an expansion would result. Consequently, clays will always shrink in use if they are then heated to a temperature higher than that attained in the firmg. This is very important in connection with refractory clays, which should be fired at a sufficiently high temperature to prevent an excessive contraction in use. Mellor has shown that if it is assumed that the change of contraction during the firing is proportional to the square of the contraction which has yet to take place, the effect of repeated heating at 1130°-1150° C. (Cone 2a—4a) approximately 566 EFFECT OF HEAT follows the law for bimolecular reactions and the contraction may be calculated as follows :— a*kt L = ———., 1 + akt where « is the contraction after any number of firings ¢, a is the maximum contraction after an indefinite number of firings, and & is a constant depending on the nature of the material. He found that fireclay bricks contracted more in a reducing atmosphere than in an oxidising one, as shown in the following figures :— TaBLe CXLIV.—Contraction of Firebricks E Contraction Percentage. Expansion. Burning Material. Temperature, | 2A | Cone. Oxidising Reducing Oxidising Reducing Atmosphere. | Atmosphere. | Atmosphere. | Atmosphere. Fireclay bricks . 14 = $e 0-21 nil. 3 : 14 sh 0°33 5 : 14 1-12 1-27 The increased contraction is probably due to the greater fluxing effect of the iron in a reducing atmosphere. The specification of the Institution of Gas Engineers directs that first-class fireclay bricks shall not contract more than 1 per cent. when reheated for two hours at Cone 14, whilst second-class fireclay bricks shall not contract more than 1-25 per cent., the test-pieces in each case being 44 inches square. A noteworthy expansion or contraction is very harmful in the case of retorts, and should specially be avoided by using suitable materials and burning them at a sufficiently high temperature. To avoid troubles which would otherwise occur, the Institution of Gas Engineers specifies that a test-piece 44 inches square, cut from a clay retort, when heated at 1350° C. (Cone 12) for two hours should not contract more than 1 per cent. in length. It is also important that the reversible expansion or contraction of retorts when in use should also be as low as possible, as it is the changes in volume of the retort whilst it is hot which are one of the chief causes of its disintegration. This is avoided in some bricks, retorts, etc., by using a siliceous clay in such proportions that the expansion of the silica is neutralised by the contraction of the clay, and, consequently, the material is almost constant, in volume. G. A. Loomis has found that firebricks which can retain their shape when heated to a temperature of 1350° C., under a pressure of 40 lb. per square inch, seldom show SHRINKAGE 567 more than very slight permanent changes in volume when they are heated to a temperature of 1425° C. If the porosity of a ceramic material decreases more than 5 per cent. and the change on firing exceeds 3 per cent. by volume, or 1 per cent. of the original length of the test-piece, the latter will not withstand the load or pressure mentioned. He considers that the porosity and volume tests form a useful means of checking the ability of fireclay bricks to withstand the load test (see also p. 166). When an appreciable proportion of fused matter is produced—either as a result of the very high temperature attained or of the presence of fluxes—its effect on the shrinkage is very marked. For this reason, semi-vitrified and vitrified clay wares shrink more than articles made of refractory clays which do not contain much flux and so are only slightly vitrified. As the fusion proceeds the material, as a whole, shrinks rapidly at first and more slowly later, the amount and rate of shrinkage depending largely on the nature of the flux, soda, potash, and whiting being the most active in this respect. The effect of lime compounds on an earthenware body is shown in Table CXLV, due to H. HE. Ashley. TaBLe CXLV.—Effect of Lime Compounds on Earthenware Linear Shrinkage, Nature of Flux. Flux, per cent. per cent. Fluorspar : iy 10-5 : ; ‘ 0-1 11-1 i { : 0:3 11-1 ee : : 1-0 10-8 A : : 3:0 11-4 Whiting . ; : 0-13 11-1 ae : : : 0-4 11:3 a ‘ ‘ , 1-0 E38 3 eee 3-0 12-7 As the shrinkage is due to the combination of the flux with the clay, resulting in the production of a molten fluid, any materials which will produce such a fluid will cause the ware to shrink during the firing. Hence, it is scarcely necessary to describe the action of any fluxes in detail, though the following observations may be mentioned. :— H. Hope? has observed that barium and zinc oxide tend to increase the firing shrinkage of china bodies. Various investigators have noted that mica slightly increases the kiln shrinkage of clay, but if a large proportion of muscovite is present 1 Trans. Amer. Cer. Soc., 8, 148 (1906). 2 Ibid., 11, 522 (1909). 568 EFFECT OF HEAT a slight expansion may occur at about Cone 10 (1300° C.). Magnesia and magnesite also increase the shrinkage of clays and also the vitrification range. Silica bricks expand considerably during the burning, in course of manufacture, on account of the conversion of quartz to tridymite or cristobalite (p. 329), which involves a volume-expansion of more than 20 per cent. or at least § inch per linear foot. The complete conversion is never effected in commercial practice during the firing of the bricks, but a slow and constant expansion continues whilst the bricks are in use at high temperatures, so that if the bricks are to be as constant in volume as possible when in use, they must be heated sufficiently during the first firing to effect as much as possible of the total expansion. Bricks which have not been heated sufficiently during the firmg may fail—as the result of continued expansion— when in use. The Institution of Gas Engineers (1912) in its standard specification requires that silica bricks when heated for two hours at Cone 14 (1410° C.) shall not show more than 0-75 per cent. linear expansion or contraction, the test being made on a sample 44 inches square. More recently, K. Endell,1 after examining many bricks and numerous specifica- tions, has suggested that silica bricks should not expand more than 2 per cent. linear if they are heated to 1600° C. in one and a half hours, and maintained at that temperature for a further period of half an hour. The change in volume, in burning silica bricks at various temperatures, observed by R. M. Howe and W. R. Kerr,? is shown in Table CXLVI, which clearly indicates the importance of a high-finishing temperature for silica bricks. Taste CXLVI.—Average Residual Expansion on Burning Silica Bricks to Various Temperatures Burning Temperature. Residual Expansion. Cone. na per cent. 11 1320 4-2 14-15 1410-1435 2°0 16-17 1460-1480 1:3 17 1480 0-8 18 1500 0-4 19 1520 0-2 Table CXLVII, due to K. Endell,? shows a comparison between the after-expansions (on re-heating) of silica bricks made by different firms. 1 J. Amer. Cer. Soc., 5, 209 (1922). ® Loc. cit., p. 217. ® Ibid., 5, 216-17 (1922). EXPANSION OF SILICA BRICKS 569 TaBLeE CXLVII.—After-Expansion of Silica Bricks Linear Expansion Type of Bricks. after Heating } hr. at Unaltered Quartz, 1600° C., per cent. See German— Average of 8 basic open-hearth furnace bricks : : : 3°7 28 Average of 3 coke-oven bricks . 4:7 32 Glass furnace bricks. : 4-0 40 Average of 4 bricks made by German steel works for their own use 0-8 13 American— Medina Quartzite (Star) . ; 0-5 14 English— Ganister brick ; 3°5 22 Swedish— Quartzite brick : , ; ; 2:8 16 According to J. W. Mellor,! silica bricks expand less in a reducing atmosphere than in an oxidising one, as shown in Table CXLVIII; this is probably due to the greater fluxing effect of the iron compounds when heated in an oxidising atmosphere, with the production of ferrous silicates which fill the interstices of the bricks. TasLeE CXLVIII.—Ezpansion of Silica Bricks (Mellor). Expansion per cent. Temperature of Burning. Oxidising Reducing Atmosphere. Atmosphere. Cone 12 . ; 0-20 0-13 ar é : 0-13 0-12 Cone 14 . : : 0-58 0-48 2. : . | contracted 0:44 | contracted 0-77 9 Bauxite bricks shrink greatly when fired during their course of manufacture even though the raw material has been calcined at a very high temperature. It is, therefore, important that bauxite bricks should be properly fired during manufacture, or they may shrink excessively when in use. The chief changes which accompany and probably cause the shrinkage are the decomposition of the bauxite with evolution of combined water, together with a later change due to the polymerisation of the resulting alumina. ‘The finest grained bauxite bricks usually shrink less in use than those made from coarser materials. 1 Trans. Eng. Cer. Soc., 16, 268 (1916-17). 2 This brick was an unusually fine-grained one. 570 EFFECT OF HEAT Table CXLIX, due to Howe and Ferguson,! shows the shrinkage of various aluminous materials (diaspore, bauxite, and gibbsite). Taste CXLIX.—Shrinkage of Aluminous Materials A B C. D. E F G. Percentage of alumina . . | 56-31 | 52-48 | 67-21 | 60-89 | 60-66 | 61-98 | 73-70 Percentage of water ; . | 28-08 | 17-90 | 13-74 | 13-41 | 9-88 | 14-98 | 14-24 Shrinkage on drying. . | 8-30] 5-50 | 11-60] 5-80] 8-40] 3-70 | 10-20 Burning shrinkage at Cone 3 .. | 18-00 | 12-70 | 8-20 | 12-20 | 12-20 | 16-70 | 1-30 Burning shrinkage at Cone 18 . | 42-60 | 30-80 | 29-90 | 16-30 | 55-20 | 38-20 | 16-30 Fused alumina shows no permanent changes in volume when in use, as the high temperature required for its fusion enables all the reactions which result in a change in volume to be completed prior to its use. Sand-bauxite bricks (which are made of a mixture of bauxite and crushed quartz) shrink less in use than ordinary bauxite bricks as the expansion of the quartz neutralises some of the contraction of the bauxite. Unfortunately, the refractoriness of the latter is considerably reduced by the addition of sand. Magnesia bricks, made from dead-burned magnesia, should not shrink in use, but if the material of which they are made has not been fully dead-burned, shrinkage will continue whilst the bricks are in use until the whole of the magnesia has been converted into periclase. : Magnesia bricks of the best quality should not shrink more than 5 per cent. linear or 15 per cent. by volume when in use and if sufficient care is taken a much smaller percentage contraction in use will be experienced. Other refractory wares, such as bricks, crucibles, etc., made of carbon, carbide, chromite, zirconia, etc., contract very little during the firing process and have a constant volume when in use. The kiln-contraction during manufacture is almost wholly due to the changes in volume of the bond. Such bricks, if properly made, show practically no after-contraction or expansion when in use, and are, therefore, very valuable as refractory materials. REVERSIBLE VOLUME-CHANGES Reversible changes in volume are those which occur when a material is heated and cease—with restoration of the original volume—when the material is cooled to its original temperature. Reversible changes are measured by the coefficient of expansion, which varies with different materials according to the (a) Chemical composition. (6) Texture. (c) Temperature of the material. 1 J. Amer. Cer. Soc., 6, 496 (1923). REVERSIBLE CHANGES IN VOLUME 571 Clays.—Kaolin and bauxitic fireclays have, after calcination, according to Houldsworth and Cobb,? a regular reversible expansion which does not vary much with the temperature of calcination. They found that fireclays calcined at Cones 14-20 have a regular reversible expansion similar to kaolin as the quartz is destroyed by interaction with the fluxes present. The regular expansion is most readily attained with fine-grained and rather fusible materials, whilst it is retarded in the presence of a large amount of quartz. Fireclays and glass-pot mixtures calcined at Cones 06-9 show a large expansion on heating between 500° and 600° C., according to Houldsworth and Cobb, on account of the quartz present and on cooling the contraction is larger than the corresponding expansion. The expansion between 100° and 250° C. after calcination at Cone 9 also exceeds the average. Table CL shows the coefficient of expansion of various kinds of clays after calcination at various temperatures. Taste CL.—Coefficient of Expansion of Clays (Houldsworth and Cobb), coeff. of Exp. x 10-8 Temperature of Firing prior to Test. Substance. ceca ay Cone 06. Cone 9. Cone 14. Cone 20. Kaolin 2 : 100-250 me 764 578 - ~ ; : 15-1000* 402 531 ATT 441 Farnley fireclay . 100-250 of 1023 875 re . : 15-500 481 769 676 ir i : 500-600 1331 1275 1075 a re : 600-1000* 217 183 250 4s 6 A : 15-1000* 491 583 540 305 Ball clay. 15-500 542 - ‘ : 500-€00 980 > ; , 600-S00 431 bh Ss ; : 15-1000* 554. 575 436 Ayrshire Bauxitic 100-250 ‘ 726 652 Ay clay. { 15-1000* 480 605 561 418 Glasgow fireclay . 100-250 by: 986 801 BS i : 15-500 401 745 542 me 500-600 1276 1220 886 iC 2 600-1000* 279 294 364 ae m . 15-1000* 457 611 554 395 1 J. Soc. Glass, Tech., 5, 16 (1921). 572 EFFECT OF HEAT The effect of porosity on the coefficient of expansion of fireclay is shown in Table CLI due to Houldsworth and Cobb. TaBLE CLI.—Effect of Porosity on Coefficient of Expansion of Fireclay Temperature of Coefficient of Ex- a Calcination. See pansion Xx 10. 15-500 769 500-600 1275 28-7 Cone 9 | 600-1000 183 15-1000 583 15-500 602 500-600 1053 44-6 2 600-1000 281 15-1000 517 15-500 550 500-600 997 50-2 D | 600-1000 295 15-1000 491 , 15-500 481 500-600 1331 31-7 Cone 06 600-1000 204 15-1000 455 15-500 441 500-600 1220 47-7 r | 600-1000 211 15-1000 426 15-500 406 500-600 1203 62-7 r 600-1000 195 15-1000 401 It will be seen that up to 50 per cent. an increase of porosity causes an appreciable diminution in the coefficient of expansion, but over 50 per cent. the change is comparatively small. The average temporary expansion of a well-burned firebrick when heated to 1300° C. is about 0-000006 per 1° C. The coefficient of other non-vitrified claywares is similar to that of firebricks. Mellor 2 found the coefficient of expansion of various kinds of tiles to be as follows :— 1 Loc. cit., p. 571. 2 Trans. Eng. Cer. Soc., 5, 159 (1905-6). REVERSIBLE EXPANSION IN VITRIFIED WARES 573 Taste CLIL.—Coefficient of Expansion of Tiles Material. Range of Coefficient of Temperature. Expansion. Soft burned clay floor tiles . 15-100° C. 71-4 10-7? Properly burned clay floor tiles 7 70°3 x 10-7 Hard burned clay floor tiles ’ . 69 x10" The average is 0-000007 per 1° C. Boeck ! found the coefficient of expansion of a ball clay at different temperatures to be as follows :— TaBLe CLIIL.—Coefficient of Expansion of Ball Clay Temperature Range. A. B. 24-100° C. 1316 x 10-8 1380 x 10-8 100-200° C. 1710 x 10-8 1610 x 10° 200-900° C. te 567 x 10-8 200-700° C. 558 x 10-8 580 x 10-8 Vitrified Claywares.—The coefficient of expansion of vitrified claywares varies considerably according to their nature and the extent of vitrification, but it is generally less than that of non-vitrified claywares. Much work has been done on the coefficient of expansion of porcelains and much data has been compiled as to the effect of different factors upon it. The following information is a brief summary of what has been done. Chemical composition has an important influence on the reversible changes in volume, as the proportions of the various materials present, in conjunction with the firing, determines the structure of the ware. Porcelain consists essentially of three constituents : (a) felspar, (b) clay, and (c) flint. Felspar decreases the coefficient of expansion of porcelain. The effect of increasing the proportion of felspar in whiteware bodies, whilst keeping the proportion of flint constant is stated by Purdy and Potts ? to be :— With 30 per cent. of Flint.—The coefficient decreases from 0-0000065 with 1 felspar and 9 clay to 0-000004 with 4-5 felspar and 5-5 clay ; it increases rapidly with equal parts of felspar and clay and then slowly to 7 parts felspar and 3 parts clay at which the coefficient of expansion is 0-000007. 1 Loc. cit., p. 521. 2 Trans. Amer. Cer. Soc., 13, 431 (1911). 574 EFFECT OF HEAT With 40 per cent. of Flint.—As the felspar increases to a ratio of 4-5 felspar : 5-5 clay the coefficient decreases gradually and then rises rapidly. It will be seen that when the flint is constant between 30 and 40 per cent. the felspar decreases the coefficient of expansion as it increases up to 4:5 parts of felspar and 5-5 parts of clay, but that more felspar causes an increase in the coefficient. When the clay-content is constant the effect of felspar is as follows :— Clay. Behaviour. per cent. 25‘ The coefficient of expansion falls with a felspar : flint ratio falling from 2: 8 to 3-5: 6-5 and rises again with ratios up to 6: 4. 35 The coefficient falls from 1:5: 8°5 to 3: 7 is constant from 4: 6, it falls shightly to 4:5 : 5:5 and rises to 5:5, after which it remains constant to 6: 4. 45 The coefficient falls from 2: 8 to 5:5 and then rises rapidly. 55 ~—‘ The coefficient is fairly constant from 2 : 8 to 4 : 6 and then falls to 5-5 : 4-5 and rises to 6: 4. 60 The coefficient falls from 3-5: 7:5 to 6: 4 and then rises gradually. These results show that there is a critical relation between the proportion of flint and felspar with any constant content of clay and that either more or less felspar than this critical amount decreases the coefficient of expansion. Similarly, an increase of felspar at the expense of flint in a mixture with a constant flint : clay ratio decreases the coefficient of expansion. As the percentage of clay is increased, more felspar and less flint is required to give a minimum coefficient of expansion. According to Bleininger and Riddle,? the thermal expansion of beryllium porcelain is lower than that of felspathic bodies (Table CLIV). TaBLe CLIV.—Thermal Expansion of Berylliwm Porcelain Temperature Range, ° Lda ian Range, Thermal Expansion. Thermal Expansion. C. C. 26-200 1-63 x 10% 400-520 36 x10 200-400 2-95 x 10% 26—400 2°33 x 10-8 Clay, according to Seger,? decreases the coefficient of expansion in porcelain, but Purdy and Potts * found this is only true when the total amount of clay present exceeds 1 J. Amer. Cer. Soc., 2, 564 (1919). 2 Collected Writings, 2, 576. 3 Loc. cit., p. 573. REVERSIBLE EXPANSION OF PORCELAIN 575 55 per cent. They found, contrary to Seger, that in a mixture containing less than 45 per cent. of flint, the addition of clay slightly increases the coefficient of expansion. Clay added at the expense of felspar, the percentage of flint being kept constant, also increases the coefficient of expansion. Purdy and Potts, therefore, conclude contrary to Seger, that, within the range of composition of useful porcelains, the addition of clay increases the coefficient of expansion. Flint—added to the extent of 5 per cent. when the ratio of felspar : clay is kept constant—increases the coefficient of expansion from 0-000041-0-000052. The addition of a further 5 per cent. causes another increase in the coefficient of expansion, but the addition of a third 5 per cent. results in a slight decrease. A porcelain body containing 45-60 per cent. of flint has a large coefficient of expansion. In porcelain bodies with a felspar : clay ratio of 1 : 4, the coefficient increases when 25-35 per cent. of flint is present, then decreases to 45 per cent. and from that proportion increases to 65 per cent. Ina mixture witha 1:1 felspar : clay ratio, the addition of 20-50 per cent. of flint causes an irregular decrease and the addition of 50-60 per cent. causes an increase in the coefficient of expansion. With a felspar : clay ratio of 3: 7, the coefficient of expansion increases rapidly with the addition of 15-28 per cent. of flint, remains fairly constant with 28-45 per cent. of flint and then increases with 45-70 per cent. of flint. Purdy and Potts also found, contrary to Seger, that within the range of composi- tion which produces good porcelain, the addition of flint slightly decreases the coefficient of expansion and that the hard porcelain body with the lowest coefficient of expansion consists of 30 per cent. each of flint and felspar and 40 per cent. of clay. A. 8. Watts + has published the following information respecting the influence of the chemical composition upon the coefficient of expansion of European porcelain :— (a) The addition of flint at the expense of felspar or kaolin increases the coefficient of expansion. (6) Felspar and clay are interchangeable without any appreciable effect on the coefficient of expansion. (c) The addition of ball clay at the expense of kaolin slightly increases the coefficient of expansion, whilst the substitution of English china clay for Zettlitz kaolin produces a marked increase in the coefficient of expansion. (d) Finely-ground flint, added at the expense of the quartz sand occurring in kaolin, increases the coefficient of expansion. (e) Calcined kaolin and flint are interchangeable without any great variation in the coefficient of expansion. The coefficients of expansion of various porcelains at ordinary or comparatively low temperatures is shown in Table CLV. 1 Trans. Amer. Cer. Soc., 13, 406 (1911). [Taste CLY. 576 EFFECT OF HEAT TaBLe CLV.—Coefficient of Expansion of Porcelain Coefficient of Expansion. Temperature Porcelain. Range. Authority. Linear. Cubical. Bayeux . : 0° C. 0-000002522 ee Tutton. y F 0:000005500 | 0:000016— | Deville and Troost. 0-000017 Meissen . ; . | 0°-100° C. | 0-000002690 a Weinhold. Berlin . , . | 23°-200° C. | 0-000003430 ss Rieke. is 16°-191° C. | 0-000001770 oe F. Henning. ” “4 0:000004.000 4, Holborn and Wien. Rosenthal laboratory Me 352 x 10-8 ve Singer and porcelain. Rosenthal. Seger porcelain (6833) | 20°-100° C.| 380 x 10-8 ny , The coefficients of expansion of stoneware (p. 577), Marquardts’ porcelain, magnesic porcelain, and ordinary glass (p. 581) are greater than the figures given above. It has been found that the porcelains which have the greatest coefficient of expansion are those which are lowest in fluxes or which are least vitrified. Texture appears to have an important influence on the coefficient of expansion, though little experimental work has been done to determine its limitations. It has, however, been found that, when other conditions are constant, porcelains fired at a very high temperature, or otherwise thoroughly vitrified, have a lower thermal expansion than others, on account of the increased vitrification, though Purdy * failed to find any definite relationship between the total porosity and the coefficient of expansion of various porcelains. Temperature has a great influence on the coefficient of expansion of porcelains. According to Purdy, the graph representing the expansion of porcelain bodies con- taining 30 or more per cent. of flint at temperatures below 500° C. forms almost a straight line. At 500°-600° C., the rate of expansion increases, but above 600° C. it decreases. The higher the proportion of flint in the ware, the more pronounced are the changes in the rate of expansion. When less than 30 per cent. of flint is present the rate of expansion only changes slightly between 500° and 600° C. | : Table CLVI shows the effect of temperature on the coefficients of expansion of various porcelains. 1 Trans. Amer. Cer, Soc., 15, 499 (1913) REVERSIBLE EXPANSION OF PORCELAIN 577 TasBLte CLVI.—Coefficient of Expansion of Porcelain at Various Temperatures Temperature Range, Coefficient of Linear Porcelain. Oo”. Bcpaseions Authority. Bayeux 0 0-000002522 Tutton. . 50 0-000003265 a 3 100 0-000004008 a se 120 0-000004305 £ Berlin 23-200 0-000003430 Rieke. i 23-400 0-000003530 3 * 23-600 0-000003550 - 23-700 0-000003560 5 Berlin 191-16 0-000001770 F. Henning. 16-250 0-000003360 ~ ; 16-500 0-000003640 ‘ : ; 16-1000 0-000004340 A. 8. Watts ! obtained some very irregular results for the coefficient of expansion of porcelain at different temperatures as shown in Table CLVII, but with most porcelains a fairly regular increase is obtained. Taste CLVII.—Coefficient of Expansion of Porcelain at Various Temperatures aie Coefficient of Expansion. pone ih Coefficient of Expansion. 16 0-000005357 139 0-000004650 55 0-000006127 196 0-000004860 72 0-000005738 243 0-000005350 102 0-000005416 According to R. Rieke,” a sample of stoneware containing about 25 per cent. of flint and fired at Cone 7 (1230° C.) had the following coefficients of expansion at the temperatures stated :— TasLe CLVIII.—Coefficient of Expansion of Stoneware Coefficient of Coefficient of Temperature, ° C. Temperature, ° C. Expansion. Expansion. 15-200 130x107 500-600 140 x10-7 200-500 70 x10-7 600-1000 50 x 107 1 Trans. Amer. Cer. Soc., 9, 86 (1909). 2 Ber. der Tech. Wiss. Abt. des Verb. Keram. Gewerke, 5, 8-15 (1919). 37 578 EFFECT OF HEAT As shown in the table, Rieke’s results do not show so regular an expansion as most porcelains ; the difference is probably due to the fact that stoneware is intermediate in structure between an unvitrified, porous structure and a true porcelain or glass, and that the coefficient of expansion of such stoneware must depend on the relative proportion of each of these forms of structure. Various formule have been devised for calculating the expansion of porcelain at different temperatures, amongst which are shown in Table CLIX, compiled by Purdy and Potts 1 :— Taste CLIX.—Formule for the Expansion of Porcelain f Temperature Porcelain. Rane Formula. Authority. Bayeux. . oy Lt=L,(1-+-at) Deville and Troost. Electric in- sulator . 19-243 hp Watts. Berlin . 0-1500 is Holborn and Wein. Bayeux. 0-830 Lt=L,(1 +(3425t +1-07¢7)10-*) Bedford. * 0-83 Lt=L,(1-+(2824t-+6:17t?)10-) Chappins. , 10-120 Lt=L,(1 +(2522¢ +7-43¢?)10-°) Tutton. Berlin . | 250-625 Lt =L,(1+(2945¢-+1-125t?)10-*) | Holborn and Day. (Lt=coefficient at temperature ¢° C., Lp=coefficient at 0° C., and @ is a constant.) T. G. Bedford 2 states that the length of a sample of porcelain at any temperature may be found from the formula 1(1+34-25 x 10-%-+10-7 x 10-1%?), where / is the length at 0° C. and ¢ is the desired temperature in ° C., the curve ex- pressing the elongation being of the form (ax?+-bz+1)K=y, but Deville and Troost, as well as Holborn and Wien, consider that the curve takes the form (av+1)K=y. Glazes must have a coefficient of expansion very similar to that of the body to which they are attached or they will either ‘“‘ craze” or “ peel’ according as the expansion is greater or less than that of the body. Some authorities, however, do not attach so much importance to the coefficient of expansion as to other properties, such as the toughness or tensile strength of the glaze. Thus, F. Singer and EH. Rosen- thal * consider that the fitting of the glaze depends on the coefficient of expansion, elasticity, tenacity, pliability, and resistance to stress and strain of the two com- ponents, and Purdy and Potts! have suggested that the coefficient of expansion of glazes and bodies cannot have the great importance often attached to it in connection with crazing as they have found, as also have others, that some glazes behave equally 1 Loc. cit., p. 573. 2 Brit. Assoc., 1899. 3 Ber. deut. Keram. Ges., 1, 3 (1920); Sprech., 54, 250 (1921). REVERSIBLE EXPANSION OF SILICEOUS MATERIALS 579 well on bodies high in felspar with a vesicular structure as on bodies high in flint which are not vitrified. Such behaviour would be explained: if the glazes had sufficient tensile strength to withstand the strains set up in them during the cooling of the ware. According to R. Rieke, the coefficient of expansion of porcelain glazes varies from 27107 to 42x10. The latter figure is very similar to the coefficient of expansion of porcelain, which at 700° C. is about 35 x 107. According to the same authority, white-ware glazes have a coefficient of expansion between 57 x 10-7 and 96 x 107’. Table CLX shows the capacity of various oxides for increasing the coefficient of expansion of enamels; the results are not always quite accurate, but give some idea as to the effect of various fluxes on the expansion. TasLe CLX.—Relative effect of Oxides on the Expansion of Glazes Oxide. Relative Activity. Oxide. Relative Activity. Soda . : : 10 Lead oxide ; 3°0 Potash ; : 8-5 Barium oxide ; 3:0 Alumina . . 5-0 Silica ; : 0-8 Lime . ; , 5-0 Boric oxide. : 0-1 Siliceous materials vary in their reversible expansion according to their origin. Houldsworth and Cobb? give the following figures for the coefficient of expansion of various siliceous materials in the raw state :— Taste CLXI.—Coefficient of Expansion of Raw Siliceous Materials Coefficient of Material. Temperature Range. Expansion x 107, Meanwood Ganister (no bond) : 15-1000 180 a (lime bond) . ; x 136 Welsh quartzite (lime bond). ; : E 136 Flint (lime bond) ; a 174 Precipitated silica with 5 nee ee tie 15-700 132 They found the average reversible thermal expansion of manufactured silica bricks between 150° C. and 1000° C. was 1-1-1-3 per cent. 1 Loc. cit., p. 577. * Trans. Eng. Cer. Soc., 21, III, 227, (1922). 580 EFFECT OF HEAT The effect of the temperature of firing siliceous materials is shown in Table CLXII, due to Houldsworth and Cobb. TaBLe CLXII.—Effect of Firing on Silica. (Coefficient of Linear Expansion x 10~*) Temperature of Firing. Material. bean Cone 06. | Cone 9. | Cone 14. | Cone 20. Meanwood ganister (no bond) . 15-1000 Bes 107 104 107 2 (lime bond) . is 75 68 55 62 Welsh quartzite (lime bond) : . 122 128 155 153 Flint (lime bond) . : : 15- 900 136 146 146 168 Pure amorphous silica : 15-1000 1661 4 224 Precipitated silica with 5 per eae soda . : : ‘ : Me 92 According to J. W. Mellor,? at high temperatures the coefficient of expansion of silica bricks is greater than at lower temperatures, the average difference being as follows :— 15-940° C. : : . 00000051 15-1180°C. , . 0-0000064 Fused silica has a very low coefficient of expansion at 1100° C.; it is only 0-00000059 or + that of ordinary glass, so that it is very insensitive to sudden changes of temperature and can be quenched from red heat in cold water without fracture. Table CLXIII shows the coefficient of expansion of fused silica when heated to different temperatures. TaBLe CLXII.—Coefficient of Expansion of Fused Silica Coefficient of Linear Temperature, ° C. Authority. Expansion. —191-16 0-000000256 F. Henning. 16-250 0-000000539 om 16-1000 0-000000540 5 200 0-000000518 Randall. 500 0-000000563 as 900 0-000000538 ‘“ 1100 0-000000583 Sa 0-1000 0-000000700 Le Chatelier. 1 Fired at 1170° C. 2 Trans. Eng. Cer. Soc., 16, 268 (1916-17). SUDDEN CHANGES IN TEMPERATURE 581 Holborn and Henning later found that the expansion of silica glass between 0° and 1000° C. is proportional to the temperature, and approximated to 54x 10-8. Common glass has an expansion of 600—900 x 10°. Fused bauxite has, according to B. Bogitch,1 a smaller thermal expansion from 0°-1600° C. than clay, silica, chromite, and magnesia. Alundum, according to Boeck,? has a coefficient of expansion of 866 x 10-8 between 100° and 900° C. Magnesia bricks, up to 1400° C., have a temporary expansion of about 1-9-1-95 per cent. Recrystallised crystolon between 100° and 900° C. has, according to Boeck, a coefficient of expansion of 474 x 10°8. It will be realised that the changes which occur in the volume of ceramic materials when heated cannot be expressed in any simple terms. They are affected by so many factors that they cannot be summarised in any simple “law.” The nearest approach to such a summary is that suggested by J. W. Mellor, and described on _p. 566, but its use is, unfortunately, limited. EFFECTS OF SUDDEN CHANGES IN TEMPERATURE The ability of ceramic materials to resist sudden changes in temperature depends upon : (a) Their permanent and reversible expansion or contraction. (6) Their texture, including the size and shape and grading of the grains (see also Chapter IT). (c) Their porosity (see also Chapter II). In a perfectly burned material the resistance would depend wholly upon the reversible expansion, but in practice this is never the case (except possibly with fused silica), because, unless the burning is carried to completion (t.e. to the fusion of the whole of the material), the reactions between the various components will not be complete and will proceed further whenever the material is heated to a sufficiently high temperature. Hence, the so-called “ permanent ”’ change in volume often has a great influence on the sensitiveness of the ware to sudden changes in temperature. Fine-grained masses are less resistant to temperature changes than coarse- grained ones, because the interstices between the fine grains are too small to enable the particles to accommodate themselves to the stresses placed upon them by local contractions or expansions, and also because the bond uniting the particles is weaker than the particles themselves. Masses composed of much coarser grains have larger interstices between the grains and are able to rearrange themselves as the temperature rises or falls. The difference in size of grain is of less importance when the particles are tightly interlocked with very little space between them, as they are then unable to move freely when sudden changes of temperature occur and, consequently, shattering of the mass results along the line of least resistance. 1 Comptes Rend., 173, 1358-60 (1921). 2 Loc. cit., p. 521. 582 EFFECT OF HEAT Whilst the foregoing statement is correct in the majority of cases, it must not be supposed that fine grains and low porosity are always accompanied by great suscepti- bility to sudden changes of temperature, because some articles having such a structure are very durable in use, even under violent changes in temperature. Fireclay bricks are not so susceptible to sudden changes of temperature as are silica bricks,! but if heated or cooled too rapidly they crack. The most resistant fireclay bricks are those which are most porous. Grog bricks are less likely to crack than fireclay bricks made wholly of clays, and sillimanite bricks—in which most of the clay has been converted into crystalline sillimanite—are still more resistant to sudden changes in temperature. Fireclay bricks of good quality and reasonably porous should not lose more than 12 per cent. by weight when subjected to the spalling test described on p. 523, but bricks having a very close texture may lose up to 65 per cent. by weight. Large blocks are more susceptible to sudden changes in temperature than are smaller ones, but if properly made they should resist all ordinary conditions of heating and cooling. To secure the necessary resistance they should have a more open texture (p. 38) than is needed for bricks or small blocks, so that the pores may take up any strains which occur. Saggers made of fireclay must be reasonably constant in volume when in use or they may warp and lose their shape. This will cause them to fit badly on one another and possibly endanger the stability of the “ bungs”’ of saggers placed one above another in the kiln. Saggers should be as porous as is consistent with the necessary strength (p. 78). Glass-melting pots and retorts require to be very resistant to sudden changes in temperature, but it is difficult to secure this property at the same time as a high resistance to corrosion. Moderately porous materials are the most suitable for the purpose where the corrosion is not excessive. Crucibles must be very resistant to sudden changes of temperature, as when in use they are withdrawn rapidly—from a furnace (which may have a temperature up to 1800° C.) into the open air. This resistance is attained if the crucible has an open, porous texture (p. 39), which may be obtained in the case of fireclay crucibles by introducing a sufficiently large proportion of grog into the mixture of which the crucibles are made. The number of times which a crucible can be re-heated varies according to its contents and the manner in which it is used. The following figures show the average “ life ’ if the crucibles are properly handled :— - TaBLeE CLXIV.—Durability of Crucibles when Melting Metals Nature of Contents. | Number of Heatings. Nature of Contents. | Number of Heatings. Brass . . : 70-100 Iron . : ; 70-90 Bronze : : about 50 Steel . ; : 6-10 1 The manufacturers of Glenboig firebricks claim that these bricks may be dropped whilst red hot into cold water without being cracked. No silica bricks will withstand so severe a test. SUDDEN CHANGES IN TEMPERATURE 583 The crucibles used for making steel do not last long on account of the very high temperatures (about 1550° C.) reached in melting the metal. Silica bricks are very susceptible to sudden changes in temperature and soon develop fine cracks, which later increase in size and cause the bricks to become weak or to spall and flake. Most silica bricks are so sensitive to changes in temperature, that if a current of air is drawn through them during the cooling of the kiln they are liable to crack. For this reason, after the firing of the bricks is completed, the lalns are closed and allowed to cool in such a manner that the whole of the heat is lost by radiation through the walls and floor of the kiln. Continuous kilns are seldom used for silica bricks, because they require a current of air to pass through them. The great liability of silica bricks spalling at temperatures below 500° C. is due to the a-f transition range described on p. 329. As cristobalite and tridymite are the forms of silica which are stable at high temperatures, whereas quartz is the unstable form, silica bricks which have been burned in such a manner that most of the quartz has been converted into tridymite and cristobalite are more constant in volume than those which have not been sufficiently fired, and the former are much less likely to crack when subjected to sudden changes in temperature. Silica bricks are not resistant to sudden heating and cooling, and when subjected to the test described on p. 523 they are destroyed in less than ten heatings. Bauxite bricks are liable to crack when exposed to sudden changes of tem- perature unless they have been made of very well-burned material. Bricks made of fused alumina are much more resistant to sudden changes in temperature than bricks made of calcined bauxite, the best results being obtained when the alumina is in large crystals. Maégnesia bricks are particularly subject to spalling, this being due, according to J. W. Mellor,} to: (a) The shrinkage caused by the conversion of a- into B-magnesia (i.e. of lightly calcined magnesia into periclase, p. 356), and (b) The shrinkage caused by the closing of the pores. Bricks of low porosity are less likely to spall, but all bricks made of this material suffer to a greater or less extent from this very serious drawback. By enclosing the material in iron cases—as in the “‘ Metalkase ”’ bricks—the drawbacks due to excessive cracking are largely prevented. Zirconia bricks and crucibles are very resistant to temperature changes and, for this reason, their use may increase in the near future. Chromite bricks, according to Hartmann and Hongen, are very susceptible to sudden changes of temperature, and are wholly disintegrated by the spalling test described on p. 523. Carbon bricks are quite insensitive to sudden changes of temperature as their coefficient of expansion is very low. Carborundum bricks are also very insensitive, especially those made from 1 Trans. Eng. Cer. Soc., 16, 85 (1916-17). 584 EFFECT OF HEAT carbofrax, which Hartmann and Hongen ! found to lose only 0-3-8 per cent. of their weight when subjected to the very stringent test described on p. 523. Table CLXV shows the results obtained by Hartmann and Hongen for the resistance of various refractory materials to spalling, the test-pieces being heated to 1350° C., and then cooled rapidly by means of a blast of air. TaBLeE CLXV.—Effect of Rapid Cooling on Spalling Loss by Spalling, Kind of Brick. No. of Coolings. per cent. Bonded carborundum (Carbofrax A). 10 0:3 = ie Ceres Bhisiex 10 6 “fi 3 easy C) ‘ 10 8 Recrystallised carborundum (Refrax) . 10 12 Bauxite. : , ‘ : 10 43 Zirconia (natural) : F : 10 53 Fireclay, Grade A 10 9 . pt : 10 65 5 aot : 10 90 Chromite bricks . 7 100 Silica bricks : : ; 4 100 Magnesia bricks . 3 : ; 3 100 Errect or Heat oN THERMAL CONDUCTIVITY The effect of heat on the thermal conductivity (p. 514) of ceramic materials and the various other equivalents of this property, such as resistivity, diffusivity, etc., are very important when the materials are required either to act as heat insulators and prevent the escape of heat from furnaces, etc., or as conductors which allow heat to pass through them from a source of heat situated on one side or externally to the article or materials to be heated on the other side or internally. The use of heat-insulators is typified in the case of furnaces and kilns of all kinds, as the heat is required to be applied to their contents and not allowed to escape unnecessarily to the open air. The use of ceramic materials as thermal conductors is typified by their employment as retorts, muffles, saggers, or crucibles, the contents of which are heated, all flames, etc., being arrested by the ceramic material. The rate at which heat passes through ceramic materials may be expressed in different ways, according to the purpose for which they are used, see p. 515. Conclusions and deductions based on the thermal conductivity or resistivity of a ceramic material must be used with great caution, as various other factors may 1 Brick and Clay Record, 56, 934 (1920). THERMAL CONDUCTIVITY 585 require to be taken into consideration. For instance, the thermal conductivity of firebricks used under oxidising conditions is quite different from that when the same bricks are employed under reducing conditions ; in the latter case they may quickly become coated with a layer of carbon (“retort graphite’) which entirely alters their behaviour with respect to the transmission of heat. As explained on p. 514, the rate at which heat passes through a ceramic material either by radiation or conduction is directly proportional to its temperature, though this statement must usually be modified on account of the influence of other factors. The effect of heat on the thermal conductivity of a material may be considered with respect to : (i) The heat applied in course of manufacture (2.¢. in the burning of ceramic ware). (ii) The temperature of the material when in use. The heat applied during the course of manufacture is very important, as the thermal conductivity usually increases with the temperature of firing. This is often due to the pore spaces being reduced in size and number by being filled with the fused material produced during the burning of the ware at a high temperature. Prolonged burning has a similar effect. The best heat-insulating qualities are obtained, accord- ing to A. L. Queneau, by burning the materials at the lowest possible temperature consistent with their satisfactoriness in use. Table CLX VI! shows the effect of the burning temperature on the thermal con- ductivity of various refractory materials. Taste CLXVI.—Hffect of Burning Temperature on Thermal Conductivity Baring Porosity, Temperature, Gm.-cals. Kg.-cals. oC. per cent. Fireclay bricks .. ; 1050 0-0035 1-25 29-4 - 1200 0-0030 1-07 =i + 1300 0-0050 1-81 24-1 - : : 1300 0-0042 1-50 30-2 Fireclay retorts. 1300 0-0038 1-37 27°3 Bauxite bricks : 1050 0-0031 1-11 41+5 _ : : 1300 0-0033 1-19 38-4 Silica bricks . ; 1050 0-0020 0-71 42-5 : : : : 1300 0-0031 1-12 42-9 Magnesia bricks. 1050 0-0058 2-08 35:1 * : : 1300 ° 0-0065 2°35 41-0 Chromite : : ‘ nf 0-0066 2°37 Graphite ‘ ; : re 0-0250 9-00 #y Kieselguhr. ; ; a 0-0018 0-64 58-0 Hard porcelain. ; 1400 0-0043 1-55 ee 1 Wologdine. 586 EFFECT OF HEAT The thermal conductivity of carborundum increases very rapidly when it is burned at higher temperatures, thus :— TaBLeE CLXVII.—Thermal Conductivity of Carborundum Burning Temperature, Thermal Conductivity, S gm.-cals. 1050 0:0033 1300 0-0145 This represents an increase of nearly 400 per cent.; the thermal conductivity of magnesia under the same conditions and within the same range of temperature increases only 5 per cent. The temperature of a ceramic material when in use has also a very important influence on the thermal conductivity. In most cases, the conductivity mecreases at high temperatures, the extent of the increase depending on the nature of the material. Thus, the thermal conductivity of fireclay and grog bricks increases considerably when they are heated to 1200° C., whereas that of chromite bricks is hardly affected, and that of magnesia bricks decreases slightly at high temperatures. Table CLXVIII, due to R. H. Horning, shows the relative thermal conductivity of various refractory materials. TasLeE CLXVIII.—Thermal Conductivity of Refractory Materials at Various Temperatures Temperature Difference in ° C. Wt. in Material. Ibs. per Ss ook 200. | 300. | 400. | 500. | 600. | 700. | 800. | 900. |1000.|1100.|1200.|1300./1400.|1500. Magnesia bricks . | 164-0 |268 |273 |276 |280 |284 |288 |292 |296 |300 |302 |305 |307 |309 {310 Silica bricks . : 97-0 |123 |127 |1382 |136 |139 |142 |145 |148 |151 |153 |154 |156 |158 |159 No. 1 firebrick . | 131-0 | 52 | 57 | 62 | 66 | 70 | 74 | 77 | 81 | 83 | 85 | 86-5] 87 | 87-5) 87-5 Repressed burned kieselguhr brick . 23-2 | 24 | 25 | 26 | 26 | 26:5] 27 | 27-5) 27-7) 28 | 28-1) 28-2) 28-5] 28-5) 28-5 Natural kieselguhr ; brick (perpend. | } 33-0 | 19-9} 20 | 20:5] 21 | 21-5) 21-9} 22-3) 22-7| 23-1| 23-5) 23-9] 24-3) 24:7) 25-2 to grain) . , Nonpareil insulat- ing brick . : 24:0 | 13 | 14:4) 15-6} 16-8] 17-8] 18-8] 19-6) 20-1] 20-6) 21 | 21-4} 21-7) 22-0} 22-2 The following information relates specifically to the particular ceramic articles mentioned :— THERMAL CONDUCTIVITY 587 Clay wares have a thermal conductivity depending on their texture, those which are vitrified being usually better conductors than those which are more porous. The thermal conductivity of fireclay bricks is almost always less than 0-0050 gram- cals., and is usually about half this figure; the thermal conductivity of building bricks is similar. Table CLXIX, due to L. R. Ingersoll,! shows the average diffusivity of fireclay bricks in comparison with other materials, including some metals :— TaBLeE CLXIX.—Thermal Conductivity of Various Materials Material. Diffusivity. Material. Diffusivity. Air. : : 0-1800 Gold . ; ; 1-1800 Building brick. 0-0050 Tron . 0-1700 Cast steel . 0-1200 Silica brick 0-0030 Copper : 1-1300 Silver. 1-7400 Firebrick . ; 0-0067 The figure given above for a firebrick differs considerably from those obtained by A. T. Green? and shown in Table CLXX, but it agrees fairly closely with the figures obtained by Dougill, Hodsman, and Cobb and shown in Table CLXXI. The thermal conductivity rises on heating the material, especially if the tempera- ture rises above 900° C. Thus, Table CLXX shows the figures obtained by A. T. Green 2 for the increase in thermal conductivity and diffusivity of fireclay bricks at the temperatures mentioned :— Taste CLXX.—Hffect of Temperature on Conductivity and Diffusivity of Fireclay Bricks Thermal Conductivity, Temperature, ° C. eel ee ee Diffusivity. 500 0-0010 0-0020 600 0-0011 0-0021 700 0-0013 0-0023 800 0-0015 0-0025 900 0-0016 0-0026 1000 0-0018 0-0027 1100 0-0025 0-0034 1 Communication to Harvard. 2 Loe. cit., p. 515. 588 EFFECT OF HEAT Dougill, Hodsman, and Cobb obtained much higher figures for both thermal conductivity and diffusivity, as shown in Table CLXXI. Taste CLXXI.—Effect of Temperature on Conductivity, ete. Heat ; A moseia | MERE, ondantinay, | Somme | Speco | | Meat gm.-cals. per sec. Firebrick : 500 0-0028 1-95 0-23 0-0062 3 : 1000 0-0040 ge 0-26 0-0079 Silica brick : 500 0-0024 1-74 0-26 0-0053 » : 1000 0-0046 if 0-27 0-0098 Magnesia brick . 500 0-0141 2-40 0-26 0-0226 a : 1000 0-0085 Ae 0-28 0-0126 They suggest the following formula for calculating the thermal conductivity of fireclay bricks up to 1000° C. :— Kt = 0-00155 + 0-25 x 10°, where K¢ is the thermal conductivity of the material in grams-cals. per sec. at ¢° C. Table CLX-XII gives the results obtained by Heyn, Bauer, and Wetzel ! for the thermal conductivity of grog bricks in comparison with other refractory materials, at different temperatures. Taste CLXXII.—Thermal Conductivity of Grog and Other Bricks Thermal Conductivity in gm.-cals. per cm. per sec. Material. ae 200° C. | 400° C. | 600° C. | 800° C. | 1000° C. | 1200° C. Grog brick . | 1:88 | 0-0014 | 0-0018 | 0-0022 | 0-0024 | 0-0026 | 0-0027 * . | 1:30 | 0-0011 | 0-0014 | 0-0016 | 0-0019 | 0-0021 . . | 1:77 | 0-0009 | 0-0011 | 00012 | 0-0013 | _ .. Seen , . | 1:90 | 0-0021 | 0-0024 | 0-0027 | 0-0027 | 0-0027 | 0-0027 Dinas brick . | 2:04 | 0-0013 | 0-0016 | 0-0017 | 0-0017 | 0-0018 | 0-0021 Magnesia brick . . | 2°35 | 0-0011 | 0-0015 | 0-0012 | 0-0013 | 0-0014 | 0-0014 Carbon brick wl Veoh gy DOr Retorts, muffles, and saggers require to have as high a thermal conductivity 1 Berlin Bur. Stand., 1914; Sprech., 52, 499-501 (1919). THERMAL CONDUCTIVITY 589 as possible, so as not to waste fuel in heating them rather than their contents. Com- plete satisfaction is very difficult to obtain as the highest thermal conductivity is associated with materials which are dense in texture, whereas resistance to the changes in temperature to which such articles are exposed necessitates the use of a porous mass. The influence of temperature on the thermal conductivity of gas retorts is shown in Table CLXXIII, due to Wologdine, whilst in Table CLXXIV corresponding figures obtained by A. T. Green are shown. Taste CLXXIII—Thermal Conductivity of Retorts Temperature of Temperature of Thermal Conductivity, Upper Surface, ° C. | Lower Surface, ° C. | gm.-cals. per cm. per sec. 140 1120 175 1110 160 1050 150 1010 140 990 151 870 105 685 TaBLe CLXXIV.—Thermal Conductiwnity of Retorts Thermal Conductivity, Thermal Diffusivity. gm.-cals. per sec. Temperature, ° C. 500 0-0008 0-0017 600 0-0009 0-0018 700 0-0010 0-0020 800 0-0012 0-0022 900 0-0013 0-0022 1000 0-0014 0-0023 1100 0-0017 0-0026 The thermal conductivity of refractory porcelain is about 0-002—0-004, or rather higher than that of glass. Lees and Chorlton found the thermal conductivity of a sample of porcelain tested to be 0-00248 between 92° C. and 98° C. Silica bricks are usually considered to be better conductors of heat than fireclay bricks, though opinion is divided on the matter. Thus, some Dinas silica bricks 590 EFFECT OF HEAT have a thermal conductivity of about 0-0013 gram-cals. at 200° C., to about 0-0021 - gram-cals. at 1200° C. (see Table CLX XII), but Goerens and Giles found that the thermal conductivity of other silica bricks is greater than that of fireclay bricks, except those made from Lias clay, as shown in Table CLXXV, whilst G. H. Brown 1 gives the relative conductivities of silica, quartzite (ganister), and fireclay bricks as 1017, 986, and 933 respectively. This represents the conductivity of silica bricks as 9 per cent. greater and quartzite bricks as 52 per cent. greater than that of fireclay bricks. Other investigators, however (including Wologdine, Dougill, Hodsman and Cobb, Heyn, Bauer and Wetzel, and A. T. Green), consider that silica bricks sometimes have a lower conductivity than fireclay bricks. TaBLE CLXXV.—Thermal Conductivity of Silica and other Bricks Average Coefficient of Average Coefficient of Material. Thermal Conductivity, Material, Thermal Conductivity, cal. per metre per hour cal. per metre per hour per 1° C. per 1° C. Semi-grog . : 0-90 Lias clay . 1-76 Grog . 0-91 Silica : bas Shale : : 0-99 Commercially, there is probably very little to choose between the thermal con- ductivities of fireclay and silica bricks, as the figures obtained with different samples vary as a result of differences in texture, burning temperature, etc. Hence, the greater speed with which heat appears to pass into silica retorts is probably due to their greater diffusivity rather than to their thermal conductivity, as is commonly supposed. The influence of the temperature of burning on the thermal conductivity of silica bricks is shown in Table CLXVI. Silica bricks with the lowest thermal conductivity are those in which the quartz is not converted into tridymite or cristobalite. Accord- ing to A. L. Queneau, silica bricks burned at about 1050° C. are as resistant to the passage of heat as kieselguhr bricks, and have only one-half the conductivity of fireclay bricks. This may not be entirely correct, as the variations in thermal conductivity in different bricks are so great. Silica bricks increase considerably in conductivity when heated, as shown by the results obtained by A. T. Green in Table CLX XVI. Other results are given in Tables CLXIX and CLXXI, including those of Dougill, Hodsman, and Cobb, who have obtained much higher results for both diffusivity and thermal conductivity. Inght-weight silica bricks have a lower thermal conductivity than ordinary ones. 1 Trans. Amer. Cer. Soc., 16, 382 (1914). THERMAL CONDUCTIVITY 591 TaBLeE CLXXVI.—Hffect of Heat on Conductivity and Diffusivity of Silica Bricks | Teisporatire, Conductivity, gm.-cals. per sec. Diffusivity. ge A B. A B 500 0-0007 ag 0-0017 600 0-0008 0-0009 0-0018 0-0024 700 0-0009 0-0011 0-0019 0-0026 800 0-0010 0-0012 0-0019 0-0028 900 0-0011 0-0014 0-0020 0-0030 1000 0-0013 0-0017 0-0022 0-0033 1100 0-0016 0-0020 0-0025 0:0037 Those made by J. H. Sankey & Son, Ltd., are claimed to have a heat conductivity of 0-0005 C.G.S. units. Kieselguhr bricks have a thermal conductivity of about 0-0018 gram-cals., which is about half that of fireclay bricks. Silica glass or fused silica has (when cold) a thermal conductivity of about 0-002-0-003 C.G.S. units. Maégnesia bricks have about twice the thermal conductivity of fireclay bricks, and also a very high heat-diffusivity, but their great sensitiveness to sudden changes of temperature is a serious disadvantage. The temperature at which magnesia bricks have been burned has only a slight influence on the thermal conductivity as shown in Table CLXVI. The results obtained by A. T. Green are shown in Table CLX XVII. TaBLeE CLXXVII.—Conductinity and Diffusivity of Magnesia Bricks Thermal Conductivity, Diffusivity, eceeerare, C- : gm. ee per sec. : C.G.8. Units. 500 0-0017 0-0023 600 0-0017 0-0021 700 0-0017 0-0020 800 0-0017 0-0019 900 0-0016 0-0017 1000 0-0016 0-0017 1100 0-0016 0-0016 592 EFFECT OF HEAT It will be seen that the thermal conductivity of magnesia bricks is almost constant, or decreases very slightly at high temperatures. Dougill, Hodsman, and Cobb, in Table CLXXI, show a similar decrease, but their results are much higher. They suggest the following formula for calculating the thermal conductivity of magnesia bricks up to 1000° C. :— Kt =0-0285—0-379 x 10-%—0-179 x 10-722, where K¢ is the thermal conductivity in gram-cals. per sec. at ¢° C. Heyn, Bauer, and Wetzel show a slight increase in conductivity at high temperatures in Table CLX XII. Chromite bricks, according to A. L. Queneau, have practically the same thermal conductivity at all temperatures, namely, 0-0057 C.G.S. units. Carborundum bricks have a high thermal conductivity ; even when mixed with as much as 20 per cent. of clay they conduct three times as much heat as magnesia bricks, seven times as much as fireclay bricks, and twelve times as much as silica bricks in a given time. Graphite bricks, according to Heyn, Bauer, and Wetzel (Table CLXXII), have about five times the thermal conductivity of fireclay bricks, viz. about 0-0012 C.G.8. units. TaBLteE CLXXVIII.—Thermal Conductinty of Powders Thermal Conductivity Material. in gm.-cal. sec. per cm.? per 1° C. White Calais sand . ' 0-00060 Fine carborundum ; 0-00050 Coarse BS ; : : 0-00051 Enamel quartz. : : : ; 0-00036 Fused quartz : : 3 ; 0-00039 Fireclay brick ; : 0-00028 Retort graphite. , . 0-00040 lime . : ? ; 0-00029 Fused magnesia. ; : : : 0-00047 Mabor magnesia brick. 0-00050 Calcined Greek magnesia 0-00045 Calcined Veitsch magnesia. at 0-00034 Pattinson’s high calcined magnesia . ; 0-00016 Kieselguhr_ . . : 0-00013 When bricks or blocks are cemented together to form a wall, as in a furnace, the THERMAL CONDUCTIVITY 593 thermal conductivity may be much altered, and Dougill, Hodsman, and Cobb have pointed out that the thermal conductivity of the joints of brickwork is only one-tenth that of the bricks themselves, so that segmental retorts would appear to require more heat than those in one piece, if both kinds of retorts are made of the same material. Table CLXXVIII, due to Harvard, shows the thermal conductivity of various raw ceramic materials in C.G.S. units between 20° C. and 100° C., the materials in each case being in the form of a powder which passed entirely through a sieve with 600-meshes per square cm. (approximately 60 meshes per linear inch). Table CLX XIX, also due to Harvard, shows the thermal conductivity of various commercial refractory, and other bricks and blocks, between 1000° and 1200° C. Taste CLXXIX.—Thermal Conductivity of Bricks : Gm.-cal. sec. per Kg.-cal. hour per Relative eeeae em.* per 1° é a per 1° c Conductivity. Graphite brick A 3 0-0250 9-00 100-0 Carborundum brick. : 0-0231 8-32 92-4 Magnesia brick : 0-0071 2-54 28-4 Chromite brick ; : 0-0057 2-05 22:8 Fireclay brick ; ; 0-0042 1-50 16-7 Checker brick : : 0-0039 1-42 15:8 Gas retort brick . : 0-0038 1-36 15:2 Building brick ; 0-0035 1-26 14-0 Glass pot ; 0-0033 a3 13-2 Bauxite brick : ; 0-0027 0-96 12-4 Terra-cotta . : : 0-0023 0-84 9-3 Sihca. y : : 0-0020 0-71 7:8 Kieselguhr 0-0018 0-64 T1 There is an unfortunate lack of information on the thermal conductivity of refractory materials at the temperatures at which they are chiefly employed, viz. above 1000° C. This is the more regrettable, as a large amount of work has been done at lower temperatures, which is, apparently, of little practical importance. As the thermal conductivity varies greatly at different temperatures, it is seldom possible to obtain an accurate result by extrapolating from data relating to lower temperatures. It is often convenient to express thermal conductivity as its converse or reciprocal,. t.e. as thermal resistivity (p. 515) as in Table CLXXX compiled by Hering, which. shows the thermal resistivity of various materials. 38 594 EFFECT OF HEAT TaBLeE CLXXX.—Thermal Resistivity Thermal Ohms. Material. Authority. in. cube. | cm. cube. Silver, 0-100° C. . : ‘ : . : 0-094 0-24 | Landolt and Boernstein. Platinum, 18-100°C. . 5 ; ; : 0-55 1-4 & au a Acheson graphite, 100°-390° C. (mean) . 5 0-28 0:71 | C. Hering. iS 100°-914° C._,, ; ; 0-32 0-82 os Electrode carbon, 100°-360° C. __,, : : 0:05 2:7 v5 a 100°-942° C.__,, : : 0-72 1°9 is Ehnshare brick, 1000° C. approx. . , ‘ 3°8 9-6 | Wologdine and Queneau. Carborundum brick ie : : : 4.1 10-3 > i Quartz . : : : é : : 5-9 15-0 | Landolt and Boernstein. Retort carbon, 0° C. : < B : 9-1 23-0 ry Magnesia brick (about 1000° C. ) . : 3 13-0 34-0 Wolosdina and Queneau. Chromite brick a : 5 : 16-0 42-0 o a Firebrick a 4 5 : 22-0 57-0 3 Chequer brick Pe : ; : 24-0 61-0 os a Gas retort brick : 3 : . 25-0 63-0 “5 fe Building brick re : : 3 29-0 72-0 a - Glass pot ys : : . 35-0 89-0 7 7 Porcelain, 95°C. . : : . A : 38-0 96:0 | Landolt and Boernstein. Firestone c ; P i 39-0 99-0 a 4 Terra-cotta (about 1000° C. eee ; ALG 104.0 | Wologdine and Queneau. Silica brick “ 5 : A : 47-0 120-0 a + Kieselguhr brick ,, 52-0 133-0 ée re Plumbago, 200-155° C., 26-1 ee cent. oli nation 96-0 240-0 | Ordway. Fine sand ae 51-4 Py A - 109-0 276-0 ‘5 Coarse sand + 52-9 a3 a 110-0 280-0 3 Pumice 34-2 a i. 219-0 558-0 en Asbestos os 8-1 a at 139-0 353-0 is Kieselguhr + 11-2 = a 435-0 1110-0 a 5S a 6-0 Ap a 472-0 1200-0 1 Magnesia, calcined ,, 28-5 = 3 160-0 407-0 3 26, a Seen: Ps = 544-0 1380-0 e. is a Pig ns 4 i AF 544-0 1410-0 oe EFFect oF Heat ON THE SpeciIFIC Heat oF CERAMIC MATERIALS. The heat capacity of ceramic materials is often very important and especially when they form part of the walls of furnaces, etc., as all the heat absorbed by such material is, in a sense, wasted and ought to be used in heating the contents of the furnace. The various terms “heat capacity, atomic, molecular”? and “ specific heats ” have been defined on pp. 510-514, and the methods of determining them have also beeu described on p. 512. 99 66 99 66 SPECIFIC HEAT 595 The specific heat of most materials increases with the temperature, so that the fuel consumption when heating ceramic materials at high temperatures is much greater than that at low temperatures, the same weight of material being used in each case. Thus, in fireclay bricks, according to Bradshaw and Emery,! the relation between the fuel consumption at 1200°-1400° C. and that at 100°-300° C. is nearly 3 : 2, Clays.—J. M. Knote ? found the specific heat of raw clay to be about 0-237, that of clay heated to 650° C. 0-204, and that burned at 1050° C. to be 0-200 C.G.S. units. It will be seen that the dehydration of clay causes a decrease in the specific heat. The specific heat at 1050° C. is practically the same as that at 650° C., the difference being due probably to the water not completely removed during the decomposition at 650° C. The specific heat of kaolin at different temperatures is shown in Table CLXXXI according to various authorities. TaBLeE CLXXXI.—Changes in Specific Heat of Kaolin Temperature, ° C. Specific Heat. Authority. .. 0-235 J. M. Knote. 22-98 0-2242 L. Boernstein. 440-1000 0-235 Bleininger and Moore. 650 0-204 J. M. Knote. 1050 0-200 bs The specific heat of fireclay bricks at different temperatures may, according to S. T. Wilson and A. D. Holdcroft, be calculated from the formula : Sp. ht.=0-193-++0-00006, where ¢ isthe temperature in°C. Table CLX XXII shows some experimental results obtained by the same investigators. Taste CLXXXII.—Variations of Specific Heat with Temperature Temperature, °C. | Average Specific Heat. Temperature, ° C. Average Specific Heat. 700 0-233 1100 0-255 800 0-241 1200 0-261 900 0-246 1300 0-264 1000 0-263 1 Trans. Eng. Cer. Soc., 19, 88 (1919-20). 2 Trans. Amer. Cer. Soc., 14, 394 (1912), 3 Trans. Eng. Cer. Soc., 12, 279 (1913). 596 EFFECT OF HEAT Bradshaw and Emery! have found that the specific heat of fireclay bricks at different temperatures is correctly shown by the formula : 0-193 -+0-000075¢, where ¢ is the temperature in °C. This is about 25 per cent. higher than Wilson and Holdcroft’s figure (p. 595). Table CLX XXIII, due to Bradshaw and Emery,! shows the specific heats of various materials between 25° and 1400° C. TaBLeE CLXXXIII.—Specific Heats of Various Bricks Stourbridge Temperature, | Coarse Silica Fine Silica Hirebrok Zirconia Firebrick i U8 Brick. Brick. : Pure. (Wilson and Holdcroft). 600 0-226 0-228 0-228 0-137 0-227 1000 0-263 0-262 0-265 0-157 0-263 1200 0-282 0-283 0-284 0-167 0-262 1400 0-293 0-295 0-297 0-175 Table CLXXXIV, due to Heyn, Bauer, and Wetzel,? shows the specific heat of grog bricks in comparison with various other materials at different temperatures. Taste CLXXXIV.—Specific Heat of Refractory Bricks ; Specific Heat at Material tie ; Gravity. |} 200° C. | 400°C. | 600°C. | 800°C. | 1000° C. | 1200° C. Grog brick 1 SL-SS 0-225 0-250 0-272 0-287 0-298 0-305 ‘ Sos He rs) 0-216 0-254 0-273 0-287 0-295 0-300 , tr 0-217 0-243 0-263 0-281 0-295 0-304 E _ | 1-90 | 0-223 | 0-262 | 0-284 | 0-291 | 0-292 | 0-293 Dinas brick . | 2-04 0-237 0-270 0-282 0-285 0-288 0-291 Magnesite brick | 2°35 | 0-258 | 0-275 | 0-291 | 0-307 | 0-324 | 0-340 Carbon brick . | 1-27 0-312 0-358 0-377 0-395 0-412 3 The specific heat of unglazed Berlin porcelain is, according to W. Steger, 0-202, C.G.S. units, between 20° and 200° C. and 0-221 between 200° and 400° C. 1 Loc. cit., p. 595. 2 Loc. cit. p. 588. SPECIFIC HEAT 597 Siliceous Materials.—Table CLXXXV, due to W. P. White, shows the specific heat of various forms of silica. TaBLeE CLXXXV.—Specific Heat of Various Forms of Silica Temperature, ° C.| Quartz Glass. a-Quartz. B-Quartz. Cristobalite. 100 0-202 0-204 250 0-236 0-244 500 0-266 0-294 550 aah 0-313 i re 750 0-280 “hs 0-277 0-278 1000 0-290 dis 0-288 0-285 1100 oe * es 0-287 Ulrich ? gives the specific heat of quartz sand between 20° and 98° C. as 0-191. The specific heats of silica and fireclay bricks are very similar, generally about 0-26. The specific heats of various silicates is shown in Table CLXXXVI, due to W. P. White.” TaBLeE CLXXXVI.—Specific Heat of Various Substances Pseudo- Tempera- Wollas- eer oes Orthoclase Soft a a owas - Pea: Orthoclase.| Diopside. Quartz. Pier Pee 100 “i 0-1833 nl 0-1919 0-1840 a 0:1977 500 0-2159 0:2180 0-2248 0-2310 0-2372 0:2291 0:2400 700 =A 0:2286 ae 0-2420 0:2547 fs 0:2646 800 a3 is 0:2401 we aie 0:2465 a 900 oe 0:2354 Se 0-2499 0:2597 re 0:2791 1100 0-2380 0:2423 0-2505 02562 0:2643 02588 0:2907 1300 0-2422 reg cs 0-2613 ee ae 0:2945 1500 os es en or ie oe 0-:2999 The increase in the specific heat of a material at high temperature is very noticeable. Barus gives the specific heats of solid and molten diabase as shown in Table QLXXXVII. 1 Wollny’s Forsch., 17, 1 (1894). 2 Amer. J. Sci., 28, 334. 598 EFFECT OF HEAT TaBLeE CLXXXVII.—Effect of Fusion on Specific Heat Temperature. State. Specific Heat. 800°-1100° C. : ’ solid 0-304 1200°-1400° C. : : liquid 0-350 The specific heats of various silicates, etc., present in bricks naturally modify, the total specific heat, but they are usually present in such small quantities that they do not cause any appreciable difference in the specific heat of the material. The molecular heats of quartz and various silicates are shown in Table CLX XXVIII. TaBLtE CLXXXVIII.—Molecular Heat of Silica and Silicates (between 15° and 100° C.) Substance. Molecular Heat. Observer. Quartz . : : : 11:1 White. CaSiO, . : : : 21-4 7 PbSiO, . : é } 22-1 Schulz. Na,OALO,6810, . . 104-9 Joly. K,0ALO4SO, a ae 84-3 : Li,OAl,048i0, . .. 80-8 Schulz. Alumina has a specific heat (according to Russell 1) of 0-200 between 3 and 48° C. and (according to H. v. Wartenburg and G. Wetzel?) a molecular heat of 10-8 at 230° absolute (—48° C.) and 29-0 at 1308° absolute (1035° C.). Lime has (according to Laschtschenko *) a specific heat of 0-113 between 0° and 150° C. and a molecular heat (according to H. v. Wartenburg and G. Wetzel ?) of 11-6 at 559° absolute (386° C.) and 13-0 at 1369° absolute (1096° C.). Magnesia has a specific heat of 0-258 to 0-340 and a molecular heat, according to H. v. Wartenburg and G. Wetzel,? of 10-2 at 415° absolute (342° C.) and 14-5 at 1683° absolute (1410° C.). Zirconia has a remarkably low specific heat; it is, according to Nelson and Petterson * only 0-108 between 0 and 100° C. Holdcroft and Mellor give the following figures for the specific heat of zirconia at different temperatures :— 1 Phys. Zeit., 13, 59 (1912). 2 Zeits. Hlectrochem., 25, 209-212 (1919). 8 J. Russ. Phys.-chem. Soc., 42, 1604 (1910). 4 Ber., 13, 1459 (1880). HEATS OF REACTION 599 TaBLE CLXXXIX.—Specific Heat of Zirconia Temperature, ° C. Specific Heat. Temperature, ° C. Specific Heat. 600 0-137 1200 0-164 1000 0-157 1400 0-175 Carbon bricks have a specific heat of about 0-312 at 200° C. and 0-412 at 1000° C. Heats oF REACTION IN CERAMIC PROCESSES The various reactions which take place as a result of chemical affinity cause ‘different heat effects, as described on p. 528. These reactions are often of great importance in the ceramic industries. The principal ones as far as they have been investigated are described in the following pages. R. C. Ray + found that the heat of solution of coarsely powdered (20-40-mesh) quartz in a 34:6 per cent. solution of hydrofluoric acid was 30,300 cals. per molecule, whilst that of silica glass in similar sized pieces was 37,300. When more finely ground material (passing a 200-mesh) was used, the results were 32-46, and 36-95 respectively. This suggests that the material is made partly amorphous by grinding it. The higher figures are probably the more correct. The heat of formation of various silicates is shown in Table CXC. TaBLE CXC.—Heat of Formation of Silicates, ete. Silinate, Moe te Seetee Biicate: Heat it rete FeOSi0, ed 10,600 A ose eae 14,900 Os00! 22.236 3Ca0Al,0,2810, 33,500 Sono. 17,850 NeOsic eee 45,200 feos, 28 300 Sn0 Algae an are 450 sCa08i0, « (iti‘( 3) Day and others. Gurther. 39 O. Nielson. Day and others. Hermann. Day and others. Hilpert and Kohlmeyer. Pascal. Day and others. Ruff. Grum Grzmailo. Ruff. REFRACTORINESS 607 TABLE CXCIV— Continued. Melting-point or Mineral. angotac: Authority. Fluorite 1361 Pascal. Forsterite . 1460 Hermann. Galena 1115 Gehlenite . 1280-1300 Grossular . 1150-1250 Hornblende 1180-1220 ie Labradorite 1477 Day and others. Lepidolite . 925-945 ~ Leucite 1320-1370 Magnesium metasilicate 1565 Magnesium orthosilicate 1900 Magnetite . 1538 Ruff. Monticellite 1435 Hermann. Muscovite . 1255-1290 : Nepheline . 1223 Ginsberg. Nephite 1180-1210 py Olivine over 1600 P. Lebedew. Orthoclase 1200 Dittler. Potassium fetahorate. 947 Van Klooster. Rhodonite . 1210 P. Lebedew. Scapolite 1120-1140 rs Silimanite 1816 Day and others. Sodium metaborate 960 Van Klooster. Sodium metasilicate 1018 R. C. Wallace. Spinel 1360 Spodumene 1380 Strontium ese licate 1287 Strontium orthosilicate 1593 Titanite 1200-1300 Titanium oxide . 1610 Rieke. Titanium orthosilicate 1650 S Tourmaline 1000-1100 Wollastonite 1250-1300 Zinc metasilicate 1479 Zinc orthosilicate 1484 From the foregoing it will be seen that the effect of heat on ceramic materials is by no means simple. Much is known of its effects in several directions, but on the whole there is a lamentable lack of exact information which can only be supplied by a large number of experiments and prolonged research. CHAPTER XIV ELECTRICAL AND MAGNETIC PROPERTIES OF CERAMIC MATERIALS Tue electrical and magnetic properties of ceramic materials are important in con- nection with both their manufacture and uses. During the manufacture of ceramic materials—-particularly those made of clay—the electrical properties of the colloids present are very important (see Chapter VI). Thus, Blake, Morscher, and Swarte have proposed to use the electrical conductivity of raw clays as an aid to their purification, but though the process they suggested has not been very satisfactory, Schwerin’s electro-osmose process (p. 289) is being used for this purpose, especially in Germany, and has attracted much attention in this country. The magnetic properties of various substances—particularly metallic iron— which occur as undesirable impurities in ceramic materials are often important, as they afford a simple means whereby such impurities may be removed. In the use of ceramic materials their electrical conductivity is chiefly important in connection with furnaces, etc., which are heated electrically and in insulators, which are widely used in different forms of electrical apparatus. In both cases, the electrical properties of principal interest to the reader are the electrical conductivity and its converse the electrical resistivity of ceramic wares. ELECTRICAL CONDUCTIVITY AND RESISTIVITY oF CERAMIC MATERIALS The electrical conductivity and resistivity of ceramic materials are expressed in different ways according to convenience :— 1. The electrical conductivity may be expressed directly in C.G.S. units as mho or reciprocal units per cubic centimetre. 2. The electrical resistance may also be expressed in C.G.S. units as ohms per cubic centimetre (the temperature being also stated both in this and the electrical conductivity). 3. The puncture voltage, or the voltage at which a piece of the material of definite thickness is unable to act as an insulator, and, therefore, allows the electric current to pass through it. This is also referred to as the dielectric strength. According to H. F. Howarth, there is no direct relation between the specific ohmic resistance and the puncture voltage. 608 ELECTRICAL CONDUCTIVITY 609 4. The “ Te” value or the temperature at which the electrical resistance is reduced to 1 megohm per c.c. 5. The specific inductive capacity or the relative dielectric strength of the article or material, that of an equal thickness of air being taken as unity. The general factors influencing the electrical conductivity and resistivity of ceramic materials are :— 1. The composition of the material. 2. The size of the article or test-piece at the time. 3. The density and texture of the material. 4. The manner of heating and the extent to which the material has been heated during the “ burning ”’ of the ware. 5. The piezo-electric effect. 6. The temperature of the ware. 7. The length of exposure to the electric current. The composition of the ceramic material largely determines its electrical properties, some materials being much better conductors than others. In composite materials, such as porcelain, the chemical composition is usually important only in so far as it modifies the structure, compositions giving a dense vitreous mass being usually better insulators than those which produce semi-vitreous or porous wares. Taste CXCV.—Electrical Conductivity of Various Minerals Good Conductors. Moderate Conductors. Bad Conductors. Magnetite. Ferriferous amphiboles | Siderite. Apatite. Titaniferous magnetite. and pyroxenes. Xenotime. Andalusite. Magnetic hematite. Biotite. Epidote. Sillimanite. Pyrrhotite. Tourmaline. Olivine. Fluorite. Chromite. Titanite. Staurolite. Diamond. Ilmenite. Rutile. Garnet. Topaz. Hematite. Anatase. Monazite. Spinel. Wolframite. Brookite. Gypsum. Cyanite. Spinel. Cassiterite. Quartz. Corundum. Ferriferous cassiterite. Chalcedony. _Celestite. Tantalite. Felspars. Zircon. Iron pyrites. Calcite. Sandstone. Chalcopyrite. Dolomite. Granite. Cordierite. Porphyry. Barytes. Schist. Phlogopite. Fluorspar. Muscovite. Silicates. Tremolite. Clays. 39 610 ELECTRICAL AND MAGNETIC PROPERTIES Table CXCV shows the electrical conductivity of various materials used in the ceramic industries or liable to occur as impurities in ceramic materials. The size of the article is often important with respect to its insulating power, as, according to B. 8. Radcliffe, the dielectric strength of porcelains and other ceramic insulating materials is directly proportional to their thickness. The density of a ceramic material is important, as A. 8. Watts 2 has proved that the dielectric strength of porcelain and similar materials increases with their density, and, consequently, vitrified articles have a greater dielectric strength than those which are porous. The heat-treatment of the ware during manufacture does not, according to B. 8S. Radcliffe,1 affect its dielectric strength, provided such treatment does not cause the formation of blebs, cracks, or other flaws. If, however, any of these defects are produced the resistivity may be greatly diminished. This is in opposition to Bleininger and Riddle,? who consider that the rate of cooling has an important influence on the “ Te”’ value of porcelain, as rapid cooling tends to produce a closer and more vitreous structure than slow cooling. Moreover, according to these investigators, the “'Te”’ value of porcelain increases with the maturing temperature, so that the electrical resistance at high temperatures varies inversely with the felspar content, though in a very irregular manner, as shown in Table CXCVI. Taste CXCVI.—Variation of Resistance with Felspar Content Maturing Per cent., Felspar. Temperature, “Te” Value. Seger Cone. 16 16 560 18 15 390 20 14 440 28 14 (4 over) 370 30 13 450 According to A. 8. Watts,? porcelains have a smaller resistance to puncture by electric currents when under- or over-fired than when it is correctly. burned, the least resistance being offered by seriously under-fired materials. The vesicular structure produced by over-firing gives a weak material, but if the vesicles are small and not too numerous, so that the mass as a whole is dense and glassy, the puncture-voltage may be very high. Watts gives the following figures as typical of correctly fired porcelains :— 1 Trans. Amer. Cer. Soc., 14, 575 (1912). * Ibid., 9, 615 (1907). 3 Loc. cit., p. 574. PIEZO-ELECTRIC EFFECT 611 Taste CXCVII.—Puncture Voltage of Porcelain Voltage per 1/100 in. Porcelains maturing at Cone 6. 424 3 i = 9 ; 426 p. ie 1S ve ie 5 f 395 The pvezo-electric effect, in which an electric current is produced by pressure, sometimes seriously reduces the electrical resistance of an insulator if the latter is under compression. Some specimens of quartz and porcelain exhibit this phenomenon, especially in the case of quartz crystals, when the pressure is applied to two diametri- cally opposite faces parallel to the major axes ; a potential difference is then set up in the faces perpendicular to those in which the pressure is applied, this difference varying directly as the pressure. The converse of this may also occur when some quartz crystals are subjected to the prolonged action of an electric current and a change in the dimensions of the crystals may then result. If these changes are hampered by the surrounding magma, great local stresses may occur ; consequently, when a piece of porcelain is placed in an alternating field of electrostatic force, a vibratory movement results, owing to the repeated changes in the dimensions of quartz or other crystals similarly affected, and these vibrations may cause a rupture along the cleavage planes of the crystals and also between the crystals and the matrix. The rupture may result in a leakage of current through the spaces so formed, and the dielectric strength then rapidly deteriorates. The piezo-electric effect appears to be exhibited only in connection with certain crystals ; it is, therefore, at a minimum in porcelains in which the quartz crystals originally present in the raw materials have been most completely dissolved in the felspathic ground mass by prolonged heating at a suitable temperature. So far as can be ascertained, sillimanite crystals—which are also formed in most well-burned porcelains—do not show this effect. The temperature of the ware when in use has a notable effect on the electrical resistance or insulating power, as most ceramic materials become much better conductors of electricity when at a high temperature than when cold. This is well shown on pp. 614-615. The duration of exposure to an electric current has a noteworthy effect on the apparent resistivity or dielectric strength of porcelains, as a much lower voltage applied for a long time will puncture the material in the same manner as a current of high voltage applied only for a very short time. In making comparative tests it is, therefore, important to state the time during which the current was applied. Clays.—The electrical conductivity of raw clay is usually regarded as fairly constant, but that of burned clay varies considerably according to the extent of the burning, the changes which have taken place during that process, and the 612 ELECTRICAL AND MAGNETIC PROPERTIES temperature of testing. Thus, according to Hartmann, Sullivan, and Allen,1 the resistivity of fireclay bricks at different temperatures is as follows :— Taste CXCVIII.—Electrical Resistivity of Fireclay Bricks Temperature. Electrical Resistivity. Temperature. Electrical Resistivity. oc ohms per c.c. aC. ohms per ¢.c. Cold Less than 137,000,000 1200 Less than 4,160 800 , 57,600 1300 ee 2,460 900 vA 20,600 1400 5 1,420 1000 i 10,800 1500 = 890 1100 as 6,590 Porcelain.—The electrical insulating power of porcelain and stoneware depends largely on the texture and density, and these are controlled by the chemical composi- tion and the manner of burning. Porcelains high in quartz have a low resistance, because of the piezo-electric effect described on p. 611; when the quartz is replaced by sillimanite or clay, the dielectric strength increases. Porcelains high in quartz also tend to be more porous than those richer in fluxes, and, consequently, have a lower dielectric strength. According to Bleininger and Riddle,? the replacement of quartz by kaolin increases the “ Te ” value of a porcelain, but the use of ball clay reduces the electrical resistance. Fused alumina notably increases the “ Te” value, as also does artificial sillimanite, provided it is not in large crystals. According to B. 8S. Radcliffe,’ high-grade fireclays mixed with felspar produce materials with as high a dielectric strength as potash porcelains vitrifying at the same temperature. Ceramic wares in which lime is used as flux instead of potash felspar have, according to B. 8. Radcliffe, a lower dielectric strength. Thus, a body containing 6-8 per cent. of lime having the same porosity, burned at about the same temperature. as a felspar-porcelain, was found to have only about half the dielectric strength of the latter. B.S. Radcliffe also found that porcelains made with soda-felspar have a greater dielectric strength than those made with potash felspar, though Minneman considers the difference to be too slight to be of importance, provided the porcelain is well vitrified. According to Bleininger and Riddle,? when magnesia is used as a flux in porcelain it increases the ‘‘ Te” value in an irregular manner, probably as a result of the increased formation of sillimanite. Bleininger and Riddle have also found that when beryllium oxide replaces felspar in a porcelain, it increases the electrical resistance and “‘ Te”’ values as shown in Table CXCIX. 1 J. Amer. Electrochem. Soc., 38, 279 (1920). * Loc. cit., p. 574. § Loc. cit., p. 610. ELECTRICAL RESISTIVITY OF PORCELAIN _ 613 Taste CXCIX.—Effect of Beryllia on “ Te” Value . . . M t i tr) Beryllium Oxide. Clay. Flint. Mera Costas “Te” Value. Per cent. Per cent. Per cent. Seger Cone. 25 50 25 12 624 35 50 15 17 784. 45 50 5 ll 798 The proportion of flux in a porcelain also affects its electrical resistance. Thus, according to Gilchrist and Klinefelter,1 when the felspar content is high the dielectric strength varies directly with the proportion of felspar present, but when the felspar is low the dielectric strength varies directly with the clay-content. It also varies inversely with increases of flint or china clay and increases rapidly with the maximum temperature attained in firing. These investigators found that the greatest dielectric strength is obtained with a porcelain containing a high percentage of felspar and a low percentage of flint, whilst the lowest strength in a porcelain is with a low percentage of felspar and a high percentage of flint. Weimer and Dun ? found that (i) at high temperatures porcelains high in felspar have a lower dielectric strength than those which have less felspar, probably on account of the former softening more readily ; (ii) the addition of clay at the expense of flint increases the dielectric strength. Purdy and Potts,? however, consider the highest dielectric strength is obtained with a porcelain containing 25-35 per cent. of felspar and not less than 40 per cent. of clay. The following figures may be regarded as typical of the porcelains investigated, but porcelains vary so greatly that no figures of general application can be given. At ordinary temperatures, H. F. Haworth‘ has found the specific electrical resistance of the porcelain he examined to be as follows :— TaBLE CC.—Specific Resistance of Porcelain a Specific Resistance, 5 Specific Resistance, Temperature, ° C. Aaa. Temperature, ° C. olime, per 6.0: 1-63 143-0 x 10 17-00 50-8 x 104 2°10 141-0 x 10" 18-65 42:3 x10" 16-40 BL 10 20-50 35:5 x10 ! The puncture voltage of porcelain at 25° C. is, according to G. Weimer and C. T. Dun,? 64,500-67,500 volts for a thickness of 0:15 inch. It is not generally less than 70,000 volts for pieces } inch thick and 100,000 volts for pieces 4 inch thick. E. Rosenthal has stated that the puncture voltage of Berlin porcelain 0-1 inch thick 1 Flec. J., 15, '77 (1918). 2 Trans. Amer. Cer. Soc., 14, 280 (1912). 3 Loc. cit., p. 573. 4 Proc. Roy. Soc., Series A., 81, A. 547. 614 ELECTRICAL AND MAGNETIC PROPERTIES is 40,000 volts. The dielectric constant of Berlin porcelain, according to H. Starke, is 5-73 megohms per c.c. That of Seger porcelain is 6-61, and that of statuary porcelain (Parian ware) is 6-84 megohms per c.c. If the porcelain is heated, its dielectric strength—like that of quartz, mica, horn- blende, quartz-glass, and ordinary glass—decreases as the temperature rises. With a rise in temperature of only 100° C. the reduction in the dielectric strength is consider- able, and with a rise of 300° C. porcelain becomes only a very poor insulator. This is still further shown in Tables CCI, CCII, CCIII, CCIV, and CCV. TaBLe CCI.—The Electrical Conductivity of Berlin Porcelain at Different Temperatures Temperature, ° C. Electrical Conductivity. Authority. 50 0:465 x 10° Foussereau. 70 0-25 x10" 160 0-582 x 10-22 Dietrich. 189 0:26 x10" 400 0:05 x10% Goodwin and Mayley.? 600 06 x10% Pirani and Siemans.® 727 0-62 x10 : -6 a ae cc | Goodwin and Mayley.? 1100 eae 8 TaBLeE CCIl.—Specific Electrical Resistance at Different Temperatures (H. F. Haworth *) Specific Resistance, Specific Resistance, Temperature, ° C. Temperature, ° C. ohms per ¢.c. ohms per c.c. 1-63 143-0 x 10” 40-86 5-7 x 108 2:10 141-0 ,, 43-03 4:85 ,, 16-40 Dla: 47-00 Al} aie 17-00 50-8, 50-40 2°64 ,, 18-65 sy 54:01 1-78 20-5 35°5,, 56°51 1-44 ,, 27°32 PAYA) Shy 58-04 1-Khows 30-87 1) 59-12 1-08 ,, 34:70 LSisa, 62-72 O:7 lane 37-03 8-24 ,, 64-84 0-64, 40-62 6-25 ,, 81-93 0-15 3 1 Phys. Zeit., 11, 187 (1910). 2 Phys. Rev., 27, 322 (1908). 3 Zeit. Hlectrochem., 13, 969 (1907). 4 Loc. ctt., p. 613. ELECTRICAL RESISTIVITY OF PORCELAIN 615 Taste CCIII.—Decrease of Dielectric Strength of Porcelain when Heated (Henderson and Weiner ) (The average thickness of the test-pieces was 0-21 in.) Temperature, ° F. Puncture Voltage. 70 47,500 Tested in oil. 190 44,000 i 240 42,700 2 75 60,700 Tested in electric furnace. 125 60,100 43 ¥ 175 57,300 an rs 225 42,500 $ 275 30,250 * .: 325 19,750 > he 375 10,500 \ re 425 4,000 4 a 475 2,500 5 é. 525 2,000 : Taste CCIV.—The Puncture Voltage of Porcelain at Temperatures wp to 300° C. (G. Weimer and C. T. Dun *) (The test-pieces were 0-15 in. thick) Temperature, ° C. Puncture Voltage. Temperature, ° C. Puncture Voltage. 25 64,500-67,500 175 39,000—41,000 50 64,250-67,000 200 - 26,500-29,500 75 63,000-66,500 225 15,000-22,000 100 62,000-63,500 250 7,500-15,000 125 57,000-—60,000 275 4,500-11,500 150 49,500-52,500 300 3,000— 7,000 Taste CCV.—“ Te” Value of Ceramic Insulators (F. B. Silsbee and R. K. Honaman?). Material. | « Te” Value, ° C. Material. “Te” Value, °C. Fused silica. 890 Aviation porcelain . 650 Best porcelain . ; 790 Automobile porcelain. 490 Mica plug. 720 1 Trans. Amer. Cer. Soc., 13, 469 (1911). 2 Loc. cit., p. 613. 8 Nat. Advisory Comm. Aeronautics, 5th Ann. Rept., 77-89 (1919). 616 ELECTRICAL AND MAGNETIC PROPERTIES The electrical resistivity of insulators at high temperatures may be calculated from the formula E xk, where His the observed resistance and k is a constant depending on the shape of the insulator. In cup-shaped insulators, md? k = ag 9 where d is the diameter of the bottom and ¢ the thickness of the cup. In the case of tubular insulators, pie 271 R 2-30 logy, ns il where | is the length of the external conducting band and R, and R, are respectively the external and internal radii of the insulator. Silica Bricks.—The electrical conductivity of silica bricks is similar to that of fireclay bricks (p. 612). The electrical resistivity of silica bricks is shown in Table CCVI, due to Hartmann, Sullivan, and Allen! These, however, are unduly different from those of Stansfield, M‘Leod, and M‘Mahon, as shown in the same table. TaBLeE CCVI.—Electrical Resistivity of Silica Bricks Saree Resistivity Temperature, ° C Se (Stansfield, M‘Leod, as : : and M‘Mahon), ohms per c.c. ohms per ¢.c. Cold Less than 125,000,000 800 _ 2,380,000 900 . 765,000 1000 a 300,000 1100 ie 126,000 1200 ‘es 62,000 Ke 1300 . 30,900 9,700 1400 + 16,500 2,400 1500 y 8,420 710 1550 ae 22 1565 £ 18 Fused silica has electrical insulating properties higher than those of glass and porcelain, as shown in Table CCVII, which tabulates results obtained at the National Physical Laboratory. 1 Loc. cit., p. 612. ELECTRICAL RESISTIVITY OF MAGNESIA 617 TasLeE CCVII.—Comparative Resistivities Fused Silica. Soda-Lime Glass. Jena Glass Combustion Tubing. Tempera- Resistivity, Tempera- Resistivity, Tempera- Resistivity, ture, ° C. Megohm-cm. ture, ° C. Megohm-cm. ture, ° C. Megohm-cm. 15 over 200,000,000 18 500,000 16 over 200,000,000 150 », 200,000,000 145 100 115 ,, 36,000,000 230 », 20,000,000 ae ie 150 », 18,000,000 250 m 2,500,000 oe whe 750 0-1-0-4 350 ss 30,000 ae ah 450 re 800 800 about 20 According to tests by the National Physical Laboratory, silica glass has a specific inductive capacity of 3-5-3-6 and a dielectric strength of over 30,000 volts for a thickness of 1-2 mm., the exact figure not being found on account of a sufficiently high voltage not being available. Magnesia bricks examined by Hartmann, Sullivan, and Allen, yielded electrical resistivity curves which differed very considerably on heating and cooling, there being a peculiar variation in the heating curves which indicate a probable physical change in the structure of the magnesia between 1000° C. and 1500° C., probably due to the formation of an allotropic form of magnesia, viz. periclase. The electrical resistivity of magnesia bricks, as determined by Hartmann, Sullivan, and Allen, and also by M‘Leod and M‘Mahon, are shown in Table CCVIII. Taste CCVIII.—Electrical Resistivity of Magnesia Bricks Resistivity Resistivity (H., S., and A.), (M‘L. and MM.) Temperature, ° C. ener Aas AGS ohms per c.c¢. Cold Less than 137,000,000 800 be 5,000,000 900 + 1,240,000 1000 + 708,000 1100 x 560,000 aa 1200 y 193,000 6,200 1300 3 67,400 ie 1400 a 22,400 1500 “ 2,500 se 1550 is 30 1 Loc. cit., p. 612. 618 ELECTRICAL AND MAGNETIC PROPERTIES According to Fr. Patent, 425,977 (1911), a mixture of 1-2 parts of powdered magnesia or alumina, 5 of magnesia, 3-4 of water-glass or other silicate or 3 of quartz, has an electrical resistance at a red heat of 10 megohms per c.c., and is stated to be quite non-conductive at ordinary temperatures. Zirconia bricks were found by Hartmann, Sullivan, and Allen? to yield heating and cooling curves with curious differences between them. They also found that on heating up to 1200° C. the resistivity fell rapidly, but above this temperature it fell slowly, as shown in Table CCIX. Taste CCIX.—Resistivity of Zirconia Bricks when Heated means Ohms per c.c. Sa Ohms per ¢.c. Cold Less than 134,000,000 1200 Less than 7,710 800 ¥ 558,000 1300 # 2,100 900 ay 224,000 1400 2 968 1000 “i 131,000 1500 “ 412 1100 _ 53,800 Chromite bricks, according to Hartmann, Sullivan, and Allen,t have a low electrical resistivity at all temperatures. Between 1100° and 1200° C. they remain fairly constant, increasing from 1200°-1350° C., and again decreasing between 1300° and 1500° C., as shown in Table CCX. The difference between the results of these observers and those of M‘Leod and M‘Mahon are shown in the same table. TABLE CCX.—Resistivity of Chromite Bricks M‘L. and M‘M., Temperature, °C. | H.,S., and A., ohms per c.c. ohms per ¢.c. Cold 48,000,000 fe 800 803 2,800 900 525 % 1000 171 1100 78 1200 63 1300 77 x. 1400 85 320 1500 41 1 Loc. cit., p. 612. ELECTRICAL PROPERTIES OF CLAY SLIPS _ 619 Carborundum bricks have the electrical resistivities shown in Table CCXI,:due to Hartmann, Sullivan, and Allen.1 Taste CCXI.—Electrical Resistivity of Carborundum Bricks Temperature, ° C. Carbofrax. Refrax. A. B. Cold Less than 127,000,000 107,200 106-90 800 re 835,000 12,550 6:45 900 ry 477,000 8,220 3°75 1000 = 197,000 7,420 4-1] 1100 24 75,000 6,320 GN 1200 x 29,500 4,160 2°45 1300 rs 15,200 2,420 2:05 1400 a3 10,100 1,435 1-74 1500 o 8,590 745 1-62 Clay slips possess electrical properties which are often important in connection with the purification of these materials and in the production of ware by the casting process. Thus, whilst the electrical conductivity of pure, distilled water is 10° reciprocal ohms (or mhos) at 18° C., the presence of even minute quantities of soluble salts causes large differences in the conductivity. When larger proportions are present, the conductivity does not conform to simple rules, but is dependent on several complex considerations. The conductivity of electrolytes at different temperatures varies considerably ; it may be calculated from the formula : C,= C,,(1 + &(é + 18)), where C, is the conductivity at any given temperature ¢ in ° C., C,, is the conductivity at 18° C., and & is the temperature coefficient. For salts, the temperature coefficient varies from 0-02-0-023 for acids, and for some acid salts it is 0-009-0-016, and for caustic alkalies about 0-02 mhos per c.c. The electrical conductivity of clay slips depends chiefly on the proportion of soluble salts present in the water. Thus, Bleininger and Kinnison ? found the results shown in Table CCXII. The presence of calcium sulphate in solution in clay slips greatly decreases their electrical resistivity. In one case, examined by Bleininger and Kinnison, the presence of 0-072 per cent. in a kaolin slip decreased the resistance from 4440 to 720 ohms per c.c. 1 Loc. cit., p. 612. 2 Trans. Amer. Cer. Soc., 15, 523 (1913). 620 ELECTRICAL AND MAGNETIC PROPERTIES TaBLE CCXII.—Electrical Resistivity of Slips Soluble Salts, Resistance in ohms, per cent. reduced to 60° F. Surface clay, Cleveland . : 2:10 2110 a ,, Curtice ; ’ 1:05 2160 © No. 3 Fireclay, Aultman . : 0-94 3790 Shale, Canton ; : ; 0-77 3050 Shale, Independence ; 0-60 3970 Determination of Electrical Conductivity, Resistivity, etc—A rough separation of minerals into good and poor conductors may be rapidly made by a method suggested by T. Crook.! The apparatus consists of two copper plates a few inches square, one of which has one surface coated with a layer of shellac, which is continued over the edge of the plate, forming a narrow strip on the opposite surface. The shellac-coated surface of one copper plate is placed next to the uncoated surface of the other, but is separated from it by two pieces of glass coated with shellac. The upper plate is charged electrically by means of an electrophorus consisting of a plate of ebonite, resin, sealing wax or shellac, on a metal base and a circular metal disc of the same diameter with an insulated handle. If the plate of the electrophorus is rubbed with a flannel or piece of fur, a negative charge of electricity is induced in it, so that, on placing the metal disc on its lower surface, it is charged positively and a complementary negative charge is given to the outer surface. This is removed by touching it with the finger. A small quantity of the sample to be examined is placed on the upper side of the lower copper plate of the pair previously mentioned, and the disc to which the insulating handle is attached is placed upon the upper copper plate. The minerals which are good conductors of electricity will immediately adhere to the upper plate and can be removed therefrom, whilst the non-conductors remain on the lower plate. For accurate work, a larger apparatus operated by a more powerful current is preferable, but the simple device just described is often useful. The electrical resistivity of an article may be determined by passing an electric current through it and measuring the resistance by means of a potentiometer. The puncture-voltage (i.e. the voltage required to break down the resistance and allow a current to pass readily) is generally determined by placing a sample of known thickness between two electrodes and applying a gradually increasing voltage until puncture occurs and noting the maximum voltage applied, the test-piece being usually immersed in oil, unless the test is made at a high temperature, when oil cannot, of course, be used. 1 Economic Mineralogy (Longmans, Green & Co.). MAGNETIC PROPERTIES 621 MAGNETIC PROPERTIES OF CERAMIC MATERIALS Clay, silica, most of the silicates, and many other ceramic materials are non- magnetic, but some of the impurities are susceptible to magnetic attraction, and, consequently, this property is sometimes used in their removal. Table CCXIII shows the magnetic properties of different minerals. TaBLe CCXITI.—Magnetic Properties of Minerals Highl Moderatel eb - Magnetic. Re hid sai eo Secu Magnetite. Hypersthene. | Chlorite. Zircon. Rutile. Titanoferite. Augite. Staurolite. Corundum. Barytes. Ilmenite. Garnet. Epidote. Galena. Most iron-free Pyrrhotite. Siderite. Limonite. Fluorspar. minerals, Hematite. Olivine. Actinolite. Pyrite. Clay. Hornblende. Cyanite. . Cassiterite. Silica. Chromite. Some burned fireclays are feebly magnetic on account of the presence of magnetic iron oxide or other minerals which are attracted by a magnet. All ferrous silicates are magnetic, and Zirkel found that fused phyllite (FeOAI,0,Si0,) is also magnetic. The magnetic properties of minerals, etc., may readily be determined by means of a small electro-magnet. A convenient one, suggested by T. Crook,! consists of two limbs, each 1 inch diameter and 4 inches long, wound with seven layers of 16-gauge wire, each layer having about forty turns. The two adjustable pole-pieces should be 14 inches wide and 4 inch thick, slotted so as to be moved nearer to, or farther from, each other, and secured by screws to the limbs. An 8-volt battery is quite sufficient for this instrument. In use, the magnet is suspended over a smooth cardboard tray containing the sample to be examined. If desired, the most magnetic particles may be removed with a permanent magnet, and the ‘“‘ moderately magnetic” grains then removed by means of an electro-magnet, with its poles about $ inch apart. After- wards, the poles of the magnet may be placed only } inch or rather less apart and the “‘feebly magnetic” minerals may then be separated. The residue may be regarded as practically non-magnetic. Alternatively, the minerals may be suspended in water, forming a slip, which can then be stirred with the magnet until all the magnetic particles have been removed. Magnets, arranged in series, are extensively used for removing minute particles of metallic iron from clay and body slips, these particles being largely derived from the machines used to grind the clay and other ingredients of the slips. 2 Loc. cit., p. 620. CHAPTER XV OPTICAL PROPERTIES OF CERAMIC MATERIALS THE optical properties of ceramic materials are important in two ways : (a) As an aid to identifying the various crystals and other substances present in a ceramic material. (b) In order to produce a desired appearance such as a particular colour or lustre, — translucency, transparency, etc., in the finished product or ware. IDENTIFICATION OF CERAMIC MATERIALS BY OPTICAL PROPERTIES As mentioned in Chapter X, an examination of the optical properties of various materials in a mixture affords a very useful method of determining the amount of some mineral. The examination may be made by means of a hand-lens or, more usually, by means of a microscope. The principal properties which can readily be determined by this means are as follows : ; = . The colour, lustre, etc. (see Chapter IIT). . The crystal form (see Chapters I and X). . Reflection. . The refractive index. . The birefringence. . The extinction angle. . The optical sign. . The pleochroism. . The interference figures. co conten Orr & bb Reflection occurs when a ray of light falls on a surface and, instead of being absorbed, much of it is returned from the absorbent surface at the same angle to a line perpendicular to the surface of the material as the entering ray. When the amount of absorption is high, no reflection occurs ; when little absorption occurs, most of the light is reflected and the substance appears to be glossy or shining. The refractive index 1 often affords a convenient method of identifying minerals. All transparent substances have the power of bending or refracting a beam of light passing obliquely through them to a varying degree, according to their nature and density. This property is termed refraction, and the amount of refraction is termed the ‘ ‘ 1 Care must be taken not to confuse the term “refractive” with the term “ refractory ” the former relates to light, the latter to heat. 622 REFRACTIVE INDICES OF MINERALS 623 ae ee sin 2 2 refractive index, which is found from the equation ~ = ——, where 7 is the angle which sin 7 the entering ray makes with a line perpendicular to the surface of the substance, r is the angle which the ray passing through the substance makes with the perpendicular, and p is the refractive index as shown in fig. 51. The refractive index of any substance is constant and is independent of the angle at which the light falls on the substance. The refractive indices of various minerals found in ceramic materials is shown in Table CCXIV. TaBLeE CCXIV.— Refractive Indices. Mineral. Max. | Min. Mineral. Max. | Min. Anatase 2-489 Hornblende . 1-64 1-68 Andalusite 1-643 | 1-632 || Hypersthene 1-705 | 1-682 Albite . 1-534 Kaolinite 1-563 Andesine 1-558 Labradorite . 1-555 Anorthite 1-582 Lepidolite 1-60 Apatite 1-638 1-634 || Leucite 1-508 Augite . 1-723 1-698 || Microcline 1-526 1-519 Barytes 1-647 1-636 || Monazite 1-841 1-796 Biotite 1-6 1-56 Muscovite 1-601 1-563 Brookite 2-741 2-583 || Nepheline 1-543 Bytownite 2-74 Oligoclase 1-544 Calcite 1-658 1-486 || Olivine 1-689 | 1-654 Cassiterite 2-093 1-997 || Opal 1-45 Celestite . 1-631 1-622 || Orthoclase 1525 | 1-519 Chalcedony . 1-55 Phlogopite 1-60 Chromite Very high Pyrophyllite 1-57 Cordierite 1-544 1-535 Quartz. 1-553 1-544 Corundum . 1-769 1-760 Rutile . 2-903 2-616 Cristobalite . 1-484 Serpentine 1-57 . Cyanite 1-729 | 1-717 || Silliimanite 1-682 | » 1-660 Diamond 2-42 Sodalite 1-48 Diopside 1-70 Spinel . 1-72 Dolomite 1-682 1-503 || Staurolite 1-746 | 1-736 Enstatite 1-67 1-66 Strontianite . 1-52 Epidote 1-746 1:714 || Titanite 2-008 1-899 Fluorite 1-434 Topaz . 1-627 1-618 Garnet High Tourmaline . 1-64 1-62 Glaucophane 1-639 | 1-621 || Tridymite 1-477 Gypsum 1-53 Xenotime 1-816 1-721 Halloysite 1-53 Zircon . 1-993 1-931 Hematite 3°22 | 2-94 624 OPTICAL PROPERTIES The refractive index of crystals which are sufficiently large may be found by measuring the relative angles of the entering and emerging rays directly in a refracto- meter, but for the minute crystals usually found in ceramic materials, other methods must be used, the crystals being viewed through a microscope. A very convenient method devised by Schroeder van der Kolk consists in immersing the coarsely Fic. 51.—REFRACTION oF LIGHT. powdered mineral on a thin glass slide in various liquids having different, but known, refractive indices. When the mineral has the same refractive index as the liquid in which it is immersed, the grains will be practically invisible, so that by using different liquids one will finally be found of which the refractive index approximates to that of the mineral, and the refractive index of this liquid may be taken as that of the mineral. Table CCXV, due to Schroeder van der Kolk, shows the refractive indices of various liquids which are useful for this purpose. TaBLeE CCX V.—Refractive Indices Material. ee Material. is Ethylene chloride. 1-450 Monobrombenzol : 1-561 Olive oil . , 1-469 Orthotoluidine . : 1-571 Benzol *. s E : 1-501 Aniline . : = : 1-583 Cedarwood oil . : 1-505 Bromoform . : : 1-590 Monochlorbenzol 1-523 Cinnamon oil . : ; 1-605 Ethylene bromide. 1-536 Moniodobenzol. : : 1-619 Clove oil . : ; ; 1-544 a-Monochlornaphthlene 1-635 Nitrotoluol . ; , 1-546 a-Monobromnaphthlene . 1-655 Nitrobenzol . ; 1-552 Methylene iodide. : 1-740 Dimethylamine 1-558 Sulphur in methylene iodide 1-839 Another ingenious method of determining the refractive index of a substance was devised by Becke, who found that if the particle to be examined is immersed in a REFRACTIVE INDICES OF MINERALS 625 liquid and the objective lens of the microscope is raised until the grain is out of focus, a bright line will move from the material having the lower refractive index to the one having the higher value, so that with any liquid and a given material it is possible to determine readily which has the higher refractive index. By using liquids of different refractive index that of the mineral will eventually be found. Another useful method consists in tilting the mirror of the microscope so as to cut off part of the light from the field of view and to cast a shadow so that the crystals appear dark on one edge and light on the other. If the dark edge lies on the side of the crystal opposite to the shadow, the crystal has a lower index of refraction than the liquid in which it is immersed, whilst if the dark edge is on the side nearest to the shadow, the mineral has a higher refractive index than the liquid. By using several liquids of known refractive index in succession, the liquid corresponding to the refractive index of the mineral can be found. . A very refined and accurate method, devised by A. B. Dick, is especially valuable for determining the nature of minerals whose refractive indices are very close to each other, as in the case of tridymite and cristobalite (p. 623). In this method, a small quantity of the coarsely powdered sample is immersed in a suitable liquid with a refractive index equal to that of one of the constituents present. When the sample is viewed through a microscope, by means of a dark ground illuminator, using yellow monochromatic light, coloured fringes will be seen around the grains, and these fringes will be of an ultramarine tint if the minerals and the liquid have exactly the same refractive indices. If the liquid has the higher refractive index, the fringes will be paler and brighter and sometimes even white, whilst if the liquid has the lower refractive index, the fringes will be of a red or orange shade. The colour of these fringes thus distinguishes between the particles of higher and lower refractive index and makes it possible to estimate the proportions of grains of different minerals when their refractive indices are very close. For example, in the examination of a mixture of tridymite and cristobalite a solution of mercury-potassium-iodide may be used, as this has, with a monochromatic yellow sodium light, a refractive index of 1-477 which is equal to that of tridymite. Any cristobalite present, having a refractive index of 1-484, will be readily discriminated from the tridymite, the test being extremely delicate. The use of the refractive index is by far the simplest and most direct method of distinguishing substances of the same chemical composition but different physical properties, such as tridymite and cristobalite, calcite and aragonite, lightly burned magnesia and periclase, etc. It is the most important method for investigating the proportion of unaltered quartz in a silica brick. Double refraction is produced when a single ray of light entering a substance emerges as two distinct refracted rays, one of which, measured by the fraction — is termed the ordinary ray, whilst the other is termed the extraordinary ray. The difference between these two refractive indices—the birefringence—is often used in the identification of minerals. Table CCXVI shows the most important minerals in order of birefringence. 40 626 OPTICAL PROPERTIES TaBLE CCXVI.—Birefringence of Minerals (Milner and Part) Uniaxial Positive. Pennine . Quartz . Zircon Cassiterite Rutile Biaxial Positive. Zoisite Albite. Enstatite Topaz Staurolite Chloritoid Augite Sillimanite Diallage Diopside Olivine . Sphene . Brookite 003 009 ‘062 099 287 006 008 009 ‘009 010 ‘015 021 ‘021 024 ‘030 036 121 -160 Uniaxial Negative. Idocrase. Apatite . Nepheline Melilite . : Corundum . Tourmaline Scapolite Cancrinite Anatase . Calcite Dolomite Biaxial Negative. Orthoclase Microcline Andesine Oligoclase Kaolin Cordierite Labradorite Axinite . Andalusite Serpentine Anorthite : Hypersthene . Wollastonite . Cyanite Actinolite Tremolite Muscovite Epidote . Biotite . Hornblende Aragonite 006 007 007 008 008 ‘008 ‘008 009 ‘O11 ‘O11 ‘012 013 014 ‘016 025 028 038 040 044 ‘072 156 POLARISED LIGHT 627 Polarised light is obtained by passing a ray of ordinary light through two prisms composed of certain minerals (such as tourmaline and iceland spar) ! placed in such a position relative to each other that they totally reflect or polarise any light projected along their axes, with the result that no light is transmitted through them. If, however, a fragment of certain minerals is placed between the two prisms, an amount of light corresponding to the optical properties of the crystal will be transmitted, but if one of the prisms is rotated on its axis, a position will be reached at which the light is again completely polarised and is extinguished, the field of view becoming quite dark. The angle at which this extinction occurs is termed the extinction angle, and is often useful in identifying minerals, particularly plagioclase felspars (see Table CCXVIT). TaBLteE CCXVII.—Extinction Angles of Felspars (Milner and Part) Extinction Angle measured po epcecn pneie mcesured tN stot from the Albite Lamelle. irom phe Tong es Microlites. Albite . , : : 6°-16° 10°-20° Oligoclase ‘ 0°-5° 0°- 7° Oligoclase (basic) . : 6°-16° 6 Andesine . : : 16°-22° 8°-20° Labradorite . : ; 27°—45° 30°-42° Bytownite . ; 45°—50° 49°-56° Anorthite : : oa 50° and over. 58°-64° If a number of flakes of any felspar are examined, two series of approximately similar extinction angles will be obtained, corresponding to the fragments which are split parallel to the two principal faces of the crystal, and from a comparison of these angles with the corresponding ones obtained with known complex felspars, an unknown felspar may be identified. The colours and forms of the minerals viewed by means of polarised light are also characteristic, and are useful in determining their nature. When the dark field of view produced by the prisms alone remains black, even after a given mineral has been placed between them and continues so at any angle of the polariser, it is said to be zsotropic. Crystals belonging to the cubic or isometric system are always isotropic (except under abnormal circumstances), and so are transverse sections of tetragonal and hexagonal crystals, but no sections of ortho- rhombic, monoclinic, or triclinic crystals have this property. Hence, only crystals 1 These prisms are usually known as “‘nicols,”’ after their discoverer. For convenience they are usually fitted to a microscope as, by this means, they may be used for examining much smaller particles than would otherwise be possible. 628 OPTICAL PROPERTIES of the three last-named systems, and sometimes those of the tetragonal and hexagonal systems, are visible in polarised light. Optical Sign.—For the observation of compensation effects and the determination of the optical sign, a quartz wedge and a simple gypsum plate should be kept at hand, as they are extremely useful. The ordinary quartz wedge used in examining sections will, as a rule, be found too feeble, owing to the thickness of the grains. The best form of quartz wedge for use in the optical examination of sand grains is the graduated thick wedge devised by Dr Evans. Quartz wedges are usually made so that the vibration-direction along the length of the wedge is that of the extraordinary (slow) ray. Such a wedge is called positive, and in reporting results a positive wedge is always assumed to be used. The thicker wedges usually needed for examining small grains should be graduated so that each interval corresponds to 1000 micro-millimetres of relative retardation. Gypsum plates, on the contrary, are usually made so that the direction of the length is the vibration-direction of the fast ray. It matters little which way the quartz wedge or gypsum plate is made, so long as its nature is determined before use. In no case should it be assumed that a quartz wedge is positive, as the maker may, purposely or inadvertently, make it negative. If one doubly refracting substance is placed above another in such a way that the direction of vibration of the slow ray of one coincides with the direction of vibra- tion of the fast ray of the other, the effect is subtractive, the order of the colours will be lowered, and if the relative retardation of the two substances is the same, the result—if viewed between crossed nicols—will be darkness, as each plate exactly counteracts the other. This is known as the position of compensation. The compensation test consists in placing a small positive crystal (say of zircon) in the position of extinction between crossed nicols, then turning either the nicols or the stage through 45° (7.e. into the position of greatest illumination), and insert a positive quartz wedge between the analyser and the objective of the microscope. If the long edge of the wedge lies parallel to the prism edge, and the crystal is thin enough to show a definite interference colour, it will be noticed that the order of the colour increases, indicating that this is the additive position. If now the crystal or polarising system be turned through 90°, so that the wedge is inserted at right angles to the prism edge, compensation will take place, and the order of the interference colours will get gradually lower until the position of complete compensation is reached, indicating that this is the subtractive position. This fact shows that the vibration-direction of the crystal parallel to the prism edge is that of the slow ray, and since it is also that of the extraordinary ray the crystal must be positive. If a negative crystal is used (e.g. apatite), the compensation takes place with the long edge of the quartz wedge along the prism edge. This simple compensation test is sufficient to show the optical sign of a uniaxial prism. For biaxial prisms other tests are required to ascertain the optical sign. In all crystals where there is a prismatic cleavage with straight extinction or a low extinction. 1 Mineralogical Mag., 14, p. 90. OPTICAL ACTIVITY 629 angle, it is useful to apply the compensation test in order to ascertain whether the direction of elongation of the cleavage fragment is that of the fast-ray or slow-ray vibration. With a biaxial figure the position of the quartz wedge should be noted in relation to the optic axial plane when the wedge is in a compensating position. The biaxial crystal (such as a cleavage plate of mica which is optically negative) should be placed on the stage of the microscope, a gypsum plate being inserted diagonally across the centre so as to produce the characteristic black cross when the nicols are crossed. The stage or the nicols should then be turned through an angle of 45° so as to produce the hyperbolic brushes and to illuminate the centre of the field. The quartz wedge is then inserted and will lie either parallel to or at right angles to the optic axial plane. If it lie along the optic axial plane, addition will result and the colour will rise ; if it he at right angles to this plane, compensation will take place. If a positive biaxial mineral is used, the reverse result will be obtained, ¢.e. com- pensation occurs when the quartz is inserted along the optic axial plane and addition or rise in colour when it is inserted at right angles to this plane. If compensation takes place when the wedge is inserted at right angles to the optic axial plane, the mineral is negative; if it occurs when parallel to the optic plane, the crystal is positive. Optical activity is a property characteristic of certain substances which rotate a ray of polarised light to either the right (dextro-rotatory) or to the left (levo- rotatory). It has been found that all carbon compounds which possess this property contain an asymmetric atom, 7.e. one which is united to four different elements or groups of elements. Hitherto, no optical activity has been observed in clay—probably because the crystals are much too small—but both dextro- and levo-rotatory quartz are known, though their constitution has not been ascertained. The optical activity of quartz must not be confused with that of some carbon compounds, because there are two classes of optically active substances: (i) those which, like quartz, depend for this power on their crystalline state, that is, on the grouping of the molecules in the crystals ; and (i1) those in which the rotatory power is inherent in each molecule, since it is not affected by solution. It is this molecular rotatory power which is of such great importance in organic chemistry in determining the molecular constitution of complex substances. Unfortunately the optical activity of quartz does not appear, at present, to be of much assistance in determining its constitution. A property which is very useful in identifying some minerals is that termed pleochroism, and is due to the fact that some crystals absorb light unequally in different directions, and so when viewed from one angle they have a different colour from that when they are viewed from another angle. In most cases, the absorption of light is only for certain colours, only the residual colour being transmitted, but sometimes almost the whole of the light is absorbed. Thus, biotite varies from a light brown in one direction to deep brown or nearly black in another. The various colours may readily be seen by rotating the prism placed below the stage of the microscope (7.e. the polariser), but without using the upper prism (¢.e. the analyser), 630 OPTICAL PROPERTIES and watching the mineral during the rotation. If preferred, the mineral may be rotated. The following minerals which occur in ceramic materials are :— Biotite. Cyanite (if coloured). Epidote. Hornblende. 7 Hypersthene. Tourmaline. Chlorite. Moderately pleochroic ! Corundum (if coloured). Sphene. Slightly pleochroic . ; Some augites. Intensely pleochroic . Minerals are termed dichroic or trichroie according to the number of colours visible when the substance is viewed in the direction of its various crystalline axes. Isotropic crystals belonging to the cubic or isometric system are never pleochroic. Coloured uniaxial crystals belonging to the tetragonal and hexagonal systems are dichroic, whilst coloured biaxial crystals belonging to the orthorhombic, monoclinic, or triclinic systems are trichroic. Interference.—If two waves of light of the same wave-length and amplitude, and travelling in the same direction, meet crest to crest, and trough to trough, the result is to give the particles affected by the wave double amplitude. I, however, the waves meet crest to trough and trough to crest, the particles are motionless, and the waves are said to interfere and extinguish one another. If ordinary white light is used, the waves overlap by interference and the colours of the spectrum are seen. Unless the light is first passed through a very narrow slit the edges of the colours will be blurred instead of sharply distinguished. It is impossible to deal fully with the effect of light in passing through minerals in the present volume ; further information will be found in the standard text-books on mineralogy and petrology. A particularly useful summary of the optical properties of minerals occurring in ceramic materials will be found in An Introduction to Sedi- mentary Petrography, by H. B. Milner (Murby & Co.). Some of the commoner properties of the various minerals found in ceramic materials are mentioned in the descriptions of the various minerals in Chapter X., and these properties, in conjunction with the other tests mentioned in that Chapter, will generally enable the more usual minerals to be identified. The use of X-ray spectra for this purpose has been described on p. 322. Ultramicroscopic particles are so minute that they are invisible when viewed through a microscope of the usual type, even though it be of the highest power. Their movements may, however, be observed by suspending the particles in a suitable fluid, such as water, and passing a powerful beam of light horizontally through the liquid. If the latter were perfectly clear it would, when viewed through a microscope, appear to be black and void, but if any particles are present in suspension they are TRANSPARENCY AND OPACITY 631 illuminated by the light and present the appearance of bright globules, resembling the motesinasunbeam. A combination of a microscope with a device for illuminating in the manner just described is known as an ultramicroscope (fig. 52). By its use the movement of particles (but not their shape) may be examined even when the particles are as small as 0-0000004—0-0000006 mm., and as these dimensions approach. Fic. 52.—ULTRAMICROSCOPE. the molecular dimensions of complex compounds, the ultramicroscope affords considerable scope for investigation of the behaviour of such molecules. The ultramicroscope is particularly valuable in the examination of clays, as these are among the most minute particles found in the ceramic materials, and by its aid the Brownian movement (p. 242), characteristic of colloids, has been observed in many clays. OptTicAL PROPERTIES OF MANUFACTURED CERAMIC ARTICLES The principal optical properties of ceramic articles in the manufactured state are :—- 1. Colour and lustre, which are dealt with in Chapter ITI. 2. Transparency, as in the case of silica-glass. 3. Translucency, in connection with fused silica, porcelain, china-ware, etc. 4, X-ray spectrum. | Transparency is so well known a term that it needs no description. The most transparent solid is glass, which is not usually regarded as a ceramic material, except in some parts of the United States. The most transparent substance which is gener- ally regarded as a ceramic material is fused silica, quartz glass, or silica glass. Transparency is obtained by the use of the purest materials and prolonged heating after complete fusion. Any cloudiness in a substance which is normally transparent is usually due either to impure materials or to imperfect fusion. As silica-glass is the only completely transparent ceramic material, it is the only one possessing such optical properties as refractive index, dispersive power, etc., but individual particles present in other ceramic materials possess them even when the mass as a whole may appear to be opaque. The principal optical properties of silica glass are shown in Table CCXVIII. Opacity is the converse of transparency and is the property of a substance which prevents it allowing any light to pass through it. Opaque substances may be examined under the microscope with direct or reflected light by shielding the light from the 632 OPTICAL PROPERTIES mirror below the microscope stage and illuminating the substance by means of direct light or by reflecting light on to it by means of a white card held above it. TaBLeE CCXVIII.—Optical Properties of Silica Glass Refractive index 1-45848 Dispersive power ; 0-01472 Refracting power 0-45848 Recriprocal __ relative Mean dispersion. 0-00675 dispersion. 67-92 Translucency is a very desirable property in porcelain and china-ware used for domestic purposes. It is intermediate between transparency and opacity, so that whilst a translucent substance will allow some light to pass through it, the shape of solid objects cannot readily be distinguished through it. Translucency is chiefly due to the formation of so large a proportion of clear transparent fused material that the ware becomes semi-transparent or translucent. It depends chiefly upon the following factors :— 1. The chemical composition. 2. The manner and duration of firing the ware. 3. The extent of crystallisation. . 4, The shape, and particularly the thickness, of the sample. The chemical composition has a very important influence on the translucency of ware, as it determines the amount of transparent glassy material which can be pro- duced and, therefore, the chemical composition may be said to control the trans- lucency. For the same reason, the nature of the siliceous material present may have a marked effect, especially as silica from different sources does not always produce the same translucency. Thus, according to Steger,! sand and geyserite, when substituted for Norwegian quartz in porcelain, decrease the translucency of the ware. Fluxes increase the translucency of wares. H. HE. Ashley ? measured the thickness of ware and of various compositions through which the light from a 16 candle-power electric light or a Welsbach gas-burner was visible and found that the addition of 3 per cent. of fluorspar to a white-ware body increased the thickness from 1:65 mm. to 2.4mm. The addition of 0-4 per cent. of whiting had about the same effect, whilst 3 per cent. of whiting only increased the thickness to 2-0 mm. Gilchrist and Klinefelter * found that, in general, the translucency of porcelains is increased in proportion to the felspar and inversely to that of the clay present. The kind of clay used also has a great influence on the translucency, ball clays affording a greater translucency than china clay, but tending to spoil the colour of the ware. The use of talc in porcelain increases the translucency. The manner and duration of firing should be such as to secure the maximum 1 Ber. der Deut. Keram. Gesellschaft, 3, Part II, 50-3 (1922). 2 Trans. Amer. Cer. Soc., 8, 150 (1906). 3 Elec. J., 15, 77 (1918). TRANSLUCENCY 633 amount of vitrification if translucent ware is required, because, as explained on p. 632, the translucency depends on the proportion of “ glassy’ matter. That the temperature attained in the firing and the duration of the firing can increase the glassy matter is clear from what has been stated in Chapter XIII. Moreover, it is well known that raising the temperature or prolonging the firing will make the ware more translucent, some mixtures which are opaque at Cone 7 (1230° C.) being very translucent at Cones 10-12 (1300°-1350° C.). K. Roth ! found that the translucency of porcelains rich in sodium and potassium silicates increases up to Cone 12 (1350° C.), remains constant from Cones 12-14 (1350°-1410° C.), and again increases above Cone 14 (1410° C.) on account of the solution of quartz by the molten material at or near 1400° C. The eatent of crystallisation which occurs in ceramic ware to some extent depends on the burning process. It is found that the larger the proportion of sillimanite present, the greater is the translucency of the ware, because the lathlike sillimanite crystals form a felted mass or skeleton which enables a larger proportion of fused or vitrified matter to be present without the ware losing its shape. It should be clearly understood that the translucency is primarily due to the glassy or vitrified ware and not to the crystals ; the latter are merely a form of reinforcement. The thickness of the sample or article obviously affects the translucency, as no ceramic wares are wholly transparent, like glass. W. Steger? has found that the translucency of porcelain is inversely proportional to its thickness and that the translucency increases as the proportion of clay decreases, the ratio of translu- cency : clay in any given porcelain being constant for all thicknesses. Translucency may be measured in various ways, though these do not give results comparable with each other. As translucency is generally regarded as the extent that light can pass through a substance, the simplest measure of translucency would appear to be the intensity of the light which can just be seen through the ware. Hence, Priest,* and others have, independently, suggested measuring the translucency by means ofa photometer. To obtain comparative results two lights of equal and standard intensity must be employed. These lights are placed on opposite sides of the piece to be examined and are moved backwards and forwards until the light which has passed through the ware is of the same intensity as that which shines directly on to the surface of the ware. Parmelee and Lawrence‘ measure the translucency by allowing ultraviolet rays to pass through the test-piece on to a sensitive photo-electric cell, which thereupon produces an electric current of an intensity corresponding to that of the light passing through the test-piece. The potential of the current as measured by a galvanometer is, therefore, proportional to the translucency. The photo-electric cell consists of a vessel filled with argon and silvered internally, having a thin film of potassium deposited on the silver to form the cathode of an electric circuit and a loop of platinum 1 Sprech., 55, 533-34 (1923). 2 Ber. Deut. Ker. Ges., 2, 63 (1921). ’ Trans. Amer. Cer. Soc., 17, 150 (1915). 4 J. Amer. Cer. Soc., 6, 630 (1923). 634. OPTICAL PROPERTIES wire to form the anode. An ultraviolet ray falling on the potassium surface causes it to become electrically charged, and the ions then liberated pass to the anode and so cause a definite deflection of the galvanometer. Another method of measuring translucency is that of R. H. Hursh,! which consists in determining the smallest mesh of gauze which can be seen through the sample by placing it in contact with the gauze and illuminating the latter by an incandescent electric light of known candle-power placed at a distance of 3 inches behind the gauze and measuring the thickness of the sample. By this test, the best porcelain with a thickness of 1-1 mm. has a translucency equivalent to the use of a 20-40-mesh gauze ; the same porcelain 3 mm. thick has a translucency corresponding to the use of an 8-10-mesh gauze. 1 Trans. Amer. Cer. Soc., 13, 103 (1911). INDEX a-Alumina, 339. a- B Change in quartz, 328. Abrasion, effect of porosity on, 73. effect of texture on, 38. ; resistance of bauxite bricks to, 122. of bricks to, 128. of carbon bricks to, 133. of chrome bricks to, 133. of earthenware to, 133. of floor tiles to, 130. of glazed ware to, 133. of magnesia to, 132. of refractory bricks to, 130. of refractory materials to, when hot, 131. of silica to, 132. of zirconia bricks to, 133. resistance to, 124, 125. testing, 134. Abrasive tests, 134. Abscisse of graphs, 455. Absolute scale of temperature, 533. Absorption, determination of, 81. effect of porosity on, 72. of size of pores on, 71. of heat, 519. Absorptive power of colloidal gels, 235. of dry clay, 75. of plastic clay, 76. a-cristobalite, nature of, 328. specific gravity of, 216. Accrington bricks, strength of, 173. Acid defined, 316, 318. radicle, defined, 317. salt, defined, 320. water in silicates, 338. Acids and bases, product of reaction of, 434. effect of, on clay, 243. on clay paste, 275. on osmotic pressure of colloids, 233. removal from solution by clay, 241. Ackermann, A. S. E., 126, 273, 274. Actinolite, birefringence of, 626. magnetic properties of, 621. Action of heat on colloidal gels, 235. selective, see Selective action. Activity and temperature, 322. Adamantine, lustre of, 95. Adhesion, limit of, 268. Adhesive bond, 153. Adsorption, 235. v. lattice structure, 324. by clays, 238, 242. by colloidal gels, 235. characteristic equation for, 236. electro-, 230. of gases and vapours, 238. of liquids by clay, 239. of solids by clay, 239. from solution by clay, 240. selective, by clay, 240. Affinity, see Chemical affinity. constants of reactions, 449. different, 442. disposing, 442. After-expansion of silica bricks, 568, 569. Agate, 424. Ageing, 274. effect of, on strength, 156. of, on water required, 269. Aggregate, effect of, on strength, 160. Aggregates, nature of, 2. Aggregation, effect of, on plasticity, 261. states of, 14. Air, importance of, in burning clays, etc., 546. thermal conductivity of, 587. -separation, 53. Akermanite, 482. latent heat of fusion of, 601. melting-point of, 601. Alabama, graphite in, 23. Albite, 342. -anorthite phase diagram, 457. birefringence of, 626. effect of fusion of, on specific gravity, 211. formation of, 480. isomorphism in, 334. melting-point of, 606. -orthoclase-anorthite phase diagram, 463. properties of, 416. refractive index of, 623. 635 636 Alcohol, effect of clay on, 238. Aleksiejeff, 261. Alexander, 242. Alge, a cause of colour, 97. Alkali defined, 320. effect of, on clays, 242, 248, 275. on osmotic pressure of colloids, 233. on silica glass, 505. on silicates, 505. in chemical analyses, 360. in clay, 364. volatilisation of, 551. Alkaline glazes, 383. silicates, 364. Allen, 468, 612, 616, 617, 618, 619. B. J., 249, 284. Allophane, 20, 345, 412. structure of, 7. thermal curve of, 352. water in, 423. Allotropic changes, effect of size of grains on, 31. on cooling, 560. on heating, 545. forms of silica, 327. Alteration of structure, 25. Alum as bond in silica bricks, 399. Alumina, 339. affinity of fluxes for, 315. artificial, structure of, 14. as base or acid, 322. -barium oxide-silica system, 481. -base ratio, 479. -base-silica system, 474, 478. colloidal, 251. combined water in, 340. constitution of, 339. decolorising effect of, 101. effect of, on colour of clay wares, 101. on expansion of glazes, 579, on porosity, 68. on silica bricks, 497. on titanic oxide, 368. free, in clays, 364. occurrence of, 426. fused, 403. constancy in volume of, 570. garnets, 342. heat of formation of, 531. hydrous, decomposition of, by heat, 545. in calcined clay, 351. raw clays, 364. -iron oxide system, 471. -lime-silica system, 478. -lime system, 470, 471. -magnesia system, 471. -silica system, 481. melting-point of, 605, 606. molecular heat of, 598. INDEX Alumina continued— polymerisation of, 350, 351. polymerising temperature of, 339. -potash-silica system, 480. replacement of, by other oxides, 335, 389. -silica eutectic, 472. ratio in clays, 371. in glazes, 386. system, 472. -soda-silica system, 480. specific gravity of, 221. heat of, 598. use of, in glazes, 385. -zinc-silica system, 482. Aluminates, 322, 356, 421. in clays, 415, 421. Aluminium, effect of, on silica glass, 498. hydroxides, types of, 339. graphic formula for, 308. minerals in clays, 421. Alumino-silicates, 364, 367. as cementing materials, 16. chemical constitution of, 325, 340, 341. corrosiveness of, 441. effect of sulphuric acid on, 504. ferric, 495. in chromite, 430. in clay, 415. thermal curves of, 349, 352. viscosity of, 441. -silicic acids, classification of, 342. anhydrides, 342. Aluminous bricks, materials used for, 402. strength of, 191. materials, hardness of, 126. shrinkage of, 570. minerals, impurities in, 427. occurrence of, 426. Alundum cements, melting range of, 605. coefficient of expansion of, 581. specific gravity of, 221. strength of, 191. a-magnesia, 220, 356. American Ceramic Society, 197, 219, 522. Foundryman’s Association, method of com- paring texture, 57. Gas Institute, 180, 187, 373, 400, 602, 603. hard porcelain, strength of, 178. National Brick Manufacturers’ Association, 197. Society for Municipal Improvements, 128, 176. for Testing Materials, 76, 82, 174, 175, 196, 197, 201, 500, 501, 523. Standard sieves, 45, 47. Ammonium carbonate, use of, in purifying clay, 288. hydrate, effect of, on silica glass, 505. INDEX Amorphous carbon, 358. definition of, 5. ' magnesite, structure of, 8. materials, effect of heat on, 8. silica, 328, 329. occurrence of, 424. use of, 425. solids, 487. substances, defined, 487. nature of, 5. Amphiboles, 416. Amphoteric electrolytes, effect of, on clay, 243. Analcime, 341, 342. formation of clay from, 354. Analcite, see Analcime. Analyses, typical, misuse of, 314. Analysis, chemical, 359. interpretation of, 360. mechanical, 44, 49. of clay, after calcination, 410. rational, 409. recalculated, 410. Anatase, 429. birefringence of, 626. crystalline form of, 4. electrical conductivity of, 609. in clay, 421. refractive index of, 623. Andalusite, 342, 413, 480. birefringence of, 626. crystalline form of, 4. electrical conductivity of, 609. formation of clay from, 354. melting-point of, 606. refractive index of, 623. thermal curve of, 352. Andesine, 417. birefringence of, 626. refractive index of, 623. ngstrom units, 323. Angular grains, 27. Anhydride defined, 317. Ankerite, as a cementing material, 15. Annealing, 559. Anorthite, 342, 367, 416, 478. -albite-orthoclase phase diagram, 463. phase diagram, 457. birefringence of, 626. -bytownite, formation of, 440. crystallisation of, 461. latent heat of fusion of, 601. melting-point of, 601, 606. refractive index of, 623. Anorthoclase, formation of clay from, 354. Anthophyllite, 416. structure of, 25. Apatite, 421, 422. action of hydrochloric acid on, 503. 637 Apatite continued— birefringence of, 626. electrical conductivity of, 609. in clay, 422. melting range of, 606. refractive index of, 623. Apparent density, 203, 205. of various ceramic materials and articles, 212-223; see also under their various names. porosity, 62. determination of, 81. Apparent specific gravity, 203. determination of, 221, 224. effect of heat on, 208. a-quartz, 328. specific gravity of, 216. heat of, 597. Aragonite, 367, 420. birefringence of, 626. replacement of, by quartz, 5. Aron, 20, 102. Arrested reactions, 487. Arrhenius, 240, 245. Arsenic oxide as opacifier, 395. Artificial colours, producing, 115. Arzuni, 334. Asahara, 358. Asbestos, bricks, porosity of, 79. crystalline form of, 4. * fuel,” 25. melting range of, 606. specific gravity of, 220. structure of, 25. thermal resistivity of, 594. Asch, W. & D., 237, 309, 327, 332, 337, 338, 339, 340, 343, 346, 347, 348, 350, 351, 352, 353. Ashes, a source of scum, 121. Ashley, H. E., 199, 240, 245, 248, 251, 265, 267, 278, 350, 567, 632. Association of particles, 445. Aston, F. W., 299. Atmosphere during firing, importance of, 554. in kiln, effect of, on colour, 118. in kiln, effect of, on shrinkage, 566. in kiln, effect of, on strength, 162. of furnace, effect of, 502. reducing, in kiln, a cause of discoloration, 121. Atomic compounds, 302. heat, definition of, 512. of various materials, 513. number, 301. structure, 301. of amorphous substances, 322. volume of basic elements in glazes, 384, 385. weights, 303. list of, 303. 638 Atoms, 300. cubic arrangement of, 324. mobility of, 437, 438. structure of, 301. a-tridymite, 328. specific gravity of, 216. Atterberg, 260, 268, 269, 282. number, 268. Attraction, molecular, effect of, on plasticity, 261. Augite, 342, 415, 459. birefringence of, 626. magnetic properties of, 621. melting range of, 606. pleochroism of, 630. refractive index of, 623. Austin, 163. Aventurine glazes, 398. Avogadros number, 234. Axinite, birefringence of, 626. Aylesbury sand, texture of, 42. Aylesford red bricks, strength of, 173. Ayrshire bauxitic clay, coefficient of expansion of, 571. specific gravity of, 213. B-Alumina, 339. Back reaction, 449. Bacteria, effect of, on clay, 275. Baddeleyite, 429. colour of, 120. hardness of, 126. Bagshot clays, colour of, when burned, 109. Baking v. burning, 557. Balanced reaction, 451. Ball clays, 414. binding power of, 284. blue, 108. Brownian movement in, 242. burned, colour of, 109. carbonaceous matter in, 422. casting, 284. cause of colour of, 106. coefficient of expansion of, 571, 573. colloidal matter in, 263. colour of, 95, 107. composition of, 372. dry, strength of, 159, 171, 172. effect of, on electrical resistance, 612. iron sulphides in, 419. ivory, 108. size of grains in, 34. translucency afforded by, 632. water required to. develop plasticity of, 269. Baraboo quartzite bricks, specific gravity of, 219. INDEX Barium carbonate as a cementing material, 15. " as a scum preventor, 97. effect of, on silica glass, 505. compounds, 368. felspar, 417. metaborate, melting-point of, 606. metasilicate, melting-point of, 606. minerals in clays, 421. orthoborate, melting-point of, 606. oxide-alumina-silica system, 481. effect of, on expansion of glazes, 579. on shrinkage, 567. heat of formation of, 531. in glazes, 387. in porcelain, 377. -lime-silica system, 474. -lithia-silica system, 477. -silica system, 469. -soda-silica system, 477. pyroborate, melting-point of, 606. silicate, 421, 477. sulphate, 421. as bond in silica bricks, 399. reaction of, with soda, 451. Barratt, 235. Barringer, 391. Barus, 486, 597. Barysilite, 333. Barytes, 421. as a cementing material, 15. electrical conductivity of, 609. refractive index of, 623. replacement of, by hematite, 5. Base-alumina ratio, 479. -silica system, 474, 478. -base-silica systems, 474. Bases and acids, product of reaction of, 434. defined, 316, 319. effect of, on clay, 243. on fusing-point, 387. in glazes, 386. reaction of, with silica, 505. removal of, from solution by clay, 241. soluble, to be fritted, 387. Basic materials, effect of, on silicates, 466. effect of sulphuric acid on, 504. silicates, 332. slag, corrosive action of, 496. Bastard ganister, Scottish, texture of, 40. Bath bricks, finishing temperature of, 553. Bauer, 518, 588, 590, 592, 596. Baugh, 244, 247. Bauschinger, 134. Bauxite, 339, 402, 427. action of hydrochloric acid on, 503. bricks, 402, 499. and spalling, 584. effect of lime on, 499. INDEX Bauxite bricks continuwed— effect of slag on, 499. firing, 556. temperature of, 558. hardness of, 132. hot, strength of, 165. melting range of, 605. permeability of, 90. porosity of, 80. refractoriness of, 603. resistance of, to abrasion, 130, 131. to slags, 500. to sudden changes of temperature, 583. shrinkage of, 569. strength of, 190. texture of, 43. thermal conductivity of, 593. effect of burning temperature on, 585. eolour of, 97, 119. combined water in, 340. constitutional formula of, 340. decomposition of, by heat, 490, 545. dehydrated, 339. effect of heat on, 340. fused, coefficient of expansion of, 581. hardness of, 126. in clay, 421. melting range of, 605. nodular structure of, 25. shrinkage of, 570. specific gravity of, 221. structure of, 19. Bauxitic clay, coefficient of expansion of, 571. melting-point of, 605. fireclays, 421. Bayeux porcelain, coefficient of expansion of, 576, 577, 578. B-cristobalite, 328. specific gravity of, 216. Beading, 386. Becke, 624. Beckenkamp, J., 326, 327, 328. Bedford, T. G., 578. Beecher, M. F., 153, 181. Beilby, Sir G., 274. Bell, M. L., 130, 134, 169, 187, 206. Bencke, A., 240. Bending temperature, 165. Bentonite, 264, 412. Berdel, E., 393. Berkshire, red-burning clays of, 109. Berlin porcelain, 375, 378. coefficient of expansion of, 574, 576, 577, 578. dielectric strength of, 614. Berthelot’s law, 530. Bertrand, L., 364. Beryl, melting range of, 606. 639 Beryllium metasilicate, melting-point of, 606. orthosilicate, melting-point of, 606. oxide, effect of, on electrical resistance, 612. melting-point of, 605, 606. source of, 21. spinel, graphic formula of, 356. Berzelius, 341. Biedermann, 260. Bigot, A., 79, 340. Bijvoet, J. M., 357. Bilz, 230. Binary silicates, 466. systems in ceramics, 466. Binding agent, 507. changes in, during burning, 547. particles of aggregate, 507. power, 141. and plasticity, 281. of clay, 281. measuring, 202, 282. Bingham, 272. Binns, C. F., 241, 389, 396. Biotite, 342, 417. as catalyst, 443. birefringence of, 626. electrical conductivity of, 609. formation of, 440. melting range of, 606. pleochroism of, 629, 630. refractive index of, 623. Birefringence, 411, 625. Bischof, 279, 381, 382. Biscuit ware, colour of, 112. discoloration of, 121. finishing temperature of, 553. porosity of, 80. producing, 554. Bisilicates, 332. effect of, on basic materials, 466. Bismuth oxide in crystalline glazes, 397. Bisque, see Biscuit ware. Bitter spar, 427. : Black articles, production of, 102, 105, 116. bricks, 111. core in clays, 423. hearts, production of, 423. spots, causes of, 121. tiles, 111. Blackening of lead glazes, 555. Blake, 260, 265, 608. Blasberg, 497. Blast furnace bricks, permeability of, 90. furnaces, resistance of, to abrasion, 125. bricks for, porosity of, 77. Bleininger, A. V., 67, 74, 145, 149, 153, 171, 177, 179, 180, 183, 195, 207, 210, 213, 270, 277, 288, 292, 331, 364, 373, 443, 482, 574, 595, 610, 612, 619, 640 Blistering, 392. Blisters in articles, 22. Bloated ware, 369. Bloating, 161, 562, 565. Blocks, hollow, porosity of, 77. large, importance of porosity in, 75. susceptibility to sudden changes in tem- perature, 582. texture of, 38. Blotches, due to pyrites, 103. in crystals, 3. in firebricks, 115. Blount, 225. Blows, effect of, in reducing strength, 169. Blue articles, production of, 102. ball clays, 108. bricks, 111. apparent density of, 214. bond in, 148. cause of colour of, 103. of strength of, 148. finishing temperature of, 555. porosity of, 76. strength of, 173, 174. colour, effect of impurities on, 105. produced by iron compounds, 97. producing, 116. discolorations, cause of, 121. glazes, 395. tiles, 111. Blueing clays, cause of, 104. Blumenthal, G., Jun., 133, 136. Blunging, 60. B-magnesia, 356. Bodies, composition of, 382. raw, identified by colour, 106. Bodin, V., 165, 179, 180, 188, 189, 190, 191, 192, 195, 548. Body-centred cube lattice, 323. Boeck, P. A., 5, 21, 573, 581. Boernstein, 594, 595. Bogitch, B., 41, 127, 135, 145, 186, 190, 225, 329, 581. Boilers, corrosive action of flue-dust on, 496. Boiling, 561. point, 523, 528. Bole, 413. Bole, G. A., 263. Bond, adhesive, 153. clays, dry, strength of, 171, 172. effect of, on porosity, 68. on strength, 147, 160. hydraulic, 154. in silica bricks, corrosion of, 498. plastic, 153. power of the, 153. proportion of, 18. vitrified, 154. INDEX Bonds for silica bricks, 399. Bone, W. A., 238. china ware, 112; see also China-ware. Boric acid as catalyst, 443. oxide, effect of, on expansion of glazes, 579. in glazes, 388, 389. Born, M., 513. Bornite, 420. Boswell, P. G. H., 42, 48. Boswell’s grading graph, 57, 58. Bottiger, 246. Bottom Busty fireclay seam, effect of weather- ing on, 255. Boudouard, 468. Boulder clay, structure of, 21. Bourry, 270. Bowen, N. L., 481. Boys, C. V., 252. B-quartz, 328. specific gravity of, 216. heat of, 597. Bradshaw, L., 132, 135, 157, 186, 595, 596. Braesco, 329. Bragg, W. H., 8, 11, 19, 259, 265, 307, 322, 327, 334, 358. W. L., 327. Bravais, 323, 325. Brearley’s sentinel pyrometers, 540. Breunnerite, 427. colour of, 97, 120. texture of, 43. true specific gravity of, 220. Brick clays, apparent density of, 212. composition of, 374. felspar in, 417. texture of, 34. water required to develop plasticity of, 269. earths, burned, colour of, 109. colour of, 108. Bricks, American, strength of, 174. Brinell hardness of, 127, 128. clay, structure of, 17. cracking of, caused by lime, 374. crushing strength of, 173. crystals in, 323. effect of burning on, 438. of frost on, 73. of repeated heating on, 163. of shape of grains in, 29. finishing temperatures for, 555, from colliery refuse, 370. made of paste, 9. minerals in, 415. modulus of rupture of, 176. porosity of, 76. testing, 82. resistance of, to abrasion, 128, 130. INDEX Bricks continued— resistant to acid open-hearth and heating furnace slags, 497. steamed, effect of frost on, 74. strength of, suggested minima for, 174. texture of clay for, 37. transverse strength of, 175. See also under their various names. Brickwork, apparent density of, 213. strength of, 172. Bridgeman, 325. Briggs, T. R., 230. Brindled bricks, apparent density of, 214. Brinell ball test, 135. hardness of bricks, 127, 128. of raw clay, 126. Briquetting ores, reactions in, 505. Bristol stoneware glaze, 392. British Standard sieves, 45, 46. Thermal Unit, 509. Thompson-Houston Company, 379, 498. Brittleness, 141. due to overheating, 562. explained, 139. Brody, E., 513. Bronzite, 415. melting range of, 606. Brookite, 429. birefringence of, 626. crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. in clay, 421. refractive index of, 623. Brown, G. H., 149, 176, 179, 189, 288, 292, 364, 373, 501, 590. colours produced by iron compounds, 97. production of, 117. discolorations, cause of, 121. goods, producing, 115. magnesite, cause of, 120. scum, 123. Brownian movement, 11, 54, 232, 242. B-tridymite, 328. specific gravity of, 216. B.T.U., 509. Bubbles in crystals, 3. Buckner, O. 8., 159, 161. Buddington, 482. Building bricks, American, strength of, 174,175. finishing temperature of, 553. German, strength of, 175. hardness of, 127. permeability of, 89, 90. porosity of, 76, 80. resistance of, to weathering, 166. specific gravity of, 215. strength of, 172, 174. 641 Building bricks continwed— thermal conductivity of, 587, 593. resistivity of, 594. volume-weight of, 215. Buff burning clays, 114. colour, cause of, 99. coloured goods, producing, 114. Bulk specific gravity, 203. Bull dog, corrosive action of, 496. Bunting, E. N., 85, 86. Bunzli, 483. Burchartz, H., 77, 215. - Burned clay, constitution of, 348. clays, colour of, 108. hardness of, 127. iron compounds in, 420. mineralogical nature of, 412, 415. specific gravity of, 213. siliceous materials, minerals in, 426. Burner’s skill exemplified, 440. Burning carbonates, 490. cement produced by, 16. ceramic materials, 544. clays, 544. effects of heat in, 544. effect of, on colour, 105. on strength, 159. the temperature on thermal conductivity, 590. formation of glassy material in, 558. manner and duration of, 632. period, 163. phase conditions in, 466. temperature, effect of, on porosity, 69, 70. on thermal conductivity, 585. Burt, 389. Burton, 231. Bytownite, 417. refractive index of, 623. Calcareous cement in rocks, 15. clays, vitrification range of, 553. sandstone, bond in, 18. Calcined china clay, action of hydrochloric acid on, 503. clay, adsorbing power of, 238. constitution of, 348. magnesia, 428. practically insoluble, 503. Calcining, alteration of structure by, 26. furnaces, porosity of bricks for, 78. Calcite, 367, 420. action of hydrochloric acid on, 503. as a cementing material, 15. birefringence of, 626. crystalline form of, 4. distinction of, from aragonite, 625. 41 642 Calcite continued— electrical conductivity of, 609. hardness of, 126. refractive index of, 623. replacement of, by hematite, 5. by quartz, 5. space-lattice of, 324. Calcium aluminates, 421. hydraulic properties of, 503. in matte glazes, 396. melting-points of, 470, 606. alumino-silicates, 367. in matte glazes, 396. biborate, melting-point of, 601, 606. borate, latent heat of fusion of, 601. ‘Calcium carbonate, 367. as a cementing material, 15. conditions of equilibrium of, 453. decomposition of, by heat, 367, 433, 447, 451, 490. heat of formation of, 531. occurrence of, 428. chloride as bond in silica bricks, 399. compounds in clays, 367, 420. mineralogical nature of, 428. felspars, 417. ferrates, 421, 503. hydroxide, effect of, on strength of fired clays, 155. metaborate, melting-point of, 606. metaferrate, 471. metasilicate, 333, 467, 478. forms of, 468. melting-point of, 606. minerals in clay, 420. orthoferrate, 471. orthosilicate, 333, 467, 468. melting-point of, 606. oxide, see Lime. phosphate, 367, 421. melting-point of, 606. pyroborate, melting-point of, 606. silicates, 367, 421, 434, 476, 479. and lithia, 476. formation of, 462, 468. fusing-point of, 467. hydraulic properties of, 503. in phase diagram, 475. sulphate, 367, 369, 421. and sodium carbonate, interaction of, 436. as bond in silica bricks, 399. effect of heat on, 367. heat of formation of, 531. in clay slips, effect of, on electrical resis- tivity, 619. in glazes, 387. occurrence of, 428. spoils slip, 285. INDEX Calculation of molecular formule, 312. Caldwell, D. R., 241. Callendar, 327. Calorie, 509. Calorimeters, 508, 510. Calorites, 540. Cambridge & Paul Instrument Co., Ltd., 537. Campbell, 356, 503. Cancrinite, birefringence of, 626. Capillary structure, 24. of clay particles, effect of, 76. tube, flow of water in, 87. Caramel, effect of, on plasticity, 275. Carbide bricks, burning, 557. shrinkage of, 570. crucibles, 404. Carbides, 404. and carboxides, impurities in, 430. constitution of, 357. crystalline structure of, 2. decomposition of, 491. effect of, on magnesia bricks, 498. hardness of, 126. structure of, 14. use of, 430. Carbofrax, effect of rapid cooling on, 584. Carbon, affinity of, for oxygen, 493. allotropy of, 358. amorphous, 358. bricks, 404, 604. burning, 557. hot, strength of, under load, 179. inertness of, 500. resistance of, to temperature changes, 583. specific gravity of, 596. heat of, 596, 599. strength of, 189. structure of, 19. thermal conductivity of, 588. colour of, 119. compounds, commercial nature of, 430. deposition of, on fireclay bricks, 169. heat of formation of, 531. effect of, on magnesia bricks, 498. on strength, 169. heat evolved on burning, 530. mineralogical nature of, 430. monoxide, decomposition of, by red-hot clay, 238, 443. by fireclay bricks, 497. heat evolved on burning, 530. heat of formation of, 531. oxidation of, 493. retardation of oxidation of, 493. specific gravity of, 221. thermal resistivity of, 594. Carbonaceous matter as a source of colour, 106. decomposition of, by heat, 491, 545. INDEX Carbonaceous matter continued— effect of, on colour, 105. in burning clays, 423. on texture, 370. in clay, 370, 422, 430. occurrence of, 430. removal of, 491. use of, to increase porosity, 65. Carbonated water, action of, on carbonates and silicates, 504. Carbonates as cementing materials, 15. burning, 490. decomposition of, by heat, 490, 545. Carbonic acid, action of, on carbonates and silicates, 504. Carborundum, 430. analyses of, 405. bricks and spalling, 584. burning, 557. effect of molten iron on, 500. of silica on, 500. of slags on, 500. electrical resistivity of, 619. hardness of, 133. hot, resistance of, to abrasion, 131. permeability of, 90. porosity of, 80. refractoriness of, 604. resistance of, to abrasion, 130, 133. to temperature changes, 583. strength of, 191. structure of, 17. texture of, 44. thermal conductivity of, 592, 593. resistivity of, 594. colour of, 119. composition of, 404. constitution of, 357. crystalline structure of, 14. decomposition of, 491. decrease in strength of, when hot, 165. effect of hydrofluoric acid on, 504. of soda on, 500. lustre of, 95. melting-point of, 605. space lattice of, 324. specific gravity of, 221. thermal conductivity of, 586, 592. Carboxide bricks, 404. Carboxides, 357. crystalline structure of, 2. hardness of, 126. inertness of, 500. use of, 430. Carlow, chert beds at, 506. Carlsson, F., 149, 363, 364, 366. Carnalite, 413. Cassiterite, 422. 643 Cassiterite continued— birefringence of, 626. electrical conductivity of, 609. refractive index of, 623. space lattice of, 324. Casting, deflocculants for, 285, 286. electrolytes for, 285. process, 283. slips, 10, 283. temperature of, 287. water in slips for, 285, 286. Catalysis, 443. Catalysts, 443. action of, 444. effect of, 442. Catalytic action, 442. of clay, 238. of fireclay, 73. agent, effect of, 238. Caustic magnesia, soluble in water, 503. soda, use of, in purifying clay, 288. Cavities in crystals, 3. Celestite, 421. electrical conductivity of, 609. refractive index of, 623. Cellular materials, 23. structure, 23. production of, 23. substances, 7. Cellulose, as a bond, 153. Celsian felspar, 417, 342. Celsius grade of temperature, 509, 533. Cement, 507. calcareous, in rocks, 15. effect of, on zirconia bricks, 500. ferruginous, in rocks, 15. a result of weathering, 507. in plastic materials, 19. in sandstone, 18. Portland, as a bond, 154. precipitation of, in rocks, 15. production of, in burning, 16. silica, 15. . Cementation, 507. Cementing materials in rocks, 15. Cements, alumino-silicate, 16. for silica bricks, 498. glassy, 16. in finished goods, 16. siliceous, porosity of, 79. texture of, 44. Centigrade and Fahrenheit scales, 534. scale of temperature, 533. unit, 509. Centres of crystallisation, effect of, 489. Ceramic materials defined, 1. properties of, see under their various names. 644 INDEX Cerium, effect of, on silica glass, 498. oxide, melting-point of, 605. source of, 21. Ceylon, baddeleyite in, 120, 403. graphite in, 23, 404. Chabazite, 342. Chain compounds, 308. formule, 343. Chalcedony, 253, 424. as a cementing material, 15. colloidal nature of, 7, 13. colour of, 119. effect of repeated heating on, 218. electrical conductivity of, 609. refractive index of, 623. specific gravity of, 216. Chalcopyrite, 420, 507. electrical conductivity of, 609. Chalfont St Peters, bricks made at, 373. Chalk, 367, 374. flint, specific gravity of, 216. Chalybite, 420. as a cementing material, 15. Change, electrical, on particles, 228. Changes due to water in natural minerals, 253. during vitrification, 552. effected by weathering, 253. in physical properties effected by heat, 528. state caused by heat, 523. effected by water, 227. in temperature, 519. in volume caused by heat, 520. of state, 444. Chapman, 325. Chapney, 118. Checker bricks, permeability of, 90. porosity of, 80. thermal conductivity of, 593. resistivity of, 594. Chemical action, 432. and physical changes, 433. avoidance of, 432. effect of light on, 444. oi pressure on, 440. of temperature on, 445. of vapour pressure on, 440. prevention of, 432. rate of, 446. resistance to, and porosity, 76. types of, 435. in weathering, 506. affinity, 433. analysis, 359. interpretation of, 360. changes, cause of, 433. during vitrification, 552. in cooling, 559. combination, laws of, 302. Chemical continued— composition, 146. effect of, on coefficient of expansion, 573. on strength, 146. compound defined, 299. compounds, definite, 462. constitution, effect of, on true specific gravity, 209. of ceramic materials, 299; see also Constitution. equations, 316. formule, 306. furnaces, porosity of bricks for, 78. notation, 306. porcelain, strength of, 178. reaction and porosity, 446. velocity of, 448, 449, 450. of increasing, 450. reactions and time, 439. factors influencing, 437. occurrence of, 435. occurring at high temperatures, 490. tendency of, 435. rearrangement, 436. systems, phase conditions in, 452. ware, permeability of, 90. Chert, 424, 506. colour of, 119. Chiastolite, 342. China clay, 414. apparent density of, 206. birefringence of, 411. Brownian movement in, 242. calcined, action of hydrochloric acid on, 503. colour of, 109. capillary structure of, 24. clay-substance from, 344. colloidal matter in, 263. colour of, 107. composition of, 372. crystalline nature of, 11, 411. structure of, 19. effect of heat on density of, 206. on porosity, 68. -felspar mixtures, effect of heat on, 66. heating curve of, 349. index of refraction of, 411. interference figure of, 411. kaolinite in, 20. mica in, 20. molecule, 347. organic matter in, 422. polarisation colours of, 411. purification of, 288. quartz in, 20. size of grains in, 33. tourmaline in, 20, 107. INDEX 645 Chromite continued— hardness of, 126. impurities in, 430. magnetic properties of, 621. melting-point of, 605, 606. occurrence of, 429. permeability of, 90. polymerisation of, 358. porosity of, 80. refractive index of, 623. sand, 20. segregation of, 25. shrinkage of, 358. Chromium compounds, colour depends on at- mosphere in kilns, 554. colours produced by, 118. effect of, on colour, 116, 117. minerals in clay, 422. oxide in crystalline glazes, 398. melting-point of, 605. spinel, 400. Chrysoberyl, 356. Chrysotile, structure of, 25. Chwarele ganister, texture of, 40. Cimolite, 345. Cinder tap, penetrative power of, 497. Clamp bricks, 546. Clark, F. W., 333, 347. HL. HL, 293. Clarke, 309. Claus-Chance process, 238. Clausmann, 504. Clay and salt, effect of heat on, 352. beds, effect of percolating water on, 506. and spalling, 584. bricks, structure of, 17. burning, 557. decomposition of, an irreversible reaction, electrical resistivity of, 618. 451. fusing temperature of, 558. definition of, 35, 344. hot, strength of, 165. dehydration of, effect of, on specific heat, inertness of, 500. 595. China clay continued— translucency afforded by, 632. of, measuring, 634. true specific gravity of, 210. vitrification range of, 553. water required to develop plasticity of, 269. X-ray spectrum of, 11, 19. -ware, 112. apparent density of, 214. burning, 554. cause of discoloration in, 121. clays for, 375. hardness of, 133. porosity of, 80. strength of, 177, 199. Chinese porcelain, 376, 377, 378. sea-green glaze, 394. Chipping, resistance of china-ware to, 199. Chlorides, a source of scum, 122. Chlorinated atmosphere, effect of, 118. Chlorite, 342, 418. action of hydrochloric acid on, 503. as cementing material, 15. magnetic properties of, 621. pleochroism of, 630. Chloritoid, birefringence of, 626. Chocolate colour, production of, 118. Chorlton, 589. Chrome bricks, see Chromite bricks. ores, mineralogical composition of, 429. use of, 429. -tin pinks, 395. Chromite, 404. bricks, 404. melting range of, 605. permeability of, 90. porosity of, 80. refractoriness of, 604. resistance of, to abrasion, 130, 131, 133. to temperature changes, 583. shrinkage of, 570. specific gravity of, 221. strength of, 150, 181, 189. structure of, 17. thermal conductivity of, 592, 593. of effect of burning temperature on, 585. resistivity of, 594. transverse strength of, at high tempera- tures, 181. determination of felspar and quartz in, 40. -felspar-flint mixtures, fusion of, 486. formule, 346, 347, 348. in aluminous minerals, 427. ironstone, chalybite in, 420. concretionary, 25. oil from, 370. pastes, best consistency of, 267. drying, 295. physical changes in, 257. strength of, 170. possible origins of, 354. purpose of, in porcelain, 377. -sand mixtures, dry, strength of, 171, 172. separating colloidal matter from, 263. composition of, 404. crystalline form of, 4. electrical conductivity of, 609. separation of quartz and felspar from, 409. of sand and silt from, 343. -silica bricks, structure of, 18. 646 INDEX Clay continued— Clays continued— slips, effect of salts on electrical resistivity of, colloidal properties of, 13, 228, 236. 620. water in, 370. electrical conductivity of, 619. properties of, 619. resistivity of, 620. nature of, 10. soluble in felspar, 483. substance, 343. suspensions, protection of, 249. swelling of, in water, 237. synthesis of, 352. use of, in glazes, 386. variable behaviour of water in, 338. wares, apparent density of, 214. thermal conductivity of, 587. true specific gravity of, 214. -with-flints, 424. Clays, absorption of water by, 76, 254, 337. adsorption by, 238. of dyes by, 240. of gases by, 239. of grease by, 239. of liquids by, 239. of oil by, 239. of salts by, 240. of solids by, 239. from solution by, 240. of soluble substances by, 240. adsorptive power of, 236. alkalies in, 364. apparent density of, 212. as bond for carbides, 405. as catalysts, 443. as emulsoids, 237. baked, structure of, 20. ball, colour of, 107, 108 ; see also Ball clays. binding power of, 281. bleaching, 506. Brinell hardness of, 126. buff burning, 114. burned, colour of, 108. iron compounds in, 420. specific gravity of, 213. texture of, 36. true specific gravity of, 214. calcining, 544. purpose of, 26. calcium compounds in, 367. calcareous matter in, 370. chemical components of, 361. constitution of, 340, 343. chromite in, 430. coefficient of expansion of, 571. colloidal, 11. gel in, 265. matter in, 13. measuring, 240, 251. colour of, 95, 97. colouration of, by carbonaceous matter, 106. by iron oxide, 99. colouring by reducing, 111. materials in, 107. common, 343. crushing, 254. strength of, 171. decomposition of, by heat, 350, 351, 438, 490, 491, 545. by lime, 479. deflocculation of, 247. disintegration of, 254, 256. distribution of water in, 255. dry, see Dry clays. drying, 295. effect of acid on, 275. of added colloidal matter on, 271. of alkalies on, 248, 275. of alumina on refractoriness of, 364. of bacteria on, 275. on carbon monoxide, 238. of colours on, 250. of drying on strength of, 158. of electrolytes on, 242. of frost on, 254. of fusion with sodium carbonate, 322. of heat on, 337, 338, 348, 350, 351, 472, 508. on porosity of, 69. on specific gravity of, 212. of iron compounds on colour of, 103. strength of, 160. of light on, 507. of lime on, 368, 495. of magnesia on shrinkage of, 568. on vitrification range of, 568. of, on coefficient of expansion of porcelain, 574. of, on crazing, 387. of silica in, on melting-point, 363. on refractoriness, 364. of soluble substances on, 242. of steam on, 256. of sulphuric acid on, 504. of water of constitution on, 337. of water on, 227, 228, 256. of weathering on, 253. electrical conductivity of, 609, 611. properties of, 242. evolution of water of constitution of, 350. extensibility of, 276. felspars in, 417. ferruginous minerals in, 418-420. fired, ideal structure of, 29. Clays continued— firing in oxidising atmosphere, 493. flaky, 22. flow of, under pressure, 273. for bricks, 374. for china-ware, 375. for earthenware, 374. for firebricks, 373. for porcelain, 375. free alumina in, 364. Gault, colour produced by, 112. grinding, 254. hardness of, 123, 125. heating curves of, 349. hydrolysis of, 236, 263. hygroscopic nature of, 254, 298. impure, effect of heat on, 414. impurities in, 362. removing, 287. iron compounds in, 366. oxide in (state of), 359. lime in, 367. magnetic properties of, 621. manganese compounds in, 368. microscopical examination of, 406. mineralogical composition of, 406, 411. miscibility of, 250. mobility of, 276. moisture in, 370. mottled, causes of colour of, 108. nature of, 13. not mixing, cause of, 239. oiliness of, 276. order of changes leading to fusion, 485. organic colloid matter in, 284. matter added to, 284. over-drying, effect of, 336. -heating, effect of, 336. oxidation of, effects of, 256. processes in, 507. peptisation of, 247. permeability of, 90. phosphorous compounds in, 369. plastic, porosity of, 75. plasticity of, 257-274. pure, 414. purification of, 287. purified, 412. purpose of adding sand to, 374. pyrites in, 369. rate of wetting, 240. raw, colours of, 107. strength of, 148, 170. red-burning, 109, 110. causes of colour of, 109. rehydration of, 337, 338, 349. removal of iron from, by magnets, 621. reversible changes in volume of, 571. INDEX Clays continued— Ruabon, colour of, 109. sampling, 360. semi-permeability of, 250. shrinkage of, 565. silica as an impurity in, 363. size of grains of silica in, 363. slaking of, 227, 257. specific heat of, 595. stoneware, colour of, 108. structure of, 19. sulphur in, 369. surface factor of, 56. tensile strength of, 171. titanic oxide in, 368. true specific gravity of, 212. ultramicroscopic examination of, 631. vanadium compounds in, 369. vitrification range of, 553. vitrified, structure of, 20. water of constitution in, 337. of crystallisation in, 337. of hydration in, 337. required to develop plasticity of, 269. weathering, 255. white, 108. yellow, colour in, cause of, 108. Clews, F. H., 353. Clinker bricks, Dutch, strength of, 173. finishing temperature for, 555. German, strength of, 175. hardness of, 128. strength of, 173, 174. true specific gravity of, 215. volume-weight of, 215. Clincenstatite, 481. Coagulation by added colloid matter, 231. causes of, 232. Coagulative power of a salt, 231. 647 Coal ash, action of, on fireclay bricks, 496. Coarse sand, definition of, 35. Coating ware with engobe, 283. Cobalt-blue glazes, 389, 395. zeolites in, 389. compounds, colour produced by, 118. effect of, on colour, 116, 117. oxide as colouring agent, 113, 118. as decolorant, 113. in crystalline glazes, 397, 398. prepared, 395. Cobaltite, space-lattice of, 324. Cobb, J. W., 329, 339, 340, 352, 462, 466, 468, 470, 495, 515, 516, 522, 467, 571, 572, 579, 587, 588, 590, 592, 593. Coefficient of expansion or contraction, 521. of clays, 571. of glazes, 578. of porcelain, 577. 648 Coefficient of expansion continwed— of stoneware, 577. of linear expansion, 580. Cohesion, 140. limit, 268. of clay to iron, 239. Cohn, 260. Coke bricks, 404. structure of, 17, 19. texture of, 43. colour of, 119. in clay mixtures, 371. ovens, formation of glaze in, 87. porosity of bricks for, 78. use of, for increasing porosity, 66. Colliery refuse, bricks made from, 370. Collins, J. H., 353. Colloidal alumina, 251. clays, 11. gels, 7, 12. changes in, during burning, 547. contraction of, 235. heating of, 13. in clay, 265. properties of, 235. structure of, 12. swelling of, 12. iron, effect of, on sands, 252. hydrate, effect of, 271. magnesia, 253. matter and binding power of clay, 281. and plasticity, 262, 277. behaviour of, on drying, 295. coagulative action of, 231. effect of adding, 271. in clays, 13, 228, 251. measuring, 240. separation of, 263. organic matter, 253. particles, size of, 11. phenomena, 228. precipitates, nature of, 232. properties of clays, 236. reactions, 236. importance of time on, 439. silica, 252, 363, 424. as a cementing material, 15. effect of, 271. inversion of, 13. silicic acid, 252. effect of drying on, 333. sols, 11. action of heat on, 235. of salts on, 11. adsorption by, 235. Brownian movement in, 233. diffusivity of, 234. electric charge on, 228. INDEX Colloidal sols continwed— electrical conductivity of, 230. electro-osmosis in, 230. kataphoresis in, 229. osmotic pressure of, 233. precipitation of, 230. properties of, 228. protection of, 232. reversibility of, 235. specific gravity of, 234. volume of, 204. stabilising, 233. viscosity of, 234. solution, 11. state, defined, 11. nature of materials in the, 228. suspension, coarse, 11. systems, 12. water, 336, 358. in clay, 370. Colloids, active, 251. atomic structure of, 322. Brownian movement in, 11. coagulable, measurement of, 277. coagulated, dispersion of, 232. crystalline, 11. flocculation by, 243. of, 11. irreversible, 253. molecular structure of, 322. nature of, 1, 10. precipitation of, 11. by electrolytes, 231. relative, 278. Collyrite, 20, 345, 412. structure of, 7. Coloration by oxidation, 107. Colour changes during burning, 547, 549. in smoking, 544. control of, 110. due to alge, 97. irregular, cause of, 111. measurement of, 123. mottled, causes of, 111. natural sources of, 96. nature of, 94. of ceramic materials and articles, see under their various names. glaze, altering, 391. streaks of, as decoration, 118. variation of, 94. variegated, 118 Coloured engobes, 395. glazes, 395. oxides for, 397. Colouring materials in clays, 107. oxides in glazes, 389. Colourmeter, use of, 123. INDEX Colours, artificial, producing, 115. blue, producing, 116. brown, producing, 117. buff, cause of, 99. chocolate, producing, 118. effect of alkalies on, 102. firing temperature on, 100, 110, 118. iron on, 98. kiln atmosphere on, 118. lime on, 102. magnesia on, 102. mode of burning on, 96. on clays, 250. finishing temperatures for, 555. flowing of, 118. green, producing, 116. grey, producing, 116. iron, effect of temperature of firing on, 100. olive-green, producing, 116. organic, in clays, 106. produced by carbonaceous matter, 106. by chromium compounds, 116, 117, 118. by cobalt compounds, 113, 114, 116, 117, 118. by copper compounds, 118. by impurities, 96. by iron chromate, 117. compounds, 97, 105, 118. destruction of, 101, 102. effect of acid on, 98. by iron silicates, 116, 117. by manganese compounds, 116, 117, 118. by nickel compounds, 118. oxide, 116, 117. by oxidation, 108. by titanium compounds, 118. by zinc oxide, 116, 117. purple, cause of, 111. red, causes of, 100. effect of alumina on, 101. minerals on, 101. producing, 117. temperature required to produce, 110. use of engobes for, 96. violet, producing, 116. volatilisation of, 118. yellow, in clays, cause of, 108. producing, 117. Comber, N. M., 244, 245, 278. Combination, chemical, laws of, 302. direct, 435. ° heat of, 528, 529. Combined water in alumina, 340. Common bricks, see Building bricks. Compensation, position of, 628. test, 628. Component, definition of, 453. 649 Composition and colour, 113. electrical properties, 609. fusing point of glazes, 384. refractoriness, 381. utility of clays, 371. changes in, due to heat, 528. effect of, on strength, 154. on translucency, 632. from molecular formula, 311. of clays, 361. of engobes, 382. of glazes, 382. adjustment of, 389. of porcelains, 378. of Seger cones, 379. of silica bricks, 398. -temperature diagrams, 465. Compounds, 299. atomic, 302. chain, 308. chemical, 299. decomposed by electricity, 444. formation of, 462. molecular, 302. ring, 308. saturated, defined, 305. unsaturated, defined, 305. Compressibility and plasticity, 280. Compressive strength, determination of, 195. Concretionary structures, 25. Conductivity, contact, 518. electrical, 230, 444, 608, 609. changes in, during burning, 548, 549. determination of, 620. effect of porosity on, 74. of clay slips, 611, 619. of silica bricks, 616. of water, 619. of heat, 514. thermal, 514. changes in, during burning, 548, 549. effect of burning on, 590. of heat on, 584. in magnesia bricks, 591. in silica bricks, 591. of permeability on, 89. of porosity on, 72, 74. of temperature on, 587, 588. of ceramic materials, see under their various names. of powders, 592. Conductors, bad, 609. good, 609. moderate, 609. Cones, Seger, 540. composition of, 379. temperatures corresponding to, 379. 650 INDEX Consistency, 268. of clay paste, 267. of pastes, 278. Constitution, chemical, of burned clay, 348. of calcined clay, 348. of ceramic materials, 299. of clays, 340, 343. of iron oxides and hydroxides, 357. of lime, 357. of magnesia, 355. of silica, 326. of silicates, 332. of silicic acid, 332. of silicon carbides and oxycarbides, 357. of spinels, 356. of sundry refractory materials, 358. of water, 337. Contact, area of, effect of, 446. conductivity, 518. of particles, effect of, on chemical action, 445. resistance, 518. Contraction, 520. effect of, on melting-point, 526. harmful, in retorts, 566. of colloidal gels, 235. Contraction on drying, 295. Convection, 519. Cook, 260. Cooling, 559. changes in, 559. curves, 489. v. heating curves, 466. effect of, on allotropic forms, 560. on composition of products, 489. on strength, 560. of kilns or ovens, 162. effect of, on strength, 162. rapid, 488. effect of, 563. on spalling, 584. rate of, 162. effect of, on crystallisation, 488. on specific gravity, 211. repeated, effect of, 563. slow, 488. Copper alloys, melting-points of, 541. -blue glazes, zeolites in, 389. compounds, colours produced by, 118. discoloration produced by, 121. glazes, 389. -iron sulphides, 420. melting-point of, 541. metallic, in aventurine glazes, 398. oxide as colouring agent, 118. effect of, on silica glass, 498. on zirconia bricks, 500. in crystalline glazes, 397, 398. in glazes, 395. Copper oxide continued— penetrative power of, 497. spinel, 400. sulphide, effect of weather on, 507. thermal conductivity of, 587. Coprolites, 367, 421, 422. Cordierite, 418, 479. birefringence of, 626. electrical conductivity of, 609. in clay, 421. refractive index of, 623. Cores, black, cause of, 105. in clay, 423. Cork, use of, to increase porosity, 66. Cornish stone as bond in silica bricks, 399. effect of, on porosity, 67. erroneous replacement by, 314. in glaze, 387. Cornu, 339, 474. Cornwall, china clay in, 414. Corrosion, 494. effect of graphite on, 371. of porosity on, 73. of texture on, 38: measurement of, 500, 501. of refractory materials, 481. rate of, 446. Corrosive substances, 494. Corundum, 339, 421, 426, 480. articles, texture of, 43. artificial, 426. birefringence of, 626. bricks, hot, strength of, 165. structure of, 17. crystalline form of, 4. structure of, 2, 14. . electrical conductivity of, 609. hardness of, 126. melting-point of, 605. range of, 606. pleochroism of, 630. refractive index of, 623. with sillimanite, 480. with soda, 480. Cracking bricks by lime, 374. due to large grains, 16. effect of grading on, 32. Cracks, 163. caused by cooling, 559. in glaze, preventing, 390. Crawford, J. L., 212. Crazing, 384, 385, 386, 387, 389, 390, 392. and expansion, 578. caused by cooling, 559. Cremiatschensky, 261. Cristobalite, 328, 426, 475. and sillimanite eutectic, 473. determination of refractive index of, 625. INDEX Cristobalite continued— formation of, 329, 331, 443. from tridymite, 331. in furnace hearths, 17. in silica bricks, 16. latent heat of fusion of, 600. melting-point of, 331, 603. produced by cooling fused silica, 331. rate of formation of, 330. refractive index of, 623. solubility of, in hydrofluoric acid, 504. specific gravity of, 216. heat of, 597. Critical density, 485. point in cooling fused material, 458. pressure, 485, temperature, 438, 485. volume, 485. Crocidolite, structure of, 25. Cronshaw, 474. Crook, T., 620, 621. Cross, 315, 413. Cross-breaking tests, 197. Crucible clay, American, strength of, after drying, 159. German, strength of, after drying, 159. furnaces, porosity of bricks for, 78. Crucibles, 404. and their contents, 433. burning, 557. carbon, shrinkage of, 570. casting, 284. corrosion of, 495. durability of, 582. effect of repeated heating on, 563. firing temperature of, 558. interaction of, with contents, 491. laminated structure in, 23. permeability of, 90. resistance of, to sudden changes in tempera- ture, 582. strength of, 183. when hot, 165. tensile strength of, 140. texture of, 39. zirconia, 79. casting, 285. ** Crumbs ”’ of clay, 261, 344. Crushing strength, 144. determination of, 195. effect of exposure on, 167. of frost on, 167. of bricks, 173. under load, heating schedule for, 197. when hot, 164. See also Strength. Cryolite, effect of, on zirconia bricks, 500. melting-point of, 606. 651 Cryptocrystalline magnesite, 427. nodular, 25. structure of, 8, 19. texture of, 43. true specific gravity of, 220. structure, 15. Crystalline forms, 2. glazes, 397. masses, nature of, 14. materials, effect of heat on, 8. matter in glaze, increasing, 390. silica, occurrence of, 425. solids, stability of, 488. structure, coarse, 15. disadvantages of, 14. fine, 15. value of, 14. systems, 4. Crystallisation, causing, 488. degree of freedom necessary for, 487. during cooling, 559. effect of dust on, 489. of number of centres of, on, 489. of nuclei of, 489. of rate of cooling on, 488. of temperature on, 488. of viscosity on, 488. extent of, 633. heat of, 528. production of, 551. water of, 371. Crystallising agents, 397. in matte glazes, 396. Crystallites, 3. nature of, 2. Crystallographic axes, 4. Crystallography, 4. Crystalloid, nature of, 11. Crystals, ‘‘ bricks ” of, 323. cause of growth of, 324. combination, 4. formation of, 2, 563. forms of, 4. imperfect, in ceramic materials, 2. imperfectly formed, 2. in ceramic materials, 2. in clay, 412. incipient, 3. isotropic, 627. mixed, 324, 334, 335, 336, 489. See also Mixed crystals. nature of, 2. optical identification of, 622. perfectly formed, 2. refractive index of, measuring, 624. simple, 4. structure of (space-lattice), 323. systems of, 4. 652 Crystals continued— zoned, 459. Crystolon, 357, 404. coefficient of expansion of, 581. effect of alkalies and alkaline carbonates on, 504. of hydrofluoric acid on, 504. of metallic oxides on, 504. Cube-lattice, 323. Cubic expansion, 521. Cunningham, E., 54. Cupolas, resistance to abrasion, 125. Cuprite, space-lattice of, 324. Curie, 326. Cushman, A. S., 271. Cutter bricks, strength of, 173. Cyanite, 342, 413, 480. birefringence of, 626. crystalline form of, 4. electrical conductivity of, 609. formation of clay from, 354. hardness of, 126. magnetic properties of, 621. pleochroism of, 630. refractive index of, 623. thermal curve of, 352. Damour, 385. Dana, 216, 605. Daubrée, 260. Dauphin, 65. Davidson, Charles, & Co., Ltd., 18. Davies, H. E., 290. Day, 331, 467, 525, 606, 607. Dead-burned magnesia, 428. composition of, 401. Deccan, laterite in, 19. Decolorising effect of alumina, 102. of fluxes, 102. of lime, 102. of magnesia, 102. Decomposition, 490. double, 436. stage in burning of ceramic materials, 545. Decorating ware, 283. by colours, 96. Decoration, over-glaze, 96. under-glaze, 96. Deflocculants for clay, 285, 286. Deflocculation, 246. Deformability and plasticity, 277. explained, 139, 144. tests, 199. Deformation of fireclay bricks at high tem- peratures, 179. Degree (unit of temperature), 533. Dehydration of clays, 237. INDEX Density, 203. and dielectric strength, 610. apparent, 203, 205. effect of mode of preparation and of manufacture on, 206. of moisture on, 206. of porosity on, 72, 205. of temperature on, 205. of texture on, 205. factors influencing, 205. See also Apparent density. changes in, during burning, 548. critical, 485. See also Apparent density. vapour, and molecular weight, 304. Dental porcelain, 376, 378. Derbyshire, pocket clays of, 506. Des Cloizeaux, 412. Desvignes, E., 131, 136. Determination of. electrical conductivity, 620. resistivity, 620. hardness, 134. magnetic properties, 621. melting-point, 526. puncture voltage, 620. refractoriness, 526. softening-point, 526. texture, 44-59. Deveraux, 337. Deville, 578. furnace, 527. Devitrification, 488, 563. avoiding, 336, 389. due to overheating, 562. Devon, ball clay in, 414. china clay in, 414. Dextrin as a bond, 153. Dextro-rotation, 629. Diabase, specific heat of, 597. Diallage, 415. birefringence of, 626. Dialysis, 251. Diamond, electrical conductivity of, 609. lustre of, 95. refractive index of, 623. Diaspore, 339, 402, 427. bricks, resistant to slags, 499. constitutional formula of, 340. in clay, 421. shrinkage of, 570. Diatomaceous earth, 7. _ Diatoms, 7. Dichroic minerals, 630. Dick, A. B., 625. Diddier insulating bricks, apparent density of, 219. specific gravity of, 219. Didymia, source of, 21. INDEX 653 Dielectric strength, 608, 610. effect of heat on, 614. of lime on, 612. of porosity on, 74. of porcelain, 613. decrease of, when heated, 615. of silica glass, 617. Dietrich, 614. Diffusion, coefficient of, 517. columns, use of, 408. Diffusivity, 234, 517. effect of heat on, in magnesia bricks, 591. in silica bricks, 591. of temperature on, 587. of fireclay bricks, 587. Dimetasilicate, 332. Dimetasilicic acid, 333. Dimorphous crystals, 5. Dinas brick, specific gravity of, 596. specific heat of, 596. thermal conductivity of, 588. rock, heat treatment of, 218. sand, 20. Diopside, 415, 474. birefringence of, 626. latent heat of fusion of, 601. melting-point of, 601, 606. refractive index of, 623. specific heat of, 597. Diorthosilicic acid, 333. Direct combination, 435. Discoloration, blue, cause of, 121. brown, cause of, 121. by kiln atmosphere, 121. pyrites, 121. soot, 121. causes of, 121. effect of porosity on, 74. green, cause of, 121. of china, cause of, 121. of porcelain, cause of, 121. pink, cause of, 121. red, cause of, 121. yellow, 122. Disintegration by weathering, 506. of clays, 254, 256. Disperse phase, 11. Dispersion medium, 11. Displacement, chemical, 436. Disposing affinity, 442. Dissociation, 436, 490. heat of, 531, 599. pressure of calcium carbonate, 448. Distortion, 560. and rapid heating, 440. cause of, 377, 550. preventing, 285. Ditte, 220, 356. Dittler, 607. Doelter, 461, 487, 488. Dolomite, 402, 428. action of hydrochloric acid on, 503. water on, 503. as a cementing material, 15. birefringence of, 626. bricks, 402. strength of, 190. texture of, 43. true specific gravity of, 221. calcining, purposes of, 26. colour of, 120. crystalline form of, 4. decomposition of, 490. electrical conductivity of, 609. formation of, 506. hardness of, 126. in clay, 421. in magnesite, 427. isomorphism in, 5. refractive index of, 623. replacement of, by hematite, 5. sands, 20. sandstone, 25. space-lattice of, 324. specific gravity of, 221. structure of, 14. Domestic pottery, hardness of, 124. Dorfner, 364, 368, 376. Dorset, ball clay in, 414. Double decomposition, 436, 490. oxides, 480. refraction, 625. salts, 341. Douda, H. W., 155. Dougill, 515, 516, 587, 588, 590, 592, 593. Drain-pipes, finishing temperature for, 555. Dry clay, effect of moisture on strength of, 166. hardness of, 126. nature of, 297. strength of, 148, 170, 171. structure of, 20. water absorbed by, 75. pastes, nature of, 298. substance in slip, determination of, 226. Dryer white, 122. Drying, 294. changes in volume during, 297. effect of, on permeability, 89. on strength, 158. heat in, 543. size of grains on, 30, 296. irregular, 297. rate of, 296. effect. of porosity on, 75. shrinkage on, 295. Ductility, explained, 139, 192. 654 Dulong, 512. Dummel, K., 281. Dun, 613, 615. Dunts, 162. — caused by cooling, 559. Dupuy, M. E. L., 183. Durability and the slag test, 502. effect of texture on, 38. explained, 139. of crucibles, 582. Durham fireclays, copper-iron sulphides in, 420. effect of weathering on, 255. ganister, texture of, 40. Dust, effect of, on crystallisation, 489. resistance to, 125. sand, definition of, 35. size of grains in, 35. surface factor of, 56. Dutch clinkers, strength of, 173. Dyes, absorption of, by clay, 240. Dyne, 509. Earthenware, clays for, 374. effect of lime on, 368. finishing temperature of, 553, 555. glazes, 393. hardness of, 133.° shrinkage of, 565. strength of, 177. texture of, 40. Efflorescence, 75, 122. Elastic modulus of glasses, 188. of glazes, 192. Elasticity explained, 139, 143. Electric current produced by pressure, 611. changes due to heat, 532. charge on colloidal particles, 228. Electrical conductivity, 230, 444, 608, 609. determination of, 620. effect of porosity on, 74. of clay, 611. of clay slips, 619. of silica bricks, 616. of water, 619. insulating power of porcelain, 612. of stoneware, 612. insulator porcelain, coefficient of expansion of, 578. precipitation, 230. properties of ceramic materials, 608; see also under their various names. of clay slips, 619. of clays, 242, 608. pyrometers, 537. resistance, 608. pyrometers, 536. resistivity, 609. INDEX Electrical resistivity continued— and temperature, 611. at high temperatures, 614. determination of, 620. effect of calcium sulphate on, 619. of electric current on, 611. of fluxes on, 613. of carborundum bricks, 619. of chromite bricks, 618. of fireclay bricks, 612. of magnesia bricks, 617. of silica bricks, 616. of zirconia bricks, 618. Electricity, effect of, 444. Electro-adsorption, 230. Electro-osmosis, 230. and purification of clays, 289. of clay, 289. Electrodes, apparent density of, 221. carbon, effect of, on magnesia bricks, 499. specific gravity of, 221. Electrolytes, action of, on clay, 242. conductivity of, at different temperatures, 619. effect of adding, 231. on porosity, 68. on strength, 155. on viscosity of clay, 286. of colloids, 234. flocculating power of, 245. flocculation by, 243. precipitation by, 231. used for casting, 285. Electronic layers, 305. Electrons, 301. Electrostatic separation, 409. Elements defined, 299. Ellam, H., 112. Elutriation, 50. Elutriators, 51-53. Emery, 65, 132, 135, 157, 186, 189, 192, 280, 330, 496, 497, 498, 595, 596. Emley, W. E., 280, 293. Emulsion, colloidal, 12. Emulsoid, 12. Enamel kilns, finishing temperatures for, 555. Enamels, 383. Endell, K., 16, 179, 216, 217, 218, 568, 603. Endomorphs, 3. Endosmose, 230. Endothermal changes, 350, 520. Enfield bricks, strength of, 173. Engelhorn, F., 400, 403. Engineering bricks, finishing temperature of, 553. strength of, 173. vitrification of, 551. English Ceramic Society, 522. INDEX Engobes, adjustment of composition of, 389, 390. application of, 283. coloured, 96, 395. composition of, 382. effect of altering, 385. fusibility of, adjusting, 390. influence of constituents on, 385. shrinkage of, adjusting, 390. slips for, 283. Enstatite, 415, 459. birefringence of, 626. latent heat of fusion of, 601. melting-point of, 601, 606. refractive index of, 623. Enzymes, effect of, on plasticity, 275. Epidote, 418. action of hydrochloric acid on, 503. as a cementing material, 15. birefringence of, 626. electrical conductivity of, 609. formation of clay from, 354. magnetic properties of, 621. melting-point of, 606. pleochroism of, 630. refractive index of, 623. Equations, chemical, 316. Equilibrium dependent upon temperature, 438. diagram, 454. incomplete, 484. production of, 434. proportions necessary to produce, 451. result of disturbing, 439. stable, 434. tendency to, 439. thermal, 509. Equivalent proportions, law of, 303. Erg, 509. Erosion, effect of porosity on, 73. Erratic boulders, 18. for silica bricks, 17. Erubescite, 420. effect of weather on, 507. Eskola, P., 469, 476. Estuarine beds, refractory sands in, 20. Etching, use of, 407. Eutectic, 459. composition of, 461. point, 459, 461. Eutectics in clay-magnesia system, 481. lime-silica system, 467. potash-alumina-silica system, 480. soda-alumina-silica system, 480. Evans, 628. Ewell bricks, 373. Excessive heating, effects of, 560. Exchange, mutual, 436. 655 Exothermal changes, 520. reaction on adsorption of water by clay, 239. Expansion, 520. and insensitiveness, 580. coefficient of, effect of chemical composition on, 573. of felspar on, 573. of flint on, 573. of texture on, 576; see also Coefficient of expansion. cubic, 521. linear, 521. of ball clay, coefficient of, 573. of firebricks, coefficient of, 572. effect of porosity on, 572. of fused silica, coefficient of, 580. of glazes, coefficient of, 578. effect of oxides on, 579. of porcelain, coefficient of, 577. of tiles, coefficient, of, 573. of vitrified claywares, 573. permanent, 565. residual, of silica bricks, 568, 569. Exposure, effect of, on crushing strength of bricks, 167. prolonged, effects of, 507. to an electric current, effect of, on apparent density, 611. Extensibility, 142. of clay, 276. testing, 142, 194. Extension and plasticity, 279. Extinction angle, 627. Extrusion and plasticity, 279. critical pressure of, 274. Face-centred cube lattice, 323. Facing bricks, control of colour of, 110. hardness of, 128. strength of, 174. Factors, 510. Fahrenheit, 509. and Centigrade scales, 534, Faience glazes, 393. Farnley fireclay, coefficient of expansion of, 571. Fayalite, 333, 416, 470, 478. formation of, in silica bricks, 426, 497. fused, 487. in furnace hearths, 17. melting-point of, 366, 606. with zinc, 482. Feathered glazes, 555. Feathering caused by cooling, 559. Federov, 335. Feel of ceramic materials, 137. Feldenheimer, W., 288. 656 Felspar, 302, 416. and flint, eutectic of, 483. and spinel, separation of spinel from, 461. as a bond in silica bricks, 399. as a cementing material, 15. content and ‘“‘ Te”’ value, 610. corrosive action of, 496. crystalline form of, 4. structure of, 2. determination of, in clay, 410. dissolves clay, 483. effect of fineness of, on porosity of ware, 67. of fusion of, on specific gravity, 211. of, on coefficient of expansion, 573. of, on crazing, 387. of, on porosity, 66. of, on refractoriness, 364. electrical conductivity of, 609. eutectics with iron oxide and lime, 482. extinction angles of, 627. formation of, 440. fusing-point of, 462. in clay, 360. in glazes, 387. in zirconia ore, 429. isomorphism in, 334. -kaolin mixtures, fusion curve of, 365. particles, fusion of, 486. phase diagram of, 463. plagioclase, crystalline form of, 4; see also Plagioclase felspar. replaceability in, 334. replaced by hematite, 507. by limonite, 507. by zeolites, 507. separation of, from clay, 409. Felspathic glazes, 383. mixtures, fusion of, 486. Felted mass, production of, 551. Fenner, 216, 328, 329, 331, 332. Feret, M., 57. Feret’s triangular diagram, 58. Ferguson, R. F., 166, 330, 495, 497, 498, 503, 570. and Buddington, 482. and Merwin, 474. Ferments, effect of, on plasticity, 275. Ferrates, 332, 503. formation of, 471. Ferric compounds, discoloration caused by, 121, 122. in raw materials, 97. hydroxide in raw materials, 97. sol, effect of, on sand, 252. oxide, 303, 357. as base or acid, 322. effect of, on porosity, 68. heat of formation of, 531 INDEX Ferric oxide continwed— in analyses, 359. in clay, 366. in raw materials, 97. melting-point of, 606. sulphide a cause of colour, 97, 103. Ferriferous amphiboles and pyroxenes, elec- trical conductivity of, 609. cassiterite, electrical conductivity of, 609. Ferro-alumino-silicates, effect of, on clay, 367. -carbonyl compounds, discoloration caused by, 121. -silicates, effect of, on clay, 367. Ferrous carbonate a cause of colour, 97, 103. in clay, 366. blue colour produced by, 97. compounds, colours produced by, 97. discoloration caused by, 121. effect of distribution of particles on colour of, 105. of heat on, 492. of size of particles on colour of, 105. green colour produced by, 97. in clays, 103. in raw materials, 97. metasilicate, melting-point of, 606. oxide, 303, 357, 419. colour produced by, 103. heat of formation of, 531. in clay, 366. in raw materials, 97. melting-point of, 606. oxidation of, 492. production of, in burning, 104. reaction of, with silica, 470. phosphate a cause of colour, 97, 112, 121. sulphate, 419, 420. scum caused by, 97. water in, 423. Ferruginous cement in rocks, 15. minerals in clays, 420. Féry optical pyrometer, 538. radiation pyrometer, 539. Fibrous structures, 25. Fibrox, 404. true specific gravity of, 221. Fiebelhorn, 354. Filaments in crystals, 3. Filters, permeability of, 89. to be highly porous, 77. Findlay, 453. Findlings quartzite, 17. structure of, 18. Fine sand, definition of, 35. Fineness, effect of, on shrinkage, 565. Finishing stage of firing, 544, 549. temperatures of ceramic materials, 553. INDEX Firebricks, resistance of, to frost, 73. texture of, 38. Fireclay and magnesia bricks, interaction of, 498. and salt, effect of heat on, 352. articles, effect of sulphur on, 68. attack of, by lime compounds, 494. Fireclay bricks and spalling, 584. apparent density of, 214. as catalyst, 443. blotches of slag in, 115. burning, 556. catalytic action of, 443. changes of strength in, 149. contraction of, 566. corrosive action of flue-dust on, 496. of iron compounds on, 495. of slag on, 495. of steam on, 495. deformation of, at high temperatures, 179. diffusivity of, 587, 588. effect of exposure on, 167. of fluxes on, 149. of lime on, 149. of Portland cement on, 495. of repeated heating on, 163. of weathering on, 167. of whiting on, 495. expansion of, coefficient of, 572. firing, 556. temperature of, 558. hardness of, 128, 130. hot, resistance of, to abrasion, 131. strength of, 165. of under load, 179, 180. interaction of, with other bricks, 491. melting range of, 605. minerals in, 415. permeability of, 90. porosity of, 77, 80. refractoriness of, 602. relation of fusibility and composition, 149. resistance of, to abrasion, 130. softening of, at high temperatures, 165. specific gravity of, 588. heat of, 588, 595, 596, 597. strength of, 178. susceptibility of, to sudden changes of tem- perature, 582. thermal conductivity of, 585, 587, 588, 593. effect of burning temperature on, 585. resistivity of, 594. transverse strength of, at high temperatures, 181. Fireclay cement, refractoriness of, 602. mortar, refractoriness of, 602. tiles, transverse strength of, 182. 657 Fireclays, 343, 414. bauxitic, 421. burned, colour of, 114. casting, 284. chalybite in, 420. colours of, 108. composition of, 373. copper-iron sulphides in, 420. effect of heat on apparent specific gravity of, 208. on porosity of, 68, 70. of rate of heating on, 526. felspars in, 417. iron sulphide in, 419. magnetic properties of, 621. Northumbrian, barytes in, 15. plastic, apparent density of, 212. dry, strength of, 171, 172. refractoriness of, 602. size of grains in, 34. specific gravity of, 213. vitrification range of, 553. water required to develop plasticity of, 269. Fired clay, minerals in, 415; see also Burned clay. Fireproof floors, porosity of, 77. Firesand, 431. colour of, 119. Fire-shrinkage, 565. Firestones, structure of, 19. thermal resistivity of, 594. Firing clays, oxidising atmosphere in, 493. duration of, 162. effect of carbonaceous matter on, 370. of duration of, on strength, 162. of heat in, 544. of, on colour, 105, 110. of rate of, 161. manner and duration of, 632. temperature and crushing strength, 161. effect of, on crushing strength of clay, 161. on modulus of rupture of silica bricks, 162. See also Burning. Firth, E. M., 70, 71, 206, 207, 210, 213. Fischer, M. H., 286. Fissile structure, 22. Fissility of clays, 137. Five Quarter fireclay seam, effect of weathering on, 255. Flaky clays, 22. effect of, 61. particles, 27. structure, 22. Flaming, 118. Flashing, 106, 114, 118. Flat particles, 27. 42 658 Fletton bricks, strength of, 173. Flexibility explained, 139, 143. Flint, 424. bricks, specific gravity of, 219. calcining, purpose of, 26. clays, apparent density of, 212. water required to develop plasticity of, 269. coefficient of expansion of, 579. colloidal nature of, 7. colour of, 119. effect of, on alkaline solutions, 505. on coefficient of expansion of porcelain, 573, 575. repeated heating on, 218. formation of, 506. nodular, 25. recognition of, in bricks, etc., 415. soluble in felspar, 483. specific gravity changes in, 217. specific gravity of, 216, 217. use of, in silica bricks, 40. Flocculation, effect of, on plasticity, 246. of clay, 243. after purification, 289. rate of, 243. Floor tiles, coefficient of expansion of, 573. hardness of, 129. measuring strength of, 199. resistance of, to abrasion, 130. to traffic, 125. Floors, fireproof, porosity of, 77. porous, 77. Flour as a bond, 153. Flow and plasticity, 273. mass, 274. surface, 274. Flower pots to be highly porous, 77. Flowing colours, 118. Flue-dust, action of, on firebricks, 330, 496. effect of, on strength, 169. Fluidity, 290. and pressure, 274. and water content, 273. limit of, 268. pressure of, 126. Fluorides as catalysts, 443. Fluorine as catalyst, 443. vapours, action of, on alumina, 443. Fluorite, electrical conductivity of, 609. refractive index of, 623. Fluorspar, effect of, on shrinkage of earthen- ware, 567. on terra-cotta, 148. on zirconia bricks, 500. electrical conductivity of, 609. replacement of, by hematite, 5. space-lattice of, 324. INDEX Fluxes, 362. affinity of, for alumina, 315. for silica, 315. and corrosion, 494. basic, effect of, on silica bricks, 497. chemical activity of, 445. combination of, 436. decolorising effect of, 102. effect of, on claysandrefractory materials,149. on electrical resistivity, 613. on porosity, 66. on silica bricks, 498. on strength, 147, 160. on terra-cotta, 148. on translucency, 632. in brick clays, 374. in glazes, 386. Maximum effect of, 478. Fluxing, cause of, 481. power of glaze fluxes, 386. production of, by reduction, 493. oxides, use of RO for, 307. Foliated structure, 23. Fontainebleau sand, sand-calcites in, 15. Foote, 560, 605. Formula weight, 305. Formule, 306. chemical, 306. molecular, 309. objections to, 314. rational, 308. structural, 308. Forsterite, 333, 416, 468, 478, 481. melting-point of, 607. Fossilised animal excreta, 367. Foster, H. D., 65, 82, 196. Fourier’s Law, 517. Foussereau, 614. Fox, 385, 389, 395. Foxwell, 332. Frankenheim, 323. Free silica, occurrence of, 424. Freedom, degree of, definition of, 453. Freezing, effect of, 168. tests, 201. Freundlich, 231, 236, 246. Friability, 138, 142. Frit, defined, 116. kilns, porosity of bricks for, 78. Fritted glazes, 385. Fritting, 387. glaze materials, precautions in, 388. Frost, effect of, on clay, 254. on crushing strength, 167. on firebricks, etc., 73. on glazed ware, 73. on steamed bricks, 74. on vitrified articles, 73. INDEX ** Fuel ”’ for gas fires, 25. Full-fire stage in burning ceramic ware, 548. Fuller, 286. Fuller’s earth, 412. Fulton, 270, 478. Furnace gases, resistance to, 125. hearths, cristobalite in, 17. fayalite in, 17. shape of grains in, 29. texture of, 42. tridymite in, 17. linings, 433. interaction of, with contents, 491. sand for, 21, 42. Furnaces, porosity of bricks for, 78. siliceous hearths in, 17. Fused alumina, 403. changes in volume of, 570. effect of, on electrical resistivity, 612. bauxite, coefficient of expansion of, 581. magnesia, hardness of, 132. masses, constitution of, 486. material, cooling of, 468. effect of temperature on amount of, 485. solvent action of, 442. matter, effect of, on shrinkage, 567. mixtures, changes in, on cooling, 457. quartz, 400, 484. ribbon-like crystallisation of, 327. specific gravity of, 220. thermal conductivity of, 592. See also Fused silica. silica, 400. . coefficient of expansion of, 580. effect of repeatedly heating, 563. electrical insulating properties of, 616. glass, formation of, 331. hardness of, 132. increasing strength of, 150. melting-point of, 603. permeability of, 89. thermal conductivity of, 591. Fusibility explained, 139. of engobes, adjusting, 390. of glazes, increasing, 390. order of, 485. Fusible metals and salts as pyroscopes, 541. Fusing-point, 484. temperature of refractory materials and articles, 558. Fusion, 484. curve of felspar-kaolin mixtures, 365. of mica-kaolin mixtures, 365. curves, 463. effect of duration of heat on, 485. on alumino-silicates, 322. 659 Fusion continued— effect of duration of heat on silicates, 336. on specific gravity, 211. heat, 598. first signs of, 527. latent heat of, 600. order of changes leading to, 485. partial, 485. point, 525. effect of bases on, 387. of glazes, lowered by boric oxide, 389. range, 486, 561. rate of, 486. superficial, 423. Gadolinite, 488. Gahnite, 356, 482. Galena, melting-point of, 607. Ganister, 17, 425, 472. bastard, Scottish, texture of, 40. bricks, 17. after-expansion of, 569. American, 186. Chwarele, texture of, 40. coefficient of expansion of, 579, 580. colour of, 119. composition of, 400. Durham, texture of, 40. North Wales, texture of, 40. Yorkshire, texture of, 40. Sheffield, texture of, 40. South Yorkshire, silification of, 19. texture of, 40. Garnet, electrical conductivity of, 609. formation of, 440. of clay from, 354. magnetic properties of, 621. refractive index of, 623. Gary, M., 179. Gas thermometers, 536. Gaseous bubbles in crystals, 3. Gases in furnaces, resistance to, 125. Gasification, latent heat of, 524. Gault bricks, strength of, 173. clays, colour of, after burning, 112. coprolites in, 422. Gautier, 504. Gebauer, 166. Gehlenite, 479, 482. melting range of, 607. replacement in, 335. Gelatin, effect of, on plasticity, 275. Gelatinous silica, solubility of, in hydrofluoric acid, 504. Geller, P. F., 182, 241, 337. Gels, colloidal, 12. water in, 12; see also Colloidal gels. 660 German Admiralty, 602, 603. silica bricks, specific gravity of, 219. Geyserite, 253, 424. effect of repeated heating on, 218. Gibbs, W., 453. Gibbsite, 339, 427. constitutional formula of, 340. shrinkage of, 570. Gilchrist, 613, 632. Gilles, S. W., 214, 590. Gillet, 493. Ginsberg, 607. Gioberite, 427. Glamorganshire, Dinas sand in, 20. Glasgow fireclay, coefficient of expansion of, 571. Glass a solid solution, 336. annealing, 559. coefficient of expansion of, 576. colours, finishing temperature of, 555. cooling, 559. corrosive action of, 496. crystallisation of, 355. devitrification of, 355. electrical resistivity of, 617. formation of, during vitrification, 552. hardness of, 132. modulus of elasticity of, 188. pot mixtures, coefficient of expansion of, 571. pots, 183. burning, 557. casting, 284. firing temperature of, 558. ideal structure of, 17. porosity of, 78. refractoriness of, 604. resistance of, to sudden changes of tem- _ perature, 582. texture of, 39. thermal conductivity of, 593. resistivity of, 594. production of, 485. quartz, 6; see also Silica glass. specific heat of, 597. tank blocks, porosity of, 78. texture of, 39. Glasses, 487. chemical constitution of, 354. tendency of, to crystallise, 489. Glasson, J. L., 325. Glassy bonds, 154. cements, 16. lustre, 95. material, action of, in burning, 558. matrix in bricks, 16. in rocks, 16. matter and translucency, 633. in ceramic materials, 485. INDEX Glassy bonds continued— state, see Vitreous state. substances, 6. use of, in ceramic materials, 6. Glauconite, 416. Glaucophane, 342, 415. refractive index of, 623. Glaze fluxes, power of, 386. formation of, in coke ovens, 87. slip, absorption of, by clay, 239. Glazed bricks, finishing temperature for, 555. ware, firing, 554. permeability of, 90. resistance of, to abrasion, 133. Glazes, 383. adjustment of composition of, 389. alumina-silica ratio in, 386. as solid solutions, 336. aventurine, 398. avoiding crystallisation in, 336. boiling of, 561. Bristol, 392. chemical constitution of, 354. clarifying, 385. coefficient of expansion of, 578. colour of, altering, 390, 391. coloured, 395. oxides for, 397. colouring oxides in, 389. composition and fusion-point of, 384. crystalline, 355, 397. matter in, increasing, 390. crystallisation of, 355. devitrification of, 355. dulling of, 561, 562. earthenware, 393. effect of altering composition of, 385. of oxides on coefficient of expansion of, 579. elastic modulus of, 192. excessive heating of, 562. faience, 393. feathered, 555. for various kinds of ware, 391. fritting, precautions in, 388. fused in burning, 548. fusibility of, increasing, 390. fusion point of, lowered by boric oxide, 389. hardness of, 133. influence of constituents on, 385. lead v. leadless, 388. matte, 355, 396. nature of, 6. opalescent, 395. pink, production of, 117, 395. poisonous, 388, porcelain, 393. proportion soluble in acid, 388. INDEX Glazes continued— salt, 391. sanitary ware, 392. shrinkage of, altering, 390, 391. slips for, 283. soda in, effect of, 398. soluble, salts in, 387. stoneware, 392. strength of, 192. tendency of, to crystallise, 489. tensile strength of, 140. terra-cotta, 392. typical, 391. use of alumina in, 385. of clay in, 386. of fluxes in, 386. of silica in, 385. zeolites in, 389. Glenboig clay, halloysite in, 412. firebricks, resistance of, to sudden changes of temperature, 582. Globulites, 3. Glossiness, cause of, 622. Glost-firing, 554. ware, colour of, 112. Glue as a bond, 153. Glycerin, as bond for carbides, 405. Goecke, 605. Goerens, P., 214, 516, 590. Gold alloys, melting-point of, 541. melting-point of, 541. metallic, in aventurine glazes, 398. Goodwin, 605, 614. Gosrow, 133. Gothite, 419. Gouy, 232. Grading, 31. effect of, on cracking, 32. on permeability, 31. on porosity, 31, 32. on spalling, 32. on strength, 152. on texture, 31, 32. on voids, 32. silica bricks, 41. Graham, Thomas, 10, 11, Grain-size, effect of, on crushing strength, 151. on inversion of silica, 330. Grains, angular, 27. rounded, 27. — shapes of, 27. size of, 29. Granger, A., 43. Granite, components of, 454. electrical conductivity of, 609. Grant, 280. Granular structure, 15. types of, 16. 661 Graphic formule, 308. Graphite, Alabama, 23. bricks, permeability of, 90. porosity of, 80. structure of, 17. thermal conductivity of, 592, 593. effect of firing temperature on, 585. Ceylon, 23. colour of, 119. crucibles, burning, 558. effect of, on corrosion, 371. for crucibles, 404. hardness of, 126. Madagascar, 23. occurrence of, 430. Pennsylvanian, 23. slabs, texture of, 43. specific gravity of, 221. structure of, 7, 8, 23. thermal conductivity of, 592. resistivity of, 594. use of, 430. Grease adsorbed by clay, 239. Greasy feel of ceramic materials, 137. Grecian magnesite, colour of, 120. Green, A. T., 515, 517, 587, 589, 590, 591. Green, 8. A., 57, 244, 247, 272. colour in burned clays, 369. produced by iron compounds, 97. producing, 116. discoloration, cause of, 121. glaze, 395. Greening & Son, N., 45. Greensand beds, phosphatic nodules in, 422. Greiger, C. F., 184. Griffiths, 516. Grey articles, producing, 116. colours produced by iron compounds, 97. scum, 123. Grinding, alteration of structure by, 26. effect of, on strength, 154. mills, shape of grains produced by, 29, Grog, 18. bricks, apparent density of, 214. refractoriness of, 603. specific gravity, 214, 596. heat, 596. strength of, 178. structure of, 18. - thermal conductivity of, 588, 590. casting, 284. constitution of, 348. effect of size of, on strength, 151. firing temperature of, 558. size of, for retorts, 39. texture of, 36, 37. use of, for increasing porosity, 66. Gross, 253. 662 INDEX Grossalmerode fireclay, specific gravity of, 213: Grossular, melting range of, 607. Groth, 309, 348. Grout, F. F., 221, 262. Growth of crystals, cause of, 324. Grum Grzmailo, 329, 606. Griinerite, 470. Guest, 325. Guldberg, 448. Gum, effect of, on plasticity, 275. Gurther, 606. Guy, J. P., 249. Gypsum, 367, 369, 421. as a cementing material, 15. decomposition of, by heat, 545. effect of weather on, 507. electrical conductivity of, 609. occurrence of, 428. plates, 628. refractive index of, 623. replacement of, by quartz, 5. Hematite, 418. as a cementing material, 15. concretionary, 25. corrosive action of, 496. crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. magnetic properties of, 621. red colour due to, 97, refractive index of, 623. replacement of, by quartz, 5. space-lattice of, 324. Halifax fireclay, specific gravity of, 213. Hall, A. D., 245. Halle porcelain, 375. Halloysite, 20, 345, 412. action of hydrochloric acid on, 503. drying, 337. refractive index of, 623. structure of, 7. thermal curve of, 352. water in, 423. Hampshire, red-burning clays of, 109. Hansen, J. E., 310. Hard bricks, resistance of, to frost, 169. -burned ware, true specific gravity of, 215. volume-weight of, 215. -paste porcelain, 375. porcelain, apparent density of, 214. wares, hardness of, 127. Hardness, Brinell’s ball test for, 135. changes in, during burning, 547. in during smoking, 545. determination of, 124, 134. Hardness continwed— Mohs’ scale of, 124. of ceramic materials and articles, see under their various names. Shore scleroscope for testing, 136. Hardy, 245. Harker, 461. Harmotine, 342. Hartmann, M. L., 130, 131, 132, 134, 181, 583, 584, 612, 616, 617, 618, 619. Harvard, 80, 186, 492, 593. Hatch, 507. Hauerite, space-lattice of, 324. Hauschofer, 348. Haiiy, 323. Hauyne, formation of clay from, 354. Heat, 508. absorption of, 519. and optical properties, 532. atomic, 512. calculations, 510. capacity, 510. of ceramic materials, 594. changes effected by, 519. in chemical composition, effected by, 528. conductivity, 514. decomposition of clay by, 438. dissociation of materials by, 436. effect of,in burning orfiring ceramic materials, 544. in drying, 543. on alumino-silicates, 322. on amorphous materials, 8. on ceramic materials, 508, 543. on chemical reactions, 437. on china clay-felspar mixtures, 66. molecule, 347. on clay, 348, 350, 351. on colour, 100, 105. on crystalline materials, 8. on iron oxide, 357. on limestone, 440. on permeability, 88. on porosity, 66. on quartz, 330. on refractoriness, 601. on shape of grains, 407. on souring pottery mixtures, 275. on specific gravity, 211. of silica bricks, 217. heat, 594. on thermal conductivity, 584. on volume of a material, 563, 564. withdrawing, 559. electrical changes caused by, 532. evolution of, 528. evolved when clay is wetted, 239. general effects of, 519. INDEX Heat continwed— latent, 523. molal, 524. of fusion, 523. of vaporisation, 524. loss of, through furnace walls, 518. measurement, 509. of combination, 528, 529. of crystallisation, 528. of dissociation, 531, 599. of formation and mode of formation, 531. of silicates, 599. of various oxides, 531. of neutralisation, 530. of reaction, 530. physical changes effected by, 528. quantity of, 510. rate of passage through ceramic materials, 584, 585. recorders, 540. relation of, to temperature, 508. sensible, 520, 523. transmission of, 514. treatment and dielectric strength, 610. Heath, A., 57, 66, 83, 85. Heating curve of china clay, 349. curves v. cooling curves, 466. diagrams, 465. duration of, effect of, on fusion, 485. on specific gravity, 209. effect of, on composition of products, 489. excessive, 560. phase conditions in, 466. prolonged, effect of, 439, 550, 562. on melting-point, 526. on silica bricks, 497. rapid, results of, 440, 563. rate of, effect of, on melting-point, 526. repeated, 163. effect of, 563. on specific gravity of siliceous materials, 218. schedule for crushing test under load, 197. substances, reasons for, 445. too rapid, 371, 492. Heats of reaction in ceramic processes, 599. obscured, 531. of solution, 529, 599. of transition of silicates, 600. Heavy liquids, use of, 408. Hedvall, 468, 498. Heinecke’s porcelain, 378. Helmholtz, 229. Hempel, 605. Henderson, 615. Henning, F., 580, 581. Hercynite, 471. Hering, C., 593, 594. 663 Herman, 264, 606, 607. Hermodorfer hard porcelain, strength of, 178. Hertwig-M6hrenbach, 376. Herzfield, 260. Hess’s law, 531. Heyn, 518, 588, 590, 592, 596. Hill, E. C., 148. C. W., 123. Hilpert, 606. Hind, S. R., 284, 287. Hirsch, 495. Hodkin, F. W., 71, 206, 207, 213. Hodsman, 515, 516, 522, 587, 588, 590, 592, 593. Hohenbocke sand, heat treatment of, 218. Holborn, 578, 581. -Kurlbaum pyrometer, 538. Holdcroft, 309, 337, 348, 350, 351, 595, 596, 598. Holdcroft’s thermoscopes, 540. Holland, 19. Hollow blocks, porosity of, 77. bricks, strength of, 150. refractory ware, strength of, 183. Homogeneity, 59. essential, 21. Homogeneous mixtures, preparation of, 283. structures, 21. Honaman, R. K., 615. Honda, 357. Honey, effect of, in souring pottery mixtures, 275. Hongen, O. A., 131, 150, 164, 401, 468, 583, 584. Hope, H., 377, 567. Hopkins, 384. Hornblende, 415. as a cementing material, 15. birefringence of, 626. crystalline form of, 4. magnetic properties of, 621. melting range of, 607. pleochroism of, 630. refractive index of, 623. Horning, R. H., 586. Hornung, 516. Hostetter, J. C., 357. Houldsworth, 329, 339, 340, 352, 571, 572, 579. Hovestadt, 385. Howarth, H. F., 260, 608, 613, 614. Howat, W. L., 145, 171, 288. Howe, R. M., 79, 156, 161, 166, 187, 189, 217, 495, 497, 498, 499, 502, 503, 568, 570. Hromatko, J. S., 178, 198. Hruda, T., 123, 369. Hull, 424. Humber silt, colour of bricks from, 110. Humus, 232. Hursh, R. H., 634. Hutchings, 15. 664 Hyalite, 253. Hyalophane, 417. Hydrargillite, 339. Hydrate defined, 319. Hydrated silica, 363. effect of repeated heating on, 218. Hydration, 507. Hydraulic bonds, 154. properties of silicates, 503. Hydrochloric acid, action of, on calcareous and ferruginous substances, 503. heat of solution of, 529. Hydrofluoric acid, effect of, 504. Hydrogel, 12. Hydrogen sulphide, effect of clay on, 238. Hydrolysis, 507. an endothermal reaction, 520. and mass action, 448. of clay, 236, 263. of silicates, 507. Hydromagnesite, colour of, 120. hardness of, 126. structure of, 8. true specific gravity of, 220. Hydromica, 412. Hydrosol, 12. Hydrous minerals, decomposition of by heat, 545. silicates, 336. Hydroxide defined, 319. Hydroxyl group, 319. Hygroscopicity of clays, 237, 298. Hypersthene, 415. birefringence of, 626. magnetic properties of, 621. pleochroism of, 630. refractive index, 623. Hysteresis of silica gel, 252. Ice, effect of, 254. Iceland, geyserite in, 422. Iddings, 315, 413, 486. Identification of ceramic materials by optical properties, 622. Idocrase, birefringence of, 626. Ilmenite, 429. crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. in clay, 421. magnetic properties of, 621. Impact strength, determination of, 198. tests, 198. Impermeability, 87. and finishing temperature, 558. of chemical ware, 90. of crucibles, 90. INDEX Impermeability continued— of glazed ware, 90. of retorts, 90. of stoneware, 90. Impermeable ware, 76. Impurities and refractoriness, 362. carbonaceous, as colouring agents, 106. colour caused by, 96. -producing, 96. conversion of, 491. dissociated by heat, 436. effect of, on blue colour, 105. on melting-point, 485, 526. on specific gravity, 209. in aluminous minerals, 427. in carbides and carboxides, 431. in ceramic materials, 362. electrical conductivity of, 610. in chromite, 430. in clays, 362, 414-423. oxidation of, 492. in graphite, 430. in kieselguhr, 426. in magnesite, 427. in silica rocks, 426. in tripoli, 426. in zirconium minerals, 429. removal of, from clay, 287. volatilisation of, 491. Incandescence, 519. Incipient crystals, 3. Inclusions in crystals, 3. Incomplete equilibrium, 484. reactions, 449. Indentation, resistance to, 124. tests, 135. Indigo, adsorption of, by clay, 240. Inductive capacity of silica glass, 617. Indurated clays, 237, 370. Ingersoll, L. R., 587. Insensitiveness and expansion, 580. Institute of Gas Engineers, 78, 186, 195, 196, 373, 400, 527, 557, 566, 568, 602, 603, 604. Mining and Metallurgy, 45. I.M.M. standard sieves, 45, 46. Insulating bricks, thermal conductivity of at various temperatures, 586. power, electrical, and temperature, 611. of porcelain, 612. of stoneware, 612. thermal, and porosity, 515. and texture, 515. Insulators, electrical, 608. resistance of, at high temperatures, 616. “Te” value of, 615. Interference, 630. Interlocking grains, effect of shape on, 28. INDEX Intimacy of association, 445. Inulin, effect of, on plasticity, 275. Ion, common, effect of, on eutectic composition, 461. defined, 306, 317. Ionisation theory, 444. Ionised solutions, 530. Tridescence, 94. Tron as a catalyst, 443. -bearing minerals, 418. carbide, effect of on colour of clay, 104. carbonate (ferrous) a cause of colour, 97. as a cementing material, 15. carbonyl, effect of, on colour of clay, 104. chromate, effect of, on colour, 117. compounds, colour of, effect of firing on, 100. produced by, 97, 118. corrosive, 495. decomposition of, by heat, 491, 546. destruction of colour due to, 101, 102. effect of carbonaceous matter on, 370. on colour, 98, 103. compounds, hydrolysis of, 507. in aluminous minerals, 421. in burned ware, 98. in clays, 366, 418. in fireclays, 373. in magnesite, 427, 428. in raw materials, 97. in silica bricks, 426. oxidation of, 491, 492. produced by burning, 100, 103, 104, 105. reduction of, 104. size and distribution of particles of, 105. See also Ferric compounds and Ferrous compounds. hydrates a cause of colour, 97. in fireclays, form of occurrence of, 103. -magnesium-mica system, 417. metallic, in aventurine glazes, 398. oxide-alumina system, 471. effect of, on strength, 160. -lime system, 471. -magnesia-silica system, 478. system, 474. magnetic, in raw materials, 97. -silica systems, 470. oxides, action of hydrochloric acid on, 503. as causes of colour, 97. as cementing materials, 15. concretionary, 25. constitution of, 357. effect of, on colour of magnesia, 120. of heat on, 357. on silica bricks, 497. on zirconia bricks, 500. eutectics with felspar, 482. in analyses, 359. 665 Tron oxides continwed— in aventurine glazes, 398. in crystalline glazes, 397, 398. in magnesite, 401. in zirconia ore, 429. reduction of, in burning, 104. resistance of, to basic open-hearth slag, 498. state of, in clays, 359. penetrative power of, 497. phosphates, colour produced by, 97, 112. pyrites, electrical conductivity of, 609. removal of, by magnets, 621. salts, artificial colouration by, 99. silicates, effect of, on colour, 116, 117. spinel, 400. sulphide (ferric), a cause of colour, 97. sulphides, 419. action of, 495. as cementing materials, 15. effect of weather on, 507. on zirconia bricks, 500. Irreversible colloids, 235, 253. reactions, 450. volume-changes, 520. Isle of Rum, ultrabasic rocks in, 461. Tso-electric point, 231. Isomeric, 302. Isomorphism, 5, 302, 334. ~ Isomorphous, 302. elements, 335. minerals, 5. mixture, 334, 341. formule of, 335. silicates, 334. Isotopes, 299. Isotropic crystals, 627. minerals, 406. Ivory ball clay, 108. Jachum, P., 281. Jackson, W., 56, 57. Jaeger, 469, 470. Jameson, A. B., 354. Japanese porcelain, 376. glazes for, 394. Jaune de cuisson, 121. Jelly, nature of, 12. Jena glass, modulus of elasticity of, 188. Johnson, 260, 265. Joints of brickwork, thermal conductivity of, . 593. Joly, 598. Jones, J. C., 69, 168. Joule, 300, 509. Kallauner, O., 123, 369. Kanolt, C., 527, 604, 605. 666 Kaolin, 345. adsorption of alkali by, 241. birefringence of, 626. burned, colour of, 109. casting, 284. clay-substance from, 344. coefficient of expansion of, 571. colour of, 107. composition of, 372. crushing strength of, 187. dehydration of, 599. dry plastic, strength of, 171, 172. primary, strength of, 171, 172. effect of Cornish stone on porosity of, 67. of heat on porosity of, 71. of magnesite on, 368. of vanadium on, 369. melting range of, 605. organic matter in, 422. size of grains in, 33. specific gravity of, 210. heat of, 595. variable behaviour of water in, 338. vitrification range of, 553. water required to develop plasticity of, 269. See also China clay. Kaolinite, 412. crystalline form of, 4. effect of heat on, 350. graphic formule for, 346. hardness of, 126. in china clay, 20. refractive index of, 623. water in, 423. Kataphoresis, 229, 249. Keane, L. A., 101. Keele, J., 67. Keeler, 386. Keppeler, 339. Kerl, B., 76. Kerr, W. R., 156, 161, 217, 568. Kieselguhr, apparent density of, 220. bricks, strength of, 188. structure of, 18. thermal conductivity of, 591, 592. resistivity of, 594. carbonaceous matter in, 430. colour of, 119. green, 119. permeability of, 90. porosity of, 79, 80. specific gravity of, 216. structure of, 7. thermal conductivity of, 585, 586, 593. at various temperatures, 586. resistivity of, 594. use of, for increasing porosity, 66. INDEX Kiln atmosphere, a cause of discoloration, 121. effect of, on colour, 118. contraction, 570. gases, a cause of scum, 122. gases, effect of, 493. white, 122. Kilns, atmosphere in, 162. cooling, 162. porosity of bricks for, 78. resistance of, to abrasion, 125. Kilwinning aluminous shale, specific gravity of, 213. fireclay, specific gravity of, 213. Kinney, 57. Kinnison, 619. Klein, A. A., 488. Klinefelter, 613, 632. Knett, 352. Knollman, H. J., 151, 214, 215. Knote, J. M., 212, 213, 350, 351, 595. Kobler, F. E., 130, 132, 134, 181. Koerner, 263, 397. Kohl, H, 248. Kohlmeyer, 606. Kolk, 624. Kolthoff, I. M., 230, 242. Kopp, 514. Kowalke, O. L., 150, 164, 401, 468. Krehbiel, 397, 398. Krehbiel’s elutriator, 52. Kyropoulos, 329. Labradorite, 417. birefringence of, 626. effect of fusion on specific gravity of, 211. fused, 487. melting-point of, 607. refractive index of, 623. Laclede-Christy bond clay, 242. Lacroix, 331. Ladd, 280. Laevo-rotation, 629. Laird, J. 8., 177, 337. Lamb, 229. Laminated structure, 22. nature and cause of, 22. Lamination, effect of size of grains on, 31. in crucibles, 23. in finished goods, 22. in graphite, 23. in raw materials, 22. in retorts, cause of, 22. in saggers, cause of, 23. Lampen, 493, 605. Lancashire red-burning clays, 109. Landolt, 594. Landrieu, P., 324. Lange, O., 16. INDEX Langenbeck, 281, 409. Langmuir, 305, 324. Lanthanum oxide, melting-point of, 605. source of, 21. Laschtschenko, 598. Latent heat, 523. of fusion, 523, 600. of gasification, 524. of vaporisation, 524. Laterite, 427. colour of, 97. in clay, 421. nodular structure of, 25. structure of, 19. Lattice-structure of compounds, 323. Laumonite, 342. Lauschke, G., 79. Law of mass action, 446, 449, Lawrence, 633. Laws of chemical action, 432. Lead Commission, results of, 388. compounds in glazes, 388. poisonous nature of, 388. glazes, 383. blackening of, 555. effect of alumina on, 385. oxide, effect of, on expansion of glazes, 579. on silica glass, 498. soluble in glazes, 388. Leadless v. lead glazes, 388. Lebedew, P., 607. Le Chatelier, F., 144. H., 135, 144, 145, 186, 187, 189, 213, 225, 260, 329, 331, 356, 439, 538, 580. Lechatelierite, 424. Lees, 589. Leicestershire red bricks, strength of, 173. red-burning clays of, 109. red marls, 109. Leighton Buzzard sand, texture of, 42. Lenzenite, 412. Lepidolite, 342, 417. melting range of, 607. refractive index of, 623. Leucite, 342, 480. action of hydrochloric acid on, 503. formation of, 440. formation of clay from, 354. latent heat of fusion of, 601. melting-point of, 601. melting range of, 607. refractive index of, 623. Leucoxene, 429. Lewis, W. H., 237. L’ Hermite, 250. Lias clay bricks, thermal conductivity of, 590. true specific gravity of, 214. Libman, 470. 667 Lickey quartzite, silicification of, 19, Lienau, 427. Liesbach, 488. Liesgang, 24. rings, 24. Light, absorption of, 622. bricks, carbonaceous matter in, 65. clay, porosity of, 80. effect of, on chemical reactions, 444. reflection of, 622. -weight silica bricks, thermal conductivity of, 590. effect of, on clays, 507. Lime, allotrophic forms of, 357. -alumina-silica system, 478. -alumina system, 470. and alumina, interaction of, 470. and silica, interaction of, 434. as a bond, 154, 399, 405. -barium oxide-silica system, 474. bricks, strength of, 192. structure of, 17. burning, 490. effect of vapour-pressure on, 441. colour of, 120. compounds as impurities in clay, 428. in brick clays, 374. mineralogical nature of, 428. constitution of, 357. corrosive action of, 496. crystalline, 357. decolorising effect of, 102. effect of, on bauxite bricks, 499. on clay, 495. on earthenware body, 567. on electrical resistivity, 612. on expansion of glazes, 597. on firebricks, 149. on porosity, 66, 68. on strength, 147. on silica glass, 498. water on, 503. eutectics with felspar, 482. felspars, 367. firing temperature of, 558. formation of, 367. fusing-point of, 467, 600. glazes, 385. heat evolved in slaking, 529. heat of formation of, 531. in clays, 367. effect of, 368. in glazes, 387. in phase diagram, 475. in porcelain, 377. -iron oxide system, 471. kiln, effect of vapour-pressure in, 441. latent heat of fusion of, 600. 668 Lime continued— -lithia-silica system, 476. -magnesia-silica system, 474. melting-point of, 605. molecular heat of, 598. olivine, 467. production of, 434. -silica system, 367, 467. -soda-silica system, 476. specific heat of, 598. -strontia-silica system, 476. Limestone, 367, 402. calcining purpose of, 26. Limestones, concretionary, 25. crystalline, structure of, 14. decomposition of, by heat, 433, 440, 545. effect of carbonated water on, 504. of percolating water on, 506. occurrence of, 428. oolitic, 25. structure of, 7. Limonite, 358, 359, 418. a result of hydrolysis, 507. as a cementing material, 15. brown colour due to, 97. colloidal, 419. concretionary, 25. decomposition of, by heat, 545. magnetic properties of, 621. melting-point of, 607. water in, 423. Linbarger, S. C., 184. Lindstrom, R. L., 41. Linear expansion, 521. Liquid bubbles in crystals, 3. to solid state, passage from, 487. Liquids and gases, reactions between, 435. effect of, on physico-chemical reactions, 445. specific gravity of, determining, 226. Litharge, effect of, on zirconia bricks, 500. Lithia-barium oxide-silica system, 477. -lime-silica system, 476. -magnesia-silica system, 477. -potash-silica system, 477. -soda-silica system, 476. -strontia-silica system, 478. Lithium biotite, 417. metasilicate, 326. -potassium mica, 417. silicate, 477. Lithomarge, 20, 413. structure of, 7. Little, 477. Loams, size of grains in, 34. Lomas, J., 44. London clay, 267, 273. selenite in, 421. . grey stocks, crushing strength of, 173. INDEX Loomis, G. A., 166, 566. Losev, 236. Loss of strength at high temperatures, 165. on ignition, 362. Lovejoy, C. H., 179. Lowenstein, E., 337. Lower Greensand, refractory sands in, 20. Lowry’s elutriator, 52, 53. Ludwig, 526. Ludwig’s chart, 364, 382. volumeter, 84. Lundy, 219. Lustre, 95. Lustres, finishing temperatures for, 555. Lutecite, 253. M‘Dowell, J. S., 14, 63, 79, 189, 218, 498, 499. M‘Gee, E., 188. M‘Leod, 616, 617, 618. M‘Mahon, 616, 617, 618. Mackler, 391, 398. Madagascar graphite, 23. Magnesia, allotropic forms of, 355. -alumina-silica system, 481. system, 471. amorphous, 356. and fireclay bricks, interaction of, 498. apparent density of, 219. as a bond in silica bricks, 399. as a flux, 486. as an opacifier, 395. bricks, 401. and spalling, 584. coefficient of expansion of, 581. diffusivity of, 588. effect of, burning temperature on, 591. of carbides on, 498. of carbon electrodes on, 499. of carbon on, 498. of conductivity of, 591. of silica on, 498. of steam on, 400, 499. electrical resistivity of, 617. Eubeean, crushing strength of, 187. firing, 556. temperature of, 558. heat diffusivity of, 591. hot, resistance of, to abrasion, 131. strength of, 165. under load, 179. melting-point of, 605. permeability of, 90. porosity of, 79, 80. practically insoluble, 503. refractoriness of, 604. resistance of, to abrasion, 130, 132. to basic slag, 498. to sudden changes in temperature, 583. INDEX 669 Magnesia bricks continwed— shrinkage of, 570. spalling of, 583. specific gravity of, 221, 588, 596. heat of, 588, 596. strength of, 160, 189. Styrian, crushing strength of, 187. texture of, 42, 43. thermal conductivity of, 586, 588, 591, 592, 593. effect of burning temperature on, 585. thermal resistivity of, 594. transverse length at high temperatures, 181. calcined, properties of, 428. thermal resistivity of, 594. chemical constitution of, 355. colloidal, 253. crystalline, 356. dead-burned, properties of, 428. dead-burning, 556. decolorising effect of, 102. distinction of, from periclase, 625. effect of iron oxide on, 498. effect of, on electrical resistance, 612. on porosity, 66, 68. on terra-cotta, 148. on vitrification range, 553. water on, 503. fused, hardness of, 132. thermal conductivity of, 592. fusing-point of, 467, 600. heat of formation of, 531. in glazes, 387. in phase diagrams, 475. in porcelain, 377. -iron-oxide-silica system, 478. system, 474. latent heat of fusion of, 600. -lime-silica system, 474. -lithia-silica system, 477. melting-point of, 605. molecular heat of, 598. polymerisation of, 356. reaction of, with iron oxide, 474. -silica system, 468. -soda-silica system, 476. specific gravity of, 219. heat of, 598. thermal conductivity of, 586, 592. Magnesite continued— bricks, see Magnesia bricks. calcining, purpose of, 26. colour of, 97, 120. crypto-crystalline, nodular, 25. structure of, 8, 19. crystalline form of, 4. effect of heat on specific gravity of, 220. of iron oxide on, 401. on colour of, 120. of silica in, 401. on refractoriness, 368. fluxing oxides in, 401. hardness of 126. impurities in, 427, in clay, 421. occurrence of, 427. segregation of, 25. spar, 427. texture of, 43. true specific gravity of, 220. structure of, 14, 21. texture of, 42. true specific gravity of, 211. Magnesium aluminates, hydraulic properties of, 503. carbonate as a cementing material, 15. decomposition of, 490. heat of formation of, 531. chloride, heat of formation of, 530. compounds, 368. mineralogical composition of, 427. effect of, on silica glass, 498. metasilicate, 333. melting-point of, 607. mica, 417. minerals in clays, 421. e orthosilicate, melting-point of, 607. silicates, hydraulic properties of, 503. fusing-point of, 467, 468. spinel, 400. graphic formula of, 356. Magnetic hematite, electrical conductivity of, 609. oxide of iron, colours produced by, 103. formation of, 531. in clay, 366. in raw materials, 97. See also Magnetite. Magnesian limestone, 402, 428. Magnesic porcelains, 378. coefficient of expansion of, 576. separation, use of, 409. refractory materials, true specific gravity of, | Magnetite, 303, 357, 418. 220. crystalline form of, 4. Magnesio-ferrite, 474. electrical conductivity of, 609. Magnesite, 401. formation of, 440. action of hydrochloric acid on, 503. hardness of, 126. amorphous, structure of, 8. in silica bricks, 426. properties of ceramic materials, 608, 621. of minerals, determining, 621. 670 Magnetite continued— inversion of, 357. magnetic properties of, 621. melting-point of, 607. replacement of, by hematite, 5. space-lattice of, 324. structure of, 7. Mailey, 605. Majolica ware, finishing temperature of, 555. Major calorie, 509. Making ceramic articles, 227. Malachite green, use of, 251. Malade jaune, 121. Mallard, 216. Malleability explained, 139, 142. Malms, London, colour of, 114. size of grains in, 34. Manchester red bricks, strength of, 173. Manganese compounds, colour produced by, 118. in clays, 368. discoloration produced by, 121. effect of, on colour, 116, 117. minerals in clays, 422. oxide-silica system, 470. in crystalline glazes, 397, 398. silicates, melting-points of, 470. Mansfield fireclay, specific gravity of, 213. Marcasite, 359, 419. effect of weather on, 507. Marls, effect of heat on porosity of, 69. hardness of, 125. red, 109. size of grains in, 34. Staffordshire, texture of, 34. Marquardt’s porcelain, 375. coefficient ofsexpansion of, 576. Marriotte-Gay Lussac Law, 533. Martin, G., 326. Mass action, 446. and hydrolysis, 448. law of, 449. effect of, on reactions, 446. flow, 274. Masses, effect of time on, 439. Massive structure, 21. Matrix, glassy, 16. non-glassy, 18. Matte glazes, 396. Matteness in glazes, cause of, 396. Matter, forms of, 1. Maw, 102. Mayer, A., 248. H. C., 183, 231. Mayley, 614. Mazzetti, 356. Measurement of binding power, 282. colour, 123. INDEX Measurement of plasticity, 276. viscosity, 291. Mechanical analysis, 44, 49. Medina quartzite, 219. bricks, after-expansion of, 569. specific gravity of, 219. Meerschaum, 333. Meissen porcelain, 375, 378. coefficient of expansion of, 576. Melilite, birefringence of, 626. formation of, 440. Mellor, J. W., 48, 57, 83, 85, 112, 121, 163, 165, 188, 192, 196, 220, 244, 247, 261, 263, 265, 270, 309, 330, 337, 339, 340, 342, 345, 348, 349, 350, 351, 356, 388, 394, 483, 496, 497, 498, 522, 550, 565, 569, 572, 580, 581, 583, 598. Melting-point, 484, 523, 524. determination of, 526. effect of impurities on, 485, 526. on specific gravity, 211. pressure on, 485, 526. prolonged heating on, 526. quantity heated on, 525, 526. rate of heating on, 526. shrinkage on, 526. size of particles on, 525. factors influencing, 525. Melting-points of ceramic materials, 605. metals and alloys, 541. Mene, C., 343. Merwin, 330, 339, 471, 474, 481. Metallic lustre, 95. vapours, destruction of 495. Metasilicate, 332. Metasilicic acid, 333. Metasomatic replacement, 506. Meusser, 505. Mho, thermal, 518. Miall, S., 301. Mica, 417. as a cementing material, 15. as an impurity in clays, 445. crystalline form of, 4. determination of, in clay, 410. optical sign of, 629. effect of fusion on specific gravity of, 211. of heat on dielectric strength of, 614. on refractoriness, 364. on shrinkage, 567. in china clay, 20. in clay, 360. -kaolin mixtures, 365. Microcline, birefringence of, 626. crystalline form of, 4. effect of fusion of specific gravity of, 211. firebricks by, INDEX Microcline continued— formation of, 440. formation of clay from, 354. refractive index of, 623. Micro-crystalline structure, 15. Microliths, 3. Microscopical examination of clay, 406. structure of fired ceramic materials, 407. Midland clays, burned, colour of, 110. tile clays, effect of weather on, 255. Millard, E. B., 512, 514. Milner, H. B., 627, 630. Mineral impurities in clays, 414-423. Mineraliser, 489. Mineralogical composition of alumina, 426. of aluminous materials, 426. of calcic materials, 428. of carbon and carbon compounds, 430. of ceramic materials, 406. of chrome ores, 429. of clays, 406, 411. of lime materials, 428. of magnesia materials, 427. of silica, 423. of siliceous materials, 423. of titanic materials, 428. of zirconium ores, 429. Minerals, arrangement of, 411. containing water, 423. in burned clay, 415. optical identification of, 622. similar to clay, 413. Minneman, 612. Minor calorie, 509. Miscibility of clays, 250. ’ Miscible liquids, phase diagram of, 457, 458, 459. Mitscherlich, 334. Mixed crystals, 324, 334, 335, 336, 456, 480, 487. composition of, 489. formule of, 335. heat effect in production of, 599. silicates, 334. Mixing, effect of, on strength, 155. methods of, 60. Mixtures, 300. preparing, 283. Mobility, 290. effect, of, on chemical action, 441. increase in, 561. of atoms, 437, 438. of clay, 276. Modulus of elasticity of glasses, 188. glazes, 192. rupture, 145. and porosity of bricks, 182. determination of, 197. effect of firing temperature on, 162. 671 Modulus rupture continued— of bricks, 176. at high temperatures, 181. of cold silica bricks, 187. Mohs’ scale of hardness, 124. Moissan, H., 220. Moisture, 362. absorption of, 254. distribution of, 254. effect of, on apparent density, 206. on strength, 166. in clay, 370. in raw materials, influence of, 256. Molal latent heat, 524. Molasses as a bond, 153. Mole defined, 600. Molecular attraction, effect of, on plasticity, 261. compounds, 302, 341. formule, 309. and norms, 315. calculation of, 312. composition from, 311. objections to, 314. use of, 306. heat, 513. of alumina, 598. of lime, 598. of magnesia, 598. of silica and silicates, 598. of vaporisation, 524. solution, 11. structure of colloids, 322. of solids, 322. weight, 304. Molecules, 300. Moler, 7. Molten mass, cooling of, 487. masses, solidification of, 487. material, solvent action of, 442. mixture, changes in, on cooling, 457. solidification of, 457. Molybdic acid as catalyst, 443. oxide in crystalline glazes, 397. Monazite, action of hydrochloric acid on, 503. crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. refractive index of, 623. sands, 21. Monosilicates, 332. Montgomery, R. J., 186, 207, 220, 364, 478. Monticellite, 474. melting-point of, 607. Montmorillonite, 20, 345, 413. Moore, B., 112, 121, 249. J. K., 67, 81, 84, 168, 196, 210, 211, 232, 492, 595. 672 Morscher, 608. Mortar, effect of salts in, on scumming, 122. Mortars for silica bricks, 498. Moseley’s law, 301. Mottled clays, causes of colour of, 108. colour, cause of, 111. Mould, 122. Moulded articles, effect of porosity on, 75. Moulding, effect of size of grains on, 30. of small grains on, 64. processes, water required for, 270. sands, porosity of, 75, 79. texture of, 41. Mucilage as a bond, 153. Muffles, permeability of, 89. porosity of, 78. refractoriness of, 604. texture of, 38. thermal conductivity of, 588. Multiple proportions, law of, 302. Murray, H. D., 231. Muscovite, 342, 417. and alumina eutectic, 483. birefringence of, 626. decomposition of, by heat, 545. electrical conductivity of, 609. melting-range of, 607. refractive index of, 623. water in, 423. Myelite, 413. Mylius, 505. Nacrite, 20, 412. structure of, 7. Naphthalene, use of, to increase porosity, 66. Nascent action, 444. Naterleuss, kieselguhr at, 119. National Brick Manufacturers’ Association, 128, 176, 200. Physical Laboratory, 163, 616, 617. Natrolite, 342, 345. Navias, 329. Nepheline, 416, 480. action of hydrochloric acid on, 503. bifringence of, 626. formation of clay from, 354. melting-point of, 607. refractive index of, 623. with silica and sodium silicate, 480. Nephelite, see Nepheline. Nephrite, melting-range of, 607. Nesbitt, C. E., 130, 134, 169, 187, 206. Neumann, 300. Neutral salt defined, 321. substances defined, 321. Neutralisation an exothermal reaction, 520. heat of, 530. INDEX Neutrality or form of equilibrium, 435. New Caledonia, chromite in, 430. Newark, gypsum and selenite at, 428. Newtonite, 345. Nickel-chrome alloys, corrosive action of, on silica glass, 498. compounds, colours produced by, 118. effect of, on silica glass, 498. oxide, effect of, on colour, 116, 117. in crystalline glazes, 397, 398. Nicol prisms, 627. Nielson, O., 606. Nitrates, a source of scum, 122. Nitre, 122. in glazes, 387. Nitric acid, effect of, 504. Nodular flint, 25. structure, 25. Nomenclature of acids, bases, and salts, 316—- 318. of silicates, 332. Nonpariel bricks, apparent density of, 219. specific gravity of, 219. thermal conductivity of, 586. Non-plastic materials, casting, 285. colours of, 118. effect of, 270. Nontronite, 367, 418, 419. as a source of colour, 97. decomposition of, 450. water in, 423. Normal salt, defined, 320. Norms, 315, 483. Northrup, 498. Northumberland fireclays, barytes in, 15. copper-iron sulphides in, 420. North Wales ganister, texture of, 40. red-burning clays of, 109. Yorkshire, texture of ganister of, 40. Notation, 306. Nottinghamshire, red-burning clays of, 109. Noyes, 229. Nuclei of atoms, 301. Ober, 235. Obsidianite bricks, structure of, 18. O’Connor, F. B., 129, 134, 199. Odour of clays, 137. Office, L. R., 186, 208. Ogden, L., 144. Ohm, thermal, 518. Ohmic resistance, 608. Oil as bond for carbides, 405. absorbed by clay, 239. clays, 370. in clays, 276. shale, origin of oil in, 422. INDEX Orthoclase continued— Oiliness of clays, 276. Oils, mineral, as bonds, 153. Oligoclase, 367, 417. birefringence of, 626. effect of fusion on specific heat of, 211. refractive index of, 623. Olive-green colour, producing, 116. Olivine, 416. action of hydrochloric acid on, 503. birefringence of, 626. crystallisation of, 461. electrical conductivity of, 609. formation of, 440. latent heat of fusion of, 601. magnetic properties, of, 621. melting-point of, 601. -range of, 607. refractive index of, 623. Onyx, 424. Oolitic limestone, 25. Opacifying agents in Se 395. Opacity, 631. Opal, 253. as a cementing material, 15. modulus of elasticity of, 188. refractive index of, 623. specific heat of, 511. structure of, 7. Opalescence, 94. in glazes, 392. Opalescent glazes, 395. Optical activity, 629. properties, changes in, due to heat, 532. of ceramic materials, 622. pyrometers, 536, 538. rotation of quartz, 425. sign, 628. Optically active quartz, 629. Ordinates, 455. Ordway, 594. Organic colloid matter in clay, 284. colouring matter in clays, 106. matter and plasticity, 264. souring, 275. colloidal, 253. effect of, on colloidal properties, 271. in clay, 284, 422. Orthoclase, 342, 416. -albite-anorthite ternary phase, diagram of system, 463. birefringence of, 626. chemical formule for, 307. crystalline form of, 4. structure of, 2. effect of fusion of, on specific gravity, 211. graphic formule for, 309. isomorphism in, 334. melting-point of, 607. 6738 -quartz-plagioclase phase diagram, 464. refractive index of, 623. specific heat of, 597. Orthosilicates, 332. Orthosilicic acid, 333. Orton, E., 98, 101, 102, 110, 128, 129, 260, 396. Osann, B., 90. Osceola insulating bricks, apparent density of, 219. specific gravity of, 219. Osmosis, 230. Osmotic pressure, 232. Ostwald, Wo., 11, 231, 234, 249, 504, 509. Ostwald viscosimeter, 293. Ounces weight per pint of slips, 283. Over-burning, 560. effects of, 442, 560. production of vesicular structure by, 610. Overglaze decoration, 96. Overheating, effect of, 442, 560. Oxford clay, burned colour of, 110. coprolites in, 422. selenite in, 421. Oxidation, 491, 507. a cause of colour, 107, 108. effect of, on clays, 256. objects of, 491. retardation of, 493. Oxides, effect of, on coefficient of expansion of glazes, 579. iron; see Iron oxides. nomenclature of, 320. Oxidising atmosphere, colours produced in, 118. Oxycarbides, constitution of, 357. Oxygen ratio, in silicates, 332. strain, 385. Paragonite, 417. Parmlee, 392, 393, 633. Parravano, 356. Part, 627. Particles, shape of, effect of, on reactions, 446. Pascal, 606, 607.: Pastes, consistency of, 278. dry, nature of, 298. effect of acid on, 275. nature of, 1, 8. physical changes in, 257. preparing, 59, 283. refractory, 10. removal of water from, 295. retention of air by, 225. souring, 274. true specific gravity of, determination of, 225. Patches in crystals, 3. 43 674 Paving bricks, 176. apparent density of, 214. finishing temperature for, 555, hardness of, 129. permeability of, 89. resistance of, to traffic, 125, Pearly lustre, 95. Peaslee, W., 177. Peeling and expansion, 578. Penetrability, 86. Penetration and durability, 502. of slag into bricks, 501, 502. Pennine, birefringence of, 626. Pennsylvania, graphite in, 23. Peptisation defined, 246. Peptising agent, amount of, 288. Perforated bricks, strength of, 150. v. solid bricks, 150. Periclase, 356, 428, 468. crystalline form of, 4. structure of, 14. effect of size of grains on formation of, 43. formation of, 438. Peridotites, chromite in, 430. Perimorphs, 3. Period of weakness, 165. Permanent volume-changes, 564. Permeability, 86. changes in, during burning, 547, 549. smoking, 544. determination of, 91. effect of quartz on, 37. of heat on, 88. of, on drying, 89. of, on thermal conductivity, 89. of, on uses, 89. shape of grains on, 28. size of grains on, 30. of pores on, 72. of ceramic materials and articles, see under their respective names. test, 91. Perrin, 230, 232. Perrot, 57. Peters, 149. Petit, 512. Phakelite, chemical formulz for, 307. Phase conditions in ceramic processes, 466. in chemical systems, 452. diagram, 454. of lime-alumina-silica system, 471, 477. -ferric oxide system, 475. -magnesia-silica system, 476. of magnesia-alumina-silica system, 479. -lime-silica system, 475. of soda-alumina-silica system, 478. Phase rule, 453. Phases, solid, liquid, and gaseous, 452. INDEX Phelps, S. M., 166, 495, 497, 498, 503. Phenakite, 333, Phillipon, M., 41, 64, 147, 151, 154, 219. Phlogopite, 417. electrical conductivity of, 609. refractive index of, 623. Pholerite, 20, 412. Phosphate minerals in clays, 422. Phosphates, action of hydrochloric acid on, 503. _ as cementing materials, 15. colour produced by, 112. Phosphatic nodules, 422. Phosphoric acid, combination of, with silica, 504. as catalyst, 443. effect of, 504. Phosphorite, 422. Phosphorus compounds in clay, 369. Phyllite, 621. Physical changes and chemical action, 433. cause of, 433. in cooling, 560. form, effect of, on specific gravity, 208. properties, effect of, on strength, 150. state, changes in, effected by heat, 523. effected by water, 227, states of ceramic materials, 227. structure, 1. of clays, ete., 1. Physico-chemical reactions between ceramic materials, 432. Picolite, 427. Pierson, 315, 413. Piezo-electric effect, 611. Pink discoloration, cause of, 121. glazes, production of, 117, 395. Pipes, porosity tests of, 83. strength of, 177. transverse strength of, determination of, 198. Pirani, 614. Plagioclase felspar, 417. crystalline form of, 4. formation of clay from, 354. -quartz-orthoclase phase diagram, 464. Plaster of Paris in glazes, 387. Plastic bond, 153. Plastic clays and colloidal substances, 228. dried, strength of, 159. organic matter in, 422. porosity of, 75. size of grains in, 35. materials, 258. cement in, 19. Plasticity, 257-274. actual v. potential, 277. and best consistency, 267. and binding power, 277, 281. and carbonaceous matter, 370, Plasticity continwed— and chemical composition, 259. and colloid matter, 262, 277. and compressibility, 280. and deformability, 277. and extensibility, 279. and extension, 279. and flow under pressure, 273. and organic matter, 264. and shear test, 281. and viscosity, 273. and water content, 277, 279. effect of aggregation of particles on, 261, of colloidal iron hydroxide on, 271. of gum, etc., on, 275. of intermolecular attraction on, 261. of non-plastic materials on, 270. of on flocculation, 246. of pressure on, 270. of sand on, 268. of shape of particles on, 260. of size of particles on, 260. of soluble salts on, 241, 364. of surface area on, 261. increase and reduction of, 266. increased by hydrolysis, 236. increasing, 271. measurement of, 276. number, 268. potential v. actual, 277. proportion of water required, 267. pseudo, 275. range of, 268. reducing, 272. water of, 336. required to develop, 262, 269. v. cohesion, 9. v. colloidal content, 9. Platinum alloys, melting-point of, 541. melting-point of, 541. thermal resistivity of, 594. Plauson mill, 13, 251. Play of colours, 94. Pleochroism, 95, 629. Plumbago bricks, texture of, 43. thermal resistivity of, 594. crucibles, burning, 558. for crucibles, 404. occurrence of, 430. thermal] resistivity of, 594. use of, 430. Plumbiferous glazes, 388. Pocket clays, 506. structure of, 21. Podszus, E., 251, 253. Polarised light, 627. for detecting structure, 3. use of, 18, 406, INDEX Polymerisation of alumina, 351. of magnesia, 356. Polymorphism, 325. Polymorphous crystals, 5. Popplewell, 173. Porcelain, apparent density of, 214. American, strength of, 178. 675 Bayeux, coefficient of expansion of, 577. Berlin, coefficient of expansion of, 577. beryllium, coefficient of expansion of, 574. burning, 554. casting, 284. chemical composition of, 378. chemical, strength of, 178. Chinese, 376. clays for, 375. coefficient of expansion of, 577. dielectric strength of, 613. dure, 375. effect of clay on coefficient of expansion of, 574. of heat on dielectric strength of, 614. electrical conductivity of, 609. insulating power of, 612. European, coefficient of expansion of, 575. expansion of, 573. firing, 559. temperature of, 558. fluxes in, 377. glazes, 393. hard, thermal conductivity of, 585. Heinecke’s, 378. Hermodorfer, strength of, 178. ideal, 375. Japanese, 376. magnesic, 378. coefficient of expansion of, 576. Marquardt, 375. ’ coefficient of expansion of, 576, materials used for, 376. Meissen, 375. microscopical structure of, 488. mixtures used for, 378. piezo-electrical effect in, 611. porosity of, 80. puncture voltage of, 613, 615. purpose of clay in, 377. red discoloration in, 121. refractoriness of, 603. resistance of, to puncture, 610. Seger, coefficient of expansion of, 576, strength of, 178. Sévres, 375. shrinkage of, 565. sillimanite in, 17, 377. specific gravity of, 214. heat of, 596. resistance of, 613. 676 Porcelain continued— steatite, 378. strength of, 177. structure of, 17. tendre, 375. texture of, 40. thermal conductivity of, 589. resistivity of, 594. translucency of, measuring, 634. types of, 375. Viennese, 375. Pore-space, insulating properties of, 515. Pores, effect of, on permeability, 88. sealed, 204, determination of, 83. effect of heat on, 207. formation of, 71. proportion of, 205. sealing, 546. size of, 71. effect of, on permeability, 72. Porosity, 61. and carbonaceous matter, 370. and chemical action, 446. and density, 205. and modulus of rupture of bricks, 182. and permeability of refractory bricks, 88. and resistance to changes in temperature, 581. to chemical action, 76. and texture, 205. apparent, determination of, 81 ; Apparent porosity. changes and pressure, 567. in, during burning, 547, 549. smoking, 544. vitrification, 552. classification of ware by, 76. coefficient, 62. determination of, 81. effect of vacuum on, 82. determining, rough method of, 84. effect of bonds on, 68. of burning temperature on, 69, 70. of Cornish stone on, 67. of electrolytes on, 68. of fineness of felspar on, 67. of fluxes on, 67. of fluxes on, 66, 68. of grading on, 31, 64. of heat on, 66, 69-71. of, on absorption, 72. on apparent density, 72. on coefficient of expansion, 572. on discoloration, 74. on electrical conductivity, 74. on moulded articles, 75. on rate of drying, 75. see also INDEX Porosity, effect of, continwed— on refractoriness, 74. on resistance to abrasion, 73. to corrosion, 73. to erosion, 73. to weathering, 73. on scum, 75. on spalling, 72. on strength, 74, 152. on thermal conductivity, 72, 514. on uses, 75. of pressure on, 62, 65. of sawdust on, 72. of shape of grains on, 28, 29. of size of grains on, 30. of texture on, 62, 63, 64. increasing, 65. low, testing, 80. materials for increasing, 65, 66. modes of expressing, 62. of ceramic materials and articles, see under their various names. reducing, 66. true, 61. determination of, 83. Porous bricks, carbonaceous matter in, 65. strength of, 182. siliceous goods, density and specific gravity of, 219. ware, 76. Porphyry, 18. electrical conductivity of, 609. Portland cement as bond for carbides, 405. in silica bricks, 399. effect of, on firebricks, 495. Pot clay, washed, apparent density of, 212. Potash-alumina-silica system, 480. effect of, on expansion of glazes, 579. heat of formation of, 531. in crystalline glazes, 398. in glazes, 387. in porcelain, 377. -lithia-silica system, 477. volatilisation of, 562. Potassium carbonate, use of, in purifying clay, 288. effect of, on silica glass, 498. hydrate, effect of, on silica glass, 505. metaborate, melting-point of, 607. mica, 417. Potters’ flint, specific gravity of, 216. Pottery clays, composition of, 372. texture of, 37. vitrification range of, 553. water required to develop plasticity of, 269. manufacture, china clay for, 372. Potts, 214, 215, 574, 575, 578, 613. INDEX Pouillet effect, 532. Poulson, A., 252. Powders, determination of permeability of, 93. mineralogical examination of, 407. porosity of, 75. thermal conductivity of, 592. true specific gravity, determination of, 224. Precipitated silica, occurrence of, 424. Precipitates, colloidal, nature of, 232. Precipitation by electrolytes, 231. electrical, 230. Preparation, effect of, on strength, 154. of homogeneous mixtures, 283. Prepared cobalt, 395. Pressure and chemical action, 440. critical, 485. effect of, on cementation, 507. on melting-point, 485, 526. on plasticity, 270. on squatting, 561. Pressing, effect of, on strength, 157. Pressure-equilibrium, 440. -flow and plasticity, 273. of fluidity, 126, 274. partial, of reacting constituents, 447. Priest, 633. - Product, final, of reaction, 530. Progress of reactions, 448. Properties depending on structure, 27. Proportion of elements in compounds, 302. Proportions, equivalent, law of, 303. fixed, law of, 302. multiple, law of, 302. Protection of colloids, 232. of clay suspensions, 249. Protective colloids, 232. Protons, 301. Pseudo-acids and bases, effect of, on clay, 243. Pseudo-plasticity, 275. -wollastonite, 467. specific heat of, 597. Pseudomorphism, causes of, 5. Pugging, 60. effect of, on strength, 155. Pukall, W., 345, 347, 352, 368, 371, 376, 397. Pukall’s salt, 353. Pulfrich, 443. Pumice, 7. formation of clay from, 354. thermal resistivity of, 594. Puncture voltage, 608. determination of, 620. of porcelain, 611, 613, 615. Purdy, R. C., 56, 57, 81, 83, 84, 168, 211, 237, 243, 246, 248, 261, 262, 264, 266, 385, 387, 389, 392, 395, 397, 398, 483, 492, 574, 575, 576, 578, 613. 677 Pure clays, 414. Purification of clay, 287. Purple colour, cause of, 111. Pycnometer, 225. Pyrites, 359, 369, 419. a cause of discoloration, 121. of scum, 123. as a cementing material, 15. colour produced by, 103. cupiferous, discoloration produced by, 121. decomposition of, by heat, 545. effect of heat on, 366, 492. grey colour caused by, 97. production of spots by, 547. replacement of, by hematite, 5. scum due to, 419. Pyrometer, Féry, 538. Le Chatelier, 538. Wanner, 538. Pyrometers, electrical, 536, 537. optical, 536, 538. radiation, 536, 539. Pyrometry, 509, 536. | Pyrophyllite, 345, 413. refractive index of, 623. thermal curve of, 352. Pyroscopes, 536, 539. Pyroxenes, 415. crystallisation of, 461. effect of heat on, 459. formation of, 440. Pyrrhotite, 419. electrical conductivity of, 609. magnetic properties of, 621. Quadrisilicates, 332. Quantities, relative, effect of, 446. Quarternary systems, 482. phase diagram of, 464. Quartz, 328, 480. a- B-change in, 328. arrangement of atoms in, 327. as a cementing material, 15. birefringence of, 626. colour of, 119. conversion of, 329. crystalline, specific gravity of, 216. structure of, 2. determination of, in clay, 410. dextro- and levo-rotatory, 629. effect of fusion of, on specific gravity, 211. of heat on, 329. dielectric strength of, 614. of repeated heating on, 218. electrical conductivity of, 609. formation of, 329. glass, 6, 487. coefficient of expansion of, 580. 678 INDEX Quartz glass continwed— Quartzites continwed— effect of repeated heating on, 563. useful, structure of, 14. permeability of, 89. useless, structure of, 14. reaction of, with lime, 468. Queneau, A. L., 88, 92, 223, 585, 590, 592, 594. ribbon-like crystallisation of, 327. Quensel, 443. specific gravity of, 216. heat of, 597. strength of, 188; see also Silica glass. Race, 419. hardness of, 126. Radcliffe, B. S., 389, 395, 610, 612. heat of solution of, 599. Radiation, 519. in aluminium minerals, 427. pyrometers, 536, 539. in china clay, 20. Radicle, defined, 317. in chromite, 430. Radiolaria, 7. in magnesite, 428. Rainwater, action of, on carbonates and in zirconia ore, 429. silicates, 504. lustre of, 95. Rakusin, 240. made amorphous by grinding, 599. Rammelsberg, 307. melting-point of, 331, 605. Range of fusion, 486. molecular heat of, 598. of plasticity, 268. occurrence of, 425. of vitrification, 552. optical activity of, 629. Rankin, 339, 471, 472, 473, 479, 481, 580. rotation of, 425. Raoult, F. M., 525. -orthoclase-plagioclase phase diagram, 464, Rastall, 507. particles, signs of fusion of, 485. Rate of chemical reaction, 446. piezo-electrical effect in, 611. firing, 161. properties of, 425. Rational analysis, 409. recognition of, in bricks, etc., 415. formule, 308. refractive index of, 623. Rattler test, 129, 199. replacement of, by hematite, 5. loss of weight in, 177. rocks, structure of, 14. Raw clays, see Clays. separation of, from clay, 409. Ray, R. C., 599. solubility of, in hydrofluoric acid, 504. Rayleigh, 327. specific gravity of, 217. Reacting masses, distribution of, 451. changes in, 217. Reaction, existence of, 433. heat of, 510, 597. heat of, 530, 599. thermal conductivity of, 592. obscured, 531. resistivity of, 594. increasing velocity of, 450. transformation of, 330. products, solubility of, 442. unaltered, identifying, in silica bricks, 625, Reactions accompanied by a change in tem- volatilisation of, 561. perature, 529. wedges, 628. arrest of, 550. Quartzine, 253. balanced, 451. Quartzite bricks, 186, 400. chemical, 435. after-expansion of, 569. factors influencing, 437. Findlings, 17. critical temperature of, 438. structure of, 18. due to acids, 503. ideal structure of, 18. to alkalies, 503. Lickey, silicification of, 19. to water, 503. Stiperstones, structure of, 19. effect of prolonged heating on, 562. Quartzites, 425. of temperature on, 435. alteration of structure of, by grinding, 26. of time on, 439. calcining, purpose of, 26. equilibrium, 452. coefficient of expansion of, 579, 580. final product of, 530. colour of, 119. incomplete, 449. composition of, 399. involving displacement, 436. crystalline structure of, 14. irreversible, 450. specific gravity of, 216. occurring at lower temperatures, 503. structure of, 14, 19. at high temperatures, 490. INDEX 679 Reactions continwed— occurring in burning, 546. in sand-lime bricks, 505. of substances with each other, 432. possible, 435. progress of, 448. reversible, 451. simple, 452. speed of, 449. tendency of, 439. thermo-chemical, 530. velocity of, 449. Rearrangement, chemical, 436. Rebuffat, O., 218, 330. Recalculated analysis, 410. Rectorite, 345. Recuperators, porosity of bricks for, 78. Recrystallisation of rocks, 2. Red bricks, apparent density of, 214. finishing temperature for, 555. importance of colour of, 547. burning clays, 109. cause of colour of, 109. colour, effect of alumina on, 101. of minerals on, 101. of articles, cause of, 100. colours, due to alge, 97. produced by iron compounds, 97. production of, 97, 117. discoloration, cause of, 121. marls, 109. Redlich, 339. Reducing action, desirable, 493. harmful, 493. agents, 492. atmosphere a cause of discoloration, 121. colours produced in, 118. Reduction, 492. of iron compounds, 104. processes and refractory materials, 493. production of colour by, 111. Reflection, 622. Refraction, 622. double, 625. index of, 411, 622, 623. Refractive index, 411, 622, 623. determining, 624, 625. indices of liquids, 624. of minerals, determination of, 623. Refractoriness and composition of clays, 381. defined, 525. determination of, 526. effect of alkali on, 364. of alumina and silica on, 364. of felspar on, 364. of heat on, 601. of impurities on, 362. of iron compounds on, 366. Refractoriness continwed— effect of lime on, 367. of magnesite on, 368. of porosity on, 74. of soluble salts on, 364. of ceramic materials and articles, see under their various names. of mixtures, 601. of powdered mixtures of bricks with slag, 495. of slags, 496. Refractory articles, effect of frost on, 73. firing, 556. porosity of, 77. bricks, resistance of, to abrasion, 130. specific heat of, 596. structure of, 17; see also under their various names. coefficient, 381. indices, 381. materials and reduction processes, 493. causes of colour of, 106. electrical conductivity of, 74. importance of porosity in, 72. resistance to corrosion, 500. strength, when hot, 164. thermal conductivity of, 586. pastes, 10. porcelain, 378. thermal conductivity of, 589. silica sands, 400. Refrax, effect of rapid cooling on, 584. Regenerators, porosity of bricks for, 77. Regnault, 513. Rehydration of clay, 237, 349. Reinforcement in ware, 377. Relative colloids, 278. quantities of reacting substances, effect of, 446. Removal of water, changes following the, 294. Renegade, E., 131, 136. Repeated cooling, effect of, 563. heating, effect of, 563. on specific gravity, 218. on strength, 163. Re-pressing, effect of, on strength, 157. Residual expansion of silica bricks, 568, 569. Resilience explained, 139. Resin as bond for carbides, 405. Resinous lustre, 95. Resistance, contact, 518. electrical, 608. and temperature, 611. at different temperatures, 614. variation of, with felspar content, 610. ohmic, 608. pyrometers, 538. to abrasion, importance of, 124. 680 Resistance continued— to acids and finishing temperature, 558. to indentation, measurement of, 124. to spalling, 583, 584. to sudden changes in temperature, 581. effect of size of grains on, 30. Resistivity, 515. electrical, 608, 609. and exposure to electric current, 611. determination of, 620. effect of calcium sulphate on, 619. of carborundum bricks, 619. of chromite bricks, 618. of fireclay bricks, 612. of magnesia bricks, 617. of silica bricks, 616. of zirconia bricks, 618. thermal, 584, 593, 594. Retort bricks, thermal conductivity of, 593. resistivity of, 594. carbon, thermal resistivity of, 594. settings, porosity of bricks for, 78. Retorts, 557. casting, 284. effect of frost on, 73. firing temperature of, 558. interaction of, with contents, 491. jointed, thermal conductivity of, 593. laminated, cause of, 23. permeability of, 90. porosity of, 78. refractoriness of, 604. resistance of, to abrasion, 125. to sudden changes in temperature, 582. size of grog for, 39. strength of, 183, 188. texture of, 39. thermal conductivity of, 588, 589. effect of burning temperature on, 585. Reverberatory furnaces, porosity of bricks for 78. Reversible colloids, 235. expansion of ceramic materials and articles, see under their various names. reactions, 450, 451. volume-changes, 520, 570. Reversibility, 235. Rhodochrosite, space-lattice of, 32¢. Rhodonite, 470. melting-point of, 607. Rhyolite, formation of clay from, 354. Rice, B. A., 480. Richard, 600. Riddle, F. H., 177, 574, 610, 612. Riddling test, 45. Rieke, R., 140, 143, 192, 217, 268, 269, 337, 364, 366, 367, 368, 481, 483, 577, 578, 579, 607. INDEX Ries, H., 20, 63, 102, 151. Rigg, G., 479. Riley, 373. Ring compounds, 308. of ware, 137. of well-fired goods, 556. R.O. defined, 307. Robertson, 189. Rock crystal, specific gravity of, 216. quartz, structure of, 14. salt, replacement of, by hematite, 5. by quartz, 5. Rocks, action of water on, 506, 507. cementing of, 15. Rohland, 76, 238, 240, 246, 250, 255, 262, 263, 269, 270, 272, 275, 277, 284. Rolling-out limit, 268. test, 278. Roofing tiles, colour of, 110. finishing temperature of, 553. hardness of, 129. permeability of, 89. texture of clay for, 37. Roozeboom, 488. Roscoe, 424. Rosenow, 264, 265, 279. Rosenthal, E., 177, 578, 613. laboratory porcelain, coefficient of expansion of, 576. Rosler, 354. Ross, D. W., 188, 331. Rotation, dextro- and levo-, 629. Rotatory power of quartz, 425. Roth, E., 633. Rouleaux in china clay, 19, 411. Rounded grains, 27. Ruabon aluminous fireclay, specific gravity of, 213. clay, colour of, 109. Rubber, effect of, on plasticity, 275. Rubbers, friability of, 142. strength of, 173. Ruff, O., 79, 120, 231, 605, 606, 607. Rugby red bricks, strength of, 173. Rum, Isle of, ultrabasic rocks in, 461. Rupture, see Modulus of rupture. electrical, of porcelain, 611. Russell, 598. Rutile, 429. birefringence of, 626. crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. in clay, 421. in crystalline glazes, 398. in glazes, 398. refractive index of, 623. space-lattice of, 324. INDEX Saggers, burning, 161, 557. carborundum, a cause of discoloration, 121. constancy of volume of, 582. effect of frost on, 73. of repeated heating on, 563. firing, 161, 577. temperature, 558. laminated, cause of, 23. permeability of, 89. porosity of, 78, 582. refractoriness of, 604. strength of, 184. of increasing, 161. texture of, 37. thermal conductivity of, 588. “Salt,” 122. Salt, corrosive action of, 496. defined, 316, 320. glaze, 352, 383, 391. alumina: silica ratio in, 391. -glazed articles, finishing temperature of, 553. bricks, finishing temperature of, 555. glazing, 468. Salts, effect of absorption of, 75. on clays, 243, 494. on electrical resistance of clay slips, 620. on osmotic pressure, 234. nomenclature of, 321. removal of, from clay, 241. from solution by clay, 241. Samples, preparation of, for microscopical examination, 407. Sampling, 360. Sand-bauxite bricks, shrinkage of, 570. weakness of, 149. Sand-blast for testing hardness, 128, 134. resistance of certain bricks to, 129. test, 134. Sand bricks, strength of, 185. -clay mixtures, dry, strength of, 171, 172. coarse, definition of, 35. collodial properties of, 252. dust, definition of, 35. size of grains in, 35. effect of, on plasticity, 268. fine, definition of, 35. size of grains in, 35. surface factor of, 56. Fontainebleau, sand-calcites in, 15. for furnace linings, 21, 42. porosity of, 79. for moulding, texture of, 41. hardness of, 126. -lime bricks, 1'7. reactions occurring in, 505. structure of, 17. See also Lime-sand bricks. 681 Sand continued— moulds, texture of, 41. separation of, from clay, 343. size of grains in, 35. thermal conductivity of, 592. resistivity of, 594. use of, in clays, 374. Sands, chromite, 20. dolomite, 20. incoherent, use of, 426. monazite, 21. origin of, 21. refractory, 20, 400. zircon, 20. Sandstones, calcareous, bond in, 18. calcining, purpose of, 26. cement in, 18. dolomitic, 25. electrical conductivity of, 609. greyness of, 430. siliceous, structure of, 18. Sanitary ware, 392. glazes for, 392. Sankey, J. H., and Son, 591. S&o Paulo, zirconia ore in, 403. Sargent, 219. Satin spar, lustre of, 95. Satoh, P., 350. Saturated compounds defined, 305. Saunders, 493, 605. Sawdust, effect of, on porosity, 72. in clay mixtures, 371. use of, to increase porosity, 65. Saxe, C. W., 159, 161. Scapolite, 342. birefringence of, 626. formation of clay from, 354. melting-range of, 607. Scheelite, crystalline form of, 4. Scheffler, W., 385, 392. Schists, electrical conductivity of, 609. structure of, 23. Schloesing, T., 263. elutriator, 52, 53. Schlossberg, A., 252. Schoeffer, 231. Schoene’s elutriator, 51. Scholes, 8. R., 353. Schorlemmer, 424. Schory, 286. Schott, 397. Schramm, E., 199. Schroeder van der Kolk, 624. Schrotterite, 345. Schulz, 598. Schurecht, 55, 68, 155, 261, 266, 269, 398, 471, 482. Schwartz, 216. 682 Schwarz, 326, 504. Schwerin, B., 252, 253, 608. Scotch bastard ganisters, texture of, 40. fireclays, aluminous, 421. copper-iron sulphides in, 420. Scott, A., 68, 210, 330, 474. Scratching tests, 123, 135. Screening tests, 45. Scum, 24, 75, 242, 367, 369. blue-green, caused by ferrous sulphate, 97. brown, 123. cause of, 122. due to pyrites, 419. grey, 123. in brick clays, 374. prevention, 97. of, by reduction, 494. white, 122. yellowish, 123. Sealed pores, 264. determination of, 83. formation of, 71. proportion of, 205, 207. Seaton clays, barytes in, 421, Seaver, 330, 331. Sectility, effect of heat on, 138. of clays, etc., 137. Sedimentation, 53. tests, 53, 54. Seeds, use of, to increase porosity, 66. Seger, 35, 98, 101, 102, 121, 150, 269, 363, 364, 381, 382, 384, 386, 389, 393, 395, 409, 472, 527, 540, 541, 574, 575. cones, 540. composition of, 379. temperatures corresponding to, 379. porcelain, coefficient of expansion of, 576. dielectric strength of, 614. strength of, 178. Seger’s imitation of Oriental porcelain, 376. volumeter, 84. Segregated structures, 25. Selch, E., 479. Selective action, 442. Selenite, 367, 421. effect of weather on, 507. water in, 423. Semi-grog bricks, apparent density of, 214. true specific gravity of, 214. -permeability of clay, 250. -porcelain, apparent density of, 214. porosity of, 80. -silica bricks, 400. effect of heating on volume of, 564. Sensible heat, 520, 523. Sentinel pyrometers, 540. Serpentine, 416. action of hydrochloric acid on, 503. INDEX Serpentine continuwed— as a cementing medium, 15. birefringence, of, 626. in chromite, 430. in magnetite, 427. refractive index of, 623. rocks, chromite in, 430. Sesquisilicates, 332. Settling of particles in water, 54. Sévres imitation of Oriental porcelain, 376. porcelain, 375, 378. Sewer bricks, 175. American, strength of, 176. porosity of, 76, 77. Shale bricks, apparent density of, 214. thermal conductivity of, 590. true specific gravity of, 214. Shales, 237, 370, 414. colour of, 108. dry, strength of, 171, 172. effect of heat on apparent specific gravity of, nature of, 22. size of grains in, 34. vitrification range of, 553. water required to develop plasticity in, 269. Shaly structure, 22. Shape, effect of, on squatting, 561. on strength, 150. loss of, and rapid heating, 440. of grains, 27. effect of grinding on, 29. on drying, 296. on permeability, 28. on porosity, 28. on texture, 27. for furnace hearths, 29. Shaping, effect of, on strength, 157. Shattering due to changes of temperature, 581. Shaw, J. B., 196. Shear test and plasticity, 281. Shearer, G., 349. Sheffield ganister, texture of, 40. Shells, 376. Shepherd, 467, 472, 525. Shiners, production of, 117. Shivering, 385. Shore scleroscope, 136. Shrewsbury, Stiperstones quartzites at, 19. Shrinkage, 520. and carbonaceous matter, 370. effect of fineness of grains on, 565. of fluxes on, 567. of fused matter in, 567. on melting-point, 526. on strength, 160. in drying, 295. in kiln, measurement of, 541. INDEX Shrinkage continued— measurement of, as heat recorder, 541. of aluminous materials, 570. of bauxite bricks, 569. of carbon crucibles, 570. of engobes, adjustment of, 390. of glaze, increasing or reducing, 390, 391. of refractory bricks, 570. permanent, 565. Shropshire fireclays, copper-iron sulphides in, 420 red-burning clays in, 109. Siderite, 420. electrical conductivity of, 609. magnetic properties of, 621. replacement of, by quartz, 5. space-lattice of, 324. Siemans, 614. Sienna, 413. Sieurin, E., 149, 363, 364, 366. Sieves, kinds of, 45. precautions with, 45. relation of apertures in, 48. standard, 45, 46, 47. Sieving, 408. test, 45. . dry, 48. Sigur insulating bricks, apparent density of, 219. specific gravity of, 219. Sil-o-cel insulating bricks, apparent density of, 219. specific gravity of, 219. Silber, P., 338. Silfrax, 357, 404. Silica, affinity of fluxes for, 315. allotropic changes in, 328. forms of, 327. -alumina-barium oxide system, 481. -alumina eutectic, 472. -lime system, 478. -magnesia system, 481. mixtures, refractoriness of, 472. -potash system, 480. -alumina ratio in clays, 371. in glazes, 386. rings, 472. amorphous, 328, 329. conversion of, to cristobalite, 329. specific gravity of, 219. and lime, reaction of, 434. as an impurity in clays, 363. -barium oxide-lime system, 474. -soda system, 477. system, 469. -base-base ternary systems, 474. bricks, 162. acid reaction of, 497. 683 Silica continued— bricks, after-expansion of, 569. American, strength of, 186. and spalling, 584. apparent density of, 218. bond in, corrosion of, 498. bonds for, 399. Brinell hardness of, 128. cements for, 498. coefficient of expansion of, 579. composition of, 398. crushing strength of, 187. cristobalite in, 16. diffusivity of, 588. effect of alumina on, 497. of ashes on, 497. of basic fluxes on, 497. of bonds on porosity of, 68. of exposure on, 167. of firing temperature on strength of, 162. of fluxes on, 498. of heat on conductivity of, 591. on diffusivity of, 591. of iron oxide on, 497. of lime on strength of, 147. of slags on, 497. of texture on strength of, 185. of tridymite on, 16. electrical conductivity of, 616. resistivity of, 616. expansion of, 568. firing, 556. temperature of, 558. forms of silica in, 426. grading, 41. hardness of, 132. hot, strength of, 165. under load, 179. increase of refractoriness in, 497. influence of burning on thermal conduc- tivity of, 590. iron compounds in, 426. melting range of, 605. mortars for, 498. permeability of, 88, 90. porosity of, 63, 64, 78, 80. resistance of, to abrasion, 130, 132. specific gravity of, 210, 218, 219, 588. effect of burning temperature on, 217. specific heat of, 588, 596, 597. strength of, 185, 186. structure of, 16. susceptibility of, to sudden changes of tem- perature, 582, 583. texture of, 40, 41, 63. thermal conductivity of, 588, 589, 590. at various temperatures, 586. 684 INDEX Silica continwed— Silica continued— bricks, effect of burning temperature on, in graphite, 430. 585. in lime glazes, 388. resistivity of, 594. in phase diagram, 475. transverse strength of, at high tempera- in solution, 506. tures, 181. -iron oxide-magnesia system, 478. tridymite in, 16. system, 470. use of flint in, 40. -lime-barium oxide system, 474. cement, 15. -lithia system, 476. porosity of, 79. -magnesia system, 474. refractoriness of, 603. -soda system, 476. chemical formule for, 307. -strontia system, 476. coefficient of expansion of, effect of heat on, system, 467. 580. -lithia-barium oxide system, 477. colloidal, 252. -lime system, 476. as a cementing material, 15. -magnesia system, 477. inversion of, 13. -potash system, 477. constitution of, 326. -soda system, 476. crystalline, 328. -magnesia-lime system, 474. effect of common salt on, 468. -lithia system, 477. grain size on inversion of, 330. -soda system, 476. on expansion of glazes, 579. system, 468. on magnesia bricks, 498. -manganese oxide system, 470. on refractoriness of clay, 364. materials, 400. phosphoric acid on, 504. mineralogical composition of, 423. flour, effect of, 219. mortar, refractoriness of, 603. forms of, 328. : occurrence of, 424. in bricks, 16. -potash-lithia system, 477. free, 363. precipitation of, in rocks, 506. fused, coefficient of expansion of, 580. reaction of, with bases, 505. hardness of, 132. reduction of, 493. permeability of, 89. by metals, 498. fusing-point of, 467. retorts, strength of, 188. gel, 252. rocks, 425. absorptive power of, 252. fused, specific gravity of, 216. glass, 328, 400. lime in, 399. attack of, by lime, 498. potash in, 399. by metallic oxides, 498. structure of, 21. by metals, 498. sand, melting range of, 605. crystallisation of, 488. -sillimanite system, 473. dielectric strength of, 617. slightly soluble in water, 506, effect of phosphoric acid on, 504. -soda-alumina system, 480. electrical insulating properties of, 616. -barium oxide system, 477. formation of, 331. -lime system, 476. optical properties of, 631. -magnesia system, 476. ribbon-like crystallisation in, 327. -strontia system, 477. solubility of, 505. system, 468. in alkalies, 505. solubility of, 504. in hydrofluoric acid, 504. specific gravity of various forms of, 216. specific gravity of, 216, 220. heat of various forms of, 511, 597. inductive capacity of, 617. -strontia-lime system, 476. thermal conductivity of, 591. -lithia system, 478. graphic formule for, 308, 326, 327. system, 469. hardness of, 126. thermal conductivity of, 593. heat of formation of, 531. triplets, 327. hydrated, effect of alkaline solutions on, 505. true melting-point of, 525. in clays, 414. use of, in glazes, 385. in combination, 363. volatilisation of, 491, 498, 561. INDEX Silica continued— ware, 400. with sillimanite, 480. with sodium silicate, 480. -zine oxide-alumina system, 482. -zine oxide system, 470. -zirconia system, 470. Silicates, 303. action of water in, 503. alkaline, effect of acid on, 333. as cementing materials, 15. chemical constitution of, 325, 332. effect of fusing, 336. of phosphoric acid on, 504. of pressure in formation of, 440. electrical conductivity of, 609. formation of, 466. fusing-point of, 467. heat of formation of, 599. hydrated, 336. as cementing materials, 15. hydrolysis of, 507. in chromite, 430. clay, 367, 415. latent heat of fusion of, 601. mixed, 334. - mobility of, 441. nomenclature of, 332. oxygen ratio in, 332. specific heat of, 597. Siliceous materials, burned, minerals in, 426. coefficient of expansion of, 579. colour of, 119. effect of temperature of firing on, 580. mineralogical composition of, 423. refractoriness of, 603. specific gravity of, 215, 216. heat of, 597. texture of, 40. sandstones, structure of, 18. sinter, 20, 253. structure of, 7. Silicic acid, colloidal, 252. constitution of, 332. water absorbed by, 237. Silicification of ganisters, 19. of quartzites, 19. Silicon carbides, constitution of, 357. manufacture of, 493. fluorides, volatilisation of, 504. hydrate, graphic formula for, 308. nitride in carboxides, 357. Silit, 357, 404. Silky lustre, 95. Sillimanite, 333, 342, 413, 478, 480, 481. -alumina eutectic, 473. birefringence of, 626. bricks, structure of, 17. 685 Sillimanite continued— crystalline form of, 4. structure of, 2, 14. crystals in porcelain, 377. effect of, on electrical resistance, 612. electrical conductivity of, 609. formation of, 350, 472. of clay from, 354. hardness of, 126. in glass-pots, 17. in porcelain, 17, 377. melting-point of, 607. refractive index of, 623. -silica system, 472. thermal curve of, 352. with corundum, 480. with silica, 480. Siloxicon, 357, 404. decomposition of, 491. effect of hydrofluoric acid on, 504. bricks, burning, 557. true specific gravity of, 221. Silsbee, F. B., 615. Silt, definition of, 35. Humber, colour of bricks from, 110, separation of, from clay, 343. size of grains in, 35. surface factor of, 56. Silundum, 357, 404, 431. colour of, 119. true specific gravity of, 221. Silver alloys, melting-point of, 541. melting-point of, 541. thermal resistivity of, 594. Simonis, M., 247, 277, 292, 364, 381. Sinclair, 337. Singer, F., 347, 389, 395, 578. Single oxides, 480. Singulosilicates, 332, 466. Size of grains, 29. determination of, 44-59. effect of, on allotropic forms, 31. on chemical action, 445. on drying, 30. on lamination, 31. on melting-point, 525. on moulding, 30. on permeability, 30. on plasticity, 260. on porosity, 30. on resistance to temperature changes, 30. on strength, 30, 150. on surface, 31. on texture, 29, 30. in china clay, 33. in clays, 33. in goods, see Texture. in kaolin, 33. 686 Size of grains continued— in raw materials, see Texture. of iron compounds, effect of, on colour, 99. of silica in clays, 363. Size of pores and absorption, 71. effect of, on permeability, 72. Skin on bricks and tiles, 123. etc., producing, 38. Slag blotches in firebricks, 115. depth of penetration of, 501, 502. -like masses in bricks, 416, 420. (zinc) penetrability of, 502. test, 495, 500. Slags, action of, on bricks, testing, 501. and silica, refractoriness, of, 497. basic, effect of, on bauxite bricks, 499. calcic, 368. corrosive action of, 441, 494. effect of, 169. of alumina on, 479. on chromite bricks, 500. on diaspore bricks, 499. on magnesia bricks, 499. on silica bricks, 497. magnesic, 368. refractoriness of, 496. resistance of bauxite bricks to, 500. of bricks to, 497. of carborundum bricks to, 500. of magnesia bricks to, 498. of zirconia bricks to, 500. rich in lime, 497. Slaking of clay, 227, 257. Slate, nature of, 22. Slip clay, dried, strength of, 159. Slips, 282. casting, 10, 283. clay for, nature of, 10. consistency of, determination of, 226. nature of, 1, 10. preparation of, 60. properties of, 290. purification by means of, 287. removal of water from, 294. solid matter in a pint of, 226. use of, 10. viscosity of, 282. volume-weight of, determination of, 226, Slurries, 282; see also Slips. Smithson, 341. Smits, A., 325, 357. Smoking, 544. Soaking, 439. effects of, 550. prolonged, effects of, 562. stage of firing, 550. Society of Glass Technology, 78. INDEX Soda-alumina-silica system, 480. as a flux, 481. -barium oxide-silica system, 477. compounds in silica bricks, 399. effect of, on clay, 247. on crazing, 387. on expansion of glazes, 579. on strength, 155. on zirconia bricks, 500, 506. -felspar, 416. heat of formation of, 531. in crystalline glazes, 398. in glazes, 387, 398. in porcelain, 377. -lime glass, corrosive action of, 496. -lithia-silica system, 476. -magnesia-silica system, 476. -orthoclase, 417. -silica system, 468. -strontia-silica system, 477. use of, in casting, 285. in purifying clay, 288. volatilisation of, 562. with corundum, 480. Sodalite, formation of clay from, 354, refractive index of, 623. Sodium carbonate, effect of, on alumino- silicates, 322. on silica glass, 505. on zirconia bricks, 500. chloride as a corrosive agent, 494, 495. effect of, on silica glass, 498. hydrate, effect of, on silica glass, 505. on strength of dried clays, 155. on zirconia bricks, 500. metaborate, melting-point of, 607. metasilicate, 333, 468. melting-point of, 607. -mica, 417. nitrate, space-lattice of, 324. phosphate, effect of, on silica glass, 505. silicate as a bond, 154, 405. crystalline form of, 337. effect of, on strength of dried clays, 155. with nepheline and silica, 480. with silica, 480. spinel, 400. sulphate, corrosive action of, 496. tetrasilicate, 468. tungstate as catalyst, 443. Soft porcelain, 375. wares, hardness of, 127. Softening-point, 164. determination of, 526. factors affecting, 525. prior to fusion, 524. Soil as colouring agent, 111. INDEX Sokoloff, A. M., 257, 350. permeability apparatus, 91. Solid and gas, reactions between, 435. and liquid, reactions between, 435. bricks, strength of, 150. cooled, composition of, 487. matter, weight of, in a pint of slip, 226. solutions, 300, 336, 354, 382, 456, 460. and eutectics, 459. substances, nature of, 1. v. perforated bricks, 150. Solidification of molten masses, 487. Solidifying point, 484. Solids, reactions between, 435. Sols, colloidal, 11, 12, 228; see also Colloidal sols. Solubility, effect of, on reactions, 441. of products of reaction, 442. Soluble bases to be fritted, 387. iron compounds, 367. salts, 364. a cause of scum, 122, effect of, on clay, 242. on plasticity, 364. on refractoriness, 364. in brick clays, 374. in glaze, 387. Solution and replacement by water, 506. colloidal, 11. heat of, 529, 599. v. colloidal sols, 11. Solutions, composition of, and the tempera- ture of equilibrium, 600. heats of reaction in, 530, ionized, 530. molecular, 11. nature of, 11. solid, 300. Solvent, action of molten material on, 442. Soot a cause of discoloration, 121. Sortwell, 394, 397. Sosman, R. B., 325, 327, 328, 329, 357, 367, 424, 469, 471, 472, 475, 486, 488, 562. Souring, 274. Space-lattice, 323. types of, 323. units, 323. Spade mixing, 60. Spalling due to lamination, 22. to large grains, 16. effect of grading on, 32. of porosity on, 72. of rapid cooling on, 584. of magnesia bricks, 583. _ resistance to, 523, 584. Specific electrical resistance at different tem- peratures, 614. 687 Specific continued— gravity, 203. apparent, 203; see also Apparent specific gravity. bottle, 225. changes in, 204. during burning, 548, 549. determination of, 221. effect of heat on, 207. factors influencing, 207, 208. of ceramic materials and articles, see under their various names. true, 203, 205; see also True specific gravity. heat, 510. determination of, 512. of ceramic materials, effect of heat on, 594. variations of, with temperature, 595. heats of various substances, 511. inductive capacity, 609. ohmic resistance, 608. resistance of porcelain, 613. volumes, 204. Speckled bricks, 111. Speed of reaction, 499. Sphene, 429. birefringence of, 626. crystalline form of, 4. in clay, 421. pleochroism of, 630. Spinel, 40, 471, 481. crystalline form of, 4, electrical conductivity of, 609. formation of, 440. hardness of, 126. in clay, 421, 422. melting-point of, 607. refractive index of, 623. Spinels, 400, 421. constitution of, 356. Splichal, J., 232, 243. Spodumene, 342, melting-point of, 607. Spongy ware, 369. Spots, black, in ware, 121. due to pyrites, 547. Spring, 440. Sproat, I. E., 145, 288. Spurrier, H., 121, 264, 275. Squatting, 560. effect of pressure on, 561. Stability at various temperatures, 520. of crystalline solids, 487. of vitreous solids, 487. Staffordshire blue bricks, cause of colour of, 103. strength of, 173. 688 Staffordshire continuwed— fireclays, copper-iron sulphides in, 420. red marls, 109. Staining liquids, use of, 407. Stains, 507. Staley, H. F., 178, 198, 201, 315. Standard sieves, 45, 46, 47. American, 45, 47. British, 45, 46. Stanger, 225. Stansfield, 616. Star silica bricks, 187. after-expansion of, 569. crushing strength of, 187. Starch paste as a bond, 153. Starke, H., 614. State, change of, effect of, on reactions, 444. States, physical, of ceramic materials, 227. Staurolite, 416. birefringence of, 626. electrical conductivity of, 609. magnetic properties of, 621. refractive index of, 623. Steam, action of, on magnesia bricks, 499, 503. as a catalyst, 443. corrosive action of, 495. Steaming, 371. clays, 256. Steatite porcelain, 378. Steel, thermal conductivity of, 587. Steger, W., 192, 596, 633. Stelzner, 482. Stickiness, 267, 276. Sticky clays, 267. Stiperstones quartzite, structure of, 19. Stirlingite, 482. Stock bricks, strength of, 173. Stoke’s Law, 53. limitations of, 54, 55. triaxial diagram, 315. Stoneware, 392. clays, colour of, 108. coefficient of expansion of, 576, 577. electrical insulating power of, 612, firing, 554. glazes, 392. permeability of, 90. porosity of, 80. vitrification of, 551. Stony clays, size of grains in, 35. Stormer, 281. Stourbridge firebricks, specific heat of, 596. fireclays, 254. specific gravity of, 213. Stratified masses, 22. Streak, 94. Streaks in crystals, 3. of colour as decoration, 118. INDEX Strength, 139. and refractoriness, 166. at high temperatures, 164. measuring, 196. at low temperatures, ratio of, to that at high temperatures, 165. changes in, during burning, 547, 549. during smoking, 545. during vitrification, 552. crushing, 144. determination of, 193. effect of ageing on, 156. of blows on, 169. of bond on, 153. of burning on, 159. of chemical composition on, 146. of deposited carbon on, 169. of drying on, 158. of electrolytes on, 155. of flue-dust on, 169. of fluxes, 147. of frost on, 167. of grading on, 152. of grinding on, 154. of lime on, 147. of method of mixing on, 155. of preparation on, 154. of shaping on, 157. of physical properties on, 150. of porosity on, 74, 152. of pressure on, 157. of repeated heating on, 163. of repressing on, 157, 158. of shape and size on, 150. of grains on, 29. of size of grains on, 30. of slags on, 169. of sudden changes of temperature on, 163. of temperature during use on, 163. of texture on, 150, 185. of vitrified bond on, 160. of weathering on, 166. factors affecting, 146. lack of, causes of, 161. minimum allowed in Germany, 175. for building bricks, 174. of ceramic materials and articles, see under their various names. tensile, see Tensile strength. transverse, see Transverse strength; see also under the various kinds of strength. Stringer, 280. Strontia in porcelain, 377. -lime-silica system, 476. -lithia -silica system, 478. -silica system, 469. -soda-silica system, 477. Strontianite, refractive index of, 623. INDEX Strontium metasilicate, melting-point of, 607. - minerals, 421. orthosilicate, melting-point of, 607. silicate, 477, 478. melting-point of, 469. sulphate, 421. Structural formule, 308. Structure, alteration of, 25. capillary, 24. of clays, 76. cellular, 23. coarse, crystalline, 15. concretionary, 25. crypto-crystalline, 15. crystalline, 14. disadvantages of, 14. value of, 14. examination of, under microscope, 407. fibrous, 25. fine, crystalline, 15. fissile, 22. foliated, 23. granular, 15. types of, 14. homogeneous, 21, 59. ideal, of fired clay, 29. laminated, 22. massive, 21. micro-crystalline, 5, 15. nodular, 25. of atoms, 301. of ceramic materials and articles, see under their various names. of crystals, 323. physical, 1. of clays, 1. properties depending on, 27. segregated, 25. shaly, 22. shown by the microscope, 15. by polarised light, 18. unstratified, 21. Stull, 389, 395. Sturm, 326. Sublimation, 485. pressure, 441. Subsilicates, 332, 466. Substances in chemical reactions, effect of, 448. Sudden changes in temperature, effects of, 581. Suffolk bricks, production of, 102. white bricks, 112. ° Sugar, effect of, in souring pottery eee 275. Sullivan, 612, 616, 617, 618, 619. Sulphates a cause of scum, 122. as cementing materials, 15. decomposition of, by heat, 369, 491, 545. = 689 Sulphates continued— in clay, effect of, 241. spoil slips, 285. Sulphides, decomposition of, by heat, 492, 545. effect of hydrochloric acid on, 503. of nitric acid on, 504. Sulphur, corrosive action of, 495. dioxide, effect of, 493. effect of, on porosity, 68. in clay, 369. Sulphuric acid, effect of, on aluminosilicates, 504. on basic materials, 504. on clay, 409, 504. minerals soluble in, 409. Sumach, effect of, on plasticity, 275. Summering, 253. Sunlight, effect of, on clays, 507. Sunning, 253. Supercooling, effects of, 466, 484. Supersaturation, effects of, 484. Surface area, effect of, on plasticity, 261. clays a cause of scum, 122. colour of, 108. effect of heat on apparent specific gravity of, 208. combustion, 238. effect of size of grains on, 31. factor, 56, 278. flow, 274. tension, effect of, on chemical reactions, 441. of clay, changing, 247. Suspensions, 282; see also Slips. colloidal, 12. Suspensoid, 12. Sven Oden, 232, 243, 255, 264. Swarte, 608. Swedish silica bricks, specific gravity of, 219. Swelling, 562. of clay, 237. Symbols, defined, 306. Syneresis, 252. Synthesis of clay, 352. System, definition of, 453. Systems, colloidal, 12 Tableware, glaze for, 393. strength of, 178.>:> , eee ine a. oom ane: material, 16. Taléspary 427° Tian 488, 489. Tannj ig qcid,, aftgct.of; on sty ength of dried clays, roo 165. Tantalive, cletaral eonductivity n 609. hardness of, 126. Tantalum pentoxide, melting-point of, 605. 44 690 INDEX Tensile strength continued— of glazes, 192. Tension, surface, 441; see also Surface tension. Tephroite, 470. Ternary system, phase diagram of, 463. Tap cinder, corrosive action of, 496. penetrative power of, 497. Tar as a bond, 153, 405. Temperature, absolute scale of, 533. advantages of slow rise in, 440. and chemical activity, 322. in ceramics, 474. attained in the firing, effect of, on strength, | Terra-cotta clay, felspar in, 417. 160. colouring, 100. Celsius scale of, 533. control of colour of, 110. Centigrade scale of, 533. discoloration of, 121. changes, effect of, on strength, 163. effect of frost on, 73. of, 163, 519. of fluorspar on, 148. of, and strength, 163. of fluxes on, 148. sudden, effect of, 581. finishing temperature of, 553. sudden, resistance to, 581. glazes, 386, 387, 392. -composition diagrams, 455, 456, 465. importance of colour of, 547. critical, 485. minerals in, 415. due to molecular movement, 509. porosity of, 77. during use, 163. strength of, 177. effect of, on cementation, 507. texture of clay for, 37. on ceramic materials, see under their thermal conductivity of, 593. various names. resistivity of, 594. on chemical reactions, 435, 437. Terrome, 231. on crystallisation, 488. Tests, see under the various kinds. on density, 205. Tetmaier, 150. on diffusivity, 588. ““Te’’ value, 609. on intensity of chemical action, 445. effect of beryllia on, 613. on reaction, 438. Texture, 27. on specific gravity, 209, 588. and carbonaceous matter, 370. heat, 588. and porosity, 205. on thermal conductivity, 588. coarse, 27, 36. ‘on vitreous solids, 487. objections to, 37. raising the temperature, 439. producing, 36. Fahrenheit scale, 533. finishing, of burning, 553. measurement, 532. of casting, 287. determination of, 44-59. effect of, 151. grading on, 31. on abrasion, 38. of reaction, 438. of ware and electrical resistivity, 611. and insulating power, 611. optimum, for souring, 275. relation of, to heat, 508. required to produce good colours, 110. resistance of, to changes in, 581. scales of, 509. -time diagrams, use of, 465. variation of specific heat with, 595. working range of, 438. Temperatures, high, reactions at, 490. homogeneous, 59. _ low, reactions at, 503. * ’ Ba oxi hie medium, 36. Témpering, OB oie | gt Vee sce Sete a eocs so] cs “producing, 36. «effect ‘of, on siliéa PER 166. oer: “of ceramic materials and articles, see under on strength, 155. their various names. Temporary’ ‘bonds fdr seksi ds 405. relation of, to porosity, 63. Tensile stv ength, "140, apis es Textures, comparison of, 56. determination of, 193. Thermal capacity, 510. of clays, variations in, according to mode of condition of a body, 508. drying, 159. conductivity, 514. on coefficient of expansion, 576. on corrosion, 38. on durability, 38. on porosity, 64. on strength, 185. on thermal conductivity, 514. shape of grains on, 27. size of grains on, 29. fine, 27, 35. objections to, 35. producing, 33. INDEX Thermal conductivity continued— determination of, 515. effect of heat on, 584, 585. of temperature on, 586, 587, 588. of permeability on, 89. of porosity on, 72, 74. factors influencing, 514. | formula for, 588. of ceramic materials and articles, see under their various names. units of, 515. curve of clay, 349. curves of alumino-silicates, 352. equilibrium, 509. mho, 518. ohm, 518. resistivity, 593. value of a reaction and heats of formation, 531. Thermo-chemical reactions, 530. -couple pyrometers, 536, 537. Thermometers, 536. gas, 536. Thermometry, 509, 536. Thermoscopes, 540. Thickness of sample, effect of, on translucency, 633. Thomas, S. G., 401, 402. Thompson, H. V., 353. Sir J. J., 301, 305. Thoria, melting-point of, 605. source of, 21. Thorpe, T. E., 388. Thugutt, 307, 338. Tiles, apparent density of, 214. black, 111. blue, 111. coefficient of expansion of, 573. control of colour of, 110. fireclay, transverse strength of, 182. floor, resistance of, to traffic, 125. hardness of, 129. importance of colour of, 547. porosity of, 77. resistance of, to abrasion, 130. roofing, permeability of, 89. resistance of, to weathering, 166. strength of, 177. Time and mass, relationship of, 439. effect of, on chemical reactions, 439. of tempering, effect of, on strength, 156. -temperature curves of ceramic materials, 464. of fused substances, 489. diagram, use of, 465. Tin-bearing minerals in clays, 422. oxide as opacifier, 395, 396. Titaniferous magnetite, electrical conductivity of, 609. 691 Titanite, crystalline form of, 4. electrical conductivity of, 609. hardness of, 126. melting range of, 607. refractive index of, 623. Titanium compounds, colours produced by, 118. in clay, 368. in zirconia ore, 429. mineral nature of, 428. materials, use of, 428. in clays, 421. orthosilicate, melting-point of, 607. oxide, effect of, on alumina, 368. on colour of zirconia, 120. on porosity, 68. on refractoriness, 368. in crystalline glazes, 397. in fireclays, 373. in glazes, 397. melting-point of, 605, 607. Titanoferrite, magnetic properties of, 621. Topaz, birefringence of, 626. electrical conductivity of, 609. formation of clay from, 354. refractive index of, 623. Torsion tests, 199. Toughness explained, 139, 143, 281. Tourmaline, 418. as catalyst, 443. birefringence of, 626. electrical conductivity of, 609. in china clay, 20. effect of, 107. melting range of, 607. pleochroism of, 630. refractive index of, 623. Traffic, resistance to, 125. Transition point of eutectic, 459. Translucency, 632. and finishing temperature, 376, 558. measurement of, 633. Transmission of heat, 514. Transparency, 631. Transverse or. cross-breaking tests, 145, 197. of bricks, 175. at high temperatures, 181. of clays, 170-172. of pieces of sagger, 184. Travers, M. W., 183. Treading plastic materials, 59. Tremolite, 416. birefringence of, 626. electrical conductivity of, 609. structure of, 25. Trials as pyroscopes, 541. Triangular grading diagram, Feret’s, 58. Triaxial diagrams, 315. 692 Tricalcium aluminate, melting-point of, 606. ° ferrate, melting-point of, 606. silicate, 333. Trichites, 3. Trichroic minerals, 630. Tridymite, 328, 426, 475, 480. bricks, strength of, 188. conversion of, into cristobalite, 331. crystalline form of, 4. structure of, 2, 14. distinction of, from cristobalite, 625. formation of, 329, 330, 443. hardness of, 126. in furnace hearths, 17. in lime-silica system, 467. in silica bricks, 16. melting-point of, 331, 603. produced from fused silica, 332. refractive index of, 623. of determination of, 625. solubility of, in hydrofluoric acid, 504. specific gravity of, 216. Trimorphous crystals, 5. Trins, 2. Triple oxides, 480. point in a system, 453. Triplet, atomic, 327. crystals, 2. Trisilicates, 332. Trisilicic acid, 333. Troost, 578. Trouton, 524. True clay, 343. determination of, 410. porosity, 61. determination of, 83. specific gravity, 203, 205. determination of, 224. Truesite, 413. Tschermak, 307, 309. Tucker, 493. Tufts in clays, 3. Tungstic acids, as catalyst, 443. oxide in crystalline glazes, 397. Turgite, 419. Turner, W. E.8., 70, 71, 135, 206, 207, 210, 213. Twinned crystals, 2. Two gases, reactions between, 435. liquids, reactions between, 435. Tyler, W. S., & Co., 45. Ullmanite, space-lattice of, 324. Ulrich, 597. Ultrabasic rocks, chromite in, 430. of Isle of Rum, crystallisation in, 461. Ultramicroscopic particles, 630. Umber, 413. INDEX Under-burning, 560. -cooled liquids, 354. cooling, 487. -glaze decoration, 96. -heating, effect of, 407. Unfired bodies, identified by colour, 106. Unglazed ware, porosity of, 77. U.S. Bureau of Standards, 45, 181, 182. Unmixing, 283. Unsaturated compounds, defined, 305. Unstratified masses, 21. Unwin, 173. Uranium oxide in crystalline glazes, 397, 398. Uses, effect of permeability on, 89. of porosity on, 75. Utility and composition of clays, 371. Vacuum, effect of,in porosity determination, 82. Valencies of principal elements, 305. Valency, 305, 308. Vale of Neath, Dinas sand in, 20. - Van Bemmelen, 235, 237, 264, 421. Hise, 354. Klooster, 469, 470, 607. Van ’t Hoff’s Law, 461, 467. Vanadium compounds, discoloration produced by, 121. in clay, 369. oxide a cause of discoloration, 123. in crystalline glazes, 397. in matte glazes, 396. Vaporisation, latent heat of, 524. molecular heat of, 524. Vapour density and molecular weight, 305. pressure, effect of, on chemical reaction, 440. Vapours, destruction of firebricks by, 495. Variation, definition of, 453. Variegated colour, effects, 118. Vein quartz, segregation of, 25. structure of, 14. Velocity constants of reactions, 449. of a reaction, effects of heat on, 528. of chemical reactions, 448, 449, 450. of reaction, increasing, 450. Venetian red, use of, 117. Vermicules in china clay, 19, 411. in clays, 3. Vermiculites, 19, 411. Vernadsky, 307, 309, 413. Very porous firebricks, apparent density of, 214. Vesicular structure, 610. development of, 207. Vicat needle, 199, 200, 280. Viennese porcelain, 375, 378. Violet colours, producing, 116. Viscosimeters, 279, 291-293. INDEX Viscosity and plasticity, 272, 273. defined, 290. ‘effect of, on chemical action, 441. on crystallisation, 488. electrolytes on, 286. measurement of, 290. of colloidal sols, 234. of slips, 282, 290. increase of, 244. Vitreosil, 400. Vitreous lustre, 95. solids, stability of, 487. state, energy in, 487. production of, 487. substances, 6. use of, in ceramic materials, 6. Vitrification, 551. and balanced reactions, 452. and fluxes, 552. effect of, on electrical conductivity, 74. range, 486, 552. of clays, 552. rate of, 552. Vitrified articles, effect of frost on, 73. bonds, 154. bricks, 175. finishing temperature of, 553. hardness of, 128. resistance of, to abrasion, 128. claywares, coefficient of expansion of, 573. material, 484. ware, production of, 463. testing porosity of, 80. Vivianite, 420. Vogt, J. H. L.,-440, 443, 461, 511, 600. Voids, effect of grading on, 32. Volatilisation, 551, 561. of colour, 118. silica, 491. Volatilised substances, action of, 495. Volume-changes due to heat, 520. during burning, 547. vitrification, 552. permanent, due to heating, 564. reversible, 570. which occur during heating, 563. critical, 485. weight, 203, 222.° determination of, 221. of bricks, 215. of slips, 226. Volumeter, Ludwig’s, 83. Seger’s, 84. Waage, 448. Waele, A. de, 272, 279, 281, 290. Wallace, 476, 477, 478, 480, 606. Wall white, 24, 182. Wanner, pyrometer, 538. Ware, biscuit, colour of, 112. glost, colour of, 112. white, 112. Wartenburg, H. v., 598. Wartha, 309. Washburn, E. W., 61, 85, 86, 329, 470, 600. Washing test, 48, 49. Washington, 315, 413. Watanabe, 24. Water, a source of scum, 122. absorption of, by clays, 254. absorbed by dry clay, 75.. acid, in silicates, 338. action of, on rocks, 507. and mobility of clay, 276. and plasticity, 277, 279. and rate of flow, 273. carbonated, effect of, 504. changes effected by, 227, 253. colloidal, 336. combined, in alumina, 340. disintegrating effect of, 254. effect of, on ceramic materials, 503. on clay, 228. on dolomite, 503. on lime, 503. on magnesia, 503. on silicates, 503. on strength, 154. Water-glass, 468. as a bond, 154, 399, 405. use of, in purifying clay, 288. heat of formation of, 531. in clays, 423. in minerals, 423. of constitution, 336, 337, 358. in clay, 371. clay, evolution of, 350. of crystallisation, 336, 337. in clay, 371. of hydration, 336, 337. of plasticity, 336. phase diagram of, 455. proportion of, in slips, used for casting, 285, 286. rate of flow of, in capillary tubes, 87. removal of, by evaporation, 288. during smoking, 544. from pastes, 294. from slips, 294. required, effect of texture on, 30. for different moulding processes, 270. to develop plasticity, 262, 267. Watkin, 152. Watkin’s pyroscopes, 540. Watts, A. S., 165, 171, 275, 376, 481, 486, 575, 577, 610. 693 694 Weather, exposure to, 506. Weathering, 227, 253. a cause of scum, 122. alteration of structure by, 26. artificial, 256. chemical actions in, 506. effect of, on pyrites, 419. on strength, 166, 167. porosity on, 73. Weber, 287. Wedge pyrometer, 538. Wedging clay pastes, 59. Wedgwood, 542. Weight per pint of slips, 283. Weimer, G., 613, 615. Wein, 578. Wenzel’s Law, 484. Wernicke, 14. Western clays, burned, colour of, 110. Wetting clay, rate of, 240. Wetzel, G., 518, 588, 590, 592, 596, 598. Wheeler, 260, 552. Wherry, E. T., 334. White, W. P., 597, 598. bricks, 112. strength of, 173. clay, 108. scum, 122. Whiteware, 112. effect of iron oxide on, 366. finishing temperatures for, 553, 555. Whitewash, 122. Whiting, effect of, on firebricks, 495. on shrinkage of earthenware, 567. Witherite as a cementing material, 15. Whitmore, 395. Whitney, 235. Wholin, R., 339, 340. Wilhelmy’s Law, 449. Willemite, 333, 397, 470, 482. Williams, 392, 393. Wilson, S. T., 595, 596. Windsor firebricks, 373. loam, 373. Wintering, 253. Woestyn, 514. Wolframite, crystalline form of, 4. electrical conductivity of, 609. Wollastonite, 363, 415, 421. birefringence of, 626. forms of, 468. melting range of, 607. specific heat of, 597. Wologdine, S., 92, 594. Worcester, W. G., 77, 110. Wright, 286, 468, 471, 473. Wyoming, bentonite in, 412. 223, 515, 589, INDEX 590, Xenotime, electrical conductivity of, 609. refractive index of, 623. X-ray spectra, drawback to, 325. spectrum of china clay, 19. use of, 299, 307, 322, 324, 344, 349. structure of colloidal gels, 252. X-rays and crystal structure, 322. structure shown by, 8. Yaichiro Kitamura, 394. Yates, W. H., 112. Yellow colour in clays, cause of, 108. produced by iron compounds, 97. production of, 97, 117. discoloration of, 122. films, 507. -green stain, cause of, 123. goods, producing, 114. scum, 123. stains, 507. Yellowstone Park, geyserite in, 424. Yielding point, 164. Yorkshire, red-burning clays of, 109. Yttria, melting-point of, 605. source of, 21. Zeolites as cementing materials, 15. in glazes, 355, 389. Zinc-alumina-silica system, 482. -blende, space-lattice of, 324. corrosion of furnaces by, 495. metasilicates, 482. melting-point of, 607. orthosilicate, 482. melting-point of, 607. oxide as opacifier, 395. effect of, on colour, 116, 117. shrinkage, 567. in crystalline glazes, 397. in matte glazes, 396. in porcelain, 377. silica system, 470. use of, for increasing crystalline matter in glaze, 390. silicate in crystalline glazes, 397. in matte glazes, 396. melting-point of, 470. slag, penetration of, 502. spinel, 400, 482. graphic formula of, 356. Zinnewaldite, 417. Zircon, 333, 403. as catalyst, 443. birefringence of, 626. colour of, 120. crystalline form of, 4. Zircon continued— electrical conductivity of, 609. fusing-point of, 470. hardness of, 126. melting-point of, 605. occurrence of, 429. refractive index of, 623. sand, 20, 403. space-lattice of, 324. specific gravity of, 221. Zirconia, 403. articles, porosity of, 79. as opacifier, 395. bricks, 403. and spalling, 584. electrical resistance of, 618. hardness of, 133. hot, strength of, 165. resistance of, to abrasion, 130. INDEX 695 Zirconia continued— colour of, 120. crucibles casting, 285. porosity of, 79. effect of titanium in, 120. melting-point of, 470, 605. occurrence of, 429. polymerisation of, 358. reduction of, 493. -silica system, 470. sources of, 20. specific gravity of, 221. heat of, 596, 598, 599. structure of, 21. water required to develop plasticity of, 269. Zirconium compounds, mineralogical nature of, 429, impurities in, 429. silicate, melting-point of, 470. to cement, 500. to fluorspar, 500. to oxides, 500. to slags, 500. to temperature changes, 583. shrinkage of, 570. strength of, 192. Zirkel, 621. Zoellner, 483. Zoisite, 342, 418. birefringence of, 626. formation of clay from, 354. Zschokke, 143, 195, 262, 279. 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