^¥im» - ^ w 3» d f A -i jewEST™ .,, PRACTICAL Dental Metallurgy A TEXT- AND REFERENCE-BOOK FOR STUDENTS AND PRACTITIONERS OF DENTISTRY Embodying the Principles of Metallurgy, and their Applica- tion to Dentistry, including an Addendum of Collateral Literature, and Experiments. BY JOSEPH DUPUY ^HODGEN, D. D. S. Assistant to the Chair of ^Dental Chemistry and Metallurgy, College of ^Dentistry, University of California; Late Editor of Pacific Coast 'Dentist. SECOND EDITION COMPLETELY REVISED, REARRANGED, AND ENLARGED. SAN FRANCISCO : THE HTCKS-JUDI) CO, PRINTERS, PUBLISHERS, AND BOOKBINDERS'! 1897. TWO COPIES RECEIVED Entered according to Act of Congress, in the year 1897, By Joseph D. Hodgen, In the office of the Librarian of Congress, at Washington, D. C. TO CLARK LA MOTTE GODDARD, A. M., D. D. S., Professor of Orthodontia, College of Dentistry, University of California, THIS WORK IS INSCRIBED, IN ADMIRATION OF HIS TALENTS, AND GRATITUDE FOR HIS TEACHINGS, CRITICISMS, AND FRIENDSHIP. PREFACE TO FIRST EDITION. In presenting this little volume to the practitioner and student of the dental profession, the author does not natter himself that he is filling a void in such literature, or that a crying need has been felt in the profession for this particular production. It has, however, grown out of the exigencies of the writer's own classroom and laborator} 7 , after several years' practical experience as an instructor on its subject. The endeavor has not been to furnish a scientific and ex- haustive treatise on metallurgy, but rather to present, in a clear and practical manner, the principles of that subject as the author sees them related and applicable to the every-day wants of the dentist. Keenly appreciating the reluctance with which this and the analogous study of chemistry have been pursued by the aver- age student, the author has sought to awaken a deserving interest by doing away with the usual lectures, and employing the work as a text-book, subject to explanatory elaboration during the recitation; and to further make it so practical that it may be taken into the metallurgical laboratory and used as a manual for practical and experimental work. It presupposes the student to possess a fair knowledge of the principles of inorganic chemistry, comprehending the reading and writing of formulae, atomic affinities, and the expression of equations. An addendum refers the interested student to the opinions of others, and more elaborate essays, papers, and discussion by authors who have made a particular study of some principle merely hinted at in the text. In the selection of these, the object has been to refer to those most available to all students, and not to intimate that other publications are devoid of equally scientific and instructive productions. The author has freely consulted and quoted from whatever works on metallurgy and allied subjects were in his reach, especially the exceptionally scientific papers on amalgams by Prof. G. V. Black, published in the Dental Cosmos; and the val- uable contributions to the American System of Dentistry, entitled Dental Metallurgy, by Dr. Edward C. Kirk ; Brannt's Metallic Alloys; the works on metallurgy by Makins, Fletcher, Essig, Mitchell ; the chemistries of Roscoe, Bloxam, and many others, found in the library of the California Mining Eureau, through the kindness of the State Mineralogist, Mr. J. J. Crawford ; together with the Denial Cosmos, Denial Review, International Denial Journal, and several others. To the authors and editors of these, the author takes this opportunity to express his grate- fulness for the liberties taken. For valuable criticisms and suggestions the author is especially grateful to Prof. C. L. Goddard, and also wishes to express his obligation to the firm of J. H. A. Folkers & Bro. for courtesies so kindly extended; also to Dr. S. Eldred Gilbert, Hastings & Co., Hood & Reynolds, and a number of other Eastern manufacturers, for their prompt responses to inquiry. JOSEPH D. HODGEN. PREFACE TO SECOND EDITION. The kindly reception and success accorded the first edition of Practical Dental Metallurgy has made a second edition necessary to fill the wants of the numerous colleges which have adopted the work as a text for their classes in this branch of science. The present edition is completely revised, rearranged and somewhat enlarged. The chapters on amalgams have been placed after the discussion of the metals individually, with the belief that such an arrangement will facilitate a more compre- hensive grasp of this most important subject. These chapters have been wholly revised with a conscientious endeavor to present the newest facts and most accurate data attainable, thus keeping the book abreast with the scientific investigations of the day. The more than kind words which have come to me so fre- quently through the professional press, from fellow teachers, and the profession generally have been a source of much pleasure to me and I wish to express my gratitude for all the encouragement which has been so generously extended. JOSEPH D. HODGEN. No. 1005 Sutter Street, San Francisco, August 30, 1897. CONTENTS. Chapter Page I. Introduction 9 II. The Properties of Metals 19 III. Combination of Metals with Non-Metallic Elements. 35 IV. Melting Metals 50 V. Alloys 83 VI. Lead 98 VII. Antimony. .. 108 VIII. Tin 114 IX. Bismuth 125 X. Zinc 134 XI. Cadmium 148 XII. Copper 153 XIII. Iron 165 XIV. Aluminum 185 XV. Mercury 198 XVI. Silver 211 XVII. Iridium 229 XVIII. Palladium 233 XIX. Platinum 238 XX. Gold 246 XXI. Amalgams 293 XXII. Classified Amalgams 318 ADDENDUM— Collateral Literature 335 PRACTICAL DENTAL METALLURGY. CHAPTER I. INTRODUCTION. CHEMISTRY is that branch of science which treats of the atomic conditions of matter, and espe- cially of atomic changes. It comprehends the combina- tion of diverse forms of matter producing new compounds, and the separating of already existing compounds into simpler ones, or resolving them into their ultimate princi- ples, which are called — ELEMENTS. — Substances whose molecules con- tain one kind of atoms only, and which all physical or chemical processes have as yet failed to break up or decompose into two or more dissimilar substances. It is not asserted that these substances are absolutely simple or elementary, or that they may not be found hereafter to yield more than one kind of matter, but merely so far as our knowledge extends it is so; indeed, recent spectro- scopic researches favor the impression that some, at least, of the elements are, perhaps, compounds of simple bodies. Sixty-six elements are at present known to us, of which the following is a list, arranged according to their electropositive and negative quality, or the electrochem- ical series. The most important are distinguished in the table by capitals, whilst those which at present are of slight importance, on account of their rare occurrence, or of our insufficient knowledge of their properties, are given in italics. 10 PRACTICAL DENTAL METALLURGY. TABLE OF ELEMENTS. NEGATIVE END. S3^mbol. o s N F CI Br I Se P As Cr V Mo W B C Sb Te Ta Cb Ti Si H Au OS Ir Pt Rh Ru Pd Hg Ag Cu Name. OXYGEN SULPHUR NITROGEN FLUORINE CHLORINE BROMINE IODINE Selenium PHOSPHORUS j ARSENICUM | CHROMIUM j Vanadium | Molybdenum Tungsten (Wolfram) j BORON I CARBON ANTIMONY (Stibium) . . . . Tellurium Tantalum Columbium (Niobium) Titanium '. SILICON HYDROGEN GOLD (Aurum) Osmium Iridium PLATINUM Rhodium Ruthenium Palladium MERCURY(Hydrargyrum) SILVER (Argeutum) COPPER (Cuprum) Valence. II II, IV, VI I, III, V. . I I, III, V, VII ... I, III, V, VII... I, III, V, VII. . . II, IV, VI III, V Ill, V II, (Cu 2 ) V[ , VI.. III, V II, IV, VI II, IV, VI Ill II, IV III, V II, IV, VI Ill V III, V IV IV I I, III II, IV, VI, VIII II, IV II, IV II, IV II, IV, VI. VIII II, IV II, (Hg 2 ) ir I II, (Cu a ) TI Atomic Weight. 15.96 31.98 14.02 18.98 35.37 79.76 126.55 78.79 30.95 74.91 52.00 51.25 95.52 183.61 10.94 11.97 119.95 127.96 182.14 93.81 47.99 28.19 l.OO 196.15 198.49 192.65 194.41 104.05 104.21 105.73 199.71 107.67 63.17 INTRODUCTION. 11 TABLE,' OF ELEMENTS— Continued. NEGATIVE] END. Symbol. Valence. u Bi Sa In Pb Cd Tl Co Ni Fe Zn Ga Mn La D Ce Th Zr Al Er Y Gl Mg Ca Sr Ba Li Na K Rb Cs Name. Uranium BISMUTH TIN (Stannum) Indium LEAD (Plumbum) CADMIUM Thallium COBALT NICKEL IRON(Ferrum) ZINC Gallium MANGANESIUM .... Lanthanum Didymium Cerium Thorium Zirconium ALUMINUM Erbium Yttrium Glucinum (Beryllium). . MAGNESIUM CALCIUM STRONTIUM BARIUM LITHIUM SODIUM (Natrium). .. POTASSIUM (Kalium) Rubidium Ccv.sium , II, IV, VI III, V II, IV Ill II, IV II I, III II, (Co,)* 1 II, (Ni 2 )^....,. II, VI, (Fe 2 ) vr ... II (G«.) VI . II, IV, VI, VIII III III, V IV, (Ce 2 ) vr IV IV Ill, or (A1 2 ) VI ... Ill, or (Er 2 ) VI ... Ill II II II, IV II, IV II, IV I POSITIVE END Atomic Weight. 238.48 207.52 117.69 113.39 206.47 111.83 203.71 58.88 57.92 55.91 64.90 68.85 53.90 138.52 144.57 140.42 233.41 89.36 27.00 165.89 89.81 9.08 23.95 39.99 87.37 136.76 7. 22.99 39.01 85.25 132.58 12 PRACTICAL DENTAL METALLURGY. To these may be added *Davyum and ^Terbium. Some ten or twelve other substances thought to be ele- ments are sometimes given, but as their identity has not yet been thoroughly established, it is thought better to omit them. These sixty-six elements are considered under two great divisions, which are known as metallic and non- metallic. METALLIC ELEMENTS, the metals, or, as they are frequently termed, the positive elements, are fifty-two in number (denoted in the table by the *), and the study of these constitutes — METALLURGY.— The science of economically ex- tracting metals from their ores, and to this strict definition may be added the art of applying them to useful purposes. AN ORE is a substance containing one or more metals in their natural state. The metal exists chiefly as a sulphide, oxide, or carbonate, and often times as a chloride, arsenide, sulphate, phosphate, or silicate. Such metals as goM and platinum are usually found in a free or metallic state, then they are termed " native." Tin, silver, copper, and some other metals are occasion- ally found native. GANGUE. — The foreign material or impurity in which minerals are found embedded is variously known as "gangue," "veinstone," or "matrix." This may consist of such carbonates as calc-spar, limestone; such silicates as feldspar, hornblende, and mica; such sulphates as heavy-spar; and such fluorides as fluor-spar. This is separated from the mineral by the miner in crushing, sorting, and washing operations known as "dressing," after which the ore is sent to the metallur- gist. INTRODUCTION. 13 SLAG is the refused fused metallic dross or recre- ment separated from the metal baring compounds when the minerals of iron, copper, silver, nickel, and cobalt are fused with arsenic, sulphur, and silica. Oxides unite with silica and form a part of the slag. REGULUS.— When the minerals of iron^ copper, and silver are smelted or fused with substances con- taining sulphur the resulting sulphide is known as 4 ' regulus " or " matte. " SPEISS. — When the minerals of nickel and cobalt are similarly fused and converted into arsenides the combination is termed " speiss." REDUCTION is the process of freeing a metal from its combinations. The substance effecting this result is called a " reducing agent." The chief reduc- ing agents are carbon, hydrocarbons, carbon monoxide, and hydrogen. In this process metallic compounds are usually converted into oxides, if they do not already exist as such. This is generally accomplished by heat- ing in contact with atmospheric air. For example, when zinc carbonate is thus treated the reaction or reduction is as follows: ZnC0 3 ( + heat)=ZnO + C0 2 . Then by addition of the reagent carbon the metallic zinc is obtained thus: ZnO+C=Zn + CO. Sulphides are reduced by partially converting the metallic sulphide into an oxide, with the aid of heat, when the remaining metallic sulphide reacts with the oxide produced, freeing the metal, as for example: PbS + 2PbO=S0 2 -!-3Pb. These various processes are called " smelting." 14 PRACTICAL DENTAL METALLURGY. ROASTING. — When metalliferous substances are reduced to oxides by heating, in contact with atmos- pheric air, the process is called "roasting." When they are similarly heated in contact with chlorine gas or with common salt, the operation is known as "chlorin- izing roasting." r CALCINATION is the process of heating a sub- stance at a temperature below its melting point. The object is to expel all volatile and organic matter, and, in the ease of ores, to render it more porous prepara- tory to roasting or smelting. DISTILLATION.— The act of separating the more volatile portions of a substance by heat in the form of vapor, and subsequently condensing it to a liquid in some cooling receiver or worm. Mercury and zinc are extracted from their ores by this process, and the former metal is purified by redistillation. SUBLIMATION is an analogous process, except that the substance separated as a vapor is condensed as a solid. For example, arsenic is sublimed from ores containing it. SCORIFICATION is the process of converting the foreign substance present in a metallic compound into scoria or slag by oxidation and union with silica. The vessel in which the operation is effected is termed a scorifier. OCCLUSION is the property possessed by some metals of absorbing and retaining certain gases, thus — iron absorbs carbonic oxide readily, silver occludes oxygen, platinum will absorb considerable quantities of oxygen and hydrogen, and it has been demonstrated that palladium foil under certain circumstances will absorb 982 volumes of hydrogen. INTRODUCTION. 15 CEMENTATION is the reaction which takes place between two substances without fusion. — Thus, wheu iron is heated with charcoal — carbon — a reaction takes place and the iron is said to become carburized. Such a reaction is known as " carburizing cemen- tation." When iron is heated with red haematite, Fe 2 3 , as an oxidizing agent, the impurities contained in it are modi- fied or removed by the cement powder. Such a process is known as an "oxidizing cementation." DRY PROCESS.— The operation of separating metals from metallic combinations or metalliferous matter by the agency of heat. WET PROCESS.— The operation of separating metals from metallic combinations or metalliferous matter by suitable solvents, such as the ordinary acids, etc., and then precipitating those desired with proper reagents or by an electrochemical process. Of the fifty-two elementary substances known as metals only fourteen are employed in their true metallic co?idi- tion. These are: Iron, Antimony, Copper, Magnesium, Lead, Bismuth, Zinc, Gold, Tin, Silver, Aluminum, Mercury, Nickel, Platinum, About twelve are more or less useful in the preparation of medicines, in the arts for coloring pigments, and for alloying purposes. These are: Potassium, Arsenicum, Sodium, Chromium, Calcium, Cobalt, Lithium, Cadmium, Barium, Titanium, Manganesium, Uranium. 16 PRACTICAL DENTAL METALLURGY. While the remaining twenty-six are more or less rare, and as yet of little or no practical value in the metallic state. The metallurgist groups the metals into two classes, which are known as noble and base; NOBLE METALS are those whose compounds with oxygen are decomposable by heat alone, at a temperature not exceeding redness. These are: Mercury, Rhodium, Stiver, Ruthenium, Gold, Osmium, Platinum, Iridium, Palladium. BASE METALS are those whose compounds with oxygen are not decomposable by heat alone, retain- ing oxygen at high temperatures. The base metals are further subdivided with reference to their affinity for oxygen and other chemical properties. THE FIRST DIVISION Contains five metals. They are very readily oxidized, and their oxides are all soluble in water, giving it a strongly alkaline reaction; so also are their phosphates and carbonates, with the exception of lithium phosphate, which is quite insoluble, and the carbonate, which is only sparingly soluble. They all energetically decom- pose water at ordinary temperatures, liberating hydrogen, and forming hydrates in solution. They are soft, of low specific gravity, and fusible at low temperatures. These are: Potassium, Rubidium, Sodium, Caesium. Lithium . INTRODUCTION. 17 THE SECOND DIVISION Contains four metals, all of which decompose water at ordinal temperatures, except magnesium, combining with the oxygen. Their oxides are more or less soluble in water, rendering it alkaline; but their neutral carbon- ates and phosphates are insoluble. These are: Barium, Calcium^ Strontium , Magnesia m . THE THIRD DIVISION Contains thirteen metals, of which but three are of much importance. Those which have been isolated do not de- compose water at ordinary temperatures without the addition of a weak acid or a slight rise of temperature. Their oxides and carbonates are insoluble in water. These are : Aluminum, Erbium, Chromium, Cerium, Titanium, Lanthanum, Glucinum, Didymium, Thorium, Tantalum, Yttrium, Columbium. Zirconium, THE FOURTH DIVISION Contains nine metals, the chief of which decompose water at a red heat. These are : Iron, Uranium , Nickel, Vanadium, Cobalt ', Thallium, Manganesutm , Indium. Zinc, 18 PRACTICAL DENTAL METALLURGY. THE FIFTH DIVISION Contains four metals, which do not decompose water at any temperature. These are : Cadmium, Lead, Bismuth, Copper. THE SIXTH DIVISION Contains six metals. All the higher oxides of these metals have acid properties. These are : Tin, Molybdenum, Antimony \ Tungsten, Arsenic, Tellurium. The non-metallic elements may be divided according to their physical states at ordinary temperatures, thus: Gases. Oxygen, Nitrogen, Fluorine. Hydrogen, Chlorine, Solids. Carbon, Sulphur, Phosphorus Boron, Selenium, Iodine. Silicon. Liquid. Bromine. CHAPTER II. PROPERTIES OF METALS. A METAL is an elementary substance, solid at ordinary temperatures, with the single exception of mercury (a liquid solidifying at — 39° C), having a peculiar luster, called a " metallic luster," and the property of replacing hydrogen in chemical reac- tions, as for example: Zn + H 2 S0 4 =ZnS0 4 +H 2 , insoluble in water, a good conductor of heat and electricity, and possessing the quality of uniting with oxygen to form a basic oxide. No line can be sharply drawn between metals and non- metals; just as none can be drawn between soluble and insoluble, poisonous and non-poisonous, substances. The two elements, arsenic and tellurium, may well be considered the intermediate links between the two classes. Sir Henry Roscoe says:* "Arsenic closely resembles phosphorus in its chemical properties and in those of its compounds, although in physical characters, such as specific gravity, luster, etc., it bears a greater analogy to the metals; indeed, it may be considered the connect- ing link between these two divisions of the elements, antimony and bismuth being closely connected with it on the one hand, and phosphorus and nitrogen on the other." Bloxam evidently does not regard arsenic as a metal. Of it he says:f "This element is often classed among the metals, because it has a metallic luster and conducts electricity, but it is not capable of forming a base with oxygen, and the chemical character and composition of * Wessons in Elementary Chemistry, p. 148. f Chemistry, Inorganic and Organic, p. 272. 20 PRACTICAL DENTAL METALLURGY. its compounds connect it in the closest manner with phosphorus." Of tellurium Roscoe says:* ' 'Although resembling a metal in its physical properties, [it] bears so strong an analogy to sulphur and selenium in its chemical relations that its compounds are best considered in this place" (under the head of non-metallic elements). Again , iodine is a crystalline solid, with bright metallic luster, but low specific gravity (4.95), and other properties of the non- metallic elements. Hydrogen, while thought to be a metal, owing to the similarity of its chemical properties to other metals, does not, at ordinary temperatures at least, present the physi- cal qualities of metals. All metals, when exposed in an inert atmosphere to a sufficient temperature, assume the form of liquids and present the following characteristic properties: They are practically non-transparent and reflect light in a peculiar manner, producing what is called metallic luster. When kept in non-metallic vessels they take the shape of a con- vex meniscus. When exposed to greater temperatures, some sooner, others later, pass into vapors. What these vapors are like is not known in many cases, since, as a rule, they can be produced only at very high tempera- tures, precluding the use of transparent vessels. Silver vapor is blue, that of potassium green, and many others — mercury, for example — colorless. The liquid metals, when cooled down sufficiently, some at lower, others at higher temperatures, congeal into compact solids, en- dowed with relative non-transparency and the luster of their liquids. NON-TRANSPARENCY.— Metals as a rule are non- transparent, or opaque, yet some have proven to possess * Wessons in Elementary Chemistry, p. 131. PROPERTIES OF METALS. 21 the property of transparency in a low degree at least. In the case of gold : Through the leaf, or thin films pro- duced chemically on glass plate, a light green color is transmitted. Also very thin films of mercury are said to transmit light with a violet-blue color, and copper, it is claimed, is somewhat translucent; while silver in in- finitely thin films is absolutely opaque. COLOR. — Most metals range from the pure white of silver and tin to the bluish hue of lead. Bismuth is a light gray, with a delicate tinge of red. Copper is called the " red metal." Gold is a rich yellow; barium and strontium a straw color, while calcium exhibits a little deeper shade. LUSTER. — Polished metallic surfaces, like those of other solids, divide any incident ray into two parts, of which one is refracted, while the other is reflected, with this difference, however, that the former is completely absorbed, while the latter, in regard to polarization, is quite differently affected, which fact, in all probability, accounts for the peculiar property of metallic luster. ODOR AND TASTE.— Most metals are destitute of odor and taste. Peculiar odors are, however, evolved from some of them when heated; in fact, one of the means of discriminating arsenic consists in its characteristic smell of garlic when heated. Iron, copper or zinc when heated also evolve peculiar odors. The taste which is perceived in some is no doubt due to some peculiar char- acter, although in some cases it may depend upon voltaic action set up by the chemical agency of the saliva, the metal not being perfectly pure. If a piece of zinc be placed upon the tongue, and a piece of silver under it, and the edges joined, a metallic taste will be perceived dependent on slow solution of the zinc under electric 22 PRACTICAL DKNTAL METALLURGY action. The odor, Dr. Kssig says,* " may be noticed in a marked degree when holding in the hand a mass of an alloy composed of gold, platinum, tin, and silver pre- pared for use as amalgam. The moisture of the hand, aided by its heightened temperature, seems to promote the electrical action." CRYSTALLINE FORM.— Most, if not all, metals are capable of crystallization, and their crystals belong to the following systems: Regular — Silver, gold, palladium, mercury, copper, iron, lead; quadratic — tin, potassium; rhombic — antimony, bismuth, tellurium, zinc, magnesium. Perhaps all metals assume a crystalline structure on congealing, differing only in degree of visibility. Anti- mony, bismuth, and zinc exhibit a very distinct crystal- line structure plainly visible in broken ingots. Tin is also crystalline, which fact is evinced by the " tin cry " when a bar of the metal is bent, causing the crystal faces to slide over one another; but the bar is not easily broken, and exhibits an apparently non-crystalline frac- ture. Gold, silver, copper, aluminum, cadmium, iron, lead, cobalt, and nickel are practically amorphous, the crystals being so closely packed as to produce a virtually homogeneous mass. MALLEABILITY, DUCTILITY, AND TENAC- ITY are those properties possessed by some metals by virtue of the cohesive power of their molecules, and are to that extent kindred. Malleability is that quality which admits of a metal being hammered or rolled into thin sheets without breach of continuity. Many metals possess this property relatively, it being most wonderfully ex- * Dental Metallurgy, p. 20. PROPERTIES OF METALS. 23 emplified in gold; leaves of it have been produced the 1-370, 000th of an inch in thickness, each grain of which is capable of covering a square of 75 square inches. DUCTILITY is that property possessed by some metals by virtue of which they may be drawn into wire. The operation consists of forcibly drawing the metal through a series of gradually decreasing holes in a hard steel draw-plate. Gold is also the most ductile of all metals, a single grain of it having been drawn into a wire 550 feet in length. This was accomplished by cov- ering the gold wire with silver, which is also remarkably ductile, thus making a composite wire of greater thick- ness. After drawing them to the greatest possible atten- uation, the silver was dissolved off by nitric acid, leaving a gold wire l-5000th of an inch in diameter. These properties, together with that of tenacity, are shown relatively for some of the more important metals in the following table: Malleability. Ductility. Tenacity. 1. Gold. 1. Gold. 1. Iron. 2. Silver. 2. Silver. 2. Copper. 3. Copper. 3. Platinum. 3. Platinum 4. Tin. 4. Iron. 4. Silver. 5. Cadmium. 5. Copper. 5. Gold. 6. Platinum. 6. Zinc. 6. Zinc. 7. Lead. 7. Tin. 7. Tin. 8. Zinc. 8. Lead. 8. Lead. 9. Iron. 9. Nickel. 10. Nickel. 10. Palladium. 11. Palladium. 11. Cadmium. The two properties of malleability and ductility are closely related to each other, yet, as may be seen from the above table, they do not always parallel each other, 24 PRACTICAL DENTAL METALLURGY. for the reason that ductility in a higher degree than malleability is determined by the tenacity of the metal; for example, cadmium, though quite malleable, is but very slightly ductile, and iron, while ninth in point of malleability, is fourth in ductility. In the quality of malleability the granular particles of the metal are flat- tened and spread in all directions, while in that of ductility each granular particle is elongated into a fiber. Annealing. — Pure iron, copper, silver, and other metals are easily drawn into wire, rolled into sheets, or flattened under the hammer. But all these operations render the metals harder, and detract from their plastic- ity. Their original softness can be restored to them by annealing, i. e., by heating them to redness and then plunging them into cool water, oil, etc. In the case of iron, however, this applies only if the metal is perfectly pure. If it contains a few parts carbon per thousand, the annealing process, instead of softening the metal, gives it a " temper," meaning a higher degree of hard- ness and elasticity.* Welding. — The process of joining two clean surfaces of a metal together by pressure is called welding. This property is possessed by iron at white heat, but lead and gold will cohere at ordinary temperatures in proportion to their purity. Iron may be welded by a current of electricity sent through the junction, when the metal is heated by the resistance offered to the passage of the current. Forging. — The process of hammering metals out into various shapes. It illustrates the solid flow of metals. * See chapter on Iron. PROPERTIES OF METAES. 25 TENACITY is that property possessed by metals, in consequence of which they resist rupture when exposed to tension. This is ascertained by preparing wires of exactly equal diameters and comparing the number of pounds weight each will sustain before rupture. There are several conditions which materially modify the propeities Of malleability, ductility, and tenacity, the most important of which are — Purity. — Gold is the most malleable of all metals, yet if the merest trace of lead, itself a soft metal, be con- tained in it, the gold becomes too brittle to be worked, and especially is this the case if the gold has any silver also with it, as most gold has. This destruction of mal- leability and tenacity is yet more pronounced when antimony or similar metals are mixed with gold, even in minute quantities.* Temperature also exercises a very great modifying influence over these properties; for example, a bar of zinc obtained by casting is exceedingly brittle, but when heated to 100° or 150° C. it becomes sufficiently plastic to be rolled into thin sheets or drawn into wire. Such sheet or wire then remains malleable and ductile after cooling. The explanation of this remarkable fact is, that the originally only loosely cohering crystals have become intertwisted and forced into absolute contact with each other, and this is supported by the fact that the rolled zinc has a somewhat higher specific gravity than the original ingot. If the temperature be carried to 205° C it again becomes so brittle that it may be powdered in a mortar. Extreme care, therefore, must be exercised in the handling of hot zinc dies, for if by accident one be dropped upon a hard surface it is likely to be ruined. * See chapter on Gold. 26 PRACTICAL DKNTAL METALLURGY. Aluminum, magnesium, and some other metals, which at ordinary temperatures possess little or no ductility, may be drawn into wire when heated. These qualities are greatly diminished in alloys by heating. Some forms of brass, for example, which are soft, tenacious, and ductile at ordinary temperatures, are made quite brittle by heating to dull redness. Again, it is quite certain that 18-carat gold solder is rendered brittle at red heat. The tenacity of metals in general is greatly diminished by heating. The exceptions to this are in the cases of iron, steel, and gold. The following table shows the results obtained by Wertheim* in his experiments on a number of the metals at temperatures from 15° to 20° C. For Wire 1 Square Mm. Section, Weight in (in Kilos) Causing Name. Permanent Elon- gation of Breakage. Iron, drawn " annealed Copper, drawn Platinum, drawn " annealed Silver, drawn " annealed Gold, drawn " annealed " annealed Tin, drawn 32. Under 5. 12. Under 3. ll"3 2.6 13.5 3. .75 1. .45 .2 .25 .2 61. 47. 40. 30. 34. 23. 29. 16. 27. 10. 13. *2!45 Lead, drawn . . 2.1 " annealed 1.8 * Annales de Chimie et de Physique (III.) Vol. XII. PROPERTIES OP METALS. 27 ELASTICITY.— All metals are elastic to this extent, that a change in form brought about by stresses not ex- ceeding certain limit values, will disappear on the stress being removed. Strains exceeding the "limit of elas- ticity " result in permanent deformation, or, if suffi- ciently great, in rupture. This property may be in- creased in some metals by compounding and alloying. Thus, iron compounded with the proper amount of car- bon, has its elasticity increased to the very highest degree, while the metal itself is almost devoid of the quality. The same is true of copper and zinc, in some forms of brass, also in gold and platinum; both are soft and possessed of little elasticity, yet when combined in proper proportions — for example: If 1 part of platinum be added to 23 parts of 21-carat gold (alloyed with cop- per and silver) an alloy of 20-carat fineness will be pro- duced, which will be found to be quite elastic and is much used for clasps for artificial dentures. SONOROUSNESS.— This is a property possessed by the harder metals, and is quite marked in certain alloys, such as those of copper and tin, known as bell-metal. Lead, which is but feebly, if at all, sonorous, may become so, it is claimed, if cast in the shape of a mushroom. Alumi- num emits a characteristic sound when struck. The first article known to have been made of aluminum was a baby rattle for the infant prince imperial of France, for which purpose it was well fitted on account of its sonorousness. Impurities sometimes increase the sonorousness of a metal, as in the case of antimony in lead. FUSIBILITY AND VOLATILITY.— All may be fused, and most of them are capable of being volatilized, but the temperature at which they become fluid differs greatly in different metals, as the following table shows: 28 PRACTICAL DKNTAL METALLURGY. Name of Metal. Fusing Point Centigrade. Fusing Point. Fahrenheit. Authority. Mercury Caesium 39 ^ + 26 to 27! 30.0 38.5 62.5 95.5 176. 180. 228. 264. 290. 320. 325. 415. 425. 525. —38.2 + 78.8 86. 101.3 144.5 203.9 348.8 356. 442.4 507.2 554. 608. 617. 779 797. 977. Setterberg L. de Boisbaudran Bunsen Bunsen Bunsen Richter (?) ( ? ) Gallium Rubidium Potassium Sodium Indium Lithium . Tin Rudberg Rudberg Lamy Rudberg Bismuth Thallium Cadmium Lead Zinc Person Antimony Incipient Red Heat Magnesium Pouillet Aluminum 700. 700. 1040. 1100. 1100. 1200. 1300 to 1400. higher-1600. 1400. 1600. (?) 1500 to 1600. 1600. 1292. 1292. 1904. 2012. 2012. 2192. 2372 to 2552. 2912. 2552. 2912. (?) 2732 to 2912. 2912. Cherry Red Heat . . Silver Gold Pouillet Bacquerel Yellow Heat Pouillet Copper Iron, wrought .... Iron, chemically pure Cobalt Uranium Dazzling White Heat Pouillet Palladium x y hy d r o g e n Flame Platinum 2000. 3632. Iridium Rhodium Ruthenium Max. Temp, of xy hy dr ogen Flame 2870. 5198 *Bunsen Osmium does not melt at 2870°, i. e., is as yet infusible. * Jahresb. f. Chem. 1867, p. 41; Phil. Mag. XXXIV, PROPERTIES OF METALS. 29 Metals maybe characterized as ' fixed" and "volatile." Of their volatility we have little precise knowledge. The boiling points of a few are given in the following table: Name of Metal. Boiling Point. Authority. Mercury Cadmium 357.3° C 860.0° " 1040.0° " Below 1040.0° " Above 1040.0° " Regnault Deville and Troost Zinc Potassium Deville and Troost Dewar and Dittmar 2. 3. For practical purposes the volatility of metals may be classed as follows: 1. Distillable below redness: Mercury. Those distillable at red heats : Cadmium, Potassium, Zinc, Sodium. Magnesium, Those which are volatilized more or less readily when heated beyond their fusing points in open crucibles : Antimony (very readily), Tin, Lead, Silver. Bismuth, 4. Those which are with very great difficulty volatilized, if at all: Gold, Copper (?). 5. Those which are practically ' 'fixed, ' ' or non-volatile: Copper {?), Aluminum, Iron, Lithium, Nickel, Strontium, Cobalt Barium. Calcium, "In the oxy hydrogen flame silver boils, forming a blue vapor, while platinum volatilizes slowly, and osmium, though infusible, very readily."* * William Dittmar. 30 PRACTICAL DENTAL METALLURGY. " It is doubtful, " says Makins, " if it [gold] is volatile per se. But if gold be alloyed with copper, it has been shown by Napier to be considerably volatalized, so that quantities, amounting to 4^ grains, could be col- lected during the pouring out of 30 pounds weight from a crucible. * * * That mixtures of gold, silver, and lead, when cupelled together, volatize considerably." SPECIFIC HEAT.— Equal weights of different met- als have been found to absorb different amounts of heat when subjected to the same temperature. They, indeed, possess different capacities for heat. Thus, the amount of heat necessary to raise a given weight of water has been found to be 31 times as great as that required to raise an equal weight of platinum through the same interval of temperature; or, in other words, the amount of heat required to raise a given weight of water through 100° C. will raise 31 times the same weight of platinum through 100° C. of temperature. Thus, water being taken as the standard or unit, the specific heat of platinum is x / 3li or 0.032 that of water. TABLE OF SPECIFIC HEATS. 1. Iron 1138 2. Nickel 1086 3. Cobalt . 1070 4. Zinc 0956 5. Copper 0952 6. Palladium . 0593 7. Silver 0570 8. Cadmium 0567 9. Tin 0562 10. Antimony 0508 11. Mercury 0333 12. Gold 0324 13. Platinum 0322 14. Lead 0314 15. Bismuth 0308 PROPERTIES OF METALS. 31 EXPERIMENT No. 1. — Prepare bullets of exactly equal weights of several of the above metals, such as zinc, silver, cadmium, tin, and lead; expose them to the same temperature, for the same length of time, and then drop them simultaneously upon a sheet of wax placed across an open side of a pasteboard box. They will be observed to melt their way through or into the wax in the order named. EXPANSIBILITY.— The expansion of metals by heat varies greatly. The coefficient of expansion is constant in metals that crystallize in the regular system only; the others expand differently in the direction of the different axes. To eliminate this source of uncer- tainty, these metals are employed as compressed powders. The following table gives the linear expansion from 0° to 100° C, according to Fizeau, the length at 0° being taken as unity:* Name of Metal. Platinum, cast Gold, cast Silver, cast Copper, native Copper, artificial , Iron, soft , Steel, cast Bismuth, mean expansion Tin, compressed powder Lead, cast Zinc Cadmium, compressed powder, Aluminum, cast Mercury Expansion. 0° to 100° C. .000907 .001451 .001936 .001708 .001869 .001228 .001110 .001374 .002269 .002948 .002905 .003102 .002336 .018153 "The high rate of expansibility of zinc renders it par- ticularly valuable as a metal for dies upon which to form plates for the mouth in many cases. The metal is cast while fluid and at its extreme limit of expansion, which upon cooling returns to its minimum dimensions, and thus furnishes a cast a little smaller than the plaster model which it represents. It has been found that this * William Dittmar. 32 PRACTICAL DENTAL METALLURGY. contraction of the zinc die a trifle more than compensates for the expansion which takes place in the plaster model in setting, and in the majority of cases a plate made thereon adapts itself more accurately to the mouth than one made upon a die of less expansible metal. Even if the contraction undergone by the zinc is so great as to produce a die somewhat smaller than the mouth, so far from being a detriment, it is a positive advantage in most cases of full upper replacement, as under such con- ditions the pressure of the finished plate is greater upon the alveolar ridge than upon the central portions of the hard palate — a state of affairs the advantages of which are sufficiently obvious without explanation."* CONDUCTIVITY.— Metals are good conductors of heat and electricity, but the quality — whether thermic or electric — is very differently exhibited in different metals. An exact knowledge of these conductivities is of great scientific and practical importance to the dentist, and too much attention cannot be given their consideration. The following table gives the thermic and electric con- ductivities of some of the more important metals and alloys : Relative Conductivity. Names of Metals. Thermic. Electric, at 0° C Silver 100.0 73.6 53.2 14.5 11.9 8.5 8.4 1.8 23.6 11.6 7.3 2.8 100.00 Copper 99.95 Gold Tin Iron Lead 77.96 12.36 16.81 8.32 Platinum 18.80 Bismuth Brass Steel 1.24 German Silver 7.67 Rose Fusible Metal Pianoforte Wire 14.40 *Dr. E. C Kirk, Am. System of Dentistry, Vol. Ill, p. 793, PROPERTIES OF METALS. 33 Makins states that amongst the results of Dr. Mat- thiessen's experiments upon the electric conductivity of metals "are the facts that impurity of a metal or alloy- ing it greatly diminishes its conducting power. Rise of temperature again has the same effect. Thus between 32° F. and 212° (or 0°C. and 100°) great diminution takes place, and that not uniformly, as some lose it much more in proportion than others, by thus raising the tempera- ture. Many lose as much as twenty-five per cent, of their conducting power." An illustration of the comparative conductivity of the metals is observed in the incandescent lamps with plati- num coils. The electricity is readily transmitted from its source by the copper efferent wire, but when it meets the platinum that metal offers so much resistance to the passage of the current, on account of its low conducting power, that it becomes white-heated — incandescent. SPECIFIC GRAVITY.— This property varies in dif- ferent metals from .594 (lithium) to 22.48 (osmium), as the following table shows: Name of Metal. Specific Gravity. Authority. Lithium .594 .875 .9735 1.52 1.578 1.743 1 88 2.1 2.5 2.583 Over 4. 4.15 5.5 5.9 Bunsen Potassium Baumhauer Sodium Baumhauer Rubidium Bunsen Calcium Bunsen and Matthiessen Magnesium Bunsen Caesium Setterberg Debray Glucinum Strontium Aluminum Mallet Barium Clarke Zirconium Troost Vanadium Gallium Roscoe Lecoq de Boisbaudran {Table continued on following page.) 34 PRACTICAL DENTAL METALLURGY. TABLE— Continued. Name of Metal. Specific Gravity. Authority. Lanthanum 6.163 6.544 6.728 6.715 6.81 6.915 7.14 7.29 7.42 7.844 8.279 8.546 8.5 8.6 8.94 9.823 10.4 11.25 11.4 11.86 12.1 12.26 13.595 16.54 18.33 19.265 21.46 22.4 22.477 Lecoq de Boisbaudran f Hillebrandt and \ Norton j Hillebrandt and I Norton Marchand and Scheerer Didymiuui Cerium Antimony Chromium Wohler Zinc Karsten Mansfanesium Brunner Tin Indium Richter Iron Nickel Berzelius Richter Cadmium Schroder Cobalt Molybdenun Copper Debray Eismuth Holzmann Silver Holzmann Lead Deville Palladium Deville and Debray Thallium Crookes Rhodium Bunsen Ruthenium Deville and Debray Mercury H. Kopp Wohler Tungsten Uranium Gold Peligot Matthiessen Platinum Iridium Osmium Deville and Debray CHAPTER III. COMPOUNDS OF METALS AND NON-METALS. Metals combine with each other indefinitely to form alloys, preserving the metallic appearance and properties. They combine with non-metals in definite chemical pro- portions to form compounds of a more precise nature, in which case the metallic characters are almost invariably lost. These definite compounds include the Oxides, Fluorides, Sulphides, Cyanides, Chlorides, Selenides, Bromides, Tellurides. They also combine with Nitrogen, Silicon, Phosphorus, Carbon, Boron, forming nitrates, phosphates, and phosphides, borates, etc. METALLIC OXIDES.— All metals combine with oxygen to form oxides, and most of them in several proportions. As a class they exhibit a greater disposi- tion to unite directly with oxygen than the non-metals, though few of them will do so in their ordinary condition and at ordinary temperatures. Several metals, such as iron and lead, are superficially oxidized when exposed to the air under ordinary conditions, but this would not be the case unless the air contained water and carbon dioxide, which greatly favor oxidation. Among the more important metals, five only are oxidized in dry air at ordinary temperatures, viz., potassium, sodium, bar- ium, strontium, and calcium. The affinity of these metals for oxygen is so great that they must be kept 36 PRACTICAL DENTAL METALLURGY. under naphtha (C IO H I6 ) or some substance containing no oxygen. EXPERIMENT No. 2.— With a knife cut off a small piece of metallic sodium; observe it exhibits a brilliant luster but speedily tarnishes by com- bining with the oxygen of the air, forming the oxide (NaO) of sodium. Plunge the sodium into a jar of oxygen: it takes fire and burns with a bril- liant yellow flame. Zinc on the other hand exhibits no disposition to com- bine with oxygen at ordinary temperatures, but is in- duced to do so at a moderate heat (1040° C. or 1900° F.), when it burns with a beautiful greenish flame, produced by the union of its vapor with the oxygen of the air, forming zinc oxide — ZnO. EXPERIMENT No. 3.— With a piece of zinc foil form a tassel, gently warm the end, dip into a little flowers of sulphur, kindle, and let down into a jar of oxygen, when the flame of the burning sulphur will ignite the zinc, which burns with great brilliancy, forming oxide of zinc. A large number of the metals are oxidized during fusion. I^ead, for example, may be entirely transformed into its oxide by continued exposure to sufficient heat. The oxides of others may be formed by heating a car- bonate or nitrate of the metal to redness. For example, if ZnC0 3 be heated to a red heat C0 2 is evolved, leaving the pure zinc oxide (ZnO). Again, the oxide of copper may be obtained by digesting that metal in nitric acid, — 3Cu+8HN0 3 =3Cu(N0 3 ) 2 + 4H 2 + 2NO— forming the nitrate of copper, which then may be decomposed by heat into nitric and cupric oxides. They are also formed from some salts; for example, if to a solution of sulphate or chloride of iron .ammonium hydrate be added the hydrated sesquioxide of iron (Fe 2 3 3HO), the antidote for arsenic is formed. And zinc oxide may be obtained by adding caustic potassa to a solution of zinc sulphate. Deflagrating some metals with an oxidizing agent pro- duces an oxide of the metal. Advantage is taken of this COMPOUNDS OF METALS AND NON-METALS. 37 in rendering brittle gold malleable by roasting it with nitrate of potassium.* The contaminating tin , lead , zinc, antimony, etc., are extracted from the noble metal, as oxides by the oxygen of the nitrate, and dissolved in the molten flux. Other metals, such as gold, platinum, iridium, rhodium, and ruthenium, do not combine directly with oxygen, their combination being effected only by indirect means, and with difhculty. Oxidizing Agents are substances such as — Oxygen (O), Potassium and (KC10 3 ) Air (O and N), Sodium Chlorates(NaC10 3 ), Water (H 2 0), Iron Tetroxide (Fe 3 4 ), Potassium and (KN0 3 ) Iron Trioxide (Fe 2 3 ), Sodium Nitrates (NaN0 3 ), Carbon Dioxide (C0 2 ), which, imparting a part or the whole of their oxygen to another substance, cause it to become oxidized; con- versely — Deoxidizing Agents are substances such as — Carbon (C), Compounds of hydrogen Carbon monoxide (CO), and carbon — carbo-hydrides, Hydrogen (H), and some times metals, which reduce oxides by combining with the oxygen which they may contain. Examples of oxidation — Zn+0=ZnO 3FeCO s + 0= Fe 3 4 + 3C0 2 3FeO + C0 2 =Fe 3 4 + CO Examples of deoxidation — 2KC10 3 +C 3 =2KC1 + 3C0 2 Fe 2 3 +H 6 =Fe 2 +3H 2 Fe 2 3 + 3CO=Fe 2 + 3C0 2 * See chapter on Gold. 38 PRACTICAL DENTAL METALLURGY. Substitution or Replacement. — Just as chlorides are derived by substitution from hydrochloric acid, HC1, so may oxides be represented as being derived from one or more molecules of water, H 2 0, by the substitution of a metal for hydrogen; with this difference, however, that water contains two atoms of hydrogen; therefore, the replacement may be only partial, producing the hydra ted oxide, or complete, forming the oxide. Thus the mon- oxides may be formed by the replacement of both atoms of hydrogen by a monad, as Na 2 0, Ag 2 0, or a dyad, CuO, ZnO; while the higher oxides may be regarded as two or more molecules of water, in which the hydrogen in a similar manner is replaced by its equivalent of meta 1 , as Mn0 2 , A1 2 3 . The oxides may be classed as Basic Oxides and Acid- forming Oxides. Basic Oxides. — When the replacement of the hydro- gen is complete, the resulting compound is a basic oxide— K 2 + H 2 0==K 2 + H 2 . Hydroxides. — When the replacement of the h3'drogen is incomplete, the resulting compound is a hydroxide — K+H 2 0=KH0 + H, or with the dyad calcium, Ca + 2H 2 0=Ca2HO+H 2 . Bases neutralize acids either partially or entirely, replacing either a part or all of their hydrogen, thus we have KHS0 4 and K 2 S0 4 . An Alkali is only a particular species of base, and might be denned as a base which is very soluble in water, as K 2 and Na 2 0. It will be observed that metals are capable of form- ing bases by combining with oxygen, or salts by com- bining with salt-radicals. Many metals,* however, form acid-forming oxides or anhydrides; thus tin forms stannic * See Sixth Division, Chapter I. COMPOUNDS OF METALS AND NON-METALS. 39 anhydride (SnOJ, and antimony forms antimonic anhy- dride (Sb 2 5 ), and it is always found that the acid-form- ing oxide of a metal contains a larger proportion of oxy- gen than any of the other oxides which the metal may happen to form, thus: The Acid-forming Oxides are those metallic oxides, or anhydrides which form acids with water, as in the case of non-metallic oxides. A number of metallic oxides are found in nature as ores from which the metals are reduced. Tin occurs as tinstone, Sn0 2 , iron as Fe 2 3 , and Fe 3 4 , etc. REDUCTION OF METALLIC OXIDES.— The variable affinities exhibited by the metals for oxygen groups them into two classes already known as noble and base metals. Reduction of the Oxides of the Noble Metals. — So feeble is the affinity of the noble metals for oxygen that their oxides are easily decomposed and the metals reduced without the aid of any other agency than that of simply heating to redness — about 600° F. Reduction of the Oxides of the Base Metals. — On the other hand the base metals exhibit a very strong affinity for oxygen and the mere application of heat will not reduce them, indeed, in many instances a decided increase of temperature serves only to strengthen their affinity and hence increase the proportion of oxygen in the compounds previously formed. Therefore, in addition to heat the assistance of some substance is required whose affinity for oxygen is stronger than that of the metal and will, when favored by heat, abstract the oxygen from the oxide. Thus, the oxide of lead may be formed by heating the carbonate: PbC0 3 (+ heat)=PbO + C0 2 , and driving off the carbon dioxide (C0 2 ). The lead 40 PRACTICAL DENTAL METALLURGY. oxide (PbO), however, cannot be further reduced to metallic lead by heat; on the contrary, if the heating be continued, the production of a higher oxide only will be effected. But if, in addition to heating, the oxidized lead be covered with a layer of pulverized charcoal, which will abstract the oxygen for its own conversion into carbon dioxide, the lead will be reduced or liberated. Such a reduction is accomplished by the reducing or deoxidizing agent, carbon favored by heat: 2PbO + C (+heat)=2Pb+C0 2 . When the lead or zinc used for counter-dies and dies in the laboratory are overheated or subjected to frequent or long continued meltings, they become partially oxidized and covered with an earthy looking mass con- sisting of partially oxized metal. A continued exposure to heat would, as we have observed, have the effect of converting this into an oxide of a higher degree, but if the molten metal be covered with pulverized charcoal (C) or other carbonaceous substance, such as oil, fat, suet, or scraps of beeswax (hydro-carbons), the oxygen of the oxide will be abstracted, carbon dioxide formed and evolved, while the metal will be reduced to a free state. Reduction with Hydrogen. — Other oxides which cannot be reduced by deoxidizing agents favored by the conditions as stated above may, by the assistance of proper apparatus and heat, be reduced by a current of dry hydrogen. EXPERIMENT No. 4. — Pass the delivery-tube of an ordinary hydrogen generator (A, Fig. 1) into one end of a drying tube (B), well filled with frag- ments of calcium chloride, for the purpose of absorbing the moisture which may be carried over with the gas; connect the other end of the drying tube with a tube (C) upon which a bulb (D) has been blown for the reception of the metallic oxide. After the gas has completely driven the air out of the appara- tus, heat is applied to the bulb containing the oxide. As the dry hydrogen COMPOUNDS OF MF/TALS AND NON-METALS. 41 flows over the heated oxide in a strong stream it combines with the oxygen — favored by heat — and passes out of the tube (E) as aqueous vapor, while the metal is left free. Fig. 1. Reduction with Sulphur. — Some oxides may be best reduced by beating with sulphur, in which case sulphur exhibits a greater affinity for the oxygen than the metal does, and, abstracting it, forms sulphur dioxide (S0 2 ). A portion of the sulphur, however, combines with the metal, converting it into a sulphide or sulphate, or a mixture of both. The reduction of such compounds is treated under metallic combinations with sulphur. Reduction with Chlorine. — There are a few oxides which may be reduced by chlorine gas. Thus platinum oxide is reduced in a current of dry chlorine. EXPERIMENT No. 5.— Repeat experiment No. 4, using calcium oxide in the drying tube, and dry chlorine gas. METALLIC SULPHIDES.— Metals combined di- rectly with sulphur to form a class of compounds which, in a chemical and economical point of view, are almost as important as the oxides, since the ores of many of the 42 PRACTICAL DENTAL METALLURGY. most important metals are found as sulphides, for ex- ample, galena (PbS); stibnite (Sb 2 S 3 ); zinc-blende (ZnS); greenockite (CdS); copper-glance (CuS); iron pyrites (FeS 2 ); cinnabar (HgS); silver glance (Ag 2 S), etc. These are generally brittle solids possessing so high a degree of luster that some of them have been mistaken for gold, hence iron pyrites has been called ''fool's gold." In composition they resemble the oxides and hydroxides, with many of which they are analogous. The exceptions to this analogy being the alkalis and alkaline earths, there being but two oxides of potassium, sodium, and barium, while there are no less than five sulphides of these metals. All the metallic sulphides are solid at ordinary temperatures; most of them fuse at red heat, and some sublime unchanged. When roasted in air at high temperatures they are converted into sul- phates; (ZnS + 4 favored by high heat=ZnS0 4 , or, if they are exposed to higher and continued heat, into oxides. They may be prepared by heating the metals or their oxides with, sulphur, from the sulphates by heating them with charcoal, deoxidizing them, and from their soluble salt solutions by adding sulphuretted hydrogen: EXPERIMENT No. 6.— To the following salt solutions in several test- tubes add a few drops of sulphuretted hydrogen: Pb2C 2 H ;J 2 + H 2 S = PbS+ 2HC 2 H 3 2 Lead acetate Lead sulphide (Black) 2AsCl 3 + 3H 2 S = ASoS 3 + 6HC1 Arsenious Arsenious Chloride Sulphide (Lemon yellow) Cd2NO,,+ H.,S = CdS+ 2HNO r , Cadmium Cadmium Nitrate Sulphide (Yellow) COMPOUNDS OF METALS AND NON-METALS- 43 2SbCl 5 + 5H 2 S = Sb 2 S s + 10HC1 Antimonic Antimonic Chloride Sulphide (Orange yellow) Zn2C 2 H 3 2 + H,S= ZnS+ 2HC 2 H 3 2 Zinc acetate Zinc sulphide (White) HgCl 2 + H 2 S = HgS+ 2HC1 Mercuric chloride Mercuric sulphide (1st White) (2d Yellow orange) (3d Brown) (4th Black) REDUCTION OF THE METALLIC SUL- PHIDES. — Since the ores of many of the most impor- tant metals are sulphides, and it is from such compounds that we derive our chief supply of copper, lead, mercury, silver, antimony, and several other metals, the subject of their reduction is of great importance. Reduction by Heat. — The reduction of some of the metallic sulphides, such as gold, platinum, silver, and mercury, is effected by heat alone. The oxygen of the air unites with the sulphur, which is evolved as sulphur dioxide, S0 2 . In some instances, however, a portion of the oxygen combines with the metal, and an oxide instead of the free metal is obtained. In some cases the sulphide is oxidized and converted into a sulphate, which, in turn, may be decomposed at high tempera- tures, separating into sulphur dioxide and free metal, or, at times, a metallic oxide. Then, again, some of the sulphides may, when roasted in air, be converted into permanent sulphates capable of resisting high degrees of heat. Reduced with Iron. — Iron exhibits a strong affinity for sulphur and when favored by heat will abstract it from several metals, such as silver, lead, etc. Thus, if 44 PRACTICAL DENTAL METALLURGY. the sulphide of lead (galena) be heated with scraps of iron, metallic lead is freed : PbS-f-Fe=FeS + Pb or in the case of silver : Ag 2 S+Fe=FeS + 2Ag. Rsduced with Hydrogen. — The sulphides of such metals as antimony, bismuth, copper, tin, and silver are decomposed by passing a current of dry hydrogen over them at a red heat, the metal being reduced, while the hydrogen combines with the sulphur, forming sulphu- retted hydrogen : CuS + 2H=H 2 S+Cu. Reduced with Chlorine. — Dry chlorine gas also decomposes some metallic sulphides, combining with both metal and sulphur. Reduced with Acids. — Nitro-hydro chloric acid con- verts the sulphides into chlorides, and hydrochloric acid, in a few instances, acts similarly: its hydrogen combining with the sulphur is evolved as sulphuretted hydrogen. Strong nitric acid also decomposes them, the sulphur being oxidized and the liberated metal combines with the acid to form a nitrate. Mercuric sulphide is the only one that can not be thus reduced. METALLIC CHLORIDES.— All metals combine with chlorine, and some of them in several proportions; thus we have stannous (SnClJ and stannic chlorides (SnCl 4 ). Some of the chlorides occur in nature, those of silver (AgCl) and mercury (Hg 2 Cl 2 ) as minerals, and those of sodium and potassium in enormous quantities in the solid state and dissolved in waters. They may be regarded as derived, like the oxides, from a type — HC1 — substituting for the hydrogen of one or COMPOUNDS OF METALS AND NON-METALS. 45 more molecules of hydrochloric acid an equivalent in metal, thus: From HC1 are derived monochlorides like KC1. " H 2 C» 2 " " dichlorides " SnC1 2 . " H3CI3 " " trichlorides " AuCl 3 . " H 4 C1 4 " " tetrachlorides " SnC) 4 . Preparation. — They may be prepared by the action of hydrochloric acid upon the metals. Zinc, tin, cadmium, iron, nickel, and cobalt may be readily dissolved by hydrochloric acid, forming chlorides respectively and liberating hydrogen: Zn+2HCl=ZnC1 2 +H 2 They are also prepared by the action of nascent chlo- rine developed by the mixture of nitric to an excess of hydrochloric acid. Gold and platinum are dissolved in this mixture (aqua regia) and stannic chloride is formed by its action on tin. Some are also prepared by subjecting the metal or its oxide to a current of dry chlorine gas. In this manner the chlorides of titanium, aluminum, and chromium may be formed. Sometimes a chloride is prepared by the substitution of one metal for another, thus stannous chloride may be made by distilling metallic tin with mercuric chloride: HgCl 2 + Sn=SnCl 2 + Hg. Other chlorides may be prepared by dissolving the oxides, hydroxides, or carbonates of the metals in hydro- chloric acid. REDUCTION OF METALLIC CHLORIDES — The chlorides of gold and platinum may be decomposed by heat alone. Gold possesses so feeble an affinity for chlorine that solutions of the chloride of gold may be decomposed by mere exposure to light or atmospheric 46 PRACTICAL DENTAL METALLURGY. air. Solutions of sugar, gum arable, oxalic acid, etc., readily decompose it.* Silver chloride yields pure silver and emits an odor of hydrochloric acid when heated strongly on charcoal. When placed in water acidulated with hydrochloric or sulphuric acid, silver chloride may be reduced by stirring with small pieces of iron or zinc; the reaction is as fol- lows: Fe+H 2 S0 4 =FeS0 4 +H 2 , and 2H-f2AgCl=2HCl+2Ag.t With the exception of the chlorides of the alkalis and alkaline earths all other chlorides may be decomposed by heating them in a current of hydrogen, hydrochloric acid and the pure metal being the result; but the evolu- tion of the hydrogen must be well maintained, in order to drive off the hydrochloric acid formed, or it will react with the pure metal, forming fresh chloride. Some chlorides may be decomposed by heating them with a metal which has a more powerful affinity for chlorine; thus, aluminum chloride may be reduced by heating it with sodium. Sulphuric acid decomposes some chlorides and con- verts them into oxides, the oxygen being supplied by the water present. METALLIC BROMIDES.— Bromine, though less active than chlorine, unites directly with most of the metals, forming compounds analogous to the chlorides and resembling them closely in general properties. A silver bromide (Ag Br) analogous to the chloride is found as a natural mineral. Nearly all bromides are soluble, and those of the alkali metals are found abun- dantly in sea water and in many saline springs. *See chapter on Gold. f See chapter on Silver.. COMPOUNDS OF METALS AND NON-METALS. 47 REDUCTION OF METALLIC BROMIDES.— The bromides are decomposed by oxidizing agents with liberation of bromine. The affinity of bromine for the metals being inferior to that of chlorine, the latter will, with the aid of heat, displace the bromine and form chlorides, but bromine can not be displaced in a like manner by iodine: KBr + Cl=KCl + Br. METALLIC IODIDES.— Many metals unite directly with iodine, forming compounds analogous to the chlorides and bromides. The iodides of potassium and sodium exist abundantly in sea water and in some springs, and the iodide of silver occurs as a natural mineral. Most of them are soluble in water, lead iodide being only slightly so, while the iodides of mercury and silver are quite insoluble. A few of them, gold, silver, platinum, and palladium, are decomposed by heat alone, giving up their iodine. Ozone promptly decomposes all iodides, while atmos- pheric oxygen decomposes those of iron and calcium slowly. The superior affinity of chlorine and bromine enables these elements to displace iodine and form analogous chlorides or. bromides: KI+Cl=KCl-fI, or KI + Br=KBr + I. METALLIC FLUORIDES are formed by heating certain metals in the presence of hydrofluoric acid ; by the action of that acid on metallic oxides; by heating electro-negative metals, such as antimony, with the fluoride of lead or mercury. Volatile metallic fluorides may be prepared by heating fluor-spar with sulphuric acid and the oxide of the metal. With some metals fluorine occurs as a natural mineral, as with calcium 48 PRACTICAL DKNTAL METALLURGY. (CaF 2 ), and the double fluoride of aluminum and sodium (A1 2 F 6 , 6NaF). The fluorides are devoid of metallic luster; most of them are easily fusible, and for the most part resemble chlorides. METALLIC CYANIDES are formed by the union of metals with the compound radical cyanogen, CN. Potassium and some other metals are converted into cyanides by heating them in cyanogen gas or the vapor of hydrocyanic acid. Cyanides very closely resemble the chlorides, bromides, iodides, and fluorides. METALLIC SELENIDES— The element selenium very closely resembles sulphur in its chemical properties; hence, it combines with metals in much the same manner. Native selenides are rarely found. REDUCTION BY ELECTRICITY.— Probably the most powerful means of reducing metals from their com- binations with non-metallic elements is obtained through the agency of electricity. To accomplish this, a solution of the metallic salt is subjected to the action of the galvanic current, and decomposed thereby. This is simply and beautifully demonstrated by hanging a strip or coil of zinc in a solution of lead nitrate. After a few hours the zinc passes into solution, and exquisite crystals of lead have taken its place. The electric furnace of Eugene H. and Alfred H. Cowles, of Cleveland, Ohio, has greatly advanced the production of such metals as aluminum from corundum, boron from boracic acid, and silicon from quartz. The furnace is constructed in the form of a rectangular box of fire-resisting material, lined with a mixture of fine charcoal and lime. It has a removable cover, which is perforated with openings to allow the escape of gases COMPOUNDS OF METALS AND NON-MKTALS. 49 evolved. In the sides of this furnace the electrodes — two plates of gas carbon — are let in by means of which a current of a powerful dynamo-electric machine is intro- duced. The charge consists of the coarsely crushed ore and coke fragments. The essential feature of the process consists, therefore, in employing in the furnace a substance like carbon, whose high resistance to the passage of the current causes the production of a pro- digiously high temperature; and which, at the same time, is capable of exercising a powerful reducing action on the ore. CHAPTER IV. MELTING METALS. REFRACTORY MATERIALS.— Furnaces designed to withstand the strain of high temperatures should be lined inside with a material capable of withstanding the heat and scorifying action of the material operated upon, without melting or decomposing. Such materials are either used in the natural state, such as sandstone or quartz, oxides of iron, and fire-clay, or they are prepared by certain methods. Refractory materials are divided into three classes with reference to their reaction: ist, those of acid char- acter, such as ganister and Dinas clay; 2d, neutral, such as fire-clay, chrome ironstone, and graphite; jd } basic, such as dolomite, bauxite, alumina, etc. Such sub- stances are termed acid, neutral, or basic when the acid present is greater, equal to, or less in equivalence than the base,* Fire-bricks are usually made of fire-clay mixed with burnt clay and white sand, which prevent the bricks cracking and do not increase the fusibility. The com- position differs with the purposes for which they are * Ganister is composed of Si0 2 89.5, Al 2 3 4.8, Fe02.8, CaO.l, K 2 0.1, H 2 02.2. Dinas Clay = Si0 2 98.3, Al 2 O s .7, FeO.2, CaO.2, K 2 0.1, H 2 0.5. Kaolin, which is the purest form of fire-clay, contains Si0 2 40, Al 2 3 45, H 2 015. Dolomite = CaC0 3 and MgC0 3 . In this case the carbon dioxide is removed by heat, leaving the oxides of calcium and magnesium, which are entirely basic. Bauxite = Al 2 O s 52, Fe 2 3 27.6, H 2 20.4. Fire-clays are essentially hydrated silicates of alumina, which resist exposure to high temperatures without melting or softening. They contain varying amounts of lime, magnesium, oxide of iron, potash, etc., and some mechanically mixed silica. Graphite (Cumberland) = C 91.55, volatile matter 1.1, ash 7.35. Hiorns. MEI/TING METALS. 51 designed — some are required to withstand high and pro- longed temperatures without softening; some to with- stand great pressure; some to resist the corrosive action of metallic oxides, and others to withstand great and sudden changes of temperature. Crucibles. — These are vessels made of various mixtures of clay, in the raw and burnt state, mixed with coke dust or plumbago, and are designed for calcining or fusing substances which require high temperatures. A good crucible should be tough, infusible, capable of withstanding sudden changes of temperature without fracture, and should not be readily corroded by metallic oxides. The most infusible crucibles are those made with clays containing the largest amount of silica, and the smallest quantity of calcium and iron oxides. A good crucible may be made with two-thirds fire-clay and one-third burnt fire-clay and coke dust, which prevent it being distorted when burnt. The power of resisting corrosion may be tested by melting copper in the crucible and adding a little borax. The latter unites with any copper oxides that may be formed, and will corrode the crucible rapidly unless it is of good quality. Graphite, black-lead, or plumbago crucibles are made of fire-clay mixed with varying proportions (25 to 50 per cent.) of plumbago or coke dust. The best ones are made with purified plumbago, as the natural material often contains impurities in the ash which would act injuriously in the clay. Instead of using black-lead crucibles, clay ones lined, or " brasqued " with charcoal paste are often employed. The graphite crucible is the most enduring of all crucibles, but they should never be used in melting or alloying noble metals without first being tested by subjecting them to a red heat, as a crack 52 PRACTICAL DENTAL METAIJJJRGY. or other imperfection may exist that escapes the notice while the vessel is cold. Again, bubbles of air or parti- cles of organic substances occasionally become mixed with the material, which, upon being heated, cause the crucible to be broken, thereby risking the loss of the metal. There are a variety of clay crucibles, the most impor- tant of which are: ist, French — -Of excellent quality, smooth, carefully made, but somewhat brittle; 2d, London — Close-grained, reddish-brown, refractory, and resist well the corrosive action of metallic oxides; $d y Cornish — Quite refractory, but are of a more acid char- acter than the preceding, and hence are more readily attacked by metallic oxides; 4th, Hessian — These are exceedingly useful, refractory, not readily corroded. They are composed of SiO 2 70.2, Al 2 3 24.8, Fe 2 3 3.8. They may be used for rough fusions, but when precious metals or their alloys are to be fused in them they should be first thoroughly lined with a surface of borax, or the rough, porous sides will absorb a considerable portion of the molten metal. Being of acid character, they are also subject to corrosion by basic fluxes, with which they form fusible compounds. They are well adapted to the fusion of noble metals where no fluxes are introduced for chemical action. Though they do not show a great resistance to extreme heat, they are very slightly affected by sudden alterations in temperature, as they may be plunged cold into a strongly heated furnace, or, white- hot, into cold water, without cracking. The Cornish crucible, though very similar to the Hessian variety, is not quite so rapidly perforated by corrosive fluxes. Crucibles are also made of porcelain, gold, silver, platinum, iron, etc., but their use is confined almost entirely to the chemical laboratory. MELTING METALS. 53 Platinum is fused either in a crucible of gas carbon or in. a concavity carved in a block of quicklime, the latter of which forms part of the furnace described in the chapter on platinum. FLUXES are certain fusible substances which, when heated with metalliferous matter, assist in the fusion and aggregation of the metallic globules by cleansing and protecting them from foreign matters, such as gangue, oxides, sulphides, chlorides, etc. With these foreign substances the flux forms a fusible slag from which the metals held as oxides, sulphides, chlorides, etc., may be subsequently reduced. Like the refractory materials, fluxes may be classified as acid, neutral, and basic in their reaction. Thus, when gold quartz is fused with sodium carbonate, the quartz, a siliceous or acid gangue, reacts with the carbonate forming sodium silicate, liberating carbon dioxide, and separating the gold which is held mechanically. A number of fluxes are used for the specific purpose of removing certain impurities or debasing elements from molten metal. This they accomplish in two ways- — first, by acting as simple solvents for the impurity, as mentioned previously, and forming a slag; second, by forming com- pounds, such as oxides, sulphides, chlorides, etc. with the debasing elements, which are either volatile or solu- ble in the molten flux. Others act in a reverse manner; these are reducing agents, the function of which is to reduce to a metallic state such metallic oxides as are dis- solved in the molten metal, and which confer friability or brittleness upon the metal when cast. The following may be enumerated as the fluxes of most common application, with their uses denned: Borax, sodium tetraborate, Na 2 B 4 7 , 10B 2 O. This salt is of almost universal use, but should be first fused 54 PRACTICAL DENTAL METALLURGY. to drive off its tea parts water of crystallization, and the glassy mass thus obtained is to be powdered. When highly heated it is of acid reaction, combining with metal- lic oxides to form borates; at lower temperatures it takes up foreign matters generally, setting the metal free and so cleansing its surface as to allow of complete aggrega- tion of the particles into a button form. It is found native in abundance in California, Europe, Peru, and other localities. It is also artificially prepared by neu- tralizing boric acid with soda ash. Sodium Carbonate, Na 2 C0 3 , 10H 2 O. This salt may be preferred to potassium carbonate from the fact that the latter is quite deliquescent. It decomposes silicates, as already instanced, and much easier when charcoal is present. It forms fusible compounds with metallic oxides and decomposes some chlorides, for example, silver chloride. Potassium Carbonate, K 2 C0 3 , is quite similar to the sodium salt; it dissolves the earthy impurities, with which it forms an exceedingly liquid flux, thus enabling the heavier particles of metal to sink through the fluid mass and collect in a button at the bottom of the crucible. Potassium Nitrate, saltpetre, nitre, KN0 3 , is an exceedingly useful flux in the purification of noble metals. When used as a flux, and heated, it energeti- cally gives up a portion of its oxygen to base metals which are thus oxidized, and the alkaline nitrate becomes a nitrite. Sodium Chloride, NaCi, powdered and heated, to prevent its decrepitation, is sometimes added to molten substances which induce much ebullition, in order to check the latter and protect the substance operated upon from the action of atmospheric oxygen. Like amnionic and mercuric chloride, it forms chlorides with some metals. MELTING METALS. 55 Black Flux, a mixture of potassium carbonate and pulverized charcoal, is an excellent reducing agent and assists in the fusion of substances. Lime, Silica, and Alumina, or lime with the silicate of aluminum, are employed together; the silica to abstract certain bases by forming with them fusible silicates; while the two bases, lime and alumina assist in the fusion of the silicates thus formed. A single silicate with one base is generally less fusible than a double or multiple silicate with two or more bases — hence the two bases, lime and alumina, are used with the silica. Plumbic, Cupric and Ferric Oxides are used as fluxes in some metallurgical operations; the first forming an alloy of lead and silver; the copper oxide for purifying gold and the ferric oxide as a flux for silica. Many prepared fluxes have been introduced from time to time for dental soldering operations, but none possess any great advantage over pulverized dehydrated borax. Dr. H. A. Parr has prepared a very useful, efficient, and convenient flux powder; and also a flux wax which affords a means for holding clasps, teeth, and other metal- lic parts together while they are being invested, and also for conveniently fluxing the surfaces to be soldered; the latter is obviously accomplished by burning out the wax, the flux which it carries remaining. A liquid flux used by jewelers and found useful in dental solderings is made by dissolving equal parts of borax and boric acid in about sixteen parts of water. FUEL. — Combustible substances that may be quickly burned in air, producing heat capable of being applied to economic purposes. Fuels are chiefly compounds of carbon and hydrogen, known as hydrocarbons. Most of them contain other elements, but are essentially carbon and hydrogen. If 56 PRACTICAL DENTAL METALLURGY. oxygen is contained the proportion of hydrogen may be equal to, or greater than, but never less than that required to form water with oxygen. Calorific Energy. — The amount of heat a unit weight of a body is capable of yielding when completely burned. It is usually measured by the number of the units of weight of water it will raise one centergrade degree of temperature. Thus in the subjoined table the calorific energy of wood charcoal, for example, is given as 8080, that is to say, one pound of wood charcoal when completely oxidized to carbon dioxide will yield sufficient heat to raise 8080 pounds of water through one centergrade degree; so with the other substances composing the table. The calorific energy of a fuel containing carbon, hydrogen, and oxygen is the sum of the calorific energies of the carbon and that of the disposable hydrogen* * Disposable Hydrogen. — The amount of hydrogen which may be com- bined with oxygen is not available as a source of heat, and is called "non- disposable " hydrogen; the excess of hydrogen over the amount which may be combined with oxygen being available is called " disposable " hydrogen. EXAMPLE No. 1 . — Determine the calorific energy of marsh gas (CH 4 ). C = lxl2=12 (The atomic weight of carbon is 12). H 4 =4x 1= 4 (The atomic weight of hydrogen is 1). CH 4 = 16 (The molecular weight of marsh gas). In one pound of CH 4 there is then Jf or y A lb. of carbon, the calorific energy of carbon is 8080 (see table) hence: %x 8080= 6060.0 In one pound of CH there is T % or ty lb. of hydro- gen, the calorific energy of hydrogen being 34462 % x 34462= 8615.5 Therefore the calculated calorific energy of marsh gas is 14675.5 EXAMPLE No. 2. — Determine the calorific energy of defiant gas (C 2 H 4 ). EXAMPLE No. 3.— Determine the calorific energy of ethine (C 2 H 2 ). EXAMPLE No. 4.— Determine the calorific energy of alcohol (C 2 H 5 HO). EXAMPLE No. 5.— Determine the calorific energy of bisulphide of car- bon (CS 2 ). MELTING METALS. 57 The experimental and calculated calorific energies of substances do not agree. This is probably on account of the heat absorbed in their decomposition. The calorific energies of different substances obtained experimentally, by the method mentioned previously, is given in the following table: TABLE OF CALORIFIC ENERGIES. Hydrogen Burned to water, H 2 34462 Carbon (wood charcoal) " " Carbon dioxide, CO„ 8080 " " " " " " monoxide, CO 2474 Silicon " " vSilicic anhydride, Si0 2 7830 Phosphorus " " Phosphoric anhydride P 2 5 5747 Sulphur " " Sulphurous anhydride, S0 2 .. .. 2140 CARBON COMPOUNDS. Marsh gas, CH 4 Burned to C0 2 and H,0 13063 Olefiant gas, C 2 H 4 " " " " " 11857 Illuminating gas, H, CO, CH 4 , C 2 H 4 , C 6 H 6 , etc. Crude petroleum Wax Tallow Alcohol " about 12000 " 10190 " 10496 " 9000 " 7183 Carbon monoxide " " " 2403 Calorific Intensity is the pyrometric degree of heat obtained when a substance is completely burned. Pyrometry is the measurement of high temperatures, and is accomplished by means of an instrument called a pyrometer. The fuels most used in dental laboratories are: wood, coal, charcoal, coke, petroleum, gasoline, and coal gas. Coal is variously classified, usually into four varieties: anthracite, bituminous, cannel r and lignite. Of these only anthracite is suitable for dental use. It is the nearest approach to pure carbon (about 90 per cent carbon); it burns with a small flame, intense heat, and no smoke. It should be carefully selected, clean, free from slate, and not yield a fusible ash. Charcoal is obtained by heating wood to the temper- ature of from 350° to 400° C out of contact with the air, 58 PRACTICAL DENTAL METALLURGY. thus the water, acetic acid, tar, and various gases are driven off, leaving a black, sonorous, hard mass known as wood charcoal. This is of two classes, hard and soft wood charcoal, the former being best adapted to dental purposes, is made from the beech, oak, alder, birch, elm, etc. The soft variety is made from the pine, larch, linden, willow, and poplar. Charcoal is particu- larly indicated for dental use for maintaining high tem- peratures in a small compass. It should be kept protected from moisture, which it will absorb, impairing its calorific energy. Coke is a carbonaceous residuum obtained when coal is strongly heated in a closed space with a limited supply of air, the volatile products of the coal being driven off leaving a substance of variable qualities depending on the nature of the coal used and the mode of coking. It may be porous and light or dense and compact; soft and ten- der or hard and resisting; varying in color from black to gray; its luster sometimes dull, at others, of metallic brightness. It is less inflammable and less combustible than charcoal, but yields a higher temperature on burn- ing. Professor Richardson observes: "The best coke for furnace use is that used by brassfounders, and has a steel-gray color, with a somewhat metallic luster." Coke does not easily ignite, and usually requires a little admixture of charcoal to kindle it; a strong draught is also necessary to burn it. It has been much used in continuous-gum work and analogous operations. Petroleum. — Kerosene or coal oil is one of the prod- ucts in distilling crude petroleum, and is much used where gas is not available. Since most dental lamps and stoves are of metallic construction, due precaution must be exercised to use only good " high test" oil, i. e., that which has been properly freed from the volatile products MELTING METALS. 59 of petroleum and is capable of withstanding the maxi- mum temperature developed by the lamp-flame without evolving dangerously combustible gases. It has been found that 5 per cent, of crude naphtha reduces the flash- ing-point from 118° to 70° F. Gasoline. — A colorless, volatile, inflammable liquid; one of the products of the distillation of crude petroleum, having a specific gravity of . 629 to .667 at 60° F. It is so volatile that if a current of air be passed through it at ordinary temperatures a highly dangerous combustible gas is formed by the mixture of gasoline vapor and atmospheric air. It is much used as a fuel in vapor stoves and for carburizing air-gases, etc., see Figs. 8 and 19. Coal Gas. — Illuminating gas as it is frequently called is a distillatory product of the varieties of coal known as bituminous and cannel. EXPERIMENT No. 7.— Fill the bowl of an ordinary clay pipe with small fragments of bituminous coal, lute over with clay and place in a bright fire; immediately smoke is seen to issue from the stem which pro- jects beyond the fire. The smoke soon ceases, and if a lighted taper is then applied to the orifice of the stem, the issuing gas burns with a bright steady flame, while a proportion of a black, thin, tarry liquid oozes out from the stem. After the combustion ceases there is left in the bowl of the pipe a quantity of char or coak. This simple experiment is, on a small scale, an exact counterpart of the process by which the destructive dis- tillation of coal is accomplished in the manufacture of gas. The products of this distillatory process are classed in the gas works as gas, tar, ammoniacal liquor and coak. The gas is purified by removing the tar and ammoniacal liquor, and then passed into the pipes for consumption. It is composed of a variety of sub- stances divided into two classes, viz.: ist — Non-lumi- nous supporters of combustion, embracing hydrogen (H), marsh gas (CH 4 a lightly carburetted hydrogen), 60 PRACTICAL DKNTAI, METALLURGY. and carbon monoxide (CO); 2d — The lumzniferous con- slituents, which include the hydrocarbon gases acet- ylene (C 2 H 2 ), olefiant gas (C 2 H 4 , a heavy carburetted hydrogen), propylene (C 3 H 6 ), butylene (C 4 H 8 ), and most important of all the vapors of the benzol (C 6 H 6 ), and naphtbalin (C IO H 8 ) series. As a source of light and heat gas is most extensively used in dental laboratories. See gas furnaces. REDUCTION OF ORES.— Occasionally metallic ores are obtained in compact masses of comparatively pure metal, from which the accompanying matrix or gangue can be detached by the hand or hammer. But such in- stances are rare. In most cases the ore comprises but a small percentage of the gangue. Hence it is expe- dient to purify it as much as possible before attempting to liberate the metal. This is accomplished generally by crushing and washing out the earthy matter as far as practicable. The ore is then subjected to roasting, amalgamating, or dissolving operations for the reduction or liberation of the metal. The great majority of metals are reduced by heat. In this process the ore, along with some kind of flux, is exposed to the direct action of a powerful fire, which in most cases has a chemical as well as a physical function. It is intended, with the assistance of the flux, to break up or burn away some chemical compound or component, or it is meant to deoxidize the ore. For these firey operations immense furnaces are con- structed of brick, granite, or other building stone, and lined with refractory or fire-resisting clay, brick, etc. FURNACES are best classified, by the method adopted for supplying air, into two classes, viz., (1) blast-furnaces, (2) chimney-draught furnaces, the latter are also known as air and wind furnaces. MELTING METALS. 61 Blast-Furnaces are supplied with air from a source under pressure (B f Fig. 2) sufficient to overcome the resistance to its free passage presented by the packed Fig. 2. Sectional View of Blast-Furnace. columns of fuel, flux, and ore. These are the oldest and simplest forms of metallurgical contrivance. The open- 62 PRACTICAL DENTAL METALLURGY. hearth blacksmith's forge is a simple type of the same principles involved in the completely closed-in blast-furnaces of gigantic dimensions* in use for work- ing and producing the various com- pounds of iron. Fig. 2 is a ver- tical section of a blast-furnace. The upper cone D Cis known as the stack proper, the lower one Fig. 3. Reverberatory Furnace. from the broad- est part C to the tuyeres B, as the boshes, and the lower cylindrical part A, B, as the hearth. A Chimney Draught, Air, or Wind Furnace is supplied with air drawn through it by a partial vacuum in the chimney formed by the heated gases on their way to the atmosphere. The reverberatory furnace is a type of this class. Fig. 3 represents a vertical sec- tion of the reverberatory furnace. The characteristic point in this furnace is, that the fire-chamber A is separate from the one in which the material to be operated upon is placed — the heat and flame passing over the charge, as from A, D, E. B is a low wall divid- ing the fireplace from the working bed C, and is known as the fire-bridge. At the opposite end Fig. 4* Crucible Furnace. * In the Middlesborough district, England, is a furnace 103% feet in height, and of 33,000 cubic feet capacity. MELTING METALS. 63 there is sometimes a second bridge of less height called the flue-bridge, E. The ore is introduced from hoppers at H, the slag is withdrawn at A", and the metal run out by a tap-hole at L. For melting gold and silver, as for all ordinary melt- ing operations, Mr. Makins recommends one after the styleof Fig. 4, which should form a part of the fitting of all metallurgical labora- tories. This may be built in an ordinary house-flue with a chimney about thirty times the diameter of the fit r 71 a c e — o r thirty feet in height, for a furnace of one foot in diameter. A third class of furnaces is known as Muffle- Furnaces, and under this head are to be found the assay- er's furnace and the contimwus-gum fur- nace, Fig. 5. The principle is to avoid contact with either fuel or flame, and in the case of continu- ous-gum work even the products of combustion are care- fully excluded. Dental Laboratory Furnaces. — For melting metals FrG. 5. Continuous-Gum Furnace. 64 PRACTICAL DENTAL METALLURGY. in the dental laboratory, the small, compact, blast-furnace devised by Mr. Fletcher, and shown in Fig. 6, is the simplest and most convenient. It consists of a cylindrical casing and perforated cover made of fire-clay which has been mixed with three or ?>£Slfc.UV<..^.>U=i. Fig. 6. four parts by bulk of sawdust and burned. Through a hole near the bottom of the casing the mixed air and gas is injected, the latter being regulated by a check near the mixing chamber. The gas is received from as large a supply-pipe as convenient, and the air driven in by Fig. 7. means of the foot-bellows. The crucibles used should not exceed 2 by 2% inches. According to Mr. Fletcher, " With half-inch gas-pipe and the smallest foot-bellows the smallest sized furnace will melt a crucible of cast iron in seven minutes, tool steel in twelve minutes, and MELTING METALS. 65 nickel in twenty-two minutes, starting with all cold." Gold, silver, or copper may be readily fused in one of these furnaces where gas is accessible. Where gas is not convenient, the metals or dental- amalgam alloys may be melted very satis- factorily in a near-by blacksmith's forge, or in a coke or coal fire in an ordinary stove or open fire- place if the draught is sufficiently strong. If the draught is weak the combus- tion of the fuel may be better accom- plished by improvis- ing a blast by pass- ing a small piece of gaspipe between the grate-bars of the fire- place or stove and attaching to this the hose from the foot- bello ws . In this manner a consider- able quantity of gold, silver, copper, or alloy may be melted FlG 8 with little trouble. A modified type of Fig. 6 has been devised (Fig. 7) retaining all its peculiar advantages, but burning petro- 66 PRACTICAL DENTAL METALLURGY. leum, instead of gas, as fuel. The burner dispenses with a wick, by being constructed on the principle of an atomizer. It is supplied with a device for regulating the supply of oil, which is operated by the milled nut at A, and for the supply of an annular jet of air which is regu- lated by turning the sleeve B. The construction is such that it may be taken apart and cleaned, in case of any obstruction. The furnace stands are interchangeable for either gas or petroleum. Where illuminating gas is not attainable a much more convenient form of furnace than that shown in Fig. 7 may be had as illustrated in Fig. 8. The gasoline generator placed beneath the bench is attached to foot-bellows and furnace. The small crucible gas-furnace without blast, illus- trated by Fig. 9 will be found most convenient for melting the more infusible metals of the dental labora- tory. It will receive a crucible not to exceed 2% by 2^ inches, and when supplied with a 6-foot chimney, will melt copper, gold, silver, etc., in a very few minutes, or cast iron in 30 minutes, all started cold. The furnace is so con- structed that the gas enters a chamber at the bottom of the burner through a device similar to a Bunsen burner, mixing with air as it enters, and burned at the upper ends of a series of concentric tubes, furnishing air spaces alternately with those supplying the mixture of gas and air. The whole burner is constructed of iron, and will be found Fig. 9. MELTING METALS. 67 better able to withstand an intense heat, more durable and quicker in its operations than the old pattern, with gun-metal tubes. In case metal should be spilled into the burner, it can be easily taken apart for its removal. Downie's crucible furnace, Fig. 10, is especially de- signed for melting metals, such as gold and silver, making dental-amalgam alloys, experimental work, etc. Fig. 10. It is also very useful for brazing, soldering, heating up bridge-cases or metal plates to solder, etc. It has two removable rings of different widths, which set on above the flaring base to carry the heat up around the crucible, the wide or narrow ring being used, according to the size 68 PRACTICAL DKNTAL METALLURGY. of the crucible; or both rings may be put on at the same time. It also has a conical-shaped top which can be set on above the rings to confine the heat when it is desired to fuse any high fusing substance. This furnace can be used for baking continuous-gum work, or any other porcelain work. Fig. 11. For those metals which fuse much before redness, such as zinc, lead, tin, and their alloys, iron ladles are usually employed. In the dental laboratory for melting zinc, lead, or alloys, for making dies and counter-dies, iron melting-pots (Fig. 11), capable of holding from 6 to 10 pounds of metal, are used. The metal may be most con- veniently melted over one of Fletcher's solid-flame gauze- top stoves, shown in Fig. 12. The stove is so con- structed that the gas mixed with the proper proportion of air from below is burned above the gauze top, yielding a blue flame, intensely hot and perfectly solid and uniform. The consumption of gas is about two cubic feet per hour for each square inch of gauze surface. It will melt an ordinary pot of lead in 12 minutes, depending on the gas supply. An MELTING METALS. 69 apparatus in which gasoline may be used, when gas is not available (much used by plumbers for melting p IG# 13 solder), is recommended by Dr. Kirk* for melting zinc and lead in the dental laboratory. In the absence of gas supply it is probably more convenient to melt these metals in the open fireplace or in the stove. BLOW-PIPES.— For minor melting operations, such as melting small quantities of gold, silver, or copper, or in soldering, the blow-pipe in some of its variously modi- fied forms is usually employed. These in- struments are classified as simple and com- pound. A simple blow-pipe of plainest pattern is shown in Fig. 13, A. It consists of a tube of brass or other metal tapering gradu- ally from the larger end, which is inserted in the mouth, to the other extremity, which is curved and mounted with a cone- shaped tip to protect it from the action of the flame; the caliber of the instrument terminates here in a very small orifice. The point of the instrument is frequently tipped with a more refractory metal, such as plati- num, and the end to be received in the mouth is sometimes plated with a less oxi- dizable metal, such as silver. The whole is usually from twelve to fourteen inches in length, and the large extremity from one- half to three- fourths of an inch in diameter. * American System of Dentistry, Vol. Ill, p. 816. 70 PRACTICAL DENTAL METALLURGY. As more or less moisture accumulates in the tube from the mouth, a second form has been devised (Fig. 13, B), to the stem of which, nearer its smaller extremity, is adjusted either a spherical or cylindrical chamber, which collects and retains the moisture as it forms within the pipe. The moisture is prevented from flowing into the Fig. 14. smaller end of the tube beyond by the projection of that portion of the stem a slight distance into the chamber. Fletcher has much improved upon this simple form of blow-pipe by coiling the smaller extremity of the stem into a light spiral over the point of the jet (Fig. 14). The air as it traverses the coil is heated, producing a hot blast instead of a cold one, as in the old form. Such an instrument enables the operator to produce a higher temperature than that produced with the ordinary pipe with the same amount of energy. The same pipe may Fig. 15. be fitted with a hard-rubber mouth-piece, which is less tiresome to grip in the mouth. Another form by the same inventor is illustrated in Fig. 15. This is wholly unlike any mouth blow-pipe yet devised, and admits of considerable latitude of MELTING METALS. 71 movements in the application of heat by the rubber tubing connected with it. The mouth-piece is so constructed that a shield protects the lips in such a manner that long-con- tinued blowing may be practiced without undue strain on the lip?, while the opening is well under the control of the tongue. It is also provided with a condensing chamber and interchangeable tip, either plain or coiled. FLAME. — Flame consists of a sheet of burning gas. The burning candle presents a type of all other flames, serving to illustrate its general structure. If such a flame be examined closely it will be found divisible into four separate parts. The por- tion which immediately surrounds the wick, represented in the figure by A, B, is a deep blue. This portion becomes thinner as it ascends, until it gradually disappears. It owes its color to the combustion of carbon monoxide, and is the coolest portion of the flame. The center (C) of the flame is dark, i. e., non-luminous, and consists of the gases produced by the decomposition of the fat, and is, in the flame we are considering, FlG - 16, highly charged with carbon. These gases do not come in contact, at this point, with sufficient oxygen to burn them, and therefore remain unchanged. Surrounding the dark cone-shaped center is the bril- liant yellowish- white flame D. Here the gases are enabled to combine with the oxygen of the air. The hydrogen burns, and the intense heat generated by its combustion ignites the minute paiticles of carbon which are held in suspension by the ascending gas, rendering them highly incandescent; hence the luminosity. All the gas, however, is not burned in the central cone of flame. A portion of it escapes beyond and burns more 72 PRACTICAL DENTAL METALLURGY. slowly, in consequence of its being mixed with steam, carbon dioxide, and other products of combustion, to- gether with a little unburnt carbon. This forms the outer dimly luminous envelope E. The greatest heat is found in the external ring EE, for here the air has free access to the exterior. The heat, however, decreases from Eto E t and from Eto A, B. The Use of the Blow-pipe consists in injecting a stream of atmospheric oxygen into this inner cone of gas, so as to cause the free combustion of it and the luminous particles of carbon evolved from it; while at the same time the operator directs the flame over the object to be heated. The Flame of the Blow-pipe consists of three parts (Fig. 17). A, an inner A "1"> B ^> cone of unburned gases mixed with Fig. 17. oxygen (air) from the blow-pipe; a second, B, blue, pointed, and well defined; and a third, C, yellowish, and somewhat vague. The Reducing Flame (R. F.). — Just beyond the tip of the inner blue cone is the reducing flame, so called from the fact that if a metallic oxide be immersed in it the oxygen of the metal will be abstracted by the heated carbon from the hydrocarbon gas, to form carbon diox- ide, and the metal liberated. At the same time the metal is protected from reoxidation by being thoroughly cov- ered with the flame. To produce this flame the jet of the pipe should be fine, and placed a little above and to the side of the flame. The Oxidizing Flame (O. F.). — Just beyond the tip of the less distinct cone C is the position where, if a metallic bead be exposed, it will combine with the oxy- gen of the air, becoming an oxide. The object is to MELTING METALS. 73 heat the metal hot enough to favor a rapid chemical com- bination with the oxygen of the air, at the same time to draw as large a quantity of external air to the point as possible; hence, the bead of metal is not only imme- diately melted, but is oxidized by the external air; therefore the farther it can be kept from the tip and sufficient heat be maintained, the more perfect will be the oxidation. This flame is formed by placing the jet, which should have a tolerably wide opening, imme- diately over the wick or burner, and injecting the air into the flame. The cone then loses its yellow color and becomes an intensely hot, long, narrow, blue flame. Reduction on Charcoal. — Reduction is much more easily effected by the employment of a block of charcoal as a support. It not only assists in heating the bead of metal by becoming hot, but it also assists in the reducing action by combining with the oxygen of the oxide, form- ing carbon dioxide, and liberates the metal. LAMPS. — Flames for soldering may be derived from oily spirit, or gas lamps. Oil Lamps. — The fluid hydrocarbon, petroleum, or coal-oil is very inexpensive, and where gas is not avail- able is much used. Fig. 18 rep- resents a soldering lamp. An oil- lamp to be satisfactory should hold about one to two pints and should have a tapering spout from three to five inches in length. The spout should be well filled with wick, but not too tightly, for fear of preventing free saturation Fig. 18. 74 PRACTICAL DENTAL METALLURGY. with the oil. Proper care should be exercised to guard against all accidents occasioned by ill-fitting parts, filling and adj usting. With a good lamp, an entire artificial den- ture can be sol- dered, or one or two ounces of gold melted. Pure sweet oil or lard oil may also be used in these lamps for soldering. Fig. 19 illus- trates a com- pound blow-pipe (D) used with gasoline gas. It is provided with a geiiera- tjr(^) and bel- lows (B) with which it is con- nected^) simi- larly to that in Fig. 8. Spirit Lamps. — Much the same lamps, as illustrated in Fig. 18, may be used for alco- hol, which is much preferable to coal-oil on account of its cleanliness and the less liability to accident. With a lamp similar to the one represented in Fig. 20, the Fig. 19. MELTING METALS. 75 spirit is entirely uninfluenced by the heat of the flame, and explosion is rendered almost impossible. In Fig. 21 is represented a self-acting lamp and blow- pipe. The lamp reservoir and the boiler will each hold about a half-pint of alcohol. Light- ing the flame under the boiler vaporizes the alcohol in it rapidly, the pressure forcing the vapor through the pipe into the large flame at the side Fig. 20. of the lamp, forming a very practicable and efficient blow-pipe. The force of the blast is regulated by raising or lowering the boiler; the spread of the flame by using the larger or smaller nozzle. The ap- pliance is substantially made of spun brass, and the boiler is provided with a safety-valve. A set-screw on the upright permits the boiler to be raised or lowered or swung to one side. One of the nozzles is carried on the top of the safety-valve; the other in position on the pipe. Gas Lamps. — The gas may be most effectively used by an apparatus on the principle of the one illustrated in Fig. 22, which consists of a sort of duplex Bunsen burner. 76 PRACTICAL DENTAL METALLURGY. Compound Blow-pipes. — The difficulty in maintain- ing a flame of uniform size and intensity, owing to the fact that the blow -pipe and lamps are separate, the lack of latitude allowed the op- erator by the fixed position of the blow-pipe, and the introduction of gas in the experimental laboratory led to a form known as the com- pound blow-pipe, Fig. 19. This instrument is so con- structed that it virtually consists of a lamp and blow- pipe all in one. In general it consists of two metallic Fig. 22. concentric tubes, one a smaller, terminating in a fine jet and placed within the first, so that the finer jet is accu- rately centered in the orifice of the larger tube, Fig. 23. Gas is supplied to the larger tube by an offset tube on the side of the nearer end, and flowing through the space in the large tube on all sides of the enclosed smaller tube to the opposite end, where it is ignited. Air from the lungs or other source is transmitted through the MEI/TING METALS. 77 inner tube to the center of the flame. The supply of both gas and air may be regulated in most of the later Fig. 23. patterns by checks within reach of the fingers of the same hand holding the instrument. The blast from the A n n 7 mouth is most convenient for small heating, but when high temperatures are desired for some time, one of the 78 PRACTICAL DENTAL METALLURGY. various forms of mechanical blowers is necessary. A mechanical blower devised by Dr. Burgess is illustrated in Fig. 24. It consists of a cylindrical metallic reservoir connected beneath with a pump cylinder, worked by means of a heel-and-toe treadle. The air is forced into the reservoir through a valve, and escapes through a small opening on the side near the top to a flexible rub- ber hose which conveys it to the blow-pipe. A far more satisfactory apparatus is found in the foot-bellows de- vised by Mr. Fletcher and shown in Fig. 25. SUPPORTS.— When sol- dering or melting gold or silver with the blow-pipe flame, it is necessaryjto place the articles to be soldered, or the metals to be melted, upon some sort of a support. Such supports may be improvised of blocks of charcoal, if the temperature is not to be too high, or large blocks of pumice- stone encased in plaster, giving the whole a variety of forms. One, de- Fig. 25. MELTING METALS. 79 signed by Professor C. L- Goddard which the author uses very comfortably, was made by making a mold of a hemisphere from a smooth croquet-ball, by pouring some plaster of pans into a pasteboard box about five inches square, and then dipping the ball, and removing it when the plaster had hardened. The mold thus made was then varnished and filled with soft plaster, on the top of which was imbedded a large piece of pumice- stone. When this hardened, the hemisphere was sepa- rated from the concavity, and the block containing the latter cut down until it covered but little over half of the hemisphere, when it was reinserted. The whole was then varnished, and presented a very compact, con- venient soldering-block, fitted in a socket which permits it to be poised at almost any angle. Blocks of charcoal may also be covered on all sides but one with about one-half inch thickness of plaster. They then furnish clean and convenient supports for small sol- derings and meltings. , Fig. 26 forms a convenient asbestos | soldering-block, and j Fig. 27 represents ■ another more easily p handled. The block is of carbon and is fur- nished with a wooden handle. Fig. 28 represents a circular asbestos soldering- block or -tray, with raised rim, set in a brass box, mounted on a wooden handle. The four holes are for the reception of brass pins, to hold the work in place. The investing material is made of a prepared asbestos fiber. This Fig. 26. 80 PRACTICAL DENTAL METALLURGY. material is simply dampened. When the objects to be soldered consist in part of artificial teeth, such as a denture Fig. 27. or bridge, a support of the style of Fig. 29, a small hand- furnace or soldering-pan, is very satisfactory. It consists Fig. 28. Fig. 29. of a funnel-shaped receptacle made of sheet iron, with a light grate or perforated plate of the same material adjusted MEETING METALS- 81 near the bottom, and an opening on one side, underneath the grate, for the admission of air. The upper part of the holder is surrounded by a cone-shaped top, which may be readily removed by a handle attached to it; while to the bottom of the furnace is attached an iron rod, 5 Fig. 30. or 6 inches in length, enclosed in a wooden handle at its unattached end; when the case is sufficiently heated, the top may be lifted off, and the case remaining in the fur- nace soldered with the blow-pipe in the usual manner, the furnace then serving the place of a support. INGOT MOLDS are usually made of iron in various forms to suit the requirements, those for the noble Fig. 31. metals generally having the form shown in Fig. 30, which is so constructed that the side next to the handle, which also acts as a set-screw, can be moved laterally upon the opposite side, so that the intervening slot may be made narrow or wide. Ingots are also frequently cast 82 PRACTICAL DENTAL METALLURGY. into molds of sandstone, charcoal, compressed carbon, pumice-stone, or asbestos preparations. Fig. 31 repre- sents such an apparatus suitable as a support for melt- ing, and as an ingot mold for the molten metal by merely- tipping the slab and placing a cold, flat surface over the still heated metal in the mold. Fig. 32 is an arrangement for melt- ing and molding noble metals without the use of a furnace. Referring to the engraving: A is a crucible of molded carbon supported in position by an iron side-plate. C the ingot mold. D a clamp holding the crucible and ingot mold in position, and swiveling on the cast iron stand B. The metal to be melted is placed in the cruci- ble A, and the flame of a blow-pipe is directed on it until it is perfectly fused. The waste heat serves to make the ingot mold hot, and the whole is tilted over by means of the upright handle at the back of the mold. A sound ingot may be obtained at any time in about Fig. 32. two minutes. CHAPTER V. ALLOYS. AN ALLOY is the compound or mixture of two or more metals effected by fusion. AN AMALGAM is an alloy of two or more metals, one of which is mercury. Few metals are employed in the pure state, with the exception of iron, copper, lead, tin, zinc, platinum, alum- inum; they are more frequently used in the form of alloys for technical purposes. Every industrial application necessitates special qualities that may not occur in any isolated metal, but which may be produced by the proper mixture of two or more of these. For example: silver and gold are much too soft and pliable for plate, coin or jewelry, but by the addition of certain amounts of cop- per they are rendered harder and more elastic, while their color and other valuable qualities are not impaired. Copper is also too soft and tough to be wrought in a lathe, but when alloyed with equal parts of zinc it forms a hard, beautiful, yellow-colored alloy known as brass, of great usefulness, and more easily worked than the pure metal. Alloys are equally interesting, from a scientific stand- point, for they may be regarded not only as mere mix- tures of metals, but in many instances as true chemical compounds. Matthiessen* regarded it as probable that the condition of an alloy of two metals in a melted state may be either that of : 1. — a solution of one metal in another; 2. — a chemical combiiiation; 3. — a mechanical mix- ture; or, 4. — a solution or mixture of two or all of the above; and that similar differences may exist as to its condition in the solid state, defining a solid solution as " a perfectly homogeneous diffusion of one body in another." * British Association Reports, 1863, p. 97. 84 PRACTICAL DENTAL METALLURGY. 1. A Solution of One Metal in Another. — Some metals when melted together will apparently unite in the same manner that water mixes with alcohol, in all pro- portions and indefinitely, forming a perfect homogeneous mass and exhibiting no tendency to separate on cooling. The mixture thus formed will, as regards chemical and physical properties, be a mean of the two components; that is to say, it will partake of the properties of both, those of the one predominating just as one or the other may be in excess. Lead and tin form such an alloy. 2. A Chemical Combination. — Other metals when melted together do, without doubt, form true chemical compounds. In the phenomena which accompany such union, and in the properties of the resulting products, we observe that which characterizes the manifestation of affinity, that is, an evolution of heat and light, resulting in the formation of substances having a definite composition, distinct crystalline form, and a variety of properties differ- ent from those of the constituents. Thus, if a piece of clean sodium be rubbed in a mortar with a quantity of dry mercury, the sodium combines with a hissing sound, and a considerable increase of mass temperature is noticeable on the addition of each successive piece of sodium. EXPERIMENT No. 8.— Throw a small piece of clean, dry sodium, upon the surface a small amount of clean, dry, and warmed mercury; a chemical union takes place immmediately, accompanied by heat and incan~ descence, forming crystalline amalgam. When the mass cools, long needles of a white, brilliant alloy of definite composition crystallizes from the middle of the liquid, and the excess of mercury may be sepa- rated by decantation. Platinum, iridium, gold, and silver unite with tin, accompanied by an evolution of heat. If the tin is in excess, upon cooling, the mass very much resembles that metal, but if the ingot be treated with strong hydrochloric acid, the excess of tin is dissolved, and crystals of a definite alloy of tin and the ALLOYS. 85 precious metals remain.* Examples of such union by definite proportion often occur in nature, as, for instance, we have the native alloys of gold and silver, in which four, five, six, or twelve atoms of gold are found com- bined with one of silver. Several other metals? such as iridium and osmium, as iridosmine, palladium, and plati- num and others occur as native alloys. 3. A Mechanical Mixture. — It must be admitted that, in the case of mixing substances, or of dissolving one in another, the result is much dependent upon the affinity — or compatibility — existing between them. Thus we may attempt to dissolve camphor in water, but here the affinity is so feeble that an exceedingly small propor- tion will be dissolved; while, on the other hand, if we employ alcohol instead of water, a large quantity of the solid camphor may be taken into solution. Then, if water be subsequently added to such a solution, the spirit, having a greater affinity for the water than for the camphor, will leave the latter, to be separated again and assume a solid form. Thus, silver or gold will not unite with iron, nor zinc to any great extent with lead. Other metals melted together which possess little or no affinity for each other, do not readily unite, but remain separate and distinct; alloys of lead and copper, "pot-metal alloys," show on their fracture surfaces, when viewed under a strong glass, a network of copper and a small amount of lead, enclosing irregularly globular masses of nearly pure lead in its meshes. Such alloys are subject to liquation, or separation, by heat — the lead separating out, leaving the copper in a porous mass. 4. A Solution or Mixture of Two or All of the Above. — It is obvious that most alloys may be correctly classed under this head, when we consider the almost infinite proportions in which metals are combined. * See chapter on Tin. 86 PRACTICAL DENTAL METALLURGY. THE PHYSICAL PROPERTIES OF ALLOYS cannot be anticipated, and are only determinable by actual experiment. Very minute proportions of some metals added to others will produce an alloy with properties foreign to either of the constituents. Thus, a small quantity of lead fused with gold will produce a brittle alloy, though each metal is malleable. Specific Gravity. — If this property be calculated from that of the components — assuming that there is no con- densation of volume — the resulting number may be greater than, equal to, or less than, the experimental result. Thus, the alloys of silver and gold have a less spe- cific gravity than the theoretical mean of the components; whereas copper and zinc vary in the opposite direction. The following table,* by Th^nard, shows examples of this variation : Alloys Possessing a Greater Specific Gravity than the Mean of Their Components. Alloys Having a Specific Gravity Inferior to the Mean of Their Components. Gold and Zinc Gold and Silver < i >i Tin < i i < Iron < i i< Bismuth it << Lead ■ < < < Antimony << <( Copper (i (< Cobalt << <( Iridium Silver (i Zinc « < <. Nickel it < c Lead Silver << Copper «( << Tin Copper << Lead < i << Bismuth Iron « < Bismuth (< < < Antimony < t < < Antimony Copper «( Zinc <( << Lead 1 1 << Tin Tin « < Lead i < cc Palladium < < < c Palladium tc < < Bismuth < < a Antimony <( << Antimony Nickel «> Arsenic Lead ( « Bismuth Zinc << Antimony u 1 ( Antimony Platinum (I Molybdenum Palladium" Bismuth * Phillip's Metallurgy. ALLOYS. 87 It is common among authorities who publish determi- nations upon specific gravities of the alloys to give the calculated as well as the observed specific gravity. The Color of an alloy is usually resembling or par- taking of that metal which predominates. Some few exceptions are quite notable, for instance gold 2 to 6, and silver 1 part produces an alloy of a greenish color, and it is said that % 4 of silver is sufficient to modify the cclor of gold. Nickel and copper form alloys varying from copper-red to the bluish-white of nickel. With a content of 30 per cent, of nickel the alloy is silver white ; while with zinc, copper yields a variety of shades, from the silver white of copper 43, and zinc 57 parts, to that of red brass, which contains 80 per cent, or more of copper. Malleability, Ductility, and Tenacity. — These prop- erties are generally very much modified by alloying. As a rule the malleability and ductility are decreased, even when two malleable and ductile metals, such as gold and lead, are alloyed together — a very small content of lead destroying the malleability and ductility of the noble metal. Again, copper 94 and tin 6 parts form an ex- ceedingly brittle alloy. Generally the ductility decreases, while the hardness as compared with that of the con- stituent metals increases to a considerable extent; for example, gold and platinum, two very ductile and soft metals, afford an alloy much harder and of greater elas- ticity than either. Gold and silver, being too soft for currency, are alloyed with 10 per cent, of copper, which gives them the required hardness. A few metals, anti- mony, for instance, possess the property of making metals harder. Mr. Makins states that l-1900th part of this brittle metal will make gold quite unworkable. As a rule, a brittle and a ductile metal afford a brittle alloy; yet copper and zinc yield a malleable and ductile brass. 88 PRACTICAL DENTAL METALLURGY. The tenacity is generally very much increased , as is shown by the following results of Matthiessen's experi- ments. Wires of the same gauge were employed, and the weights causing their rupture before and after alloy- ing noted as follows: Lbs. at Rupture. Copper, unalloyed 25 to 30 Tin, " , under 7 Lead, " " 7 Gold, " 20 to 25 Silver, " 45 to 50 Platinum, " , 45 to 50 Iron, " 80 to 90 Lbs. at Rupture. Copper, alloyed with 12 per cent. Tin 80 to 90 Tin, •' " " " Copper 7 Lead, " " Tin 7 Gold, " " Copper 70 Silver, <; " Platinum 75 to 80 Steel (iron compounded with carbon) . above 200 Fusibility. — The fusing point of an alloy is always lower than the least fusible metal entering into its com- position, and is sometimes lower than that of any of the components. Thus an alloy composed of 10 parts lead and 4 parts tin fuses at 470° F., melting lower than the less fusible lead (617° F.), but at a greater temperature than tin (442° F.); and an alloy composed of 4 parts lead, 2 parts tin, 5 to 8 parts bismuth, and 1 to 2 parts cadmium (Wood's metal) melts at 140° to 161° F., lower than that of any of its constituents — tin being the most fusi- ble (442° F.). Alloys of lead and silver, containing a small quantity of the latter, are more fusible than lead, and sodium and potassium form a fluid alloy at ordinary temperatures. Matthiessen* explains why the fusing point of alloys is uniformly lower than the mean of those of their con- stituents: "It is generally admitted that matter in the * Makins' Metallurgy, p. 65. ALLOYS. 89 solid state exhibits excess of attraction over repulsion, whilst in the liquid state these forces are balanced, and in the gaseous state repulsion predominates over attrac- tion. Let us assume that similar particles of matter attract each other more powerfully than dissimilar ones attract each other. It will then follow that the attrac- tion subsisting between the particles of a mixture will be sooner overcome by repulsion than will the attraction in the case of a homogeneous body; hence, mixtures should fuse more readily than their constituents." Sonorousness. — This property is most wonderfully developed in some instances. Copper and tin, two metals which possess the quality in but a small degree comparatively, unite to form an alloy known as "bell metal," the normal composition of which is, copper 72 or 85, and tin 15 or 26. Copper and aluminum also yield alloys of remarkable sonorousness. Conductivity. — The property of conductivity, either for electricity or heat, in an alloy is much inferior to that of the pure metals. Advantage is taken of the high electrical resistance in some of the alloys, such as Ger- man silver, for measuring the resistance of long lines of telegraph wire, the electromotive force or working power of batteries, for making rheostats and other apparatus for controlling the electric current, etc. Decomposition. — Heat decomposes alloys containing volatile metals like mercury or zinc. It requires a tem- perature much above the boiling point of the metal, however, to completely separate all traces of it from an alloy, and in most instances this cannot be accomplished even then without the assistance of chemical agency. When gold is contaminated with tin, the latter cannot be removed entirely by roasting; but if heated with small quantities of potassium nitrate, which serves to oxidize 90 PRACTICAL DENTAL METALLURGY. the base metal, it may be entirely removed. Mercury may be completely separated by roasting; it volatilizes at about 675° F. When endeavoring to expel it from old amalgam fillings, however, the plug should be heated to a bright red. ANNEALING AND TEMPERING.— Annealing is a process employed in the working of various metals and alloys to reduce the brittleness usually resulting from a rapid or important change of molecular structure, such as is produced by hammering, long continued vibration, rolling, and sudden cooling. Bell metal is brittle, and cracks under the hammer, cold as well as heated. If it be repeatedly brought to a dark-red heat and quickly cooled by immersion in water, its brittleness is so far decreased that it can be hammered and stamped. The dentist, in swaging a flat sheet of gold to conform to his dies, must stop at intervals and anneal the piece of metal to prevent its splitting under his blows and pressure. Wheels and axles of railway coaches, from the constant vibration to which they are subjected, become in course of time dangerously brittle; and they require to be re- worked and annealed anew to restore the required tough- ness to the material. It is said sudden changes of temperature have the effect, almost invariably, of rendering metals brittle. Gold, silver, platinum,, etc., should be heated for a re- arrangement of their molecular structure and allowed to slowly cool, rather than to be immediately plunged into a cold bath, if the best results are desired. Lead, tin, and zinc are annealed by immersion in water, which is made to boil and then cool slowly. Steel should not be annealed in an open fire, as the carbon which enters the iron as an element combines with the oxygen of the air to the detri- ment of the steel. ALLOYS. 91 Annealing may be said to be the inverse process of — Tempering, which latter is the fixing of the molecular condition of steel by more or less sudden cooling from a particular temperature. Oxidation. — Alloys are usually more easily oxidized than their constituents. Mr. Makins says:* " The supe- rior oxidizability of one constituent of an alloy appears to be assisted by galvanic action set up. This is always the case where an electronegative, or acid-forming metal, is alloyed with an electropositive, or base-producing one. Chemical action is, therefore, generally more energetic on an alloy than upon a simple metal; and, indeed, metals which are untouched by an acid when alone will be acted upon by the same acid when alloyed with another which is soluble in the acid employed. Thus platinum is quite insoluble in nitric acid, but if it be alloyed with a large proportion of silver, it will be dissolved with the silver by the nitric acid, and that to the extent of a tenth of the weight of silver." Nearly all metals in a state of fusion have a tendency to dissolve a greater or less amount of their oxides; and this is particularly true of alloys, as then the metals are in a state of solution, a condition most favorable to chem- ical change. A striking illustration of this came under the author's notice in a dental-amalgam alloy prepared by Dr. S. E- Knowles, consisting of 2 parts tin and 1 part each of silver and aluminum. There was no excep- tional difficulty in thoroughly blending the constitu- ents, and the alloy resembled the ordinary dental-amal- gam alloy when filed and ready for mercury, but upon the addition of mercury the oxidation of the whole was so rapid that a very considerable heat was evolved, and so complete that nothing remained but a black stain. * Makins' Metallurgy, p. G4. 92 PRACTICAL DENTAL METALLURGY. "In some alloys, as those of copper and tin," Dr. Kirk says,* " as much as from 2 to 5 per cent, of the oxides formed will be dissolved, unless means are taken to prevent it." Such a solution of the oxides greatly diminishes the cohesive property of the alloy by prevent- ing perfect contact of the particles; hence, much of the strength and toughness of the mass are lost. The best preventative against this formation of oxides and their subsequent absorption is to protect the molten alloy by a layer of pulverized charcoal or some of the fluxes. A reduction- of much of the oxide formed may be effected by vigorous stirring with a stick of green wood. The careful addition of not more than % 000 to x /xooo parts of phosphorus has been found an excellent agent for the deoxidation of the oxides dissolved in bronze. The zinc and alloys used in the dental laboratory for making dies, after repeated melting and casting in con- tact with the air, often become thick and mushy from dissolved oxides; and their valuable working qualities are so seriously impaired that they fail to copy the fine lines of the mould and produce a perfect die. Their prop- erties may be restored to a great extent by melting under pulverized charcoal or tallow, and vigorously stirring with a stick of green wood. INFLUENCE OF CERTAIN METALS IN ALLOYS. — Certain metals when present in an alloy confer upon it definite properties which are in many in- stances characteristic; thus, in a general way, mercury, cadmium, and bismuth increase fusibility; tin, hardness and tenacity; antimony and arsenic, hardness and brit- tleness. * American System, of Dentistry, Vol. Ill, p. 801. ALLOYS. 93 A SOLDER is an alloy or metal used for cementing or binding metallic surfaces or margins together, and the process is usually effected by heat. Ordinary solders are divided into hard and soft classes. The Hard Solders comprising those which require a red heat for their melting. The Soft Solders being those used by plumbers and tinsmiths, and consisting principally of lead and tin, with sometimes an addition of bismuth. Brazier s Solder, for uniting the surfaces of copper, brass, etc., is usually composed of copper and zinc, nearly equal parts, with a small addition of tin, and sometimes antimony. For fine jewelry, alloys of gold, silver, and copper are used; silver solder is employed for the inferior qualities, and even soft solder finds extensive use in jewelry estab- lishments. Silver is the proper solder for German silver articles, and gold for platinum. In Soldering, the surfaces or edges to be united must be kept free from oxidation and dirt. To keep them unoxi- dized during the operation several fluxes are used, such as dehydrated borax, or some of the reliable prepared compounds on the market, for gold, silver, brass, or copper soldering; rosin, or a solution of zinc chloride, for tin plate; zinc chloride for zinc, and rosin and tallow for lead and tin. Among the requirements of a good gold solder the most important are carat, color, strength, and fusing point. In fineness it should be equal, or nearly equal, to the plate, its color and strength as near as possible the same, while the fusing point should be a trifle lower — the nearer the melting point of the plate the better the results. To obtain these qualities, it is necessary to prepare a solder by the addition of some metal which will fuse at 94 PRACTICAL DENTAL METALLURGY. a lower temperature than any of the constituents of the plate. Zinc is admirably suited for this purpose, and is generally used, since it permits of a solder as fine, or nearly as fine, as the plate. In addition to this it also possesses the advantage of yielding a less fluid solder than that of copper and silver, permitting it to bridge over slight spaces. This is very probably on account of the oxidatioa or volatilization which takes place, for it is observable that any subsequent fusing requires a greater heat. An advantage is also obtained here in this fact, since it enables more perfect second solderings with the same alloy. The process of soldering is a cementation by superficial alloying, and is admirably illustrated in the instance of soldering platinum bases for continuous-gum dentures. By means of the blow-pipe the pure gold is flowed over the surfaces of platinum, joining them, but if the joint is not well made, and the intervening space is filled with gold, it is not as strong as it might be. This, however, is all remedied during the process of baking the body and enamel, as the high heat required for this so diminishes the cohesive power of the platinum that it readily and completely alloys with the gold, producing a stronger joint of a platinum-gold alloy, which is observed to be the same color as the platinum. Autogenous Soldering is a process of soldering by direct fusion of the contiguous parts, without the inter- vention of a more fusible alloy. It is extensively used in large plumbing work. The sources of heat in soldering are the alcohol lamp or gas flame, intensified by the mouth or foot-bellows blow-pipe. In hard soldering the objects to be soldered and their investment are heated over a charcoal or gas furnace to equally heat all parts to a greater or less ALLOYS. 95 extent preparatory to using the blow-pipe. When the furnace heating is carried to a high point the blow-pipe is needed but to slightly raise the temperature and direct the flow of the solder. Apparatus for heating and fusing metals and alloys have been studied under a special head, while the composition and management of various solders will be treated under appropriate heads, such as gold, silver, tin, etc. PREPARATION OF ALLOYS. —Casually this would seem but a simple task, but in order to produce an accu- rate result it is far from being as easy as it may seem. Most alloys are prepared by directly melting the metals together, but much skill, judgment, and experience are required to determine when it is best to add each constit- uent, and the amount of each to be used; to protect the molten mass, and to handle it generally. The metal having the highest fusing point is generally melted first, and the others are added in accordance with their points of fusibility. For making large quantities of an alloy the reverber- atory furnace is used, special precautions being taken to preserve a deoxidizing flame within the furnace. For preparing alloys in a small way a crucible is used, and the alloy is covered with a suitable flux to protect it from the action of atmospheric air. Four sources of loss must be guarded against: 1 — loss by oxidation; 2 — loss by volatilization; 3 — loss by chemical combination with the flux; 4 — loss by fracture or solution of the crucible. The first may be prevented by the use of one of the various fluxes,* or covering the surface with pulverized charcoal. The second loss usually occurs through an endeavor to alloy a metal of a high fusing point with one which fuses at a low temperature. Under such circum- *See chapter on Melting Metals. 96 PRACTICAL DENTAL METALLURGY. stances the one requiring a high temperature should be fused first and well covered with flux melted to extreme fluidity ; the more fusible metal should then be added in as large a piece as convenient and quickly thrust beneath the molten surface. The third source of loss is prin- cipally caused by the use of borax as a flux for some base metals. It is well known that in much borax a portion of the boric acid is not perfectly saturated, and this is especially true of the prepared article; and if melted with some base metals the free acid is absorbed, which, with the sodium borate, forms double salts of a glassy nature. Hence, by fusing some metals and alloys under borax, a certain portion will be lost in chemical combination. The fouith cause is guarded against by careful selection of crucibles. If alloys of low fusing metals are to be made, the ordinary clay or Hessian crucible is all that is neces- sary, and, indeed, with proper care, noble metals may be alloyed in them without danger of loss; but they are subject to perforation by corrosive fluxes, allowing the molten alloy to escape. Therefore, for the preparation of expensive alloys from noble metals, the employment of tried graphite or graphite and clay crucibles often saves much trouble and expense. In some instances, especially when metals are known to form chemical combinations, it maybe best to melt the one of lowest fusing point first, and then dissolve the other components in it. Or, those of low fusing point may be melted in one crucible, while those more difficult of fusion are melted in another, then combined in the molten state. When two metals of varying specific gravity are alloyed the mass should not be allowed to become quiescent just before pouring. And if any incompatibility exists be- tween the metals, such as in the case of zinc and lead, accompanied by a great difference in specific gravity, an ALLOYS. 97 intimate admixture should be effected by vigorously stirring the molten mass with sticks of soft, dry wood, which become more or less carbonized, according to the temperature of the mixture. In consequence of this dry dis- tillation of the wood there is evolved an abundance of gases, which, by ascending in the fused mass, contribute to its intimate mixture. The stirring should be con- tinued for some little time, and the alloy poured as quickly as possible. "Many alloys," says Mr. Brannt,* "possess the property of changing their nature by repeated remelting, several alloys being formed in this case, which show con- siderable differences, physically as well as chemically. The melting points of the new alloys are generally higher than those of the original alloy, and their hardness and ductility are also changed to a considerable extent. This phenomenon is frequently connected with many evils for the further application of the alloys, and in preparing alloys showing this property the fusion of the metals and subsequent cooling of the fused mass should be effected as rapidly as possible." Although most of the heavier metals are at present used in the preparation of alloys, copper, zinc, tin, lead, silver, and gold are more frequently employed than all others. Alloys containing nickel have become of great importance, as well as those in which aluminum forms a constituent. Mr. Brannt recommends for experimentation that metals be added to each other in certain quantities by weight, which are termed atomic weights, and claims that in this manner alloys of determined, characteristic prop- erties are, as a rule, produced; or, if such does not answer the demands of the alloy, the object maybe attained by taking two, three, or more equivalents of the metal, excep- tion being made in the cases of arsenic and such elements. * Metallic Alloys, p. 87. CHAPTER VI. LEAD. Plumbum. Symbol, Pb. Valence, II, IV. Specific gravity, 11.25 to 11.36. Atomic weight, 206.47. Malleability, 7th rank. Melting point, 325° (617° F.). Tenacity, lowest (8th) rank. Ductility, 8th rank. Chief ore, galenite. Conductivity (heat), 8.5. Conductivity (electricity), 8.32. (Silver being 100.) Specific heat, 0.0314. Crystals, octahedral. Color, bluish-white. OCCURRENCE.— This abundant and very useful metal is almost wholl}' obtained from its native sulphide, (1) Galenite (PbS) or galena, and is rarely, if ever, found free. Its other more widely distributed ores are (2) Ceru- site (PbC0 3 ), lead carbonate, sometimes called white-lead ore, and (3) Crocoisile (PbCr0 4 ), lead chromate. There is also a (4) Wulfenite (Mo0 4 Pb) and a (5) Sulphate (PbS0 4 ). Galenite often carries silver, as AgS, in suffi- cient quantities to be well worth extracting, the propor- tion of the noble metal varying from about 0.01 to 0.03 per cent., and in rare cases amounting to 0.5 or 1 per cent. Such ore is called Argentiferous Galena. Lead ore frequently occurs accompanied by copper, iron pyrites, and zinc-blende. Galenite is found in the United States, Great Britain, Spain, and Saxony. REDUCTION OF GALENITE is effected in a rever- beratory furnace, into which the crushed lead ore is in- troduced and roasted for some time at a dull-red heat. In the roasting a portion of the lead sulphide is oxidized to the oxide and sulphate — PbS + 30=PbO+S0 2 and PbS+0 4 =PbS0 4 . LEAD. 99 The contents of the furnace are then thoroughly mixed and the temperature raised, whereupon the sulphate and oxide react with the remaining sulphide, forming sul- phurous oxide and metallic lead — 2PbO+PbS=SO,+ 3Pb and PbS0 4 + PbS=2S0 2 + 2Pb. Contaminating metals, which render the lead hard, are removed by melting and partially oxidizing in a rever- beratory furnace with a cast-iron bottom. EXPERIMENT No. 9.— (a)— Heat galenite with small pieces of iron or in an iron ladle. Result — metallic lead. PbS+Fe=Pb-fFeS. (b) Heat galenite on charcoal with sodium carbonate. Result— metallic lead. PROPERTIES.— Pure lead is a feebly lustrous, bluish- white metal, endowed with a high degree of softness and plasticity and almost entirely devoid of elasticity. A wire 1-10 of an inch in thickness is ruptured by a charge of about thirty pounds. It is said to be the least tena- cious of all metals in common use. Its specific gravity, as determined by Deville, is, for that "very slowly frozen," 11.254, and that " quickly frozen in cold water," 11.363. It melts at 325°C* or 617° F. At a bright-red heat it vaporizes, and at a white heat boils. Its specific heat is .0314f , that of water at 0° C. being taken as unity. Lead exposed to ordinary air is rapidly tarnished, forming a suboxide, as is thought; but this thin film once formed is very slow in increasing. The same sup- posed suboxide is formed upon lead kept in a state of fusion in the presence of air, when at the same time the metal rapidly absorbs oxygen; then the monoxide (PbO) is formed, the rate of oxidation increasing with the tem- perature. By slowly cooling, lead may be obtained in * Rudberg. + Regnault. 100 PRACTICAL DKNTAL METALLURGY. octahedral crystals. Dilute acids, with the exception of nitric, act but slowly on lead. DENTAL APPLICATIONS.— Its chief dental use is in the laboratory as a counter-die. It may be rolled into a thin foil, and at one time was used for filling carious teeth, and in conical points is now used in filling the apices of pulp-canals. It is an important component of soft solders and various alloys.* COMPOUNDS WITH OXYGEN.— There are four compounds of lead and oxygen: The Diplumbic Oxide, or Lead Suboxide, Pb 2 0, a gray pulverulent substance, is formed when the monoxide is heated to dull redness in a retort, and is supposed to cor- respond with the dull coating formed on bright, freshly cut surfaces of lead when left exposed to the air. EXPERIMENT No. 10.— Melt old, partially oxidized lead in a ladle under powdered charcoal — metallic lead. The Monoxide, Litharge or Massicot, PbO, is very heavy, and of a delicate straw-yellow color, slightly soluble in water, melting at a red heat, with a tendency to crystallize on cooling, and is easily reduced when heated with organic substances of any kind containing carbon or hydrogen. It is the product of the direct oxi- dation of the metal, but is more conveniently prepared by heating the carbonate to dull redness. PbCO s ( + heat) — Pb0 + C0 2 . EXPERIMENT No. 1 1.— Heat the monoxide in reducing flame on char- coal — metallic lead. v The Dioxide, Puce or Brown Lead Oxide, Pb0 2 , is a heavy brown powder, insoluble in water, having an acid reaction, and may be regarded as the anhydride of plum- bic acid, H 4 Pb0 4 . It is easily obtained by digesting the red oxide in nitric acid. * See Literature — Lead. LEAD. 101 Red Oxide, or Red Lead ', is a compound of the mon- and dioxides, not very constant in its composition, but is generally regarded as having the formula 2PbO,Pb0 2 (Pb 3 4 ). It is a heavy, bright-red powder, and may be regarded as lead plumbate, Pb 2 Pb0 4 . It is used as a cheap substitute for vermilion. When treated with dilute nitric acid the monoxide dissolves, forming soluble lead nitrate, leaving the puce-colored oxide behind. It is prepared by exposing the monoxide, which has not been fused, for a long time to the air at a very faint red heat. EXPERIMENT No. 12.— To a small amount of red lead placed in a test- tube add a small quantity of dilute HNO... The 2PbO is dissolved and the PbO, is left. EXPERIMENT No. 13.— Heat litharge to red heat in the presence of air — red lead. (Note. — The product soon loses its additional oxygen when heated but for a short time, returning to the yellow oxide.) ACTION OF ACIDS ON LEAD.— The presence of carbonic acid in a water does not affect its action on lead. Aqueous non-oxidizing acids generally have little or no action on lead in the absence of air. Sulphuric Acid, when dilute (20 per cent, solution or less), has no action on lead, even when air is present, nor on boiling. Stronger acid doss act, slowly in general, but appreciably, the more so the greater its concentra- tion and the higher its temperature. Pure lead is more readily acted upon than that contaminated with antimony or copper. Boiling concentrated sulphuric acid converts lead into the sulphate, with evolution of sulphurous oxide. EXPERIMENT No. \4.— To small pieces of lead foil in a test-tube add concentrated sulphuric acid and boil. Pb-f2H.,SO +PbSO + + SO.,-(-2H,0. Nitric Acid. — The metal is readily dissolved in dilute nitric acid, nitrogen dioxide being evolved and plumbic nitrate formed. 102 PRACTICAL DENTAL METALLURGY. EXPERIMENT No. 15.— To several pieces of lead foil in a test-tube add dilute nitric acid and warm to hasten action. 3Pb-f8HNO. s +3Pb (N0. J )., + 2N0-f4H,0. (Preserve) Hydrochloric Acid. — Strong and hot hydrochloric acts but slowly upon lead, forming the dichloride and liberating hydrogen. EXPERIMENT No. 16.— To small pieces of lead foil in a test-tube add strong HC1 and boil. Pb+2HCl + PbCL+2H. ACTION OF AQUEOUS REAGENTS ON LEAD. — Water, when pure, has no action on lead per se. In the presence of free oxygen (air), however, the lead is quickly attacked, forming a hydrated oxide, Pb2HO= PbOH 2 0, which is appreciably soluble in water, render- ing the liquid alkaline. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbon- ate — PbC0 3 (which is slightly soluble in water containing carbon dioxide) — so there is room made, so to say, for fresh hydrated oxide, and the corrosion of lead pro- gresses. Now, all soluble lead compounds are strongly cumulative poisons; hence the danger involved in using lead pipes or cisterns in the distribution of PURE waters. We emphasize the word " pure," because experience shows that the presence in water of even small propor- tions of bicarbonate or sulphate of lime prevents its action on lead. This little sulphate, almost invariably present, causing the deposition of a very thin but closely adherent film of lead sulphate upon the surface of the metal, which protects it from further action. ALLOYS. — Pure lead unites with almost all metals. Mercury readily amalgamates with it, and, in proper proportions, crystallizes, forming a very white but brittle alloy. This union is said to be of a definite chemical proportion, and is expressed as Pb 2 Hg. Very small LEAD. 103 quantities of lead admixed with the noble metals destroy completely their malleability, and hence renders them unworkable. It is said that l-1920th part of lead in gold will greatly impair its coining property, and that gold containing l-500th part of lead is "rendered unfit for coinage. " The gold drawer in the dental laboratory is often so situated that it is almost impossible to pre- vent particles of lead from accumulating with the gold scraps and filings. These, however, may be easily re- moved by roasting with potassium nitrate and sulphur. * Silver in certain proportions with lead forms an alloy which has a lower fusing point than that of lead. Pat- tinson, taking advantage of this fact, invented his proc- ess for recovering the silver from argentiferous galena. A quantity of the silver-lead ore is melted in one of a series of iron pots. After complete fusion it is allowed to slowly cool, when the poorer lead crystallizes and is ladled off to another pot, leaving the rich silver-bearing lead behind. This is carried on through the whole series of some twelve pots, until the lead-silver alloy has been reduced to proportions by which the noble metal may be recovered by the process of cupellation.f Platinum with equal weight of lead gives a purplish- white, brittle, and granular alloy. So great is the affinity these metals have for each other that lead oxide heated in a platinum crucible with reducing flux is broken up and the lead combines with the platinum vessel. I^ead can only be separated from platinum by the humid process of refining platinum. Palladium and lead form a green alloy which is very hard and brittle. The more common alloys of lead are those with tin, antimony, etc. * See chapter on Gold, t See chapter on Silver. 104 PRACTICAL DENTAL METALLURGY. Tin unites with lead in almost any proportion with slight expansion.* The following table gives an idea of the melting points of alloys of lead and tin : An Alloy of— Fuses at— Lead 1, Tin 2 340° F. " 1, " (i 382° F. " 2, " 1 442° F. " 4, " 1 498° F. " 17, " 1 557° F. With tin 1 part and lead 5 partsf Dr. Haskell makes counter-dies to be used with his Babbitt-metal dies. It fuses at a lower temperature than the die alloy, and also has the advantage of being harder than lead, which he claims is too soft for counter-dies. Tin-lead alloys are used largely in soldering. The following are compositions and melting points of frequently used compounds^: Grade. Tin. I^ead. Melts at — Fine Solder... 2 1 340° F. Common " .... 1 1 370° F. . Coarse " . . . . 1 2 442° F. Pewter may be said to be substantially an alloy of the same two metals; but small, quantities of copper, anti- mony, and zinc are frequently added. Common pewter contains about 5 parts of tin for 1 of lead. In France a tin-lead alloy, containing not over 18 per cent, of lead, is recognized by law as being fit for measures for wine or vinegar. " Best pewter" is simply tin alloyed with a mere trifle (}4 per cent, or less) of copper. Antimony. — L,ead contaminatedwith small proportions of antimony is more highly proof against vitriol than the * Kuppfer. | The author has found the fusing point of this alloy to be S7b r F. X Tomlinson. LEAD. 105 pure metal. An alloy of 83 parts of lead and 17 parts of antimony is used as type metal; other proportions are used, however, and other metals added besides antimony (e. g., tin, bismuth) to give the alloy certain properties. Arsenic renders lead harder. An alloy made by the addition of about } y 56 of arsenic is used for making shot. Lead forms a very important part in " fusible alloys."* TESTS FOR LEAD IN SOLUTION.— In testing various solutions, first pour some of that which you have reason to suspect in a test-tube, to the height of an inch or so, and add a few drops of the selected reagent. Sometimes the precipitate is soluble in an excess of this reagent, and sometimes in excess of either solution or reagent. If there be reason to suspect either, proceed cautiously, adding but a drop at a time, until a sufficient precipitate has been thrown down. If the first few drops of the reagent added cause a precipitate which is imme- diately redissolved, it shows that it is soluble in an excess of the solution, and if it be also soluble in an excess of the reagent, an equilibrium must be attained. After the precipitate has thoroughly settled note its color and general appearance; then decant the supernatant liquid as thoroughly and carefully as possible, and divide the precipitate in as many other test-tubes as may be desired for testing its solubility in the various reagents. Sulphuretted hydrogen is one of the most important reagents used in tests for salts of metals in solution. To the suspected solution add, drop by drop, the saturated solution of sulphuretted hydrogen (U 2 S); a black pre- cipitate is quickly formed, which is insoluble in an excess of the reagent. To the suspected solution add a few drops of ammonium hydrosulphide, (H 4 N)HS; a black precipi- tate, insoluble in an excess of the reagent, is formed. * See chapter on Bismuth. 106 PRACTICAL DENTAL METALLURGY. Potassium hydrate or ammonia throws down a white precipitate — hydrated oxide. This is soluble in an ex- cess of the potassa, but not of the ammonia. Alkaline carbonates precipitate the white plumbic carbonate, which is quickly blackened by sulphuretted hydrogen. Sulphuric acid is a characteristic test, precipitating a white sulphate. Hydrochloric acid or a chloride gives a white pre- cipitate soluble in an excess of potassa. EXPERIMENT No. 1 7.— Test a lead-salt solution as above. BLOW-PIPE ANALYSIS.— A lead-salt is easily re- duced on a piece of charcoal before the blow-pipe, a bead of lead ultimately resulting in the center of the point of fusion, around which the charcoal will be seen to have absorbed a portion of the yellow monoxide of lead. The bead may be readily recognized as metallic lead, which is soft and may be readily flattened or cut with a knife. "If the lead contains silver, the latter is easily detected by the use of bone-ash. Fill a bowl-shaped cavity in the charcoal with finely powdered bone-ash, pressed down well, so as to fill the cavity with a compact mass, smooth, and slightly hollowed on the surface. In this, place a small quantity of the lead, hold the charcoal horizontally, and direct the extreme point of the outer (oxidizing) flame upon the metal. The bone-ash will absorb the lead oxide formed, leaving a metallic globule of silver. The latter may be covered with a thin film of oxide, showing rainbow tints. When the color ceases, and the globule no longer diminishes in size, it is pure silver. The process is hindered by the presence of tin."* On charcoal in either flame lead is reduced to a malle- able metal, and yields near the assay a dark lernon- *Dr. Clifford Mitchell. LEAD. 107 yellow coat, sulphur-yellow when cold, and bluish-white at border. With bismuth flux: On plaster, a chrome-yellow coat, blackened by ammonium sulphate. Interfering Elements. — Antimony. — Treat on coal with boracic acid, and treat the resulting slag on plaster with borax flux. Arsenic Sulphide. — Remove by gentle O. F. Cadmium. — Remove by R. F. Bismuth. — Usually the bismuth flux test on plaster is sufficient. In addition the lead coat should color the R. F. blue. ELECTRO-DEPOSITION OF LEAD.— In a solu- tion of hyponitrite, nitrate or acetate of lead, zinc re- ceives a coating, or its place may be taken entirely by the lead. EXPERIMENT No. 18.— Dissolve one dram of the nitrate or acetate of lead in about two pints of distilled water and put the solution in a bottle. Suspend a piece of granulated zinc or a spiral of zinc wire in the center of the solution and let it stand. The lead will be deposited slowly in a crystalline form, known as arbor phimbi. At the same time the zinc will pass into solu- tion, the lead simply replacing the zinc. After the tree has been formed filter off some of the solution and see whether or not zinc is contained in it. There will probably be some lead left. In order to detect the zinc the lead w T ill have to be removed. This may be done by adding sulphuric acid (form- ing the sulphate) and alcohol (to prevent its being redissolved). Filter off the lead sulphate, and to the filtrate add just enough ammonia to neutralize the sulphuric acid, and then test with ammonium hydrosulphide; white zinc sulphide is precipitated. CHAPTER VII. ANTIMONY. Stibium. Symbol, Sb. Valence, III, V. Specific gravity, 6.715. Atomic weight, 119.95 Malleability, brittle. Melting point, 425° (797°F.). Tenacity, brittle. Ductility, brittle. Chief ore, stibnite. Specific heat, 0.050S. Crystal, rhombohedral. Color, bluish-white. OCCURRENCE —Antimony is found in the metallic state to a small extent in many of the localities from which its ores are derived. It occurs alloyed with other metals, such as silver, nickel, copper, and iron, and usually contaminated with arsenic. Commercial antimony is obtained almost entirely from its chief ore stibnite, the sulphide, Sb 2 S 3 , which is found in great abundance in Borneo, New Brunswick, and Nevada. This ore usually occurs in veins, and has a leaden-gray color, with a metallic, sometimes iridescent, luster. REDUCTION.— The metal is easily reduced by heat- ing the ore in a furnace with about half its weight in scraps of metallic iron, whereupon it gives up its sulphur, which unites with the iron, forming ferrous sulphide, and liberates antimony. Sb 2 S 3 + 3Fe=Sb 2 + 3FeS. The metal is so frequently contaminated with arsenic that it cannot be safely used for dental purposes until it has gone through a refining process. Chemically pure antimony may be best obtained as follows : — Four parts of metallic antimony are powdered with two parts of sodium carbonate and five parts sodium nitrate; the whole is then heated. The arsenic, if any ANTIMONY. 109 be present, and antimony are converted to the oxides at the expense of the oxygen of the nitrate, and then sodium arsenate and antimonate are formed with the sodium of the carbonate. Upon cooling, these com- pounds are powdered and thrown into boiling water; the soluble arsenate is dissolved, while the insoluble anti- monate remains. The latter after having been thoroughly washed with hot water is dried and heated with half its weight of potassium bitartrate. The product of this fusion is then broken up and cast into water, when the potassium of the tartar oxidizes, liberating hydrogen and leaving the antimony as a powder contaminated with any iron or lead that may have been contained in the original metal. These latter are gotten rid of by heating about one-third of the powder with nitric acid, oxidizing it; this portion is then dried, mixed with the remainder and fused in a covered crucible; pure antimony separates and subsides under a slag composed of these foreign oxides. PROPERTIES.— Pure antimony is a brilliant, some- what iridescent, bluish-white metal, readily crystallizing in rhombohedrons, which form large stellate figures on the fused surface when cooled. It fuses at 425° C. (797°F.)» and may be distilled at a white heat in an atmosphere of hydrogen. When heated to redness it takes fire, burning with a brilliant white flame. It undergoes no change in air at ordinary temperatures, but is easily oxidized when heated to fusion. It is an important metal in the manu- facture of alloys, increasing their hardness even when mixed in very small quantities. The finely powdered metal takes fire spontaneously when thrown into chlorine gas, forming chlorides. COMPOUNDS WITH OXYGEN.— Antimony forms two distinct oxides : 110 PRACTICAL DENTAL METALLURGY. The Trioxide, or Antimonous Oxide ', Sb 2 3 , occurs native, though rarely. It may be prepared by burning metallic antimony at the bottom of a large red-hot cruci- ble. It is a pale buff-colored powder, fusible, volatile, of basic reaction, and absorbing oxygen at a high heat is changed into the tetroxide, or Sb 2 3 ,Sb 2 O s . When boiled with potassium bitartrate it is dissolved, and the solution yields on evaporation crystals of tartar emetic^ KSbOC 4 H 4 6 . The Pentoxide, or Antimonic Oxide, Sb 2 5 , is obtained by the action of strong nitric acid on antimony. It is a pale straw-colored powder, of acid reaction, insoluble in water or acids, decomposes on being heated, passing to the intermediate oxide, Sb 2 3 ,Sb 2 O s . The Intermediate Oxide, or Tetroxide, Sb 2 4 , as it is sometimes called, Sb 2 3 , Sb 2 O s , is obtained by heating the pentoxide in the air, and is recognized as an infusible, non-volatile and insoluble, grayish-white powder. ACTION OF ACIDS ON ANTIMONY.— Hydro- chloric acid, boiling and concentrated, slowly dissolves powdered antimony, forming antimonous chloride and liberating hydrogen- — Sb+3HCl=SbCl 3 + 3H, but when the metal is in the compact state it resists this acid. Sulphuric acid, boiling and concentrated, slowly con- verts it into antimonous sulphate with an evolution of sulphur dioxide — 2Sb+ 6H 2 S0 4 =Sb 2 3S0 4 + 6H a O + 3S0 2 , Nitric acid rapidly oxidizes the metal, the dilute acid forming chiefly antimonous oxide — 2Sb+ 2HN0 3 =Sb 2 3 -f H 2 + 2NO, while the concentrated form yields some antimonic oxide — 6Sb+ 10HNO 3 =3Sb 2 O 5 + 5H 2 0+ 10NO. ANTIMONY. Ill For the most part the intermediate is the result of this action — 6Sb+8HN0 3 =3Sb 2 4 + 4H 2 + 8NO. Nitro-hydrochloric acid converts it into soluble antimonous chloride and insoluble oxides. Tartaric acid in a boiling solution slowly dissolves precipitated antimony— 2Sb+H 2 (C 4 H 4 6 ) + 2H 2 0=(SbO) 2 C 4 H 4 6 +6H. Alkalis do not dissolve it. ALLOYS. — The metal is chiefly valuable for the alloys it yields with other metals, and, as has been said, pos- sesses the quality of increasing the hardness of those alloys. Antimony also causes expansion in most alloys, thereby copying fine lines and sharp casts; hence, its great value in the manufacture of type. In many cases it renders the alloy very brittle, and is especially injuri- ous to the noble metals or copper, destroying their mal- leability, ductility, etc. Mercury. — The amalgam of antimony is soft and easily decomposed. Experiments have been made, with a view to using this element in dental-amalgam alloys, but to no profit. Gold. — One grain of antimony to 2000 will greatly injure the malleability of gold. Copper containing l-1000th of this metal can no longer be worked for sheet-brass. Tin. — Antimony is added to tin alloys to give hard- ness and expansion, but renders most of them very brittle. Bismuth forms with antimony a grayish, brittle, and lamellar alloy. In order to remove the brittleness vary- ing quantities of tin are added, as is also lead, and both. The fusibility then rather increases, instead of decreasing. 112 PRACTICAL DKNTAL METALLURGY. Some alloys containing antimony: Cliche metal, tin 48, lead 32.5, bismuth 9, and antimony 10.5. Babbitt metal, copper 4, tin 12, and antimony 8, melted separately. The antimony is added to the tin, then the copper, and 12 parts more tin after fusion. TVPE METAL— TABLE OF COMPOSITION.* Metal Parts. i. ii. III. IV. v. Antimony 3 1 10 2 70 18 2 6 4 100 30 8 1 2 Zinc 90 Tin 10 20 8 Britannia Metal (Wagner's). — Tin 85.64, antimony 9.66, copper 0.81, zinc 3.06, and bismuth 0.83. Queen's Metal. — Tin 88.5, antimony 7.1, copper 3.5, and zinc 0.9. TESTS FOR ANTIMONY IN SOLUTION.— Sulphuretted Hydrogen added to an acidulated solu- tion of antimony occasions an immediate precipitate of very characteristic orange-red color. Potassa, or ammonia, or their carbonates, throw down a bulky white hydrate, of which that formed by potassa is soluble in excess of alkali, but the hydrate formed by the ammonia or alkaline carbonates is nearly insoluble. If a hydrochloric acid solution be treated with a quantity of water, an immediate precipitate of oxychloride falls, soluble in tartaric acid distinguishing it from bismuth. 3SbCl 3 + 4H 2 0=Sb 3 4 Cl+ 8HC1. * Table from Brannt. - ANTIMONY. 113 ELECTRO-DEPOSITION OF ANTIMONY.— Antimony may be electro-deposited by simple immer- sion by placing a piece of zinc in contact with a piece of antimony in a solution of the chloride of antimony, or a piece of platinum may receive a coating of antimony on being immersed in a solution of the chloride in contact with a piece of tin. It may also be deposited from an acid solution of the chloride by the separate current process, producing ex- plosive antimony. CHAPTER VIII. TIN. Stannum. Symbol, Sn. Valence II, IV. Specific gravity, 7.29. Atomic weight, 117.69. Malleability, 4th rank. Melting point, 228° (442° F.). Tenacity, 7th rank. Ductility, 7th rank. Chief ore, tinstone. Conductivity (heat) 14.5. Conductivity (electricity), 12.36. (Silver being 100.) Specific heat, 0.0562. Crystals, isometric and quadratic. Color, brilliant white. OCCURRENCE. — Tin occurs chiefly as ti?istone, cassit- erite, or native oxide, Sn0 2 , which forms in very hard quadratic crystals, usually discolored by the presence of ferric or manganic oxide. The pure ore is colorless and very scarce. Another native form known as " wood tin " occurs in roundish masses, with a fibrous, radiating fracture. The metal is rarely, if ever, found free. The ore is mined from veins or layers within the older crys- talline rocks and slates, associated with copper ore, iron arsenide and other minerals, and as alluvial deposits, mixed with rounded pebbles, in the beds of streams. The former is called mine-tin, and the latter stream-tin. REDUCTION.— The ore is first washed to separate it from earthy impurities, and then stamped, and again washed to separate the lighter gangue. It is then roasted at a low heat to volatilize the arsenic and sulphur, with- out at the same time fusing the ore. The copper ore, copper pyrites, is, during this time, joined with subse- quent exposure to air and moisture, changed to copper sulphate, and is then dissolved out by water, the copper afterwards being reduced by iron and thereby saved. The ore is finally washed to separate all lighter oxides, TIN. 115 and is then ready for smelting. The purified ore, known as "black tin," is mixed with about 15 to 20 per cent, of anthracite "smalls," the mixture moistened to pre- vent its being blown off by the draft, then fused in a reverberatory furnace for five or six hours, and, after thorough stirring, the metal is run off — Sn0 2 + 2C=Sn+2CO. The tin obtained from Malacca is almost chemically pure, while that from England almost invariably contains traces of arsenic and copper. Most of the tin consumed in this country is shipped from Singapore, having been mined from the Malacca regions. Two varieties of the commercial metal are known, called grain and bar-tin- The first is the better; it is prepared from the stream ore. EXPERIMENT No. 19. — Roast tin oxide in crucible with charcoal or heat on charcoal block with sodium carbonate in reducing flame— globule of tin. Pure Tin. — Tin used in dental operations should be chemically pure. Much of that which we have just de- scribed is still greatly contaminated with arsenic, copper, iron, etc., and to obtain it pure a further refining process must be gone through with. For this purpose good commercial tin may be dissolved in hydrochloric acid. Hydrogen is evolved, and the metals are all converted into chlorides, with the exception of antimony and arsenic. If either of these be present it will combine with hydrogen, forming a gas and be evolved. The liquid is now evaporated to a small bulk, and to it is added nitric acid, which will convert the tin into the insoluble, white, crystalline, metastannic acid, H IO Sn 5 O l5 . The whole is now evaporated to dryness, washed with water acidulated with hydrochloric acid, filtered, re- washed, dried, and melted in a crucible with charcoal, when a button of pure tin will result. 116 PRACTICAL DENTAL METALLURGY. EXPERIMENT No. 20.— Dissolve commercial tin in hydrochloric acid , evaporate and add nitric acid. Kvaporate to dryness and add water acid- ulated with hydrochloric acid, wash, place on filter and rewash, melt in crucible with charcoal, and obtain pure tin button. PROPERTIES.— Pure tin is white (except for a slight tinge of blue); it exhibits considerable luster, and is not subject to tarnishing on exposure to normal air. It is soft and exceedingly malleable; indeed, it is said it may be beaten into foil 1-40 of a mm. in thickness; at 100° C. it may be drawn into wire, but is almost devoid of tenacity. That it is elastic, within narrow limits, is proven by its clear ring when struck with a hard body under circumstances permitting free vibration. Though it is seemingly amorphous, it has a crystalline structure con- sisting of an aggregate of quadratic octahedra, hence the crackling noise known as the "tin cry" which a bar of tin emits on being bent. This structure can be rendered visible by superficial etching with dilute acids. The crystalline structure must also account for the strange fact that an ingot, when exposed to the temperature of — 39° C for a sufficient length of time, becomes so brittle that it falls into powder under pestle or hammer, and, indeed, sometimes crumbles into powder spontaneously. At some temperature near its fusing point it again becomes brittle. Tin fuses at 228° (442.4° F.).* At a red heat it begins to volatilize slowly; at 1600° to 1808° C. it boilsf and may be distilled. The hot vapor produced combines with the oxygen of the air, forming the white oxide, Sn0 2 . The specific gravity of the cast metal is 7.29 to 7.299; of that which has been crystallized by the galvanic current from solutions 7. 178. J Its specific heat is 0.0562. *Rudberg. f Williams. JW.H. Miller. TIN. 117 DENTAL APPLICATIONS.— Tin-foil is very highly recommended as a filling-material for carious teeth, on account of the ease with which it may be inserted; its sup- posed therapeutic effect (which is doubtful), and because of its comparatively low conducting power of heat, tin, as compared to silver, which is taken as the unit, being 14.5, while gold rises to 53.2, and, absolutely pure gold much higher. In the conduction of electricity the com- parison is still greater: tin 12.36 and gold 77.96.* The combination of tin and gold foil is said to have a very low conducting power of heat. It is claimed that dis- integration of the tooth structure by galvanic action at the margins of the cavity so filled is rendered impossible, from the fact that any such action is confined to the metals which form a closed circuit, tin-foil being more electropositive to gold than tooth structure. It is also claimed that consolidation of the two metals occurs sub- sequent to their insertion as a filling. Of this Dr. W. D. Miller, Berlin, says: "Without entering into a pro- longed discussion of the causes of this consolidation, I will say that it is owing to electro-chemical process, through which the tin is dissolved and redeposited upon the surface of the gold. By this means the material becomes rigid and all parts of the filling thoroughly bound together. * * * It has neither therapeutic nor antiseptic action. "f (See Addendum.) Models of tin are used to vulcanize upon, and plaster models are often covered with tin-foil to give a clear and finished appearance to the denture after the process of vulcanization. COMPOUNDS WITH OXYGEN.— There are two oxides of tin : * Figures from Matthiessen. f Cosmos, Vol. XXXII, p. 714. 118 PRACTICAL DENTAL METALLURGY. Tin Monoxide, or Stamious Oxide, SnO, is a black- ish-brown powder of feeble basic reaction prepared by- heating stannous hydrate, Sn2HO, in an atmosphere of carbon dioxide. It is unstable, and burns when heated in the air, becoming stannic oxide. Tin Dioxide, or Stannic Oxide, Sn0 2 , occurs native as tinstone, or cassiterite, the common ore of tin, and is easily formed by heating tin, stannous oxide, or stannous hydrate, in contact with air. According to the manner in which it may be prepared, it is either a white or yel- lowish-white amorphous powder, or it may be obtained crystalline. It is infusible and insoluble in the acids or alkalis, and is known as " polishing putty," being used for polishing glass, hard metals, granite, and similar substances. It forms two isomeric hydroxides (stannic and metastannic), which differ somewhat in their prop- erties; both, however, are acids, and capable of ex- changing their hydrogen for metal, thereby forming salts: Stannic Acid, H 2 Sn0 3 , is precipitated by an alkali from stannic chloride as a white powder, soluble in the stronger acids and alkalis, and is capable of exchanging the whole of its hydrogen for metal-forming stannates, as: Na 2 Sn0 3 . These salts are quite stable. Metastannic Acid, H IO Sn 5 O l5 , may be written H 2 Sn0 3 , is prepared as a white crystalline powder by the action of dilute nitric acid upon tin. It is insoluble in water and the acids, but dissolves slowly in the stronger alkalis, and has the property of exchanging only one-fifth of its hydrogen for metal-forming metastannates, very- unstable, as K 2 H 8 Sn 5 O l5 . ACTION OF ACIDS ON TIN.— The three mineral acids act upon tin. Sulphuric acid, concentrated, acts rather energetic- ally at first, but, owing to the stannic sulphate coating TIN. 119 which is soon formed, its action is greatly hindered. The dilute form acts slowly, but converts the whole of the tin into stannic sulphate with a liberation of hy- drogen. Sn + H 2 S0 4 =SnS0 4 -f-2H. EXPERIMENT No. 21.— Place small pieces of tin-foil in test-tube, and add dilute sulphuric acid— stannic sulphate. Nitric Acid. — In its concentrated form this acid acts but feebly upon tin, and, if heated, produces the nitrate; but the dilute is energetic, and, instead of dissolving, oxidizes it into the crystalline powder, hydroxide, known as metastannic acid — H IO Sn 5 O l5 . 3Sn + 4HN0 3 =3Sn0 2 + 2H 2 + 4NO, and then 5Sn0 2 + 5H 2 0=H IO Sn 5 O l5 . EXPERIMENT No. 22.— To small pieces of tin in test-tube add dilute nitric acid — metastannic acid. Hydrochloric Acid. — Strong, warm hydrochloric acid acts energetically upon tin, the cold and dilute forms acting more slowly, but converting it into stannous chloride and liberating hydrogen. Sn + 2HCl=SnCl 2 + 2H. EXPERIMENT No. 23.— Dissolve tin in hot, strong, hydrochloric acid adding more tin than will be dissolved. Filter and preserve. Nitro-hydrochloric acid dissolves tin very energetic- ally, producing stannic chloride, SnCl 4 . Sn+4C1 (nascent chlorine)=SnCl 4 . EXPERIMENT No. 24.— To small pieces of tin-foil in a test-tube add a small amount of nitro-hydrochloric acid — stannic chloride is formed. Filter and preserve. Caustic Alkalis. — Boiling solutions of caustic soda or potassa act upon tin, producing stannates with an evo- lution of hydrogen. Sn+2KHO + H 2 0=K 2 Sn0 3 + 2H 2 . 120 PRACTICAL DKNTAL METALLURGY. ALLOYS. — Mercury and tin readily unite as an amal- gam, under ordinary circumstances, and, it is said, form a definite chemical compound having the formula, Sn 2 Hg. Tin is a very important component of dental-amalgam alloys.* Of it Dr. J. Foster Flagg says, in his work on Plastics and Plastic Filling : "All such alloys as I should favorably regard, have from 35 to 48 per cent, of tin; it is found that by the addition of copper and gold, both antagonists of "shrinkage," the most deleterious of the effects of tin can be counterbalanced; that under this control sufficient silver can be used to obviate a detri- mental loss of edge-strength; that the retardation of 11 setting" is prevented, and that the tin not only loses its power for harm, but becomes an ingredient of mani- fold utility; it greatly augments the facility of amal- gamation; it aids in producing a good color and in preventing discoloration; and it diminishes conduc- tivity." The amalgam of tin is also largely used in the manu- facture of mirrors. Gold and tin form a malleable alloy, provided the tin be pure and does not exceed in quantity 10 per cent. Platinum and tin in equal proportions form a hard, but brittle, alloy, fusing at a comparatively low tempera- ture. Palladium, says Mr. Makins, forms a very brittle alloy with tin. In view of the fact that gold, platinum, and palladium so readily unite with tin to form alloys whose fusing points are so comparatively low, and in view of the behavior of tin with other metals, and of metals in general toward each other, there is little reason to doubt a chemical affinity of tin * See chapter on Amalgams. TIN. 121 for these metals. The affinity of tin for gold in particu- lar has been clearly demonstrated by Dr. Matthiessen. Into a crucible of molten tin a rod of gold and one of copper were dipped, the latter having been previously tinned to ensure perfect contact. The gold united readily and rapidly with the tin, while the copper rod remained unaffected. A gold wire which has been super- ficially tinned will melt like one of tin when held in the flame of a Bunsen burner. A wire of tinned copper exposed to the same heat, under like circumstances, remains unaffected, except that the tin is burned off. The affinity of tin for platinum is so great, states Clarke, that if tin and platinum foils be rolled together and heated before the blow-pipe combination takes place explosively. The affinity of tin for gold is unquestionably an interest- ing subject for the dentist, in view of the place these two metals occupy in operative dentistry. Silver alloys with tin, and, in the proportion of 80 of the former to 20 of the latter, it is said produces a very tough alloy. Dr. G. F. Rees's alloy for artificial dentures, con- structed by the cheoplastic process, is composed of tin 20, gold 1, and silver 2 parts. * Other alloys much used in cheoplastic work are composed largely of tin. Bean's alloy, intended for casting inferior dentures, is composed of tin 95, and silver 5 parts. Antimony 1 and tin 16 parts forms another alloy, which is intended for the same purpose, and was intro- duced by the late Dr. William B. Kingsbury. Brittannia metal is made under a great variety of for- mulae; one known as English is composed of antimony 7.8, tin 90.7, and copper 1.5. It sometimes contains lead or bismuth. ♦"Amalgams and Alloys Chemically Considered," by J. Morgan Howe, M. D., Transactions New York Odontological Society, 1880. 122 PRACTICAL DENTAL METALLURGY. Type metal, generally speaking, consists of lead, antimony and tin — lead 55, antimony 30, and tin 15 parts. Dr. L. P. Haskell's Babbitt metal for dies is com- posed of the following: "Copper 1 part, antimony 2 parts, tin 8 parts. These should be melted in the order named, as tin would oxidize badly before the first was melted, if all were placed in the crucible together. Melt, and turn off into ingots, and remelt. If it should not be found to run freely from the ladle, when making a die, add a small amount of tin, as it is presumable that some of that metal has oxidized."* Babbitt metal is made under a great variety of formulae; but one in the same proportions as the above (tin 12 paits, antimony 3 and copper 2 parts) is given by Dr. Kssig, which, he states, is sometimes used in the dental laboratory for dies, and is thought by many to be superior to zinc for that purpose, f Copper and tin form a large number of alloys of great importance. Bronze. — Copper and tin unite in almost any propor- tion to form bronze. Copper. Tin. Phosphorus. Zinc. U. S. Ordnance Bronze. 90. 90.34 84.42 10. 8.90 4.30 Statuary Bronze 0.76 11.28 Actual speculum-metal is supposed to have the formula, Cu 4 Sn, and the centesimal composition of copper 66.6 parts and tin 33.4 parts. Bell metal is copper 72 to 85 parts, and tin 15 to 26 parts. * Dr. L. P. Haskell, f Dental Metallurgy. TIN. 123 With iron, in the process of tin-plate manufacture, tin is said to alloy. Lead and tin alloy freely in all proportions, tin gener- ally imparting greater resistance to the lead. Such alloys constitute certain forms of pewter, an important class called "soft solders," and counter-dies.* TESTS FOR TIN IN SOLUTION.— To the sus- pected solution add a few drops of caustic potash or soda. A white precipitate is thrown down, soluble in excess of the reagent. Ammonia also gives a white precipitate /^soluble in excess of the reagent. Ammonium or hydrogen-sulphide throws down a brown, in the case of stannous, and yellowish-brown pre- cipitate with stannic chloride, both of which are soluble in excess of the reagent. Gold trichloride added to a dilute solution of the tin chlorides gives the characteristic purple precipitate, known as the purple of Cassius.f EXPERIMENT No. 25.— Test a tin-salt in solution as above. BLOW-PIPE ANALYSIS. — On charcoal, with sodium carbonate, tin is reduced to malleable lustrous globules. Under the O. F. these become incrusted with white stannic oxide. With Cobalt Solution. — Moisten the coal in front of the globules with the solution, and blow a strong R. F. upon the whole. The white oxide coat will become bluish-green when cold. In Borax Bead. — A faint blue bead is made reddish- brown or ruby-red by heating a moment with a tin com- pound in R. F. * See Lead Alloys, t See Gold. 124 PRACTICAL DENTAL METALLURGY. INTERFERING ELEMENTS.— Alloys of Lead or Bismuth. It is a fair proof of tin if such alloy oxidizes rapidly with sprouting and cannot be kept fused. Zinc also interferes with the above tests. ELECTRO-DEPOSITION OF TIN.— Tin is easily deposited upon small articles of brass or copper by simple immersion, as by the following experiment: EXPERIMENT No. 26.— Place the articles in layers between sheets of grain tin in a saturated solution of potassium bitartrate and boil. A little stannous chloride may also be added, if necessary. The metal may be crystallized out of its solution and rendered pure by the following: EXPERIMENT No. 27.— Immerse a bar of tin in a strong solution of stannous chloride and pour on carefully, so as not to disturb the tin solution, some distillled water. Pure tin will be deposited on the bar of tin at the point of junction of the water and tin solution. CHAPTER IX. BISMUTH. Bismuthum. Symbol, Bi. Valence, III, V. Specific gravity, 9.82. Atomic weight, 207.52. Malleability, brittle. Melting point, 264° (507° F.). Tenacity, brittle. Ductility, brittle. Chief ore, native metal. Conductivity (heat), 1.8. Conductivity (electricity), 1.24. (Silver being 100.) Specific heat, 0.0308. Crystals, rhombohedral. Color, white with reddish tint. OCCURRENCE.— Practically the only ore of this ele- ment is the Native Metal found disseminated in veins through slate rock associated with the ores of copper, iron, cobalt, nickel, silver, gold, and arsenic. It also occurs as Bismuthine, or bismuth glance, a sulphide, Bi 2 S 3 , and the ore called Bismuth ochre, the trioxide, Bi 2 O s . It is a comparatively rare metal inasmuch as the supply has not kept pace with the demand, and its com- mercial value has risen considerably. It is found chiefly in Saxony, Transylvania, United States, England, Peru, Norway, and Sweden. REDUCTION.— This is simple and may be accom- plished by a process of sweating. The crushed ore is introduced into large iron tubes or tubular retorts, built in the furnace. These tubes are placed in an inclined position over a wood fire. At the upper end the ore is introduced, and as the metal is sweated out it accumu- lates at the lower end, where it is drawn off into iron vessels. The siliceous residue is then raked out of the tube at its upper end and the retort recharged. Commercial bismuth frequently contains arsenic and iron, besides gold and silver; hence is not fit for dental 126 PRACTICAL DENTAL METALLURGY. usage until it is purified. When silver exists in bismuth in sufficient quantity to repay for extracting, it is cu- pelled just as lead is, during which the bismuth is oxi- dized, leaving the silver as a molten button on the cupel. The oxide of bismuth is afterwards recovered by strongly heating under powdered charcoal. At the same time the arsenic is gotten rid of, in that it volatilizes. The bis- muth is protected from oxidation by the covering of charcoal. The metal is also frequently fused with potas- sium nitrate, which removes the arsenic and iron by oxidizing them. To produce the pure metal, however, it is best to employ some humid method of refining, as: "EXPERIMENT No. 28.— Place a small amount of Bi. in a test-tube and add to it HNO s ; after the action of the acid has ceased, filter the solution into a beaker filled with distilled water. The bismuth will be precipitated as the subnitrate — BiONO s ; filter and wash first with a solution of caustic pot- ash, then distilled water, dry and heat in a crucible with about one-half its bulk of powdered charcoal — pure bismuth. PROPERTIES.— Bismuth is a highly crystalline, hard, and very brittle metal, having a grayish-white color, with a decided reddish tint. Its specific gravity is 9.823 at 12° C* and fuses at 264° (507.2° F.). It expands about 1-32 of its volume upon cooling, and imparts this property to its alloys. It crystallizes in large, beautiful, iridescent rhombohedra, which near^ approach a cube, their angles being nearly 90°, (87°40'). These crystals may be obtained by melting a quantity of the metal and allowing the bulk to cool slowly, the surface being pre- vented from more rapid solidification by covering the pot with a sand bath filled with glowing coals. As soon as a crust has formed on the sides and top, it is pierced with a hot iron, and the still molten metal poured out. When quite cold the upper surface is sawed off, exposing the beautiful crystals in the interior. The metal volatilizes * Holzmann. BISMUTH. 127 at a high temperature, and has a specific heat of 0.0308. It is the most diamagnetic of all substances. Exposed to the air at ordinary temperatures, it is unaffected, but when heated to a red heat it rapidly oxidizes, forming a play of beautiful colors. COMPOUNDS WITH OXYGEN.— Bismuth com- bines with oxygen to form two oxides: Bismuthous Oxide, the Trioxide^ Bi 2 3 , is found na- tive as bismuth ochre, and may be prepared by roasting the metal in air or by gently igniting its nitrate. It is a straw-yellow powder, insoluble in water; is fusible at a high temperature, and in that state acts toward siliceous matter as a flux. Bismuthic Oxide, the Pe?itoxide, Bi 2 O s , may be ob- tained by dissolving the trioxide in caustic potash and passing chlorine through the liquid; the water decom- poses, forming hydrochloric acid, and the trioxide is con- verted into the pentoxide. It is then washed with dilute nitric acid to separate any remaining trioxide. The pentoxide is a reddish-brown powder, which is insoluble in water. ACTION OF ACIDS ON BISMUTH.— Sulphuric acid when cold has but slight action on bismuth, but dissolves it more readily when heated, forming the sul- phate, and giving off sulphurous anhydride. Hydrochloric acid, hot or cold, but feebly attacks bismuth. Nitric acid dissolves bismuth very energetically, giv- ing off red fumes copiously and forming the nitrate or ternitrate, as it is generally termed, Bi3N0 3 , which is a white crystalline, soluble mass. If the ternitrate be added to a large quantity of water, a white precipitate is thrown down known as the sub- nitrate of bismuth, BiON0 3 , which is much used in 128 PRACTICAL DENTAL METALLURGY. medicine and as pearl white or bla?ic de fard in cosmetics. It is a heavy white powder, insoluble in water or alcohol. It is darkened by sulphuretted hydrogen. ALLOYS. — Bismuth unites readily with other metals, the alloys being remarkable for their ready fusibility, and by their property of expanding on solidification. These two properties render it most valuable as an ingredient to alloys used for making casts or dies where it is essential to copy fine lines, and in alloys when a very low fusing point is desirable. With copper it forms a pale-red, brittle alloy. With zinc it easily unites, producing an alloy ll more brittle, presenting a larger crystallization with less adherence than zinc or bismuth taken singly."* It is, however, says Dr. Kirk, sometimes employed in the dental laboratory for the formation of dies, such an alloy having a lower fusing point than pure zinc, and being free from contraction on cooling. With tin bismuth alloys in any proportion. A very small quantity of the metal imparts to tin more hard- ness, sonorousness, luster, and a fusibility lower than either of the metals taken separately possesses. An alloy of equal parts of the two metals fuses at 212° C. With lead bismuth alloys very easily, producing an alloy which is malleable if the proportion of bis- muth does not exceed that of lead. The specific gravity is greater than the mean of the two taken sepa- rately. Its alloys are white, lustrous, harder than lead, and more malleablef up to a certain proportion. Bis- muth 1 and lead 2 gives a very ductile and malleable alloy fusing at 330° F. * William T. Brannt. t Ibid. BISMUTH. 129 With antimony it produces a grayish, brittle, lamellar alloy. Lead and tin added renders it malleable, but its fusibility is increased rather than decreased. Such alloys are very frequent and much used in the prepara- tion of Britannia and Queen's metal. Bi. Sb. Sn. Pb. Cliche-metal 9. 8. 1. 10.5 Type-metal 1. 3. 48. 4. 32.5 1 1 5. < < 8. Alloys of bismuth, tin, and lead are known as the triple alloys, and are very numerous and useful. Newton's alloy consists of bismuth 8, lead 5, and tin 3 parts, and fuses at 202° F. Rose's fusible alloy is composed of i. ii. Bismuth 2 8 parts. Tin 13" Lead 1 8 " The first fuses at 200.75° F. and the second at 174.2° F.* They were used as safety-plates and inserted in the top of steam boilers, intended to prevent the explosion of boilers by allowing the steam to escape at a certain tension. Wood's metal consists of lead 4, tin 2, bismuth 5 to 8 and cadmium 1 to 2, melts at 140° to 161.5° F., in color resembles platinum, and is, to a certain extent, malleable. f Onion's fusible alloy contains lead 3, tin 2, and bis- muth 5 parts, and melts at 197° F. "La Nation" describes a new fusible alloy, of which the following is the formula: * William T. Brannt. f Ibid. 130 PRACTICAL DENTAL METALLURGY. Bismuth 48, cadmium 13, lead 19, and tin 26. It melts at 158° and resists great pressure. Hodgen's fusible alloy, for making dies and counter- dies by the dipping process, is composed of the following: Bismuth 8, lead 5, tin 3, and antimony 2. It is a light, lustrous alloy, very hard, slightly malleable, expands slightly on cooling, copying the finest of lines, takes a high polish and resists great pressure, melting at 224° F. Dr. Mathews Fusible Alloy. — This alloy is com- posed of bismuth 48, cadmium 13, and tin 19 parts. It melts below the boiling point of water and may be packed with the fingers. It may be poured into plaster impressions immediately after they have been taken, producing sharp, bright, hard dies, with which shot may be used for the counter- die. Darcet's fusible alloys are a series of proportions of bismuth, tin, and lead, and their melting point varies as per the following table : Parts. Melts. Bismuth. Tin. L,ead. 7 16 8 4 7 2 2 4 6 212°F. 212 C F. 205° F. Most of these fusible alloys are of much value in the dental laboratory in the hands of a practical, quick-witted man. The cleaner ones may, when lack of time will not permit of a more perfect repair, be used to mend a dentuie or replace a tooth or block of teeth on a vulcanite plate, and the more fusible ones may be used for the same pur- pose, even though the base be celluloid. In replacing teeth undercuts may be made with a file, or preferably BISMUTH. 131 with a large bur in the engine, the tooth placed in posi- tion and the alloy packed in with warm instruments, smoothed and afterwards polished. These alloys are also valuable baths for tempering steel instruments. They give a very exact temperature, which may be adjusted to the purpose intended. They are used, according to Thurston,* by placing the articles on the surface of the unmelted alloy and gradually heating until fusion occurs and they fall below the surface, at which moment their temperature is right ; they are quickly removed and cooled in water. f "An alloy of 3 parts each bismuth, fine gold, and platinum with 15 of fine silver, and 10 of tin, is very similar to precipitated palladium, and has been used as a substitute for this costly metal. One curious point about this alloy is, asserts Mr. Fletcher, that if it contains the merest trace of palladium it is almost worthless; and as ordinary fine silver is rarely, if ever, free from palladium, this alloy can only be made from silver reduced direct from the chloride." In amalgams. — ' ' The addition of bismuth to amalgams makes them excessively sticky and adhesive, necessitat- ing, at the same time, an increase in the proportion of mercury required. "J The same author, continuing, says: " Amalgams con- taining a trace of bismuth will build and adhere to a flat dry surface, and may be used as a metallic cement for joints in apparatus which require to be perfectly air tight and to stand heavy pressures. A good alloy for this pur- pose is 1 bismuth, 15 tin, 15 silver, fused and filed up, and then mixed in the proportion of 1 alloy to 4 of mer- * Brasses, Bronzes, and other Alloys, p. 196. t Metallic Alloys, Brannt. t Dental Metallurgy, Thomas Fletcher, p. 65. 132 PRACTICAL DENTAL METALLURGY. eury. This alloy is so excessively sticky as to be useless for fillings."* Commenting on the above, Dr. Kirk says:f " The effect of bismuth in dental-amalgam alloys does not seem to have been fully studied." And, further, "it would seem that the power of bismuth to overcome the contraction of alloys in solidifying would render it valuable as an in- gredient in certain dental-amalgam alloys if it conferred no objectionable qualities other than adhesiveness upon them." TESTS FOR BISMUTH IN SOLUTION.— Makins states: " The salts of this metal are for the most part devoid of color; some are soluble, others insoluble, the soluble salts redden litmus paper." Sulphuretted hydrogen or ammonium sulphide when added to a solution produce a black precipitate — sulphide of bismuth — insoluble in dilute acids or alkalis, but dissolves in strong hot nitric acid. The alkalis precipitate from bismuth solutions — in the absence of certain organic substances — the white bismuth hydrate, Bi3HO, insoluble in excess of the reagents, con- verted by boiling to the yellowish-white oxide, Bi 2 O s . The carbonates — as K 2 C0 3 — precipitate the white basic bismuth carbonate, Bi 2 2 C0 3 , insoluble in excess of the reagents. Water precipitates from acidulated bismuth solutions white basic salts, which contain less of their acid radicals in proportion as greater quantities of water are added, as — 1st. Bi3N0 3 + 2H 2 0=BiON0 3 . H 2 + 2HN0 3 . 2d. 4Bi3N0 3 + 6H a O=Bi 4 O s 2N0 3 . H 2 0+ 10HNO 3 . BLOW-PIPE AN ALYSIS.— On charcoal with sodium carbonate, before the blow-pipe, bismuth is easily reduced * Dental Metallurgy, Thomas Fletcher, p. 65. t American System of Dentistry, Vol. Ill, p. 931. BISMUTH. 133 from all of its compounds. The globule is easily fusible, brittle (which fact distinguishes it from lead), and is gradually oxidizable under the flame, forming an incrus- tation — Bi 2 3 — orange yellow while hot, and pale yellow when cold. In borax bead it gives a faint yellowish color when hot, and is colorless when cold. ELECTRO-DEPOSITION OF BISMUTH.— This metal may be deposited by simple immersion or by means of an extremely feeble current; in the former case zinc, tin, lead, and iron deposit bismuth upon themselves. CHAPTER X. ZINC. Zincum. Symbol, Zn. Valence II. Specific gravity, 6.915. Atomic weight, 64.9. Malleability, 8th rank. Melting point, 415° (779° F.). Tenacity, 6th rank. Ductility, 6th rank. Chief ore, Calamine. Specific heat, 0.0956. Crystals, rhombohedral. Color, bluish-white. OCCURRENCE.— Zinc is a somewhat abundant metal, but never occurring in the native state. It is found as a carbonate, sulphide, silicate, etc, associated with lead ores in many districts; large supplies are obtained from Silesia and from the neighborhood of Aachen. The native carbonate, (1) Calamine, Smithsonite, ZnC0 3 , is the most important of its ores. Asa rule this ore is a light-gray, yellow, or buff in color, having a specific gravity of 4 to 4.5. (2) Zinc-Blend, the sulphide, ZnS, is second in importance only to the above; it is ex- tensively mined, and much of the zinc of commerce is procured from this ore. Its color is green, yellow, or red, but mostly brown, having a specific gravity of 3.9 to 4.2. There is also a (3) Red Zinc Ore, an impure oxide, ZnO; an (4) Electric Calamine, one of the silicates, ZnO.Si0 2 + H 2 and (5) Willemite, an anhydrous sili- cate, ZnO.Si0 2 . REDUCTION.— -Calamine, ZnC0 3 ,is generally reduced by first roasting, to expel the water and carbon dioxide. This leaves the oxide, ZnO, which is then mixed with fragments of coke or charcoal, and distilled at a full red heat in a large earthen retort. The carbon unites with the oxygen to form carbon monoxide and escapes, while the reduced metal volatilizes and is condensed by suitable zinc. 135 means, generally contaminated with minute quantities of arsenic. Zinc-blend is roasted to drive off the sulphur, but when it contains any foreign sulphide, as of lead, it is more difficult, and requires some twelve hours' roasting. EXPERIMENT No. 29.— Melt old, partially oxidized zinc in a crucible, and when molten, cover the surface with pulverized charcoal; heat at a strong temperature, and stir with a stick of wood. After a few minutes the zinc may be poured, and will be found to be quite free of the oxide. Thus old zinc may be refined or cleaned for use again in the laboratory. PROPERTIES.— Zinc is a bluish-white metal, which but slowly tarnishes in moist air, usually forming a super- ficial carbonate; it has a lamellar, crystalline structure, a specific gravity of 6.915, and is, under ordinary circum- stances, quite brittle, but when heated to 100° or 150° C. it may be rolled or hammered into thin sheets, or drawn into wire; and, what is very remarkable after such treat- ment, it retains its malleability when cold; the sheet zinc of commerce is thus made. If the temperature be carried to 205° C. it again becomes so brittle that it may be easily powdered in a mortar. Care should be exercised in handling hot zinc dies, for if by accident one be dropped upon a hard surface it is likely to be ruined. The metal melts at 415° C. or 779° F. It boils and volatilizes at 1040° C. or 1904° F., and, if air be admitted, burns with a splendid greenish incandescence, forming the oxide. In boiling water zinc is said to be attacked appreciably, but no more, forming the hydroxide, Zn2HO, with evolu- tion of hydrogen. EXPERIMENT No. 30.— Test zinc for malleability at ordinary temper- atures; when heated to from 100° to 150° C; and also at 205° C. IN THE ARTS. — Zinc meets with extensive applica- tion. It is much used for the positive element in galvanic batteries, and in the form of sheet zinc it is greatly employed in manufacturing industries. 136 PRACTICAL DENTAL METALLURGY. DENTAL APPLICATIONS.— Zinc has long been very extensively used in the dental laboratory for making dies. Its comparatively low fusibility, hardness, and other properties eminently fit it for this purpose. DIES. — " In passing from a low to a higher tempera- ture zinc increases in volume in a greater ratio than any of the metals in common use. The coefficient of its cubical expansion between zero and 100° C, which represents the rate of increase of its unit volume between these temperatures, has been found to be 0.000088251, or nearly three times that of cast iron. The rate of expansion of liquids being greater than that of solids, and as this rate is not constant, but increases with the temperature, the rate of increase in volume which zinc undergoes in passing from the solid to the fluid condition would be represented by a figure some- what higher than that given above. From the fact that metal plates for entire dentures which have been swaged upon dies made of zinc generally fail to fit the plaster model accurately, it is held by some practitioners that the high rate of expansibility of zinc is an undesirable feature; but as the absolute contraction in the size of a zinc die is but slight, and as the difference in the size of a plate made upon it and that of the mouth for which it is intended is to a certain extent reduced or counteracted by the expansion which the plaster model undergoes in setting, it is questionable whether the contraction which takes places in zinc on passing from the fluid to the solid condition is of any detriment. It is held by many, and for potent reasons, that in most cases the contraction which occurs in a zinc die is of positive benefit. A plate swaged upon a zinc die is, by reason of the contraction which the metal undergoes in passing from the fluid to the solid state, slightly smallet than the mouth it is intended to fit, thus bringing the greatest pressure to bear upon the alveolar ridge. Should the plate be made to fit upon the plaster cast, it would be a trifle larger than the mouth, as plaster expands in setting, and two expan- sions have taken place in taking the impression and making the cast. The pressure exerted by such a plate zinc. 137 would be expended upon the bony arch of the hard palate. Usually, the tissues covering the alveolar ridge are thicker, and therefore more yielding, than those covering the hard palate, and a plate swaged upon a zinc die would be of positive advantage, as the slight absorp- tion of the tissues covering the alveolar ridge which result from the increased pressure would soon bring about a perfectly uniform bearing over the entire area covered by the plate. But one class of cases arises, and their occurrence is infrequent, where the quality of ex- pansibility of zinc is detrimental to the fit of a plate when swaged upon it — namely, where the tissue covering the bony arch of the hard palate are thick and spongy, while the alveolar ridge is hard and covered by a thin un- yielding membrane. When this set of conditions pre- sents, it is usually in combination with a high V-shaped arch. In such cases a die of Babbitt metal gives better results, though even with a zinc die the difficult}' can be readily overcome and a proper adaptation secured by properly manipulating the plaster cast or impression, i. e., by scraping those portions of the cast which repre- sent the soft, yielding portions, or by treating the im- pression in like manner at those positions which represent the hard or unyielding parts of the ridge. "* Counter-Dies. — Zinc is frequently used for making the counter-die as well ; being hard and unyielding, copying the finest lines, it secures a perfect and ready adaptation of the metal to the die. In working platinum- gold or iridium-platinurn the lead die is entirely inade- quate to perfectly swage the metal to the die, especially where the palatine arch is very high or the rugae promi- nent, and it is then that a zinc counter-die is especially serviceable. It is also of great assistance in conforming plates to dies for partial dentures, as it more perfectly forces the metal snugly about the necks of the teeth than lead can be made to do. * Dr. E. C. Kirk, Am. System of Dentistry, Vol. Ill, p. 922. 138 PRACTICAL DENTAL METALLURGY. The zinc counter is formed similarly to the manner of making a lead counter, except that the die should be quite cool* — not cold — and thinly coated with a solution of whiting", which is allowed to dry, or with a deposit of carbon, obtained by smoking the die over a candle flame. In experienced hands the coating may be dispensed with and zinc heated just to complete fusion , and quickly poured in a?i uni?iterrupted stream upon the cool die. Dr. Essigf recommends, where swaging is likely to be attended with difficulty, "at least three sets of dies and counter-dies. " For the most imperfect of these he pours a lead counter-die and uses it for the preliminary swaging of the metal to the die. A partial lead counter is also exceedingly serviceable in the preliminary con- formation of the plate to the buccal and labial portions of the process. Such a counter is held in position by vari- ous means, the most practical of which, that has come to the writer's notice, is one used by Dr. Thomas N. Iglehart, consisting of a large steel screw-clamp, the bow of which may be represented by the letter U laid on one side, thus p. The end of the lower side termi- nates in a large circular section of iron, provided with a flat base, perforated for screwing into the bench. The upper arm carries a screw, similar to those used in letter presses, and when the die is placed on the heavy iron below the screw centers the partial counter from above, thus hold- ing the two in a perfect grip, permitting swaging with the hammer or tracer all around. Such a device is especially serviceable in turning the rim. A second die is furnished with a zinc counter, and when the plate is * If melted zinc is poured at 800° F. upon a zinc die at 70° F., the fused zinc by contact with the iron ring and by radiation will lose heat enough to cause its temperature to fall far below the fusing-point, and it will probably not impart to the die more than 400° F. — Essig. t Dental Metallurgy, p. 232. zinc. 139 so far conformed to the shape of the die as to preclude all probable wrinkling or folding, this counter is adjusted and the plate more perfectly driven to the die and the finer lines accurately copied. As the die is necessarily much marred by the unyielding quality of the counter, a third and very perfect die provided with a lead counter should be used to complete the swaging. The writer prefers to put the counter-die down finally under a steady pressure of from 1000 to 5000 pounds by means of a screw press. THE COMPOUND WITH OXYGEN.— Zinc oxide, ZnO, is the only known compound of this metal and oxygen. It is a strong base, forming salts isomorphous with those of magnesium. It may be prepared by the combustion of the metal, heating it to 1900° F., exposed to the action of the atmosphere. Soon alter melting it begins to be covered with a film of gray oxide. When the temperature nearly reaches redness it takes fire and burns with an intense white light, generating the oxide in the form of very light and white flocculi, resembling carded wool, which quickly fill the crucible, and are in part driven into the atmosphere by the current of air. It may also be prepared by heating the carbonate, ZnC0 3 , to redness, driving off the water and carbon dioxide, C0 2 . Too high a temperature will discolor the oxide a light yellow, and, partially vitrifying it, will give to it a harsh, gritty feel. A good quality should present a soft, white, flaky, impalpable powder, permanent in air, odorless and tasteless, insoluble in water or alcohol, but soluble in acids without effervescence. When strongly heated the oxide assumes a deep lemon color, but turns nearly white again on cooling. At a low white heat it fuses, and at a full whiteness sublimes. If it be contaminated with white lead or chalk, it will not be entirely soluble in 140 PRACTICAL DENTAL METALLURGY. dilute sulphuric acid, but an insoluble sulphate of lead or of lime will remain. If not properly calcined, and any carbonate remains, it may be detected by treating with hydrochloric acid, causing effervescence. In medi- cine it is used as a tonic, astringent, and applied ex- ternally as an ointment. As a cosmetic it has a great advantage over lead in not being poisonous. It is also used as a substitute for white lead in paints, and has the advantage of not being discolored by sulphuretted hydrogen. BASIC ZINC CEMENTS.— The basic zinc cements used in deutistry are the phosphate, the oxychloride, and the oxysulphate. The powder is prepared by calcining a quantity of the purest zinc oxide, luted in a sand or French clay crucible, for several hours, at a white heat. Every precaution should be taken to obtain pure oxide, and each specimen should be carefully tested before cal- cining. The commercial metallic zinc, of which most of the oxide is made, contains, among several other impur- ities, arsenic. So that arsenic compounds are apt to be contained in cheap qualities of oxide of zinc. Dr. Kirk recommends Hubbuck's English as a preparation most likely to be reliable. EXPERIMENT No. 31 A.— Into a test-tube half filled with water place eight or ten grains of pure zinc oxide and boil, add a few drops of hydrochloric acid, and then a small quantity of a solution of sulphuretted hydrogen. Note no precipitate appears. EXPERIMENT No. 31B.— Into a test-tube half filled with water place eight or ten grains of zinc oxide, to which has been added one grain of arsenious oxide, boil, add a few drops of hydrochloric acid, and then a small quantity of a solution of sulphuretted hydrogen. Note that the experiment is distinguished from No. 31A by the formation of a lemon-yellow precipitate, the sulphide of arsenic. This is a test for the presence of arsenic, and may be used in testing the oxide of zinc. ZINC. 141 After the oxide has been properly calcined it is found to be greatly contracted in mass, semi-vitrious, and light yellow in color. When cool it is removed from the crucible, broken up and ground to a fine powder between mill-stones. The powder is bolted through a fine bolting cloth, and then placed in tightly stopped bottles ready for use. The bottles are tightly corked, for if exposed to the air the oxide absorbs carbon dioxide and a portion of it is con- verted to carbonate of zinc. Other oxides are frequently mixed with the oxide of zinc, such as the oxides of aluminum, magnesium, and tin, with a view to improving its properties. The native oxide of titanium, powdered rutile, slate, etc., are frequently used to give it a variety of shades to meet the demand. Ground glass, silica, borax, etc., are sometimes added to improve its wearing qualities. Their value is questionable. The Liquid. — For the phosphate cements the liquid is usually made by dissolving glacial (metaphosphoric) acid, HP0 3 , in distilled water and evaporating to a syrupy consistence. The commercial acid contains such impurities as sodium and magnesium phosphates in vari- able amounts, and since these impurities, like the acid itself, are soluble in water, but form no chemical com- bination with zinc oxide, they remain in the cement to be dissolved out by the saliva. After standing they sometimes recrystallize, owing to a lack of water. When this tendency is noticed, a drop or more of water should be added. They also sometimes become turbid or cloudy after a few days or weeks stand- ing, showing deterioration. The liquid for the oxychloride is prepared by deliques- cing half an ounce of crystalline chloride of zinc with two or three drams of distilled water. Some heat is 142 PRACTICAL DENTAL METALLURGY. generated during the process; therefore, the bottle con- taining it should not be too tightly stopped. Any residue should be allowed several days to settle, when the clear supernatant liquid is decanted off for use. Mixing. — The prepared oxide of zinc is mixed on a slab with enough of the liquid to make a semi-thick, plastic, putty-like mass, when it is ready for introduction. The deliquesced chloride and prepared zinc oxide are mixed to form the oxychloride, but the paste should not be worked as stiff as the phosphate. It is best mixed to a thick, creamy consistence, and immediately introduced. EXPERIMENT No. 32.— Each student should prepare a small quantity of oxide by heating metallic zinc to about 1900° F. in an uncovered crucible. EXPERIMENT No. 33.— Calcine at a white heat for two hours a small quantity of oxide of zinc; powder in a mortar, and pass through a fine sieve. EXPERIMENT No. 34.— Prepare a small quantity of glacial phosphoric acid solution, as previously described. EXPERIMENT No. 35.— Prepare a small quantity of chloride of zinc solution, as previously described. Oxysulphate. — What is known as the oxysulphate of zinc to dentists, is merely a mixture of oxychloride of zinc and zinc sulphate. A true zinc oxysulphate is pre- pared by saturating a solution of zinc sulphate with zinc oxide. It forms a white paste, sets quickly, and attains about the same hardness as plaster of Paris. It is prin- cipally used as a capping for exposed pulps. It is bland and non-irritating, a non-conductor, and faintly and per- sistently astringent. ACTION OF ACIDS ON ZINC— Pure zinc dis- solves very slowly in acids (or alkalis), unless in contact with copper, platinum, or some less positive metal. Any metallic impurity in zinc renders it quite soluble in the acids or (alkalis). It is rapidly oxidized in water con- ZINC. 14o taitiing air, when in contact with iron, but the water does not dissolve it, unless aided by certain salts. All agents which dissolve the metal, also dissolve its oxide and hydroxide. In Sulphuric acid dilute, it dissolves slowly, forming zinc sulphate, and evolving hydrogen — Zn+H 2 S0 4 =ZnS0 4 +H 2 . In strong sulphuric a coating of zinc sulphate is quickly formed over the metal, retarding, if not altogether pre- venting, further solution. In Hydrochloric acid it is also slowly dissolved when pure, more rapidly when contaminated, forming the chloride of zinc and evolving hydrogen — Zn + 2HCl=ZnCl 2 + H 2 . The Chloride is a nearly white, translucent, fusible substance, very soluble in water and alcohol, and very deliquescent. It is used in dentistry when melted, or melted and diluted as liquid for oxy chloride cements; as an obtundent to sensitive dentine, an antiseptic, dis- infectant, etc. In Acetic acid zinc slowly dissolves, forming the acetate, and evolving hydrogen — Zn + 2HC 2 H 3 0=Zn (C 2 H 3 2 ) 2 + H 2 . In Nitric Acid. — In very dilute nitric acid it dis- solves without evolution of gas — 4Zn+ 10HNO 3 =4Zn(NO 3 ) 2 + H 4 NN0 3 + 3U 2 0, forming the nitrates of ammonium and zinc. In moderately dilute cold nitric acid, it dissolves with evolution of nitrous oxide — 4Zn+10HNO 3 =4Zn(NO 3 ) 2 + N 2 O + 5H 2 O. In a less dilution it dissolves with evolution of nitric oxide — 3Zn + 8HN0 3 =3Zn(N0 3 ) a + 2NO + 4H 2 0. 144 PRACTICAL DENTAL METALLURGY. In concentrated nitric acid zinc is but slightly soluble. IN ALKALIS. — In Potassium Hydrate, and in all the caustic alkalis, zinc slowly dissolves, evolving hydrogen — Zn+2KHO=K 2 OZnO+H 2 . EXPERIMENT No. 36.— In each of five test-tubes place a small piece of zinc, and add respectively sulphuric (dilute), hydrochloric, acetic, and nitric (dilute) acids, and a strong solution of potassium hydrate, and note the action. ALLOYS. — Mercury and zinc amalgamate quite readily to form a definite compound, having the formula Zn 2 Hg.* With Gold, zinc readily unites. The malleability, brilliancy, and color of gold is impaired by a content of zinc. Platinum. — Small pieces of platinum may be dissolved in molten zinc, and the union is attended with consid- erable energy, owing to the formation of a definite chemi- cal compound. The alloy is hard and brittle. An alloy may be prepared of platinum, 16; copper, 7; and zinc, 1; which very much resembled gold in color, specific gravity, and ductility. Silver and zinc have a great affinity for each other. This fact, with the knowledge that zinc and lead are so comparatively incompatible, led to the process of desil- vering lead by the assistance of zinc. The alloy of silver and zinc is best obtained by throwing the required quan- tity of zinc wrapped in paper into, molten silver, stirring thoroughly with an iron rod, and pouring the fused mass at once. The alloy of two parts zinc and one part silver is flexible, ductile, and has nearly the color of pure silver. Larger proportions of zinc produce brittle alloys. * See chapter on Amalgams. zinc. 145 Copper and Zinc Alloys. — (See chapter on Copper.) Iron and zinc unite to form a very interesting as well as somewhat useful and brittle alloy. On account of its brilliant light it may become of considerable value in pyrotechnics. It is best prepared by heating zinc in a crucible and adding anhydrous sodium ferrous chloride upon the surface of molten zinc, immediately covering the crucible. An energetic reaction takes place during the union. Iron plate and ware when perfectly cleaned may be immersed in molten zinc and the surface alloyed slightly, forming what is known as "galvanized iron," the name being derived from the circumstance that the coating is analogous to that produced by electrical means. Zinc alloys with the iron melting-pots of the laboratory; the admixture rendering the zinc less fluid when molten and more difficult to fuse. This contamination may be pre- vented by coating the pot with whiting. With Lead zinc does not alloy, except to a very slight degree. ' ' Matthiessen found* that on melting equal parts of zinc and lead, and, after well mixing, allowing the alloy to cool slowly, they separate, but the heavier lead on subsiding retains 1.6 per cent, of the zinc alloyed with it; while on the other hand the upper layer of zinc thrown out retains 1.2 per cent, of lead." It often occurs that lead and zinc will become mixed in the laboratory, and is seldom discovered until the molten mixture is poured. Then the lead, owing to its greater specific gravity, falls to the bottom of the mold, forming the alveolar ridge of the die, rendering it worth- less. Many times the counter-die is poured before the mistake is noticed, resulting in a union of the die and counter-die. *Makins' Metallurgy, p. 62. 146 PRACTICAL DENTAL METALLURGY. Tin and zinc alloy in almost any proportion. Mr. Fletcher* recommends an alloy of zinc 2 parts and tin 1 part for making dies for swaging, claiming the impres- sion from the sand is much finer, and the shrinkage on cooling is greatly reduced. It melts much lower than zinc alone, hence some care must be exercised in pouring the counter-die. The die should be perfectly cold and the lead should be just hot enough to pour, but not sufficiently heated to char a slip of paper. EXPERIMENT No. 37.— Form an amalgam of zinc. EXPERIMENT No. 38.— Melt 1 ounce of zinc with 1 ounce of lead; cast in a long ingot and notice separation. EXPERIMENT No. 39.— Make sufficient alloy of zinc 2 or 3 parts and tin 1 part to form a small die, and mold. EXPERIMENT No. 40.— Form an alloy of zinc and copper (brass) in any proportion. (Best to melt in separate crucibles and pour together while molten.) TEST FOR ZINC IN SOLUTION.— The Caustic Alkalis all precipitate the white hydroxide of zinc, Zn2H0, soluble in excess of either precipitant with the formation of sodium zinc oxide, etc. ZnCl 2 +2NaHO=Zn2HO+2NaCl. Zn2H04-2NaHO=Na 2 OZnO+2H 2 0. Ammonium sulphide completely precipitates zinc as a sulphide. Alkaline carbonates precipitate it as basic carbonate soluble in ammonia. EXPERIMENT No. 41.— Add to a solution of any of the zinc salts (ZnS0 4 ) in a test-tube a few drops of— a. Caustic potassa: white precipitate, soluble in excess; b. Caustic soda: " " " " " c. Ammonia: " " " " " d. Ammonium sulphide: " " (if pure) insoluble in excess. * Dental Metallurgy, p. 69. zinc. 147 BLOW-PIPE ANALYSIS.— On Charcoal with Sodium Carbonate before the blow-pipe compounds of zinc are reduced to metallic zinc. The metal on charcoal is easily oxidized: film yellow when hot and white when cold. With Cobalt Solution. — Moisten the coal in front of the lead with a drop or so of solution of cobalt nitrate and blow a strong reducing flame upon the partially oxidized bead. The coal will be a bright yellow-green when cold. INTERFERING ELEMENTS.— If the zinc is not pure, the interfering elements are a?iiimony, cadmium, lead, bismuth, or tin. Cadmium, lead, or bismuth will not prevent the cobalt solution test, however. EXPERIMENT No. 43.— Heat a small piece of zinc on charcoal with the O. F., and notice the yellow oxide in front of the assay, which turns white when cold. EXPERIMENT No. 43.— Moisten the coal in front of the assay with cobalt solution, and heat in R. F., and the oxide will have a bright yellow- green color. ELECTRO-DEPOSITION OF ZINC— This metal is too electropositive to be readily set free by simple immersioo, except by means of metals more electroposi- tive than itself, such as magnesium. It is best deposited by means of a separate current, preferably from the sul- phate solution, using a large zinc anode, yielding a very good deposit from two small cells feebly charged. Iron was formerly so coated to protect it from rusting, arid called "galvanized iron." It is not so coated at present. CHAPTER XI. CADMIUM. Cadmium. Symbol, Cd. Valence, II. Specific gravity, 8.54. Atomic weight, 111.83. Malleability, 5th rank. Melting point, 320° (608° F. ). Ductility, 11th rank. Tenacity, 10th rank. Chief ore, Greenockite. Specific heat, 0.0567. Crystals, octahedral. Color, tin-white. OCCURRENCE.— This metal does not occur native. There is but one mineral known which could be called an ore of cadmium, and which contains it in any quan- tity; namely, the sulphide, CdS, greenockite, which is found near Bishopstown, Renfrewshire. This ore is crystalline, belonging to the hexagonal system, and is of an orange-yellow color. Cadmium is, however, often associated with zinc-blende, ZnS, and Calamine ZnC0 3 , and from these two ores it is principally obtained, vary- ing in amount from 1 to 5 per cent. The metal very much resembles zinc, especially in its chemical properties. REDUCTION.— A humid method (Stromeyer's) is to dissolve the zinc ore containing cadmium in dilute sulphuric acid and precipitate the metal as the orange- yellow sulphide by means of sulphuretted hydrogen. The sulphide is then dissolved in hydrochloric acid, the excess of the solvent evaporated, and the cadmium thrown down as the carbonate by adding ammonium carbonate. By heating this to redness the carbon dioxide is driven off, leaving the oxide, which is mixed with carbon and distilled from an earthen retort. PROPERTIES.— Cadmium is a tin-white, lustrous metal, tough, very volatile (next to mercury), and some- what harder than tin, which it very much resembles CADMIUM. 149 physically. It fuses at 320° (608° F.)* and boils at 860° C.,f giving off a yellowish-brown colored vapor. Its specific heat is 0.05669J; electric conductivity some- what lower than that of zinc and its specific gravity 8.546 (ingot) and 8.667 (hammered). § It is malleable, ductile, and somewhat tenacious, breaking under an increasing strain, with fibrous scaly fracture; may be readily crys- tallized in regular octahedra; is unalterable in the air at ordinary temperatures, but when heated strongly in the presence of air, burns, emitting the yellowish brown fumes of cadmium oxide, CdO. COMPOUNDS WITH OXYGEN.— Cadmium forms a single oxide, CdO, a yellowish-brown powder, which is easily volatilized, or may be readily reduced with hydrogen or carbon, at a high temperature, but below that point necessary for the reduction or volatilization of zinc oxide. It is strongly basic and forms a series of salts similar in constitution to those formed by the oxide of zinc. It may be formed by burning the metal in the air, or by calcining the nitrate or carbonate, differing somewhat in shade according to the manner of prepara- tion. ACTION OF ACIDS ON CADMIUM.— In hot sulphuric or hydrochloric acid, moderately diluted, it is slowly dissolved, forming the salts CdS0 4 or CdCl 2 , and liberating hydrogen. In Nitric acid it is readily soluble, forming the nitrate, and generating nitrogen oxides. COMPOUNDS OF CADMIUM.— The most impor- tant of these is the sulphate, CdS0 4 , which is used in medicine as an astringent and stimulating remedy, * Rudberg. f Deville and Troost. X Regnault. \ Schroder. 150 PRACTICAL DENTAL METALLURGY. especially in diseases of the eye. The next of impor- tance is the sulphide, CdS, which occurs native as green- ockite, and is used as a superior yellow pigment by artists. The iodide, Cdl 2 , is used in photography. There is also a chloride, CdCl 2 . ALLOYS. — The metal is of little use except as a constituent of certain alloys, especially those fusing at a low temperature. With mercury cadmium combines readily to form a silver-white mass, which readily crystallizes, and under certain circumstances is said to be malleable. When in- troduced into a dental-amalgam alloy, it imparts the property of malleability. Such dental alloys, however, cannot be too strongly condemned. " In 1848, Dr. Thomas W. Evans of Paris introduced his amalgam, which was composed of pure tin, cad- mium, and mercury; but it was soon found that cad- mium was one of the very worst metals that could be used in a dental alloy, and its use was soon discon- tinued."* Of it Dr. J. Foster Flagg, Philadelphia, says: " The promises of this alloy were certainly alluring. It was easily amalgamated; the amalgam was readily inserted; it did not discolor; it 'set' with surprising celerity; it made a sufficiently resisting filling. What wonder, then, that the gentleman who introduced it was pleased with the material? * * * My satisfaction was, however, very short-lived, for only three or four months passed before sundry indications presented, which aroused my suspicions as to the uniform integrity and durability of the material — these were, an occasional, but evident crevicing at edges; a gradual softening and disintegra- * Relative Merits of Filling-materials, by E. T. Darby, M. D., D. D. S. Dental Cosmos, Vol. XXXVI, p. 178. CADMIUM. 151 tion of some fillings; and the yellowish discoloration sometimes apparent in adjoining tooth structure." He further states that in some cases the " dentine had be- come thoroughly decalcified, and stained to a bright orange-yellow color — sulphide of cadmium." He ex- plains that pulps under such fillings were " devitalized," except " where thick septa of dentine existed between the bottoms of the cavities of decay and the pulp cavities."* " Cadmium shares with bismuth the property of strongly reducing the melting points of alloys, there being some whose melting point is so low that they can be liquefied in hot water. But while bismuth alloys are nearly all brittle, many alloys of cadmium possess con- siderable ductility, and can be worked under the hammer as well as between rolls. They act, however, very differ- ently in this respect, there being alloys which are ductile, and others again, though containing besides cadmium the same metals, only in different proportions, which are very brittle, "f These alloys are usually made up of cadmium, tin, lead, bismuth, and sometimes mercury, the latter being added chiefly to lower the melting point still more. The follow- ing are a few cadmium alloys with their melting points. J Alloy. L,ipowitz's Alloy. . . Wood's Metal Other Alloys, No. 1 a 2 »< 1 ( << o Color, bluish-white. Crystals, octahedral. OCCURRENCE.— With the exception of silicon and oxygen, aluminum is the most abundant element in the earth's crust. It is found, combined with silicon and oxygen, as marl, clay, slate, pumice-stone, feldspar, mica, and nearly all rocks, with the exception of lime- stone and sandstone. As the crystallized oxide — alumina — it occurs as corundum, emery, ruby, sapphire, emerald, topaz, and amethyst, which are used as gems. The metal is further found in combination with nearly two hundred different minerals. REDUCTION.— It was first isolated in 1828, by Wohler, who obtained it as a gray powder by decompos- ing aluminum chloride with potassium. It remained a laboratory product until St. Claire Deville, about 1858, succeeded in improving the mode of production, so as to render the operations capable of management on a manu- facturing scale. The process consists in heating to a red heat the double chloride of aluminum and sodium with metallic sodium. A vigorous action takes place, chloride of sodium being formed and the metallic aluminum sep- arated — AlC1 3 .NaCl+3Na=Al+4NaCl. 186 PRACTICAL DENTAL METALLURGY. On a large scale the reduction is effected by throwing a mixture of 10 parts of the double chloride, 5 parts of the double fluoride (AlF 3 .3NaF, cryolite), and 2 parts sodium on the hearth of a reverberatory furnace. Im- mediately after the action, the fused metal and slag, con- sisting of common salt and fluoride of aluminum, are run out, and a new quantity of the previous mixture in- troduced. The various patents which have been secured in reference to this manufacture have all regard to the saving of the metal sodium. The metal is extensively reduced through the inven- tion of the Cowles' electrical furnace, which consists of a rectangular box of fire-brick lined with limed charcoal. The crushed ore is mixed with fine charcoal, and the iron cover of the furnace adjusted. A powerful current from a dynamo-electric machine is then passed into the furnace by means of two carbon electrodes. After about five hours the furnace is allowed to cool, and the metallic aluminum and slag removed. PROPERTIES.— Aluminum is a bluish-white metal, somewhat resembling silver in appearance. It is also said to be as malleable, of the same tenacity, and equal to. that metal in the conduction of heat and electricity. It is harder than tin, but softer than copper. By ham- mering in the cold it may be made as hard as soft iron, but is softened again by fusion. It is remarkably sonor- ous, and has been used for making bells. It is one of the lightest of metals, being approximately only 2}i times heavier than water, and 4 times lighter than silver. It fuses at 700° C, or about 1300° F.; does not oxidize in air, even at a red heat; has no action on water at ordi- nary temperatures, nor is it acted upon by the compounds of sulphur, thus preserving its luster where silver would be tarnished and blackened. It is without odor or taste. ALUMINUM. 187 IN THE ARTS aluminum is used in the manufacture of weights of small denomination, such as the milligram; its low specific gravity rendering it particularly well adapted to that use. It is further used, on account of its lightness and resistance to atmospheric action, for the manufacture of delicate physical, mathematical, and optical apparatus, as well as ornamental articles, such as medalions and badges of a souvenir character; also for parts of bicycles, tablewear, and cooking utensils. The apex of the Washington Monument is of highly polished aluminum. IN DENTISTRY this metal is employed as a base for artificial dentures. Its many valuable proper- ties, chiefly conductivity, lightness, malleability, cheap- ness, and unalterableness in dry or moist air, render it applicable for such a purpose. The base is swaged between a zinc die and lead counter-die, but will not stand the rough swaging sometimes given to gold or platinum. Caution must be used to prevent contamination with the lead or zinc. It being difficult to determine during the progress of the conformation whether or not any con- tamination has occurred; the pattern is best swaged be- tween thin tissue paper, removing the paper as it becomes broken. The metal should be occasionally annealed by coating with pure sweet-oil or tallow, and passing through the flame until the oil or fat is carbon- ized, when at the moment the last trace of black (carbon) disappears from the metal, it may be dropped into water. Before applying the oil and annealing, however, the metal should be thoroughly brushed with pumice-stone, to remove any contaminating lead or zinc which might otherwise become alloyed with the base, causing small holes or pits to appear on its surface, or perhaps the occurrence of galvanic action. 188 PRACTICAL DENTAL METALLURGY. Vulcanite is attached to such a base by spurs made with a sharp-pointed graver, counter-sunk holes, or loops made with a punch along the alveolar ridge. After waxing up the denture the base may be varnished to protect it from the plaster during vulcanization, after which the varnish is removed with alcohol, and the plate polished with pumice-stone and whiting, but it cannot be well burnished. The process of making cast aluminum dentures was first introduced by Dr. J. B. Bean of Baltimore, who cast the metal through tall conduits lined with clay and attached to the gates of his flask, the entire apparatus being first heated to an elevated temperature. The pres- sure of the column of metal thus produced overcame the sluggish flow due to an inherent lack of fluidity and lightness of the metal, and forced it into the finer parts and irregularities of the mold. The cast bases were finally abandoned, because of cor- rosion and decomposition. Dr. C. C. Carroll, following the efforts of Dr. Bean, has devised an apparatus by which very 6ne castings of aluminum may be secured through the agency of pneu- matic pressure. To control shrinkage he has alloyed the aluminum slightly so that it can be cast directly on the teeth. He gave the writer the following as the composition of his two bases: Base No. 1 for superior dentures, to be cast under pressure. Aluminum 98 per cent. Platinum ) Silver V 2 " " Copper J Specific gravity, 2.5; fusing point, 1300° F. ALUMINUM. 189 Base No. 2 is composed of aluminum, tin, copper, and silver; specific gravity, 7.5; fusing point, 700° F. This is intended for lower dentures, and is cast without pressure,* The alloy is melted in a specially constructed plumbago crucible, which has the general form of a thick-walled cylinder, closed at one end t which serves as a bottom. "A channel is formed within the wall of the crucible, one orifice of which terminates within the crucible at the side and close to the bottom. Starting from this orifice, the channel rises in the crucible wall to near the top, making a sharp return upon itself, and descends in a parallel course after the manner of a siphon, and makes its exit at the base and near the side of the crucible. Here it terminates in an iron nipple that fits into a cor- responding socket in the gateway of the molding-flask. A cylindrical plug of soapstone, which fits the open mouth of the crucible, is provided with a central tube of brass, to the free end of which is, connected by a short length of rubber tubing, a large rubber bulb. When the metal has been brought to a state of fusion and the cruci- ble connected by means of the iron nipple at its base with the gateway of the flask, which has been previously heated to near redness, the soapstone plug is inserted in the mouth of the crucible and the rubber bulb is steadily but forcibly compressed. The atmospheric pressure forces the fluid metal out through the siphon-like chan- nel and into the minutest lines of the mold, yielding a fine casting; but in this, as in Bean's process, the con- traction of the metal on cooling almost invariably causes fracture of the teeth, or the shrinkage will show itself in portions of the plate, causing objectionable, or at least unsightly, defects/' * Prof. C. Iy. Goddard. 190 PRACTICAL DENTAL METALLURGY. Carroll's improved crucible, made of iron and lined with asbestos-fiber, is somewhat funnel-shaped, and pro- vided with a screw-cut stem, which is pierced by a small hole. The flask-gate is also screw-cut to receive the stem of the crucible, thus making an air-tight joint. The flask and crucible thus attached are placed in a gas- furnace, so constructed that a greater heat is applied to the crucible than to the flask. When the aluminum is melted, the crucible cover, made of iron and lined with asbestos-fiber, is clamped on. The cover is also perfor- ated by a small hole passing through a nipple, or stem, projecting from its outer surface. A bulb is connected with this stem by a flexible rubber tube, and by its use the requisite amount of pneumatic pressure is secured to force the molten aluminum into the mold. Dr. Carroll also made a foil of aluminum, of which Dr. Dwindle* said: "It is easily worked, crimped, folded, twisted into coils, or shaped into pellets, and treated like other foil; that it is subject to varying tem- pers obtained by annealing, and has the advantage that it approaches the color of the teeth more nearly than any other metal." Bridges are cast similarly to the making of cast plates, and seamless crowns, as those of gold, are prepared by swaging. Thinly rolled aluminum makes a very ser- viceable matrix in filling. THE COMPOUND WITH OXYGEN.— Aluminum oxide, Alumina, the Sesquioxide of Aluminum, A1 2 3 , is found crystallized in hexagonal prisms in nature, as ruby, sapphire, corundum,f etc., colored by admixtures. * Dental Cosmos, Vol. XXXI, p. 655. f Chemical formulae of some of the oxides of aluminum: Corundum (Ruby and sapphire the same) Al 2 O s . Garnet (CaMgFeMn) 3 Al 2 Si 3 12 . Cyanite Al 2 Si0 5 . ALUMINUM. 191 It may be prepared by treating a solution of alum with an excess of ammonia, by which an extremely bulky, white, gelatinous precipitate of aluminum hydrate is formed. This is washed, dried, and ignited to whiteness. Thus obtained, alumina constitutes a white, tasteless feebly basic coherent mass, very little acted upon by acids, and fusible in the oxyhydrogen flame. Emery is impure corundum, containing iron and aluminum oxides, Feldspar is regarded as the double silicate of po- tassium and aluminum, and as having the formula A1 2 3 .K 2 0.6Si0 2 . It is much used in the preparation of bodies, frits, and enamels for the manufacture of porcelain teeth. Kaolin is known as a hydrated silicate of aluminum, (2A1 2 3 .3Si0 2 ) + 3H 2 0, and is the purest form of clay. It is much used in the preparation of bodies for the man- ufacture of porcelain teeth. ACTION OF ACIDS AND ALKALIS ON ALUM- INUM. — Sulphuric acid, concentrated and boiling, dis- solves aluminum, but it is not soluble in the dilute acid. Nitric acid does not affect aluminum. Hydrochloric acid, hot or cold, readily dissolves it, forming aluminum chloride, and evolving hydrogen — 2Al+6HCl=Al 2 Cl 6 -f-H 6 . In Potasshrm or sodium hydrate it is soluble, forming aluminates and liberating hydrogen — Al+3KHO=K 3 A10 3 +H 3 . ALLOYS. — Aluminum alloys with nearly all metals, except lead; indeed, the wonderful alloys it is capable of producing gives it, perhaps, its greatest value. Aluminum may be melted in a graphite crucible without flux, but great care must be taken not to heat it too hot. On account of its high specific and latent heat, alumi- 192 PRACTICAL DKNTAI, METAUJJRGY. num requires a long time to melt; but, unlike some other metals, it soon becomes fluid after the melting point is reached. With mercury alone aluminum is said to form an unsta- ble amalgam. A series of dental-amalgam alloys of alumi- num are prepared by Dr. Carroll of which it is claimed that: the amalgams made of them set quickly, do not shiink, and make a dense, fine-grained filling of white luster nearer the color of the teeth than any other mate- rial; that they do not tarnish nor change color from wear and have a strong, tough edge, that will not break by burnishing or mastication. The experiments made by Dr. Black recently showed aluminum in the proportion of 1 to 5 per cent, in silver-tin amalgam alloys to so increase the expansion in amalgams made of them as to exclude this metal as a component in dental-amalgam alloys. Gold and aluminum unite, forming a hard and brittle alloy. One per cent, of aluminum in gold destroys the ductility of the noble metal and gives it a greenish cast; 5 per cent, of aluminum with gold yields an alloy brittle as glass, and 10 per cent, of aluminum produces a white, crystalline, and brittle alloy. Nurnberg gold, an alloy, for cheap goldware, very much resembling gold, and unchanged in air, is com- posed of aluminum 7.5, gold 2.5, and copper 90 parts. Silver and aluminium readily unite, forming alloys of beautiful whiteness, and unchangeable on exposure to air. Their hardness is considerably greater than alumi- num, but they are more easily worked. An alloy of 100 parts of aluminum and 5 parts of silver differs but little from pure aluminum, save that it is considerably harder and takes a beautiful polish. An alloy of aluminum 169 parts and silver 5 parts possesses considerable elasticity, ALUMINUM. 193 and has been recommended for watch springs, dessert ' and fruit knives. Equal parts of the two metals pro- duce an alloy equal to that of bronze in hardness. Copper and aluminum form some exceedingly impor- tant alloys, differing according to the quantity of alumi- num they contain. Those of a small content of copper cannot be used industrial^. With 60 to 70 per cent, of aluminum they are very brittle, glass-hard, and beauti- fully crystalline. With 50 per cent, the alloy is quite soft ; but under 30 per cent, of aluminum the hardness returns. The usual alloys are 1, 2, 5, and 10 per cent, of aluminium. These are known as aluminum bronze. The 10 per cent, bronze is a bright golden, and keeps its color and polish in air; it may be easily engraved, shows a greater elasticity than steel, and can be easily soldered with 20-carat gold solder. When first made, it is brittle, acquiring its best qualities after three or four meltings, after which it may be melted several times without sensi- ble change. It casts well in sand molds, but shrinks greatly. It has a specific gravity of 7.68, about equal to soft iron. Its strength when hammered will equal that of the best steel. Annealing makes it soft and mal- leable. It does not clog a file, and may be drawn into wire. It melts at about 1700° F. Aluminum bronze as a base for artificial dentures: " In the proportion of aluminum 100 and copper 900 it oxi- dizes but superficially in the mouth, and is as strong and resistant to attrition as 18-carat gold; it may be swaged as easily as 20-carat gold, but it must be annealed fre- quently, and it is necessary to carry the heating almost to whiteness, for if the bronze be merely heated until it assumes a dark-red color it remains as hard as before." (Prof. Souer.) 194 PRACTICAL DENTAL METALLURGY. The alloys of copper and aluminum are prepared in the Cowles' electric furnace by fusing together the oxides of aluminum and metallic copper with enough carbon and flux to reduce them. The oxides and all to be as finely divided as possible. Solders. — The following alloys may be used as solders for articles of jewelry made of 10 per cent, aluminum bronze: HARD SOLDER. Gold 88.88 per cent. Silver 4.68 " Copper 6.44 " " MEDIUM HARD SOLDER- Gold 54.40 per cent. Silver 27.00 " " Copper 18.00 " " Mr. Wm. Frismuth of Philadelphia recommends the following solders for aluminum, with vaseline as the flux: SOFT SOLDER. Pure Block Tin from 90 to 99 parts. Bismuth < 10 " 1 HARD SOLDER. Pure Block Tin from 98 to 90 parts. Bismuth " 1 " 5 Aluminum " 1 " 5 " Schlosser recommends the following for dental labora- tory use: PLATINUM-ALUMINUM SOLDER. Gold 30 parts . Platinum 1 " " Silver 20 " Aluminum 100 ' ' GOLD-ALUMINUM SOLDER. Gold 50 parts . Silver 10 " Copper 10 " Aluminum 20 " ALUMINUM. 195 O. M. Thowless has patented the following solder for aluminum and method for applying it: Tin 55 parts . Zinc 23 " Silver . .. 5 " Aluminum 2 " First melt the silver and aluminum together then add the tin and zinc in the order named. The surfaces to be soldered are immersed in dilute caustic alkali or a cyanide solution, and then washed and dried. They are next heated over a spirit lamp, coated with the solder, and clamped together; small pieces of solder being placed at the points of union, the whole is then heated to the melting point. No flux is used. The following are use- ful as solders. i. ii. in. Zinc 80 parts 85 parts 92 parts Aluminum 20 " 15 " 8 " The flux used in soldering is composed of 3 parts balsam of copaiba, 1 part Venetian turpentine, and a few drops of lemon juice. The soldering iron is dipped into the mixture. So far, the soldering of aluminum in the dental laboratory is very difficult and unsatisfactory. Another solder for aluminum, recommended by the Scientific American, is composed of the following: Cadmium 50 parts. Zinc 20 " Tin 30 " The zinc is first melted in a suitable vessel; then the cadmium is added, and then the tin, in small pieces. The proportions of the various ingredients may be varied, in accordance with the use to which the article is put. For instance, when a strong and tenacious solder- ing is required, a larger proportion of cadmium can be used; where great adhesion is desired, a large propor- 196 PRACTICAL DENTAL METALLURGY. tion of zinc should be used, and where a nice and durable polish is desired, a greater per cent, of tin should be used. An alloy of zinc, copper, and aluminum has been intro- duced as a dental base. (See also Carroll's alloys for cast dentures, pp. 188 and 189.) It is said to be unaffected by the oral fluids. Tin and aluminum form alloys little affected by acids. With 100 parts aluminum and 10 parts tin an alloy is produced much whiter than alluminum and but little heavier. It can be welded and soldered like brass. Iron and aluminum unite readily. Ostberg, a Swedish inventor, discovered that an exceedingly small content of aluminum (5-1000th of 1 per cent.) in wrought iron served to lower its fusing point about 500° F., so that castings may be made from it as readily as from the highly carburized cast iron. Iron may be coated with aluminum much as it is with tin. Zinc and aluminum unite to form alloys very useful for soldering the latter. They are prepared by first melt- ing the aluminum and adding the zinc gradually, after which some fat is introduced to prevent oxidation, and the alloy is stirred rapidly with an iron rod. Aluminum may be frosted by immersion in a solution of potassa. TESTS FOR ALUMINUM IN SOLUTION.— Sulphuretted hydrogen does not produce a precipitate when added to a solution of a salt of aluminum. Ammonium hydro-sulphide produces a white pre- cipitate of aluminum hydrate and evolves sulphuretted hydrogen. Ammonium hydrate throws down a bulky, gelatinous aluminum hydrate, slightly soluble in the precipitant. BLOW-PIPE ANALYSIS.— Compounds of alumi- num are not reduced to the metal, but most of them are reduced to the earth, by ignition on charcoal. If this ALUMINUM. 197 residue is moistened with a solution of cobalt nitrate, and strongly ignited, it assumes a blue color. Silica gives the same reaction, but the color is paler and thus distinguished. ELECTRO-DEPOSITION OF ALUMINUM.— Jeancon patented a process for depositing aluminum from an aqueous solution of a double salt of that metal and potassium, by means of a current from three Bunsen's cells, the solution being at 140° F.* In order to plate aluminum it must first be coated with copper. * Telegraphic Journal, Vol. 1, p. 308. CHAPTER XV. MERCURY. Hydrargyrum. Symbol, Hg. Valence, II, (Hg 2 ) n . Specific gravity, 13.595. Atomic Weight, 199.71. Malleable at -39° C. Melting point, -39° C. Boiling point, 357.3° C. Conductivity (heat), greater Conductivity (electricity), ^ 7 th than water. of silver. Specific heat, 0.0333. Chief ore, cinnabar. Color, silver- white. Crystals, octathedral at — 39° C. OCCURRENCE. — Mercury occurs in nature chiefly as the red sulphide, HgS, cinnabar, which, as a rule, is accompanied by more or less of the reguline metal. The most important mercury mines of Europe are those of Almaden, Spain, and of Idria, in Illyria; it is also found in China, Mexico, Corsica, Peru, and California. The European mines, until lately, furnished the bulk of the mercury of commerce, but they have been eclipsed by the rich deposits of New Almaden, near San Jose, California. The mines of the latter have been the most productive in the world, yielding more than 3,000,000 pounds annu- ally, and large quantities are still taken from them. The ore of old Almaden is of a dull red color in mass; of a dull brick-red color when in fine powder, and is of 3.6 specific gravity. That from New Almaden is of a bright red color, slightly inclining to purple, and not so hard as the Spanish ore; of a bright vermilion color in powder, having a specific gravity of 4.4. The California cinnabar is richer in mercury, because purer, than the Spanish, the former yielding about 70, the latter about 38 per cent, of mercury. MERCURY. X99 Mercury is also found jree; forming an amalgam with silver; and in the form of protcchloride (native calomel). REDUCTION.— The metal is obtained almost exclu- sively from the sulphide or native cinnabar, arid is extracted by two principal methods. By the first method the mineral is picked, crushed, and mixed with lime. The mixture is then introduced into cast-iron retorts, which are placed in rows, one above the other, in an oblong furnace, and connected with earthenware receiv- ers, one-third full of water; heat is applied, the lime combines with the sulphur, forming the sulphide and sulphate of calcium — 4HgS + 4CaO=3CaS + CaS0 4 + Hg 4 , while the mercury distills over, and is condensed in the receivers. In the second method the decomposition of the cinnabar is effected by the direct exposure of the ore lo the oxidizing flame of the furnace, and the mercury vapor is recovered in more or less imperfect condensers. PURE MERCURY.— The commercial article, as a rule, is quite pure chemically, and only needs to be forced through chamois skin to be fit for ordinary pur- poses; but it frequently contains foreign metals, as lead, tin, zinc, and bismuth. It is seldom intentionally adul- terated. When impure, the metal has a dull appearance, leaves a trace on white paper, is deficient in due fluidity and mobility, as shown by its not forming perfect globules, is not totally dissipated by heat, and, when shaken in a glass bottle, coats its sides with a pellicle, or, if very impure, deposits a black powder; if agitated with strong sulphuric acid, the adulterating metals be- come oxidized and dissolved, and thus the metal may be to a limited extent purified. If sulphuretted hydrogen does not act upon hydrochloric acid, which has been 200 PRACTICAL DENTAL METALLURGY. previously boiled upon the metal, the absence of contam- inating metals is shown. Detection of Lead. — Lead may be detected by shaking the suspected metal with equal parts of acetic acid and water, and then testing the acid by sulphate of sodium, or iodide of potassium. The former will pro- duce a white, the latter a yellow, precipitate, if lead be present. Detection of Bismuth. — Bismuth is discovered by dropping a nitric solution of the mercury, prepared with- out heat, into a quantity of distilled water, when the sub-nitrate of bismuth will be precipitated. Detection of Tin. — Complete solubility of the metal in nitric acid shows the absence of tin. Lead is the chief impurity, and may be removed by exposing a thin layer of the metal to the action of nitric acid diluted with twice the quantity of water, which should well cover the surface, remaining for a day or two, with frequent stirring. The lead is much more easily oxidized and dissolved than the mercury, though some of the latter also passes into solution. The mercury is afterwards well washed with water, and dried first with blotting paper, then by gentle heat. At the same time most of the other metallic impurities are removed. Mer- cury is, however, best purified for dental use by redistil- lation. Chemically pure mercury may be obtained by decom- posing pure mercuric oxide by heat, and washing the condensed metal with dilute nitric acid. EXPERIMENT No. 46.— Into a test-tube containing equal parts of acetic acid and water, drop some mercury, suspected to contain lead, and shake thoroughly. Add a solution of potassium iodide — yellow precipitate if lead is present. EXPERIMENT No. 47.— Dissolve a little mercury, suspected to con- tain bismuth, in nitric acid, without heat. Drop into considerable quantity of distilled water — white precipitate if bismuth is present. MERCURY. 201 EXPERIMENT No. 48.— Dissolve mercury, suspected to contain tin, in nitric acid. If tin is present, a white, flaky residue, oxide of tin, remains. EXPERIMENT No. 4 9.— Boil mercury, suspected of containing metallic impurities, in hydrochloric acid, decant the acid, and add to it sulphuretted hydrogen. If no action, the absence of contamination is shown, EXPERIMENT No. 50. — Roll impure mercury over a white paper or clean watch glass: a "tail" is left where it passes. PROPERTIES. — Mercury, or quicksilver, as it is often called, is of a silver-white color, liquid at ordinary tem- perature — above- 39° C. — odorless and tasteless. Vol- atile at common temperature (see experiment No. 51), but more rapidly volatilizes as the temparature increases, and at 357.3° C. it boils, being finally volatilized without residue. When globules are dropped upon white paper they should roll about freely, without tailing, retaining their globular form. It should be perfectly dry, and present a bright surface. When perfectly pure it under- goes no alteration by the action of the air or of water, but in the ordinary state it suffers a slight tarnish. It solidifies with considerable contraction into a compact mass of regular octahedra, which can be cut with a knife, or flattened under the hammer. EXPERIMENT No. 51.— In a small vial containing a little metallic mercury, suspend a strip of gold-foil about a quarter of an inch over the metal. In a short time the lower portion of the gold will become white, owing to the condensation of the mercury upon it. EXPERIMENT No. 52.— On a clean strip of copper place a globule of mercury; the latter soon covers a considerable surface, giving it a white color. Heat the copper, and the original color will be restored, the mercury volatilizing. USES. — It is in constant requisition in the chemical laboratory, and is greatly used in the construction of thermometers, barometers, and manometers, for the de- termination of the capacity of vessels, and for many other purposes. In medicine, in the uncombined state, it is inert, but in combination acts as a peculiar and universal stimu- 202 PRACTICAL DENTAL METALLURGY. lant. When exhibited in the finely divided state it forms several preparations, producing peculiar effects; this fact, however, does not prove that the uncombined metal is active, but that in minute division it is favorable to chemical action and combination. Rubbed up with chalk, mercury forms hydrargyrum cum creta; with the confection of roses and licorice, massa hydrargyri; with lard and suet, unguentum hydrargyri. Mercurial poisoning, ptyalism, salivation, is first ob- servable by a coppery taste, a slight soreness of the gums, and an unpleasant sensation in the sockets of the teeth, when the jaws are firmly closed. In dentistry mercury is used to form alloys known as amalgams. (See chapter on Amalgams.) COMPOUNDS WITH OXYGEN.— Monoxide or Mercuric Oxide HgO, perhaps more commonly known as red oxide of mercury, or red precipitate. The compound may be prepared by several methods, the most prominent of which are: First, by exposing mercury in a glass flask with a long, narrow neck, for several weeks, at a temperature of about 315° C. The product of such exposure and heat is highly crystalline and of a dark red color. Second, as it is generally pre- pared, by cautiously and thoroughly heating any of the mercuric or mercurous nitrates to complete decomposi- tion, which latter fact is recognized by the absence of the characteristic red fumes and odor of nitrous oxide. By this means the acid is decomposed and expelled, oxidiz- ing the metal to the maximum if it happens to be in the state of mercurous salt. The product thus obtained is also crystalline and very dense, but of a much paler color than the preceding. While hot, it is nearly black. Third, by adding caustic potash in excess to a solution of mercury chloride, by which a bright yellow precipi- MERCURY. 203 tate of mercuric oxide is thrown down. This precipitate is destitute of crystalline character, and much more minutely divided than the two preceding. The monoxide is only slightly soluble in water, com- municating to the latter an alkaline reaction and metallic taste; it is highly poisonous. When strongly heated, it is decomposed into mercury and oxygen gas. EXPERIMENT No. 53.— In a test-tube place a small quantity of mer- curic oxide and close by rubber stopper, through which pass a glass tube connected with rubber tubing. Place mouth of the tube below the surface ot water and heat test-tube to dull-redness. The oxygen separates from the mercury and escapes bubbling through the water, while the mercury con- denses in a ring upon the colder part of the test-tube. HgO = Hg+0. Mercurous Oxide, Hg 2 0; Suboxide or Gray Oxide of Mercury may be prepared by adding caustic potash to mercurous nitrate. It is a dark gray, nearly black, heavy powder, insoluble in water, slowly decomposed by the action of light into metallic mercury and the red oxide. ACTION OF ACIDS ON MERCURY.— Hydro- chloric acid does not attack mercury. Sulphuric acid, boiling, converts it into mercurous sulphate, liberating sulphur dioxide. Nitric acid is the most effective solvent for mercury. It dissolves readily in the dilute acid with heat, or in the cold, if nitrous acid is present; with the strong acid, heat is soon generated, and with considerable quantities of the material the action acquires an explosive violence. At ordinary temperatures, dilute nitric acid, when ap- plied in slight excess, produces chiefly normal mercu- rous nitrate, but when the mercury is in excess, more or less of basic mercurous nitrate is formed; hot dilute nitric acid, in excess, forms chiefly mercuric nitrate; when the mercury is in excess, both basic mercurous and 204 PRACTICAL DENTAL METALLURGY. basic mercuric nitrates are formed. In all cases, chiefly nitric oxide gas is evolved. ALLOYS. — Mercury unites readily with most metals except iron and platinum. With the former it has been found to unite only indirectly; for example, by rubbing very finely divided iron with mercuric chloride, water, and a few drops of metallic mercury. The latter metal can only be combined in the spongy state. Yet both of these metallic elements combine chemically with mercury to form definite compounds, according to Bloxam and other authorities, and present the composition, FeHg and PtHg 2 respectively. Of gold and mercury, Dr. H. H. Burchard,* in an exceptionally able paper, quotes: "A gold amalgam 1 to 1000 has all the fluid mercury expressed through chamois; the residue treated with dilute nitric acid at a moderate heat. A solid amalgam is left, Au 8 Hg, which crystallizes in four-sided prisms, and does not melt even when heated until the mercury volatilizes. "f And further, "A mixture of gold and mercury was heated to a temperature a little above the boiling-point of mercury, and the heat maintained until the weight became constant, and there resulted an amal- gam containing 10.3 per cent, of mercury, giving a for- mula of Au 9 Hg." (Hiorns.) Then adds: " Guettier points out that a saturated solution of gold in mercury is Au 2 Hg, a mass of waxy consistence. Evidently, when the gold exceeds this ratio, there is not a perfect chemical compound, as, for instance, in the Au 8 Hg amalgam." Silver and mercury combine very readily, and undoubt- edly form a definite chemical compound. Joule gives its * Dental Cosmos, Vol. XXXVII, p. 989. f T. H. Henry, Philos. Mag., Vol. IX, p. 468. MERCURY. 205 formula as Ag 2 Hg, Bloxaru as Ag 2 Hg 3 . It also forms two native amalgams, having the formulae of AgHg and Ag 2 Hg 6 . With copper, zinc, tin, and lead it also forms definite chemical compounds, and their formulae may be expressed respectively as: CuHg,Zn 2 Hg, Sn 2 Hg, and Pb 2 Hg. The conclusion, then, is obvious that our dental amalgams are probably mostly — fundamentally, at least — chemical compounds, but usually with mercury, and, perhaps, some other constituent in excess. EXPERIMENT No. 54. — Throw a piece of clean sodium upon warm, dry mercury; union takes place with incandescence and evolution of heat sufficient to volatilize portions of the metals. EXPERIMENT No. 55.— Repeat the experiment, using potassium, instead of sodium: the combination is attended with even more violence. An amalgam is formed with the metal in each instance. VERMILION. — Mercuric sulphide, HgS, occurs native as cinnabar, a dull-red mineral, the most important ore of mercury. It may be prepared by several different methods, much depending upon the purity of the mate- rials employed. When mercury and sulphur are heated together the union is accompanied with much energy, and if the product be sublimed, becomes the red or mer- curic sulphide. The sulphur is best first melted and the mercury gradually added by straining through linen cloth, whereby it falls in a minutely divided state, while the mixture is constantly stirred. When the tempera- ture arrives at a certain point, the combination takes place suddenly with a slight explosion, attended with the inflammation of the sulphur, which must be extinguished by covering the vessel. The product of the combination is a black mass, generally containing an excess of sul- phur, which, before the sublimation is performed, should be gotten rid of by gentle heat on a sand-bath. Sub- limation is best carried on in a closely stopped glass 206 PRACTICAL DENTAL METALLURGY. matrass, which should be placed in a crucible contain- ing sand, and, thus arranged, exposed to a red heat. The resulting vermilion is reduced to a fine powder by levigation, the beauty of the tint depending much upon the extent to which the division is carried. It is prepared in a wet way by intimately mixing 100 parts of mercury with 38 parts of the flowers of sulphur, and the iEthiop's mineral digested, with constant agita- tion, in a solution of 25 parts of caustic potash in 150 parts of water at 45° C. (the water lost by evaporation being constantly replaced), until the preparation has come up to its maximum of fire and brilliancy, which takes a good many hours. Purely sublimed vermilion has a comparatively dull color, and must be manipulated with an alkaline (potassium) sulphide solution to give it the necessary fire. The action of the alkaline sulphide consists probably in this, that it dissolves successive in- stallments of the amorphous preparation and redeposits them in the crystalline form. Properties. — It is a fine, bright scarlet powder, per- manent in air, odorless and tasteless, insoluble in water, alcohol, dilute nitric, concentrated hydrochloric, or sul- phuric acids. Nor is it acted upon by boiling potassium, hydrate, sulphide of ammonium, cyanide of potassium or sulphite of soda. It is slightly acted upon by concen- trated hot nitric acid, and completely soluble in a solution of potassium sulphide in the presence of free alkali or a. solution of sodium sulphide. Nitro-hydrochloric acid decomposes it into mercuric chloride, which is readily soluble. It may be completely sublimed, as has been seen, without decomposition, but if exposed to a tem- perature of 315.5° (600° F.) it is decomposed into metallic mercury and sulphur dioxide. It is frequently adulterated with red lead, dragon's blood, chalk, ferric MERCURY. 207 oxide, realgar (As 2 S 2 ), and brickdust. If lead be present it will yield a yellow precipitate when digested with acetic acid and potassium iodide added. Dragon's blood may be detected by alcohol, which will take up the coloring matter of that substance if present. Chalk is detected by an effervescence on the addition of an acid. Most other impurities may be detected by subliming a small portion of the compound. The non-volatile substances used for adulteration will remain behind. Uses. — When pure it is much used as a pigment, on account of its brilliancy and color. Its unalterableness and resistance to chemical action render it particularly valuable in giving the red color to vulcanizable rubber used in the construction of artificial dentures of red and pink vulcanite in the composition of which it forms, in some cases, about one-third of the entire weight of the compound. Notwithstanding the poisonous character of mercurial compounds in general, and the frequency of troubles of an inflammatory nature of the mucous mem- brane in mouths fitted with rubber dentures, it is obvi- ously very improbable, when we consider the properties of pure vermilion, that such conditions can be in any degree attributable to the presence of this substance per se. It is quite possible that impure vermilion may contain from the start free mercury; be contaminated with arsenic bisulphide, or poisonous adulterations. Again, the practice of dissolving tin-foil off of the surface of plates with nitro-hydrochloric acid just after vulcan- ization may possibly decompose some little vermilion, forming soluble bichloride. It is highly improbable that any of these conditions can be found, yet it is possible. It is said that free mercury has been observed with the microscope in finished vulcanite. The occurrence of oral inflammatory conditions, under black rubber dentures, 208 PRACTICAL DKNTAI, METALLURGY. precisely similar to those under red rubber, practically relieves vermilion of the responsibility. Such inflamma- tory troubles are directly attributable to its rough and porous surface, lack of cleanlijiess on the part of the wearer, and the fact that rubber, being a non-conductor of heat, not only prevents proper radiation from the muc- ous membrane, but also prevents this membrane being cooled by the passage of air, fluids, or foods through the mouth. EXPERIMENT No. 56.— Test vermilion and red rubber (pieces and filings) in nitric, hydrochloric, sulphuric, and nitro-hydrochloric acid. EXPERIMENT No. 57.— Prepare red rubber and vermilion and examine under microscope. TESTS FOR MERCURY IN SOLUTION.— Sul- phuretted hydrogen, gradually added to mercuric solu- tions, forms at first a white precipitate; by further addi- tions of the reagent, the precipitate becomes yellow- orange, then brown, and finally black. Such progressive variation of color is characteristic of mercury. With mercurous compounds, sulphuretted hydrogen, and solu- ble sulphides precipitate mercurous sulphide, Hg 2 S, black, without change of color. Soluble Iodides precipitate mercuric iodide, Hgl 2 , from mercuric compounds, first reddish-yellow, then red. From mercurous solutions they precipitate mercurous iodide, Hg 2 T 2 , greenish-yellow in color. Caustic soda or potassa precipitates yellow mercuric oxide from mercuric salts, and black mercurous oxide from mercurous salts. Ammonium hydrate throws down a "white precipi- tate" of mercuric chloramide (H 2 NHgCl) from mercuric salts, but black precipitates are thrown down from mer- curous salts. EXPERIMENT No. 58.— The student should perform these tests. MERCURY. 209 BLOW-PIPE ANALYSIS.— All compounds of mer- cury, in glass tubes or on charcoal, are quickly volatile before the blow-pipe. All compounds of the metal, dry, and intimately mixed with dry sodium carbonate, and heated in a glass tube closed at one end, give a sublimate of metallic mercury as a gray mirror coat on the inner surface of the cold part of the tube. ELECTRO-DEPOSITION OF MERCURY.— If a piece of bright, clean copper be immersed in a solution of a mercuric salt acidulated with hydrochloric acid the surface will be rendered white by the deposition of mercury upon it. From solutions of mercuric chloride, cyanide, or nitrate, aluminum deposits mercury, forming an amal- gam which decomposes water at 60° F., and rapidly oxidizes and becomes heated in the air. Dr. Kirk's method for preparing copper amalgam: " Precipitate the copper directly into the mercury by electrolytic process. This may be done conveniently by pouring a quantity of mercury into a suitable glass ves- sel — a small battery jar, for example — and suspending a thick plate of copper, by means of a wooden support, some distance above the surface of the mercury. A saturated solution of cupric sulphate is then poured into the jar until the copper plate is completely submerged. The cathode pole of a battery, or other source of electrical current, is then connected with the layer of mercury, and the anode with the copper plate. All that portion of the cathode electrode in contact with the cupric sulphate solution should be insulated with gutta percha, and only the point, which is in contact with the mercury, left ex- posed. The passage of the current causes solution of the copper from the anode and deposits it in the mercury continuously as long as the foregoing conditions are 210 PRACTICAL DENTAL METALLURGY. maintained. The precipitation should be continued until the mercury is saturated, which will be evidenced by the appearance of the characteristic red color of the ex- cess of copper at the cathode pole. When the saturation point has been fully reached, the mass should be washed, first in dilute hydrochloric acid, and then in water, dried and compressed as is usual with this amalgam when pre- pared by the ordinary processes."* * Operative Dentistry, IS- C. Kirk, p. 226. CHAPTER XVI. SILVER. Argentum. Symbol, Ag. Valence, I. Specific gravity, 10.4. Atomic weight, 107.67. Malleability, 2d rank. Melting point, 1040° (1904° F.). Tenacity, 4th rank. Ductility, 2 rank. Conductivity (electricity), 100. Conductivity (heat), 100. Chief ore, silver glance. Specific heat, 0.057. Crystals, isometric. Color, white. OCCURRENCE.— Silver is widely diffused through- out the earth's crust. It is found chiefly in the United States, Mexico, Peru, and Chile; Austria, Hungary, Nor- way and Australia also furnish considerable amounts. Of the varieties of silver ores the following chiefly are metallurgically important. (1) Reguline silver, (2) horn silver, (3) silver glance, (4) silver-copper glance, (5) pyraigyrite, (6) stephanite, and (7) polybasite. Silver is also frequently met with in base metallic ores, as in lead ores and many kinds of pyrites. Reguline silver, native silver. Owing to the weak affinity of silver for other substances it is frequently found free in a metallic state, occurring in flat masses, and at times in an arborescent form, composed of numer- ous isometric crystals strung' together, or in twisted filaments. In the I^ake Superior district it occurs with native copper, showing in specks upon the surface of the latter metal. With mercury it is found as a native cry- stalline amalgam. Native silver is usually free from any considerable admixture of other metals, but it always contains gold. Horn silver, native chloride, AgCl. The ore is named from its resemblance to horn in texture aud 212 PRACTICAL DENTAL METALLURGY. appearance. It is of a pearl-gray color when freshly cut, and on exposure to sunlight turns brown. It con- tains about 75.3 per cent, silver. The corresponding bromide and iodide also occur native. Silver glance, native sulphide, Ag 2 S, is the most important ore of silver. It is a soft, gray, and somewhat malleable mineral, may be cut with a knife, is quite fusible, and when pure contains 87-1 per cent, silver. It is frequently found associated with copper, as silver-cop- per glance (AgCu) 2 S; with antimony, as pyrargyrite^ Ag 3 SbS 3 , and as stephanite, Ag s SbS 4 , with copper, anti- mony, and arsenic, as polybasite, 9(Ag 2 ,Cu 2 )S+(Sb 2 , As 2 )S 3 ; with lead, as argentiferous galena, and with iron. REDUCTION.— The method by which silver is ex- tracted from its ores depends chiefly on the nature of the admixtures, the state of the combination of the silver being as a rule irrelevant in the choice of process, because some at least of the noble metal is always present as sul- phide, and the mode of treatment for it includes all other forms. Amalgamation. — If the ore is comparatively free from base metals, amalgamation is resorted to. Most ores con- tain too great a proportion of earthy matter, etc., to admit of any other method economically, even in localities where fuel is plenty. Several methods of amalgamation are employed, varying with different localities and circum- stances, but the principles involved are similar, and a general description will suffice for all. The ore is ground and roasted at a dull-red heat with common salt, which converts the sulphide of silver into chloride — Ag 2 S + 2NaCl + 40 from the air=2AgCl + Na 2 S0 4 . The mass, along with certain proportions of water, scrap-iron, and mercury, is placed in barrels, which are SILVER. 213 made to rotate about their axis, so that the several ingre- dients are forced into constantly varying contact with each other. The salt solution takes up a small propor- tion of the chloride, which in this (dissolved) form is quickly reduced by the iron to the metallic state — 2AgCl+ Fe=FeCl 2 + 2Ag, so that there is, so to say, room made in the brine for another instalment of silver chloride, which in turn is reduced, and so on. The metal, as soon as freed, is combined with the mercury in a semi-fluid amalgam, which, on account of its specific gravity, is easily sepa- rated from the dross. The silver amalgam is then pressed in linen or some other suitable cloth bags, to separate the amount of comparatively free mercury, which, of course, is reused in the process. The remain- ing solid amalgam is subjected to distillation from iron retorts, the mercury recovered as a distillate, while the silver in a more or less impure state remains in the retort. The silver furnished by the amalgamating process is never pure, even in a commercial way. A general method of its purification is to fuse it with lead, and subject the alloy to cupellation. Cupel silver is apt to contain small quantities of lead (chiefly), bismuth, anti- mothy, copper, and more or less gold. The first three can be removed by recupellation, without added lead, at a high temperature. The gold, if present to the extent of 1 per cent, or more, is removed by treating with nitiic or sulphuric acid. The copper is allowed to remain, for commercial silver. Argentiferous Galena. — The lead extracted from galena often contains a sufficient quantity of silver to allow of profitable extraction. This is accomplished by first concentrating the lead and silver alloy by the Pat- 214 PRACTICAL DENTAL METALLURGY. ttnsoti process, which is based upon the fact that the alloy of silver and lead has a lower fusing point than lead alone, and therefore remains fluid after the purer lead crystallizes. Alloys are thus concentrated from lead containing not more than 3 or 4 ounces of silver per ton to that which contains about 300 ounces to the ton, when by cupellation the lead and other oxidizable metals are removed as oxides, leaving pure silver. Desilvering Lead. — The process is thus described by Bloxam: "Bight or ten cast-iron pots, set in brickwork, each capable of holding about six tons of lead, are placed in a row with a fireplace underneath each of them. Suppose that there are ten pots numbered consecutively, that on the extreme left of the workmen being No. 1, and that on his extreme right No. 10. About 6 tons of the lead containing silver are melted in pot No. 5, the metal skimmed, and the fire raked out from beneath, so that the pot may gradually cool, its liquid contents being con- stantly agitated with a long iron stirrer. As the crystals of lead form, they are well drained in a perforated ladle (about ten inches wide and five inches deep) and trans- ferred to pot No. 4. When about four-fifths of the metal have thus been removed in the crystals, the portion still remaining liquid, which retains the silver, is ladled into pot No. 6, and the pot No. 5, which is now empty, is charged with fresh argentiferous lead, to be treated in the same manner. " When pots Nos. 4 and 6 have received, respectively, a sufficient quantity of the crystals of lead and of the liquid part rich in silver, their contents are subjected to a perfectly similar process, the crystals of lead being always passed to the left and the rich argentiferous alloy to the right. As a final result to these operations, the pot No. 10, to the extreme right, becomes filled with a rich alloy of lead and silver, sometimes containing three hundred ounces of silver to the ton, whilst pot No. 1, to the extreme left, contains lead in which there is not more than one-half an ounce of silver to the ton." SILVER. 215 Cupellation. — The extraction of the silver from the rich alloy of silver and lead is accomplished by a process of refining or cupellation, which is based upon the prop- erty possessed by certain oxides of being absorbed by the porous cupel. The process is necessarily modified accord- ing to the quantity of alloy to be cupelled; the principle, however, remains identical. The cupel, Fig. 33, a small, shallow crucible, so named from the diminutive of the L,atin cupa, a cup, is made from prepared bone- ash, moistened with sufficient warm water to hold it together. Sometimes a little wood-ashes or potas- sium carbonate is added to the water for moistening the bone- ash. After proper Fig. 33. moistening and mix- ing the cupel is formed by packing and tamping the moistened ash into a steel mold made for the purpose, and the cupel knocked out by a gentle tap. The ash should not be too fine or packed too densely, or the cupel will want in porosity; nor should it be too coarse or too loosely packed, resulting in a cupel so porous as to cause a loss of the metal. A good cupel, well dried, should not crack on being heated, and should be capable of ab- sorbing nearly its own weight in lead oxide. The furnace used in operations of a small character is a muffle furnace, called an assayer's furnace, similar to a continuous-gum furnace. The muffle contained is identical with that employed for continuous-gum work, except that a narrow slit is provided on each side or at 216 PRACTICAL DENTAL METALLURGY. the end, for the circulation of a current of air over the cupel. The cupel is first heated in the muffle to an even tem- perature with the latter, which should be a full red heat. The weighed mass of alloy may then be gently placed on the cupel, the muffle closed, and the alloy heated to redness as soon as possible. When this degree is attained the muffle is opened and air admitted. As the air strikes the molten mass a film of oxide quickly makes its appearance upon its surface, which, waving over the melted alloy, is quickly absorbed by the cupel, only to be replaced by other oxide, which is also absorbed; this is continued, the metallic globule rapidly diminishing in size, uutil at last all of the lead has been reduced to an oxide and gotten rid of. The operator must carefully watch the process during this time, for if the mass becomes too highly heated silver will be lost by volatilization, and if insufficient heat is maintained, the mass freezes, and the proper temperature cannot be restored without loss of noble metal. A proper temperature is maintained by observing the color of the muffle and cupel, the former of which should be reddish-white, and the latter full red, the molten alloy will then appear luminous and clear, and fumes of oxide will be seen whirling over the mass. When the last of the lead has been oxidized and gotten rid of, the metallic globule remaining is observed to rapidly revolve on its axis, is covered with iridescent tints, and later assumes an exceedingly bright appear- ance, which is technically termed brightening or corusca- tion of the button. When this is observed the temperature should be somewhat increased to insure the expulsion of the last traces of lead. When the operator is assured that all the lead has been expelled the button is allowed to slowly cool to prevent spitting, sputtering or vegetatio?i of the SILVER. 217 mass, resulting in a loss of some of the silver. Remov- ing the lead, however, is not the only action; if it were, little would be gained in the process. Another action goes on whilst the lead is being oxidized in a current of air, and other metals, except gold and silver, are also oxidized and carried off with the litharge. If the lead is therefore properly proportioned the resulting button will consist of silver and gold, if the latter was present originally. Any gold present may be recovered by the parting process. (See chapter on Gold.) EXPERIMENT No. 59.— If the metallurgical laboratory contains an assay furnace or a muffle furnace that can be used as such, the instructor should cupel a small button of silver for the demonstration of the process. CHEMICALLY PURE SILVER in small quantities may be easily prepared in the laboratory by dissolving commercial or coin silver in pure dilute (50 per cent.) nitric acid contained in a Florence flask, hastening the action by gently heating over a sand-bath. After the silver has been dissolved, and the solution somewhat cooled, add an equal bulk of distilled water, and filter into a second flask. To the filtrate add a saturated solution of sodium chloride (common salt) until no more white precipitate of silver chloride is formed — AgN0 3 + NaCl=AgCl + NaN0 3 . The flask should then be stopped and shaken for sev- eral minutes, when, on being allowed to rest, the chlcride will quickly fall to the bottom, leaving a clear, super- natant liquid above, which, if copper be present, will be colored a bluish-green. If to this clear supernatant liquid the salt solution be added, the operator is enabled to determine instantly whether all of the silver has been thrown down as the chloride, or not. If so, the clear liquid is decanted off and the chloride washed until the wash-water does not assume the slightest tinge of blue 218 PRACTICAL DENTAL METALLURGY. upon the addition of ammonia. The chloride is now best transferred to a beaker, or some other wide-mouthed vessel, and about twice its bulk of water, acidulated with about 10 per cent, of sulphuric acid, added. Several small pieces of iron in some form, preferably lath-nails, may now be added to the mixture, and the whole stirred with the closed end of a test-tube. The following reactions then take place, during which ferrous sulphate and hydrochloric acid are formed and silver liberated, thus — Fe+H a S0 4 =FeS0 4 +H 2 , and 2H + 2AgCL=2HCl+2Ag. The completion of the reaction is recognized by the changing of the precipitated mass from white to a dark- gray, which is the color of the finely divided silver. The small pieces of iron are now removed, the precipitated silver washed and rewashed with dilute hydrochloric acid, then with distilled water, dried, mixed with about an equal bulk of potassium carbonate, and melted in a well-boraxed crucible. EXPERIMENT No. 60.— The student should refine a silver dime, or an equal weight of silver, by the above means, obtaining the pure silver in its stead. Pure Silver Nitrate Crystals or pure Silver may be prepared by digesting commercial, or coin silver, in nitric acid as before, and evaporating the solution over a sand-bath. After the water and free acid is driven off a greenish solid of silver and copper nitrates remains. By continued heat the former is fused and the latter is changed to black oxide of copper, CuO, by driving off the nitrogen tetroxide (or dioxide, at high temperatures, N0 2 ). When the evaporating dish has sufficiently cooled to be handled, a small quantity of distilled water is added and the contents of the dish stirred and then SILVER. 219 filtered; the soluble nitrate of silver passes through as a filtrate, leaving the insoluble black cupric oxide on the filter paper. If the preparation of pure silver nitrate crys- tals is the object of the experiment, evaporate the filtrate to crystallization. If the desire is to recover the silver, this may be done by the addition of sodium chloride to the nitrate solution, as before, or by immersing a clean strip of copper in the solution, when the silver will be precipi- tated upon the copper. Silver obtained in this manner, however, is seldom entirely free from contamination with copper. EXPERIMENT No. 61.— The student should make a pure solution of silver nitrate, recrystallize a portion of it, and recover the silver from the remainder, as instructed above. PROPERTIES.— Silver is the whitest of metals, very brilliant, tenacious, malleable and ductile, in the last two qualities being inferior only to gold; if considered weight for weight, it is superior to gold, for while one grain of gold may be beaten so thin as to cover an area of 75 square inches, a grain of silver may be made to cover 98 square inches, though the foil of the former is much thinner than that of the latter. The extent of the mal- leability of gold and silver has never been absolutely determined, as the means employed have invariably failed before the property in either was exhausted. In tenacity silver is superior to gold. It is also harder than gold, but softer than copper, and is the best-known conductor of heat and electricity. It fuses at 1040° (1904°F.), far below the fusing point of either gold or copper. It volatilizes appreciably at full red heat; in the oxyhydrogen flame it boils, with the formation of a blue vapor. The fused metal readily absorbs oxygen gas (when fused under potassium nitrate it takes up as much as twenty times its volume). As the metal cools 220 PRACTICAL DENTAI, METAUJJRGY. the oxygen escaping through the semi-solid crust on the surface of the fused mass produces very beautiful effects. Pure silver retains a trace of the absorbed oxygen per- manently. It is unaltered in the air at any temperature, but is readily acted upon by sulphur, phosphorus, or chlorine. EXPERIMENT No. 62.— Melt pure silver on a soldering block of asbestos or pumice-stone, and note the absorption of oxygen while molten, and the escape on cooling. COMPOUNDS WITH OXYGEN.— There are three oxides of silver, only one of which (Ag 2 0), however, can be regarded as a well-defined compound: The Monoxide, or Argentic Oxide, Ag 2 0, is a strong base, yielding salts isomorphous with those of the alkali- metals. It is obtained as a pale-brown precipitate on adding caustic potash to a solution of silver nitrate. Very soluble in ammonia, and slightly so in pure water, forming an alkaline solution. It is easily decomposed by heat; the sun's rays also effect a slight decomposition, as is the case in most compounds of silver. The other two oxides are the Argentous Oxide, Ag 4 0, and Silver Dioxide, Ag 2 2 . ACTION OF ACIDS ON SILVER.— Nitric Acid is the proper solvent for silver, and is most efficient when diluted about 50 per cent., but active whether con- centrated or dilute, with production of nitric oxide (N 2 O s ) and silver nitrate (AgN0 3 ). Sulphuric Acid, hot and concentrated, acts upon sil- ver, forming a sulphate which is sparingly soluble. Hydrochloric Acid, hot and concentrated, forms argen- tic chloride, slightly soluble in the concentrated reagent, but precipitated on dilution. Fused alkaline hydrates or nitre are without action upon silver; hence, it is used for the manufacture of crucibles for the fusion of caustic alkalis, etc. SIXVKR. 221 ALLOYS. — Pure silver is too soft for coinage or com- mercial purposes; it is, therefore, alloyed variously for different purposes to increase its hardness. Gold. — Formerly silver was much used to alloy gold. The metals are easily mixed together, but do not appear to form definite compounds. With certain proportions of the metals the resulting alloys are more ductile, harder, more sonorous and elastic than either metal considered singly. Copper. — The alloys of copper and silver are more useful than any of the alloys of silver. In most coun- tries it forms the silver coins. In the United States the silver for coinage is alloyed with 10 per cent, copper, the proportion of each being stated in the thousandths; thus, pure silver being 1000 fine, the coin or "standard silver" is 900 fine, with 100 parts copper added. The German and French silver coins are of the same grade, those of Great Britain are 925 fine, with 75 parts of copper added, being known as "sterling" silver. Most silverware is of "sterling" fineness. The presence of copper does not modify the color of silver so long as the proportion of the former does not exceed 40 or 50 per cent. Copper imparts to silver greater hardness, tenacity, and strength. Comparison of the silver dollar of the United States with that of Mexico: U. S. DOLLAR. MEXICAN DOLLAR. Pure Silver 371.25 grs. 377.14 grs. " Copper 41.25 " 40.65 " Total weight 412.50 " 417.79 " The Mexican dollar contains 5.89 grains more silver than the United States dollar, and .60 grains less copper. It is also 5.29 grains greater in weight than the United States dollar. 222 PRACTICAL DENTAL METALLURGY. The Mexican dollar is equal to 0.866 of a Troy ounce. Zinc and silver have a great affinity for each other, and are consequently readily alloyed. Silver solder for soldering the metal is usually com- posed of an alloy with copper and zinc. The following are well adapted for the purpose. No. 1* No. 2.t Silver 66 parts. Silver 6 parts. Copper 30 ' * Copper 2 " Zinc 10 " Brass 1 " " When the material to be united is composed of pure silver and platinum, silver coin alloyed with, one-tenth zinc may be used as a solder." " Standard " is also an excellent solder for high fusing brass and German silver. If the article is to be soldered twice, this may be used first and the silver solder after- wards, x Dr. Kirk§ recommends the following compositions: ine Silver. Copper. Brass. Zinc 4. 3. . . . 2. 1. . . . 19. 1. 10. 5. 66.7 23.3 10. 50. 33.4 16.6 . . . 11. 4. 1. These may be used for soldering the surfaces of stand- ard silver. TESTS FOR SILVER IN SOLUTION.— Hydro- chloric acid and the soluble chlorides precipitate silver chloride, AgCl. It is a white, curdy substance, quite in- soluble in water, and nitric acid; one part of silver * Richardson, Mechanical Dentistry, p. 78. t Ibid. % Professor C. I,. Goddard. g Am. System of Dentistry, Vol. Ill, p. 879. SILVER. 223 chloride is soluble in 200 parts of hydrochloric acid, when concentrated. When heated, it melts, and on cooling it becomes a grayish, crystalline mass, which cuts like horn. It is found native in this condition, con- stituting the mineral called horn silver. Silver chloride is decomposed by light, turning violet to brown (forming argentous chloride, Ag 2 Cl) both in the dry and in the wet state, very slowly if pure, and quickly if organic matter be present. It is reduced also when put in water with metallic zinc or iron. It dissolves very readily in ammonia and in a solution of potassium cyanide. This precipitation is the most delicate of the ordinary tests for silver, being recognized in solution in 250,000 parts of water. Potassium and sodium hydrate precipitate from solutions of silver salts, silver oxide, Ag 2 0, grayish brown, insoluble in excess of the reagents, easily soluble in nitric, acetic, or sulphuric acid, and in ammonia. Sulphuretted hydrogen throws down a black precipi- tate of silver sulphide, which is insoluble in potassium cyanide, dilute acids, or alkalis, but soluble in boiling nitric acid. Potassium chromate gives a red precipitate of silver chromate, Ag 2 Cr0 4 , which is soluble in ammonia, and concentrated nitric acid. EXPERI3IENT No. 63.— The student should perforin these tests. BLOW-PIPE ANALYSIS.— On charcoal, with sodium carbonate, silver is reduced from all its com- pounds in the blow-pipe flame, attested by a bright, malleable globule. Lead and zinc, and elements more volatile, may be separated from silver by their gradual vaporization under the blow-pipe. 224 PRACTICAL DKNTAL METALLURGY. ELECTRO-DEPOSITION OF SILVER.— Silver is the most important and prominent metal in electro- plating processes. The solution generally used is the cyanide, and it may be prepared by either of two methods — the battery or the chemical process. The method of procedure in the former is simple, when thoroughly understood. First must be ascertained the percentage of actual cyanide in the salt used. If, say, it contains about 50 per cent., dissolve about one ounce in each quart of distilled water; or if it contains more, add less, and vice versa in proportion. Suspend a large anode and a small cathode of silver in the liquid, and pass a strong current of electricity through, until the required amount of metal is dissolved from the anode. As this process produces some caustic potash in the liquid, some of the strongest hydrocyanic acid may now be added to form cyanide, and more of the anode dissolved in the mixture by the battery. Making solutions for deposits by the chemical process is accomplished as follows: Take four parts of pure grain silver; and reduce it by mixing with nitric acid to argentum nitrate. Dissolve this in distilled water, in the proportion of one quart to every one-half ounce of silver used. At the same time make a solution of from two to three parts of cyanide of potassium in twenty or thirty parts of distilled water. This is to be added gradually to the solution of nitrate of silver as long as it produces a white precipitate. If too much be added, however, it will cause some of the precipitate to be redissolved and wasted. In such a case the liquid should be stirred and then allowed to settle clear. A small amount of nitrate of silver dis- solved in distilled water should be added as long as it SILVER. 225 produces a white cloud. This may be better conducted by using a glass vessel and observing the precipitate as it dissolves. The liquid should now be left to settle until quite clear, and the clear portion then decanted, and the precipitate washed four or five times in a large quantity of water by simply adding the water, stirring, and allowing it to settle again and decanting as before. Next dissolve from six to eight parts of cyanide of potas- sium in twenty parts of distilled water, adding it a portion at a time, with free stirring, to the wet cyanide of silver, until the whole is barely dissolved; then add about three parts more of cyanide of potassium to form free cyanide, and sufficient distilled water to reduce the whole to the proportion of about one-quarter of an ounce of silver to the quart; finally, when all the free cyanide is dissolved, filter the solution and it is ready for use. The specific gravity of the solution should be maintained at between 1.8 and 1.15. Deposit solutions are very numerous, but, in the author's judgment, the above is best adapted for a good, reguline solid deposit. Knowledge of the management of solutions is essential. There are varying circumstances which must be noted in order to keep them in good condition for a reguline deposit. New solutions do not work as well, usually, as old ones, provided the latter is not too old. Solutions of two or three years of age work probably the best. They change from many causes; they become dirty and con- centrated from exposure; increase or decrease in their relative proportions of cyanide and metal; they acquire other metals in solution, dissolved from the anode and corroded from the cathode; plaster and plumbago accu- mulate in them, and in consequence of which they should be filtered; they gradually decompose, become brown, 226 PRACTICAL DENTAL METALLURGY. discolored, and evolve ammonia by exposure to light, especially if they contain too much free cyanide; therefore, all these deviations from the proper condition should be corrected. The specific gravity should be maintained, and the proper amount of metal and cyanide kept in solu- tion. To determine any disproportion in the latter, place 25 grams of the solution in a test-tube of proper size and add to it, at first freely, and afterwards gradually, until at last, drop by drop, with constant stirring, a solution of one gram of crystallized nitrate of silver in ten grams of distilled water. If the precipitate formed is dissolved rapidly, with but little need of stirring, there is too much cyanide. If, however, it does not dissolve, even after much stirring, there is too little cyanide; but if it who'ly dissolves (the latter part quite slowly) the proportion of silver to cyanide is about correct. Many other minor troubles not mentioned are encoun- tered, which must be corrected by means gathered only from experience in working the process. The process for making dental bases by electro-depo- sition on the plaster cast of the mouth was patented February 5, 1889, by Joseph G. Ward of Irvington, N. J. The author has had some experience in the work; in fact, was engaged in perfecting a process for the same result when Mr. Ward secured his patent. The method of proceeding in the preparation of a dental base is as follows: A true impression of the mouth is secured, and from this a cast is obtained by filling in the usual manner. After the cast has become thoroughly dry it should be soaked in hot fluid paraffin, until saturated, and before cooling the surface wiped clean of all superflous ad- hesions which might in any way destroy the exactness of the model. The cast is then coated freely where the SILVER. 227 deposit is desired with a mixture of equal parts of pure finely pulverized plumbago and the finest tin-bronze powder or any other conducting substance suitable under the circumstance. This recommended is applied with a thick, short-haired camel's-hair pencil. The cast is now so wired that perfect connection is made with the pala- tine, buccal, and labial surfaces. From these guiding wires a cathode-hook suspends the cast in the solution. After the metal has been deposited to a sufficient thick- ness, the cast, with its deposit, is to be taken from the bath, the deposit removed from the cast, trimmed and polished; but if it is desired to have the plate of increased thickness at any part to give the appearance of a turned- rim, etc., the cast, with the deposit adhering to it, may be removed from the bath, and all the exposed surface of the deposit, except the portions to be thickened, may be covered with a coating of wax or some other non-con- ducting substance, and re-submerged in the bath and left there until the required thickness of deposit is secured in the parts desired. It may then be taken from the bath, burnished, trimmed by scraping, burring and filing to the proper shape and thickness, then polished, and spurred. A thick plating of gold should now be added to the properly shaped plate, or the rubber for the attachment of the teeth will not harden and adhere to the plate dur- ing the process of vulcanization (the sulphur of the vulcanite combining with the silver). After the teeth have been attached and the vulcanite and all properly finished, a second coating of gold should be electro-plated over it all to cover portions that had been made bare in finishing the vulcanite. The denture may be made by depositing the metal directly on the teeth as in cheoplastic work, and, where 228 PRACTICAL DENTAL METALLURGY. necessary, clasps may be formed. Broken dentures have been soldered with 18-carat gold solder. Crowns and bridge-work may also be made in various ways by this process. EXPERIMENT No. 64.— The process should be demonstrated in the metallurgical laboratory by the instructor. CHAPTER XVII. IRIDIUM. Iridium. Symbol, Ir. Valence, II, IV. Specific gravity, 22.40. Atomic weight, 192. G5. Malleable, at red heat. Melting point, oxhydrogen Chief ore, Iridosmine. flame. Crystals, hexagonal. Color, steel-white. OCCURRENCE.— This metal occurs chiefly as a native alloy of iridium and osmium, known as osmiridium or iridosmine. It is also found thus combined with plat- inum, and is contained in gold from several localities, especially that from some mines of California and in the Frazer River district of British Columbia, causing much inconvenience. It is observed in hexagonal crystals, rarely in hexagonal prisms, commonly in irregular flat- tened grains, of a tin-white color, in the residues after the extraction of platinum from its ores. REDUCTION.— It is obtained from its native alloy by mixing with an equal weight of dry sodium chloride, and heating to redness in a glass tube, through which a moist stream of chlorine gas is transmitted. The mixture of iridium and osmium sodio-chlorides thus formed is dissolved in water and evaporated, and distilled with nitric acid, removing the osmium as osmic acid; when its complete removal is thus effected, ammonium chloride is added to the residual solution, which precipitates the ammonio-chloride of iridium as a dark red-brown pre- cipitate. From this, spongy metallic iridium is obtained in the same manner as the production of spongy platinum. PROPERTIES.— Iridium is a steel-white metal, exceedingly hard, brittle when cold, but somewhat malleable when at red heat, having a specific gravity of 230 PRACTICAL DENTAL METALLURGY. 22.40, unaltered in air, and fusible only in the oxyhydro- gen flame. If the precipitated metal be moistened with a small quantity of water, pressed tightly between filter- paper, and then very forcibly in a press, and calcined at a white heat, it may be obtained in the form of a very hard compact mass, capable of taking a good polish, but still very porous, and having a specific gravity not to exceed 16. COMPOUNDS WITH OXYGEN.— Iridium forms three oxides: The Monoxide, or Hypoiridious Oxide, IrO, is but little known, and upon being exposed to the air is quickly converted into a higher oxide. The Sesquioxide, or Iridious Oxide, Ir 2 O s , may be formed by exposing the metal at a red heat to the action of the oxygen of the air. It is a black powder, insoluble in the acids and much used for imparting an intense black to porcelain. The Dioxide, or Iridic Oxide \ Ir0 2 , is also a black powder, obtained by heating the tetrahydroxide in a current of C0 2 . It is insoluble in acids, and is said to be the most stable oxide of the metal. ACTION OF ACIDS ON IRIDIUM.— The pure metal itself is not acted upon by the acids, but when reduced by hydrogen at a low temperature, it oxidizes slowly at a red heat; and may be dissolved in nitro- hydrochloric acid. It is, however, usually rendered soluble by fusing it with potassium nitrate and caustic potash. Its hydroxides are also soluble. It forms two chlorides, IrCl 3 and IrCl 4 and analogous iodides, and three sulphides analogous to the three oxides. ALLOYS.— With Mercury. Dr. Kirk relates that "Bottger formed an amalgam with iridium by immers- ing sodium amalgam in an aqueous solution of chloriri- IRIDIUM. 231 date of sodium; he describes the amalgam as soft and viscid." With Gold iridium forms a malleable and ductile alloy, its color depending upon the proportions of the metals. Platinum and iridium form some very valuable and useful alloys.* Aside from these, and the use of the metal and its alloy with phosphorus for pointing gold pens, iridium is of little value. With Silver it is claimed there is no alloy; and that after exposing a mixture of these metals to a high tem- perature, or attempting to pour out the contents of the crucible, silver alone flows out and a thick mass is left in the crucible. Phosphor-iridium — f"For preparing larger pieces of iridium than found in nature, for making points for the Mackinnon stylographic pen, Mr. John Holland of Cin- cinnati has devised the following ingenious process; The ore is heated in a Hessian crucible to a white heat, and, after adding phosphorus the heating is continued for a few minutes. In this matter a perfect fusion of the metal is obtained, which can be poured out and cast into any desired shape. The material is about as hard as the natural grains of iridium, and, in fact, seems to have all the properties of the metal itself. "Phosphor-iridium, as this metal may be called, pos- sesses some very remarkable properties. It is as hard, if not harder, than iridosmine, from which it is prepared. It is somewhat lighter, owing to its percentage of phos- phorus and increase of volume. It is homogeneous and easy to polish, and forms some alloys impossible to pre- pare in any other manner. It combines with small quantities of silver, and forms with it the most flexible and resisting alloy of silver. With gold or tin no alloy has thus far been obtained. Added in small quantities to copper, it furnishes a metal possessing very small re- sistance to friction, and is especially adapted for articles * See chapter on Platinum. f Metallic Alloys, Brannt, p. 347. 232 PRACTICAL DENTAL METALLURGY. subjected to great pressure. This alloy seems to possess more than any other metal the power of retaining lubri- cants. With iron, nickel, cobalt, and platinum, phos- phor-iridium forms combinations in all proportions, which are of great importance. With iron an alio}' is obtained which retains the properties of phosphor- iridium, although its hardness decreases with a larger addition of iron. The alloy is slightly magnetic, and is not attacked by acids and alkalis, and the best file pro- duces no effect upon it, even if it contains as much as 50 per cent, of iron. With more than 50 per cent, of iron, the power of resistance decreases gradually, and the nature of the metal approaches that of iron." CHAPTER XVIII. PALLADIUM. Palladium. Symbol, Pd. Valence, II, IV. Specific gravity, 11.4. Atomic weight, 105.73. Malleability, 11th rank. Melting point, 1600° (2912° P.). Ductility, 10th rank. Chief ore. In gold and platinum. Conductivity (heat), — Conductivity (electricity), 18.4. (Silver being 100.) Specific heat, 0.0593. Crystals, fibrous. Color, platinum-white. OCCURRENCE.— Palladium is found native in com- pany with platinum from which it is distinguishable by- its fibrous structure. It is usually found, however, alloyed with platinum and with some specimens of Brazilian gold. REDUCTION.— When the solution of crude platinum from which the greater part of the metal has been pre- cipitated by sal-ammoniac, is neutralized by sodium carbonate, and mixed with a solution of mercuric cya- nide, palladium cyanide separates as a whitish insoluble substance, which on being washed, dried, and heated to redness, yields metallic palladium in a spongy state. The palladium may then be welded into a mass in the same manner as platinum.* PROPERTIES.— Palladium is a white metal, much resembling platinum, though somewhat darker in color. Its specific gravity differs greatly from that of platinum, being only 11.4. It is also very much less ductile and malleable than that metal; it is the most fusible of the platinum group, yet it barely melts at the highest wind-furnace heat, or about the temperature at which * Manual of Chemistry, Physical and Inorganic. Watts. 234 PRACTICAL DENTAL METALLURGY. malleable iron fuses, 1600° C, and when heated to redness and exposed to the air, especially in its spongy state, it acquires a blue or purple superficial film of oxide, but may be restored to its brightness and luster upon being heated to a more intense degree, the oxide being reduced. If heated and fused in air it is apt to vegetate on cooling, similarly to silver. The metal is most remarkable for its property of " oc- cluding" or absorbing hydrogen. According to Graham the compact metal when immersed in cold hydrogen gas takes up little or none of it; but at higher temperatures very considerable occlusions take place. A certain specimen of palladium-foil at 245° C. was found to absorb 526 times its own volume of hydrogen; and at between 90° and 97° C. , 643 times its volume. The hydrogen, as in the case of platinum, is retained in the metal on cool- ing. Graham views hydrogenized palladium as a true alloy, containing its hydrogen in the form of a metal — " hydrogenium." Palladium has not the power of ab- sorbing oxygen or nitrogen. If palladium be used as the negative electrode in the electrolysis of water, the coefficient of absorption is very high; especially is this the case when the palladium was produced electrolyti- cally and hydrogenized while itself in the nascent state. Such metal cold was found to contain 982 volumes of hydrogen, corresponding approximately to the formula Pd 4 H 3 . The element does not lose any of its metallic properties by being hydrogenized, but it loses nearly 10 per cent, of its specific gravity. At the same time its bulk is increased about one-tenth. DENTAL APPLICATIONS.— Formerly palladium was used to some extent as a base for artificial dentures. Its lightness (having little more than half the specific gravity of gold), hardness, and resistance to discoloring PALLADIUM. 235 and corroding influences made it desirable; but then it might be had for % its present price — which practically excludes it from the dental laboratory to-day. COMPOUNDS WITH OXYGEN. — Palladious Oxide, the Monoxide, PdO, may be prepared by evapo- rating to dryness, and cautiously heating the solution of palladium in nitric acid. It is of black color, but slightly soluble in acids, and decomposed by high temperature. Palladic Oxide, the Dioxide, Pd0 2 , is not known in the separate state. From palladic chloride solutions alkalis and their carbonates throw down a brown precipitate, hydrated palladic oxide combined with the alkali. The hydroxide is decomposed by high heat; dissolves slowly in acids, forming yellow solutions. ACTION OF ACIDS ON PALLADIUM.— In Hydrochloric or Sulphuric Acid boiling and concen- trated palladium is slightly soluble, forming palladious chloride, PdCl 2 and palladious sulphate, PdS0 4 . Nitric acid dissolves it slowly, but it is more readily soluble in a mixture of nitric and nitrous acids, forming palladic nitrate, Pd (N0 3 ) 4 . Nitro-hydrochloric acid is a ready solvent for this metal, forming palladic chloride, PdCl 4 . The solution is a deep brown color, decomposed upon evaporation with a liberation of chlorine and becoming PdCl 2 . ALLOYS. — With Mercury, in the finely divided state, it readily combines to form a gray plastic amalgam. This union is attended with the evolution of some heat, and is said to result in a definite chemical compound. (See chapter on Amalgams.) Palladium renders its alloys harder and more brittle. These are chiefly used in the manufacture of fine watches, and the most important are those with silver and the so-called palladium bearing-metal. 236 PRACTICAL DENTAL METALLURGY. Gold and palladium combine to form a hard alloy less malleable and ductile than gold in proportion to the amount of palladium it contains. Silver. — An alloy of palladium 9 parts and silver 1 part was used for dental bases, as was also the following: platinum 10, palladium 8, and gold 6 parts. Mr. Fletcher says * * * a silver alloy poor in palladium is worthless, as a large amount of the latter is necessary to protect it from sulphuretted hydrogen. And, further, that pure palladium is the best metal known for plates for artificial teeth, owing to its high specific heat, its extreme light- ness and hardness, requiring no alloy, and also to its absolute freedom from tarnish. The same author further says: " As an alloy, the pres- ence of palladium in small quantities is frequently objec- tionable, * * * almost inadmissible, even in so small a proportion as 1 to 2000 in silver for making amalgams." With nickel it forms a malleable alloy susceptible to high polish. With antimony, bismuth, tin, zinc, iron, and lead it combines to form very brittle alloys. Palladium bearing-metal. — An uncommonly hard alloy used as bearings for fine watches, and is said to produce less friction upon arbors of hard steel than the jewels generally used. The composition is, palladium 24, gold 72, silver 44, and copper 92. TESTS FOR PALLADIUM IN SOLUTION.— Salts of palladium may be discriminated by: — Sulphuretted hydrogen or ammonium hydro-sul- phide, which throws down a black precipitate of palladious sulphide, insoluble in alkaline sulphides, but soluble in hydrochloric acid. Potassium iodide gives a black precipitate of palla- dium iodide from palladious chloride. PALLADIUM. 237 Potassa or soda yields a red precipitate soluble in excess of alkali if heated. Mercuric cyanide, the characteristic test, gives a yel- lowish-white gelatinous precipitate of palladious cyanide from solutions of palladious chloride, which is soluble in hydrochloric acid. Nearly all the compounds of palladium are reduced by heat, before the blow-pipe^ to a "sponge." If this be held in the inner flame of an alcoholic lamp it absorbs carbon at a heat below redness; if then removed from the flame it glows vividly in the air, till the carbon is all burnt away — (distinction from platinum). ELECTRO-DEPOSITION OF PALLADIUM.— The metal may be deposited from a cyanide of potassium solution by the battery process, yielding thick metallic deposits in a white reguline state. CHAPTER XIX. PLATINUM. Platinum. Symbol, Pt. Valence, II, IV. Specific gravity, 21.46. Atomic weight, 194.41. Malleability, 6th rank. Melting point, oxyhydrogen name, 1770° C. Ductility, 3d rank. Tenacity, 3d rank. Conductivity (heat), 8.4. Conductivity (electricity), 18.8. (Silver being 100.) Specific heat, 0.0322. Chief ore, Polyxene. Color, bluish silver-white. Crystals, octahedral. OCCURRENCE.— The ore of platinum, "polyxene," which is a most complex mixture of a number of heavy reguline species of platinum, osmiridium, iron-platinum, platin-iridium, iridium, palladium, gold, and a number of non-metallic species, notably chrome-iron ore, mag- netic iron oxide, zircone, corundum, and occasionally also diamond, is found in the province of Choco, South America, where it was first discovered in 1736, in New Granada, Barbacos, California, and Australia, but chiefly in alluvial deposits in the Ural district. The vari- able percentages of the several components range ap- proximately as follows: Platinum, 60 to 87; other polyxene metals, 3 to 7; gold, 2 and more; iron, 4 to 12; copper, to 4; non-metallic gangue, 1 to 3. Gold is separated by amalgamation when it exists in any con- siderable quantities. Platinum is rarely found in large nuggets, but usually in granules, which are generally small, but occasionally assume considerable dimensions. The Demidoff Museum contains a native platinum lump weighing 21 pounds troy. PLATINUM. 239 REDUCTION.— The extraction of platinum is accom- plished by two distinct methods. The first, devised by Wollaston, which produces the purest metal, is more of a chemical than a metallurgical process, a modification of the method by Herseus is as follows: The ore is digested within glass retorts in dilute aqua regia, by which the platinum, palladium, part of the iridium, and more or less of the other metals pass into solution, the platinum, palladium, and iridium as tetrachlorides. The solution is then evaporated to dryness, and the residue heated to 125° C, to reduce the palladic and iridic chlor- ides to the lower stages of PdCl 2 and Ir 2 Cl 6 , which form soluble double salts with sal-ammoniac. The heated residue is dissolved in water acidulated with hydrochloric acid; the solution filtered, and mixed with a hot con- centrated solution of sal-ammoniac, when a quite pure chloroplatinate, PtCl 6 (H 4 N) 2 , comes down in a yellow precipitate, which is washed first with a saturated sal- ammoniac solution, then with hydrochloric acid. This precipitate needs only to be exposed to a dull red heat to be converted into "spongy platinum," i. e., metallic platinum in the form of a gray, porous mass. The second method, known as Deville's, is based upon the tendency of platinum to be dissolved in melted lead. The ore is fused with an equal weight of galena (sulphide of lead) and as much of the oxide of lead. The sulphur and oxygen escape as sulphur dioxide, the reduced lead dissolving the platinum, and leaving the very heavy alloy of osmium and iridium to sink to the bottom un- dissolved. The upper portion containing the platinum is then ladled out (see chapter on Silver) and cupelled, when the latter metal is left in a spongy mass, the lead having passed off as the oxide. 240 PRACTICAL DENTAL METALLURGY FUSING PLATINUM.— By means of the oxydrogen blow-pipe the spongy platinum is easily reduced to a compact mass, where formerly it was only obtained so, relatively, by the very tedious and laborious means of welding. The furnace for fusing platinum (Fig. 34) is at once a cupel and furnace, consisting of two thoroughly burned lime blocks with a basin-like concavit3 r in each, and fitted one over the other. The concavity in the lower block forms the bed of the furnace, and is provided with a gut- ter leading from the basin to the outside. Through the top is passed the oxyhydrogen blow-pipes. They each consist of two concentric tubes, or a smaller within a larger. Through the outer, or larger, tube the hydrogen or illuminating gas is passed, and through the inner, or PLATINUM. 241 smaller, the oxygen is forced into the center of the flame. The tubes are of copper, tipped with platinum. Platinum scraps are melted by first heating up the furnace and then introducing them through an opening in the side. In the case of platinum sponge the mass is introduced before heating up the furnace, and it is here that the furnace acts as a cupel; the impurities remaining in the metal are oxidized and volatilized or absorbed by the lime of the furnace. The temperature produced is supposed to be about 2870° C. Osmium does not melt at this point, but it, with palladium and gold, is volatilized, if present. Platinum may also be fused by placing the metal between the carbon tips of an arc light. EXPERIMENT No. 65. — Fuse platinum under flame of oxyhydrogen (or oxygen and illuminating gas) blow-pipe. EXPERIMENT No. 66.— Fuse platinum by means of the electric cur- rent. EXPERIMENT No. 67.— Tin a piece of platinum by dipping it into molten tin. Observe that the tinned platinum -will melt over the Bunsen flame like wax. When platinum is added to dental-amalgam alloys containing tin there need be no fear that it will not be melted. PROPERTIES.— The metal is bluish silver-white, about as soft as pure copper, and has a specific gravity of 21.46. It is tough, ductile, and malleable, and may be rolled or beaten into foil or drawn into wire of almost microscopic fineness. Dr. Arendt states* that a cylinder of platinum, one inch in diameter and five inches long, may be drawn into a wire sufficiently long to encircle the earth at the equator. The fine wire used in the microm- eter eye-piece of microscopes suggested by Wollaston is made by drawing a composite wire of platinum coated with silver to its greatest attenuation, and then dissolv- *Dr. Kirk in American System of Dentistry, Vol. Ill, p. 887, from Anorgan- ischen Chemie. 242 PRACTICAL DENTAL METALLURGY. ing off the silver in nitric acid. Platirmm possesses tenacity in a high degree, being just inferior to iron and copper. The fusing point, according to Violle, is 1779° C. When heated much above its fusing point, it soon begins to volatilize. The fused metal, like silver, absorbs oxy- gen, and consequently "spits" on freezing. At a red heat it "occludes" hydrogen gas. The volume of hy- drogen absorbed by unit volume of metal at a red heat, under one atmosphere's pressure, was found, in the case of the fused metal, to vary from 0.13 to 0.21, volume measured cold; in the case of merely welded metal from 2.34 to 3.8 volumes. Oxygen, though absorbed by the liquid, is not occluded by the solid metal at any tempera- ture, but when brought in contact with it at moderate temperatures, suffers considerable condensation at its sur- face, and in such a state exhibits a high degree of chem- ical affinity. When a jet of hydrogen gas strikes a layer of spongy platinum it causes it to glow and take fire. The most striking effect of the metal for absorbing gases is demonstrated in the finely divided state known as "platinum black." This state is produced by dropping platinum chloride solution into a boiling mixture of 3 parts of glycerine and 2 of caustic potash. Platinum black is said to absorb 800 times its volume of oxygen from the air, and is, therefore, a most active oxidizing agent, acting catalytically, i. e., after having given up its oxygen to the oxidizable substance it takes up a fresh supply from the atmosphere. DENTAL APPLICATIONS.— Platinum was intro- duced in France as early as 1820 for a base in con- tinuous-gum work. Its low rate of expansibility under increased temperature, its coefficient being about equal to that of glass, and its very high fusing point make it most useful for continuous-gum work, and for pins for PLATINUM. 243 artificial teeth. Its comparatively great resistance to chemical agents insures it against corrosive action, and places it on an equality with gold for dental bases, crowns, and bridge-work. Platinum and gold is used as a filling material in the form of platinum and gold folds, platinized gold folds, and platinum and gold foil. The color of this material inserted as a filling is not as beautiful as that of either gold or platinum alone. The yellow of gold having almost entirely disappeared leaves an ashen effect to the platinum, which very much detracts from its character- istic color and appearance. The advantage it possesses over gold is its tough, resistant property — making a sur- face that will stand the abrasive action of mastication much better than gold. Coils of platinum are useful in dental offices in various forms of electric heating devices. The heat is free from products of combustion and can be most accurately con- trolled. A device of this nature is especially valuable in annealing gold. COMPOUNDS WITH OXYGEN.— Platinum forms two compounds with oxygen. Platinous Oxide, or Monoxide, PtO, is obtained by digesting the platinous chloride with caustic potash as a black powder, soluble in excess of the alkali. It is not known in the separate state; is a feeble base, and decom- posed by heat, leaving metallic platinum. Platinic Oxide, or Dioxide, Pt0 2 , also a weak base, occasionally acting as an acid; hence, it is sometimes termed platinic acid. It is best prepared by adding barium nitrate to a solution of platinic sulphate; barium sulphate and platinic nitrate are thus formed, and from the latter caustic soda precipitates one-half of the platinum as platinic hydroxide. The hydroxide is 244 PRACTICAL DENTAL METALLURGY. a bulky brown powder, which, when gently heated, becomes black and anhydrous. If this oxide be dissolved in dilute sulphuric acid and the solution mixed with excess of ammonia, a black precipitate of fulminating (explosive) platinum is obtained, which detonates vio- lently at about 400° F. ACTION OF ACIDS ON PLATINUM.— The metal is not sensibly tarnished by sulphuretted hydrogen vapor or solution; and is not attacked at any tempera- ture by nitric, hydrochloric or sulphuric acid; but it dissolves in nitro-hydrochloric acid to form platinic chloride. It is, however, less readily soluble in this acid than gold. ALLOYS. — Platinum alloys with most of the metals. With mercury spongy platinum unites to form an ex- ceedingly unctious amalgam. It does not unite readily, and its union is best accomplished by continuous rubbing in a warm mortar. Iridium from 10 to 15 per cent, added to platinum greatly increases its hardness, elasticity, infusibility, and resistance to chemical action. Platinum alloyed with iridium can be made very useful in dentistry to strengthen weak parts of partial continuous-gum and partial vul- canite dentures. An alloy of 78.7 platinum and 21.3 iridium will withstand the action of aqua regia. Equal parts of the metals form a very brittle alloy. Gold and platinum form an alloy of great value for the construction of dental bases. Platinum gives to gold a greater hardness and elasticity. Two parts to one of gold forms a brittle alloy, while with equal parts the alloy is malleable. Prinsep found that 7 parts of gold and 3 parts of platinum. formed an alloy infusible in the strongest blast-furnace. Gold 11 parts and platinum 1 part form a grayish-white alloy, having somewhat the appearance of tarnished silver. PLATINUM. 245 Silver. — By small additions of platinum to silver its pure white color is changed to a gray, and its hardness is increased. The alloys are difficult to make, on account of the separation of the platinum, owing to its greater specific gravity. Platine au titre, an alloy composed of from 65 to 83 per cent, of silver, has been used for dental bases in pref- erence to coin silver, on account of its resistance to chemical action and its greater elasticity. Nitric acid will dissolve an alloy of silver and platinum when the latter is not present, to exceed 10 per cent. Cadmium and platinum unite, to form a definite compound, having the formula of PtCd 2 . Copper and platinum, equal parts, form a gold-col- ored alloy tarnishing in air. Lead and tin unite in all proportions with platinum. Those of tin are hard and brittle, with comparatively low fusing points. Those of lead are harder, whiter and tougher than pure lead. TESTS FOR PLATINUM IN SOLUTION.— Sul- phuretted hydrogen throws down, after heating, a blackish-brown precipitate. Potassium or ammonium hydrate each throws down a very characteristic yellow crystalline precipitate, the former soluble in large excess of precipitant, and the latter, when dried and heated, yielding metallic platinum. By the reducing blow-pipe flame, the compounds of platinum are reduced to spongy platinum. ELECTRO-DEPOSITION OF PLATINUM.— Good, thick, reguline deposits of platinum may be obtained from a cyanide solution made by dissolving the chloride in a solution of potassium cyanide. The anode is not dissolved; therefore, the salt must be replaced. Zinc and iron precipitate the metal in a finely divided state. CHAPTER XX. GOLD. Aurum. Symbol, Au. Valence, I, III. Specific gravity, 19.265. Atomic weight, 196.15. Malleability, 1st rank. Melting point, 1100° (2012° F.). Tenacity, 5th rank. Ductility, 1st rank. Chief ore — found native. Conductivity (heat), 53.20. Conductivity (electricity), 77.96. (Silver being 100.) Specific heat, 0.0324. Crystals, octahedral. Color, yellow. OCCURRENCE.— Gold is found in nature chiefly in the metallic state, or as native gold, and. less frequently in combination with tellurium, lead, and silver. It is also found combined, or, perhaps, more strictly speak- ing, minutely mixed with pyrites and other sulphides, more commonly called " sulphurettes. " Native gold occurs rather frequently in crystals be- longing to the cubic system, the octahedron being the commonest form, but other and complex combinations have been observed. Large crystals are rarely well de- fined, owing to the softness of the metal, the points being commonly rounded. The most characteristic forms, however, are the nuggets or pepites. These, when of a weight less than one-quarter to one-half an ounce, are kno wn as gold dust. Kxcept the larger nuggets, which are usually more or less angular or irregular, gold is generally found in a bean-shaped or somewhat flattened form, the smallest particles being scales of scarcely appreciable thickness, and owing to their small bulk, as compared with their surface, they are frequently suspended in water and may be washed away by a rapid current; hence, they are known afloat gold. GOLD. 247 In the museum of the Mining Bureau in San Fran- cisco are several plaster of Paris models of famous gold nuggets found in the various gold regions of the world. The largest single piece of gold ever found was taken out at Ballarat, Victoria, Australia. It weighed 2166 troy ounces, and was valued at $41,882. The second largest was discovered in the Ural Mountains district, and weighed 1200 ounces. The third largest, which was also found in Victoria, Australia, weighed 1121 ounces, and was valued at $22,000. The physical properties of native gold are quite similar to those of the melted metal and its alloys. The com- position varies considerably in different localities as shown in the following table: ANALYSIS OF NATIVE GOLD FROM VARIOUS LOCALITIES. Locality. Gold. Silver. Iron. Copper. EUROPE: British Isles — Vigra and Clogau Wicklow (River) Transylvania ASIA: Russian Empire — Brezovsk 90.16 92.32 60.49 91.88 98.96 90.05 94.00 88.05 76.41 90.12 81.00 84.25 87.78 99.25 9.26 6.17 38.74 8.03 0.16 9.94 5.85 11.96 23.12 9.01 18.70 14.90 6.07 0.65 Trace .78 Trace .05 Trace 0.77 .09 Ekaterinburg .35 AFRICA: Ashantee AMERICA: Brazil Central America Titiribi 0.87 California Mariposa Cariboo 6.15 .03 AUSTRALIA: South Australia Ballarat 248 PRACTICAL DENTAL METALLURGY. The most important minerals containing gold are: Sylvanite, or graphic tellurium, (AgAu)Te 2 , con- taining 24 to 2 6 per cent: Calaverite, AuTe 2 , containing 42 per cent.; Nagyagite, or foliate tellurium, of a complex and rather indefinite composition, and containing from 5 to 9 per cent, only of gold. The calaverite, a nearly pure telluride of gold, has been found to some considerable extent in Calaveras County, California. The minerals of the second class, called auriferous, are comparatively numerous, and include many of the metal- lic sulphides. The most important of these are iron pyrites and galena; the first of these is of great practical importance, being found in many districts exceedingly rich, and, next to the native metal, is the most prolific source of gold. A Native Amalgam of gold is found in California, but rarely in any considerable quantities. Gold is so widely distributed throughout the earth's crust that few regions may be said to be destitute of slight traces of it; yet it has been found in comparatively few localities in quantities sufficient for economical ex- traction. The principal supplies of the metal have been derived from Africa, California, Australia, Mexico, Bra- zil, Ural Mountains, Transylvania, etc. California was for many years chiefly known to the world as the region where gold was found in extraordi- narily large quantities. Great excitement was occa- sioned by the discovery of the precious metal in Januar)?-, 1848, and its subsequent extraction from the placers of the Sierra Nevada Mountains. The gold regions of Cali- fornia are the mountain counties lying between Shasta and lessen on the north, and Fresno on the south. At GOLD. 249 the time of their greatest productiveness the yield reached about $65,000,000 in value a year; this was from 1850 to 1853. The association and distribution of gold may be considered under two different heads ; namely, as it occurs in mineral veins, and in alluvial or other super- ficial deposits which are derived from the waste or disin- tegration of the former. As regards the first, it is usually found in quartz veins or reefs traversing slaty or * crystalline rocks, either alone or associated with such metals as iron, copper, tellurium, and rarely bismuth, or such minerals as magnetic and arsenical pyrites, galena, specular iron ore, and silver ore, and rarely with the sulphides of molybdenum, tungstate of calcium, bismuth, and tellurium minerals. In the second or alluvial class (placers) of deposits it is associated chiefly with those minerals of great density and hardness, such as platinum, osmiridum, and other metals of the platinum group, tinstone, chromic, mag- netic, and brown iron ores, diamond, sapphire, ruby, topaz, etc., which represent the more durable original constituents of the rocks whose disintegration has furnished the detritus. MINING AND EXTRACTION.— The simplest and oldest form of mining and extracting the precious metal is known as — Placer mining, which consists of washing the allu- vial deposits, sands of rivers, and other earthy matter, by which the lighter particles of earth and sand are washed away, while the gold in irregular and flattened grains by its gravity remains. In the early days of California, when rich alluvial de- posits were common at the surface, the simplest appli- ances sufficed, the most characteristic of which was — 250 PRACTICAL DENTAL METALLURGY. The "pan" a circular dish of sheet-iron, with sloping sides about 13 or 14 inches in diameter. The pan, about two-thirds filled with pay dirt to be washed, is held in a stream or in a pool of water. The large stones separated by hand; the pan is given a twisting lateral motion, keeping the contents suspended in the stream to remove the lighter substances, the heavier gold remaining on the bottom. This process is termed panning out. The " cradle" is a simple contrivance based upon the same principle for treating somewhat larger quantities. The " torn " is a sort of cradle with an extended sluice placed on an incline. Under certain circumstances mer- cury is used in the sluice to amalgamate the gold. The "sluice," a Californian invention, is used in work- ing on a larger scale, where the supply of water is abundant. The simplest form of this consists of a rect- angular trough of boards set up on trestles at an inclina- tion that the stream of water may carry off all but the largest stones, which are kept back by a grating, and removed by hand as they accumulate. The floor of the sluice is provided with riffles made of strips of wood laid parallel with the current, and at other points with boards having transverse notches filled with mercury for the accumulation of the gold. The length of the sluice depends, of course, upon the volume and flow of water, the ordinary ones ranging from 100 to 500, and even to 1000 feet in length, while the sluices leading from hydraulic operations are sometimes a mile in length. Hydraulic Mining. — This method is also a Californian invention, and has for the most part been confined to the placer mining of this State. The method is employed where an abundance of water is available, and where thick banks of auriferous gravel are to be removed; it consists in loosening and washing away banks of gravel, GOLD. 251 sand, and soil with powerful streams of water discharged from nozzles resembling those of a fire erjgine. It is supplemented by the use of gunpowder for breaking up and removing "bed rock," immense boulders, etc., and arrangements must be made for saving the gold without interrupting the flow of water, and for disposing of the vast masses of impoverished gravel. The stream from the site of operation laden with stones and gravel passes into sluices where the gold is recovered in the manner already described. Quartz mining does not greatly differ from the meth- ods employed in the extraction of similar deposits of other metals. The quartz is first reduced to a very fine powder; this is accomplished in the most productive regions, such as California and Australia, by means of the stamp mill. In this operation cylindrical iron pestles, weighing from 600 to 800 pounds, are lifted by means of cams, and allowed to fall some 8 or 10 inches at the rate of from 30 to 90 blows per minute upon the quartz. A stream of water carries the comminuted material in con- tact with mercury, which, on account of its great affinity for the gold, absorbs and separates it from the earthy gangue. To prevent the " sickening " and "flouring" of the mercury, which is produced by certain associated minerals in the ore, and which occasions much annoyance and some considerable loss of both gold and mercury, by greatly diminishing the solvent powers of the latter metal, a small quantity of the amalgam of sodium is added to the mercury. Before the solvent mercury becomes saturated it is re- moved and subjected to powerful pressure in leather bags, when the excess is squeezed out through the pores of the leather, leaving the more or less coherent mass of rich amalgam inside. The mass is then heated in a 252 PRACTICAL DENTAL METALLURGY. proper vessel, when the mercury is distilled over and re- condensed in iron retorts. The gold is left in a spongy state usually quite free from other metals, except silver, which is separated by the " parting process," to be sub- sequently described. The spongy gold with its silver content is then melted in plumbago crucibles with the addition of a small quantity of suitable fluxes and shipped as bullion. In some cases it has been found advantageous to smelt the ore by fusing it with lead, which latter in the fused state has a very great affinity for gold. In such opera- tions the crushed ore is mixed with suitable proportions of metallic lead or litharge and charcoal, together with some lime or clay as a flux for the silica and fused on the hearth of a reverberatory furnace. The melted lead dis- solves the particles of gold, just as mercury does, and sinks beneath the lighter slag; is drawn off and afterward separated from the gold by cupellation. Chlorination Process. — Under some circumstances it is found best to separate the gold from the quartz in a wet way by means of chlorine. The process depends upon the fact that chlorine acts rapidly upon gold, but does not attack ferric oxide, and is now adopted in Grass Valley, California, where the waste minerals, principally pyrites, have been worked for a considerable time by amalgamation. The ore is roasted at a low temperature in a reverbera- tory furnace, during which salt is added to convert all the metals present, except iron, into chlorides. The auric chloride is, however, decomposed at the elevated temperature, and the finely divided particles are readily attacked by the chlorine gas. The roasted mineral, slightly moistened, is then introduced into a wooden vat which is provided with a double bottom. Chlorine gas gold. 253 is led from a generator beneath the false bottom, and rises through the moistened ore, converting the gold into a soluble chloride which is afterwards removed by wash- ing with water. The noble metal is then precipitated by the sulphate of iron. The method is very accurate and yields metal of great purity. REFINING GOLD.— The accumulation of gold in the form of scraps, filings, etc., in the dental laboratory and operating-room frequently becomes a source of con- siderable loss to the dentist, on account of unfamiliarity with the methods of refining, and lack of convenience and apparatus necessary to its several processes. Some forms of scrap-gold, such as old fillings, need only to be melted with. the proportion of silver, copper, or both, to produce the desired alloy. Others, as scrap- plate of known carat, may be utilized by simply remelt- ing and rolling. Old crowns, plates, bridges, mixed filings containing more or less iron from the file, zinc, lead, antimony, and other base metals may be converted into malleable gold by simply roasting with such fluxes as will combine chemically with the base metals and remove them. Sweepings may be washed and then carried through the same process, which is known as THE ROASTING PROCESS.— A method for roughly refining and rendering brittle gold malleable. This pro- cess may be most satisfactorily employed where the ap- proximate carat of the bulk of the scraps is known and the gold is suspected to be unworkable, owing to the admixture of base metals. The larger pieces should be removed from the accu- mulation and the smaller ones with the filings freed from as much iron and steel as possible by a good magnet. All should then be placed in a previously well-boraxed 254 PRACTICAL DENTAL METALLURGY. and tried graphite crucible, with the addition of sufficient potassium carbonate to well cover the charge; the object of this addition being to form a thin flux, permitting the small particles and filings to sink and accumulate in one mass. The furnace should be placed beneath a fume-chimney or by a window with an outward draught, that the fumes escaping from it during the roasting may not fill the laboratory, thereby endangering the health of the students or operator and damaging such instruments and tools as may be unprotected. The most convenient place to avoid such results is the fire-place. The furnace may be placed beneath its chimney in such a manner that all fumes will be readily carried off. When the metal has become thoroughly fused, the refining process may be begun by first adding small quantities of the oxidizing agent, potassium nitrate, (KN0 3 ), accompanied with borax as needed to properly protect the mass and further the process. The object of the potassium nitrate is to furnish sufficient oxygen to oxidize the contaminating base metals beneath the flux, thus separating them from the gold. As most base metals are easily oxidized under these circumstances, a continuation of this process from ten minutes to one hour and a half, according to the quantity of material, and the proportion of base metals contained, adding the niter and borax as required, and maintaining a perfect fusion of the metal, the ingot, when made by pouring into a previously warmed and oiled mold, will be found to be quite malleable. If, however, upon examination it is found to be still brittle, it should be placed in a clean, boraxed, and tried crucible, heated, and brought to a perfect state of fusion. A mixture of equal parts of finely pulverized vegetable gold. 255 charcoal and amnionic chloride should then be added; at first sufficient to properly cover and protect the molten mass and afterwards a small quantity at a time as it is needed. When the metal has been sufficiently treated, which may be determined by removing small quantities and subjecting them to the physical tests for malleability, the crucible is to be removed from the furnace and the metal cast into an ingot or allowed to cool in the crucible as a button. The rationale of such a process is that the heat of the crucible breaks up the chloride compound, liberating the chlorine in the nascent state; which in turn combines with the metals lead, tin, and silver contained in the gold to form chlorides respectively. These are either volatilized or taken up by the flux, the gold remaining free of them. Mercuric chloride is sometimes used when the contam- ination of the gold with lead or tin is extensive, or where it is desired to remove a quantity of silver. But its use is so dangerous on account of the fumes evolved it is rarely employed. Sulphur or antimonic sulphide is used to abstract large quantities of silver from gold, by combining with the former to form the fusible sulphide of silver, leaving the gold free, or if the antimonic sulphide has been used, contaminated with antimony, which may be removed by fusing with borax and potassium nitrate, as previously described. In the process of refining by fluxes, the first step should be to determine, as far as possible, the nature of the debasing elements; this being known or reasonably approximated, the process may be confined to the par- ticular flux most likely to free the gold from its contam- ination. Iron, steel, zinc, copper, antimony, and bismuth 256 PRACTICAL DENTAL METALLURGY. are, perhaps, best removed by oxidation through the agency of potassium nitrate. Lead, tin, and silver are removed by chlorine, forming volatile compounds with that element. If, after such treatment, the alloy is found to be malle- able, but stiff or elastic, or dull in color, it very probably contains some platinum which cannot be removed by this means, but which may be gotten rid of by a wet method. When desired, such an alloy may be made direct use of as clasp gold. When the object is to produce pure gold from which to subsequently prepare desired carats by alloying the result, it is best and most conveniently attained by the process known as PARTING GOLD.— A wet method for refining gold by inquartation, or "quartation," as it is more commonly known. This is accomplished by digesting the thinly rolled or granulated alloy of silver and gold in either nitric or sulphuric acid. The student, in his choice of metal for this operation, may endeavor to obtain gold containing as much silver as possible, and, as this will require an additional quantity of the latter metal fused with it in order to carry out the operation, it is of course an object, if pos- sible, to employ silver which contains small quantities of gold, and thus, as it may be said, to carry on a double refining process at once. As the actual separation of the two is effected by digesting the mixture in hot nitric acid, which, while it is a ready solvent for other metals, is inactive upon gold, it may be asked: Why not at once treat the alloy with acid without such alloying ? Such would be quite use- less, for, the foreign metals being in so small a relative proportion, the acid would only remove the alloy at or GOLD. 257 near the surface, the metal being sufficiently close in texture to mask all the rest from the action of the acid. The sulphuric acid process is doubly recommended, especially when large quantities of the alloy are to be digested, as it is less expensive, and the gold is obtained of a greater degree of fineness. The oxidizing action of the nitric acid is of especial value, however, when tin or antimony is present in the batch of metal. Preparation of the Alloy. — The impure gold is first weighed and the approximate weight of the silver, if it contains any, subtracted; silver is then added in the pro- portion of three to one, less the amount already contained in the alloy, thus when melted forming an alloy of three parts silver and one part impure gold. Hence the term " quartation." These proportions are then fused together in a clean and boraxed crucible, well mixed, and either poured into warmed and oiled ingot- molds, to be subsequently rolled, or dropped while molten from the crucible into a wooden tub or tank of cold water for the purpose of granulation. The latter is unques- tionably the simplest method of preparing it for the digest- ing process, for, if poured into the ingot-molds, the alloy will require rolling to a very thin ribbon (No. 35 gauge), after which it must be cut into small pieces. The roll- ing many times is impossible, because of the gold that it is desired to refine being exceedingly brittle. The alloy being thus prepared, is ready for the acid. Nitric Acid Process. — For this process the prepared alloy is placed in a Florence flask and nitric acid to the amount of about one and one-half times the weight of the alloy poured on. The acid should always be tested for chlorine by adding a drop of the solution of silver nitrate (AgN0 3 ) to it, which, if chlorine be present, will in- stantly be rendered milky from the precipitated chloride 258 PRACTICAL DENTAI, METALLURGY. of silver. Heat the flask gently in a sand-bath over a Bunsen or alcohol flame. Copious red fumes of the oxides of nitrogen and ammonium will be given off, show- ing vigorous action on the alloy, and the silver and other metals will be dissolved, leaving the gold in a spongy mass of a blackish-brown color. When this evolution has entirely ceased and the flask is clear, carefully decant the solution of the nitrates of silver, etc., thus formed and preserve it, adding a fresh portion of nitric acid and boil until all fumes cease to rise, which marks the termination of the digesting process. The acid is now replaced by distilled water two or three times, for the purpose of washing the gold remaining. At length filter the contents of the flask, catching the gold on the filter paper, add a sufficient quantity of potassium carbonate, fold the paper over the whole, and place in a previously boraxed crucible, melt and pour into warmed and oiled ingot-molds. Gold thus refined may reach 998-1000ths fineness, and is ready for any desirable alloying. For the recovery of the silver, see chapter on that subject. Sulphuric Acid Process. — The use of sulphuric acid for the operation is preferred by many. For, as was stated, it is more economical; and the gold so reSned is more thoroughly freed from silver; indeed, it is said that gold having been previously refined by the means Of nitric acid may be freed of still more silver by this acid. In operating the metals are so mixed that the gold amounts, at most, to not quite half the weight of the silver; and if copper is contained, (which in small propor- tions facilitates the operation), it should be under 10 per cent, for, if too much be present, a large quantity of sulphate of copper will be formed, which latter is insolu- GOLD. 259 ble in the strong acid liquors. The process may be employed for silver containing very small quantities of gold. Thus, in France, it was found very profitable to separate the gold from old five-franc pieces, which con- tained only l-1000th to 2-1000ths of gold. The alloy having been granulated, as before described, is introduced into a digester (Florence flask) with about two and one-half times its weight of concentrated sul- phuric acid. This is allowed to boil, during which strong action is evidenced by copious evolution of sulphur dioxide, while the silver and copper are simultaneously converted into sulphates. This first boiling is continued as long as sulphur dioxide is evolved, which in large quantities of metal will commonly go on about four hours. The liquid is then removed and a smaller quan- tity of acid added, the boiling being further carried on for a short time, after which the digester is allowed to remain at rest, in order that the gold may subside. Sometimes it may be requisite to use even a third acid. Repeated washing of the gold with boiling water is now necessary, as the sulphate of silver is a very insoluble salt, and sulphate of copper, when contained in so acid a menstruum, is also somewhat so. The gold is then dried, melted, and poured, as described before. This process affords gold as pure as 998.5-1000ths. THE PREPARATION OF CHEMICALLY PURE GOLD. — The metal, either, in the form of powder, gran- ulations, thin plate, or "cornets" from the purest gold that can be obtained, is dissolved in chemically pure nitro-hydrochloric acid.* The best material to operate on is gold which has been refined in the ordinary way; this may be used in the form of a powder, as it is * One volume of nitric to two of hydrochloric acid, (or any proportion, so the latter is in excess). 260 PRACTICAL DENTAL METALLURGY. precipitated in the last process, as granulations or as plate. The acid for small quantities is best contained in an evaporating dish placed in a sand-bath upon a tripod, over the flame of a Bunsen burner, beneath a chimney or near an open window. The action will be tolerably energetic when the metal is first introduced; hence, it is not necessary to ignite the burner at the start, but as the action slackens a moderate heat may be applied. Instead of previously mixing the acids, the hydro- chloric acid may first be poured over the metal, and the nitric acid afterward gradually added in small portions, the function of the nitric acid being to oxidize the hydro- gen of the hydrochloric acid, converting it into water, while the chlorine, which is the active solvent, is liberated in the nascent state and unites with the gold, converting it into auric chloride, which dissolves.* Bach ounce of gold will require about three and one- half ounces of mixed acid for its solution. During the process of solution a sediment will be noticed in the bot- tom of the evaporating dish, which will be recognized by the operator as a silver chloride, formed .by the union of the silver contained in the gold and the liberated chlorine. It must not be expected that all the silver will be directly precipitated to the bottom as a chloride, for the liquor is strongly acid, and some may be held in solu- tion. Therefore, this must be taken into consideration, and subsequent pains taken to throw it down by the thorough evaporation of the nitric acid. The gold hav- ing been dissolved, the solution is now best transferred to a clean dish by decantation, leaving the chloride of silver in the first and the solution contained in the second dish heated to further evaporate. When about one-third is evaporated more chloride of silver will be * Dr. E). C. Kirk, American System of Dentistry. GOLD. 261 found to have been separated from the solution and precipitated. It is well, therefore, to again transfer the solution to a third dish by decantation and evaporate as before, care always being maintained during the heating not to apply so great a temperature as to decompose the auric salt. As the bulk is reduced over the gentle heat by evapora- tion, small quantities of hydrochloric acid are to be added from time to time, which has the effect of liberat- ing nitrous anhydride by decomposing the remaining nitric acid in the liquor; these additions must, however, be made very cautiously, for the action produced is very energetic, and, without due precaution, considerable por- tions of the now rich liquor will be spirted out of the dish and lost. When the liquor has become of a deep red color, and of the consistency of syrup, it is to be withdrawn from the heat and permitted to rest for a time, when the whole of the auric chloride will crystallize, forming a mass of prismatic crystals.* The bottom of the dish is now carefully wiped off to remove any sand or dirt that may have collected there from the sand-bath, and the dish and its contents im- mersed in about a half pint of distilled water, acidu- lated slightly with hydrochloric acid. It is better now to let this solution stand a week, for chloride of silver, although slightly soluble in a very strong and hot acid solution, is separated by dilution, and, by allowing this rest, it will completely subside in the vessel. At the end of this time the solution must be filtered to remove any foreign substance, together with the silver chloride. The filtrate will then be seen to be a rich straw-yellow, and the gold it contains is ready for precipitation. * Makins' Metallurgy. 262 PRACTICAL DENTAL METALLURGY. Precipitating the Gold. — The solution is now best contained in a large glass flask, and the precipitating reagent added. As gold is one of those metals which, as a base, combines with very feeble affinities, it is conse- quently not only very easily separated, but the physical conditions of the precipitate may be much modified and controlled by the nature of the precipitant, as also by the mode of operating. Thus gold may be thrown down in a powder, in scales, in more or less of a crystalline state, in a tolerably compact sheet or foil, or lastly, in a spongy condition. And these states may be attained with some degree of certainty, although the circumstances determin- ing the more compact forms are hardly yet well under- stood. Spontaneous precipitation may take place to some ex- tent in a vessel of trichloride of gold when exposed to the air; and thus the sides of the vessel containing it will slowly become covered with the deposit. This is prob- ably due to the action of the nitrogen of the air. Many elementary substances will precipitate gold from the tri- chloride. Most of the lower metals reduce it, some metallic salts throw it down, and many organic bodies readily precipitate it. Thus sugar when boiled in it gives a first a light red precipitate, which afterwards darkens in color.* Practically, however, ferrous sulphate or oxalic acid are the only precipitants used. The oxalic acid is pre- ferred, and is an excellent precipitant. The gold salt, being in solution, is broken up by the addition of a strong solution of oxalic acid, and the gold is precipitated to the bottom as either a crystalline mass or a leafy foil. It is necessary to add a slight excess, and the whole should be kept at a gentle heat in a sand- * Makins' Metallurgy. GOLD. 263 bath over a flame. Soon after the application of heat some slight bubbling is noticed, a copious evolution of gas takes place, and at the same time the body of the liquid appears filled with most delicate spangles of me- tallic gold, which become coherent as they descend, and in consequence assume most any one of the forms above mentioned. The gas noticed to escape is C0 2 , from the compound, oxalic acid. The reaction is of the simplest — an acid on a binary salt — 2AuCl 3 + 3C 2 H 2 4 =6HC1+ 6C0 2 + 2 Au. "The action of this precipitant being gradual, and capable of much regulation, by the amount and nature of heat employed, while it is also peculiar in being at- tended throughout by this evolution of gas which rises quickly through the solution, there is produced from the former cause a tendency in the metal to deposit in a crystalline or crystallo-granular state; while from the latter a more or less spongy character is given to it: hence it will be readily seen that inasmuch as we are able to modify these conditions, so we can in the same degree influence the molecular nature of the result."* Where ferrous sulphate is used about four times the weight of the gold will be necessary for precipitation. This may be dissolved quickly in hot distilled water and added to the gold solution. The precipitate thrown down is of a brown color, and will, on being gently burnished with the finger-nail, assume that metallic golden luster characteristic of the metal. The following is the reaction — 2 AuCl 3 + 6FeS0 4 =Fe 2 Cl 6 + 2Fe 2 (S0 4 ) 3 + 2 Au. After the solution has fully subsided from the disturb- ance caused by addition and precipitation a quantity of * Makins' Metallurgy. 264 PRACTICAL DENTAL METALLURGY. hot hydrochloric acid may be added, and much of the supernatant liquor removed, either with a siphon or by decantation, and the remainder of the solution and pre- cipitate poured upon the filter paper. The precipitate is afterwards washed with hydrochloric acid, distilled water, aqua ammonia, and again with distilled water. The necessity of this is apparent, especially in the use of ferrous sulphate as the precipitate will become more or less contaminated with the iron, and in the use of oxalic acid to remove the copper, as gold precipitated by oxalic acid from an acid solution containing copper is always contaminated with cupric oxalate. It is then also advisable to heat the solution with a slight addition of potassium carbonate, a soluble double oxalate of copper and potassium is formed, and the gold is left in the pure state. Gold may also be precipitated from its acid solu- tion in a state of purity in the form of brilliant span- gles by means of hydrogen dioxide, thus — 2AuCl 3 + 3H 2 2 =-6HCl+60 + 2Au. When the precipitated gold has been carefully washed and re-washed with distilled water, and the above-men- tioned reagents, it may be dried and placed in a new crucible, previously boraxed, with some potassium car- bonate and potassium nitrate melted and cast into an ingot. If iron ingot moulds are used the gold should be washed after moulding in hot hydrochloric acid to remove any trace of metallic or oxide of iron that may by chance have adhered to its surface during the process of casting the ingot. EXPERIMENT No. 68.— Each student should provide himself with not less than two and a half pennyweights of gold or alloy (old jewelry, etc.) con- taining that amount; accurately weigh and describe it. If malleable, the instructor may add small quantities of base metal to destroy its malleability, acquainting the student with the weight and character of debasing metal used. The alloy should then be roasted by the student, as described in the process, gold. 265 until malleable, after which the button or ingot should be alloyed with three times its weight of silver and carried through the parting process. The result of this operation is then to be rendered chemically pure by the third process described, then cast into a smooth ingot and rolled (between parchment or Swedish filter paper, with frequent annealing, exercising care to keep it pure and clean) to No. 60 or 30 foil, which is to be inserted in a tooth as a filling. PROPERTIES.— Pure gold is a rich, beautiful, yellow color, of strong metallic luster, unalterable in air. It is the most ductile of all metals, but ranks only fifth in point of tenacity. One grain, however, if covered with a more teuacious metal, like silver, forming a composite wire, may be drawn into a wire 550 feet in length, and only l-5000th of an inch in diameter. It is also the most malleable of all metals. One grain of it may be beaten into leaves so thin as to cover an area of 75 square inches, being of but l-370,000th of an inch in thickness. Very thin leaves of gold appear green in color by transmitted light; but when heated, the light trans- mitted is ruby-red. Gold possesses the property of welding cold. Thus, thin leaves, foil, and other forms of gold are more espe- cially adapted to the use of the dentist as a filling ma- terial. The small particles are welded together in one perfectly homogeneous mass as the plug is inserted. The finely divided metal, such as that thrown down in the preparation of pure gold from the chloride solution, may be compressed between dies in the form of disks or medals. The pure metal fuses at 1100° C. or 2012° F., and its alloys at much lower temperatures. When heated much above its melting point it slowly volatilizes and is readily dissipated in vapor by the oxyhydrogen flame. Pure gold is nearly as soft as lead, in consequence of which articles of jewelry, coin, etc., made from it are 266 PRACTICAL DENTAL METALLURGY. alloyed with copper, silver, platinum, etc, to give them, the requisite hardness, durability, and elasticity. The specific gravity of gold cast in an ingot is 19.265; when stamped, 19.31; and that of the precipitated metal from 19.55 to 19.72. Graham has shown that gold is capable of occluding 0.48 of its volume of hydrogen, and 0.2 of its volume of nitrogen. GOLD BEATING.— After the gold is precipitated from the chloride solution it is thoroughly washed, dried, and melted in a clean crucible at a temperature higher than necessary to simply fuse it, by which its mallea- bility is said to be improved. It is then poured into ingot-molds previously heated and oiled, and cast into ingots, each one inch wide, one-fourth of an inch thick, and from four to eight inches in length. These are re- moved from the mold, cleaned in dilute sulphuric acid, washed, and annealed. The ingot is next laminated by being repeatedly passed through heavy steam rollers, and annealed after each lamination, until it is formed into a ribbon one inch wide, about the thickness of tissue paper, and its length depending upon the original weight of the ingot. The ribbons are then cut into pieces of an inch square, their thickness depending upon the number of the foil, 2, 4, 6, 10, etc., it is designed to prepare from them; some allowance being made for subsequent trimming, and the necessity for leaving the sheet of correct weight. These little squares are again cleansed, taken up by wooden pliers and placed between the leaves of a (l cutch " made from vellum or ground parchment which holds about two hundred pieces. The cutch is then enclosed in parchment bands and beaten on a granite or marble block, securely and firmly set, with a hammer GOLD. 267 weighing from seventeen to twenty pounds. The ham- mer is short-handled, and is wielded by the beater with one hand, while with the other hand he holds and rotates the packet of gold. Every few moments the cutch is opened and the gold examined; then split in half and the position of the pieces reversed, so as to bring the middle ones to the out- side and those on the outside to the middle of the packet. During the beating the packet is continually rotated and turned, to distribute the force of the blows equally throughout the packet; at intervals it is taken up and rolled between the hands to overcome any adhesion that may have taken place between the leaves of metal and interposed parchment. Considerable skill is required to produce a good quality of foil, the physical properties of the metal being capable of alteration by the too great rapidity of the process. When the gold is beaten into sheets about three or three and a half inches square, or about the size of the leaves of the cutch, which requires from fifteen to twenty minutes, they are removed from the cutch by means of the wooden pliers, and placed piece by piece in a second packet of larger size made of the same material and called a " shoder." If, however, the pieces are too thick to produce the required number they are cut into four pieces, and the process repeated in the cutch. The shoder is then placed in the parchment bands, and the beating continued until the gold again equals the size of the skins, which requires about twice the length of time as before. After the last beating the pieces are carefully laid out one at a time on a calf-skin cushion lined with soft flannel; the blemished, broken and torn ones are laid to one side, and the perfect ones are cut into uniformly square sheets 268 PRACTICAL DENTAL METALLURGY. so familiar to the dentist, by means of an instrument carrying four edges of malacca reed called a wagon. The gold does not adhere to this as it does to the metal. The sheets are then accurately weighed and are ready for annealing. The annealing is an important and delicate process, and may be accomplished in several ways: in the muffle of a furnace, on heated platinum, or on platinum- wire gauze with a spirit-lamp beneath it. Whatever may be the means by which it is accomplished, it must be prop- erly done, i. e., the temperature must be sufficient to thoroughly heat and soften it uniformly in every part of the sheet, without melting and thickening the edges. From the first to the last step the most important con- sideration is absolute purity and cleanliness. Kach book of gold foil contains }i of an ounce, 2}4 pennyweights, or 60 grains. Each full sheet is 4 inches square, and the number of the foil indicates the weight of each full sheet. Thus, a book of No. 2 would contain 30 sheets, 4 inches square, of 2 grains each; No. 10, 6 sheets of 10 grains each; No. 30, 2 sheets (4 inches square) of 30 grains each, and so on. Corrugated foil is supposed to have been an outcome of the great Chicago fire. When the safes of one of the depots were opened, it was found that the paper had burned to a crisp, and the foil in the form of what is now known as corrugated foil. It was tried by some, and found many friends. In making the corrugated foil to-day a miniature Chicago fire is used to burn the paper. The gold is beaten as described before to No. 4 foil. It is then placed, sheet by sheet, between paper and enclosed in an iron box, with weights on the gold. The iron box is then placed on a slow fire and allowed to smolder, care being taken not to ignite the paper. After gold. 269 smoldering, the heat is gradually let on until the paper becomes carbonized. As the paper shrinks, the gold shrinks with it. The carbon is then blown off, sheet by sheet, and we have what is called corrugated or crystalline gold. Cylinders. — These sheets are then rolled upon them- selves in a cylindrical form of desired thickness, and cut into the size and style of cylinders required. The numbering of pellets or cylinders is so variable with the different manufacturers that it really means little, if anything. Some number them as follows: No. %, one-fourth of a sheet of No. 4 foil, rolled and cut into pellets of varying length; No. ^, one-half of a sheet of No. 4 foil, rolled and cut into pellets; No. 1, a whole sheet of No. 4, and so on. The length of the pellets is also variable, and is designated as style A, B, C, etc., by some manufacturers.* Cylinders are, in accordance with the manner in which they are rolled, known as loose and compact. The former can only be made by the manufacturers, and are composed of several sheets of No. 4 corrugated cohesive foil laid loosely upon one another and rolled lightly around a smooth needle-like piece of steel. The needle is then removed and the cylinders are cut by a peculiar sharp tool into assorted sizes or styles. The compact variety may be made by the operator in a similar manner, except that the ends of the cylinder will necessarily be more compact, on account of the manner in which it is cut. There are other forms of gold used, such as mat, block, rope, tape, ribbon, etc. Rolled Gold.— The heavier foil, as Nos. 20, 30, 60, 160, etc., is usually prepared by rolling, instead of * Hood and Reynolds. 270 PRACTICAL DENTAL METALLURGY. beating. Flattening by rolling elongates the fiber, instead of increasing its surface in all directions, and it is thought produces a foil of greater density and toughness, and, when annealed, greater softness. Such foil is exceedingly cohesive and tenacious. It is also made in the non-cohesive variety. Gold and Platinum Foils. — These are prepared in a variety of ways by several manufacturers of dental foils. Dr. C. E. Blake of San Francisco prepares a weldable, tenacious, platinum foil by electro-depositing a surface of pure gold upon it. As a rule such combinations pro- duce a filling more able to stand the stress and abrasive force of mastication. Crystal Gold. — This form of gold was first introduced by A. J. Watts, who prepared it by precipitating gold from a chloride solution by means of oxalic acid, treating the precipitate with nitric acid and neutralizing the acid by washing with ammonia. The crystalline substance was then thoroughly dried in a muffle, when it was ready for use. The preparation fell into disuse on account of its unreliable quality, frequently being con- taminated with nitric acid and other foreign substances. It is now prepared by electrolysis from the chloride solu- tion. Plates of pure gold are suspended in the solution w T hich replace the metal as fast as it is deposited upon the platinum cathode. When prepared properly it is an unobjectionable material, but certain precautions must be observed in its use, as its manipulation is very deceptive. ANNEALING GOLD.— This process has a two-fold purpose: (1) To remove any deleterious substances which may have accumulated on the gold through ex- posure, and (2) to soften and more perfectly secure its cohesive quality. GOLD. 271 After gold has been exposed for some time to the action of the air and extraneous influences, it loses to a great extent, the qualities it possessed when first pre- pared,^ e., it looses its softness, cohesiveness, etc., but is partially, if not wholly, restored to its original condition by annealing. This is accomplished by a variety of methods. The main consideration, however, is the employment of a flame of as great purity as possible, free from the evolu- tion of such substances as sulphur, chlorine, phosphorus, carbon, and their compounds. Obviously, flames from substances as free as possible from such compounds are the best; the desideratum being a pure hydrocarbon, which, all things being considered, is most conveniently found in good alcohol. The gas flame from a small Bunsen burner is one of almost perfect combustion; has become quite popular, and may be conveniently used where the gold is not annealed by passing through the flame. The most desir- able means, perhaps, is found in the pure platinum coils which are heated to incandescence by the electric current. It has been observed that more even and uniform results may be attained by annealing the gold in small quantities on heated mica, or platinum, since the por- tion of the pellet or foil held by the carriers is only slightly heated when passed through the flame. Gold is best annealed after cutting, as the action of the shears in cutting annealed foil tends to give ita " wire " edge by slightly condensing it. PROPERTIES OF GOLD FOIL .—These are various and variable with the products of different manufac- turers. The most prominent and important for the consideration of the dentist is that known as cohesiveness. 272 PRACTICAL DENTAL METALLURGY. Most metals which possess the analogous property called weldability are weldable in proportion to the length of time they will remain in a plastic condition under heat without melting, but pure gold is weldable cold. It is not to be inferred that gold is the only metal which is weldable cold; on the contrary, the clean, pure surfaces of many metals, such as lead and copper, are quite coherent, but much difficulty is encountered in obtaining and keeping them in the necessarily pure state. Gold prepared by the manufacturers is unfortunately given a variety of names, such as cohesive, semi-cohe- sive, non-cohesive, hard, soft, etc., all of which relate to its property of cohesiveness, and are perplexing and misleading to the older practitioners, not to say students. Gold is either cohesive or non-cohesive. Cohesive gold is simply pure gold. The greater its purity, the more perfect its cohesiveness. It is annealed and soft when it leaves the makers, and sheets of it laid upon each other will cohere by mere contact. As stated previously, after it is exposed to air and to extraneous influences, handled and jostled about, it loses, from con- tact with impurities, some of its cohesiveness, and from pressure, some of its softness. These two properties are, however, partially or fully restored by proper annealing. Non-cohesive Gold. — Since pure gold is cohesive by virtue of its purity, it is obviously conversely true that non-cohzsive gold is as it is from /^purity. The process of preparing non-cohesive gold is a trade secret which every manufacturer possesses as part of his stock in trade. The natural cohesiveness of gold may be over- come by slightly alloying with iridium or iron, but it is probable this property is generally destroyed by some surface treatment the foil receives. Hastings & Co. pre- gold. 273 pare a very excellent non-cohesive foil, which, from analysis has been found exceedingly pure. The process of manufacture is not known, but is probably dependent upon some surface treatment. Non-cohesive foil cannot be made cohesive by annealing, because the impurity cannot be removed by that means. Semi-cohesive foil is non-cohesive before and co- hesive after annealing. It is evident that the process of annealing purifies it, thereby restoring its natural prop- erty. The terms "'hard" and ''soft" are used by the makers and many practitioners to designate the cohesive and non-cohesive varieties, respectively. These terms are, however, erroneous, and should not be used. " The feeling of softness," says Dr. Kirk, " exhibited by non-cohesive foil under the instrument is due largely to the fact of its non-cohesiveness, whereby the several laminae slip or slide one upon another, thus conveying a yielding or soft sensation to the tactile sense, and making it possible to condense large masses at a time, the pres- sure being conveyed continuously throughout the mass and the condensation or consolidation being uniform. A similar mass of cohesive foil, treated in the same manner and under like conditions, presents a greater resistance to the instrument, and conveys the idea of hardness, from the fact that as pressure is applied the successive laminse unite or weld together from the surface down- ward into a homogeneous stratum of metal, which offers greater resistance and becomes more impenetrable by constant additions to its thickness; until the condensing instrument fails to make any further impression; but upon removing the mass of gold it will be found that that portion of it which occupied the bottom of the cavity is still in the form of foil and not homogeneously condensed." Purity. — The physical properties of gold are more apt to be influenced by lack of purity than by any other 274 PRACTICAL DENTAL METALLURGY. factor. Impurity may be occasioned by several means: first, by admixture of other metals, especially those easily oxidized. Second, by surface contamination caused by handling or exposure to vaporous gases. Third, by the absorption or occlusion of gases. The following assays were made for Dr. Kirk by Messrs. Dubois and Hckfeldt, assayers of the United States Mint, Philadelphia. The table exhibits in thou- sandths the relative fineness of some of the foils in general use.* Bach assay was duplicated: No. 1. Abbey's Non-Cohesive 998.8 998.7 No. 2. Wolrab's, from C. A. Timme. . . 999.2 999.3 No. 3. Quarter-Century, S. S. White Dental Mfg. Co 999.1 999.1 No. 4. Rowan's Decimal Foil from Gideon Sibley 999.9 999.8 Discoloration. — The unfortunate discoloration of some gold fillings is in all probability due to a slight admixture of iron obtained during the precipitation from the chlor- ide solution when ferrous sulphate is used, or from the iron ingot-mold, or from the surface of the plugger-poirit during the insertion of the filling. COMPOUNDS OF GOLD WITH OXYGEN.— Gold forms two oxides: Aurous Oxide, or Monoxide, Au 2 0, is prepared by adding a solution of caustic potassa to the monochloride. It is a green, unstable powder, being easily decomposed into metallic gold and auric oxide. Auric Oxide, or Trioxide, Au 2 3 , is prepared by adding magnesia to auric chloride; when the sparingly soluble aurate of magnesium thus formed is well washed and digested with nitric acid, auric oxide is left as an * American System of Dentistry, Vol. Ill, p. 842. GOLD. 275 insoluble reddish-yellow powder. It is easily reduced by heat or mere exposure to light; soluble in nitric, hydrochloric, and hydrobromic, and insoluble in hydro- fluoric acid. Its acid properties are marked: it dissolves freely in alkalis. When digested with ammonia, it yields — Fulminating Gold. — This compound is usually pre- pared, however, by the addition of ammonia to trichloride of gold. It is a buff precipitate, and explodes violently when gently heated. Its formula is probably — Au ? 3 4H 3 N.H 2 0. ACTION OF ACIDS ON GOLD .-Gold is not tarnished or affected by air or water at any temperature, nor by sulphuretted hydrogen. Neither is it soluble in sulphuric, nitric nor hydro- chloric acid. In nitro-hydrochloric acid, however, it speedily dis- solves, forming the trichloride of gold, AuCl 3 . It is also attacked by a vapor or solution of chlorine; By bromine, dissolving in bromine- water to form auric bromide, AuBr 3 ; and By iodine, dissolving when finely divided in hydriodic acid by aid of the air and potassium iodide, forming potassium auric iodide — 2Au + 6HI + 2KI+30=2KIAuI 3 + 3H 2 0. Potassium cyanide solution dissolves precipitated gold with the aid of air, forming potassium aurocyanide — 2Au+4KCN+0=2KAu(CN) a + K a O. Purple of Cassius. — So named from its color and discoverer. It is much employed for imparting a rich red or reddish purple to glass and porcelain, and is of especial interest to dentistry, because it is used by the 276 PRACTICAL DENTAL METALLURGY. manufacturers of artifible teeth to produce the gum tint of dental porcelain. It is a vitrifiable material composed of gold, tin and oxygen. The proportion, however, is thought to be variable. It is generally given the formula — Au 2 O.Sn0 2 ,SnO.Sn0 2 .4H 2 0. It is prepared by a variety of methods. Pelletier's is as follows: 20 grains of gold are dissolved in 100 grains of aqua regia containing 20 parts nitric to 80 parts com- mercial hydrochloric acid; the solution is evaporated to dryness over a water-bath; the residue dissolved in water; the filtered solution diluted with seven or eight deciliters of water and tin filings introduced into it. In a few minutes the solution becomes brown and turbid and deposits a purple precipitate, which merely requires to be washed and dried at a gentle heat. The purple thus prepared contains, in 100 parts, 32.746 parts of stannic acid, 14.618 of protoxide of tin, 44.772 of aurous oxide, and 7.864 of water. The precipitate obtained by the addition of stannous chloride to auric chloride is always brown. To obtain a fine purple precipitate the auric chloride should be treated with a mixture of stan- nous and stannic chlorides. The purple is now produced by a dry method, by the manufacturers of gum enamel. It consists of digesting an alloy of gold, tin, and silver in nitric acid. The method was discovered by the late Professor Klias Wild- man. The proportions are as follows: Pure silver 240 grains. " gold 24 " 11 tin 17.5 « In preparing the alloy it should be melted and granu- lated four times to insure intimate admixture of the metals. It is afterwards placed in a flask and digested by the aid of gentle heat in chemically pure nitric acid GOLD. 277 in the proportion of 2 parts acid to 1 of water. After the silver has all been dissolved the precipitate is allowed to settle, the supernatant liquid poured ofF and the precipi- tate washed several times with warm distilled water. It is then again subjected to the action of dilute nitric acid, aided by heat and continual stirring to dissolve any of the remaining silver. When all action subsides, pour the contents of the flask on a filter paper and wash all the nitrate of silver out with pure water. The filtrate should be frequently tested with sodium chloride. When the latter reagent will no longer throw down the white pre- cipitate of chloride of silver, the precipitated purple of Cassius on the filter paper is dried and is ready for use in the manufacture of gum frit. Purple of Cassius is very soluble in ammonia before fusion, after which it is insoluble. "That ignition," says Dr. Kirk, "does not effect such decomposition is proven by the fact that the ignited powder can have all of its gold extracted by aqua regia, leaving pure stannic oxide, or the gold may be extracted from it by amalga- mation with mercury, which is impossible before ignition. ' ' ALLOYS. — Gold very readily unites with most of the metals, forming alloys of varied qualities. When in the pure state gold is too soft for any great use other than for filling teeth; consequently the greater quantity of gold is alloyed with some metal that will increase its hardness and durability, without greatly impairing its more valuable qualities. The metals usually employed for this purpose are silver, platinum and copper. Silver and gold are easily mixed together, but do not seem to form definite compounds. Such alloys are more fusible, more ductile, harder, more sonorous and elastic than gold, and are generally of a greenish-white color. One-twentieth of silver is sufficient to modify the color of gold. The alloys of gold and silver are known to 278 PRACTICAL DENTAL METALLURGY. jewelers as yellow, green and pale gold, according to the content of silver. Copper and gold unite much more readily than silver and gold; indeed it is reasonable to believe from their behavior that a chemical combination is formed with 76 per cent, of gold and 24 per cent, of copper. Alloys of copper and gold are much harder, tougher, and more easily fused; less malleable and ductile, and greatly changed in color, being of a decidedly reddish tint, de- pending upon the proportion of copper with which the gold is debased. An alloy of gold 76, and copper 24, as spoken of heretofore, is distinctly crystalline and quite brittle; but a larger proportion of either gold or copper restores the malleability of the alloy. Standard Gold. — The standard alloy of most nations is one of copper and gold. Some contain small quanti- ties of silver, but this is due to imperfect parting of silver and gold, or it may be contained in the copper used for the alloy. The proportion of copper to gold varies slightly in different countries, and such proportions are stated in thousandths; thus, pure gold is one thousand (1000) fine. The following table gives the composition of standard gold, as fixed by the nations mentioned: Nation. Gold. Copper. United States ") 900 .... France Italy , Switzerland ' Spain 100 (Carat 916 .... 21.6—) Greece China Austrian Crowns 84 Ducats Hungarian 989 .... 11 Ducats, Austrian Ducats, Dutch 986 .... 14 982 18 GOLD. 279 The first United States gold coins were ten-dollar pieces, coined in 1795; they weighed 270 grains each, and were of 916.666 (22-carat) fineness. Their weight was reduced in 1834 to 258 grains, with 899.225 (21.581- carat) fineness; and in 1837 the present standard of 900 (21.599-carat) fineness was established. Alloys of gold with copper, or with silver, or with both, are much used in the manufacture of jewelry. When the gold contains copper only it is termed red gold; when silver only, white gold; if the gold contains both metals, the caratation is termed mixed. In many countries a legal standard of fineness is fixed for gold ornaments and jewelry. In England gold is stamped, or Hall Marked, 16, 18, and 22-carat; in France, 18, 20, and 22-carat; in Germany, 8, 14, and 18-carat, and, also, under the term joujou gold, a 6-carat gold used for electro -plated jewelry. The purpose of the stamping is to protect the purchaser, who is enabled to know the carat of the gold he is buying. The following alloys used by jewelers are also of much interest to the dentist: TABLE OF MIXED CARATATION.— Brannt. Parts. carats. Gold. Silver. Copper. - ... 23 .... 23 Vz .... Vz 22 .... 22 1 1 ... 20 .... 20 2 2 3 18 .... . . 18 . . 3 ... 15 .... 15 .. 3 .., 6 .... 8 8*4 ... 13 .... .... 13 .... 3 ... 12 ... . . . 10 . 9 .... .... 12 3K .... .... 10 .... 9 .... 4 W z ... 10 .... 10^ 8 .... 8 h x A .... .... 10# 9 7 .... 7 .... 8 280 PRACTICAL DENTAL METALLURGY. COLORED GOLDS.— Brannt. Parts. Color. Gold. Silver. Copper. Stee Cadmium. 2 to 6 1.0 Green 75.0 16.6 8.4 «< 74.6 11.4 9.7 4.3 <( 75.0 12.5 • ♦ • • 12.5 (< 1.0 2.0 • . • . • • • • • • ■ Pale yellow 4.0 3.0 1.0 ■ • • i Dark yellow 14.7 7.0 6.0 • • • «< (i 3.0 1.0 1.0 Pale red 10.0 1.0 4.0 • ■ • << (i 1.0 1.0 Dark red 30.0 3.0 2.( ) Gray 1 to 3 l.( ) Blue HIGHER CARAT COLORED GOLDS. Parts. Color. Carat. Gold. Silver. Copper. 15 dwt 2 dwt. 18 grs . 1 " 18 " 6 " 2 " 2 dwt. 6 grs.. 3 " 6 " 12 " 5 " 8 " Yellow tint 18 K 15 " Red " 18 K 1 oz. 16 dwt.. . 1 oz Reddish Spring Gold. Yellow tint 16 K 16 K 1 oz Red tint 16 K Jewelers usually make their solders from the gold upon which they are to be used by the addition of small quantities of copper, silver or brass, the latter greatly increasing the fusibility and flow. The follow- ing are: jewelers' solders. For 18-Carat Gold. 18-carat gold .... 1 dwt. Silver 2 grs. Copper 1 gr. Carat. — The fineness of For 16-Carat Gold. 16-carat gold 1 dwt. Silver 10 grs. Copper 8 " gold is also expressed in carats, a twenty-fourth part, formerly the twenty-fourth GOLD. 281 part in weight of a gold marc. It is now assumed that there are 24 carats in unity; whether the unit be one pound, one ounce, or one pennyweight, it is divisible into 24 equal parts, and each of these parts is called a carat to express fineness. If a quantity of gold is chemically pure, in other words contains no alloying elements, it is, as we have previously explained, 1000 fine; or, in other words, each 24th part is gold, and it is, therefore, said to be of 24-carat fineness. If, however, 2 carats, or 2-24ths of the unit quantity are composed of one or more alloying metals, the gold is said to be 22 carats fine; or if 6 carats or 6-24ths of the alloy is debasing metal, the carat is 18 fine, etc. The following table shows the equivalent of each carat in thousandths: Carats. Thousandths. Carats Thousandths. ... 1 ... .... 41.667 .... ... 13 ... ... 541.667 ... 2 ... .... 83.334 14 ... ... 583.333 ... 3 ... .... 125.001 15 ... ... 624.555 ... 4 ... .... 166.667 . 16 ... ... 666.667 ... 5 ... .... 208.333 17 ... ... 707.333 ... 6 ... .... 250.000 18 ... ... 750.000 ... 7 ... .... 291 666 19 ... ... 79L.666 ... 8 ... .... 333.333 20 ... ... 833.333 ... 9 ... .... 374.999 a 21 ... ... 874.999 ... 10 ... .... 416.667 22 ... ... 916.666 ... 11 ... .... 458.630 23 ... ... 958 333 ... 12 . 500.000 • 24 1000.000 ... GOLD PLATE. — Pure gold is rarely employed in the dental laboratory, except for soldering continuous-gum cases and such. Its extreme softness and flexibility make alloying absolutely necessary. The latter must be accomplished, however, without practically impairing either its malleability, pliancy, or purity, and at the same time endow it with that degree of hardness, elas- ticity and strength necessary to resist the stress and wear which an artificial denture is exposed to in the mouth. 282 PRACTICAL DENTAL METALLURGY. Copper and silver are much used to debase or alloy, pure gold. It is questionable, however, whether copper should be used as almost universally as it is, some regard it as exceedingly objectionable. A plate made from a gold alloy containing a large percentage of copper is more easily tarnished, and imparts an ugly metallic taste, and may become a source of injury to the soft tissues of the mouth. Silver exercises a very benign influence over copper contained in gold plate, controlling the tendency to that disagreeable redness. Equal parts of silver and copper have little or no effect upon the color of gold. Silver assistsjin imparting hardness, elasticity and durability to the alloy, without so far debasing it as copper alone. Platinum and silver are sometimes used to endow pure gold with the qualities necessary for a dental base; but the labor of swaging is very greatly increased when platinum is contained in the plate. In order to secure the best results, alloys intended for plate should not be less than 20-carat fineness; 18 -carat is sometimes used; but in such cases the alloy should contain as little copper as possible. It is positively unsafe to use a lower carat than 18. The following are some of the formulae in use for the preparation of alloys for dental bases: Number of Formula. Carat. Parts Pure. Gold. Silver. Copper. Platinum, * 1 18 18 18 19 19 20 21 22 18 dwts. 18 " 18 " 19 " 19 " 20 " 21 " 22 " 2 dwts. 3 " 4 " 2 " 3 " 2 " 1 dwt. 18 grs. 4 dwts, 3 " 1 dwt. 3 dwts. Idwt. 2 dwts, 2 " Idwt. 2 3 * 4 Idwt. 5 * 6 * 7 * 8 Idwt. 6 grs. * Richardson's Mechanical Dentistry, p. 56. GOLD. 283 Number of Carat. Formula. t 9 18 * 10 18 X ii 18 * 12 * 13 19 20 X 14 20 * 15 * 16 21 21 Parts Pure. Gold. 64y 2 dwts. ($60 00) 20 dwts. 516 grs. ($20 00) 20 dwts. 20 " 516 grs. ($20 00) 20 dwts. 20 " Silver. 13 dwts. 2 dwts. 96.45 grs. (25 c. coin. 40+grs. 20 4- grs. 10 c, coin. 13 + grs. Copper. 2 dwts. 25 grs. 18 " 6 grs. Platinum. / o-/ grs. CLASP GOLD. — Gold for clasps, elastic wires, back- ings, stays, posts, pivots, etc., usually contain a small amount of platinum to give it greater strength and elas- ticity. The following formulae are recommended by Professor Chapin A. Harris: No. 1— 20-Carat. Pure gold 20 dwts. " copper 2 " " silver 1 dwt. " platinum.. 1 " No. 2— 20-Carat. Coin gold 20 dwts. Pure copper 8 grs. " silver 10 " " platinum.. 20 " A content of platinum in gold renders the alloy more liable to oxidation, and, says Professor Harris, "This effect is so marked that such an alloy is readily acted upon by nitric acid." It is not probable, however, that the small amount contained in clasp gold would affect its integrity. CROWN GOLD.— Gold for crowns should combine strength with good color. Those alloys of a large copper content make exceedingly unsightly crowns, on account of their deep red color. Professor C. L. Goddard recom- mends the following for alloys the color of pure gold: ♦Richardson's Mechanical Dentistry, p. 56. f Johnson Bros. X Prof. C. I,. Goddard. 284 PRACTICAL DENTAL METALLURGY. No. 1— 21.6-Carat. No. 2— 21.6-Carat. Pure gold 90 parts. Coin gold 50 parts. " silver 5 " Pure gold 45 l ' " copper... 5 " " silver 5 " GOLD SOLDERS.— These are usually alloys of gold, silver, copper aud zinc, and are designed to be a trifle more fusible than the parts to be soldered; this property is conferred upon them principally by the content of zinc (or brass). They should also possess considerable strength; too much base metal, therefore, should not be added, as it will, by oxidizing, tend to very materially weaken the alloys. Their carat should be as high or nearly as high as that of the plate, and color as nearly as possible the same. The following formulae have yielded satisfactory re- sults as gold solders: Parts. u o No. 1 14 2 14 3 15 4 16 5 16 + 6 18 7 18 8 20 9 20 10 14 U.S. Coin Gold. $10.00 16 dwts. 6 " 30 parts $10 00 Pure Gold. Pure Silver. 11 dwts. j 11 dwts ) 112 grs. } 27 parts. 4 dwts. 30 grs. 3 dwts ) 6 grs. . < 3 dwts . 4 parts. 4 " 5 dwts. f (18 K. gold plate 1 J formula No. 9) 1 20 dwts. (, Johnson Bros. 12 grs. 2.5 dwts. Pure Copper. Pure Zinc. 118 20 \t I 1 112 1 dwts. dwts ( grs. I grs. I 10 grs. dwts grs. dwt - I l<>ers grs. ) J - grb 1 part. 1 part. parts. I t 1 " Spelter Solder.* 6 grs. 20 2TS. 35 grs 20.61 grs. 6 grs. A simple method for making a good solder suitable for the plate upon which it is to be used is : 5 parts of the * Composed of equal parts copper and zinc. gold. • 285 plate and one of brass or of silver solder. In the case of coin gold, or the crown alloy given on page 284, a solder thus made will be exactly 18 carat.* Zinc is best added to the alloys in the form of brass. The latter should be of a known formula, so that the desired amount of zinc may be accurately calculated; it should also be malleable and ductile; or the solder is apt to be very brittle. RULES FOR COMPUTING AND COMPOUNDING GOLD ALLOYSf AND EXAMPLES J PART I. To ascertain the carat of any given alloy, the propor- tion may be expressed as follows: As the weight of the alloyed mass is to the weight of gold it contains, so is 24 to the standard sought. Example. — Gold 6 parts, silver 2 parts, copper 1 part, total, 9 P arts - 9:6::24:? 6 9)144 16 Answer. Another method when alloyed gold is used in forming the mass, instead of pure gold, is to express the propor- tion as follows — As the weight of the alloyed mass is to the weight of the gold alloy used in its composition, so is the carat of the latter to the carat of the former — Example. — Harris No. 1 solder: 22-carat Gold 48 parts. Copper 16 " Silver 12 " Total 76 " 76:48::22 carat. Ans. 13.9 carat. * Prof. C I,. Goddard. t Rules by Prof. Geo. Watt. X Examples by Prof. C. L,. Goddard. 286 PRACTICAL DENTAL METALLURGY. EXAMPLES UNDER RULE 1ST. 1. Find the carat of 36 pennyweights of gold, 8 pennyweights of copper, 4 pennyweights of silver. Ans. 18 carat. 2. Find the carat of 9 pennyweights of gold, 2 pennyweights of copper, 1 pennyweight of silver. Ans. 18 carat. 3. Find the carat of 38 pennyweights of gold, 6 pennyweights of copper, 4 pennyweights of silver. Ans. 19 carat. 4. Find the carat of 22 pennyweights of gold, 1 pennyweight of copper, 18 grains of silver, 6 grains of platinum. Ans. 22 carat. 5. Find the carat of 22 pennyweights of gold, 2 pennyweights of copper, 1 pennyweight of silver, 1 pennyweight of platinum. 6. Find the carat of 6 pennyweights of gold, 2 pennyweights of copper, 1 pennyweight of silver. 7. Find the carat of 48 parts of 22-carat gold, 16 parts of silver, 12 parts of copper. Ans. 13.9 carat. 8. Find the carat of 20 pennyweights of gold coin, 2 penny- weights of copper, 2 pennyweights of silver. Ans. 18 carat. 9. Find the carat of 20 pennyweights of gold coin, 25 grains of copper, 40 + grains of silver. 10. Find the carat of 20 pennyweights of gold coin, 18 grains of copper, 20 + grains of silver. 11. Find the carat of 464.4 grains of gold, 5.16 grains of sil- ver, 46.44 grains of copper. PART II. To reduce pure gold to any required carat, the propor- tion may be expressed as follows: As the required carat is to 24, so is the weight of gold used to the weight of the alloyed mass when reduced. The weight of gold subtracted from this gives the quantity of alloy to be added. Example. — Reduce 6 ounces of pure gold to 16 carat, 16:24: :6 ounces: 9 ounces. 9 — 6= 3 ounces alloy to be added. To reduce gold from a higher carat to a lower carat, the proportion may be expressed as follows: As the required carat is to the carat used so is the weight of the mass used to the weight of the alloyed mass when reduced. The weight of the mass used, subtracted from this, gives the quantity of alloy to be added. gold. 287 Example. — Reduce 4 ounces of 20-carat gold to 1(3 carat : 16:20::4 ounces:? 4 16)80 5 ounces 5 ounces — 4 ounces=l ounce alloy to be added. EXAMPLES UNDER RULE 2D. 1. Reduce 6 ounces gold to 16 carat. Ans. Add 3 ounces alloy. 2. Reduce 25 pennyweights gold to 18 carat. Ans. Add 8 pennyweights, 8 grains alloy. 3. Reduce 4 ounces of 20-carat gold to 16 carat. Ans. Add 1 ounce alloy. 4. Reduce 6 ounces of 18-carat gold to 15 carat. 5. Reduce 15 pennyweights of gold coin to 20 carat. Ans. Add 1.2 pennyweights. 6. Reduce 12 pennyweights of gold coin to 18 carat. 7. Reduce 4 pennyweights of 22-carat gold to 20 carat. Ans. Add 9.6 grains alloy. 8. Reduce 48 grains 20-carat gold to 16 carat. 9. Reduce 2 pennyweights 20-carat gold to 18 carat. 10. Reduce 1 pennyweight, 8 grains 18-carat gold to 16 carat. PART IIL To reduce gold from a lower to a higher carat, add pure gold or a finer alloy. As the alloy in the required carat is to the alloy in the given carat, so is the weight of the alloyed gold used to the weight of the reduced alloy required. The weight of the alloyed gold used subtracted from this gives the amount of pure gold to be added. Example. — Reduce 1 pennyweight of 16-carat gold to 18 carat. First subtract 16 and 18 from 24 to find the amount of alloy in each carat. 24 24 JL8 _16 6 : 8 :: 1 pennyweight:? I 6)~~8 l}i pennyweight. \]A, — 1=K pennyweight of pure gold to be added. 288 PRACTICAL DENTAL METALLURGY. To reduce gold from a lower carat to a higher carat, by adding gold of a still higher carat. Subtract the lower carat and the required carat each from the highest carat (instead of from 24) and proceed as before. Example — Reduce 2 pennyweights of 16-carat gold to 18 carat, by adding 22-caret gold. First subtract 16 and 18 from 22. 22 22 18 16 4 : 6:2 pennyweights : 3 pennyweights. 3 — 2=1 pennyweight of 22-carat gold to be added. EXAMPLES UNDER RULE 3D. 1. Reduce 1 pennyweight of 16-carat gold to 18 carat. Ans. Add 8 grains of gold. 2. Reduce 2 ounces of 16-carat gold to 20 carat. Ans. Add 2 ounces of gold. 3. Reduce 11 pennyweights, 8 grains of 18-carat gold to 20 carat. 4. Reduce 9 pennyweights of 16-carat gold to 18 carat. Ans. Add 3 pennyweights of gold. 5. Reduce 2 ounces of 20-carat gold to 22 carat. 6. Reduce 18 pennyweights of 16-carat gold to 18 carat by add- ing 22-carat gold. Ans. Add 9 pennyweights of 22-carat gold. 7. Reduce 3 pennyweights of 18-carat gold to 20 carat by add- ing gold coin. Ans. Add 3 pennyweights, 18 grains. 8. Reduce 12 pennyweights, 10 grains of 16-carat gold to 20 carat by adding gold coin. 9. Reduce 20 grains of 16-carat gold to 18 carat by adding 20-carat gold. MISCELLANEOUS EXAMPLES. 1. Find the carat of 19 pennyweights of gold, 3 pennyweights of copper, 2 pennyweights of silver. Ans. 19 carat. 2. Reduce 5 pennyweights, 4 grains of gold to 20 carat. 3. Reduce 2 ounces, 4 pennyweights, 8 grains of 20-carat gold to 18 carat. 4. Reduce 12 pennyweights of 18-carat gold to 20 carat. Ans. Add 6 pennyweights of gold. GOLD. 289 5. Find the carat of 20 parts of gold coin, 3 parts of copper, 3 parts of silver (gold plate). 6. Find the carat of 30 parts of gold coin, 1 part copper, 4 parts of silver, 1 part of brass (solder). 7. Reduce 258 grains of gold coin to 20-carat gold. 8. Reduce 516 grains of gold coin to 18-carat gold. 9. Reduce 4 pennyweights of 16-carat gold to 18 carat by adding gold coin. 10. Reduce 3 pennyweights, 6 grains of 16-carat gold to 18 carat by adding 20-carat gold. ($10 gold coin weighs 258 grains — $0.10 silver coin, 38.58 grains.) 11. Add 10 cents silver to $20 gold — find weight and carat. 12. Add 25 cents silver to $20 gold — find weight and carat. {Gold coin, 20 pennyweights ] to formula for copper, 2 [- pure gold, and silver, 2 " ) find carat. {Gold coin, 20 pennyweights ) copper, 25 grains >■ do silver, 40+ " ) f Gold coin, 20 pennyweights) 15. Change^ copper, 18 grains >• do [ silver, 20+ " ) {Gold, 18 pennyweights "J to formula for Copper, 4 [■ gold coin, and Silver, 2 " J find carat. 17. A watchchain, 14 carats fine, weighs 2 ounces, 4 penny- weights, 16 grains. How much pure gold must be added to raise it to 20-carat gold ? 18. A piece of jewelry, 12 carats fine, weighs 3 pennyweights. How much U. S. gold coin must be added to make it 18-carat fine ? 19. Add 4 ounces, 16 pennyweights, 5 grains of 14-carat gold to 2 ounces, 4 pennyweights, 16 grains of 16-carat gold and find the carat of the mixture. Ans. 7 ounces, 21 grains of 14.64-carat gold. 20. How much pure gold must be added to the above mix- ture to make it 18 carat fine? 21. How much U. S. silver coin and how much copper must be added to 3 ounces U. S. gold coin to reduce it to 18-carat gold containing equal parts of silver and copper? The alloys of gold and most of the metals have been discussed under the heads of the various metals. 290 PRACTICAL DENTAL METALLURGY. TESTS FOR GOLD IN SOLUTION.— Sulphuretted Hydrogen or Ammonium Hydro-Sulphide throws down a brown precipitate of auric sulphide (Au 2 S 3 ). The second precipitant is not used, however, as the pre- cipitate is soluble in it, as it also is in the alkaline sulphides. Auric sulphide is insoluble in nitric or hydrochloric acid taken separately, but soluble in aqua regia. Ferrous Sulphate and Oxalic Acid precipitate the gold in the metallic state; it is a brown powder, darker in the instance of the former than the latter, but develops the color and luster of gold by being burnished with the finger-nail or instrument. Stannous and Stannic Chloride. — The most delicate test for gold is probably the formation of the purple of Cassius. EXPERIMENT No. 69.— To a weak solution of gold chloride (AuCl 3 ), add, drop by drop, a weak solution of the mixture of stannic and stannous chloride. An intense purple color results, which, without the solutions, have been largely diluted, or the resulting precipitate mixed with a large quantity of water; the color cannot be appreciated on account of its intensity. If the precipitate formed in the experiment above be dried and heated on charcoal a metallic globule results. Heat and Light. — Gold is reduced from many of its compounds by sunlight, and from all of them by more or less heat. ELECTRO-DEPOSITION OF GOLD— By Simple Immersion. — From an acid solution of gold chloride, the base metals, and silver, platinum, and palladium, deposit gold in the metallic state. In the double cyanide of gold and potassium, zinc will quickly become gilded, copper, brass, and German silver, slowly, and antimony, bismuth, tin, lead, iron, nickel, silver, gold, and platinum not at all. GOLD. 291 Deposition by a Separate Current. — The Solu- tion. — There are many solutions prepared for electro- gilding, some being formed by chemical means, others by a separate current from the battery; but whether they are made by chemical or electrical process, the best for a thick reguline deposit is the pure double cyanide of gold and potassium. A cyanide solution may be prepared as follows : Dissolve 120 grains of pure gold in one ounce of chemically pure aqua regia, and thus preparing the chloride of gold, as described previously.* Dissolve the chloride obtained in 32 ounces of warm distilled water, and add to it 1^ ounces of magnesia; the gold is pre- cipitated. Filter and wash with pure distilled water, digest the precipitate in 10 parts of distilled water mixed with .75 part of nitric acid to remove magnesia; then wash the remaining oxide of gold with distilled water, until the wash-water exhibits no acid reaction with test- paper. Next dissolve 3 ounces of ferro-cyanide of potassium and 6 drams of caustic potash in 34 ounces of distilled water, add the oxide of gold prepared, and boil the solution about twenty minutes. When the gold is dissolved there remains a small amount of iron pre- cipitated, which may be removed by filtering the solu- tion. The liquid, a fine, clear, golden color, is then ready for use, to be employed either hot or cold, but a better and quicker deposit is nearly always obtained from the warm solution. In electro-plating objects the first essential is a fin- ished surface, which must be made just as it is desired to be when completed. The next is cleanliness. If it be a silver denture or any other metallic object it should first be cleaned of all surface combinations, as oxides, * Preparation of Chemically Pure Gold, p. 259. 292 PRACTICAL DENTAL METALLURGY. sulphides, etc., by polishing in the ordinary way; then scrubbed with a solution of hot water and soap by means of a brass or steel scratch-brush on the lathe; then washed or boiled in a strong solution of caustic potash, afterwards washing in distilled water, and finally in an acidulated water to remove all traces of the alkali. When a sufficient coating has been formed the object is to be removed from the bath and burnished by the scratch brush or agate burnisher, moistened with a solu- tion of warm water and soap, until the surface is finished as desired. The apparatus is exceedingly meager and simple, con- sisting of a single cell and a glass bowl (preferably of perpendicular sides) to contain the solution. The latter may or may not be adjusted in a water-bath, according to whether the operator desires to work his solution hot or cold. Aside from these connecting and guiding wires, cathode and anode hooks, together with an anode, a thermometer, a scratch-brush, etc., are all that will be needed. EXPERIMENT No. 70.— A solution should be prepared by the students under the guidance of the instructor, and kept on hand for the practical use of the students in electro-gilding their " dummy work." CHAPTER XXI. AMALGAMS. An Amalgam is an alloy of two or more metals, one of which is mercury. The name is probably derived from the Greek malagma, meaning a soft material, and was applied to alloys of mercury on account of the increased plasticity and fusi- bility which it conferred upon them. Most metals, even hydrogen and ammonium, unite directly with mercury to form this very numerous and interesting series of alloys which are termed amalgams. Many are extensively used in the arts and industries; but to no art, calling, or profession can they be of more interest or importance than to dentistry. It must not be inferred that amalgams are to be given different chemical and physical theoretical consideration because they are studied thus distinctly. On the con- trary, they are to be considered in all respects alloys, differing from the usual in no general way, except that all contain mercury and are endowed with some proper- ties peculiar and dependent upon that metal. They are, therefore, subject to the same classification quoted from Matthiessen in the chapter on Alloys.* They offer an excellent opportunity for studying the behavior of metals towards each other, the examination being facili- tated by the low temperature at which their combinations are effected. f The affinities affording the union of mercury with its constituents in the formation of amalgams are not, as a ♦The student should carefully review this elassi6eatiou. f Mercury, it must be remembered, is simply a metal fused at ordinary temperatures. 294 PRACTICAL DENTAL METALLURGY. rale, strong, for many of them are decomposable by pressure, and all by considerable heat; yet, like all other metals, mercury tends to form definite chemical com- pounds with certain metals. The following have been formed by combining the metals named with mercury, and squeezing out the excess by means of hydraulic pressure to the amount of 60 tons to the square inch : Amalgam of lead, Pb 2 Hg.* " " silver, AgHg. i( " iron, FeHg. " " zinc, Zn 2 Hg. " copper, CuHg. " " platinum, PtHg 2 . Also " " gold, Au 4 Hg. " tin, Sn 2 Hg. A native compound of mercury and silver, known as Argnerite, Ag 6 Hg, is found crystallized in the form of the regular system. Beautiful crystallizations of silver amalgam (Arbor Diance) may be formed in long prisms having the com- position Ag 2 Hg 3 , by dissolving 400 grains of silver nitrate in 40 ounces of water, adding 160 minims of con- centrated nitric acid, and 1840 grains of mercury; in a few hours beautiful crystals of considerable length will be deposited. The union of mercury and other metals may be said to take place by four different means: (1st.) Some by direct co?itact, accompanied in some instances by a considerable evolution of heat. Thus, if a piece of clean sodium be thrown upon a clean, dry surface of warmed mercury, union takes place with explosiveness, accompanied by incandescence, and the * Bloxam's Chemistry, Inorganic and Organic, p. 400. AMALGAM S. 295 evolution of an amount of heat sufficient to volatilize portions of each metal. EXPERIMENT No. 71.— Repeat Experiment No. 8. (2d.) Some by the action of mercury on a salt of the metal, as the introduction of metallic mercury into a solution of a salt of the metal; and (3d.) Others by the action of the metal on a salt of mercury, as, the introduction of the metal into a solu- tion of a salt of mercury. EXPERIMENT No. 72.— Into a solution of mercuric chloride place a strip of clean aluminum. A deposition of mercury will take place on the surface of the aluminum. (4th.) By voltaic action, as when a metal is placed in contact with mercury in some acidulated solution. IN THE ARTS.— "Silvering."— The process known as "silvering on glass" was until recently a misnomer, as tin amalgam was alone employed in the manufacture of mirrors. This was accomplished on a perfectly smooth, flat, stone surface, surrounded by a wooden gutter. On this surface was smoothly spread a sheet of tin-foil somewhat larger than the glass to be operated on. A small quantity of mercury was rubbed over the surface of the tin to "quicken" the foil; the impurities were taken off, and mercury to the depth of about % of an inch, or sufficient to float the glass, was poured over the whole. The scum was then carefully removed from one end, and the glass, started there, was slid over the mer- cury-covered foil, carrying with it most of the superfluous mercury and the impurities; heavy weights were then placed upon the glass, until the greater part of the remaining mercury was pressed out. The table being tilted diagonally, all the superfluous mercury found its way to the gutter. After twenty-four hours the amalgam 296 PRACTICAL DENTAL METALLURGY. backing was sufficiently hard and adherent for the glass to be moved aside for more perfect drying, which in the case of large sheets occupied some twenty-five to thirty days. "Fire-gilding." — Before the action of the galvanic current upon solutions of metals was understood, amal- gams were greatly used in the process known as fire- gilding or fire-silvering. This was effected by coating the object to be plated with the amalgam of the corre- sponding metal, and volatilizing the mercury by the application of heat, the gold or silver remaining as an adherent coat which was afterwards burnished into a compact film. The attraction of mercury for gold and silver is taken advantage of for the extraction of those metals from their ores. The addition of a little amalgam of sodium to mercury increases its combining power, and it more readily unites with other metals, even iron. This is especially recommended in the employment of mercury in the extraction of silver or gold from their ores. An amalgam of equal parts of tin and zinc with six parts of mercury is much used for rubbers on electrical machines. DENTAL-AMALGAM ALLOYS.— The term com- prehends those alloys composed principally of silver and tin, with the addition of small percentages of one or more other metals, which, when comminuted and mixed with mercury, form a coherent mass.* A DENTAL AMALGAM may, therefore, be under- stood to be a comminuted metal or dental- amalgam * Such a distinction precludes the confusion of the terms " Dental Alloy " and ' ' Dental Amalgam, the former of which we may understand as applicable to any of the numerous alloys used by dentists, for whatever purpose, and which do not contain, nor are designed to be mixed with, mercuiy; the latter being accepted as the Dental-Amalgam Alloy mixed with mercury. — Author. AMALGAMS. 297 alloy mixed with sufficient mercury to form a cohe- rent mass. There are alloys which contain small percentages of mercury, added usually to lower their fusing points. Within the strict reading of the definition of amalgam these might be considered amalgams, but in the dental acceptation of the term they cannot be regarded as such. HISTORY.— "The introduction of amalgam," says Dr. Burchard,* " was not prompted by any specific merit that it had been demonstrated to possess, but was due solely to its properties of easy introduction, compari- tively perfect sealing and prompt hardening, qualities which apparently recommend its wide and general use to those not possessing the requisite degree of skill for the successful manipulation of gold foil." " Applied upon a bases of glaring empiricism, with an absence of technical skill, the material received the prompt and sustained condemnation which its abuse had warranted. The steps and phases of this opposition of the trained and skilled against untrained and unskilled operators may be read in the dental journals of from 1846 to 1878, and even after. It was commonly known as the ' amalgam war.' " Amalgam was probably first introduced in the year 1826 by M. Traveau of Paris, who made an amalgam of pure silver and mercury and called it "Silver Paste." For convenience the pure silver was afterwards replaced by silver coin (composed of about 9 parts silver and 1 of copper). This was introduced in America in 1833 by the Crawcours, two French charlatans, under the name of "Royal Mineral Succedaneum." These adven- * Operative Dentistiy, Kirk, p. 219. 298 PRACTICAL DENTAL METALLURGY. turers opened an office in New York and did a thriving, though unscrupulous, business for a time. This was the signal for the beginning of the heated opposition to this material already referred to. In 1841 the American Society of Dental Surgeons declared that any material containing mercury was injurious, and subsequently declared, its use malpractice. In 1845 this society exacted a pledge of its members not to use it. Many prominent members were using and advocating its use at the time, and the action on the part of the society met with such violent opposition that the requirement to sign a pledge was withdrawn. In 1849 Dr. Thomas Evans of Paris presented a formula of pure tin and cadmium. An amalgam made from this alloy was found to shrink and so greatly discolor the dentine of the teeth into which it had been introduced, that it was soon discarded by Dr. Evans himself. About this time Dr. Elisha Townsend of Philadelphia introduced his alloy of .silver 42 and tin 58. The amalgam of this alloy received an immediate endorsement and application on account of the eminence of its author, but a reaction soon occurred which brought amalgams again under the ban. Soon after this time the " New-Departure Corps " was organized and espoused the cause of amalgams, and Dr. J. Foster Flagg made marked improvements in the silver- tin alloy. To his conscientious and faithful adherence to the use of plastics is due much of the credit for the present status of dental amalgams. The later investigations of Dr. G. V. Black have placed the study of amalgams upon a thoroughly scientific basis, and much may be expected from the work he has so scientifically begun. AMALGAMS. 299 FORMATION OF DENTAL-AMALGAM AL- LOYS. — The directions for the preparation of alloys in general are equally applicable to the preparation of these special ones. The same precautions should be observed to avoid loss by oxidation, volatilization, etc. The manner of melting and pouring differs in no essential. It will, therefore, suffice to briefly illustrate the pro- cess by detailing the manner of preparing one from the metals usually employed, such as tin, silver, and gold or copper. The source of heat may be an open-grate coke or coal fire, the forge, or what is best adapted to the purpose, the small injector gas furnace devised by Mr. Fletcher for melting metals. The crucible may be the ordinary refractory sand or Hessian, a clay and plumbago, or the plumbago crucible alone, the latter being preferable after it has been tested by heat. The crucible selected should be placed in the furnace and heated to a bright red heat; then a sufficient quantity of borax should be added to properly cover the inner sides when the crucible is tipped and rotated with the tongs. The silver and gold or copper may now be added, preferably in small pieces of thinly rolled plate, and thoroughly heated until fused. Being sure that the borax is melted as thin as possible, the tin may be added in as large pieces as convenient, that they may readily sink and unite with the fused metals before oxidation can take place. The crucible should then be removed with tongs, the contents well shaken or stirred with a stick of soft wood, and quickly poured into the previously warmed and oiled ingot moulds, after which it is ready for comminution. 300 PRACTICAL DENTAL METALLURGY. Borax is used as the flux, because it more perfectly protects the metals from oxidation and volatilization, ab- sorbing the oxides that may have been previously formed or developed during the fusing; protects the molten metals from the rough and porous sides of the crucible, and facilitates the pouring. The difficulty in adding tin is to avoid volatilizing any portion of it. It has been, therefore, melted separately, and the molten silver and gold or copper poured into it. This plan very thoroughly protects the tin, but it is ques- tionable if the less fusible metals are not so chilled that their proper alloying is prevented. In such a case the ingot should be broken up and remelted under plenty of borax. An exceedingly good plan to avoid volatiliza- tion is to wrap the volatile metal in soft paper that will conform readily to the sides of the piece of metal and quickly dash it beneath the surface of molten borax. The paper quickly chars and serves both as a covering and reducing material. The addition of zinc is probably the most difficult to accomplish without loss, as it so easily oxidizes. It is, therefore, sometimes united with a small amount of gold previously by wrapping the small grains of zinc in gold foil and thrusting them beneath the molten borax. Bismuth, antimony, and the other more fusible metals, are alloyed with the mass similarly to tin. Platinum, palladium, and such, with the silver, or silver and gold melted as recounted above. The appended table of Dr. J. O. Keller gives the composition of some of the prin- cipal dental-amalgam alloys in use:* * American System of Dentistry, Vol. Ill, p. 813. AMALGAMS. 301 CO o i-J <1 <1 o < H Q O % i w to o to a o to o o »— i o to § o o .J •a a c o O a a a o a a! ^ CO "JD CO O 0-1 CO CO OS iC O H uO io co >ra co olflW'J tflrtf- >1 In 302 PRACTICAL DENTAL METALLURGY. Co I * CO O a p £33 'Sri v a c ,-vo ^* $ Tj i - * CM bo w> p. a t/2 tc:< Contrary to Dr. Flagg's views, he demonstrates that the value of ageing depends upon the formula of the alloy. For example, an alloy of silver 65 and tin 35, made into an amalgam, when freshly cuts/trunk not at all; but, instead, expanded one ten-thousandths of an inch. An amalgam made of the same alloy, after annealing shrunk ten ten-thou- sandths of an inch. On the other hand, an alloy con- sisting of silver 72.5 and tin 27.5 freshly cut and made into an amalgam expanded forty-two ten-thousandths of an inch, while the amalgam made of the same alloy annealed shrunk three ten-thousandths of an inch. As will be seen by the accompanying tables of Dr. Black's, annealing seems to decrease the expansion and increase the shrinkage. As a solution to the foregoing inquiry : If the operator prefers to use freshly cut alloy, the comminution should be followed immediately by amalgamation, and the amalgam thus made should be such as will neither shrink nor expand, or at least these changes should be as near zero as possible. On the other hand dealers and users of aged alloys must see to it that the alloy is of such a formula that age, either natural or artificial, brings these changes of expansion and contraction of the amalgams made from them down to a minimum. f *See tables, page 306. t The student should be directed to Dr. Black's article, Physical Properties of Silver-Tin Amalgams, Dental Cosmos, Vol. XXXVIII. p. 965. 306 PRACTICAL DENTAL METALLURGY. TABLE OF UNMODIFIED SILVER-TIN ALLOYS.* Formulae. How Prepared. Ph Shrinkage in Ten-Thousandths of an Inch. Expansion in Ten-Thousandths of an Inch. o •>' u /l^ilp* edge. They are so arranged as to place them on ^ ;s Mli!||!|^ the stage of the microscope, and rotate them so I