^v ^s ^ \^ r^^^Yr>\i"t\ v\v^^'^N LIBRARY OF CONGRESS. Clia}3..L-_-- Copyright No._ Shelf„.Ji___l: UNITED STATES OF AMERICA. :mm^Mmmjmmm^,^\::^,. « 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, with Experiments. JOSEPH DUPUY HODGEN, D. D. S. Assistant to the Chair of HDcntal Chemistry and Metallurgy, University of California, College of Dentistry ; late Editor of Pacific Coast ^Dentist. -,^.«^r,8. $.'^^oof«'e«';''''.v .^ ^^"^v V* Entered according to Act of Congress, in the year 1896, 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, GRATITUDE FOR HIS TEACHINGS, CRITICISMS, AND FRIENDSHIP. jprekace:. In presenting this little volume to the practitioner and student of the dental profession, the author does not flatter 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 laboratory, 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 everyday 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 contribution to the Anieyican System of Dentistry^ entitled Dental Metallurgy, by Dr. Edward C. Kirk ; Brannt's Metallic Alloys ; the works on metallurgy by Makins, Fletcher, Kssig, Mitchell ; the chemistries of Roscoe, Bloxam, and many others, found in the library of the California Mining Bureau, through the kindness of the State Mineralogist, Mr. J. J. Crawford ; together with the Dental Cos7nos, Dental Review, International Dental 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. Iv. 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. No. 1005 Sutter Street, San Francisco, September 26, 1896. CONTKNTS. Chapter Page I. Introduction 9 IE. The Properties of Metals 16 III. Alloys 31 IV. Amalgams 46 V. Amalgams — continued. 57 VI. Amalgams — concluded 71 VII. Melting Metals 85 VIII. Combination of Metals with Non-Metallic Elements. 105 IX. Lead 113 X. Antimony 123 XI. Tin 129 XII. Bismuth 140 XIII. Zinc 148 XIV. Cadmium 161 XV. Copper 166 XVI. Iron 178 XVII. Aluminum 197 XVIII. Mercury 208 XIX. Silver 220 XX. Iridium 237 XXI. Palladium 241 XXII. Platinum 246 XXIII. Gold 254 Addendum— Collateral Literature. . . . , 301 PRACTICAL DENTAL METALLURGY. CHAPTER I. INTRODUCTION. CHEMISTRY is that branch of science which treats of the atomic conditions of matter, and especially of atomic changes. It comprehends the combination of diverse forms of matter producing new compounds, and the separating of already existing compounds into simpler ones, or resolving them into their ultimate principles, which are called — ELEMENTS. — Substances whose molecules contain one kind of atoms only, and which all physical or chemi- cal processes have as yet failed to break up or decompose into two or more dissimilar substances. It is not as- serted 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 b3^ 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. Symbol; Negative End. Atomic Weight, o s N F CI Br I Se P As Cr V Mo W B C Sb Te Ta Cb Ti Si H All OS Ir Pt Rh Ru Pd Hg Ag Cu OXYGEN SULPHUR . NITROGEN FLUORINE CHLORINE BROMINE IODINE Selenium PHOSPHORUS ARSENICUM CHROMIUM Vanadium Molybdemun Tiingsten (Wolfram) BORON CARBON ANTIMONY (Stibium) Telliiriujn Tantalum Columbium (Niobium) Titanium SILICON HYDROGEN GOLD (Aurum) Osmium Iridium PLATINUM Rhodium Rutheiiium , Palladium MERCURY (Hydrargyrum) SILVER (Argentum) COPPER (Cuprum) 15.96 31.98 14.02 18.98 35.37 79.76 126.55 78.79 30.95 74.91 52. 51.25 95.52 183.61 10.94 11.97 119.95 127.96 182,14 93.81 47.99 28.19 1. 196.15 198.49 192.65 194.41 104.05 104.21 105.73 199.71 107.67 63.17 INTRODUCTION. 11 TABLE OF Eh'E'SinNrS— Continued. Symbol. Negative Eud. Atomic Weight. u ^ Bi ^-^ Sn ^ In ^ Pb ^ Cd * Tl >:= Co ^ Ni 'fi Fe ^ Zn * Ga '-K Mn >i^ La >1^ D ^ Ce >^ Th * Zr >K Al * E * Y >i< Gl >i« Mg >i^ Ca * Sr * Ba :*: Li '!< Na ^ K ^< Rb ^ •Cs * Uranium 238 . 48 BISMUTH 207.52 TIN (Stannum) 117.69 hidiu7n 113.39 LEAD (Plumbum) 206.47 CADMIUM 111.83 Thallium 203 . 71 COBALT 58.88 NICKEL 57.92 IRON (Ferrum) 55.91 ZINC 64.9 Gallium 68.85 MANGANESIUM 53 . 9 Lanthanum 138. 52 Didymium. 144. 57 Cerium 140.42 Thorium 233.41 Zirconium 89 . 36 ALUMINUM 27. Erbiictn 165 . 89 Yttrium 89.81 Glucinum (Beryllium). 9.08 MAGNESIUM 23.95 CALCIUM 39.99 STRONTIUM 87.37 BARIUM 136.76 LITHIUM 7. SODIUM (Natrium) 22.99 POTASSIUM (Kalitim) , 39.01 Rubidiicm 85 . 25 CcBsiuvi • 132 . 58 Positive End. 12 PRACTICAI. DENTAL METALLURGY. To these are added *Davyum and ^Terbium. Some ten or twelve other substances thought to be elements 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 7ion- 771 eta I lie. METALLIC ELEMENTS, the metals, or, as they are frequently termed, the positive elements, are fifty-two in number (denoted by the * ), and the study of these constitutes the science of metallurgy. METALLURGY is the science of economically ex- tracting metals from their ores, and to this strict defi- nition may be added the art of applying them to useful purposes. Of the fifty-two elementary substances known as metals only fou7'teen are employed in their true 77ietallic coTidi- tio7i. 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. INTRODUCTION. 13 While the remaining twenty-six are more or less rare and as j^et 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 oxy- gen are decomposable by heat alone, at a temperature not exceeding redness. These are: Mercury, Rhodium, Silver, Ruthenium, Gold, Osmium, Platinum, Iridium. Palladium, Base metals are those whose compounds with oxygen are not decomposable by heat alone, retaining oxygen at high temperatures. The base metals are further subdivided according to their affinity for oxygen and. other chemical properties. THE FIRST DIVISION Contains five metals, which decompose water, and com- bine with the oxygen, liberating hydrogen. They are very readily oxidized, and their oxides are soluble in water, giving it an alkaline reaction ; so also are their phosphates and carbonates. They are soft and fusible at low temperatures. These are: Potassium^ Rubidium, Sodium, Calcium. Lithium, THK SECOND DIVISION Contains four metals, some of which decompose water rapidly, combining with the oxygen. Their oxides are more or less soluble in water, rendering it alkaline ; but 14 PRACTICAL DENTAL METALLURGY. the neutral carbonates and phosphates are insoluble. These are : Barmm, Calciiun, Strontium, Mag7iesium. 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: Alummuut, Erbium, Chromium , Ceri u m , 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: Iro7i, Urajiium, Nickel, Vanadium, Cobalt, Thallium, Manganesium, Indium. Zinc, THE FIFTH DIVISION Contains four metals, which do not decompose water at any temperature. These are: Cadmium, , Bismuth, Lead, Copper. THE SIXTH DIVISION Contains six metals. All the higher oxides of these metals have acid properties. These are: INTRODUCTION. 15 Antimony, Arsenic, Molybdenum, Tungsten, Tellurium. The non-metallic elements may be divided according to their ph3^sical states at ordinary temperatures, thus: Gases. Oxygen, Hydrogen, Nitrogen, Chlorine, Solids. Fluorine. Carbon, Boron, Sulphur, Silenium, Phosphorus. Iodine. Silicon, Liquid. Bromine. CHAPTER II. PROPERTIES OF METAI.S. A METAL is an elementary substance, solid at ordi- nary temperatures, with the single exception of mercury (a liquid solidifying at —40), having a peculiar lus- ter, called a ''metallic bister,'' and the property of re- placing hydrogen in chemical reactions, as for example: Zn + H,S04=ZnS04+H„ insoluble in water, a good conductor of heat and elec- tricity, 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. ' ' Bloxan evidently does not regard arsenic as a metal. Ofithesays: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 * Lessons in EJlementary Chemistry, p. 148. f Chemistry, Inorganic and Organic, p. 272. PROPERTIES OF METAI.S. 17 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 rela- tions 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 proper- ties 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 pecu- liar manner, producing what is called metallic luster. When kept in non-metallic vessels they take the shape of a convex meniscus. When exposed to greater tem- peratures, 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 temperatures, 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, endowed 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 * Lessous in Elementary Chemistry, p. 131. 18 PRACTICAI. DENTAL METALLURGY. 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 plates, 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 ab- sorbed, while the latter, in regard to polarization, is quite differently affected, which fact, in all probabilit}^ ac- counts 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 character, 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 action. The odor, Dr. Essigsays,^ " may be noticed in a * Dental Metallurgy, p. 20. PROPERTIES OE METALvS. 19 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 follow- ing systems: Regular — Silver, gold, palladium, mercury, copper, iron, lead; quadratic — tin, potassium; 7'hombic — 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 homogenous mass. Malleability, Ductility, and Tenacity are those properties possessed by some metals by virtue of the cohesive power of their molecules, and are to that ex- tent 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 rela- tively, it being most wonderfully exemplified 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 surface of 75 square inches. 20 PRACTICAL DENTAL METALLURGY. 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 covering the gold wire with silver, which is also remarkably ductile, thus making a composite wire of greater thickness. After drawing them to the greatest possible attenuation, the silver was dissolved 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: Ductility. Gold. 1. Silver. 2. Platinum. 3. Iron. 4. Copper. 5. Zinc. 6. Tin. 7. Lead. 8. Nickel. Palladium. Cadmium. Malleability. 1. Gold. 1. 2 Silver. 2. 3' Copoer. 3' 4. Tin.^ 4. 5. Cadmium. 5. 6. Platinum. 6. 7. Lead. / . 8. Zinc. 8. 9. Iron. 9. 10. Nickel. 10. 11. Palladium. 11. Tenacity, Iron. Copper. Platinum. Silver. Gold. Zinc. Tin. Lead. 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, 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 PROPERTIES OF METALvS. 21 very slightly ductile, and iron, while ninth in point of malleability, is fourth in ductility. In the qualit}^ of malleability the granular particles of the metal are flat- tened and spread in all directiocs, while in that of ductilit}^ 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 plasticity. Their original softness can be restored to them by annealing, i. e., by heating them to redness and then plunging them into cold 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.* Tenacity is that property possessed by metals, in con- sequence 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 properties 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 * See chapter on Iron. 22 PRACTICAL DENTAL METALLURGY. 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. 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 Wertheimf in his experiments on a number of the metals at temperatures from 15° to 20° C. * See chapter on Gold. t Aunales de Chimie et de Physique (III.) Vol. XII. PROPERTlEvS OF METALS. 23 For Wire 1 Square Mm. Section, Weight in (in" Kilos) Causing Name. Permanent Elon- gation of Breakage. Iron, drawn , . . . 32. Under 5. 12. Under 3. ii!3 2.6 13.5 3. .75 1. .45 .2 .25 2 61 " annealed Copper, drawn 47. 40 " annealed Platinum drawn . ... 30. 34 " annealed 23. Silver, drawn " annealed 29. 16. Gold, drawn 27. " annealed 10. Zinc, drawn 13. " annealed Tin, drawn 2.45 " annealed 2.1 Lead, drawn 1.8 ' ' annealed Elasticity. — All metals are elastic to this extent, that a change in form, brought about by stresses not exceed- ing 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. Thus, iron compounded with the proper amount of carbon, 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 elasticit}^ yet when combined in proper proportions — gold 109, and platinum 4 part.«, 20-carat fineness, the alloy is found to be quite elastic and is much used as clasps for artificial dentures. Fusibility and Volatility. — All metals may be fused, and most of them are capable of being volatilized, but 24 PRACTICAI, DENTAL METALLURGY. the temperature at which they become fluid differs greatly in different metals, as the following table shows: Name of Metal. Fusing Point. Centigrade. Fusing Point. Fahrenheit, Authority. Mercury ......... —38.8 + 26 to 27. —39. + 78.8 86. 101.3 144.5 203.9 356. 348.8 442 507.2 554. 608. 617. 779. 797. 977. B. Stewart Setterberg L. de Boisbaudran Bunsen Gallium Rubidium 30.1 38.5 Potassium 62.5 .... .Bunsen Sodium T .ifhium . .... 95.5 180. 176. 228. 264. Bunsen ( "* ) Indium Richter (?) Rudberg Rudberg Tin Bismuth Thallium 290. Lamy Cadmium 320. 325. 415. 425. 525. . . . .Rudberg Lead Zinc Person Antimony Incipient Red Heat Pouillet Magnesium Aluminum Cherry Red Heat. . Silver 700. 700. 1040. 1100. 1292. 1292. 1904. 2012, 2012. 2192. 2372 to 2552. 2912. 2552. 2912. ( ? ) Pouillet Bacquerel Gold Yello-cv Heat 1100. Pouillet Copper 1200. Iron, wrought. . . . |1300 to 1400. Iron, chemically: pure hip-her-lfiOO. Cobalt Nickel 1400. 1600. ( ? ) Uranium .... Dazzling White Heat 1500 to 1600. Palladium 1600. 2732 to 2912. 2912 Pouillet Oxy hy drogen Flame Platinum , . . 2000. 3632. Iridium Rhodium . i Max. Temp, of Oxyhydrogen 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. 25 Metals may be characterized as ''Jixed^^ 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 357.3° C 860.0°" 1040.0° " Below 1040.0° " Above 1040.0°*' Regnault Deville and Troost Deville and Troost Dewar and Dittmar Cadmium Zinc Potassium Sodium. For practical purposes the volatility of metals may be classed as follow^s : 1. Those distillable below redness: Mercury. 2. Those distillable at red heats: Cadmium, Potassium, Zinc, Sodium. Magnesium, 3. 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 ivith very great difficulty volatilized ^ if at all: Gold, Copper (?). 5. Those which are practically '''fixed,' ^ or non-volatile: Copper (?), Aluminum, Iron, lyithium, Nickel, Strontium, Cobalt, Barium. Calcium, In the oxyhydrogen flame silver boils, forming a blue vapor, while platinum volatilizes slowly, and osmium, though infusible, very readily.* * William Dittmar. 26 PRACTICAL, DKNTAL MKTAI.LUKGY. "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 volatilized, so that quantities, amounting to 4^ grains, could be collected during the pouring out of 30 pounds weight from a crucible. * >K >p That mixtures of gold, silver, and lead, when cupelled together, volatilize considerably." Specific Heat. — Equal weights of diflferent metals have been found to absorb diflferent 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° will raise 31 times the same weight of platinum through 100° of temperature. Thus, water being taken as the standard or unit, the specific heat of platinum is K„ or 0.031 that of water. 1 TABIvE OF SPECIFIC Iron. ; HEATS. 1138 2 8 Nickel Cobalt 1086 .1070 4 Zinc . .0956 5. 6. 7 Copper . . Palladium Silver 0952 0593 0570 8 Cadmium .0567 9. 10. 11. Tin Antimony Mercury Gold. 0562 0508 0333 . . .0324 13. 14 Lead Platinum 0314 0311 15. Bismuth 0308 PROPERTIES OF METALS. 27 EXPERIMENT No. 1.— Prepare bullets of exactly equal weights of several of the above metals, such as zinc, silver, cadmium, tin and lead; ex- pose 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 dif- ferent axes. To eliminate this source of uncertainty, 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 ot Metal. Expansion. 0" to 100" C. 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 Mercurv .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. 28 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 a 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 elec- tric — 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. Thermic. Electric. Silver 100. 100. Copper Gold Tin Iron 73.6 53.2 14.5 ..' 119 99.95 77 96 12.36 16.81 Lead Platinum Bismuth. Brass , . , .... 86 8.4 1.8 23 6 8.32 18.8 1.24- Steel German Silver . . . o 1L6 7.3 7.67 Rose Fusible Metal Pianoforte Wire 2.8 14.4 *Dr. E. C. Kirk, Am. System of Dentistry, Vol. Ill, p. 793- PROPERTIES OF METALS. 29 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 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 different 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 6.163 Bunsen Potassium Sodium Baumbauer Baumliauer Rubidium Bunsen Calcium .... Bunsen and Matthiessen Mas^nesium Bunsen Caesium. Setterberg Debray Glucinum . . Strontium Aluminum Mallet Barium Clarke Zirconium Troost Vanadium Roscoe Gallium Ivanthanum Ivccoq de Boisbaudran Ivecoq de Boisbaudran ( Table continued on follozving- pa^e.) 30 PRACTICAL DENTAL METALLURGY. TA.^Y,^—Coiiiimied. Name of Metal. Specific Gravity. Authority. Didymium Cerium .... .... 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 10.54 18.33 19.265 21.46 22 4 22 "477 r Hillebrandt and \ Norton r Hillebrandt and \ Norton Antimony Chromium Zinc Wohler Karsten Manganesium Tin Brunner Indium Richter Iron Berzelius Nickel Richter Cadmium Schroder Cobalt Molybdenum Debray Holzmann Copper Bismuth Silver Lead Palladium Thallium Holzmann Deville Deville and Debray Crookes Rhodium Bunsen Ruthenium Mercury Tungsten Uranium Gold Platinum Iridium Osmium Deville and Debray H. Kopp Wohler P^ligot Matthiessen Deville and Debray CHAPTER III. 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 combination; 3. — a mechan- ical mixture; or, 4. — a solution or mixture of two or all of the above; and that similar differences may exist as * British Association Reports, 1863, p. 97. 32 PRACTICAL DENTAL METALLURGY. to its condition in the solid state, defining a solid solu- tion as " a perfectly homogenous diffusion of one body in another." 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 homogenous 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 compo- sition, distinct crystalline form, and a variety of proper- ties different 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 tem- perature is noticeable on the addition of each successive piece of sodium. 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 separated by decantation. Platinum, iridium, gold, and silver unite with tin, accompanied by an evo- lution 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 ALLOYS. 33 dissolved, and crystals of a definite alloy of tin and the precious metals remain.* Examples of such union by deOnite proportio4i 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 platinum 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 * See chapter on Tin. 34 PRACTICAL DENTAL METALLURGY. classed under this head, when we consider the almost infinite proportions in which they are united. The Physical Properties of Alloyscannot be antici- pated, 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 Thenard, shows examples of this variation: Alloys Possessir s a Greater Specific Alloys Having a Specific Gravity Gravity than th s Mean of Their Inferior to the Mean of Their Com ponents. Components. Gold and Zinc Tin Bismuth Antimony Cobalt Gold and Silver Iron Lead Copper Iridium Silver " Zinc Nickel " " Lead Silver Copper it << Tin Copper Lead (( ct Bismuth Iron Bismuth " " Antimony " Antimony Copper " Zinc " Lead Tm Tin Lead Palladium " Palladium " " Bismuth <' Antimony " " Antimony Nickel Arsenic Lead Bismuth Antimony Zinc Antimony Platinum " Molybdenum Palladium " Bismuth * Phillip's Metallurgy. ALLOYS. 35 It is common among authorities who publish deter- minations 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 partak- ing of that metal which predominates. Some few exceptions are quite notable, for instance, gold 7, and silver 3 parts produces an alloy of a greenish color, and it is said that ){^ of silver is sufficient to modify the color of gold. Nickel and copper form alloys varying from copper-red to the blue 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 de- creases, while the hardness as compared with that of the constituent 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. 36 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: Ivbs. 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, ith 12 per cent. Tin 80 to 90 Copper 7 Tin 7 Copper 70 Platinum 75 to "" Copper, alloyed w Tin, Lead, " Gold, Silver, Steel (iron compounded v^ith 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 componeuts. Thus, an alloy composed of 10 parts lead and 4 parts tin fuses at 470° F., melting lower than the less fasible 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, audi 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 * Makius" Metallurgy, p. 65. ALLOYS. 37 solid state exhibits excess of attraction over repulsioa, 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 homogenous 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 78 or 80, and tin 22 or 20. 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," says Dr. Kirk,* "such as 'German silver,' for measuring the resistance of long lines of telegraph wire, the electro- motive force or working power of batteries, etc., the low conductivity of a wire of given dimensions of such an alloy furnishing a convenient standard for comparison.'' 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 * American System of Dentistry, Vol. Ill, p. 800. 38 PRACTICAL DENTAI. METAI.LURGY. removed entirely by roasting; but if heated with small quantities of potassium nitrate, which serves to oxidize the base metal, it may be entirely removed. Mercury may be completely separated by roasting; it volatilizes at about 600° 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 de- creased 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 cracking under his blows and pres- sure. 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 toughness 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, ti7i, and zinc are annealed by immersion in water, which is made to boil and then cool slowly. Steel should not be ALLOYS. 39 annealed in an open fire, as the carbon which enters the iron as an element combines with the oxj^gen of the air to the detriment of the steel. Annealing may be said to be the inverse process of — Tempering, which is the fixing of the molecular con- dition of steel by more or less sudden cooling from a par- ticular temperature. Oxidation. — Alloys are usually more easih^ oxidized than their constituents. Mr. Makins says:* "The su- perior 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 ex- ceptional difiiculty in thoroughly blending the constitu- ents, and the alloy resembled the ordinary dental-amal- gam alloy when filed and ready for mercury, but upon * Makins' Metallurgy, p. 64. 40 PRACTICAL DENTAL METALLURGY. 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. " 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 di- minishes the cohesive property of the alloy by preventing perfect contact of the particles; hence, much of the strength and toughness of the mass are lost. The best preventative againt 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 efi^ected by vigorous stirring with a stick of green wood. The careful addition of not more than /(^^^ to %oo 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 Stirling 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 instances characteristic; thus, in a general way, mercury, cadmium, and bismuth increase fusibility; tin, hardness and tenacity; antimony and arsenic, hardness and brittleness. * American System of Dentistry, Vol. Ill, p.Soi. ALLOYS. 41 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 unoxidized during the operation several fluxes are used, such as dehydrated borax, or some of the reliable pre- pared 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 42 PRACTICAI, DENTAI. 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 oxidation 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 admirablj^ 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 dimin- ishes 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. 43 extent preparatory to using the blow-pipe. Wlien 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 will be 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 accurate 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 con- stituent, 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. 44 PRACTICAI^ DKNTAI, 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 fourth 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 necessary, 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 prepara- tion of expensive alloys from noble metals, the employ- ment 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 may be 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 difiicult 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. 45 intimate admixture should be effected by vigorously stirring the molten mass with sticks of soft, dr^'- wood, which are more or less carbonized, according to the tem- perature of the mixture. In consequence of this dry distillation 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 prop- ert}^ of changing their nature by repeated remelting, several alloys being formed in this case, which show considerable differences, physically as well as chemically. The melting points of the new alloys are generallj^ 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 farther 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 properties are, as a rule, produced; or, if such does not answer the demands of the alloy, the object may be at- tained by taking two, three, or more equivalents of the metal, exception being made in the cases of arsenic and such elements. * Metallic Alloys, p. 87. CHAPTER IV. AMALGAMS. AN AMALGAM is an alloy of two or more raetals, 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, and even hydrogen, it is claimed, 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 contrary, they are to be considered in all respects alloys, differing from the usual in no general way, except that all contain one same con- stituent and are endowed with some properties particular 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 facilitated by the low temperature at which their combinations are effected. The union of mercury and other metals may be said to take place by three different means: (1st.) Some by direct contact^ 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 AMALGAMS. 47 explosiveness, accompanied by incandescence, and the evolution of an amount of heat sufficient to volatilize portions of each metal. (2d.) Some by the action of mercicry 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 — sometimes developing a weak electric current. The affinities affording the union of mercury with its constituents in the formation of amalgams are not, as a rule, 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^Hg.* " " silver, AgHg. " " iron, FeHg. " " zinc, Zn^Hg. " " copper, CuHg. " " platinum, PtHg^. A native compound of mercury and silver, known as Arguerite, AggHg, is found crystallized in the form of the regular system. Beautiful crystallizations of silver amalgam (Arbor DiancB) may be formed in long prisms, having the com- position Ag2Hg3, 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 * Eloxam's Chemistry, Inorganic and Organic, p. 400. 48 ' PRACTICAI, DENTAL METALLURGY. few hours beautiful crystals of considerable length will be deposited. 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 flioat 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 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. AMALGAMS. 49 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 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 comprehends 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 mer- cury, form a coherent mass.* A Dental Amalgam may, therefore, be understood to be a comminuted metal or dental-amalgam alloy mixed with sufficient mercury to form a coherent 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. The history of dental amalgams dates from its first in- troduction as a filling-material for carious teeth, some sixty-five years ago, and it appears that from its earliest use it engendered, for obvious reasons, so much ener- getic opposition on the part of some, that a ten- years' war was waged against it as a tooth-conserving filling-material. During this time, in the eyes of many, * vSuch a distinction precludes the confusion of the terms " Dental Alloy " and " Dental Amalgam," the former of which we may understand as appli- cable to any of the numerous alloys used by dentists, which do not contain, nor are designed to be mixed with, mercury; the latter being accepted as the Dental-Amalgam Alloy mixed with mercury. — Author. 50 PRACTICAL DENTAL METALLURGY, it established for itself such an unenviable reputation that even at this enlightened day of scientific and prac- tical experimentation its advocates have failed to entirely eradicate the error. And this, notwithstanding amalgam and gold are the only filling-materials in general use that are supposed to endure with any sort of permanency the continual stress of mastication they are called upon to bear, and that the former has proven its superior qual- ities to the majority, if not to all, as a medium for the conservation of teeth, by probably saving more than any other material. Dental-amalgam alloys may be classified as binary^ ternary^ quaternary^ quinary, etc, according to the number of elementary constituents of which they may be made up. A Binary Dental Amalgam may be composed of mer- cury alloyed with any one of the various metals used as constituents of dental-amalgam alloys or dental amal- gams, such as silver, tin, gold, copper, platinum, zinc, palladium, cadmium, bismuth, or antimony. Copper and palladium, however, are the only ones which have thus far found any place of apparent usefulness. Silver formed the prototype of binary dental amalgams, as it did of dental amalgams in general. When finely divided, such as the precipitate, it readily and rapidly combines with mercury, evolving consider- able heat, and forming a hard mass in a few seconds. In the state of larger particles, such as filings, it combines more slowly. According to Mr. Fletcher,* "The rapid- ity of combination is reduced by the use of a mixture of precipitated silver and filings. If the precipitate is in excess, and the mass is inserted before the hardening commences, there is a risk of bursting the tooth by the * Dental Metallurg3-, p. 33. amai.ga:\is. ol gradual expansion of the mass " (which Mr. Kirbj^ claims amounts to l-40th of the diameter of the plug), and "if a great excess of mercury is used the mass only partially hardens and the results are uncertain." Silver is, however, the most important and essential component of dental-amalgam alloys, and is usually the largest component of their composition. It unites chem- ically with mercury to form definite chemical compounds* having the varying formulae of AggHg and Ag2Hg3. It is readily discolored by sulphur compounds. Tin very readily combines with mercury, forming a friable, very slowly and imperfectly hardening amalgam. It is very probable that the action of tin and mercury when combined is a decided contraction of the mass. It has been stated by some that the mass expanded, and by others that it contracted, but accurate data are wanting. The author bases his opinion entirely upon the data fur- nished by Dr. G. V. Black. f In arriving at this con- clusion, however, it was necessary to eliminate so many factors of influence, such as chemical affinities, fineness of cut of the alloy, manner of mix and manipulation, etc., that the opinion after all seems of little value. The fol- lowing notes of Dr. Black, rearranged, formed the data: Fillings, how Inserted. Silver. Tin. Per cent of How Mer- Mixed. cury. 46.36 Hand . . . 44 6 " 37.85 " 37.27 " 45.31 Mortar. . / Amount of Contraction=( -) ^ ^""^ n'ni? of ^"^^ ^ Contrac- Measurement, ExoTnsTon I Thousandth 1^-^?'^?,!'°^' of an Inch. ; ^^ ^^^>^' Hand pressure Burnished. . 42.45 42.45 35 40 57.55 57.55 —.05, +.1, —.8, 4-.2, -9, -l-.l +.1, —.1 Equalized Neither | Neutral 05 Equalized 2 I .0008 j .0008 * See page 49 of this chapter. t Contraction and Expansion of Silver-Tin Amalgams, Black, Dental Cos- mos, Vol. XXXVII, p. 648. 52 PRACTICAL DENTAIv METALLURGY. Tin and mercury show a disposition to unite — forming a definite chemical compound of a weak crystalline nature. It is second to silver in importance as a con- stituent of dental-amalgam alloys. Gold combines with mercury at any temperature, but more readily if either or both be heated slightly. Finely divided it combines even more rapidly. " Gmelin states that an amalgam of 6 of mercury to 1 of gold crystallizes in four-sided prisms, and that the mercury may be dis- tilled off from this, leaving the gold in the arborescent form."* Copper possesses the property of combining with mercury to form an amalgam which, on hardening or setting, may be softened by heat, kneaded, and inserted as a filling, and again becoming hard may be polished. It retains its metallic luster for some time when exposed to air, but blackens quickly when in contact with air or moisture containing sulphuretted hydrogen. Its peculiar properties have led to its introduction as a dental amal- gam, first known as Sullivan's Amalgam or Cement. Its preparation and properties Mr. Fletcher describes as foUowsrf " Precipitate from a weak solution of sulphate of copper by rods of pure zinc. Wash the precipitated copper with strong sulphuric acid (the addition of a small quantity of nitrate of mercury assists greatly), and add mercury in the proportion of 3 copper to 6 or 7 mercury. This alloy has the property of softening with heat and again hardening after a few hours." This amalgam has been thoroughly tried as a dental filling-material, and its use practically discontinued on account of the intense blue-black discoloration of its sur- face and the teeth containing it, and its undeniable surface * Makins' Metallurgy, p. 268. t Dental Metallurgy, p. 60. AMALGAMS. 53 disintegration. It has been thought to possess thera- peutic value; indeed, Dr. Kirk says: "Its preservative qualities render it a valuable constituent in alloys for use in teeth of a low grade of structure." Of it Dr. Black saysf-*^ " This amalgam has so many good qualities that many abandon it with much regret. I think it generally acknowledged that copper amalgam fillings 'retain good margins, when they are once made good, better or more perfectly than any other filling-material." He shows that frequent reheating deteriorates the amalgam very materially; claims that it will not flow under stress and "within the limits of its strength it is as rigid as hardened steel"; that it does 7iot cojitrad, but exhibits very slight expansion on setting, and attributes the properties heretofore assigned to its therapeutic qualities to the fact that it simply more perfectly seals the cavity. A variety of methods of preparing copper amalgam have been taught in the classrooms and described in our literature, some of which are as follows: Dr. RoUin's method, f — Distilled water, 5 gallons; sulphate of copper, enough to saturate; sulphuric acid, 1 pound. Mix, filter, and pour into a wooden firkin with wooden hoops. All the chemicals should be absolutely pure. Place 10 pounds of pure mercury in a glass jar and immerse in the copper solution. To the zinc plate of a gal- vanic battery attach a gutta-percha-covered wire, having one end bare for about an inch. This exposed end is to be immersed below the level of the surface of the mercury. Tie granulated pure copper in a bag and hang it in the copper solution, connecting with a wire to the carbon of the battery. The battery is to be kept in action till the mer- cury has absorbed enough copper to make a thick paste. Then remove, and wash thoroughly in hot water till all of the sulphate solution has been removed. Squeeze out the softer amalgam, and allow the remainder to * Copper Amalgam, Dental Cosmos, Vol. XXXVII, p. 737. t Boston Medical and Surgical Journal, Feb., 1886. 54 PRACTICAI. DENTAI. METALLURGY. harden. When it is hard, heat it, and renew the squeez- ing as before. A battery answers for home manufacture, but on a larger scale a dynamo should be used. Dr. T. H. Chandler's method for making his " No. 1 " and "No. 2" is as follows:* No. 1. To a hot solution of sulphate of copper add a a little hj^drochloric acid, and a few sticks of zinc, and boil for about a minute. The copper will be precipitated in a spongy mass. Take out zinc, pour off liquor, and wash the copper thoroughly with hot water. Pour on the mass a little dilute nitrate of mercury, which will instantly cover every particle of the copper with a coat- ing of the mercury. Add mercury 2 or 3 times the weight of the copper, triturate slightly in a mortar, and finish by heating the mixture a few moments in a crucible. No. 2. Take finely divided copper (copper dust) ob- tained by shaking a solution of sulphate of copper with granulated tin; the solution becomes hot, and a fine brown powder is thrown down. Of this powder take 20, 30, or 36 parts by weight and mix in a mortar with sulphuric acid, 1.85 specific gravity, to a paste, and add 70 parts of mercury, with constant stirring. When well mixed wash out all traces of acid and cool off. When used heat to 1300° F. It can be kneaded like wax in a mortar. Platinum. — "Worked platinum," says Mr. Makins.f ''cannot be amalgamated with mercury, and the only method of forming platinum amalgam consists in rubbing finely divided platinum (such as that reduced from the ammonio-chloride) and mercury together in a warm mor- tar; the combination of the two will be accelerated by moistening the two metals with water, accidulated with acetic acid," Zinc readily amalgamates with mercury to form a very brittle amalgam, whatever may be the relative pro- * Dental Chemistry and Metallurgy, Mitchell, p. 141. t Metallurgy, p. 304. AMALGAMS. 55 portion. With large amounts of mercury it forms an amalgam similar to that of copper, but too brittle for dental use. Zinc plates used in batteries are amalga- mated best by heating to about 482° to 500° F., and after quickly and uniformly coating them with a solution of the chloride of zinc and ammonia, dipping them at once into mercury. Amalgamation takes place immediately, and plates so amalgamated give currents of greater con- stancy^ and intensity than the ordinary zinc plates. Since the amalgam of zinc is not acted upon b}^ dilute sulphuric acid, there is no wasting while the bat- tery is not in use, the zinc being dissolved only while the circuit is closed. Palladium may be precipitated from its solution by metallic iron or zinc. It should then be washed with weak nitric acid, and dried. This character combines quite readily with mercury, attended by evolution of heat. It hardens quickly as it cools, but may be com- bined so as to set quickly or slowly, depending upon the proportion of its constituents. It turns very dark, but does not greatly discolor the tooth structure. On account of its quickly setting property it is difficult to work, and if inserted imperfectly it may harden so soon that it is almost impossible to remove it. Tombes says it shrinks less than a ny of the binary amalgams. The expensiveness of palladium has caused the use of this amalgam to be almost, if not entirely, discontinued. Mr. Fletcher says:* " It may be prepared to combine with mercury so as to set quickly or slowly by varying the strength of the solu- tion; but it must be borne in mind that unless precipi- tated palladium sets very rapidly when mixed with mercury, it is totally useless for dental purposes: the plugs fail, unless fully hard, in so short a time that the * Dental Metallurgy, Fletcher, p. 41. 56 PRACTICAL DENTAL METALLURGY. amalgam is difficult to insert whilst it remains plastic. Plugs of palladium amalgam generally contain about 70 to 80 per cent, of mercury."^ Cadmium forms a silver-white, somewhat brittle amalgam of a crystallo-granular texture, which under certain circumstances is said to be malleable, imparting that quality to its alloys. Antimony and Bismuth. — (See chapters on these sub- jects.) *XoTE. — Mr. Coleman in the subjoined gives his preparation of palladium amalgam: " About as much mercur5' as would fill the cavity to be treated is placed in the palm of the hand, and the palladium powder very gradually added. It requires some careful rubbing with the forefinger before the two become incorporated, when it should be divided into smallish pellets, and these rapidly carried one after another to the cavity, each piece being well compressed and rubbed into the inequalities of its walls by a burnishing or compressing instrument and with a rotary movement of the hand. This is continued until the cavity is quite filled, or even, if necessary, to some slight extent built out, the surface being rendered smooth and polished with a bur- nisher until it is quite set, which is generally in a very little (too short) a time." — Dental Surgery atid Pathology. ' He also states, according to Kirk, that it is probably the most durable of all amalgams, but the most difficult to manipulate. Its surface changes to a black color, but as a rule does not stain the structure of the tooth. — Am. Sys- tem of Dentistry. CHAPTER V. {^Amalgams Continued.') TERNARY DENTAL AMALGAMS. These are general!}^ alloys composed of silver and tin, comminuted by filing or turning in a lathe, and amalga- mated into a coherent mass with mercury. The com- ponents are, as a matter of course, subject to consider- able variation, the proportions of silver and tin ranging from 60 parts of the silver to 40 of the tin, to 40 of the silver and 60 of the tin, with an occasional larger pro- portion of silver. The proportions of mercury range from 30 per cent, of mercury to 70 per cent, of alloy, to equal parts by weight, or, exceptionally, a larger pro- portion of mercury may be used. The comminuted alloy is amalgamated with the mercury by rubbing them together in the palm of the hand or in a wedgewood or ground-glass mortar until a more or less smooth and co- herent mass is formed. When a considerable amount of mercury is used in the mixing, the excess is generally squeezed out through a muslin cloth or chamois skin. The mass, when packed together, acquires a metallic hardness within a few hours, and arrives at its full degree of hardness usually in twenty-four to forty- eight hours. It is then a hard, brittle mass that may be dressed with a file and polished as other metallic bodies.* By an appended table of dental-amalgam alloys it may easily be seen that silver and tin form the basis of all the formulae. With a view to overcoming the imperfec- tions in and disadvantages of this simple ternary amal- gam, and increasing its tooth-conserving qualities, and, * Dr. G. V. Black, Silver-Tin Amalgams, Dental Cosmos, Vol. XXXVII, 58 PRACTICAL DENTAL METALLURGY. therefore, its usefulness, a vast amount of experimenta- tion has been carried on by the profession and the supply concerns. Papers after papers have been written, pub- lished, and discussed, all of which have had a tendency to prove that there is good reason to believe that the addi- tion of small proportions of one or more other metals will, in a measure, overcome the objections inherent in an amalgam made up of silver and tin alone. RKQUISITS PROPERTIES OF A DENTAL, AMALGAM. 1. Permanency of Form (exhibiting as little tendency to con- tract, expand, or assume a spheroidal form as possible). 2. Sufficient Density, Hardness, and Toughness to Resist At- trition. 3. Strength and Sharpness of Edge.* 4. Complete Resistance to the Action of the Oral Secretions and Food. 5. Freedom from Admixture with Any Metal Favorable to the Formation in the Mouth of Soluble Salts of an Injurious Character. 6. Good Color. Quarternary, Quinary, Etc., Dental Amalgams. — The metals which are added to alloys of silver and tin in small proportions are as follows: Gold, platinum, iridium, copper, zinc, cadmium, bismuth, and antimony. A greater number of the dental-amalgam alloys in present use contain small proportional additions of one or more of the above-named metals. * Note.— Of stress, Dr. Black, in the July, 1895, number of the Dental Cos- mos, p. 554, says: "The stress in the ordinary use of the teeth has been shown to be from sixty to eighty pounds upon the area of two molars of medium size. This, if evenly distributed, would give from seven and a half to ten pounds on a filling occupying one-fourth the area of one of these teeth. This would be a filling of ordinary size; but it frequently happens that the filling must bear all of this stress, and occasionally such fillings must bear all of the stress that the person is capable of exerting. Therefore, while the filling itself may not have to endure a stress of more than seven and a half to ten pounds in chewing a piece of beefsteak, it is continually liable to have to bear the whole stress when some hard substance is caught upon it, or even the whole stress the person can exert. This may be anywhere from one to two hundred pounds, or even a greater stress in some cases." TERNARY DENTAL AMALGAMS. 59 Silver and Tin when alloyed readily unite with mer- cury, forming a gray-white, plastic mass. Contraction and Expansion — On hardening, they show little tendency toward coiih'actioji, and less toward expansion. In his experiments Dr. Black* tested five different alloys of silver and tin. Numbers 1, 2, and 3 were pro- portioned as follows: 1. 2. Silver 65 Silver GO Tin 35 Till 40 Mercury. . .44.60 /(^r cent. Mercury . . , .37.85 per cent. 3. Silver 70 Tin 30 Mercury 46.36 per cent. These were mixed in the hand and inserted in steel tubes, t After thoroughly hardening they were examined with the microscope and showed '' perfect margins," i. e., no space was observable between filling and tube. Where slight contraction of the mass had taken place it was equalized by a subsequent corresponding expansion. A fourth alloy was tested twice:! * Contraction and Expansion of Silver-Tin Amalgams, Dental Cosmos, Vol. XXXVII, p. 648. t The Wedelstaedt test-tubes, in which these fillings were inserted for examining them under the microscope, and for other tests, are made of hardened steel one-half inch deep and one inch in diameter, with a cavity three-eighths of an inch in diameter and one-fourth of an inch deep. The face or top of the tube is ground flat on a hone, and the margin of the cavitj- brought to a perfect edge. They are so arranged as to place them on the stage of the microscope, and rotate them so Fig. I. as to bring every part of the margin of the cavity under the lens in the same relation to the angle of the rays of light chosen for the examination.— Z^/rt;^/^. In the illustration the test-tube is empty. X Note a greater proportion of tin than in any of the three former. 60 PRACTICAL DENTAL METALLURGY. 4. 4a. Silver . , 42.45 Silver 42.45 Tin 57.55 Tin 57.55 Mercury Zl. 21 per cent. Mercury 45.31/t';- cent. (Mixed in the hand and inserted (Mixed in a mortar and inserted by hand pressure.) "by burnishing, removing all softened material.) Number 4 showed "margins open irregularly, width from .0 to 1.2 thousandths of an inch." Number 4a showed margins open .4 to 1.2 thousandths of an inch. The fifth alloy, composed of silver 80 and tin 20, mixed in a mortar with GO per cent, of mercury and inserted by hand pressure, showed ''margins much raised, with spherical surface spheroided, 11.1 thou- sandths of an inch." These experiments were conducted in a manner excep- tionally accurate and scientific, and we can but conclude: First, that an alloy composed of the ordinary proportion of silver 65 and tin 35 mixed in the hand, with an ordi- nary percentage of mercury, and inserted by hand pressure produces a filling which finally shows neither expansion nor contraciioji; second, that a greater propor- tion of tin may cause final contraction; and third, that an excess of mercury (60) and silver (80) proportionally to tin (20) may cause a spheroiding of the plug. Spheroiding of Amalgams.— It is reasonably held by most experimenters and writers that the composition, methods of mixing, and handling of some amalgams tend to promote an inherent disposition to assume a spheroidal or globular form on perfectly hardening. This, it is claimed, is manifested in the fillings made of these alloys, in that they raise out of and draw away from the sides of the cavity. No doubt much of this apparent elevation is, in reality, due to slovenly work, extending the filling-material over the margins of the cavity and build- ing it fuller than necessary; subsequently a lack of edge- TERNARY DENTAL AMALGAMS. Gl Strength in the amalgam permits the portions so extended to be broken off, giving the plug the appearance of hav- ing been raised from its cavity. Dr. Black ascribes it to the expansion and flow of the mass; it being confined, has a tendency to rise up in the center, assuming a spheroidal form, much as ice does when forming in a glass, due to the flow produced by the stress caused from the expansion of the ice against the unyielding sides of the vessel. The cause of the spheroidal tendency has commonly been ascribed to the influence of tin; but of all the differ- ent amalgams tested by Dr. Black he found but two in the number which spheroided. Their formulae are as follows: 1. 2. Silver 80 Silver 75 Tin 20 Tin 20 Mercury ^-y^ per cent. Copper 5 Mercury.. bQ.bQ per cent. No, 1. "Margins much raised with spherical surface sphe- roided, 11.1." No. 2. "Spheroided, 12.3." From such results we might conclude that the sphe- roidal tendency is caused by the excess of silver and mer- cury, or both; but such a conclusion cannot be safely drawn, nor is the problem one easy of solution, since it unquestionably involves the chemical affinities brought into play under these conditions. Resistance to Stress. — A silver-tin amalgam, when fully hardened, presents a hard, brittle, metallic appear- ance. Its brittleness is such that when subjected to stress or pressure greater than its endurance, it goes to pieces suddenly with a crash like glass. It has, there- fore, been commonly supposed that amalgam is brittle, and consequently exhibits no other change under stress. Such conclusions are but natural, since no metal or other alloy exhibits these seemingly incompatible properties of 62 PRACTICAL DENTAL METALLURGY. brittleness and ductility at the same time. Iron is malleable and ductile, as is soft steel, but hardened steel and cast iron are exceedingly brittle. Neither iron nor any of its compounds could be possessed of both of these properties at the same time. Yet Dr. Black's experiments tend to show, as he says, that "in this particular the silver-tin amalgams seem to be an anomaly among the metallic compounds, for they are at the same time both very brittle and very ductile. If struck with a hammer they fly to pieces; but if subjected to a comparatively light stress, either continuous or intermitting, they may be drawn out into thin laminae or molded into any form without breakage."* Flow. — The property of yielding under stress without breach of continuity. Dr. Black, in explaining this property of amalgams, says: " When the flow has begun it continues as long as the stress is maintained. No increase of stress is required to maintain the flow " ; that " it will go slowly with a light stress, somewhat quicker with a heavy stress, but it can- not be made to go very quickly with a very heavy stress; it will break into fragments. A silver-tin amalgam is not malleable, "t His conclusions, based upon further experiments with silver and tin separately, are, that this property cannot be imparted by either of these metals, but results from the combination of the three — silver, tin, and mercury; and further, J " that the strength of the mass depends mostly on \.\i^ perfect evenness of the dis- tribution of the mercury. Any irregularity in the work which disturbs this increases the flow," and after even distribution, ' ' any form of violence weakens the product." Finely comminuted alloys flow more than coarsely divided ones. *Dr. G. V. Black, Filling-materials, Dental Cosmos, Vol. XXXVII, p. 556. t Ibid., p. 558. + Ibid., p. 561, TERNARY DENTAI. AMALGAMS. 63 The formula of the alloy largely controls the flow of the amalgam, and is of vastly more importance than the percentage of mercury or the manner of manipulation, provided both of these be reasonable. "The standard being sixty pounds for one hour on a cube 85x85x85 thousandths of an inch, >ii * * a pure silver-tin alloy may be said to flow from two and a half per cent, to ten per cent., the difference depending on the composition of the alloy, the fineness of the cut, and the special mode of handling."* The following tablef shows some of the findings of Dr. Black from the various amalgams of silver and tin: Notes. ,^ ^ Flow. Stress. A mechanical mix of pre-( cipitates of the metals. ' "Weighed and mixed and ) used without wringing > out; fillings made at once. ) As the blocks were made\ the odd numbers were | placed in one box and the i even numbers in another, ^ so that they should be j alike, and the tests made I two days apart. J No mercury could be re- ] Silver, 70. moved from these mixes (Tin, 30. by wringing through T | " muslin. ) ' " Silver, 60. \ Tin, 40. j Silver, 60. ) Tin, 40. j Silver, 42.451 'Tin, 67.55 f 50 per cent, of mercuryX mix was a dark, semi- j ;rent powder that was I emely difficult to I With 50 per cent, of the m cohere extremely- pack. / (These mixes worked easily i and well, only that they j set very quickly.) / Silver, 80. Tin, 20. 55.61 Mortar 38.26 38.58 Hand 2.91 40. Hand 5.07 40.59 Hand 4.89 39.05 Mortar 8.61 9. 9.76 50. Hand 4.28 " Mortar 4.12 5.71 50. Hand 7.41 60. 60. Mortar Hand Mortar 9.23 2.4 3.5 4.24 4.5 160 277 230 230 235 220 300 345 275 280 330 340 315 325 * Black, Filling-materials, Dental Cosmos, Vol. XXXVII, p. 566, t Ibid., p. 564. 64 PRACTICAL DENTAI. MJ^TAI^I^URGY. " The crushing strength," says Dr. Black, "proves to be no test for the stability of an amalgam." All rea- sonable dental-amalgam alloys, when mixed with a "sufficiently low percentage" of mercury, are strong enough to withstand the stress of mastication. The flow, however, is an important test. The chief difficulty is, that while most of the alloys are capable of resisting the stress imposed, many of them will gradually change form under the same strain; thus, the appearance of the *' black ditch" which is so often seen along the margin. Gold is usually added to silver-tin dental-amalgam alloys to the extent of from 2 to 7 per cent. Dr. Bonwill regards a greater quantity very undesirable.* It is claimed for gold, in small proportions, that it de- creases the tendency to contract, increases the hardness and edge-strength, hastens the setting, gives a greater plasticity, and assists in the maintenance of a better color. Dr. Kssig found that the following alloy hardened in a few minutes, retained its edge-strength, and apparently filled all the requirements of a dental amalgam: tGold 10, Silver .... 40, Tin 50, and Mercury 50 per cent. Dr. KirkJ; claims for gold a large control over shrink- age, or contraction, but Dr. Black§ says no such influ- ence has been manifest in the amalgams containing gold he has tested, and that " the tendency of this metal to make the amalgam softer rendered a large proportion in- admissible." His tests for contraction and expansion show the following: II * Dental Cosmos, Vol. XXIV, p. 422. t Dental Metallurgy, p. 52. X American System of Dentistry, Vol. Ill, p. 810. $ Dental Cosmos, Vol. XXXVII, p. 654. II Dental Cosmos, Vol.;XXXVII, p. 648. TERNARY DENTAL AMALGAMS. 65 I Per Cent.! of Mercury. How Mixed. Contraction (— ) Kxpansion (+) Unit of ISIeasurenient, 1 Thousandth of an Inch. Amount of Final Con- traction or Expansion, if any. ( Silver, 48 ) Hand pressure. \ Tin, 48 [ (Gold, 4) (Silver, 48) :\Ial]et pressure