CHAPTER X SILVER ALLOYS § 148. Articles are very seldom manufactured from fine silver free from alloy, as pure silver is far too soft to resist the wear to which most bodies are subjected. It is therefore alloyed with some other metal, chiefly copper, to impart the requisite degree of hardness. Fine silver, in consequence of its high ductility, is used in the manufacture of silver lace and fine filigree-work, the latter being principally made in India, Sweden, Norway, and some parts of Germany, where labour is cheap. The purest commercial silver contains minute quantities of other elements, which, in the best varieties, do not materially affect its working properties. It may be necessary in some cases to obtain chemically pure silver, and this may be done in the following ways. Ordinary silver is dissolved in pure nitric acid, when any gold is left undissolved, and is removed by filtration. The silver solution is next evaporated to dryness, and the residue fused to decompose any platinum nitrate that may be present. The residue is then dissolved in dilute ammonia and filtered; and the filtered blue liquid diluted with enough water to bring the strength down to 2 per cent of silver. A sufficient quantity of normal ammonium sulphate is now added to render the solution colourless on warming, and the liquid is allowed to stand in closed stoppered vessels for twenty-four hours, when a third of the silver separates out in the crystalline form. CHAP. X SILVER ALLOYS 317 The liquid, which is still blue when cold, is poured off and heated from 60° to 70° C., when the remainder of the silver is deposited. In order to remove every trace of copper, the metallic precipitate is washed with water, and allowed to stand several days in contact with strong ammonia; it is then again washed, dried, and fused in an unglazed porcelain crucible with 5 per cent of pure borax and 5 per cent of pure sodium nitrate. Lastly, it is cast in moulds lined with a mixture of burnt and unburnt porcelain clay. The bars of silver must then be cleaned with sand, and heated with potash solution to remove every trace of adherent silicate, and finally washed with water. To prepare pure silver, Stas dissolved fine silver in dilute nitric acid, evaporated the solution to dryness, and ignited the residue until all the red fumes were evolved. The mass was dissolved in water, filtered, and diluted with rain-water (30 of water to 1 of silver) and the silver precipitated as chloride with pure hydrochloric acid. The precipitate was washed with dilute hydrochloric acid, then with pure water, and then dried, and the powder well rubbed in a clean porcelain mortar. This was then repeatedly digested with aqua-regia, and afterwards well washed. The silver chloride was then reduced to metal by boiling with dilute pure caustic potash and some milk-sugar. The reduced silver was washed with dilute sulphuric acid, then with water, then dried and fused. Metallic silver has the power of absorbing certain gases when melted in contact with them, but the gases are, for the most part, expelled during the solidification of the metal, raising blisters on the surface, or covering the same with a number of small excrescences, giving it a frosted appearance. This action is termed spitting or vegetating. `When molten silver is allowed to cool slowly out of contact with air, the gases gradually escape, and little evidence of spitting is exhibited. The gas which produces this phenomenon is principally oxygen. When silver is melted under a sufficiently thick layer of non-oxidising material, such as common salt, or potash, the metal solidifies with a bright surface, showing that oxygen had not been absorbed and afterwards emitted. When powdered charcoal is thrown on the surface of the metal, the carbon withdraws the absorbed oxygen and prevents the silver from spitting. Silver may be alloyed with gold, even to the extent of one-third of its weight, without losing its power of absorbing oxygen when melted, and of spitting during solidification. 1 Chevillot states that silver alloyed with copper, of the respective standards 990 and 995 parts of silver per 1000, exhibit the phenomenon of spitting; that silver of the standard 952 does not evolve gas in sensible quantity; and he supposes that silver of the standard 980 is the limit at which spitting occurs. Silver is capable of absorbing oxygen and other gases, and retaining them when cold, by heating the metal to redness in contact with oxygen or other gas. Such gas is said to be occluded. Graham found that pure silver occluded 545 of its volume of oxygen; and fine silver wire '002 inch diameter, yielded 289 of its volume of a gas, consisting chiefly of carbonic acid. When standard silver is heated to low-redness it becomes almost black on the surface, from the oxidation of the copper. Silver wire thus blackened was found to have occluded several times its volume of oxygen. Fine silver wire heated to redness in hydrogen, and cooled in that gas, occludes 211 of its volume of hydrogen. Fine silver is somewhat extensively used in the manufacture of fine wire and filigree-work on the Continent and in India. The Indian workman accomplishes work of a very beautiful kind, representing flowers, animals, etc., with true artistic taste. The articles are "hand-made" with the aid of a few simple tools. The work is commenced by hammering out the metal on an anvil, and when it has assumed a certain degree of thinness, it is cut into strips, and drawn 1 Memoirs of the Phil. Soc. of Manchester, second series, 1819, p. X SILVER ALLOYS 319 Indian into very thin wire through perforated steel plates, a pair of strong pliers being used for the purpose. The wire is then used for fashioning various ornamental articles. filigree-work is said to be the finest and cheapest in the world. It is of importance, in this class of work, that the various forms required in filigree-work should steadily retain their place when pressed into shape, and not rebound like metals of a highly elastic nature, hence the need of using fine silver in preference to standard silver. § 149. Silver and Arsenic. These metals are capable of uniting in several proportions, forming hard, gray, brittle, and readily fusible alloys. Gehlen produced an alloy containing 16 per cent arsenic, which is compact, brittle, steelgray, and fine-grained. Berthier describes an alloy of 14.8 per cent arsenic as dull-gray, brittle, and crystalline; by burnishing it acquires the lustre and colour of silver; it is very fusible, and not decomposed on heating. Guettier describes an alloy containing 14 per cent arsenic, formerly used for table ware. Mr. R. Smith prepared a hard and brittle though somewhat tough alloy, which became white and lustrous on burnishing. It contained 18.54 per cent arsenic, and corresponded to the formula Ag.As. Silver-arsenic alloys may be prepared by direct fusion of the constituent metals, or by melting a mixture of silver, arsenious acid, and black flux. § 150. Silver and Antimony.-Alloys of these metals may be obtained in all proportions by direct fusion. They are hard, brittle, and gray or white in colour. The white ness decreases with the proportion of antimony. The alloys are very fusible, and wholly decomposed by cupellation or by fusion with nitre, pure silver remaining. has prepared the following alloys :— 1 Mr. R. Smith -corresponding to the formula 3Ag+ Sb, 4Ag+Sb, and 6Ag+ Sb respectively. The silver was melted first under a layer of charcoal, and the antimony then added. No. I was hard, crystalline, and bluish-white. No. II was similar to No. I, but grayish-white. No. III was hard, granular, and grayish-white. The specific gravities of 48 silver-antimony alloys containing 50 per cent of silver, and upwards, has been determined by Cooke, of Harvard College, U.S., who found that the densities were above the mean densities of the constituents, the maximum being reached in the alloy containing 26.6 per cent of antimony. Cooke also found that the crystallisation of the alloys becomes marked in proportion as the same composition is approached. § 151. Silver and Bismuth.—Alloys of these metals are hard, easily fusible, brittle, and lamellar in structure. The colour of the 50 per cent silver alloy is the same as that of bismuth. An alloy containing 33.33 per cent silver is said to be steel-gray and to expand on solidification. Schneider states that when impure bismuth, containing sulphur, arsenic, iron, nickel, and silver, is fused and poured upon a cold plate, the globules of metal which are thrown up during solidification of the mass contain at least 99.5 per cent bismuth, and of the heavy metals only silver is found in the bismuth. § 152. Silver and Tin.—The smallest quantity of tin renders silver brittle. Alloys of tin and silver, according to Guettier, are harsh, very hard, and brittle. An alloy of 80 per cent tin is nearly as hard as bronze. An alloy of 52 per cent tin is somewhat malleable. These alloys are very easily oxidised. They have a specific gravity less than the mean of the constituents. Tin may be removed from silver by fusion with bichloride of mercury (corrosive sublimate), leaving the silver pure. Dentists use an alloy of 60 parts silver and 40 parts tin, in admixture with mercury, for stopping teeth. § 153. Silver and Zinc.-These metals combine very readily, forming bluish-gray and, for the most part, brittle |