Crude silver, produced by the cyanide process, or separated from lead by cupellation, or from copper or gold by electrolysis, contains small amounts of copper, gold, bismuth, lead, and other metals. When it contains a few parts of gold per thou sand it is called "dore silver." Dore silver was formerly refined by boiling in sulphuric acid (see GOLD: MINING AND METAL LURGY) but more recently most of it has been treated by electrol ysis. Both the Moebius and Balbach processes are used. Besides dore silver, scrap and residues of all kinds containing gold up to about one third of the total weight may be treated by electrolysis.
In the Moebius process the silver is cast into plates about i8in. long, loin. wide and tin. thick. Each plate is enclosed in a linen or cotton bag and a number of them are suspended in a parallel series in earthenware or porcelain vats. The plates form the anodes of an electric circuit and are supported by hooks from rods connected at one end with the positive pole of a dynamo. Cathode plates consisting of thin rolled sheets of pure silver are suspended in the vats alternately with the anodes, parallel to them and about 1i-in. distant. The electrolyte filling the vats consists of a solution containing of silver nitrate and 1-2% of nitric acid. A
current of electricity is passed from the anodes to the cathodes through the liquid, which is kept stirred. The silver, copper and some other metals are dissolved from the anodes, and pure silver is deposited on the cathodes in pulverulent and non-coherent form. The cathodes are continually cleaned by wooden scrapers moving backwards and forwards automatically and by this means are kept free from loose crystals of silver. If left undisturbed the crystals form trees which may stretch from cathode to anode and cause short circuits. The detached silver falls into canvas trays at the bottom of the tanks. In later practice glue or gelatine is added to the solution and the stirring is more vigorous, with the result that hard, compact, adherent deposits of silver are formed on the cathodes. In one form of apparatus there are rotating cylindrical cathodes, surrounded by anodes, in order to enable stronger currents to be used with more expeditious deposition.
The copper accumulates in the solution as copper nitrate and, to prevent it from being deposited on the cathodes with the silver, more nitric acid is added and the current is reduced. When the amount of copper in the liquid reaches about 5%, part of the solu tion is withdrawn and replaced by fresh solution of silver nitrate; otherwise copper would be deposited on the cathodes. The de
posited silver is removed, washed, melted and cast into ingots which should contain over 99.9% of pure metal. The slime in the anode bags consists in great part of gold which remains undis solved. In the Balbach cell, the whole bottom surface of an earthenware trough is the cathode which may consist of carbon. The anodes are placed horizontally above it and are supported on perforated earthenware trays or a grill consisting of wooden rods. The trays are covered with a filter cloth to prevent the anode slime from falling through and settling on the cathode.
The addition of a small percentage of cop per to silver lowers its melting point, prevents it from "sprouting" on solidification, and makes it harder, without sensibly impairing its malleability, altering its colour, or preventing it from taking a high polish ; thus, silver-copper alloys are used for silver coin and plate. A little cadmium is often added to them to facilitate the manufacture of wares, and other metals such as antimony have been added to produce alloys which will not tarnish. The colour of silver-copper alloys remains nearly white until the copper amounts to about 45% to 50% when the alloy is pale red and as the copper is increased further the colour becomes deeper. Silver copper alloys, like pure silver, blacken in the atmosphere of towns, from the formation of a film of sulphide of silver. They blacken also when heated to redness but the black oxide of copper thus formed can be removed from their surface by warm dilute sul phuric acid. Silver alloys with molten lead with great readiness and in all proportions, but if the melted alloy is cooled, pure lead crystallizes out and sinks in the liquid which becomes enriched until it contains 2.5% of silver, when the alloy solidifies as a whole. These properties are used in the Pattinson process (see LEAD). Zinc removes silver from molten lead, forming a zinc silver compound which is lighter than lead and floats to the top with the dross and surplus zinc so that it can be skimmed off, as in Parke's process of desilverizing lead. (See LEAD.) Silver is at once "wetted" by mercury when brought into con tact with it at the ordinary temperature (see AMALGAMATION) but the interpenetration of the metals is exceedingly slow and a true solid amalgam is difficult to form. The crystalline amalgam containing 35% of silver and 65% of mercury is produced in the beautiful branching form known as Arbor Dianae by prolonged contact between mercury and a solution of silver and mercury nitrates. Other alloys of silver are of little industrial importance.