The solutions generally employed as electrolytes from which silver and gold are respectively separated, are those of the argento-cyanide and the auro-cyanide of potassium. These compounds are what chemists call double salts; for instance, cyanide of potassium is simply a compound of potassium and cyanogen; but argento-cyanide of potassium is cyanide of silver united with cyanide of potassium. When a solution of this double salt is electrolysed silver appears at one electrode and cyanogen at the other, while a proportionate amount of the simple cyanide of potassium is formed in the solution. But if the positive electrode is of silver, the cyanogen combines with it, and forms cyanide of silver, which unites with the liberated cyanide of potassium, and so keeps up the strength of the solution.
As in the deposition of copper, the apparatus used for plating or gilding may be the single cell or the decomposition cell and battery. The necessity of economising solutions of silver and gold has, however, led to certain modifications in the apparatus. The single-cell arrangement consists, as before (see above), of an outer vessel of glass or earthenware, containing a cell of porous biscuit ware; but the object to be silvered or gilded is placed, with the cyanide solution in the latter, while the zinc is placed in the outer vessel, with the dilute sulphuric acid.[273] The zinc is usually employed in the form of a cylinder, completely surrounding the porous cell. In the battery arrangement the decomposition cell may be of porcelain or glass; the silver or gold employed to keep up the strength of the solution may be in plates, wires, or ingots. For plating small objects, a single cell of a Daniell’s battery will afford ample decomposing power; gilding may be better accomplished by using three such
cells. The battery arrangement is much more convenient, effective, and economical than the single-cell arrangement.
[273] The strength of the acid water acting upon the zinc must be regulated according to the work to be done. If the action between the acid and the zinc be too energetic, the electricity developed will be more than sufficient to release pure metal, and hydrogen will be evolved, which will interfere with the deposition.
On a large scale, electro-plating is carried out in oblong vats, occasionally holding from 200 to 250 gallons of solution. Silver plates connected with a powerful voltaic or magneto-electric battery, are placed at intervals in the vats; they form the positive electrodes, and correspond in extent of surface with the articles to be coated, and face them on both sides. The articles (tea-pots, cruet-frames, forks, spoons, &c.) act as the negative electrodes, and are suspended by copper wires from brass rods laid lengthways over the vats, and connected with the battery. The articles plated are usually formed of nickel silver or German silver, which is chosen on account of its silvery whiteness, a quality of great importance when portions of the coating of noble metal have been worn away by use.
To prepare the articles for plating, they are first boiled in a solution of potassa, to free them from grease; they are then quickly dipped in red nitrous acid, to remove any oxide that may have formed on the surface, and after this well washed in water, to remove every trace of acid. They are then suspended from copper wires, and dipped into a solution of mercury in cyanide of potassium, or some other mercurial solution, and afterwards washed in water, as before. The amalgamation of the surface effected by the last operation promotes the adhesion of the film of silver. The articles having been weighed, are now immersed in the silvering solution, and left until a sufficient amount of silver has been deposited on them. Their condition at any time may be ascertained by weighing a test-object removed from the solution. In some electro-plating establishments the silvering solution is kept constantly stirred by simple mechanical arrangements; in others, continual motion is given to the suspended articles. On being removed from the vats the plated articles are well brushed with brushes of fine brass wire attached to a lathe, and cleaned with fine Calais sand; they are afterwards polished on revolving brushes with rottenstone, then by hand with soft leather and rouge, and, lastly, with the naked female hand. A lasting polish is given to some articles by burnishing with a burnisher formed of highly polished hardened steel, bloodstone, agate, or flint. The process of electro-gilding on the large scale is nearly the same as that of electro-plating or silvering, but, of course, plates of gold are suspended in the solution instead of silver plates.
Various solutions for silvering, plating, and platinising, have been recommended. We give below those generally employed.
1. Solvent solution. Cyanide of potassium, 2 oz; distilled water or rain water, 1 pint; dissolve. Other proportions may be employed. Used as a general solvent for salts of silver, gold, and platinum.
2. Silver solution. Oxide of silver?[274] (not dried), 1 oz.; the solvent solution (No. 1), 1 pint. Used for the single-cell apparatus, its strength being maintained as the deposition proceeds by a fresh supply of oxide from time to time.
[274] Precipitated from pure solution of nitrate of silver by excess of lime water. It should be well washed, and preserved in bottles with distilled water.