ELECTRO-PLATING. .1 process by means of which salts of gold. and other precious metals are decomposed by the galvanic current, and a coating of their metals is deposited on the :surface of objects made of some cheaper metal. In 183S, Jacobi, of Saint Petersburg, published a description of a method capable of reproducing line engraved on copper into a relief by gal vanic process. Soon after, Thomas Spencer. of Liverpool, independently announced his discovery of a similar process. From these discoveries have grown the extensive eleetro-plating processes that include the deposition of gold. silver, copper. nickel. and other metals. In order to obtain a reproduction—as, for instance, a medal or similar object—a mold or cast is made on ,which the layer of metal is to be deposited by the current. This is sometimes of an inferior metal, as base or white metal; or, as in the case of type, of a mold made of sonic non-conducting material, such as wax or gutta-percha, whieh must be carefully brushed over before immersion with very finely pondered graphite. (See PRINTING.) In order that the metal to be deposited may adhere perfectly to the object to he plated. it is necessary that it shall be perfectly clean; it is therefore usually dipped in a cleansing solution, as of an acid or a caustic alkali, and then rinsed in water to remove all traces of the cleansing solution. The current for the electrolytic deposition of metals should be of great constancy. The Bunsen, Daniell, Grove, and Smee batteries are often used to supply the current for electroplating small objects: but for commercial purposes it is customary to employ a dynamo machine as the source of electricity.
The solutions employed in elcetro-plating are very numerous, but in ordinary commervial practice they consist of the following salts dissolved in distilled water: For gold, a solution of gold cyanide and potassium cyanide: for silrer, a solution of silver cyanide and potassium cyanide; for copper, an ammoniaeal solution of copper and potassium cyanide: for nickel, a solution of nickel and ammonium sulphate, or of the corre sponding chloride. The electrolysis is effected in a convenient bath. which, when small objects are to lie plated. may be of glass or porcelain. but in commercial practice is a trough lined with a non-comlueting material. The latter may lie wood or cement, in the ease of silver; zinc, in the ease of copper; and asphaltum, in the ease of nickel. The object on which the metal is to be deposited is connected by a wire with the nega tive electrode of the battery, while' the positive lade isseonneeted with an anode of the same metal which is to be deposited on the object, thus clos ing the circuit. As fast as the solution is decom posed the liberated acid dissolves the metallic anode, in (4)1Ni:queue° of which the solution is kept in about the same state of concentration.
Consult: MeM m isr on I.:tech-n-1•M' ((t•oy (London, 1890) : Partridge, .1 Poo-lira! Treatise on the .Irt of Elerteotyping (Chicago. IS99) ; Urquhart, Eiretro-Phiting. .t Practirol Handbook on the Depositinn of Copper. Silrer, Vickel, (Vold. Aluminium, Brass, Platinum. etc. (London. 1g94) ; Van Horne, Under?? Elcrtro Plating (Chicago, IS97). See El.r.•rito-Curmls TRY. N DUSTRI 1..