ELECTROPLATING, the art of plating or covering solid objects with a coating of metal by electro-deposition. This is the most common method of applying silver or gold plate for ornament, or copper or nickel plate, as for rendering an article more durable. Given a solution of the salts of a metal, say, for in stance, sulphate of copper (the constituents of which are sulphuric acid and copper oxide), in which are immersed a copper plate connected with the positive pole of a source of electro motive force and a metal plate connected with the negative pole; when an electric current is passed through the solution an action takes place which may be described as follows: First, the salt is decomposed into sulphuric acid and oxide of copper. At the same time a portion of the water of the solution is also decomposed. setting free hydrogen and oxygen. The oxygen of the oxide of copper is drawn to the negative pole, where it unites with a portion of the hydrogen just freed, forming water, and the metallic copper thus set free is deposited uni formly on the negative metal plate. Simul taneously with this action sulphuric acid and oxygen arrive at the positive plate, where the oxygen unites with a particle of the copper plate, forming oxide of copper, with which the sulphuric combines, forming sulphate of cop per; which process is continued as long as there is any metal left in the positive plate. For each atom of copper thus dissolved at the posi tive plate another is set free at the negative plate. Actions analogous to these underlie all electroplating and electrotyping operations. If it is desired to deposit nickel. silver, gold or other metal on the object, salts of those metals instead of copper will be used in the solution or both, as it is termed.
Silver is the easiest metal to use in plating. one ampere of current depositing 4.02 grams of metal per hour; with the same current copper deposits 1.17 and nickel 1.09 grams per hour.
If 10 baths are worked in series of eight hours a day, depositing each 10 pounds of copper, they will require 4,830 amperes of current all the time; and with copper anodes the pressure will be about 16 volts for the 10 baths.
The art of electroplating is extensively practised The current for the decomposition of the electrolyte, in solution, is usually sup plied by continuous-current dynamo machines which are specially designed to give large cur rents at low electromotive force, rarely exceed ing three to five volts. Sufficient electromotive force must be provided to decompose the solu tion, but the amount of chemical decomposition depends altogether on, and is proportional to, the rate or amperage of the current. If too high electromotive force is employed the plating is uneven and granular. Storage or primary batteries may also be used for this purpose, and are so used for planting on a small scale. Much care and special knowledge is required to obtain the best results in electroplating. The process is begun by thoroughly cleaning, as by pickling and scouring the article to be plated to remove all trace of oxide or other impurity from its surface. In the case, for instance, of gold, silver or nickel plating the bath or solu tion employed by some electroplaters consists of 100 parts water, 10 of cyanide of potassium and 1 of the cyanide of gold, silver or nickel, as the case may be. Plates of either of these metals are suspended in the bath as the positive pole, while the article to be plated is suspended in the bath as the negative pole. When the plating has proceeded to the desired depth or thickness the articles are taken out and bur nished. Such parts of the article as it may not be desired to plate are covered with grease, oil or wax. When non-metallic articles are to be plated they are first given a coating of wax, over which is laid a film of powdered plum bago, upon which the plating then takes place.