Electro-metallurgy is an important art based on electrolysis. A layer of metal may be deposited on a plate of any shape if this plate be in a suitable solution of the metallic salt, and if a current be passed from a rod or slab of the metal through the liquid to the plate. If the layer is to remain permanently, the process is termed electroplating; if it is to be taken off, retaining, of course, any impression that is made on its inner surface by any pattern previously cut on the plate, the process is termed electrotyping. The process of electroplating strictly applies to the deposition of silver only, but gold, platinum, copper, and other metals may also be deposited. Silver will not adhere to all metals; iron, for instance, in order to be plated, must first be coated with copper, and this with silver. The surfaces to be coated require first very careful cleansing in boiling caustic potash and then in dilute nitric acid; they are then scoured with sand. The solution of silver generally used is made up of one part of silver cyanide with 10 of potassium cyanide and 100 of water. This is contained in an earthenware trough in which hang the articles to be plated, all connected by a wire with the negative or zinc pole of the battery or dynamo employed. The positive pole is connected with a silver plate immersed in the solution, and the current in flowing through the cell removes silver from the plate and deposits a layer on the given articles. The succes of the plating depends largely on the strength of current and of the solution, and on the shape and size of the electrode. When the silver is of suffi. cient thickness, the articles are taken out of the plating-bath; they are polished with a rotating wire brush, and then a finish is given by burnishing.
Electrotyping is of great importance in many directions, for by means of it copies of woodcuts, printing type, engraved plates, medals, etc., may be taken in metal and rendered practically permanent. There are two methods available to obtain copies of, say, a woodcut. Either an impression may be taken in wax, guttapercha, or some other soft substance that will retain a clear impression; this is then made electrically conducting by being brushed over with plumbago, and hung in the plating-bath as in the electroplating process. Or the original woodcut may be itself hung in the bath, and a layer of copper deposited over it. In the former case an exact copy is at once obtained, in the latter a reversal is first produced, which may easily be removed from the woodcut and then itself used to receive a fresh layer of copper. This will be an exact copy of the woodcut.