ELECTRO - CHEMISTRY, Nlit":sTRIAL.
Flectio-•licmical processes are now employed in extracting metals from their ores and in parting them from their alloys; in the deposition of certain of the liner metals upon the surfaces of baser metals. and other in the manu facture of various chemical products employed in the arts, and for various miscellaneous pur poses. Tilt• processes of electrometallurgy, by metals are extracted from these ores or parted from these alloys, can be best in the articles describing those metals and their product ion. See Al.umINIUM; CoriER: GOLD: NICKEL: SILVER; ZINC: etc.
The electro-deposition of certain metals upon the surfaces of other substances is called electro plating. When eleetroplating is employed as a means of espying printing-type. medals, seals. woodcuts, Itc.„ it is vaned electrotyping. The metals commonly used for electroplating other metals are aluminium, copper, nickel. silver, and gold, which are in each case made the anode or positive pole of a voltaic current, while the metal to be coated is made the cathode or nega tive pole. The electrolyte or immersing bath is differently composed. according to the metal is to Ire deposited.
The eketro-deposition of aluminium was em ployed on n very large srale in yonstrueting the tower of the Philadelphia City Hall. in which the iron columns and plates were electroplated with this metal. Some of the largest columns whi•h were plated were 26 feet long and 3 feet. in diameter at the largest end. The various operathinS were as follows: (1) Preparation 01 the iron columns by boiling in caustic soda, pickling in acid, and scouring; (2) with copper or almninium bronze in an alkaline cyanide bath: (3) plating with copper in an acid sulphate bath: (4) plating with aluminium. In the final operation the bath was a concen trated solution of aluminato of soda, cyanide of potassium, and stannate of soda heated to 130' F. by a steam coil. The anode of pure alumin ium was inch thick. It) inches wide, and 4
feet long, and the cathode was of course the column or plate to be coated. The current was of 2500 amperes at vo.ts, and was supplied by a dynamo. A coating about inch thick, or • to 3 ounces per square foot, was deposited in 4S hours.
Electrically deposited copper is now largely used to protect and ornament arohiti•ctural iron work, the coating varying from S to 10 ounces per square foot for smooth plates, to 14 to 16 ounces for rough cast-iron. For interior orna ment elcetro-bronzing is often employed, using a coating of from 2 to 3 ounces per square foot. In Europe it is common practice not to deposit the plating directly on the iron, but on a suitably prepared varnish, thus avoiding the necessity of cleaning, but at the same time giving a plating which is easily knocked off: in the United States the iron is very carefully cleaned and the copper deposited directly upon it, and so firmly that the original metal with its coating may be bent or twisted without rupturing the plating.
The elect•o-deposition of nickel, or nickel-plat ing, is employed in manufacturing a multitude of useful and ornamental articles familiar to every one. The bath consists of nickel-ammo nium sulphate dissolved in boiling water. The articles to be plated are first cleaned with abrasives or acids. and after the plating is finished are polished. Gold and silver plating are employed in making jewelry. ornaments, and tableware of all descriptions. In silver-plating the bath used is a solution of chloride of silver, cyanide of potash, and water. In gold-plating either a hot or a cold solution may be used. and each is variously compounded, according to the result which it is desired to obtain. After the plating has been deposited the article is suitably polished. See article ELECTROPLATING.