ELECTROTYPING, the production of copies of types, wood-cuts, medals and other objects by means of the electro deposition of a metal upon a mould. A negative cast of the object to be copied is generally prepared in the first place, and is made from gutta-percha, plaster of paris, or wax. After being coated with a thin layer of graphite to serve as conductor the negative cast is connected to copper leads and is immersed in an electro plating bath as cathode. Forms of fusible metal may also be used, or electro-deposition may be effected on mandrels of special shapes and later stripped off.
Electrotypes may consist of copper, nickel or iron, the first metal being most usually employed. Copper electrotypes which are subjected to much wear are often "faced," however, with a thin coating of nickel, iron or chromium. In producing the rela tively thick deposits of copper required in electrotyping an acid copper sulphate bath is used, similar to that employed in refining. The electrolyte contains 200 grams CuSO4,5H20 and 3o grams per litre, and is usually kept somewhat above room temper ature. The anodes are sheets of electrolytic copper. The current density is about 2 and the bath voltage about one volt. For more rapid working the temperature is raised and the electro lyte stirred by vigorously blowing in air. In this way current den sities of 5-8 can be employed at the cost of a con siderably increased bath voltage. When the copper positive is sufficiently thick and rigid, the whole is taken from the bath, the negative cast detached, and the electrolyte backed with some low melting lead alloy to provide the necessary strength.
Other applications of electrotyping are in the Elmore process, by which seamless copper tubes are produced. The density, regu larity and evenness of the deposit are increased by continuously rotating the cathode and by causing pieces of agate, pressed on the cathode surface by springs, to travel continuously from one end of the cathode to the other and back again. To detach the tubes subsequently from their cores, they are gently heated, and worked loose by pressure. Their great tensile strength allows them to be easily drawn ; consequently a few standard sizes only are directly produced in the bath. Tubes up to 5 ft. in diameter and 16 ft. in length have been produced. In other processes, such as that devised by Cowper-Coles, a very rapid rotation of the cathode enables high current densities to be used and improves the smooth ness of the deposit. The similar production of iron tubes is described under ELECTROPLATING. An ingenious process has been described (Chem. and Met. Eng., 1914), for the production of narrow, flat copper tubing through which small supporting tubes pass transversely at regular intervals: the copper is deposited on a perforated lead band which passes continuously through the solu tion, and the lead is subsequently melted out. Finally, Thain (Trans. Faraday Soc. 1921), has described an interesting electro typing operation in which copper water jackets are built up around the cylinders of aero-engines. The metal is deposited on a fusible metal form which is afterwards melted out. A preliminary coating is put on from a cyanide solution, the main deposit being produced in the acid copper sulphate bath. (See also ELECTRO PLATING, PRINTING.) BIBLIOGRAPHY.-A. J. Allmand and F. Ellingham, The Principles of Bibliography.-A. J. Allmand and F. Ellingham, The Principles of Applied Electro-Chemistry (1924) ; W. R. Barclay and C. H. Hains worth, Electroplating. (J. N. P.)