Pipeclay, pasteboard, bladder, gold-beater's skin, parchment, sail-cloth, and certain kinds of wood may be employed, but nothing equals porcelain clay, submitted to a certain heat, which hardens the paste without destroying its porosity. Vases made with this material are just porous enough, and resist the action of con* centrated acids.
The battery will work well for 24 hours; and, for four consecutive days, it will only be necessary to add small quantities of acid and amalgamating salt, in proportion to the volume of the cells. Stir the mixture each time with a glass rod. The fifth day, throw away all the exciting liquors, and substitute fresh ones, otherwise the zinc salt will be so abundant as to crystallise upon the zincs and the cells. A cell may be clogged in two ways, (1) by the zinc sulphate, which having an insufficiency of water, crystallises in the pores; in this case boil the cells in water acidulated by sulphuric acid. (2) By deposits of copper caused by bad working; dip the cells in aquafortis until all the copper is dissolved, and rinse in plenty of water afterwards. It is also possible to clean cells by keeping them filled with water, which, escaping through the pores, pushes out the salts and the acids with which they are clogged. Cast zinc will work, but is far inferior to laminated zinc, which will be uniformly corroed instead of being perforated. It sometimes happens that zinc is scarcely attacked, even by concentrated liquors, and that a multitude of small cavities are engraved on its surface. It also becomes covered with a blackish-grey crust, and no electricity is disengaged.
These inconveniences occur when the zinc is too rich in lead.
When a bath contains too weak a solution of copper sulphate, the* electro-deposit is pulverulent, black and irregular. The same inconvenience occurs when the liquors become too acid, because they do not dissolve enough copper sulphate. When the bath is too acid, add copper carbonate until effervescence no longer takes place. The bath should then be acidified anew to increase its conducting power. The copper carbonate may be replaced by the oxide of the metal, which dissolves without effervescence. If, after very long use, a bath becomes overloaded with free acid and zinc sulphate, there is no remedy but to start a fresh one.
The depth of the bath should be sufficient to have a little liquor above and below the moulds. If the moulds are lighter than the solution of copper sulphate, sink them with lead pieces covered with varnish, with stones, or other non-conductors of electricity. When the object to be covered is metallic, and unacted upon by the solution of copper sulphate, attach the conducting wire to any part of its surface, and it will be rapidly covered with a uniform deposit; if the mould is a non-conductor of electricity, and has been covered with some conducting substance, such as plumbago, bronze powder, or reduced silver, multiply the points of contact, as much as practicable, of the electrode, by uniting the connecting wire with a number of fine copper wires, and making their bent extremities touch the mould at various places. This method gives a greater rapidity of operation, and a uniform thickness of deposit. It is especially necessary for moulds having deeply indented surfaces. As soon as the surface is entirely covered, remove the supplementary wires.
If only one face of the mould is to receive the deposit, protect the other surfaces by a resist varnish, melted yellow wax, or softened gutta-percha.
Metals are unequally qualified to receive the galvanoplastic deposit; and some are naturally unfit for it. For instance, wrought and cast iron, steel, and zinc, as soon as immersed in the solution of copper sulphate, and without the aid of electricity, decompose the salt, and are coated with a muddy precipitate of copper without adherence. It is necessary to give them previously a thick coat of copper in the bath of double salts before submitting them to the action of the sulphate. Tin, although presenting these inconveniences in a much less degree, should also be copper electroplated in the solutions of double salts before going into the bath. When the metal to be covered is unacted upon by the bath, cleanse it well, and submit it to the action of the current, which will give a rapid and uniform deposit; this should not be too thick, otherwise the surfaces may have a coarse appearance, which impairs the fineness of the lines of the mould. With a good bath, and a well-regulated electric current, the delicacy of the pattern will not be defaced by a copper coat having the thickness of stout writing paper.