Chapter VIII

HOW ELECTROPLATING AND ELECTROTYPING ARE DONE.

=81. Electricity and Chemical Action.= We have just seen, Chapter VII., that the electric current has the power to decompose certain compounds when they are in solution. By choosing the right solutions, then, we shall be able to get copper, silver, and other metals set free by electrolysis.

=82. Electroplating= consists in coating substances with metal with the aid of the electric current. If we wish to electroplate a piece of metal with copper, for example, we can use the arrangement shown in Fig. 78, in which C is the cathode plate to be covered, and A is a copper plate. The two are in a solution of copper sulphate, and, as explained in § 79, the solution will be decomposed. Copper will be deposited upon C, and the SO_{4} part of the solution will go to the anode A, which it will attack and gradually dissolve. The SO_{4}, acting upon the copper anode, makes CuSO_{4} again, and this keeps the solution at a uniform strength. The amount of copper dissolved from the copper anode equals, nearly, the amount deposited upon the cathode. The metal is carried in the direction of the current.

If we wish to plate something with silver or gold, it will be necessary to use a solution of silver or gold for the electrolyte, a plate of metallic silver or gold being used for the anode, as the case may be.

Great care is used in cleaning substances to be plated, all dirt and grease being carefully removed.

Fig. 80 shows a plating bath in which several articles can be plated at the same time by hanging them upon a metal bar which really forms a part of the cathode. If, for example, we wish to plate knives, spoons, etc., with silver, they would be hung from the bar shown, each being a part of the cathode. The vat would contain a solution of silver, and from the other bar would be hung a silver plate having a surface about equal to that of the combined knives, etc.

Most metals are coated with copper before they are plated with silver or gold. When plating is done on a large scale, a current from a dynamo is used. For experimental purposes a Gravity cell will do very well. (See "Study," § 374 to 380 with experiments.)

=83. Electrotyping.= It was observed by De La Rue in 1836 that in the Daniell cell an even coating of copper was deposited upon the copper plate. From this was developed the process of electrotyping, which consists in making a copy in metal of a wood-cut, page of type, etc. A mould or impression of the type or coin is first made in wax, or other suitable material. These moulds are, of course, the reverse of the original, and as they do not conduct electricity, have to be coated with graphite. This thin coating lines the mould with conducting material so that the current can get to every part of the mould. These are then hung upon the cathode in a bath of copper sulphate as described in § 82. The electric current which passes through the vat deposits a thin layer of metallic copper next to the graphite. When this copper gets thick enough, the wax is melted away from it, leaving a thin shell of copper, the side next to the graphite being exactly alike in shape to the type, but made of copper. These thin copper sheets are too thin to stand the pressure necessary on printing presses, so they are strengthened by backing them with soft metal which fills every crevice, making solid plates about ¼ in. thick. These plates or _electrotypes_ are used to print from, the original type being used to set up another page.