art of electrotype includes ELECTRO-PLATING, and ELECTRO-GILDING.
General Principles.—If a current from a voltaic battery be passed, by means of platinum electrodes, through water to which some sulphuric acid has been added, electrolysis, takes place, hydrogen appearing at the cathode, and oxygen at the anode. If into the acid liquid some crystals of sulphate of copper be now thrown, electrolysis will still go on, but only one of the elements of the water, namely oxygen, will be evolved; for the hydrogen, on being released, will take the place of the copper in the solution, and the copper thus liberated will be deposited on the platinum plate or wire which constitutes the negative electrode. This experiment may be continued until all the copper is extracted from the solution. Let this experiment be repeated with a copper plate for the positive electrode, and it will be found that neither of the gases will be evolved. The hydrogen, as before, will take the place of the copper in the solution; the oxygen, instead of escaping at the anode, will combine with the copper of the electrode and the sulphuric acid to form sulphate of copper. The chemical forces called into action by the current are so beautifully balanced, that in the last experiment the quantity of copper supplied by the positive electrode exactly equals the quantity withdrawn from the solution and deposited upon the negative electrode. The whole art of electrotype consists in applying the metals thus released from their solutions to artistic or useful purposes. To obtain compact and brilliant deposits, many precautions have to be observed. The solutions must be kept saturated, or nearly so; the mould to be copied, or object to be coated, must not be too small, or out of proportion to the size of the zinc plate of the battery; in fine, the power employed must be carefully regulated according to the work to be done. In all arrangements the moulds or objects which receive the deposits act as negative electrodes, and are consequently in connection with the zinc of the battery or generating cell.
Electrotype Processes. Although reguline deposits of many metals can be obtained through the agency of voltaic electricity, we shall only treat of those of copper, silver, gold, and platinum. When copper is deposited, the object is generally to produce a substantial copy of a medal, an engraved plate, or other work of art; but when solutions containing the precious metals are electrolysed, the deposits are nearly always used for covering the surface of inferior metals. We shall notice the operations connected with the deposition of copper, and those relating to electro-plating under separate heads.
1. Deposition of copper:
The moulds or models intended to receive the deposited metals may be formed of various materials. For medals and similar small works, moulds of fusible metal, white wax, stearine, stearic acid, and gutta percha, are commonly used. The first are formed by dropping or pressing the medals to be copied upon the melted metal, taking care that the former are quite cold, and that the surface of the metal is bright or free from oxide. To make a mould in gutta percha, the material must be softened in warm water, and then pressed upon the medal by means of a strong screw press. With the other materials the manipulation is very easy. A ribbon of cardboard or thick paper is placed round the medal, so as to form a rim; the material, which has been melted in an earthen vessel, is then poured on, and allowed to remain until quite cold and hard, when it is cautiously removed. For large works, moulds of plaster of Paris are usually employed; these require to be saturated with wax or tallow, by standing them in a shallow vessel containing these substances in a melted state. For copying seals and small coins, impressions in ordinary sealing-wax may be used as electrotype moulds. Non-metallic moulds must be coated with some substance which has the property of conducting electricity before they can be used as negative electrodes. The substance commonly employed is plumbago or black-lead. It must be in the condition of an impalpable powder. It is rubbed briskly over the surface of the mould (wax, stearine, plaster, &c.) by means of a strong fine camel-hair brush, till the whole presents the well-known black-lead polish. The adhesion of the plumbago may be often promoted by breathing slightly on the mould. To cause it to adhere to sealing wax impressions, the wax may be slightly moistened with spirits of wine, or exposed to the vapour of ether. Delicate moulds and objects, which cannot well be black-leaded, may be covered with a conducting film of silver, by first dipping them in bisulphuret of carbon holding about 1⁄20th part of phosphorus in solution, and then, after a few seconds, immersing them in a weak solution of nitrate of silver, and allowing them to dry in the light. Metallic moulds require no preparation.
The voltaic apparatus used may now be described. The single-cell arrangement, used for small works, is formed on the principle of Daniell’s Constant Battery. It consists of a vessel of glass, earthenware, or wood, containing a smaller cell of thin biscuit ware, or other porous material; a rod or plate of amalgamated zinc, placed within the porous cell, and a wire connecting the zinc with the mould to be copied; the latter being placed in the outer vessel. The annexed figure represents a convenient form of the single-cell:—
The battery arrangement has many advantages over that described above, and should always be employed when large objects are to be electrotyped, or when a number of small moulds are to be operated upon. In this arrangement the copper solution is electrolysed
in a separate vessel, termed the decomposition cell, and the current generated by one or more cells of a Daniell’s or Smee’s battery. This arrangement is shown in the following engraving:—
a. An oval vessel of salt-glazed earthenware or wood nearly filled with a saturated solution of sulphate of copper.