Electrotyping. This is a process for depositing a coating of metal on objects, metallic or otherwise, by the agency of a current of galvanic electricity. Before entering into any description of the methods employed, it will be necessary to give some indispensable preliminary directions, in order that the whole matter may be more clearly understood. The matter is mainly derived from the 4th edition of Napier's Manual of Electro-Metallurgy.

3661. Solution of Copper for Electro-typing

3661.    Solution of Copper for Electro-typing. Crush fine sulphate of copper in crystals, and expose to the air for some time. This oxidizes any iron that may be present. Stir the sulphate of copper into pure cold water, until the water will dissolve no more; then let it settle, and decant the clear solution ; add to it about one-fourth its quantity of water, and it is ready for use.

3662. To Amalgamate Zinc

3662.    To Amalgamate Zinc. Immerse a plate or strip of zinc of the required size in diluted sulphuric acid, for a few moments; then rub quicksilver over the surface. Whenever the surface of the amalgamated zinc employed in a battery begins to blacken and lose its quicksilver coating, the zinc must be taken out of the acid cell and amalgamated again.

3663. To Keep the Zinc Plates of a Smee's Battery Constantly Amalgamated

3663.    To Keep the Zinc Plates of a Smee's Battery Constantly Amalgamated. The trouble of renewing the coating of amalgam on the zinc plates may be obviated by a very simple contrivance. Cover the bottom of the cell with quicksilver, and let the zinc plates be long enough to dip into it. The silver plate must bo a little shorter than the zinc plates, so that it will not touch the mercury. By this arangement the zinc plates draw up the mercury as fast as it is worn off by the action of the acid.

3664. Decomposing Cell

3664.    Decomposing Cell. This is a vessel of suitable shape and dimensions, containing the plating or electrotyping solution; and is usually furnished with appliances over it for suspending and sustaining in their proper position the negative electrodes or articles to receive the metallic coating, and their corresponding positive electrodes, or plates of metal, which serve to complete the electric circuit, and whoso decomposition serves to keep up the strength of the solution. The positive electrode must always bo of the same metal as that which the solution contains.

3665. The Principles of the Galvanic Battery Explained

3665.    The Principles of the Galvanic Battery Explained. If a piece of ordinary metallic zinc bo put into dilute sulphuric acid, it is speedily acted upon by the acid, and hydrogen gas is at the same time evolved from its surface. If the zinc be taken out, and a little mercury be rubbed over its surface, an amalgamation takes place between the two metals, and the plate becomes of a beautiful bright silver appearance. If the zinc thus amalgamated be again put into the dilute acid, there is no action, for the mercury retains the zinc with sufficient force to protect it from the acid. If a piece of copper be immersed along with the zinc, and the two metals be made to touch each other, a particular influence is induced among the three elements, zinc, copper, and acid; and the acid again acts upon the zinc as if no mercury was upon it, but the hydrogen is now seen to escape from the surface of the copper; this action will go on as long as the two metals are kept in contact. Or if, instead of causing the two metals to touch, a wire be attached to each, and their opposite ends are placed in a little dilute acid in another vessel, the same action will take place between the zinc and copper as when they were in contact; but in this instance, the ends of the two wires which dip into the vessel containing acid will undergo a change; the one attached to the zinc will give off a quantity of hydrogen gas, while the one attached to the copper, supposing it to be also copper, will rapidly dissolve.

Figure 1. Represents the zinc and copper, placed in dilute sulphuric acid, brought into contact; in this experiment, gas will be seen escaping from the copper.

Fig i.         Fig. 2.                 Fig. 3.

Figure 2. Zinc and copper, placed in dilute acid, and wires attached, which, when connected, will exhibit the same effects as in the first case.

Figure 3. Shows the wires connected by means of a liquid, such as acid and water, sulphate of copper, etc., contained in a wineglass.

The copper and zinc, c and z, with the acid in the first vessel, figure 3, constitute a battery of one pair. The wine-glass in which the wires are placed, is termed the decomposing cell (see No. 3664 (Decomposing Cell)), and is the receptacle or vessel in which the process of electroplating is effected. The above description will give a tolerably clear idea of the principles of a simple galvanic battery. Different kinds of batteries are only different modifications or applications of the same principles, and have each their special excellence; but for electroplating, Smec's battery is the one usually adopted.