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and if a metal, such as zinc, which has a great affinity for oxygen, is immersed in the water, there is a tendency to decomposition; the oxygen combining with the zinc to form oxide of zinc, the hydrogen becomes liberated. But in order that this tendency may develope itself, it is necessary to place in the same liquid a piece of some other metal, such as copper, which has a less affinity for oxygen than zinc has, and to connect the copper with the zinc by a piece of wire; and so long as the decomposition of the water and the oxidation of the zinc are going on, there is a current of electricity passing silently and invisibly through the whole arrangement in a continuous circuit.
Now it happened that in a particular form of battery, devised by Professor Daniel, a solution of sulphate of copper was one of the liquids through which this electrical current passed in its progress; and the effect of the current was to decompose the sulphate, separate the copper from the sulphuric acid, with which it was combined, and deposit it in a very fine metallic state upon the inner surface of the vessel which contained the liquid. It was afterwards observed, that, on removing such a film of copper from the vessel, it presents an exact counterpart or countertype of the surface of the vessel. This led to the invention of the Electrotype process, by which medals, engravings, and bas reliefs, were copied with unerring fidelity.
Prosecuting this subject with a view to a more useful application of the principle discovered, it was soon found that not only copper, but gold and silver, as well as the cheaper metals, could be precipitated in a metallic form from these solutions, and hence, first arose the art of metallic plating or covering by the following che
mical means. The copper, or other metallic articles to be gilt, are well cleaned and immersed in a boiling hot liquid, consisting of a solution of nitro-muriate of gold, mixed with a solution of bicarbonate of potass. This adhesion of the gold to the inferior metal takes place by a singular interchange; for a portion of the copper becomes dissolved by the action of the potass, and an equivalent of gold is deposited upon the copper article instead. This mode of gilding is extensively practised in France.
But the electro-gilding, or silvering process, is far more durable, and it embraces, as we have observed, many distinct branches of manufacture. The first part consists of the designing of the article to be plated. This may be a candlestick, a vase, or a dish. The designer makes a drawing in Indian ink, and after him comes the modeller, who models from the drawing its figure in wax. The wax model is handed over to workmen who make from it a leaden mould, from this lead mould a cast is taken in brass, which is an exact copy of the original wax model. This cast is called technically, the pattern, and is carefully examined to see that all its details of ornament are properly developed. This pattern now forms the basis of a sand mould. The sand of a very fine and peculiar kind being prepared, the brass pattern is used to make an indentation in it, so exactly effected, that the cavity of the sand shall present in reverse all its details, while a central cone, or a plugging, is at the same time applied, so as to enable the metallic article to be made hollow for the purpose of saving the metal.
The metal employed is a mixture of copper, nickel, and zinc, hard in substance and white in colour. It is white, that should the plating at any time wear off at the edges it will not be so easily
discernable. This being brought into a molten state in pots or crucibles, is poured into the sand moulds, and upon being cooled and removed, the impression thus obtained is a copy of the wax model, or the brass pattern, and then constitutes the body or foundation on which the deposit of gold or silver is afterwards to take place. In this process, the lead and sand acted as moulds, but the wax, the brass, and the white metal, were three different copies of the original design itself.
We now come to the electro-chemical manipulation. In an electro-plating manufactory, we find the deposit room to consist of a range of tanks or troughs about a yard in width and the same in height. Wires and rods are placed across, and with these, tanks so arranged as to bring the contents of the tanks in connection with a galvanic battery, contained in an adjoining apartment; two wires, the positive and the negative, extend from this apparatus to the tanks, so that their contents may form part of the galvanic circuit. The tanks contain a solution of a double salt, the cyanide of potassium, and silver. The articles to be silvered, after being made perfectly bright and clean, by being boiled in caustic potass, are afterwards dipped in a solution of aquafortis, and then suspended by wires, side by side, but without contact, and the silver is gradually deposited upon them by the galvanic agency. That is, it decom
, poses the solution, liberates the silver from the other component elements, and deposits it in a beautiful, clear, and equable layer on the articles hanging in the tank; and by increasing the galvanic current, by strengthening the solution, or prolonging the time of immersion, any desired amount of thickness of silver may be deposited.