Galvanoplasm consists of deposits with sufficient thickness to form a resisting body, which may be separated from the objects serving as moulds, and which will preserve the shape and dimensions of the model. A statue of plaster of Paris, wood sculpture, an impress in wax, fruit, and similar things, may, after certain preparations, be covered with electro-deposits, for instance, which will give a deposit representing the same shape and dimensions. In galvanoplastic operations copper is almost exclusively employed. It is possible to have the deposits entirely of silver and gold; but these are exceptions, on account of the cost of the materials and of the difficulties of the operation. The following is a summary of the usual requirements; - 1. To apply upon a metallic surface conductor of electricity, a deposit of copper adhering to the metal underneath. 2. The above, operation being completed, the two metals must be separated in such a manner that they will furnish two identical productions, one of which will be in relief, and the other hollow, for casts of medals, etc. 3. To apply the electro-deposits upon substances not naturally conductors of electricity, but rendered so by the process of metallisation; upon ornaments of plaster of Paris, wax, glass, or porcelain, or upon leaves, fruits, and insects. 4. After the deposit to separate the non-metallic model to have a perfect copper copy of it.
For reproduction of type in stearine, gutta-percha, gelatine. 5. Or, if it is impossible to apply the electro-deposit of copper directly upon the model, make moulds upon which a greater or less number of copies may be obtained. This is the general case: - The imprint of the model is taken with a plastic substance, which is rendered a conductor of electricity, and upon which the galvanoplastic deposit is effected.
(1) Put into a vessel, made of glass, stoneware, porcelain, gutta-percha, or lead, a certain quantity of water, to which is added 8-10 per cent. of sulphuric acid, If in a glass vessel, or one lined with guttapercha, pour in the acid slowly and stir all the time, otherwise the acid, which is much denser than water, falls to the bottom, and slowly combining with the surrounding water, may cause an increase of temperature sufficient to break the glass or melt the gutta-percha.
(2) Dissolve in this liquor as much copper sulphate as it will absorb at the ordinary temperature. Stir frequently with a glass or wooden rod, to mix the solution; or the copper sulphate may be put into a perforated ladle of copper or stoneware, or into a bag of cloth, fixed near the surface of the liquid. When the liquid refuses to absorb any more crystals, it is saturated, and marks about 25° B. Baths of copper sulphate, while they are working, must always be kept saturated; new copper sulphate must be introduced to replace that decomposed and forming the metallic deposit; for this purpose suspend from the top of the vessel, and in the upper portion of the liquid, bags filled with crystals of copper sulphate. It is necessary to use good copper sulphate; the best is in crystals, semi-transparent, and of a fine blue colour. Its solution is also a pure blue. These baths arc always used cold, and are kept in vessels of shapes adapted to the wants of the operator. Stoneware, porcelain, and glass are the best materials for the purpose; but as it is difficult to find vessels sufficiently large, wooden troughs covered inside with coats of gutta-percha, marine glue, or with a sheet of lead, are used, painted with resist varnish.
After proper preliminary operations, the object which is to receive the deposit is connected with the conducting wire attached to the negative pole of the battery, zinc generally, and immersed in the solution; and the conducting wire starting from the positive pole, carbon or copper, is attached to a foil or plate of copper, and this anode is placed in the liquor parallel to the object connected with the other pole. This plate should have a surface at least equal to that of the article to be covered. The deposit will begin immediately, and its progress may be seen by removing the object from the solution. If upon a clean metallic substance, the deposit of copper will be instantaneous on every part of it; if, on the contrary, the surface only moderately conducts the electricity, as graphite, the deposits will begin at the points touched by the conducting wire, and then proceed forward. With a little practice it is easy to ascertain whether the intensity of the current corresponds to the surfaces to be covered. The operation will be slow with a weak current, but there is no other inconvenience, unless the substance of the mould is alterable, like gelatine.
Too intense a current results in a granular deposit, of which the particles have little cohesion between themselves, and no adherence to the mould.
Place the solution of copper sulphate in a stoneware, earthenware, or porcelain vessel, in the centre of which stands a porous cell filled with water with 2 or 3 per cent, of sulphuric acid, and 1 per cent, of amalgamating salt. This liquid must surround an inner cylinder of zinc, upon the top of which rests a circle of brass wire, by two crossed bars soldered or fixed in four notches on the top of the zinc cylinder. Suspend from this circular framework, projecting over the copper solution, a certain number of objects or moulds, immersed in the liquid in such a way as to have their faces to be covered near and opposite to the cell. Two small hair bags filled with copper sulphate crystals, should be attached to the upper edge of the vessel.
To cover large surfaces, use a bath contained in a large wooden trough, lined inside with guttapercha, or lead, or other substance unacted upon by the bath. In the middle of the trough dispose a row of cells close to each other, and each with its zinc cylinder. Connect a thin metallic ribbon with all the binding screws of the cylinders, in contact at its extremities with two metallic bands on the ledges of the trough. The metallic rods to support the moulds are placed in contact with the metallic bands of the ledges, and therefore in connection with the zincs. If the objects are in high, relief, use a circular trough, place the. cells in a circle, and the mould to be covered in the centre. Whatever the shape of the mould, its position should now and then be changed, because the lower layers of the bath give more abundant deposits, owing to the difference of specific gravity of the layers more or less charged with sulphate. As far as practicable,,maintain the liquids in the bath and in the cells at the same level; or it is better to have that of the bath slightly above that of the cell, to prevent the solution of zinc from mixing with the copper bath.