If the model or figure be composed of plaster of Paris, a mould is often taken in copper by deposition ; the figure is saturated with wax, as before described, and copper deposited upon it sufficiently thick to bear handling without damage when taken from the model. The figure with the copper deposit is carefully sawn in two, and then boiled in water, by which the plaster is softened and easily separated from the copper which now serves as the mould in whieh the deposit is to be made. When the deposit is made sufficiently thick, the copper mould is peeled off, and the two halves of the figure soldered together. The copper moulds, which are deposited upon the wax modrls taken in the elastic moulding, are often treated in the same manner ; but more generally these moulds are used for depositing silver or gold into them, to obtain facsimiles of the object in these metals, in which case the copper moulds are dissolved off by acids.
When plaster busts or figures are wanted in copper, the most usual way is to prepare the figure with wax, as described, and to coat it over with a thin deposit of copper, letting the copper remain. Some operators, when it can bo done, remove the plaster, and wash over the inside with an alloy of tin and lead melted. In this ease, the copper must previously be cleaned by washing first in a solu tion of potash, and then with chloride of zinc ; the latter mode will cause the alloy to adhere to the copper, and give it strength. In either of these eases, the deposit most not be very thick, or it will throw the figures out of proportion, such as the features of a bust. Any slight roughness of deposit may be easily smoothed down by means of fine emery.
Were any of the plaster or wax moulds attached to the zinc, and immersed in the copper solu tion, no deposit would be obtained, because neither the plaster nor the wax is a conductor of elec tricity. Some substance must now be applied to the surface, in order to give it conducting power. There are several ways of communicating this property, to be afterwards described ; but the beet and most simple for the articles under consideration is to apply common black-lead in the follow ing manner :—A copper wire is put round the edge of the model, or if wax moulde are need, a thin slip of copper may bo inserted into the edge of the mould, or, being slightly heated and laid upon the back, the two will adhere. A fine brush is now taken and dipped into fine black-lead, and brushed over the surface of the model ; the brushing is to be continued until all the face round to the wire upon the edge, or slip of copper forming connection, has a complete metallic lustre; bright polish is necessary to the obtaining a quick and good deposit.
In brushing on the black-lead, care should be taken not to allow any to go upon the back or beyond the copper connection, or the deposit will follow it, and so cause a loss of copper, and make the mould more difficult to separate from the deposit.
When the face of the mould is properly black-leaded, the copper wire connected with it is attached to the zinc plate in the porous cell, and the mould immersed in the copper solution ; the deposit will soon spread over every part, covering the black-lead polish with less or more facility, according to the state of the solution and other circumstances. When the deposit is considered sufficiently thick for removing, which, in ordinary circumstances, will require from 2-3 days, the mould is taken out of the solution, and washed in cold water, and the connection is taken off. If the deposit has not gone far over the edge of the mould, the two may be separated by a gentle pull; if otherwise, the superfluous deposit must be eased off, and if care be taken, the wax may be fit to use over again ; but when the mould is plaster of Paris, however well it may be saturated with wax or tallow, which is done by brushing it over with either substance in a melted state, the mould being cold it will not absorb the wax or tallow, hence it may he recovered again. The sulphate of copper possesses so penetrating a quality, that if the slightest imperfection occurs in the satu ration of the mould by wax, or if imperfectly ptotected, the solution will penetrate through it, and the copper will be deposited upon the face of the object, adhering to the plaster, giving to the medal a rough, matted appearance, and seriously injuring it.
A mould in fusible alloy does not require to be black-leaded, but the back and edge must be protected by a coating of wax or other non-conducting material ; it may be connected by putting a wire round its edge previous to laying on the non-conducting substance, such as tallow or wax, which should also cover the wire. Or a slip of copper or wire may be laid upon the back and fastened by a drop or two of sealing-wax ; the back is then coated, but care must be taken that the wax does not get between the connection and the mould, which will prevent deposit. The deposit on this mould goes on instantaneously. When sufficiently thick, it may be taken off in the same manner as from the wax mould, the surface having been prepared by turpentine to prevent adherence. These moulds may be used several times, if care be taken not to heat them to the melting point.