3686. To Take Moulds of Figures

3686.    To Take Moulds of Figures. 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 (see No. 3688 (To Prepare a Plaster Cast for Electrotyping)), 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 which the deposit is to bo made. It is prepared in the same way as we have described for depositing in coppermoulds. (See No. 3672 (To Obtain a Copper Mould of a Coin).) 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 models 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 fac-similes of the object in these metals, in which case the copper moulds are dissolved off by acids.

3687. To Coat Figures with Copper

3687.    To Coat Figures with Copper. When plaster busts or figures are wanted in copper, the usual way is to prepare the figure with wax (see No. 3688 (To Prepare a Plaster Cast for Electrotyping)) and to coat it over with a thin deposit of copper, letting the copper remain. Some operators, when it can be done, remove the plaster and wash over the inside with an alloy of tin and lead melted. In this case the copper must previously be cleaned by washing first in a solution 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 cases the deposit must not bo very thick, or it will throw the figures out of proportion, such as the features of a bust, etc. Any slight roughness of deposit may bo easily smoothed down by means of fine emery or glass paper. (See No. 1935 (To Make Emery Paper or Cloth).)

3688. To Prepare a Plaster Cast for Electrotyping

3688. To Prepare a Plaster Cast for Electrotyping. First dry the plaster cast in the oven thoroughly, then get equal parts of bees' wax and common resin, melt them together, and boil the cast until it will not absorb any more; when cold, get some good black lead and cover the cast entirely, not thick, but a bright surface. (See No. 3689 (To Prepare Non-Metallic Moulds to Receive Deposit).)

3689. To Prepare Non-Metallic Moulds to Receive Deposit

3689. To Prepare Non-Metallic Moulds to Receive Deposit. "Were any of the plaster or wax moulds, described above, attached to the zinc and immersed in the copper solution in the same manner as described in No. 3369, no deposit would bo obtained, because neither the plaster nor the wax is a conductor of electricity. Some substance must now be applied to the surface in order to give it conducting power. There are several ways of communicating this property, but the best and most simple for the articles under consideration is to apply common black lead (carburet of iron) in the following manner: A copper wire is put round the edge of the medal, or, if wax moulds are used, a thin slip of copper may be 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 (a small hat brush is very suitable) and dipped into fine black lead, and brushed over the surface of the metal. 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. A bright polish is necessary to obtain 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; being, as it were, incased. 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 immediately begin upon the copper connection, and will soon spread over every part, completely covering the black-lead surface. When the deposit is considered sufficiently thick for removing - which, in ordinary circumstances, will require from 1 to 3 days - the medal 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 bo separated by a gentle pull; if otherwise, the superfluous deposit must be eased off, and if care be taken the wax may bo fit to use over again; but when the mould is plaster of Paris, however well it may bo saturated with wax, it is seldom in a condition to use again. If the plaster mould bo large and thick, it is advisable to coat the back with wax or tallow, which is done by brushing it over with either substance in a melted state; the mould, being cold, will not absorb the wax or tallow; hence it may bo recovered again. The sulphate of copper possesses so penetrating a quality that if the slightest imperfection occurs in the saturation of the mould by wax, the solution will penetrate through it, and the copper will be deposited upon the face of the object adhering to the plaster, giving to tho metal a rough, matted appearance, and seriously injuring it.