But the execution of the process is not easy, as it is very difficult to ascertain that the skeleton anode is nowhere in contact with the enclosing mould; to avoid such contact, wrap all the external parts of the platinum anode with a spiral of rubber thread. As the increase of the deposit of copper reduces the distance between the mould and the anode, the latter and the deposit may come in contact, and stop the operation without any exterior sign to attract attention. Thus, if in a trough holding many moulds, one point of contact were established between the two poles, mould and skeleton, all the electricity of the battery would escape at that place, and the working of the bath would stop entirely. To obviate this inconvenience, support all the moulds of the same bath by hooks suspended from a metallic rod. These hooks must have no contact with the metallised surfaces of the moulds, which must be connected with the negative pole by metallic wires terminated above the liquid by very fine iron wires. The connecting wires of the skeleton anode are to pass through the same opening as the negative electrodes, but without contact, and are united to the positive pole.
So long as there is no contact between the skeleton and the interior of the mould, the electric fluid finds sufficient passage through the several fine iron wires which connect the moulds with the battery; but, if any contact takes place, the whole of the electricity rushes to that point, and, being too abundant for the small iron wire, it heats and burns it out rapidly. The work is thus instantaneously stopped for this mould, and continues for the others; and the broken wire shows where the defect is. The iron wire should be very short, so as to burn rapidly. In closed moulds and with an insoluble platinum anode, the solution of copper sulphate will be rapidly transformed into sulphuric acid and water. Therefore make two holes at the lower part of the mould, through which and the opening at the head left for the passage of the electrode a free circulation of the liquor in the bath may take place. When the operation is completed, remove the gutta-percha mould, and the skeleton anode must be pulled out.
Close the three holes in the statue, and file off the seams left at the junction of the different parts of the mould.
First cover the exterior with clay, plaster of Paris, or Spanish white mixed with charcoal dust, and dry in a stove-room. This coat is to prevent the copper deposit from losing its shape and being oxidised by the heat. The interior of the article is then to be filled with the softest brass solder and powdered borax, which are melted by a gas or turpentine blowpipe. All the hollow parts are soon filled with the solder, which imparts to them as much firmness and durability as is to be found in cast articles.
With a metallic mould, after having removed the useless portions of the deposits, pass a card or a blade of ivory between the model and the deposit. The operation is the same with moulds of plaster of Paris, porcelain, marble, glass, or wood; but it is difficult to save a plaster mould which has been in the bath, and which is nearly always sacrificed. Moulds of wax, stearine, fusible metal, gelatine, or gutta-percha are softened in boiling water, and their separation presents no difficulty whatever.
The articles when separated from the moulds are generally spotted with plumbago, grease, or other substances from the moulds. It is usual to heat them, so as to burn out the impurities, and to cleanse them by immersion in a pickle of dilute sulphuric acid. The heating renders the copper deposit softer and more malleable; but it may result in injury to the minute details add the fineness of the copy. Therefore, for delicate works, it is preferable to clean with alcohol, turpentine, or benzole, and to rub the surface with a stiff brush; finish with a paste of Spanish white in water, which let dry upon the object before it is wiped out. Should any Spanish white remain in the hollows, it may be dissolved in water holding one-tenth of its volume of hydrochloric acid, which does not corrode the copper. Complete the operation by rinsing in fresh water, and drying in sawdust or otherwise. When it is desired to anneal the articles without injury to their surface, plunge them into boiling colza or linseed oil, or simply grease, which will bear a heat sufficient for annealing, and will prevent the oxidising action of the air.
This annealing in fatty substances is to be recommended in the case of highly undercut moulds of gutta-percha, which may have left part of their substance in the deep recesses of the copy. The gutta-percha is first softened, and then dissolved in the fatty material.
The processes are more difficult and less effective than those for copper. In the case of non-conducting and deeply-wrought moulds, after having deposited by the ordinary process a thin coating of copper, the whole is plunged into the silver bath, which then works very well. After the separation of the copy from the mould, allow it to rest in a solution of ammonia or of very dilute nitric acid, which, after a time dissolves the copper, and leaves the silver deposit. This reproduction must be imperfect, as there is, between the mould and the precious metal, an intermediate layer of copper of unequal thickness. When the surfaces are but slightly in relief, employ moulds of lead, tin, or fusible metal, upon which silver or gold will deposit well and without adherence. Lead is preferable to the other metals, especially when the mould may be obtained by pressure. Cover the pattern with a very thin foil of lead larger than the object, the guttapercha is applied upon it, and pressed, as before explained. The lead foil, without being torn, will follow all the details of the pattern, and may be separated afterwards with the gutta-percha which it has metallised. Instead of lead, silver or gold foils may be used, and are so thin that the seams disappear by simple pressure.
A somewhat thick sheet of very pure lead may be employed for taking moulds of engravings upon copper or steel. The lead and the engraved plate are to be passed between rollers, or simply pressed under a screw press.
-The bath for silver is composed of distilled water, 1 3/4 pint; potassium cyanide, 7 ox.; nitrate of silver, fused, 2 1/2 oz. The gold bath is made of distilled water, 2 pints.; potassium cyanide, 6 oz.; neutral gold chloride, 2 oz. These baths generally work with separate batteries, and with anodes of the metal used in the solution, or the porous cells and zincs may be put into the bath itself, provided that the exciting liquor be a more or less concentrated solution of potassium cyanide. The zincs must not be amalgamated, unless in separate batteries. Green gold is obtained by mixing 10 parts gold bath with 1 of silver bath, or by employing for a time a silver anode in the gold solution. The deposits of gold and silver, after their separation from the mould, should be heated and scratch-brushed; and a proper shade may be given to them by a short sojourn in ordinary electro-gilding or silvering baths.