Plumbago (graphite) is generally preferred, and in most cases its conducting power is sufficient; it may be applied in films thin enough not to impair the sharpness of the mould. The plumbago found in the trade is rarely pure. Remove the impurities by digesting for 24 hours paste made of plumbago and water, with hydrochloric acid. Several washings with water, and slow drying in a stove, finish the operation. If the plumbago is in large lumps, it should be powdered and passed through a silk sieve. The conducting power of this substance is sufficient when the surfaces are not deeply indented; but the mould should be rough enough for the plumbago to stick to it.
Gilt Plumbago has a conducting power much greater than that of the ordinary substance. Prepare as follows: - In 1 3/4 pint sulphuric ether dissolve 1/3 oz. gold chloride, and thoroughly mingle with it 18-20 oz. good plumbago. Then pour into a shallow porcelain vessel, and expose to the action of air and light. After a few hours, the ether completely volatilises; stir the powder now and then with a glass spatula. * Finish the drying in a stove.
Dissolve 3 oz. crystallised; silver nitrate in 3 pints distilled water; mix this solution with 2 lb. good, plumbago. Dry in a porcelain dish, and then calcine at a red heat in a covered crucible. After cooling, powder and sift. Plumbago thus metallised conducts electricity nearly as well as a metal, although it is very expensive. Bronze powder mixed with plumbago is also used.
Porous substances, before being coated with plumbago, are submitted to a previous operation, to render them impervious, by covering them with a coat of varnish, or by saturating them with wax, tallow, or stearine. For instance, with a plaster cast, cut a groove on the rim of the mould, place in it a brass wire, twist the ends, which must be long enough to hold the cast by. The cast, having been previously dried, is then dipped into a bath of stearine kept at a temperature of 180°-212° F., and a number of bubbles of air will escape from the mould to the surface. When the production of air-bubbles is considerably diminished, remove the cast from the bath. When the cast is tepid, cover it with powdered plumbago, and let it get quite cold. Then, after breathing upon it, rub thoroughly with a brush covered with plumbago; and be careful that the surfaces are completely black and bright, without grey or whitish spots. When the mould is very undercut, it is difficult to employ plumbago. In such cases metallise the whole, or the deep parts only, by the wet way. Soft brushes should not be used for rubbing plumbago.
When the substances to be metallised are not porous, such as glass, porcelain, stoneware, horn, and ivory, cover them with a thin coat of varnish, which, when nearly dry, receives the plumbago.
After having varnished the portions of the piece to be covered, cover them with very finely laminated foils of lead, which bend to ail desired shapes; then connect a brass conducting wire with the lead, and dip the whole into the bath; copper is immediately deposited upon the metallic parts. Thus glass vases may be entirely covered with copper, upon which deposit layers of gold or silver. The chaser may penetrate with his tool to different depths, and uncover one after the other, first the layer of silver, next that of copper, and at last the crystal itself. The vase will appear as if set in a net of various colours. For very fine work, the gold ornament first painted with the pencil, and fixed in the usual manner by heating in a muffle, is put in contact with a very thin conducting wire, and the whole is immersed in a copper, silver, or gold bath, where the deposit takes place in the same manner as upon an ordinary metal, and the adherence is as perfect as that of the film of gold upon the porcelain.
The deposit is afterwards polished, chased, or ornamented on the lathe.
Silver, gold, and platinum, reduced from their solutions, have an excellent conducting power. Silver is generally preferred, and its nitrate is dissolved in certain liquids, variable with the substances to be covered. Apply the solution with a pencil upon the mould, and let it dry; repeat the operation two or three times. Lastly, expose the mould to the action of the sunlight, or of hydrogen, or fix it to the top of a box which closes hermetically, and at the bottom of which is a porcelain dish holding a small quantity of a concentrated solution of phosphorus in carbon bisulphide. After a few hours this solution completely evaporates, and reduces to the metallic state the silver nitrate covering the mould, which becomes black, and is then ready for the bath. When used to metallise wood, porcelain, and other resisting substances, dissolve 1 part silver nitrate in 20 of distilled water. With fatty or resinous materials, which water will not wet, use aqua ammonia. With very delicate articles, which will not bear a long manipulation, make the solution in alcohol, which evaporates rapidly. Concentrated alcohol dissolves silver nitrate but slightly; but enough will be dissolved for metallising flowers, leaves, and similar articles, if the solution if aided by grinding in a mortar.
If the conducting wire is fixed to the mould before the metallisation, the wire mast be of gold, silver, or platinum, as the other metals rapidly decompose the solution of silver nitrate; but brass and copper wires may be employed when the metallisation is completed, after the reduction by phosphorus.
Half fill a glass stoppered bottle with a large neck with bisulphide, then gradually introduce the phosphorus gently dried with blotting paper, and shake the bottle now and then. Phosphorus is added until no more dissolves. This preparation requires great care in the handling, because in drying upon combustible materials it takes fire spontaneously.