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the deposit as a mould, and this second deposit will be an accurate reproduction of the original. A great number of copies may be made from the first or reverse matrix obtained.

CHAPTER LVI.

ELECTRO-DEPOSITS UPON NON-METALLIC SUBSTANCES.

THERE is simply an artificial, or indirect adherence be tween a deposited metal and a non-conducting mould. Metals are generally, and by the usual methods, welded or soldered only to metals. When we speak of metallic deposits adhering to non-metallic substances, therefore it will be understood that, either the non-metallic material is completely enveloped by the metallic deposit, or that the mould is undercut, or held by several points of adhe

sion.

In either case, when the artificial support is removed, the metallic and non-metallic surfaces separate.

It is by this process that we are enabled to cover with galvanoplastic deposits the surfaces of porcelain, crystal, plaster of Paris, and other plastic materials, wood, flowers, fruits, animals, and the most delicate insects.

But the first difficulty to be overcome in these various operations is the absence of conductive power for electri city which characterizes these materials. It is, therefore. absolutely necessary, and before anything can be done, to render them conductors by the procedure known as metallization.

CHAPTER LVII.

METALLIZATION-PLUMBAGO— RENDERING OBJECTS IMPERVIOUS TO LIQUIDS-METALLIC POWDERS

-METALLIZATION

BY THE WET WAY.

Metallization.

THE metallization of a non-conducting substance.consists in covering all the parts, which are destined to receive the galvanoplastic deposit, with a coating of some material which will conduct electricity as well, or nearly as well, as a metal. This coating should be imperceptible in thickness, so that it may not sensibly fill up the finest details of the surface of the object.

Many methods are known for imparting the power of conducting the electric fluid to substances which do not possess it naturally. Unfortunately, however, none are perfect, and many are so expensive and difficult to apply as to forbid their employment. We shall, nevertheless, enumerate those in common use, and which produce results with which we are obliged to content ourselves.

Plumbago-Gilt Plumbago-Silvered PlumbagoCoppered Plumbago.

The substance generally employed as a metallizing material is plumbago, also called black-lead, or graphite. In a majority of cases its conducting power is sufficient; and it may be applied in films thin enough not to impair the sharpness of the mould.

Commercial plumbago is rarely pure, being contami

nated with earth, iron, and sometimes with the sulphides of this metal. The impurities may be removed by digest ing for twenty-four hours with hydrochloric acid a paste made of plumbago and water. Several washings with water, and a slow drying in a stove finish the operation. In its pure state, plumbago is simply a peculiar variety of mineral carbon. For the purpose here described, only the very best should be used, and it should be in the finest possible state of division. An extra quality is prepared and sold expressly for the galvano-plastic operator.

Plumbago is satisfactory for the metallization of moulds of a certain nature and shape, but will not suit for others. Generally, the conducting power of this substance is sufficient when the surfaces are not deeply, indented. At the same time, the object should be rough enough, or suf ficiently fatty or greasy to permit of the adherence of the plumbago.

Roseleur has lately prepared plumbago with a conducting power much greater than that of the ordinary substance, by incorporating with it gold or silver.

Gilt Plumbago is prepared as follows: In 100 parts of sulphuric ether he dissolves 1 part of chloride of gold, and thoroughly mingles with it from 50 to 60 parts of good plumbago. The whole is then poured into a shallow porcelain vessel, and exposed to the action of air and light. After a few hours the ether is completely volatilized, and the powder is now and then stirred with a glass spatula. The desiccation is finished in a stove, and the product is ready to use.

Silvered Plumbago is obtained by dissolving 10 parts of crystallized nitrate of silver in 200 parts of distilled water, and mixing with this solution 100 parts of good plumbago. The whole is dried in a porcelain dish, and

then calcined at a red heat in a covered crucible. After cooling, the product is powdered again, and sifted.

Plumbago thus metallized conducts electricity nearly as well as a metal surface. Knight's process of coppering non-conducting surfaces is very generally applicable, though especially serviceable in electrotyping, and consists in applying on the mould, after black-leading, a solution of sulphate of copper, then dusting thereon from a pepperbox some impalpably-fine iron filings, and gently going over the surface with a soft brush. A uniform and continuous precipitation of metallic pure is thus procopper duced, which covers the entire surface of the mould with an excellent conducting medium. A process devised by Mr. Adams, for accomplishing the same object, consists in applying finely-powdered tin to a wax mould, with a soft brush, until the surface presents a bright, metallic appearance.

Rendering Objects Impervious to Liquids.

Certain objects, before-being coated with plumbago, are submitted to a preparatory operation, for the reason that they are porous, and will be injured by the solution of sulphate of copper which penetrates them. Objects of carbonate of calcium (chalk and marble), sulphate of calcium (plaster of Paris), wood and other spongy materials, fall into this category. It is therefore necessary to render them impervious before metallization, by covering them with a coat of varnish, or by saturating them with wax, tallow, or, still better, with stearine.

Let us take a plaster cast, and describe this procedure: After having cut a groove on the rim of the object, connect to it a brass wire in the manner shown (Fig. 147), and long enough to facilitate the operation. The plaster cast, previously dried, is then dipped into a bath of molten

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When the production of air bubbles has considerably diminished, the cast is removed from the bath, and almost immediately dries, by the absorption of the last portions

of stearine.*

When the cast has become nearly cold, it is dusted with the finest plumbago, and allowed to cool off entirely. Then, after breathing upon it, it is rubbed thoroughly

* If this operation has been effected with great care and cleanliness (upon alum plaster, Keene's cement, plâtre aluné), it is sufficient to rub with a woollen rag, and afterwards polish the stearined cast, to impart to it a species of translucency. Thus are made those religious emblems fixed upon velvet and framed in ebony, which so well imitate ivory. In this case, the stearine employed must be very pure, or contain only a small proportion of spermaceti, and it should be melted on a water bath in glass or porcelain vessels. The plaster of Paris may receive, by absorption, various colors before it is introduced into the bath of stearine.

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