of gold or silver is their alkaline state, which prevents the use of many substances, especially of fatty materials, for the preparation of the moulds. Moreover, these baths are inferior in conducting power for electricity, and require a better metallization than that afforded by plumbago. We dare not use the reduced metals from solutions of nitrate of silver or chloride of gold, because they will be immediately dissolved. Therefore, in the case of somewhat deeply wrought moulds, we are obliged to get around the difficulty by the following artifice: After having deposited by the ordinary process a thin film of copper, the whole is immersed in the silver bath, which then deposits very well. After the separation of the copy from the mould, it is allowed to remain in a solution of ammonia or of very dilute nitric acid, which, after a certain length of time, dissolves the film of copper, and leaves the silver deposit alone.

Such a reproduction must necessarily be very imperfect, since there has been, between the mould and the precious metal, an intermediary layer of copper of unequal thickness. This is, however, the only process we know of, in operating upon non-conducting and undercut moulds. On the other hand, when the surfaces are plane, or but slightly in relief, we may employ moulds of lead, tin, or fusible metal, upon which silver or gold will deposit well and without adherence.

Roseleur describes the following process for employing non-conducting moulds for galvanoplastic operations with gold or silver: The pattern is covered with a foil of lead very thin and larger than the object, then the gutta percha is applied upon it, and the whole is subjected to pressure, as before explained. The lead foil, without being torn, follows all the details of the pattern, and may be separated afterwards with the gutta percha which it

has metallized. Instead of lead we may employ silver or gold leaf, which is so thin that the joints disappear by simple pressure. Roseleur recommends this method to amateurs, desirous of obtaining good deposits of silver or gold, as very simple and easy to acquire.

A somewhat thick sheet of very pure lead may be em ployed for taking moulds of engravings upon copper or steel. The lead and the engraved plate are passed be tween rollers, or simply subjected to pressure in a screw

press.

Galvanoplastic Baths for Silver and Gold. (Roseleur.) The bath for galvanoplastic operations with silver is composed of

Distilled water

Cyanide of potassium.

Nitrate of silver (fused)

100 parts.

20

66

7"

A more economical solution is obtained by using half of the above named quantities of cyanide and nitrate of silver for the same proportion of water.

The gold galvanoplastic bath is composed of

These baths are generally operated with separate current and with anodes of the metal used in the solution. Nevertheless, the porous cells and zines may be immersed in the bath itself, provided that the exciting liquid be a more or less concentrated solution of cyanide of potas sium, or of common salt.

Green gold is obtained by mixing ten parts of gold bath with one of silver bath, or, what is preferable, by employ

ing for a certain length of time a silver anode in the gold solution.

The galvanoplastic deposits of gold and silver, after their separation from the mould, should be heated and scratch-brushed; and it will be advantageous to give them the desired tint by a short immersion in the ordinary electro-gilding or silvering baths.

CHAPTER LXIV.

GALVANIC ETCHING-THE FUTURE.

Galvanic Etching.

WE have said that, with baths working by the battery process, the soluble anode, or plate of the same metal as that in solution, was connected with the positive pole, and was dissolved at the same time as the metallic deposit was effected at the other pole.

From this phenomenon to galvanic etching is but a single step. Indeed, it is evident that, if certain portions of the anode be covered with some insulating material, the exposed parts will be dissolved slowly and more or less uniformly, and will form the hollows of an engraving, in which the reliefs will be the covered parts.

Galvanic etching is obtained by several methods, which differ but little one from another.

The most simple process consists in covering entirely a copper plate, for instance, with an insulating varnish, which is not acted upon by the bath, and then in tracing the drawing with a graver, which should penetrate through the coat of varnish, and expose the copper. By using this plate as the soluble anode of a bath of sulphate

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of copper, and suspending another copper plate at the negative pole, the latter will receive the deposit, while the former will be eaten away at the places uncovered by the graver. The engraving produced will need only a slight touching up to be ready for use.

Instead of using wax, varnish, or similar substance as an insulating material, we may employ a film of some metal that will not be dissolved in the bath. Let us suppose, for instance, that the copper plate has been strongly gilded with the aid of the battery, and that with a graver we trace a drawing deep enough to uncover the copper. this plate, being submitted to the same electric conditions as above, will be corroded where the graver has passed, while the gilded portions will remain entirely unacted upon, since the free acid liberated in the bath of sulphate of copper does not dissolve gold.

With the same ease it is possible to produce a drawing in relief, instead of countersunk, by making the drawing with some insulating material like varnish or a litho graphic crayon. The uncovered portions surrounding the lines of the drawing will become eaten away, and the drawing will remain in relief. We may also make the drawing with the lithographic pencil or with varnish. and strongly gild the uncovered parts; then, if the var nish or the greasy lines of the pencil are removed, the etching will follow these lines, which will be countersunk.

The baths employed generally contain in solution the same metal as that to be engraved; thus, baths of sulphate of copper are used for etching copper plates, solu tions of sulphate of zinc for zinc plates, and gold or silver

baths for these metals.

Nevertheless, it is possible to engrave copper and zinc plates, with the aid of the battery, in baths of water simply acidulated with either nitric, hydrochloric, sul

phuric, or acetic acid; and this process seems to be coming more and more into use.

Herewith is given a process, not for engraving, but for obtaining a mould of a design by substitution, from which galvanoplastic copies ready for printing may be obtained: Upon a varnished plate of copper a drawing is traced; this plate is dipped into a weak " quicking" solution, and then set perfectly level. By covering it with metallic mercury, this metal becomes fixed upon the lines traced by the graver, and takes the meniscus (curved) shape, that is to say, the relief is the greater as the engraved line is larger. Therefore, all the drawing is reproduced in relief by the mercury. We may now cover the plate with a thin paste of plaster of Paris, and when the latter has set, the two moulds may be separated, the mercury still adhering to the copper. The plaster mould will be treated by one of the above-described methods, either for preparing a counter mould from it, or for directly obtaining a galvanoplastic deposit after its metallization.

The following is another similar method which gives a mould immediately ready for the bath: The copper plate is varnished as above, and with the graving tool the parts which will produce the blacks of the engraving are uncovered. A solution of neutral protochloride of zinc is poured upon the plate, followed by a quantity of Darcet's metal, fusible at about 175° to 212° Fah. (see page 467), which is melted by means of an alcohol lamp moved about under the copper plate. The operation is facilitated by spreading the fused metal with a small iron rod all over the plate. The same result is obtained as with the mercury, except that the mould may be immediately reproduced by galvanoplastic methods.

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