which should be covered their whole length, except at the points of contact.

Galvanoplastic Deposits upon Metals (with Adherence).

The simplest case the operator will meet in his practice is that of covering a metal with a coating of copper in such a manner that the object and the deposit shall form a single body.

All the metals are not equally qualified to receive the galvanoplastic deposit; and there are some which are naturally unfit for it. For instance, wrought and cast iron, steel and zinc, as soon as immersed in the solution of sulphate of copper, and even without the action of the galvanic current, decompose the salt, and become covered with a muddy, non-adherent precipitate of copper. When operating with these metals it is, therefore, absolutely necessary to give them a previous, and quite thick, coating of copper in the bath of double salts described in the first part of this work, before submitting them to the action of the sulphate of copper bath. Tin, although presenting these inconveniences to a much less degree, nevertheless reacts upon the sulphate of copper, blackens, but finally receives a deposit of slight adherence. It should also be copper-plated in the solutions of double salts before going into the galvanoplastic bath. The same observations hold good also for lead.

On the other hand, when the metal to be covered is unacted upon by the bath, it is sufficient to cleanse it well, and to submit it then to the action of the current in order to obtain a rapid and uniform deposit, which adheres more or less perfectly. The copper deposits obtained under these circumstances should not be too thick, otherwise the surfaces may have a coarse appearance, which will not satisfactorily reproduce the finer outlines of the mould.

With a bath in good condition, and a well-regulated electric current, the delicacy of the pattern will not be impaired by a copper coating having the thickness of a sheet of stout writing paper. A bright lustre will be imparted to the deposited surface by simply scratch-brushing and burnishing; or by a passage through aqua fortis and soot, and afterwards through the acid mixture for bright dipping.

CHAPTER LIV.

DEAD-GILDING BY GALVANOPLASTIC DEPOSIT.

ALTHOUGH We have already described in extenso, in Chapter XXVI., the galvanoplastic process of dead-gilding, it may be of interest to repeat the method here, and to generalize upon it by describing its application to all the metals and their alloys, as well as to non-metallic sub

stances.

Adherent galvanoplastic deposits furnish us the means of obtaining cheaply a very handsome dead-gilding which equals, if not in durability at least in appearance, that obtained with mercury, which was described in the first part

of this work.

Roseleur conducts the operation in the following manner: After having carefully cleansed the object if metallic, or rendered it a conductor, if non-metallic, immerse it in the sulphate of copper bath, and allow the deposit to acquire a dead lustre slightly in excess of that desired. After this operation, which may last from two to six hours, remove the article from the bath, rinse in plenty of water, and pass it rapidly through the compound acid mixture

for bright dipping, which diminishes the dead appearance of its surface. Next rinse in fresh water; steep in quicking solution, rinse again; and, lastly, immerse in an electrogilding bath made of

2 66

1 part.

Cyanide of potassium .

Gold (or its equivalent of chloride)

At first the current is rendered sufficiently intense by submerging the platinum anode deeply; afterwards the intensity is diminished by partly withdrawing the anode until the desired shade of gold is obtained. This gilding requires but little gold, since the frosty dead lustre is derived from the When the lustre of the copper very fine and velvety, we may dispense with the dipping in the acid mixture, but a rapid passage through the quicking solution is always desirable.

is

copper.

If the deposited gold be not uniform, or appears partly cloudy, it is proof of an imperfect deposit in the bath, or of an insufficient passage through the acid mixture for bright dipping. The piece should then be removed from the bath, washed in a tepid solution of cyanide of potassium, rinsed in fresh water, "quicked," and electro-gilded anew. This gilding bears burnishing well, but acid waters and soap, which will produce a red tint, must be avoided, and saliva only, or fresh solutions of linseed, or of marshmallow, be used. The tone of the gilding thus obtained is richer, deeper, and more durable than that produced upon frosted silver, which last may be recognized by the green color of the burnished parts.

This method is employed for dead-gilding the ornamental parts of cigar-cases, tablets, medals under glass for paper-weights, and especially chandeliers, clocks, and

other articles of ornament or utility, which, at the present day, are sold much more cheaply than formerly. Many articles of plaster of Paris and wood are also gilded by this process.

These adherent galvanoplastic deposits likewise give us the means of imparting a uniform appearance to objects composed of heterogeneous substances. For instance. a clock may have a base of copper and ornaments of lead; and the whole may be made to appear like bronze after a thin galvanoplastic deposit.

To conclude this form of deposit may be employed for binding together substances simply placed side by side, since the electro-deposited covering will form one continuous body. Certain kinds of mosaic work are thus made, of which the different pieces are simply placed side by side and united by, or set in, a galvanoplastic deposit.

CHAPTER LV.

NON-ADHERENT GALVANOPLASTIC DEPOSITS UPON METALSGALVANOPLASTIC COPIES OF DAGUERREOTYPES.

Non-adherent Galvanoplastic Deposits upon Metals. THE second galvanoplastic problem consists in deposit ing a coating of copper upon another metal, and in such a manner that, the operation being completed, the deposit may be easily separated from the object that has served as a mould, preserving at the same time, with mathemati cal accuracy, the shape and dimensions of the model, and possessing a considerable solidity.

This method is used for reproducing medals, bas-reliefs, etc., with great perfection.

We cannot give a more striking example of the capabilities of this method of procedure than the interesting experiment which we have already mentioned, of reproducing, with its aid, a daguerreotype with all its minutiæ. The manipulations, which are very simple, are as follows: Take a daguerreotype plate and coat its back and edges with varnish, then hold it by a corner and pour upon it a small quantity of alcohol, and rinse it in fresh water. After having connected it with a brass conducting wire, hang it for six or eight hours in the bath of sulphate of copper. The four edges are then cut with strong shears, and, by raising a corner with a pen-knife, or visiting card, the two sheets will easily separate, and will furnish two images, one as perfect as the other.

Returning to the general method of non-adherent galvanoplastic deposits of metals upon other metals, it will be necessary, as has been said, to cover zinc, iron, or tin with a previous coating of copper in the baths of double salts before submitting them to the simple electro-bath.

In this condition, or in the case of a metal unacted upon by the bath, we proceed as follows:

After a thorough cleaning of the object to be copied, it is rubbed with a brush charged with plumbago, or, what is better, with a soft brush slightly passed over a tallow candle. By this means, an imperceptible film of fatty substance, which prevents the adherence of the deposited copper, is left on the surface. This latter method is quite as good, and cheaper than that of covering with gold and silver, sometimes employed to prevent adherence.

It will be understood that the deposit will represent an inverted image of the pattern, in which the raised parts are seen as hollows, and conversely. This inconvenience is remedied by performing the same operation anew, using