After a few hours this solution will have completely evaporated, and will be found to have reduced to the metallic state the nitrate of silver covering the object, which has become entirely black, and is ready for the galvanoplastic bath.

To metallize wood, porcelain, and other resisting subtances, Roseleur recommends the following procedure, viz: Dissolve 1 part of nitrate of silver in 20 parts of distilled water. With fatty or resinous materials, which are water repellent, we employ aqua ammonia, which dissolves the nitrate of silver in all proportions. Lastly, with very delicate articles, which will not bear a long manipulation, alcohol, which evaporates rapidly, is em ployed as the solvent. Concentrated alcohol dissolves nitrate of silver but slightly; nevertheless, enough will be dissolved for metallizing flowers, leaves, mosses, lichens, etc., especially if the solution is assisted by trituration in a glass or porcelain mortar.

We have recourse also to metallization by silver (reduced by phosphorus in bisulphide of carbon) of those deeply indented parts of certain moulds which cannot be reached by even the slenderest pencil charged with plumbago. In this case, the mould is plunged entirely into the solution of nitrate of silver, then allowed to dry, and lastly exposed to the phosphorus fumes in the manner indicated.

This process of metallization is so perfect, that a silk cocoon thus prepared may be unspun, and the resulting fibre will possess a conducting power sufficient to immediately deflect the needle of a galvanometer, when it is used as a conductor between the instrument and the battery.

Roseleur makes allusions to laces and mousselines, which were coppered, gilded, and silvered, and which had scarcely, or not at all, lost their primitive softness.

It was even possible to burnish certain parts, and thus produce contrast of dead and bright lustres.

It is important to remark that if we fix the conducting wire to the mould before metallization, the wire must be of gold, silver, or platinum, since the other metals rapidly decompose the solution of nitrate of silver. On the other hand, brass or copper wires may be employed when the metallization is completed, that is to say, after the reduction by phosphorus has been effected.

The solution of phosphorus in bisulphide of carbon is prepared as follows: Half fill with this liquid a widemouthed flask with a tightly fitting ground glass stopper, then gradually introduce the phosphorus, gently dried with blotting paper, and shake the bottle now and then until no more dissolves. The preparation of this solution requires the exercise of great care, because in drying upon combustible materials it takes fire spontaneously. A piece of paper or of cloth moistened with it, takes fire after a few moments' exposure to the air.

It is not impossible to obtain galvanoplastic deposits, without previous metallization, upon a non-conducting substance, of which Roseleur gives the following instructive illustration: If we bind a cameo of agate with a copper wire, and hang it in the bath, the wire alone will receive the deposit, and its volume will increase in every direction. If, now, we cover with an insulating varnish the exterior of the deposit, repeating the operation as it grows, it will continue to increase only on the periphery, and by and by it will cover the whole of the cameo. After separation, the reproduction will be all the more perfect from the fact that no foreign substance has been interposed. It is from a similar expansion of the deposit that the non-metallized parts of moulds often become coated.

CHAPTER LVIII.

MOULDING-MOULDING WITH PLASTER OF PARIS-WITH STEA• RINE WITH WAX-WITH MARINE GLUE-WITH FUSIBLE METAL WITH GELATINE-WITH GUTTA PERCHA.

Moulding of the Pattern.

In cases where the original objects are used as patterns or moulds, even where they have flat or not-undercut surfaces, and with copies of medals or bas-reliefs of plaster, bronze, wax, etc., there is a risk of damaging them if we effect the galvanoplastic deposit directly upon Moreover, we thus obtain a reversed copy of the object in which the reliefs are sunken, and vice versa, and, in order to reproduce a fac-simile, we are obliged to make a second deposit upon the first, which is double work.

them.

These considerations have naturally induced galvanoplastic operators to begin by taking a mould from the pattern or object itself, with the aid of some plastic or fusi ble material, and to submit this mould to the subsequent operations. If from a bronze metal in relief, we have taken a cast in plaster of Paris, this casting will be a hollow mould, which being filled afterwards with the copper deposit, will exactly reproduce the relief of the original medal. In this operation we run no risk of dam aging a rare and valuable object, and have the advantage of being able to reproduce it many times and with the same accuracy, since we can make as many casts of moulds from the original as may be desired.

or

There are many substances which may be employed

for making moulds, but all are not equally well adapted, and it is important to know how to apply them in the manner best adapted to each special case.

Moulding with Plaster of Paris.

Plaster of Paris stands first, not in the order of merit, but as having been the earliest to be employed for this purpose. It is convenient for taking casts from plaster of Paris itself, from stucco, marble, alabaster, metals, and wood. The mode of procedure is as follows: Given, a plaster medal in relief, from which we desire to obtain a hollow mould which will furnish the true relief of the original in copper. After the original has been thoroughly soaped or black-leaded, wrap around the rim a piece of sufficiently stout paper, or of thin lead foil, and attach it in such a manner that the face of the medal to be copied is at the bottom of the receptacle thus formed. It is desirable to sink this box to a certain depth in a layer of fine sand, which prevents the escape of the semi-fluid plaster of Paris between the rim of the medal and the paper. Then in a vessel filled with a sufficient quantity of water, sprinkle fine plaster of Paris until the last portions reach the level of the water. After waiting for one or two minutes, the mass is stirred, and the resulting thin paste must be employed immediately. Take up a small quantity of this paste with a pencil or brush and spread it in a thin film carefully and smoothly over the face of the medal, then pour on the remainder of the paste up to a proper height, and allow it to set. After a few minutes the plaster heats and solidifies. Then remove the surrounding paper, scrape off with a knife what has run between the paper and the rim of the medal, and carefully separate the plaster cast from the model.

If, instead of applying the first layer with a brush, the

whole of the plaster were run at once into the recess, there would be great risk of imprisoning bubbles of air between the model and the mould, which would consequently be worthless.

We have seen that moulds of plaster of Paris cannot be introduced into the bath of sulphate of copper without having been previously rendered impervious. This difficulty prevents the more frequent use of this substance for moulding, and galvanoplastic operators, for this reason, give the preference to other materials which are unaffected by the bath, and which it is sufficient to metallize if they are non-conducting; such are stearine, wax, fusible-metal, marine-glue, and, especially, gelatine and gutta-percha.

Moulding with Stearine, Wax, and Marine Glue.

Stearine is applied in the same manner as plaster of Paris, with this difference, that the former is melted with the aid of heat, and poured upon the model when it is about to congeal. When stearine is too new or dry, it crystallizes in cooling, and this impairs the beauty of the cast. In such case it should be mixed with a few drops of olive oil, or with tallow. On the other hand, if it be, too fat, it remains soft and is difficult to separate from the mould. It should then be mixed with beeswax or spermaceti.

As stearine contracts considerably in cooling, its employment should be avoided when the copies from the model are required to be mathematically accurate reproductions of the original.

When we desire to make a cast with stearine of a plaster model, it should be thoroughly saturated with water or stearine beforehand, as explained in our description of galvanoplastic deposits upon plaster. It should also be thoroughly coated with plumbago before the melted