Guttapercha is entirely insoluble in water, in weak acids, and in the solution of copper sulphate. After purification in boiling water, plates of various thicknesses or lumps are formed. A quantity sufficient for the intended mould is cut and put in cold water, which is gradually heated, until it is soft enough to be kneaded with the fingers like dough. After having pulled the gutta-percha in every direction, the edges are turned in so as to form a kind of half ball, the convex and smooth surface of which is then applied upon the middle of the model. Then the gutta-percha is spread over and forced to penetrate the details of the object. The kneading is continued so long as the material remains sufficiently soft, when it is allowed to cool. As soon as it is lukewarm, the gutta-percha is separated from the model, and dipped into cold water, when it hardens, and may then be handled without danger of impairing its accuracy.
After the object has been carefully coated with plumbago or tallow, it is put square and firm upon the table of the press, and surrounded with a ring or frame of iron, which should be a little higher than the most raised parts of the object. A piece of gutta-perchn, at least doable the thickness of the pattern, is cut so as to fill the ring or frame of iron, and then heated, on one of its faces only, before a bright fire. When about two-thirds of its thickness have been softened, it is to be placed, soft portion downwards, in the iron ring or frame, and the whole covered with a block of metal exactly fitting. The screw to the press is made to act slowly at first, but with gradually increased force, as the gutta-percha becomes harder and more resisting.
Cast a thin block of lead upon sand; hollow out approximately with a graver the places corresponding to the reliefs of the pattern, bearing in mind the desired thickness of the gutta-percha. Spread over the pattern a plate of gutta-percha of the same thickness all through; upon, this place the lend block; compress by the screw press. This process produces excellent results.
This is con-venient for brittle articles of plaster of Paris, marble, or alabaster. The pattern is put upon a dish of iron or earthenware, a ball of gutta-percha is placed in the middle of the object to be moulded, and the whole is placed in a stove, where the temperature is just sufficient to melt the gutta-percha, which softens and penetrates all the details; when it has sunk completely, remove it from the stove, and allow to cool off until it still retains sufficient elasticity to be separated from the pattern.
The foregoing process does not suit objects which will not bear the heat of the stove; for such articles heat the gutta-percha slowly until it becomes a semi-fluid paste; pour a sufficient quantity of it upon the pattern previously placed in an iron frame or ring. After a few minutes, knead it, with wet or oiled fingers, to make it penetrate all the details of the pattern until it scarcely yields to the pressure. In removing the mould from the pattern, cut off all the useless parts of the gutta-percha, and especially those which may have passed under the pattern and bind it. Then the proper position and shape of the covered pattern must be ascertained, so as not to break the model, or tear the gutta-percha. In moulding with the press, gutta-percha of the best quality is generally employed. For moulding by sinking or kneading, gutta-percha should be mixed with certain substances to increase its fusibility, such as linseed oil, lard, tallow, or yellow wax. Their proportions should never be over one-third of the total weight. The mixture with linseed oil is made by heating in a kettle 1 part linseed oil, and when its temperature has reached 190°-212° F., add gradually, and stir in 2 parts guttapercha cut into small pieces.
When the whole is in a pasty form, and begins to swell up with the production of thick fumes, remove the kettle from the fire, and throw its contents into a largo volume of cold water, where, without loss of time, the paste must be well kneaded. While still hot, place it upon a slab of marble or stone; it may afterwards be rolled between middling warm rollers. Gutta-percha may be used for an indefinite length of time. Models of plaster of Paris, from which moulds of fusible metal or of gelatine are to be taken, will stand the operation much better if they have been hardened by being saturated with boiled linseed oil, to which a certain proportion of dryer has been added. They must be oiled again just before pouring the gelatine over them.
A cast of a human head in plaster of Paris may be rendered impervious, and then metallised. After a deposit of copper has been effected on its surface, remove the plaster by boiling) and breaking it through the opening of the neck. The copper mould thus obtained, after being slightly greased inside, serves as a galvanoplastic trough, which is to be filled with the solution of copper sulphate. Suspend bags filled with crystals of blue vitriol to the edges, and with a separate battery and soluble anode, or with a porous cell placed inside the mould, which is connected with the zinc, another deposit of copper takes place in the cavity. When the thickness of the metal is sufficient, strip off the mould or first deposit. This process is expensive, but gives sure results with large patterns having large raised parts. With small or narrow, or very crooked objects, moulds in several parts must be used, although the seams require mending.
If it is required to imitate a statue, or other large article, commence by making with gutta-percha a mould in several pieces, which, by means of proper marks, may be united together, and form a perfect hollow mould of the pattern. Cover all these parts carefully with plumbago. Make a skeleton with platinum wires, to represent the outline of the pattern; this must be smaller than the mould, as it has to be suspended in it without any point of contact. The skeleton is to be enclosed in the metallised guttapercha mould, and the whole immersed in the galvanoplastic bath; connect the inner surface of the mould with the negative pole of the battery, and the skeleton of platinum wires, which should have no point of contact with the metallised surface of the mould, with the positive pole; this decomposes the solution of copper sulphate, which must be placed in the mould. When the deposit has reached the proper thickness, remove the gutta-percha mould, inside which will be found the statue, which may be finished at a very small expense. Lead wires may be substituted for the platinum, they are cheaper, and may easily be removed, when done with, by melting.