The moulder then proceeds to work on the projecting portions of the model, making separate pieces so that they can be withdrawn and replaced at will. When he has made as many pieces as his judgment tells him are requisite, he places another iron box or flask upon the first, and having applied the parting dust to the work, he makes of the same loam the cope or case which is to retain all the pieces of the mould in position, and which is itself held together by the flask, which is merely an open iron frame. The two flasks are then turned over, and the first flask, with its loosely packed loam having been removed, the freshly exposed side is moulded in the same manner as the former, and the cope or case is formed as before. The two flasks can now be separated, and the pieces of the mould removed one by one from the figure, and accurately replaced in the mould. This gives a correct imprint of the plaster model. If the metal were cast into this it would be quite solid, therefore it must be cored. The co're is made by filling the cavity of the mould with the same loam, so that the figure is again reproduced. As, however, this loam figure occupied the entire cavity of the mould, it must be pared down, so that an empty space may remain between its surface and the interior of the mould.
This space is that which will be occupied by the metal.
Of coarse such a core has to be supported by an internal iron framework, the ends of which, projecting through the openings of the neck, the arms, and at the base, serve to keep it in position; without this it would of course fall to one or other side, and thus prevent the flow of the metal.
Openings must be provided in the mould for the admission of the metal, and for the escape of the air; but the vents for the air need not be so carefully arranged in casting on this system as in casting by waste wax, because the loam is very porous, and the joints in the mould facilitate the escape of the gases. By this process the risk in casting a statue is reduced to a minimum; no small advantage where colossal work is concerned. Nevertheless, when the bronze is cleaned from the loam, it will be found to be covered with lines from the joints of the mould; it is also not unlikely to be covered with a sort of hard skin, composed of particles of sand which have been vitrified by the intense heat. This skin and these lines have to be removed, and the various parts of the figures must be joined together. It is very frequently found necessary to go over the entire figure with the file and chasing tool, to bring the work to one tone, and in that case it is not possible for the statue to retain any claim to be considered an autograph work.
There can remain on it no touch of the original artist, and the fact of his signing it, and even numbering and dating it, will not save it from being merely a more or less excellent copy, and not an original.
Of course, if a sculptor chose to do the chasing and fitting himself he could do so, but think of the waste of time, not to mention the fact that even then the work would probably lose something of its original freshness and spirit.
This system of piece moulding for bronze is admirable for commercial purposes, and for colossal work it is all that can be desired, but otherwise its value is small, and a statue executed by this method cannot be esteemed by anyone who understands sculpture at more than a fraction of the price of the same statue if cast by the waste-wax process.
This wasta-wax process, So unimpor-tant commercially, is all-important artistically. It is by this method alone that a sculptor can produce original bronzes in his own studio, without the drawbacks incidental to the former process of piece moulding.
Roughly speaking, this process is as follows: - The sculptor makes, of plastic wax, an exact model of the work which he desires to cast in bronze. This model must be hollow, the thickness of the wax being exactly that intended for the bronze. This wax model is moulded, both inside and outside. The mould must be ail in one piece, and without seams. The wax, therefore, cannot be withdrawn without destruction either to itself or to the mould. But as the mould must be preserved till after the bronze is cast, the wax is withdrawn by melting it out. The bronze is then poured into the cavity.
Of course, this is not so simple as it sounds, and yet there is nothing in it that may not be successfully carried out by an artist in his own studio with small works, that is to say, under life-size.
The first thing that claims attention is the wax. This must be made harder for summer weather, and softer for winter use, unless, indeed, the artist is careful to keep his studio always at one temperature. This seems a small matter, but, nevertheless, it is of great importance, and much of one's comfort in working depends on it.
There are many different wax compositions, almost every founder having his favourite mixture. The chief ingredients, however, are beeswax and Venice turpentine. Some artists make use of no other ingredients and use no colouring matter. Most men, however, use in addition to the above, rosin, lard, tallow, and pitch in varying quantities. Excellent wax can be made either with or without these latter. Yellow wax is not easy to model in; it is therefore the general practice to stain it, in order to get rid of Borne of its transparency. In Italy, it is almost always coloured a brilliant red, with sulphide of mercury, which is entirely evaporated when the wax is melted out, and leaves no residue in the moulds. Almost any pure vegetable colour will do, or, indeed, any colour that will burn without leaving a residue; this is easily tested by placing a lump of the wax in a small white clay crucible, closing the top with a cover, to prevent any dirt getting in, and then burning away the wax. When the wax appears to have been consumed, gradually raise the temperature to a cherry-red heat, and then let it cool down.