CASTING AND MOULDING. Under this head we shall give the art of taking impressions from sculptures, medals, and other delicate works of art ; also the taking of casts from the face, and other natural objects. For the process of casting articles of metal in the large way, see FOUNDRY. To procure a cast from any figure, bust, medal, &c. it is necessary to obtain a mould, by pressing upon the thing to be copied some substance capable of being forced into all the cavities or hollows of the sculpture. When this mould is dry and hard, some substance, which will fill all the cavities of the mould, is poured into it : the form of the original from which the mould was taken, is now accurately repre sented. Moulding in any particular manner depends upon the form of the subject. When there are no projecting parts, but such as rectilineal angles with the surface of the body, nothing more is necessary than to cover it over with the substance of which the mould is to be formed, and to take it off clean, and without bending. It may be laid horizontally, and will bear to be oiled without injury. The substances used for moulding are various, according to the nature and the situation of the sculpture ; as wax, metal, plaster of Paris, &c. This last is prepared in a fine powder, mixed with water, and poured over the mould to a convenient thickness, after oiling it to prevent the plaster from sticking. A composition of bees' wax, resin, and pitch, makes a very desirable mould, if many casts are to be taken. If the situation of the sculpture be perpendicular, clay, or some similar substance, must be used. The best kind of clay is that with which sculptors make their models ; it is worked to a due consistence, and having been spread out to a size sufficient to cover all the surface, it is sprinkled over with whiting, to prevent it from adhering to the original. Bees' wax and dough, or the crumb of new bread, may also be used for moulding some small subjects, as impressions of seals and bijoux. When there are under-cuttings in the boa relief, they must be first filled up before it can be moulded, otherwise the mould could not be got off. When the casts are taken afterwards, the places must be worked out with a proper tool. When the model or original subject is of a round form, or projects so much that it cannot be moulded in this manner, the mould must be divided into several parts; and it is frequently necessary to cast each of them separately, and afterwards to join them together. In this case the plaster must be tempered with water to such a
consistence that it may be worked like soft paste, and laid on with some con venient instrument, compressing it till it adapt itself to all parts of the surface. When the model is thus covered to a convement thickness, the whole is left at rest till it becomes sufficiently firm to bear dividing, without falling to pieces by any slight violence ; it must then be separated into pieces to be taken from the model, which is done by cutting it with a thin bladed knife. Being now divided, it must be cautiously taken off and kept till dry ; but before the separa tion of the parts is made, they are notched across the joints at proper distances, that they may with certainty be put together again. The art of properly dividing the moulds to make them separate from the model, requires dexterity and skill. Where the subject is of a round or spheroidal form, it is beet to separate the mould into three parts, which will then easily come off from the model; and the same will hold good of a cylinder, or any regular curved figure. The mould being thus formed and dry, and the parts put together, it must be first oiled and placed in such a position that the hollow may be upwards. It is then filled with plaster mixed with water, and repaired where necessary. This finishes the operation. In larger masses, where there would otherwise be a great thickness of the plaster, a core may be put within the mould, as was observed in regard to the casting of statues, to produce a hollow in the cast : this saves expense of plaster, and renders the cast lighter. In the same manner, figures, busts, &c., may be cast of lead, or any other metal, in the moulds of plaster or clay ; the moulds must be perfectly dry, for should there be any moisture, the sudden heat of the metal will convert it into vapour, and produce by its expansion an explosion that would blow the melted metal about to the great danger of the artist.