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, etc. 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 represented. 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 principal 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, etc.
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 bas 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 convenient instrument, compressing it till it adapt itself to all parts of the surface. When the model is thus covered to a convenient 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 separation 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 best 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, etc, 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.
To take a Cast in Metal from any small Animal, Insect, or Vegetable. Prepare a box sufficiently large to hold the animal, in which it must be suspended by a string; the legs, wings, etc. of the animal, or the tendrils, leaves, etc. of the vegetable, must be separated and adjusted in their right position by a pair of pincers. A due quantity of plaster of Paris, mixed with talc, must be tempered to a proper consistence with water, and the sides of the box oiled. Also a straight piece of stick must be put to the principal part of the body, and pieces of wire to the extremities of the other parts, that they may form, when drawn out after the matter of the mould is set and firm, proper channels for pouring in the metal, and vents for the air, which, otherwise, by the rarefaction it would undergo, from the heat of the metal, would blow it out, or burst the mould. In a short time the plaster will set and become hard; the stick and wires may now be drawn out, and the frame in which the mould was cast taken away; the mould must then be put, first into a moderate heat, and afterwards, when it is as dry as it can be rendered by that degree, removed into a greater, which may be gradually increased until the whole be red hot.
The animal or vegetable inclosed in the mould will then be burnt to a coal, and may be totally calcined to ashes by blowing for some time into the charcoal and passages made for pouring in the metal and giving vent to the air. This operation, at the same time that it destroys the remainder of the animal or vegetable matter, will force out the ashes. The mould is then allowed to cool gently; the destruction of the substance that had been included in it has now produced a corresponding hollow; but it may, nevertheless, be proper to shake the mould, and blow with bellows into each of the air vents, to free it wholly from any remaining ashes: when there may be an opportunity of filling the hollow with quicksilver, it will be found an effectual method of cleaning the cavity, as all the dust and ashes must rise to the surface of the quicksilver, and be poured out with it. The mould being thus prepared, must be heated very hot when used, if the cast is to be made of copper or brass, but a less degree of heat will serve for lead and tin.
The metal being poured into the mould, it must be gently struck, and then suffered to rest till it be cold; it is then carefully taken from the cast, but without force: such parts of the matter as appear to adhere more strongly, are to be softened by soaking in water until they be loosened, that none of the more delicate parts of the cast may be broken off or bent. When talc cannot be obtained, plaster alone may be used; it is apt, however, to be calcined by the heat used in burning the animal or vegetable from which the cast is taken, and to become of too incoherent and fusible a texture. Stourbridge clay, washed perfectly fine, and mixed with an equal part of fine sand, may be employed. Pounded pumice-stone and plaster of Paris, in equal quantities, mixed with washed clay in the same proportion, make excellent moulds.
To take a Cast in Plaster from a Person's Face. The person from whom the cast is to be taken should lay down on his back, with his hair tied back, so that none may cover his face. Into each nostril a conical piece of stiff paper, open at each end, is placed to allow of breathing. The face is then to be lightly oiled over with salad oil, to prevent the plaster from sticking to the skin. Fresh burnt plaster is mixed with water to a proper consistence, and poured in spoonsful all over the face (taking care that the eyes be shut), till it is entirely covered to the thickness of a quarter of an inch. This substance will grow sensibly hot, but this inconvenience is of short duration as, in a few minutes, the plaster will set hard, and may be taken off in a complete mask, which will form a mould, in which a head of clay may be moulded, wherein the eyes may be represented as open, and such other additions or corrections made as may be found necessary. Then this second face being anointed with oil, a second mould of plaster is made upon it, consisting of two parts, jointed lengthwise along the ridge of the noise, and in this a cast in plaster is taken, which is an exact likeness of the original.
To take Casts from Medals, a mould must first be made of plaster of Paris, or of melted sulphur. After having oiled the surface of the medal with a little cotton, or a camel's hair pencil dipped in oil of olives, a hoop of paper must be put round the medal, standing up above the surface, of the thickness you wish the mould to be. Take now some plaster of Paris, mix it with water to the consistence of cream, and with a brush rub it over the surface of the medal, to prevent air-holes from appearing; then immediately afterwards make it to a sufficient thickness by pouring in more plaster. Let it stand half an hour, when it will have grown so hard that you may take it off; then pare it smooth on the back and round the edges, neatly: in cold or damp weather it should be dried before a fire. When the mould is large, if you cover its face with fine plaster, a coarse sort will do for the back; but no more plaster should be mixed up at one time than can be used, as it will soon get hard, and cannot be softened without being burned over again.
Sulphur must not be poured upon silver medals, as this will tarnish them.
To prepare your mould for casting sulphur, put plaster of Paris in it; take half a pint of boiled linseed oil, and one ounce of oil of turpentine, and mix them together in a bottle, dip the surface of the mould into this mixture, take the mould out again, and when it has absorbed the oil, dip it again; repeat this till the oil will no longer be absorbed. Then wipe off the oil with cotton wool, and set the mould in a dry place for a day or two, when it will be a hard surface. To cast plaster of Paris in this mould, proceed with it in the same manner as above directed for obtaining the mould itself, first oiling the mould with olive oil. When casts are wanted in sulphur, the material must be melted in an iron ladle
To take Casts with Isinglass, Dissolve isinglass with water over the fire. With a hair pencil lay the solution carefully over the surface of the medal, and let it dry When hard, raise the isinglass with the point of a knife, and it will fly oil" with a spring, leaving a sharp impression of the medal. The isinglass may be coloured with any of the water colours while in solution, or you may breathe on the concave side, and lay gold loaf upon it, which, by shining through, will give it the appearance of a gold medal. A little carmine mixed with the isinglass gives the appearance of copper, particularly if gold leaf be placed inside.