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plaster of Paris, mixed with talc, must be tempered to the 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, in order 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 metals, would blow it out, or burst the mould. In a short time the plaster will set, and become hard; when the stick and wires may be drawn out, and the frame or coffin in which the mould was cast taken away; and the mould must then be put, first, into a moderate heat, and, afterwards, when it is as dry as can be rendered by that degree, removed into a greater, which may be gra. dually increased, till 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, which will at the same time that it destroys the remainder of the animal or vegetable matter, blow out the ashes. The mould must then be suffered to cool gently, and will be perfect; the destruction of the substance included in it having produced a corresponding hollow; but it may nevertheless be proper to shake the mould, and turn it upside down, as also to blow with the bellows into each of the air-vents, in order to free it wholly from any remainder of the ashes; or, where there may be an opportunity of filling the hollow with quicksilver, it will be found a very effectual method of
clearing the cavity, as all dust, ashes, or small detached bodies, will necessarily rise to the surface of the quicksilver, and be poured out with it. The mould being thus prepared, it must be heated very hot when used, if the cast is to be made with copper or brass, but a less degree will serve for lead or tin. The metal, being poured into the mould, must be gently struck, and then suffered to rest till it be cold; at which time it must be carefully taken from the cast, but without force; for such parts of the matter as appear to adhere more strongly must be softened, by soaking in water till they be entirely 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; but it is apt to be calcined by the heat used in burning the animal or vegetable from whence the cast is taken, and to become of too incoherent and friable a texture. Stourbridge, or any other good 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, is said to make excellent moulds.
Method of taking a Cast in Plaster from a
whose likeness is required in plaster must lie on his back, and the hair must be tied up
so that none of it covers the face. Into each nostril convey a conical piece of stiff paper open at both ends, to allow of breathing. The
face is then lightly oiled over in every part with salad-oil, to prevent the plaster from sticking to the skin. Procure some fresh burnt plaster, and mix it with water to a proper consistence for pouring. Then pour it by spoonfuls quickly all over the face (taking care the eyes are shut), till it is entirely covered to the thickness of a quarter of an inch. This substance will grow sensibly hot, and in a few minutes will be hard. This being taken off will form a mould, in which a head of clay may be moulded, and therein the eyes may be opened, and such other additions and corrections may be made as are necessary. Then, this second face being anointed with oil, a second mould of plaster must be made upon it, consisting of two parts joined lengthwise along the ridge of the nose ; and in this a cast in plaster may be taken, which will be exactly like the original.
To take Casts from Medals.
In order to take copies of medals, a mould must first be made; this is generally either 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, put a hoop of paper
round it, standing up above the surface of the thickness you wish the mould to be. Then take 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 on more plaster. Let it
stand about half an hour, and it will in that time grow so hard, that you may safely take it off; then pare it smooth on the back and round the edges neatly. It should be dried, if in cold or damp weather, before a brisk fire. If you cover the face of the mould with fine plaster, a coarser 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 burning over again.
Sulphur must not be poured upon silver medals, as this will tarnish them.
To prepare this mould for casting sulphur or plaster of Paris in, take half a pint of boiled linseed oil, and oil of turpentine one ounce, and mix them together in a bottle; when wanted, pour the mixture into a plate or saucer, and dip the surface of the mould into it; take the mould out ugain, and when it has sucked in the oil, dip it again. · Repeat this till the oil begins to stagnate upon it; then take a little cotton wool, hard rolled up, to prevent the oil from sticking to it, and wipe it carefully off. Lay it in a dry place for a day or two (if longer the better,) and the mould will acquire a very hard surface from the effect of the oil.
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. If sulphur casts are required, it must be melted in an iron ladle.
Another method with Isinglass.—Dissolve isinglass in water over the fire; then, with a hair pencil, lay the melted isinglass over the medal : and when you have covered it properly, let it dry.
When it is hard, raise the isinglass up with the point of a penknife, and it will fly off like horn, having a sharp impression of the medal.
The isinglass may be made of any colour by mixing the colour with it; or you may breathe on the concave side, and lay gold leaf on it, which, by shining through, will make it appear like a gold medal. But if you wish to imitate a copper medal, mix a little carmine with the isinglass, and lay gold leaf on as before.
Cements require to be of various compositions, according to the substances to which they are applied, and whether they are to be exposed to heat and moisture.
Common glue is formed by extracting by boiling the gelatinous part of cuttings or scraps of coarse leather, or the hides of beasts. It is then poured out in thin cakes and dried.
Isinglass Glue. Isinglass glue is made by dissolving beaten isinglass in water by boiling, and, having strained it through a coarse linen cloth, evaporating it again, o such a consistence, that being cold, the glue will be perfectly hard and dry.
This cement is improved by dissolving the isinglass in any proof spirit by heat, or by adding to it when dissolved in water, an equal quantity of spirits of wine.