(5) Mending Models
(7) With Small Models
For making small models in plaster, gelatine is generally used. Good glue, mixed with treacle or glycerine answers every purpose. The composition that the " chro-mograph " is made of will answer very well. The model is immersed in it, and, when cool, a cut is made with a sharp knife, and the elastic nature of the composition allows the model to be taken out. The mould should be greased before the plaster is poured in; when set, it is extracted in the same manner as the original model. Large figures are poured in plaster moulds; these are made in pieces, which are fitted together with wooden pegs. The peg is inserted in one piece before the plaster sets. This piece is trimmed off, in order to prevent the wet plaster adhering to the next piece; the latter should be greased with lard; the whole of the mould is thus built up of pieces. In pouring the model, pieces of wood or wire should be placed in the legs or arms to strengthen them. To cast brass in plaster, the mould should be previously made hot, which might be fatal to the stability of the plaster.
(8) To make casts or moulds of plaster of Paris from metal types, without air-bubbles or "picks," use the finest and purest plaster of Paris obtainable. When filling a mould, learn to beat up the requisite quantity of cream quickly, and with care to avoid making it too thick. In pouring this in, use a good camels' hair brush to displace air-bubbles; a mere surface cover of this thin cream is all that is requisite. While doing this, have ready the thicker plaster, of the consistence of light syrup, and fill up the mould at once. In about 20 minutes you can open the mould, if your plaster is pure and has been properly mixed. If you do not put too much oil on the type, and have used your brush properly, -you will find clear, sharp moulds.
(9) Metal may be cast in moulds made of plaster of Paris and talc mixed; or of pounded pumice and plaster of Paris in equal quantities, mixed with washed clay in the same proportion. The mould 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. You may safely use plaster for zinc castings, taking the precaution of thoroughly drying the parts of the mould, e.g. in the kitchen-range oven ; care, however, must be taken not to use too much heat, or the plaster will be burned - just as much as is unpleasantly hot to the hand. The zinc should not be hotter than will give it sufficient fluidity for pouring. In this way 4 or 5 good castings may be taken, after which the mould gets cracked and scales on the surface; this spoils it for fine work, but is of little consequence for battery zincs.
(10) In many cases it is advisable to preserve copies of small carved objects for future use, and this is easily done by taking a plaster cast of the work. To take an impression of the object of which a cast is desired, a substance known as squeeze-wax is used, and this is made of the following ingredients, viz.: 2 lb. flour, 1/2 lb. best beeswax, 1/2 lb. linseed oil, and a small quantity of rouge; these should be thoroughly mixed together, and then exposed to the air. Should the squeeze-wax become hard at any time, it may easily be softened by slightly warming and well kneading. In taking a cast, the wax should be well presse'd into every portion of the work, and then gently withdrawn, the mould thus formed being filled with plaster of Paris, the plaster having been mixed with water until it is of the consistence of cream. After standing for a few hours the squeeze-wax can be taken off, leaving a copy of the carving in plaster. Care should be taken to obtain the plaster fresh, as after being exposed to the air it loses some of its properties, and does not harden well.
These remarks on taking plaster-casts apply only to small objects that are not deeply undercut; larger casts, and casts of subjects carved on more than one side, are taken in sections.
(11) Anatomical Specimens
Prepare the specimen by making it as clean as possible; place on oiled paper, in a position that will show it to advantage. Soft projections may be held in position with threads suspended from a frame or from a heavy cord stretched across the room. Paraffin melted on a water bath is painted over the preparetion with a soft brash, the first layer being put on with single and quick strokes, that the rapid cooling of the paraffin may not cause the brush to adhere to the preparation, thus drawing the soft tissues out of place, until the mould is formed about 1/8 in. thick; all undercuts must be well filled. When the mould is hard it can be readily separated from the preparation; it is then well washed with cold water. Stir fine dental plaster into cold water to consistency of cream, pour into the mould and out again several times, so that there will be no air bubbles on the surface, then fill the mould and let it stand until hard. Place the whole in a vessel containing boiling water until the paraffin is all melted; wash with clean boiling water. When the cast is thoroughly dry, it may be painted with oil colours by coating it first with shellac varnish.
Casts of any part of the body may be made from a living subject, if the parts are not too sensitive to bear the heat of the paraffin, which varies from 104° to 140° F.
(12) Natural Objects
Taking plaster casts of natural objects is thus explained by Prof. Boyd Dawkins: - The material of the mould is artists' modelling wax, which is a composition akin to that used by dentists; and as it becomes soft and plastic by the application of heat, though in a cold state it is perfectly rigid, it may be applied to the most delicate object without injury. As it takes the most minute markings and striations of the original to which it is applied, the microscopic structure of the surface of the original is faithfully reproduced in the cast. The method is briefly this: - Cover the object to be cast with a thin powder of steatite or French chalk, which prevents the adhesion of the wax. After the wax has become soft, either from immersion in warm water or from exposure to the direct heat of the fire, apply it to the original, being careful to press it into the little cavities. Then carefully cut off the edges of the wax all round, if the undercutting of the object necessitates the mould being in 2 or more pieces, and let the wax cool with the object in it, until it is sufficiently hard to bear repetition of the operation on the uncovered portion of the object. The steatite prevents the one piece of the mould sticking to the other.