After the original model, say a medal, has been thoroughly rubbed with soap or plumbago, wrap round the rim a piece of stout paper, or thin lead foil, and bind it in such a manner that the article to be copied, face upwards, is at the bottom of the box thus formed. Then in a vessel filled with a sufficient quantity of water, sprinkle fine plaster of Paris until the last portions reach the level of the water. After waiting for one or two minutes, stir, and the thin resulting paste must be employed immediately. With a painter's brush give a thin coat of this paste, and press into all the recesses; this is to expel the air; then pour the remainder of the paste up to a proper height, and allow it to set. After a few minutes the plaster hardens, and may be separated from the paper. Scrape off what has run between the paper and the rim of the medal, and the plaster cast will separate from the model. Plaster of Paris moulds cannot be introduced into the bath without having been previously rendered impervious.
Stearine is melted and poured upon the model when it is going to set. When stearine is too new or dry, it crystallises in cooling, and this impairs the beauty of the cast. In such case it should be | mixed with a few drops of olive oil, on with tallow, or suet; if it is made too fat, it will remain soft and difficult to separate from the mould. It should then be mixed with virgin wax or spermaceti. As stearine contracts consider-ably by cooling, its employment must be avoided when the copies are required to be perfectly accurate. When it is desired to make a cast with stearine of a plaster model, the latter should be thoroughly saturated with water or stearine beforehand, and should also be perfectly coated with plumbago before the melted substance is poured upon it, otherwise the two will stick together, and it will not be possible to separate the cast from the model. Wax may also be employed in the same manner, but its price and want of hardness interfere with its application.
This metal is a perfect conductor of electricity, and therefore well adapted to the production of homogeneous deposits of equal thickness; it is, however, seldom employed, on account of the difficulty of the operation, of its crystalline texture, and of the presence of air-bubbles.
Fusible alloys are: - (a) Pure lead, 2 parts in weight; tin, 3; bismuth, 5; fusible at 212° F. (b) Pure lead, 5 parts in weight; tin, 3; bismuth 8; fusible at 180°-190° F. (c) Pure lead, 2 parts in weight; tin, 3; bismuth, 5; mercury, 1; fusible at 158° F. (d) Pure lead, 5 parts in weight; tin, 3; bismuth, 5; mercury, 2; fusible at 125° F. For those alloys without mercury, the component metals may be melted together; when mercury is employed, it should be added when the three other melted metals have been removed from the fire. To obtain a thorough mixture the alloy should be stirred with an iron rod, or melted over and cast several times.
Run the metal into a small dish, re-move the oxide with a card, and then apply the model, give it a few taps when the setting takes place; or put the model into the dish, and pour the clean alloy upon it. Or, put the medal at the bottom of a small box of iron or copper, and bury half its thickness in plaster of Paris; then, cover the medal with the cold fusible alloy, and apply heat until melted, when it is allowed to cool off. it is easy to separate the medal from the fusible alloy, as the portion protected by the plaster of Paris may then be grasped. A well-made cast of fusible alloy is the best mould for gal-vanoplastic operations with silver and gold. Alloys containing mercury should not be used for taking casts from metallic medals, iron excepted, which would be amalgamated and injured. Copper deposits obtained upon such alloys are very brittle. Melted sulphur produces very neat and sharp casts; it is, however, very difficult to get it metallised, and it transforms the deposit of copper into sulphide.
In certain conditions, the elasticity of gelatine and gutta-percha allows of removing them from undercut or highly-wrought parts, and they reacquire the shape and position they had before the removal. This property is found in gelatine to a higher degree than in gutta-percha, but it requires a very rapid deposit, otherwise it will swell and be partly dissolved by too long an immersion in the solution of copper sulphate. Put sufficient colourless gelatine in cold water, and let it swell there for about 24 hours; then drain off the water, and heat the gelatine upon a water bath until it has become of a syrupy consistency; it is then ready to be poured upon the object, which must be encased in a box of pasteboard or of thin lead. After cooling for about 12 hours, separate the cast from the object. To enable the gelatine to remain longer in the bath without alteration, use one of the following mixtures: -
(a) Dissolve the best gelatine in hot water, and add 1/50 of the weight of gelatine in tannic acid and the same quantity of rock candy; then mix the whole thoroughly, and pour upon the model in its box. After a few hours the gelatine may be easily separated from the object.
(c) Beat, in 2 pints distilled water, the whites of 3 eggs, filter, and cover the entire surface of the gelatine mould with this liquid. After drying, operate with the solution of potash bichromate, as in (6).
(d) Pour some varnish upon the gelatine mould, drain carefully, and let it dry. The best varnish for the purpose is a solution of rubber in benzole, or in carbon bisulphide. The mould must be metallised, and, when in the bath, submitted to a galvanic current of great intensity at the beginning. When the entire surface is covered with the copper deposit, and swelling is no longer to be feared, the intensity may be reduced.