If a plaster of Paris mould is to be taken from metal, the preparation is the same as described above ; and when so prepared with the rim of cardboard or tin, get a basin with as much water in it as will be sufficient to make a proper sized mould, then take the finest plaster of Paris and sprinkle it into the water, stirring it till the mixture becomes of the consistence of thick cream ; then pour a small portion upon the face of the article, and with a brush similar to that used for oiling it, gently brush the plaster into every part of the surface, which will prevent the formation of air bubbles ; then pour on the remainder of the plaster till it rises to the edge of the rim : if the plaster is good, it will be ready for taking off in an hour. The mould is then to be placed before a fire, or in an oven, until quite dry, after which it is to be placed, back downwards, in a shallow vessel containing melted wax, or paraffin, not of sufficient depth to flow over the face of the allowing the whole to remain over a slow fire, or upon a hot plate, until the wax or paraffin has penetrated tho plaster, and appears upon the face. Having removed it to a cool place to harden, it will soon be ready for electrotyping. If the monld is large and the plaster thick, the wax may be put upon the surfaoc, and only as much as will penetrate a small way into the plaster. In both these instances the wax used is generally lost, and there is always a liability of the copper solution passing through, and causing what is termed surface deposit, making the face rough.
Moulds in fusible alloy are made from mixtures of two or more metals which melt at very low temperatures ; they suit the purpose of taking moulds of small objects very well. The following are examples of such compositions :— These all melt at a temperature below that of boiling water ; the ingredients are melted together in an iron ladle, poured out upon a flat stone, broken up, and remelted together in the same way two or three times, in order that they may be thoroughly mixed. The object from which the mould is to be taken is prepared in the same manner as wax.
The means of taking moulds with fusible metal are the following :—The fusible alloy is melted and poured into a saucer, or, what does better, a small wooden tray ; the operator now watches it till it cools down into a semifluid state, or to the point of setting, when he brings the article suddenly upon it, face downwards, and holds it there until the alloy has fairly set ; the object being moulded may be kept at a temperature that will keep the alloy melted, either by placing them into a water bath or oven ; after being kept in the melted state over the model for a few minutes they are removed and allowed to cool, when the mould and model are easily separated. Some of the finest moulds are taken by this process ; but from the constant loss of the materials by oxidation the use of such alloys is limited.
Guttapercha, as a material for moulding, serves the purpose most admirably. Moulds of this substance are equal, if not superior, to any taken in wax, and of a depth of cutting which it would have been very diffioult to have taken in wax. The method adopted for taking moulds is to beat the guttaperolia in boiling water, or in a chamber heated to the temperature of boiling water, which makes it soft and pliable. A quantity of guttapercha is pressed into the saucer, and as much added as will cause it to stand above the edge of the rim surrounding tho object ; it is now placed in a common copying press, and kept under pressure until it is quite cold and hard. Tho impressions taken in this way are generally very fine ; when the object is not deep cut, n less pressure may suffice, but when the pressure is too little the impression will be blunt.
Stereotypes and engraved plates of large size and fine patterns are copied in this way by tho oleotrotype process. Guttapercha may be softened by mixing with it a small quantity of oil, tallow, or wax. Guttapercha takes a coating of black-lead readily, and the deposit goes over it easily.
A mixture, of guttaperoha and marine glue has been recommended for moulds as superior to guttaperoha alone.
With one method of taking impressions of fern leaves and seaweeds, a piece of gutta-percha, free from blemish, and the size of the plate required is placed in boiling water. When thoroughly softened, it is dusted over with the finest bronze powder to dry the surface, to render the surface more smooth, and to prevent adhesion. The plant is then laid out upon the bronze surface, and covered with a polished metal plate, either of copper or of German silver. The whole is then to be subjeoted to an amount of pressure suffioient to imbed the upper plate in the guttapercha. When cold, the motel plate may be removed and the fern gently withdrawn from its bed. A beautiful impression of the fern will remain.
This process is well adapted for flat leaves, bnt the pressure required renders it unsuitable for many kinds of leaves, indeed it destroys the natural forms of the greater number both of leaves and seaweeds. The products of the process cannot, indeed, be compared with those electrotypes the moulds of which are taken by wax. The great merit of the process is its ease and simplicity. The method given for taking the mould of the leaf is suitable for any kind of flat mould in gutta percha. The mould of a leaf may be taken in plaster, by placing the leaf upon dry sand, and pressing the sand under and on each side to fill up the spaces under the leaf, so as to bear the pressure of the plaster, putting a collar of paper round the sand to prevent its yielding, and then pouring plaster over the whole.