In another method of making moulds of leaves and other vegetable objects, the leaf is care fully dried, and laid upon a smooth piece of milled lead, which is placed between two steel plates, and passed between rollers ; these press the leaf into the lead, and produce a complete mould. Copies from this may be taken with guttapercha or electrotype. Roseleur describes the copying of nettle and other leaves so perfect that all the hairs on their surface were to be seen. "One of the sides of a fresh leaf was covered by means of a brush with a thin paste of plaster of Paris, and after the drying of the first coat, other layers were applied, until a resisting block had been obtained with the leaf uppermost. The free side was then covered with several coats, always with a brush or pencil, of guttapercha dissolved in bisulphide of carbon, and lastly with melted guttapercha. The mould was removed from the leaf, metallized, and immersed in the galvano-plastic bath." To cast reptiles, imbed the subject in a mould made of four parts of plaster of Paris, one of unburnt lime powder, and one of Flanders brick-dust. Dry the mould carefully in an oven, then make it red hot, and burn the subject out of it, taking care to free the mould from the ashes. Before putting this mould into the oven to dry, insert two or more iron or smooth wooden pins, one touching the object inside, the other projecting outside. When the mould is dried, and before burning, these are carefully removed, to allow escape of gas and to remove the ashes ; by a proper arrangement of these holes the carbon of the article may be consumed, when the remaining ash is easily removed. Fusible metal may be cast in this mould, or a wax model may be taken of the object, pouring the wax in just before setting. The whole is now placed in water, the lime causes the mould to dissolve or break up, and the figure modelled within it may be taken and covered with copper, and the wax afterwards melted out. Flowers, insects, lizards, and other little animals may be typed in this way. In all these processes perseverauce and care are the best cure for little difficulties.
When a plaster mould is to be taken, the face of the model is prepared differently to that described, in order to prevent the adhesion of the two plasters. The best substance for this purpose is a mixture of half a pound of soft-soap put into three pints of clean water, which is set on a clear fire, and the solution kept in agitation by stirring ; when the mixture begins to boil, add from one ounce to an ounce and a half of tallow, and keep boiling till it is reduced in bulk to about two pints, when it is ready for use. The surface of the model must be washed over with this lacquer, allowing it to absorb as much as it can, when it assumes the appearance of polished marble ; it is now prepared with a rim of paper, and the mould taken as directed for taking plaster moulds from metallic articles. When hardened, they will separate easily. Wetting the plaster model with a solution of soap before taking the cast will do, or, if the plaster model has been saturated with oil or milk, it has only to be moistened with sweet oil, the same as a metal model.
If a mould of fusible metal be required from a plaster model, the plaster may he saturated either with boiled oil or the soap and tallow lacquer, and the mould taken in the same manner as from a metallic medal.
Many electro-metallurgists prefer taking a mould in copper when the medal is of plaster of Paris. This is done by the electrotype process, the plaster model is saturated with wax over a
slow fire, as already detailed, and then prepared for taking an electrotype in the usual manner. We need hardly mention that in this case the model is destroyed ; but, notwithstanding, in the case of plaster models, to take a copper mould is the most preferable way, as it may be repaired in case of slight defect, and it may be used over and over again without deterioration.
When an electrotype is required of a model that is undercut, or of a bust or figure, the process which we have described will not answer, as the mould cannot separate from the model. In such circumstances, the general method of proceeding is to cast the mould in separate pieces, and then join these together. The material used for this purpose is plaster of Paris ; the operation, however, to be done well, requires considerable experience. If the undercutting is not very great, a gutta percha mould can be taken by the process described ; but before removing the mould subject it to a heat of boiling water to soften the percha, which by careful manipulation may be removed without damage to either mould or model.
Parkes' process for taking a mould of any kind of model in one piece is excellently adapted for the electrotypist. The material is composed of glue and treacle. 12 lb. of glue is steeped for several horns in as much water as will moisten it thoroughly. This is put into a metallic vessel, which is placed in boiling. water, as a hot bath. When the glue falls into a fluid state, 3 lb. of treacle are added, and the whole is well mixed by stirring. Suppose now that the mould of a small bust is wanted, a cylindrical vessel is chosen, so deep that the bust may stand in it an inch or so under the edge. The inside of this vessel is oiled, a piece of stout paper is pasted on the bottom of the bust, to prevent the fluid mixture from going inside ; and if it is composed of plaster, sand is put inside to prevent it from swimming. It is next completely drenched in oil, and placed upright iu the vessel ; this done, the melted mixture of glue and treacle is poured in till the bust is covered to the depth of an inch. The whole must stand for at least twenty-four hours, till it is perfectly cool throughout ; after which it is taken out by inverting the vessel upon a table, when, of course, the bottom of the bust is presented bare. The mould is now cut by means of a sharp knife, from the bottom up the back of the bust to the front of the head. It is next hold open by the operator, when an assistant lifts out the bust, and the mould is allowed to recluse; a piece of brown paper is tied round it to keep it firm. The operator has now a complete mould of the bust in one piece ; but he cannot troat it like wax moulds, as its substance is soluble in water, and would be destroyed if put into the solution. A mixture of wax and rosin, with occasionally a little suet, is melted, and allowed to stand till it is on the point of setting, when it is poured carefully into the mould, and left to cool. The mould is then untied and opened up as before; the wax bust is taken out, and the mould may be tied up for other casts. Besides wax and rosin, there are several other mixtures used, deer's fat is preferable to common suet and stearine. The object is to get a mix ture that takes a good cast, and becomes solid at a heat less than that which would melt the mould.