10. Mix a proper quantity of plaster, pour it into the mould and let it harden.
Lay the mould (with the cast inside) on a cushion, or on your lap; take a half-inch chisel and a light mallet, and, beginning at the end nearest your left hand, chisel away the case of the mould, bit by bit, until you come down to the pink lining coat, which shows that you are close to the cast. Great care is necessary to avoid breaking the cast, which of course is very easily cut or broken. In cutting off the pink lining be exceedingly careful not to go too deep (Fig. 65). The purpose of this lining is to show you when you are close to the cast. If the case of the mould is quite thick, hold the chisel on a slant of about fifty degrees from a perpendicular, and pare off the upper surface gradually.
[a]Fig. 65.]—Chiseling off the Waste Mould.
Making Gelatine Moulds.—There are many objects which can not be copied in plaster by either of the above methods without great difficulty. These are hard substances, the surfaces of which are extremely irregular and full of little hollows, such as meteorites, statuettes, sculptured rocks, or models such as those of the Aztec calendar stone and sacrificial stone. When a number of copies are required, the making of a waste mould for each copy is out of the question, and the manufacture of a piece mould that will draw off is also a long task, to say nothing of making the casts themselves and cleaning them up. The solution of all such difficulties is the gelatine or "glue" mould, which is elastic, pliant, and yet keeps its shape perfectly. This is how to make it:
Let us suppose we are to make a gelatine mould of a flattened meteorite, eight inches in diameter and about three inches thick. Take some potter's clay, or modeling clay, which has been nicely worked up in a square lump, and is not wet enough to be too sticky. With a small wire, cut it into slabs about three-quarters of an inch thick, and with these cover the entire object to the depth mentioned. Put the clay on everywhere the same thickness, making it conform to the irregularities of the surface. This clay will presently be exactly replaced with gelatine.
Having coated the object as described, make a plaster Paris mould of the whole of it, in two parts, which separate horizontally around the outermost edge. When you make a plaster mould of the upper half, erect a high cone of clay over the centre of the meteorite as it lies flat upon the table, so that it will make a funnel-shaped hole in the upper half of the mould, through which you can pour in the gelatine. Of course the two pieces of this plaster mould must fit nicely together, with countersunk holes. This plaster mould of the clay-covered object is called the "jacket," and its use will soon be apparent. Now for the gelatine.
Recipe for Gelatine Moulds.—The gelatine compound is made by taking glue and glycerine in the following proportions, varying the quantity to suit the size of the object: Of best Irish glue, 3 pounds; glycerine, 1-1/4 pounds; and about 1-1/2 ounces of white-wax. Dip the glue in water, and then roll it up over night in several thicknesses of wet cloth, so as to soften it without soaking it in water, which is an element to be kept out. In the morning the glue will be soft. Procure a large-sized gluepot, or improvise one by putting a small tin pail in a larger one, with water between, and in this put the glue and glycerine and cook it up. Melt the wax separately, and pour it in after the other is well mixed and hot. A gelatine mould can be made of any degree of hardness by adding dry white zinc which has been carefully ground in a mortar, but ordinarily none is necessary.
To make the gelatine mould the clay must all be taken off the object, and the latter washed clean. Lay the lower half of the "jacket" upon the table, inside uppermost, and drive four small wire nails into it at different points, allowing each one to project just three-fourths of an inch, for the meteorite to rest upon, and give space for the gelatine to flow underneath and form that part of the mould. Now put the meteorite carefully in place, resting on these nail-heads, and then put on the upper half of the "jacket." Cord the jacket tightly together without disturbing the position of the object inside. If there are any cracks at the edges, fill them up with clay. Now pour in the hot gelatine at the funnel-shaped hole in the upper half of the jacket, until the mould is quite full. Let the mould stand two hours to cool and harden; then remove the upper half of the "jacket." To get the object out, take a sharp knife and slit the coating of gelatine fully half-way round, so that the two halves can be opened like an oyster, and the object lifted out. The inside of the gelatine mould must now have a coating to make it impervious to the water in the plaster Paris.
Mix up the following: