11. Make the countersunk holes in the edge of the second piece as before; fill in with plaster all the remaining space between pieces 1 and 2, and we have the third piece, which completes the mould. As soon as this hardens, tap the mould lightly all over with a small mallet, to loosen it from the object, then take out piece No. 3, and the mould is easily separated and the apple taken out. Take one part of gum sandarac (dissolved in ninety-five per cent alcohol) and three parts of white shellac, also dissolved, and mix them. They will form a solution a little thicker than water. Have the mould perfectly dry, and apply this solution to the entire inner surface of it with a small paint-brush. The solution will be absorbed at first, and you must continue to apply it until the inner surface has an egg-shell gloss, which is sufficient. Then oil the inside with lard oil.
[a]Fig. 61.]—Last Step in Making a Piece Mould.
12. Put together pieces 1 and 2 of the mould, cut a groove around them to hold a stout string, and tie it tightly to hold them together. Then mix some plaster, and fill each piece (1 and 2) about half full, walling it up the sides with the finger, a quarter of an inch thick, or as the plaster will make it. Then wipe off the exposed edges of the mould so they will fit snugly together with No. 3. Now mix up a little more plaster, about one-fourth the quantity first used, pour into the hollow, then put on the third piece; tie all tightly together, and turn the mould round and round slowly. This fills the third piece, and holds it in its place. Keep turning the mould slowly, and tapping it with the left hand. Leave a little of the plaster on the outside, on a piece of glass or paper, so that you can tell when it gets hard and flinty in the mould. Do not take off the mould until the cast is perfectly hard.
[a]Fig. 62.]—The Finished Mould.
How to Make a Waste Mould.—When a soft or fleshy object is to be cast, one which will yield, and draw out of the mould regardless of undercuts, a very quick and satisfactory process (provided a second copy of the cast will never be wanted) is to make what is called a waste mould. This, with a fleshy subject, is a short cut to a perfect cast, and often saves hours of valuable time. In obtaining casts of mammal heads, legs, or other parts, or casts of fishes and reptiles, it is the method par excellence. Let us learn the principles of it by making a waste mould and cast of a human hand.
1. Bed the hand in damp sand (i.e., one-half of it), as shown in the accompanying figure, No. 63.
[a]Fig. 63.]—The Beginning of a Waste Mould.