PRINCIPLES OF UNIVERSAL APPLICATION IN MAKING MOULDS AND CASTS.

The processes employed in making plaster Paris moulds and casts are very simple, and easily learned, even by one who has had no previous knowledge of the subject. To be sure, a certain degree of intelligence and skill is necessary in the operator; but we are not writing for the edification of duffers who do not know how to use their hands, or follow plain directions.

The first thing to understand is the difference between a mould that will "draw," and one that will not. A mould may be made on one side of a base-ball, and it will draw off the object at once, because there is no point on the ball behind which, or under which, the plaster can catch, and hang fast until something breaks. A mould of one full side of an apple will not draw, because the apple has a hollow at each end, and when these are set full of plaster the mould and the apple are held firmly together.

A hollow or a protuberance on an object which would prevent a mould from drawing away makes what is called an "undercut," and necessitates the making of a separate piece in the mould. To cast several copies of a human head and neck necessitates the making of a mould in several pieces, all fitting very nicely together, with countersink joints, to accommodate the undercuts behind the ears, under the chin, the hollows of the eyes, etc.

A mould made in more than two pieces is called a "piece mould." It may have any number of pieces, of course, according to the nature of the object. Perhaps the most remarkable piece mould in existence is that used by Professor H.A. Ward in making casts of the tail of the great fossil armadillo called the glyptodon. The tail is a mass of big conical tubercles, and the mould contains, as nearly as I can remember, about one hundred and twenty-five pieces, all fitting into a big "jacket" of plaster which holds them all in place while a cast is being made. In the case of a piece mould the cast is not taken out of the mould, but the latter is dissected and taken off the cast, piece by piece.

The principles involved in making moulds and casts are best explained by brief descriptions of the processes, and if they are learned by carefully following the directions here given, the operator will be able to apply them, indefinitely, to all classes of objects, large or small.

How to Make a Piece Mould.—Let us take a good-sized apple as our subject, and follow out the process, step by step.

1. In making a mould of any kind of fruit, first soak the fruit in water an hour or two, to "plump it up," so that it will not swell in the mould and cause trouble.

2. Wipe it dry, and with a small paint-brush give it a thin coat of lard oil, so that the plaster will not stick to it. Some objects should be coated with clay water, or very thin clay, instead of oil.

3. Bed one end of the apple in damp sand, precisely as shown in the illustration (Fig. 59). If you have no sand, use fine sawdust, or wet corn meal. Some objects require clay. Do not have the sand wet and water soaked, for it will not yield so readily to the palette knife.

4. Dot a line on the sand all round, to show how far out to run the plaster, and avoid making the mould too thick.

5. Take two-thirds of a teacup of water, and put in plaster Paris until it becomes about as thick as New Orleans molasses. Stir thoroughly with a spoon, and let it stand two or three minutes.

6. Dip the plaster out with a teaspoon, and cover the exposed part of the apple. It will run down all over the horizontal surface of the sand, but never mind that. Make it cover the apple everywhere to a depth of a quarter of an inch. In some places it will be thicker. After about three or four minutes the plaster will be as stiff as modeling clay. While it is in this state take your palette knife and neatly smooth and shape the outer surface. Finish smoothing with the finger, and let the plaster harden. This will take about seven minutes longer, or until the plaster has warmed and cooled again. The time varies according to the humidity of the atmosphere.

7. Take the apple from the sand, with the half mould on it, wash off the sand, and neatly trim the edge of the mould with a knife.

8. With a blunt-pointed knife make three countersunk holes in the edge of the mould.

9. With a brush, anoint the edge of the mould with thin clay, clay-water, or soft soap, or lard oil, or even lard.

10. Turn the mould up on edge, and bed half of the exposed portion of the apple in the sand, as seen in Fig. 60. Fill in this space with plaster, precisely the same as when the first piece was made, which produces the second piece of the mould.

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.

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.

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.

2. Give the exposed portion of the hand a good coat of lard oil.

3. Take two-thirds of a gill of water in a teacup, put into it half a teaspoonful of dry Indian red (to be bought for five cents at any paint store), and mix it up. This is to color some plaster with which to make a thin, colored lining for our mould, the purpose of which will be appreciated later on.

4. Mix with this red water one and one-half gills of plaster Paris, stir it up thoroughly to get it well mixed and free from air-bubbles, then with a teaspoon distribute it all over the hand until it is completely covered with a coat of the pink plaster about one-eighth of an inch thick.

5. After this thin coat has hardened, anoint the surface of it with lard oil or clay water, so that the plaster which is to be put upon it will not stick to it, but separate readily when the outer case of the mould is chiseled off.

6. Take one and one-half gills of water, and three-quarters of a pint of plaster, mix for outer case of mould, and apply on top of the pink lining to a thickness of about half an inch. Let this get perfectly hard. (Fig. 64.)

7. Take the subject out of the sand. Turn it over, anoint the edge of the mould with clay water or lard oil, and treat the other side in precisely the same manner. This is the course when the whole object is to be cast. If half the object is sufficient, as is the case in taking a record cast of one side of an animal's head and body, then take the subject from the mould, and

8. Wash the inside of the mould thoroughly to get out the clay and sand.

9. Anoint the inside of the mould with clay water, thin clay, or lard oil, and lay it in position to receive the plaster.

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.

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:

To make a cast, oil the inside of the mould with lard oil, put the plaster jacket around it, so that it fits perfectly, and tie the two pieces of the jacket firmly together to prevent a disaster when the plaster begins to heat in the mould.

Mix your plaster with ice-water for the same reason, and you will have no trouble. For irregular objects, the working of a gelatine mould is perfection itself. It yields gracefully in coming out of the undercuts and around corners, takes every detail perfectly, and in the jacket its shape is always the same. A careful operator can make from twenty to fifty copies of a cast in a single mould before its loss of sharpness necessitates its abandonment.