Plaster casting will help to satisfy the mud-pie urges, the nature curiosities, the artistic interests and the inventive genius of the young of all ages. This simple and inexpensive project can be used indoors and out and at all seasons of the year. It will serve as an excellent introduction to nature activities in the camp or playground program and its possibilities are limited only by the ingenuity, imagination and dexterity of the "doer. "
The following materials will be needed:
Plaster of Paris Lard or grease Can for mixing Water
Object to cast
Celluloid or cardboard Modeling clay Paint
Paper clips Stick
There are a few tricks to mixing plaster of Paris, but they are easily mastered. First be sure that everything is in readiness-that is, that the mould is ready to receive the plaster-before you mix the plaster and water. Plaster of Paris begins to set just as soon as it is mixed and waits for no one! It is advisable to use a tin can to mix it in, rather than a dish or pan, for the plaster hardens in the bottom while you are attending to the mould, and is virtually impossible to remove without ruining the container. Stir with a stick, not a spoon, for the same reason (Figure 34).
Put into the can the amount of water required to fill the mould. You will have to learn by experience to estimate this if you cannot measure the quantity by filling the mould. Sift the plaster in slowly, letting it sink to the bottom without stirring it. Continue until no more will sink below the surface-then add a dash for good measure. You now have the right amount and may begin to stir. When the mixture is the consistency of thick cream and free from lumps, it is ready to pour into the mould.
If you start stirring before the right amount of plaster has been poured in, additional plaster is hard to mix in.
Adding water to plaster that has started to set does not keep it from setting, but the finished cast is not as strong. If the mixture becomes too hard to pour before you can use it, throw it away rather than trying to dilute it. Adding salt will hasten the setting; adding a little vinegar will tend to retard setting.
Here are a few general hints that will be useful to the inexperienced "plaster caster":
Hooks or loops for hanging plaster casts may be made from hairpins, screw eyes, wire and string (Figure
35) . Sink the loop into the back of the cast before the plaster hardens.
Coloring may be done in several ways.
(a) Pour a little showcard color into the mixed plaster.
Stir. Add more if necessary. By giving only one or two stirring strokes, a marbled effect is achieved when the plaster is poured out and sets.
(b) Paint the still moist white or tinted cast (or moisten a dry cast) with poster paints, painting the track, leaf or twig in natural colors. Flowers are especially attractive when painted (Figure
36) . Thin the paint so that when it dries it will not peel off.
(c) Thinned oil paints are also excellent. Apply these to a "bone dry" cast, however.
(d) White shellac will preserve the casts from fingerprints and smudging of watercolor paints applied to the surface. Painted casts can be washed when necessary if they have been shellacked.
Warning! Plaster of Paris will clog drains. Do not pour extra plaster into the sink. Let it harden in the can or pour it out on a paper and discard it with other rubbish.
Labeling will add interest to the project. You can paint the names of leaves, twigs, flowers, etc. on the cast, or when the cast is made in paraffin or clay, scratch the name in the clay before the plaster is poured on it. Be sure to print it backwards so that it will be right on the finished cast (Figure 37). When the cast is just set and still moist, you can scratch the name in the plaster. This is also a good time to trim off any rough edges. Use a pocket-knife for this scratching and trimming.
In place of celluloid, tin or
 cardboard "rings" may be made. Strips made of tin or cardboard can be bent into hexagonal, triangular, square and many other shapes (Figure 38), which will give variety to the casts.
Air bubbles are sometimes formed in the plaster in the process of stirring and pouring. Jarring the cast or the table on which the cast rests will help to break many of these.
Rubber bowls such as dentists use are ideal for mixing plaster, since they can be bent to crack off the dry plaster and can be used over and over again (Figure 39). They are practical for large groups or when casts are made in large numbers or frequently. Otherwise tin cans will be entirely adequate.