It is best to purchase the finest quality of potters' plaster by the barrel.[E] It will keep indefinitely if stored in a dry place. The necessary appliances are:
- One or two large jugs for mixing, or a metal can with a spout.
- A metal spider or frying pan.
- Six feet of rubber machine belting, six inches wide, or similar strips cut from linoleum or enameled cloth.
- Two or three thin pieces of steel of various degrees of flexibility (scrapers).
- Handy knives, called vegetable knives.
- A small painter's brush.
- Two or three fine sponges.
To begin with, a size of soft soap and water is prepared. Put a quart of water into a kettle and add a piece of soap the size of an egg.[F] Simmer for an hour or until the soap is entirely dissolved and then set aside to cool. When cold the size should be of the consistency of maple syrup. This size is used whenever plaster is to be kept from sticking to a form or surface, and it has also the merit of causing clay to stick to plaster. For example, if a mold is to be taken from a clay model no size should be used, but if a plaster form is used as a foundation for clay ornament it should be well sized first. The size is laid on with a brush and wiped off with a sponge. Another sponge is then used with clean water and the sized surface is washed, all superfluous water being removed. Size is then applied a second time and washed off as before. A third application is sometimes necessary, or until the sized surface rejects water like grease does. On the last sizing, water is not applied, but the surface is polished with the sponge containing size. If the surface to be prepared be of wood or metal a single coat of size will often suffice, but if it be of plaster three or four applications are often necessary.
The first lesson may well be the manufacture of a plaster bat. The frying pan is first sized and set upon a level table. Let us suppose that a quart of water will fill it to about an inch in depth. This amount of water is put into a jug and two pounds and three-quarters of dry plaster is weighed out and allowed to trickle through the fingers into the water. This proportion has been found to be best for ordinary mixings. A smaller quantity of plaster to the quart of water will result in a very soft bat; a larger quantity will be proportionately harder. After the plaster has soaked up all the water it will take, that is in about two minutes' time, the hand is plunged in and the whole stirred to a smooth cream. All lumps must be broken up and the air bubbles removed as far as possible. Continue stirring gently and presently the mixture will be felt to grow thicker. The psychological moment arrives when the plaster forms upon the hand a white coating which cannot be shaken off. The creamy liquid is then poured into the frying pan which is gently shaken to level the surface.
If the plaster has been poured at the right moment it will set smoothly with a mat surface like sugar icing. If poured too late it will be stiff and difficult to level, and if poured too soon it will curdle on the surface and water will be seen above the plaster. A little practice will show the right moment. The jug should be washed out immediately while the plaster is soft. In the place used for plaster work a tub should be provided in which all vessels and tools can be washed, for, if allowed to flow down the waste pipe of a sink, the plaster will speedily choke the outflow.
After standing for some ten minutes, more or less, the bat in the frying pan will grow warm. This is the sign of a combination between the plaster and the water and shows the completion of the setting. The pan is now taken by the handle and, holding it upside down, the edge is rapped smartly on a brick or stone. This will cause the contents to fall out and there is a smooth disc which is one of the most useful of appliances. The edge will need to be scraped and the bat can be set aside until needed. It will be good practice to make a half dozen of these.
This process of mixing and pouring plaster is the same for all operations and the instructions will not be repeated, but when the student is told to "pour plaster" it will be presumed that this experiment has already been made.
Fig. 1. A, table. B, clay mound. C, plaster. D, rubber belt.