Slip casting is an interesting process. Most ceramic figures and dishes purchased in stores are made in this manner. Duplicates may be made, although the original shapes are made by hand.
PLATE XLV. A cake jar with a wooden lid.
First, make a model. For a one-piece mold for a cup or a tumbler, build up the form in clay. The form is built upside down and solid. Use the top of a potters hand wheel for the base. Smooth and shape the model as one makes a piece of pottery. Handles are usually applied after the cups have been cast.
Second, sponge some water glass—sodium silicate— thinned with water on the spinner top so that the plaster will not adhere to it. The clay need not be covered with water glass. Make a wall around the spinner top with cardboard, linoleum, or galvanized tin. The wall should be 2 inches higher than the model being cast. Fill the cracks with clay. Apply water glass to the wall.
PLATE XLVI. A large teapot.
Third, mix a crockful of plaster-of-Paris molding plaster. Pour the plaster over the model, tapping the side of the wheel gently to cause the air bubbles to rise. If the mold is not filled, mix more plaster and complete the mold. Be sure that it is sitting level.
When the plaster has turned hot and later cool, the wall may be removed and the water clay model pulled out in little pieces. Dry the mold in the wind and sun. Do not use an oven for drying plaster of Paris.
Fill the dry mold with slip made by mixing clay or pulverized dry clay in water. If there are lumps in the slip press it through a fine sieve. It should be the consistency of whipping cream. The plaster will absorb water from the slip and form a deposit of clay around the inside wall of the mold. Pour the mold full. Plate XLVII. The plaster will absorb the water and cause the level of the slip to lower. Tap the side of the mold gently to make the air bubbles rise. Fill the mold three or four times as the slip sinks down. To see how much clay has been deposited on the walls of the mold scrape the top edge. When lA inch has adhered to the inside of the mold pour the thin slip from the center into the jar containing slip and scrape the top edge of the mold clean.
The plaster will continue to draw water from the deposit of clay on the inside of the mold. Some of the moisture will be evaporated by the air. As the slip casting dries it will shrink away from the mold. If it tends to stick to one edge, release that edge by slipping the thumbnail under the clay.
When the casting has shrunk away from the plaster, tip the mold over and let the casting slide out on the hand. It should be dried upside down. Cover the rim with a dry cloth to prevent the edge from warping. Put the casting in a cupboard or under a waterproof cloth to dry. When it is leather hard, turn it right side up and allow it to dry completely. The edges should be sanded smooth and rounded with very fine sandpaper or steel wool. Smoothing, however, should not take the place of good work.
Another piece may be cast after the mold has dried. The drying may be hastened by placing the mold in the breeze of an electric fan. The mold should be turned from time to time so that all sides will be drying.
More complicated pieces may be cast, but pieces requiring more than a one-section mold should be designed so that the parts of the mold will fit together and can be removed without marring the casting. This requires skill and much experience. Ceramic sculpture is oftened modeled first in wet clay or plasticine and a mold with two or more pieces is made. Then grog clay is pressed in the mold to a thickness of from 1 to 3 inches. For smaller pieces, the sculpture is slip cast. Surface details may be added later. It is a safer way of making large ceramic sculpture. The solid pieces, even though hollowed out by hand, may crack in the firing.