Should a mould be made, the clay is not allowed to dry but is kept in what is known as leather-hard condition, firm and hard but still moist. From this the mould is cast, and if the piece is not too intricate and has no undercuts, the original may be saved. The clay, shrinking slightly as it drys, separates itself from the plaster mould. In order to cast a difficult mould, one with small passages and many pieces, the clay must be poured. Clay in a dry state, powdered and sprinkled into water and then stirred vigorously or blunged, makes what is known as slip (a liquid clay). Slip needs to be fairly thick to prevent too great shrinkage in the drying. The mould should be dry before the pouring, and the slip free from air bubbles. Pouring the slip very slowly over the mouth of one pitcher on to the mouth of another will break the bubbles. To make the mould ready for pouring, it should be tied together securely and wedges placed between the plaster and the coed to tighten the hold. Then the slip is poured in until the mould is full and shaken gently to raise any bubbles. The slip will adhere t.? the dry mould, the moisture soaking into the plaster, and as the amount soaks down, more slip is poured in to fill the mould again to the top. This process is repeated until the thickness of the wall can be seen by scraping across the mouth of the mould with a flat knife, and, if sufficient, the mould is then inverted on two or three small blocks or tiles and allowed to drain. When it begins to pull away from the mould, or is firm enough to stand by itself, it is safe to untie and separate the mould. It is stood on a flat level table, and the sculptor then begins to work over the surface, cutting away the seams which always show where the sections of the mould come together, and refinishing and retouching. The statue is then allowed to dry slowly and is ready for the fire.
Another method, which, if possible to use, is quicker, is to saturate the mould and then, after wedging the clay thoroughly, to roll it out with a rolling pin on a flat, dry, unpainted surface until the clay is an even thickness and not hard. The clay is then placed on each half of the mould and with a damp sponge gently patted into all of the irregularities of the mould and the edges are trimmed with a thin pointed knife. Then the two halves of the mould are tied together, and the sculptor works the clay back and forth across the seam inside the mould vigorously, adding small bits of clay where it is uneven. This is very important, because if the joining is not sufficient the clay will crack in the firing. The cast can be taken from the mould almost immediately, and the mould is ready to use again. With the heavier clay the shrinkage is less than with the slip. The retouching is the same in both methods.
The colour of the clay can be modified by blending a light clay with a red clay or by completing the objects in one clay and then by giving them a wash, or engobe, of slip of the desired colour.
Firing.—Kilns vary in their size, construction, and manipula tion and have been built to burn coal, wood, oil or gas. There are updraft, downdraft, and muffle kilns, and round, square, and oblong kilns, made of brick, stone, and sheet iron. The latest de
velopment is the electric kiln. In stacking a kiln, the unglazed ware will not stick together and the pieces can consequently be allowed to touch. Figures set in the kiln for firing should be placed on spurs, or small pieces of fire brick, which raise them slightly and allow the heat to circulate all around the figures. When the fire is started it is raised gradually to allow the moisture to escape slowly so as to prevent cracking; for although a piece may have been drying for a year, it still contains from 18 to 20% of moisture.
Terra-cotta can be fired in any type of kiln, different clays demanding different temperatures. To fire heads, sketches, statu ettes, animals, etc., the average clay fired to 1,800° F is sufficient. The higher the fire the denser the body, the darker the colour. For unglazed architectural decoration and for garden sculpture, the body needs to be well fired, as it will be exposed to the weather. Science has progressed in registering heat from the sight gauge to the Seger Pyrometric Cones, and finally to the automatic heat recording pyrometer.
When the kiln has cooled, the ware is unpacked and, if the sculptor is interested in colour by glaze, he weighs out his glaze formula, grinds the glaze and applies it to the statue as a liquid, painting it on and allowing it to dry. Then it is reset in the kiln, great care being taken to prevent the statues from touching, and refired, the temperature being determined by the type of the glaze.
There is a growing interest among modern sculptors to work in clay, some carrying their work to the height of exquisite finish and others dashing off spirited sketches which in this medium express spontaneity as they can in no other material.
Among the American sculptors, Herbert Adams, Paul Man ship, Victor Salvatore, Gleb Derujinski, Paul Jennewein, Bessie Potter Vonnoh, Alfio Faggi, John Gregory, Elsa Horn Voss, Janet Scudder, Beniamino Bufana, Carl Walters, and A. A. Weiman are the chief workers. They use various methods of finish. Adams and Manship treat the terra cotta individually with a wax and oil paint finish. Salvatore and Derujinski leave their work in the biscuit, while Jennewein, Gregory, Weiman, and Voss carry their sculpture to a glazed finish. The sculptors show great contrast, Jennewein and Gregory designing strong and decorative archi tectural details, pediments, friezes, etc., while Voss portrays animals with great charm. Rodin, Despiau, and Maillol are the outstanding figures in modern French terra cotta, and Wackerle in modern German.
The possibilities of glaze sculpture are just being investigated. The Della Robbias used it extensively, but their colour and type of glaze never varied and did not lend itself to very sensitive modelling. The modern sculptor who is interested in colour is working to make a glaze which has quality and yet is thin enough not to hide the details of the modelling. For architectural details, however, the thick brilliant glazes are suitable. (See TERRA COTTA, which includes bibliography; also SCULPTURE.) (M. R.)