TERRA COTTA Material.—Clay is decomposed felspathic and silicious rock (see CLAYS), varying in composition and colour according to its location. Pure clay or Kaolin, as the Chinese named it, is a resid ual, non-plastic, white-burning clay, not usable in its natural state but essential in the composition of porcelain and china bodies. In contrast, the impure sedimentary clays, of which all earthen ware and most terra cotta sculpture are made, are plastic and in their natural state contain all the requisites for modelling (plas ticity, porosity, vitrification). If these are lacking, the clay can be modified to obtain these essentials. Buff and red-burning clays in their green (or raw) state are grey, black, tan and red, but when burned these change to a light flesh colour which ranges to a deep chocolate brown, the tones modified according to the proportions of iron carried and also by the temperature reached in the firing.
A good modelling clay must have : (I) plasticity, with the ability of holding its form both while being worked and during the firing; (2) porosity, or sufficient looseness or openness in the clay particles to enable the moisture to escape in drying without shrink ing too much or cracking; (3) vitrification, or the proper resist ance to heat, so that it does not warp and crack in the firing or shrink excessively. The open, rather sandy clays are the more desirable.
Sculptors have worked in terra cotta since the beginning of history, using it for the modelling of small figures, for the mak ing of moulds and for pressing in the moulds, as well as for archi tectural details, friezes, tiles, roofing and facades. The amount of terra cotta objects produced is naturally governed by the workable material of the country. In Egypt and Italy it was used exten sively because of the scarcity of marble and stone.
A suitable clay having been located, it must be mined and cleaned of all foreign substances (stone, twigs, etc.) by washing, blunging, sieving, and then separating from the water. In modern efficient equipment the separating is done by means of a compressor which absorbs the water and leaves the clay in com pact workable squares, which in turn are placed in a damp box and kept moist for use by the sculptor when needed. In primitive
plants one method is to make a trough of hollow tile; into this trough the liquid clay, or slip, is poured and, because of the porous quality of the tile, the water is absorbed and evaporated, leaving the clay which can be rolled up and stored away.
Now it is ready for the sculptor. One method of constructing a figure is to build it up on an armature, with either plasticine or clay. If clay is used, it must be kept wet either by spraying water on the figure from time to time as the work advances, or by keeping damp cloths around most of the surface, and by covering it entirely and keeping it in a damp box when not being worked. If the original is not to be cast, but to be fired directly, no armature can remain in the statue because of the twisting and burning of the wire in the fire. Usually small statu ettes are made solid ; but for larger figures and groups it is best to build them hollow, keeping the thickness of the clay walls as even as possible. In this method the clay requires definite prep aration. It must be thoroughly wedged, which means that the clay is cut in two on a taut wire and vigorously thrown down, one, piece on top of the other, to expel the air. This is done repeatedly until the clay is even in texture and uniform in moisture. If a clay is too hard, water can be poured on the clay and then the clay cut and beaten again as before. It is wise to start with approximately as much clay as the size of the figure to be constructed, working as long as possible until it begins to sag on account of the moisture. While working up to this point care should be taken when adding new clay to work it thoroughly into the old clay, making sure that no air has been allowed to enter and that the added clay is securely blended to the old. When the sculptor has worked as long as the clay will permit, the statue is allowed to dry and stiffen just enough to hold its shape, and then the sculptor will proceed as before. When the figure is ultimately accomplished, and the fine details are being developed, it is helpful to spray the figure occasionally with a very fine spray of water.