The material chiefly used in large terra-cotta to check warping, unequal shrinkage, and cracking, is ground burnt clay, old ware, or potsherds. This material is sometimes called by the workmen " grog," at other times "grit." It is ground and passed through sieves of various meshes to suit the character of the work in point of magni tude and delicacy of oina mentation. Clay is some times mixed, kneaded, and burned especially for this use. The proportions to be used vary according to the size of the pieces to be made, the quantity of other sub stances (if any) to be com bined, and the fatness of the clay. After the clay is mixed and prepared by the mill, it should for all artistic works be well beaten with an iron bar, and what is termed " wedged," which consists in cutting a lump of clay with a wire, and slap ping it together repeatedly to remove the air.
The other materials used besides clay and potsherds yary according to the clay, and the class of terra-cotta it is desired to make. Among these, sand is an important substance. Ground glass, china stone, and flint are often used. Generally clay itself forms, in light-coloured ware, about 4 of the composi tion, and for good red clays, sand or flint is commonly all that is require,d to be added, unless the pieces are large.
Kilns. — Before entering upon the subject of modelling, or moulding, it will probably be beet to speak of the kilns for burning the ware (Figs. 1158, 1159). These are usually round in plan, with an internal dome and a conical chimney. They are lined with fire-bricks, and banded with iron, which is arranged to fold, and fasten over the door of the kiln, when the firing goes on. The number of fur nace-holes around the kiln is determined by its size. Flues pass from the lower parts of the furnaces, under the floor of the kiln, and are commonly in connection with a fire-clay pipe-flue in the centre, as ehown in Fig. 1158, which represents the section of a kiln with a continuous muffle, or inner lining, for the prevention of discoloration of the articles burned by vapours from the coals, also for the better protection of coloured and glazed articles. For all ordinary terra-cotta, this inner casing of brick or muffle throughout ie not necessary. Small delicate models, whether plain or painted, enamelled or gilt, should be placed in Baggers, if burnt in a kiln without a muffle. Articles beautifully wrought are made of stone-ware clay, painted, fired, and glazed with the vapours from salt, as in the manufacture of stone-ware. It has been found that burning by wood produces
different results from firing salt-glaze with coal. Furnace-holes are constructed to cheek or prevent the formation of smoke. When a large volume of smoke is formed in the furnaces, it will pass through the kiln in full fire and red hot, and cotue out black at the top.
Modelling.—Before commencing any architectural work, full-size drawings should be made of all the details, and, to aave errors, this had better be done to the terra-cotta scale of the wet clsy, allowing for shrinkage in drying and burning ; this with buff clays, is about 1 in. to 1 ft., but the extent of contraction after careful firing ahould be determined exactly, at the outset, before any work is set out, or model made. From the drawinge, sections of the mouldings may be traced on wood or metal, for formiug templates or running moulds, aimilar to those toed in working plaster cornices. Such templates, worked against the edge of a rule fixed to a beard or bench, can be used to form mouldings in clay, and, worked from a centre, for deacribing arches and circular forms. The clay used may be tbe terra-cotta olay itself, and thia mode of working is the cheapest when a small quantity is wanted of a given moulding, as the whole may be described at one time. NVhen the clay has become stiff, it can be cut into various lengths, and fitted with angles if necessary, and so left to dry for burning. If the mouldings are large, they should be hollowed out. A thicknesa of 2 in. is usually sufficient for the largest mouldings. Considerable attention should be paid to the drying of the mouldings, or they will warp, or get out of shape. By turning them about, face, and sides downwards, and then reversing their position, until they are dry, they mey be kept true. Tho clay used in running mouldinga in this way should not be coarser than would pass through a sieve of 20 meshes to the lineal inch, or the arrises will not bo brought out sharply. Mouldings may be dried before burning, uutil they are hard enough to be rubbed true with a piece of burnt terra-cotta, as masons rub the surface of Bath and other donee. They seldom twist in the fire after perfect drying and rubbing, and the mitres and junctions can be delicately finished by the chisel, so as to flt with accuracy. This operation of perfect drying and rubbing applies to all plain terra cotta surfaces, whether prepared by the process of running moulds, or by machines similar to brick-machinee, or from plaster moulds.