A very large proportion of the articles made of terra-cotta are dried imperfectly, or in draughty sheds, and hence there are many twisted and unsightly pieces. A good drying-chamber or shed properly heated, and in proximity to the kilns, is a desideratum. More ordinary terra-cotta ware is injured in drying than is spoiled in the firing.
Employment of llfachinery.—Terra-cotta mouldings or moulded bricks are sometimes made in a machine acting vertically with a die at the bottom, and as the clay passes from this die, it is caught by a bosxd or palette, and cut off in given lengths, as drain-pipes are. At other times, mouldings and ashlar pieces are driven through dies, out of a brick-machine, cut off by wires and removed On palettes. The mouldings and the ashlar work, when stiff, are trimmed clean at the edges. Ornament pressed out of plaster moulds is sometimes luted or slipped on to the plain mouldings. The completeness of machine-work depends much upon careful drying. All these machine-made mouldings and ashlar work, if dried thoroughly, may be rubbed true, the same as Bath and other stones are treated, as before mentioned. Terra-cotta tiles 12 in. sq. have been made in great quantities by maclainery, and rubbed as true as marble.
Fire-proof Terra-cotta.—By the aid of machinery, fire-proof terra-cotta for casing iron columns, girders, and for general construction of walls and floors, may be economically produced. Gas and other stoves, hearths, and backs for fire-places are made of fire-clay, coloured, and glazed. Stove backs, ornamented with crests and coats-of-arms, have been made of fire-clay in which black oxide of manganese has been introduced, so as to produce, after burning and rubbing with black-lead, the appearance of cast-iron.
Some fire-clays are of a nice pale-buff colour, and free from specks of iron, from which, columns, pilasters, soffits, facias, and slabs could be made by machinery. Such work should be of greater thickness than ordinary terra-cotts, and shonld be contrived to fit together by mortise and tenon, with lap-joints lutts1 with flre-proof cement, and filled in with fire-proof materiala, so as to picvent t lie audden destruction by flre and water of iron girdera and columns, which ahnost alwaya ensues in large fires.
All the buff claya used in ordinary terra-cotta are to a certain extent fire-clay, and will stand a temperature at whieh most of the red clays run down into a vitreous mass. Columns made hollow, of buff tLrra-cotta olay combined with fire-clay, were used by Sir Digby Wyatt to support git dere, end at the same time to act aa warming-flues through a building of three floors.
Strength of Terra-cotta.—The resulta of experimeitts made to aacertain the reaiatance of terra-cotta to a thrusting stress, comparatively with Portland and Bath stonea and common stock brick, ahow that, as a building-material, it greatly exceeds all in ordinary use. A table recording these experimenta, prepated by David Kirkaldy for J. M. Blashfield, and laid before the Royal Institute of British Architects by Charles Barry, ia given on pp. 1588-9. (The spechnens were bedded between pieces of pine in. thick.) Among the public edifices where terra-cotta haa been very largely used during the past 30 years, are the following ;—S. Kensington Muaeum, Duchy of Conawall Office, Royal Mausoleum (Windsor), Victoria and Albert Museum, Sasanon Tower, Elphinstone Circle, and National Bank of India (Bombay), London & N. W. Station, Broad St. (London), Buckingham Palace, Sandringham House, Royal Albert Hall, Wedgwood Institute, India Office, Rolla and Record Office, Dulwich New College, Natural History Museum (S. Kensington), Law Chambera in Carey St , Fine Art Museum (Boston, U.S.A.), New Librarica (S. Kensington), Life Guards Barracks (Knightsbridge).
Teseelated and Tile Pavemente.—Such pavements are frequently called mosaic, en caustic, and inlaid. Floors of tesselated work made of common claya are of great antiquity. The mosaic pavementa so frequently met with in Roman remaina in England are composed of fragments of tile and atones of various hues ; the larger tessera) for the outside margins are commonly clay, and the red invariably an. These pavements, from the inequality of hardness of the materiala, have not worn uniformly in all cases.