Terra Cotta is usually made from pure clay, which will burn to a white or yellow colour, or from impure, which will burn to a red colour, owing to the presence of oxide of iron. Pure clay is a hydrous silicate of alumina, containing 47 parts per cent. of silica, 40 of alumina, and 13 of water. Clay, in this proportion, is the Kaoline or china clay. Fire clay, which is found in the coal measures, has a larger proportion of silica than Kaoline, and from it much of the terra cotta is made. When first dug out, it is hard and compact, and of a greenish grey colour, deepening to black. It is often weathered before using. This causes it to “fall” and facilitates grinding. Old fire clay, previously burnt (“grog” as it is called) is added to the new clay to counteract the excessive shrinkage to which all close-grained clays are liable. The coarser the clay, the less the shrinkage. The colour of the clay varies according to the quantity of lime, iron, or bitumen it contains. Pure clay contracts as much as one-eighth from the size of the mould; one half of this contraction takes place in drying, the other half in burning. Clay mixed with “grog” will contract about one-twelfth.

The moulds for terra cotta are usually piece moulds, made of plaster of Paris, which absorbs much of the moisture of the clay. Sheet clay about two inches thick is used. This is carefully pressed into the mould and supported by webs of clay of the same thickness. It is essential to the clay to be uniform throughout, or the shrinkage would be unequal. It is then placed upon a flue to dry from two to six hours, when the clay will have contracted sufficiently to allow the mould to be taken off. It is then dried for a further period and burnt in a kiln. For fine work, the kiln is “muffled”—the “muffle” being a lining of bricks to keep the clay from actual contact with fire and smoke. The dry, or semi-dry process, is the pressing of clay-powder into metal moulds, which obviates the excessive shrinkage of the wet process. Encaustic tiles are made in this way, the ornament being run into the incised pattern with “slip.” Many tiles are decorated in the same way as ordinary earthenware, that is, painted and glazed.

Terra cotta was largely used by the nations of antiquity, especially by the Assyrians, whose clay tablets or books throw so much light upon Assyrian history. With the Greek, terra cotta was extensively used for “antefixa,” and the many beautiful Tanagra figures now treasured in our museums show the exquisite modelling by the Greeks, in such a material as terra cotta.

This material was used by the Etruscans for their sarcophagi and recumbent figures. The Pompeians tiled their roofs with terra cotta. It was used for votive statues and offerings, and for lamps, some of which were dipped in molten glass.

During the revival of art in Italy in the 15th and 16th centuries, terra cotta was extensively used by the Della Robbia family. Luca della Robbia, 1400-82, produced many beautiful terra cotta reliefs coated with the white tin enamel and enriched with coloured enamels. Among his numerous works were the following:—The marble Cantoria in the cathedral; five bas-reliefs in marble on the Campanile at Florence; his two first terra cotta reliefs in the tympanans of the doorway, and the doors of the sacristry of the cathedral at Florence (1443-46); with the two kneeling angels holding candelabra; the splendid monument to L’EvÊque Federighi (1455) with its beautiful recumbent figure, in the church of S. TrinitÀ, Florence; and the many fine medallions enriched with heraldic forms executed for the church of Or San Michele and the palace Quarateri in Florence. Fine examples are the medallions with the arms of King RenÈ D’Anjou, now in the South Kensington museum.

In Santa Croce at Florence, there are a series of medallions of the four evangelists and the twelve apostles, and in the South Kensington museum there are twelve medallions representing the months. Many splendid examples of Luca della Robbia’s work are now treasured in the national museums.

Andrea della Robbia (1435-1525) the nephew of Luca carried on the traditions with rare selective power and artistic skill; among his early works are the medallions for L’Hospital des Innocent, or the Children’s Hospital. The Adoration and the Annunciation were familiar subjects with Andrea, the illustrations given of the Annunciation in the Children’s Hospital, and the Virgin and child in the national museum at Florence being typical examples of his work. There is a splendid “Adoration” by Andrea in the South Kensington museum.

Giovanni della Robbia (1469-1527) son of Andrea continued this splendid tradition: his principal works being the Lavabo in S. Maria Novella, the tabernacle in S. Apostles, and the virgin and saints in Santa Croce, all in Florence. Many other beautiful works still remain which attest to the remarkable traditions of craftsmanship of the Della Robbia family.

Girolamo, brother of Giovanni, carried this tradition into France under Francis I.