Tiles, plates of baked clay, of various shapes and patterns, according to their use, some being for roofs, some for facing walls, and others for pavements. Marble and stone slabs used for covering roofs are also sometimes called tiles. Drain-tiles have been described under the head of PIPES. The small cubical pieces of burnt clay, stone, glass, or other material used for mosaic pavements are called tessere (see MOSAIC).
ENCAUSTIC TILES are ornamental tiles made of earthenware, and now extensively used for paving churches, halls, conservatories, &c. Strictly speaking, the name applies only to tiles with a pattern produced by layers of different coloured clays. By R. Prosser's process (patented in 1840) tiles of one colour are made of dried slip—i.e. the powder of carefully mixed and prepared clay. These are made by placing the coloured clay powder in strong steel moulds, and consolidating it in a semi-dry state, by means of a plunger fitting accurately into a mould worked by a screw-press. It is then removed, heated in a hot chamber, fired, and glazed if required.
Encaustic tiles are made by two quite different processes. By the older of these, which is a revival of that used in mediæval times, the clay is worked in a plastic state first into square blocks. These are cut into square slices or slabs by passing a wire through them; upon a slab so cut is put a facing of fine clay of the colour of the ground of the pattern—another layer of fine clay being added to the bottom to prevent warping. By means of metal containing-frames, fitting into each other, a plaster of Paris mould or reverse is pressed with a screw-press into the face of the soft tile, and thus indents the pattern. Clay of the requisite colour to form the pattern is now poured in a semi-fluid state into this depression, the tile being then set aside until dry enough to have its surface scraped. By this means the superfluous clay is removed, and the pattern is brought out clear and well defined, the two or more colours of clay forming one smooth flat surface. The tile is then dried and fired.
The other process of making encaustic tiles is by the use of perforated brass plates and a screw-press, clay powder or 'dust,' prepared in the same way as in Prosser's method, being used. It was patented by Boulton and Worthington in 1863, and is extensively used. By this process the device on the tile is first made by pressing clay dust into the perforations of a brass plate laid on a metal block; or if it is of more than one tint, then dust of two or more colours, with as many brass plates, is used to build up the pattern. The plates being removed, the clay powder which is to form the body of the tile is then filled into a mould round the device, and the whole consolidated by pressure.
Encaustic tiles were used for paving churches in the north of Europe from the 12th to the 16th century; but in Spain and Italy the mediæval and later paving-tiles had their patterns produced in thick enamel glaze on the surface, and were not so durable as when made with two or more layers of different coloured clays.