The durability, range of colour, and appropriateness of material and treatment to architectural conditions, has placed the art of Mosaic as the chief decorative enrichment of architecture. Its antiquity is unquestionable, for in the Book of Esther, i, 6, we read “of a pavement of red, and blue, and white, and black marble.”

Mosaic is the art of forming patterns by means of pieces of variously-coloured materials, fitted together, and is broadly divided into three classes: (1) Opus Tesselatum, or clay mosaic; (2) Opus Lithostratum, or stone mosaic; (3) Opus Miserum, or glass mosaic. These divisions are again sub-divided into: (1) Opus Figlinum, or ceramic mosaic, formed of a vitreous composition and coloured with metallic oxides; (2) Opus Signinum, small pieces of tile; (3) Opus Vermiculatum, sub-divided into (a) Majus, black and white marble, (b) Medium, in which all materials and colours were used, and (c) Minus, of minute tesserÆ, principally used for furniture inlay; (4) Opus Sculpturatum, slabs of marble hollowed out and filled in with grey or black marble; (5) Opus Alexandrinum, inlay of porphyry and serpentine; and (6) Opus Sectile, formed of different laminÆ or slices of marble of various colours.

It was in Rome that the art of Mosaic was brought to its greatest perfection, during the 1st and 2nd centuries, A.D., and many splendid examples of this period are now in the museums of the Vatican and at Naples. The finest example came from the House of the Faun, Pompeii, and represents the battle of Issus, between Alexander and Darius. This mosaic, of the 3rd century B.C., is probably a copy of a Greek painting.

Many fine Roman mosaics have been found in England at Cirencester, London, Lincoln (fig. 6), Leicester, and at Brading in the Isle of Wight.

The tradition was carried on in Italy at Ravenna and Venice, where the Opus Miserum reached its culmination. Of the Ravenna mosaics, those of the Baptistery, A.D. 450, and of S. Apollinare are typical examples of the earlier Byzantine mosaics, having dark green and gold back-grounds with tesserÆ about ? inch square. The beautiful frieze of male and female saints in S. Apollinare extends along both sides of the nave, and is 10 feet high. The vaulting and domes of St. Mark are entirely covered with the characteristic 11th century Byzantine gold ground mosaic, formed by fusing two pieces of glass together with gold leaf between. At Santa Sophia, Constantinople, other fine mosaics exist of the 6th and 7th centuries. In Italy under the Cosmati (a family of mosaicists of the 13th and 14th centuries), fine geometrical inlaid mosaics were used for the enrichment of marble tombs and altars; some good examples of this style are in Westminster Abbey on the tomb of Edward the Confessor (finished under Henry III, A.D. 1270).