In the early centuries A.D. glass mosaics brought color and decoration to the broad walls of the basilicas. By the 4th cent. the triumphal arch between nave and apse and the walls above the nave arcades received mosaic adornment, while the entire domed apse was lined with a mosaic picture, generally of Jesus surrounded by saints and apostles.
In this period Byzantium (later Constantinople) became the center of the craft, which reached perfection in the 6th cent. Hagia Sophia exhibits glittering gold backgrounds—a special feature of Eastern mosaic art, which later spread to the West. A gold tessera was produced by applying gold leaf to a glass cube and covering it with a thin glass film to protect against tarnishing; for the other tesserae the colors were produced by metallic oxides. The tesserae were set by hand in the damp cement mortar, and the resulting irregularities, causing the facets to reflect at different angles, were an essential factor of effect. In the 5th and 6th cent. Ravenna became the Western center of mosaic art, and the Ravenna masterworks (e.g., the decoration of San Vitale), as well as those in Rome, show the Byzantine characteristics of stylized rigidity in the figures.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.