Glass
Glass, which first appeared among the materials of mosaic in the Hellenistic period (3rd–1st century bce), brought unlimited colour possibilities to the art. In floors, however, it had to be used sparingly because of its brittleness. In floors, glass tesserae were used for the strongest hues of red, green, and blue, while softer tints were rendered with coloured stone. With the development of wall mosaic, glass largely took over the functions of stone, producing tints of unsurpassed intensity and leading to a continuing search for new coloristic effects.
With little knowledge of the laws of optics but with immense practical experience, mosaic makers of the Early Christian period gave the art a completely new direction with the exploitation of gold and silver glass tesserae. Like a mirror, the glass from which this kind of tesserae was made had a metal foil applied or, better, encased in it. The metal was gold leaf or, for the “silver,” probably tin. These pieces of mirror glass gave golden or white reflections of high intensity and could be used to depict objects of precious metal or to heighten the effect of other colours; but, above all, it was used as a means of rendering the light emanating from God.
Gold tesserae were first used by the Romans, in both floor and vault decoration of late antiquity. Initially, their role was simply to give a golden effect. Gold tesserae, for example, were employed to depict a golden wreath in a floor mosaic at Antioch (c. 300 ce) and gold vessels in some of the vault mosaics in Santa Costanza in Rome. Later, when this use of gold for imitation purposes had become more refined, some spectacular effects were produced in the depiction of garments. The Good Shepherd in the Mausoleum of Galla Placidia at Ravenna (c. 450 ce) is dressed in golden robes of densely set gold cubes shaded with stripes of light-yellow tesserae. The female saints in Sant’Apollinare Nuovo (c. 550–570 ce) in the same town wear costumes set with green glass cubes among which appear both patterns and large fields of gold tesserae, producing a striking similarity to rich silk brocade. Silver was used in a similar way. Christ scenes in Sant’Apollinare Nuovo (500–526 ce) employ silver tesserae in the drawn sword of Peter in the betrayal, no doubt an imitation of steel. Silver tesserae are also found in the silver jug and basin in the scene of Pilate washing his hands.
Gold cubes were distributed among the ordinary tesserae to add to the shimmer of light in ornaments and background details. To avoid an uneven gleam in the surface, the mirror effect was often moderated by setting the gold tesserae in reverse, so that the visible part of the cube is the side with the thickest sheet of glass covering the gold leaf. In the now-lost mosaics of the Church of the Dormition in Nicaea, a scholar observed another exquisite effect, which he called dark gold, created by cubes from which some of the gold leaf had been chipped off, for example, in the frontal part of Mary’s golden footstool (7th or 8th century ce).
An early instance of the use of gold for depicting light emanating from God is in a representation of Christ-Helios (Christ as the Sun God) in a 3rd-century mausoleum under St. Peter’s at Rome. Here, a few gold tesserae are seen in the rays coming from Christ’s head. The halo of gold, a feature so common in Christian art that religious pictures without it can hardly be imagined, developed in mosaic art in the 4th century ce. The gold background, signifying divine light, probably originated in Roman mosaic art, but the first preserved instances date from the advanced 4th century. The cupola mosaic of Áyios Geórgios at Thessaloníki (c. 400), for example, has a background of gold. In Italian mosaics of the 5th century, other types of background, such as a dark-blue ground or a more naturalistic landscape setting, were dominant. Only at the beginning of the 6th century did the gold background become the rule.
In addition to this massive predilection for gold, the Christian East began to use silver to depict the symbolic light emanating from Christ. First, it was used for the entire disc of his halo, later only for the cross arms. The archangels were the only figures besides Christ for whom the silver halo was used. The light of God, appearing as rays from above in scenes of the Annunciation, Nativity, Baptism, and Transfiguration, was also depicted with silver tesserae. Finally, silver and gold were used together in Byzantine representations of the infant Jesus, whose golden robes are highlighted with silver cubes (the apse and south vestibule of Hagia Sophia, Istanbul; both 9th century).