This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Walk into San Vitale in Ravenna and the walls do not read as masonry with pictures on it. They read as light. That effect is the whole point of Byzantine mosaic, and it is achieved by a specific, deliberate material decision: setting cut glass tesserae, many of them backed with a thin layer of real gold leaf sandwiched between two sheets of glass, at slightly irregular angles across a curved or vaulted surface, so that no single viewing position catches every tessera's reflection at once. The wall shimmers and shifts as a visitor moves through the space, which is not a technical accident later ages happened to admire. Byzantine theologians understood church decoration as a genuine encounter with the divine rather than an illustration of a story, and gold ground mosaic, uniquely among the period's available techniques, could make an entire architectural surface behave like an unstable, living source of light rather than a fixed, describable picture. Hagia Sophia's own mosaics, added and altered across centuries as the empire's theological politics shifted, and San Vitale's sixth century apse, with its famous flanking panels of Justinian and Theodora, remain the two sites art historians point to first when explaining why mosaic, more than fresco or panel painting, became Byzantine art's defining monumental medium.