Article: The mosaics of Hagia Sophia by C. R. Morey
The application of mosaics also changed; while the Romans favored the use of mosaic patterns for flooring, the Byzantines took the art further and applied them to walls and ceilings. They kept their smalti un-grouted to allow light to reflect and refract through them and set their pieces at slight angles to capture the play of light as it moved through the space and allow the silver or gold backing to sparkle from every angle.
The mosaic patterns and themes melded together, with Roman images being absorbed into the predominantly Christian themes favored by the Byzantines, and while some of the pieces remained purely decorative, some were used to depict Emperors much as the Romans depicted gods (Source).
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