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How did mosaic artists in Hagia Sophia brighten dark areas with gold tesserae?

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Bird mosaic — mosaic Mosaic floor fragment from a synagogue or church, cut stone with mortar from Israel, late 5th–6th century ce; in the Jewish Museum, New York City.

To brighten dark areas in Hagia Sophia, mosaic artists employed sophisticated techniques involving the strategic placement and angling of tesserae.1

In particularly dark corners, artists not only tilted the gold cubes downward to maximize light reflection toward the onlooker but also turned them slightly sideways to capture light from nearby windows. Additionally, in poorly lit areas of the vestibule and gallery, craftsmen incorporated a high percentage of silver cubes among the gold ones to enhance the overall sparkle and luminosity of the background.12

These methods were part of a broader Byzantine tradition of manipulating light through the physical orientation of materials. By carefully adjusting the angle of the gold and silver glass cubes—which were crafted by sandwiching metal leaf between layers of glass—artists could effectively illuminate spaces that would otherwise appear dim.314