Which issue challenged the longevity of mosaic artistry during high-humidity periods in Hagia Sophia?

💡 Explanation

The deterioration arose due to efflorescence on the mosaics' reverse side due to moisture; the salt crystallization from this process exerted pressure and thus weakened the tesserae adhesion, because hygroscopic salts migrate through porous materials to the surface, therefore causing decohesion rather than direct pigment change.

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