@Спас Колев This makes so much more sense to me.  Covered in plaster.  So the mosaics fair much better under plaster than say a painting does.  Of course, of course.

@Nerdboy I had no idea.  That's really cool.  It wasn't in the Lonely Planet chapter I had on Istanbul (then again I only bought one chapter and not the whole Lonely Planet Istanbul guidebook).  I am sad to have missed seeing the church.   Maybe next time...