Please take a look at this passage:
It is due to a misunderstanding that most modern sculptures are monochromatic. When ancient sculptures were exhumed years ago, they were discovered to be uncolored. No one at that time had reason to believe, as we now do, that the sculptures had originally been colorfully painted, but that centuries of exposure to moisture washed away the paint.
I can't tell exactly what's the usage of "but" in the last part of the passage. When seeing it as a conjunction, it seems wrong because it should not be preceded by a comma. This is as far as I could go.
So, then, is it an adverbial clause "but + noun clause" meaning "only"? Also, what does the second sentence really mean?
I would really appreciate any helps!