Complex sentence: The fact that the "Black Paintings" were executed as "fresco-secco" is a probable cause for their relatively quick deterioration, and it is likely that they had begun to erode even before their removal from the Quinta walls.
Split into simpler sentences: The fact that the "Black Paintings" were executed as "fresco-secco" is a probable cause for their relatively quick deterioration. It likely that they had begun to reode even before their removal from the Quinta walls.