Specifically, Wikipedia states that Sir Joshua Reynolds' "The Age of Innocence" was painted by the artist over another one of his paintings because the latter was "had suffered some paint losses," and goes on to say that "Innocence itself has deteriorated since 1859." Why?
Some pictures where lead-based paint was used tend to deteriorate due to formation of black PbS. However, I have found that the cause of this is the artist's 'defective' technique, which caused flaking of paint. It's the matter of technique, not chemistry.