Ironically, Rembrandt used[1] smalt, a glass powder with cobalt which is now known to be fugitive in some formulations; this includes Rembrandt works in the Nat

Ironically, Rembrandt used[1] smalt, a glass powder with cobalt which is now known to be fugitive in some formulations; this includes Rembrandt works in the National Gallery in London[2]. [1] https://heritagesciencejournal.springeropen.com/articles/10.1186/s40494-020-00429-5 https://heritagesciencejournal.springeropen.com/articles/10.... [2] c.f. their excellent book, Art in the Making: Rembrandt