Verdigris
A blue-green substance that forms on copper, once made deliberately as a green pigment.
Also: copper green
Verdigris is the green or bluish crust that grows on copper, brass and bronze. For centuries it was also manufactured as a pigment, by exposing copper sheets to vinegar or to fermenting grape skins and scraping off the crust. It was the brightest green available to European and Persian painters before the 19th century. It could be unstable, sometimes darkening to brown or eating into paper. The name comes from Old French and is usually explained as green of Greece. It is poisonous if swallowed.