Verdigris - copper salt, common as a layer on copper that is exposed to the elements
This degradation is to blame for the brown or bronze color of grass or foliage in many old paintings, although not typically those of the "Flemish primitive" painters such as Jan van Eyck, who often used normal verdigris.
In Kurt Vonnegut's short story Armageddon in Retrospect, published in his final book of the same name, Verdigris is featured as the headquarters of the United Nations Demonological Investigating Committee, or UNDICO.