Verdigris

Verdigris (/ˈvɜːrdɪɡriː(s)/) is a common name for any of a variety of somewhat toxic copper salts of acetic acid, which range in colour from green to a bluish-green depending on their chemical composition. Once used as a medicine and pharmaceutical preparation, verdigris occurs naturally, creating a patina on copper, bronze, and brass, and is the main component of a historic green pigment used for artistic purposes from antiquity until the late 20th century, including in easel painting, polychromatic sculptures, and illumination of maps. However, due to its instability, its popularity declined as other green pigments became readily available.  The instability of its appearance stems from its hydration level and basicity, which change as the pigment interacts with other materials over time.