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.^: 132 Once used as a medicine and pharmaceutical preparation,^: 176 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.^{: 414–423} However, due to its instability, its popularity declined as other green pigments became readily available.^: 171 The instability of its appearance stems from its hydration level and basicity, which change as the pigment interacts with other materials over time.^: 637