Verdigris

Verdigris /ˈvɜːrdɪɡriː(s)/ is a common name for any of a variety of somewhat poisonous copper salts of acetic acid, which range in colour from green to a bluish-green depending on their chemical composition.^: 132 Verdigris has been used for artistic purposes from antiquity until the late 20th century, including in easel painting, polychromatic sculptures, and illumination of maps. Verdigris was a common ingredient in colouring agents and pharmaceutical preparations.^: 414 Until the 19th century, verdigris was the most vibrant green pigment available and was frequently used in paintings. 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 changes as the pigment interacts with other materials over time.^: 637