Verdigris () 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. "green [made by action] of vinegar". The modern French writing of this word is ' ("green of grey"), sounding like the older name verdet gris ("grey greenish"), itself a deformation of verte grez. It was used as a pigment in paintings and other art objects (as green color), mostly imported from Greece, and hence it is more usually given another etymology as vert-de-Grèce ("green of Greece").
Production
Copper(II) acetate is prepared by treatment of copper(II) hydroxide with acetic acid. Another method of production was developed in 18th-century Montpellier, France, a locale which had the ideal climate to produce verdigris for pulverization. The acid in the grapes caused the copper to develop crystals. The crystals ripened into verdigris and were scraped off when matured. Other sources describe the main copper salt in natural verdigris as (brochantite). Still, other sources describe it as basic copper carbonate (), or as where n varies from 0 to 3. In marine environments, the main copper salt is tribasic copper chloride ().
Toxicity
Verdigris, which was once used as a medicine Nontoxic substitutes have been developed for some applications, such as art pigments.
Uses
Verdigris is a naturally occurring protective layer on metals such as copper, brass, and bronze. In addition to being a desirable artistic effect, it has been used primarily as a pigment and in now-outdated medicinal preparations.
When burned, verdigris produces a green flame.
Pigment
Verdigris has been used as a pigment since antiquity, including in paintings in Rome and Pompeii. The use of verdigris continued into the Middle Ages, Renaissance and Baroque paintings.
Verdigris pigment is lightfast in oil paint, as numerous examples of 15th-century paintings show. However, its lightfastness and air resistance are very low in other media. Copper resinate, made from verdigris by boiling it in a resin, is not lightfast, even in oil paint. In the presence of light and air, green copper resinate becomes stable brown copper oxide. Certain components of historical verdigris pigments, copper(II) acetates, are partially irreproducible based on the given historical recipes.