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. The instability of its appearance stems from its hydration level and basicity, which change as the pigment interacts with other materials over time.
History
Etymology The name verdigris comes from the Middle English vertegrez, from the Old French verte grez. According to one view, it comes from vert d'aigre, "green [made by action] of vinegar". The modern French writing of this word is vert-de-gris ("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. The historical methods used for producing verdigris have been recorded in artistic treaties, manuscripts on alchemy, works in natural history, and texts on medicine. The most common ingredients used were copper and vinegar. Throughout history, recipes changed. In the Middle Ages, copper strips were attached to a wooden block with acetic acid; the block was then buried in dung. A few weeks later, the block was to be dug up, and the verdigris scraped off. Another method of production was developed in 18th-century Montpellier, France, a locale which had the ideal climate to produce verdigris for pulverization. The industry there was long dominated by women, with verdigris manufactured in household cellars using copper plates stacked in clay pots that were filled with distilled wine. The acid in the grapes caused the copper to develop crystals. The crystals ripened into verdigris and were scraped off when matured. It was a profitable business, and 80% of production was sold abroad through certified female brokers. At the height of its popularity, in the 1710s, the government had to enforce inspection systems to address growing fraudulent practices. By the 20th century, the production of verdigris had moved away from Montpellier and more cost-efficient methods of producing green pigments sent the industry into decline after WWI.
Chemical composition Verdigris is a collective term for copper acetate, whose chemical varieties produce different hues. The technical literature is inconsistent in describing these variations. Some sources refer to "neutral verdigris" as copper(II) acetate monohydrate (Cu(CH3CO2)2·(H2O)) and to "blue verdigris" as Cu(CH3CO2)2·CuO·(H2O)6. Other sources describe the main copper salt in natural verdigris as Cu4SO4(OH)6 (brochantite). Still, other sources describe it as basic copper carbonate (Cu2CO3(OH)2), or as Cu(CH3CO2)2·(Cu(OH)2)n where n varies from 0 to 3. In marine environments, the main copper salt is tribasic copper chloride (Cu2(OH)3Cl). Overall, variations of verdigris can be divided into two groups: basic verdigris and neutral verdigris. The difference in colour depends on the hydration level and degrees of basicity.
Toxicity
Verdigris, which was once used as a medicine and pharmaceutical preparation is mildly poisonous. Symptoms of toxicity include nausea, anemia and death, although widespread acquired immunity has been documented, as occurred with female workers in Montpellier. 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. It has been identified in The Last Supper (1306) by Giotto. During the 15th and 16th centuries, it was used in paintings for its transparency and brilliance. It was difficult to create strong green colors in paintings due to the limitations of the existing green, yellow and blue pigments. In early Italian, Netherlandish, and German paintings, verdigris was widely used to create pure green tones for landscapes and drapery, such as the green coat of Saint John in the Mond Crucifixion by Raphael. Verdigris was used as both glaze and opaque paint. When verdigris glaze was combined with lead white or lead-tin yellow, it created a deeply saturated green. It was used in oil-based house paint for French and Dutch country houses. Verdigris also was used as an imitation of 'Chinese varnish' on European lacquer. However, during the 19th century, the use of verdigris began to decrease as alternatives such as Emerald Green and viridian became more popular.
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