verdigris

Verdigris in Prague Underground

Verdigris is the common name for the green coating or patina formed when copper, brass or bronze is weathered and exposed to air or seawater over a period of time. It is usually a basic copper carbonate, but near the sea will be a basic copper chloride.^[1] If acetic acid is present at the time of weathering, it may consist of copper(II) acetate.

Etymology

The name Verdigris comes from the Middle English vertegrez, from the Old French verte grez, an alteration of vert-de-Grèce ("green of Greece"). The modern French spelling of this word is vert-de-gris. Since it was used as a pigment in paintings and other art objects (as green color), it was required by artists in Greece. It was originally made by hanging copper plates over hot vinegar in a sealed pot until a green crust formed on the copper. Another method of obtaining verdigris pigment, used in the Middle Ages, was to attach copper strips to a wooden block with acetic acid, then bury the sealed block in dung. A few weeks later the pot was dug up and the verdigris scraped off. One method used in the early nineteenth century had to do with reacting copper sulfate solution with solutions of lead, barium, or calcium acetate. Their sulfates are insoluble, forming precipitates and leaving the copper acetate in solution.

Uses

As a pigment

The vivid green color of copper(II) acetate makes this form of verdigris a very common pigment. Until the 19th century, verdigris was the most vibrant green pigment available and frequently used in painting. Verdigris is lightfast in oil paint, as numerous examples of 15th century paintings show. However, its lightfastness and air resistance is very low in other media. Copper resinate, made from verdigris, is not lightfast, even in oil paint. In the presence of light and air, green copper resinate becomes stable brown copper oxide. This degradation is to blame for the brown or bronze color of grass or foliage in many old paintings, although not typically those of the "Flemish primitive" painters such as Jan van Eyck, who often used normal verdigris. In addition, verdigris is a fickle pigment requiring special preparation of paint, careful layered application and immediate sealing with varnish to avoid rapid discoloration (but not in the case of oil paint). Verdigris has the curious property in oil painting that it is initially bluish-green, but turns a rich foliage green over the course of about a month. This green is stable. Verdigris fell out of use by artists as more stable green pigments became available.

Other uses

It is used industrially as a fungicide, a catalyst for organic reactions, and in dyeing (The Merck Index , Ninth Ed., 1976). Verdigris has also been used in medicine.^{[citation needed]}

Chemical Properties

Copper(II) acetate is soluble in alcohol and water and slightly soluble in ether and glycerol. It melts at 115 °C and decomposes at 240 °C. It can be prepared by reacting copper(II) oxide, CuO, or copper(II) carbonate, CuCO₃, with acetic acid, CH₃COOH.

External links

• National Pollutant Inventory - Copper and compounds fact sheet

References

1. ^ Sharp, D. W. A: "Penguin Dictionary of Chemistry", page 419. Penguin Books, 1990 (2nd edition)

This color-related article is a stub. You can help Wikipedia by expanding it. v • d • e

This inorganic compound-related article is a stub. You can help Wikipedia by expanding it. v • d • e