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Verdigris
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— Color coordinates —
Hex triplet #43B5AB
RGBB (r, g, b) (67, 181, 171)
HSV (h, s, v) (175°, 63%, 71%)
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B: Normalized to [0–255] (byte)

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.

Verdigris in Prague Underground

Contents

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 It was originally made by hanging copper plates over hot vinegar in a sealed pot until a green crust formed on the copper.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 made this form of verdigris a much used pigment. Until the 19th century, verdigris was the most vibrant green pigment available and was 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.

Chemical Composition

Verdigris is a complex chemical mixture of compounds, complexes and water. The primary components are copper salts of acetate, carbonate, chloride, formate, hydroxide and sulfate. The secondary components are other metallic salts, acids (organic and mineral), atmospheric gases and water. All the components are in an ever-changing and extremely sophisticated electrochemical reaction equilibrium that is dependent on the ambient environment.

Handling Hazard

As verdigris consists of various poisonous acetates, sulfates, and chlorides, one should always wash their hands after handling.[2]

See also

External links

References

  1. ^ Sharp, D. W. A: "Penguin Dictionary of Chemistry", page 419. Penguin Books, 1990 (2nd edition)
  2. ^ McCreight, "Complete Metalsmith", page 9. Brynmorgen Press, Inc., 2004 ISBN: 1-929565-05-4

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

VERDIGRIS, a pigment, consisting of basic copper carbonates, made by acting upon copper plates with pyroligneous acid soaked up in cloths, exposing the plates to air, then dipping in water, and finally scraping off the greenish crust; the plate is re-exposed and the operation repeated till it is used up. Another method consists in exposing thin copper sheets to the acid vapours rising from the residues or "mares" of wine factories, the product being scraped off, and the plate reexposed. Both processes require several weeks. The pigment appears with several shades of blue and green; blue verdigris is chiefly CuO Cu(C 2 H 3 0 2) 2.6H 2 0, while light blue and green verdigris contain 2CuO Cu(C 2 H 3 0 2) 2.2H 2 0. Besides being used as a paint it is employed in dyeing and calico-printing, and also in the manufacture of other paints, e.g. Schweinfurt green, which is a double salt of the acetate and arsenite. A liniment or ointment is also used in medicine as a cure for warts. It is an irritant poison (hence the need that acid substances should never be cooked in copper utensils); the best antidote is white of egg and milk.


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