Paris Green

This page is about the blue green chemical copper(II)-acetoarsenite. For the color "emerald green", see Emerald (color).

Paris Green is a common name for copper(II)-acetoarsenite, or C.I. Pigment Green 21, an extremely toxic blue green chemical with four main uses: pigment, animal poison (mostly rodenticide), insecticide, and blue colorant for fireworks.

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Other names for the chemical are Emerald Green, Parrot Green, Schweinfurt Green, Imperial Green, Vienna Green, and Mitis Green. It is almost never called Paris Green when referencing its use as a pigment. Since the use of Emerald Green as a pigment has been abandoned (around 1960), if one comes across the chemical today it is usually referred to as Paris Green.

It was once a popular pigment, C.I. Pigment Green 21, used in artists' paints. When used as a pigment it was almost always given a color-based name, usually Emerald Green. The brilliance of this pigment has not been matched by modern pigment chemistry. The modern, significantly less vivid, substitution is a mixture of phthalocyanine green (blue shade), an organic lemon yellow, and white. Modern imitations either call themselves "Emerald Green" or "Permanent Green". The closest match to true Emerald Green in watercolors, Winsor & Newton's "Emerald Green", is impermanent and was recently discontinued.

It was once used to kill rats in Parisian sewers, hence the common name Paris Green. It was also used in America and elsewhere as an insecticide for produce, such as apples, around 1900, where it was blended with lead arsenate. This quite toxic mixture is said to have burned the trees and the grass around the trees. Old pianos may contain this mixture, or either of its components. It may still be found in limited use as an insecticide, primarily in the developing world, and as a fireworks colorant. It is reportedly very difficult to obtain a good blue in fireworks with any other chemical. Paris Green was once a popular pigment for painting ships, because its toxicity prevented the accumulation of barnacles.

Scheele's green is a chemically simpler, less brilliant, and less permanent, synthetic copper-arsenic pigment used for a rather short time before Emerald Green was first synthesized, which was approximately 1814. It was popular as a wallpaper pigment, and would degrade, with moisture and moulds, to arsine gas. Emerald green may have also been used in wallpaper to some extent and may have also degraded similarly. Both pigments were once used in printing ink formulations.

The color of Emerald Green is said to range from a pale, but vivid, blue green when very finely ground, to a deeper true green when coarsely ground. The molecule's vividness comes from hydrogen bonds. Similar natural compounds are the minerals Chalcophyllite Cu₁₈Al₂(AsO₄)₃(SO₄)₃(OH)₂₇·36(H₂O), Conichalcite CaCu(AsO₄)(OH), Cornubite Cu₅(AsO₄)₂(OH)₄·(H₂O), Cornwallite Cu₅(AsO₄)₂(OH)₄·(H₂O), and Liroconite Cu₂Al(AsO₄)(OH)₄·4(H₂O). These vivid minerals range from greenish blue to slightly yellowish green. The ancient Romans used one of them, possibly conichalcite, as a green pigment. The Emerald Green paint used by the Impressionists is said to have been composed of relatively coarse particles. Later, the chemical was produced with increasingly small grinds and without carefully removing impurities; its permanence suffered. It is likely that it was ground more finely for use in watercolors and inks, too. The exterior of the home of Ulysses S. Grant was painted with this pigment, making it extremely striking. It was also the pigment used to paint window shutters, signs, etc. It blackens when exposed due to the instability of the molecule, instability that appears to be increased when the chemical is produced with a very fine particle size and isn't washed to remove impurities. Much of the blackening may be due to its reactivity with sulfur, as sulfur-containing compounds are common in painting (such as cadmium sulfide) and in air due to air pollution. In oils, artists often isolated Emerald Green with varnish to reduce its tendency to darken, a practice that is suggested for all arsenic pigments, such as orpiment. It has become the norm in parts of America to paint shutters a dark green color because people mistakenly believe the tradition was to paint them such a dark hue. In fact, shutters were commonly painted with Emerald Green.

Arsenic based green also is a very old dye for cloth, but its use was eventually abandoned because those who wore clothes so dyed tended to come to early ends, without the substance's toxicity being formally recognized. To this day, French theater costumes traditionally eschew the color green.

An artist who recently produced his own oil paint with Paris Green obtained from a fireworks supplier suffered arsenic poisoning from vapors which emanated from the finished paint. Either impurities were the cause, or the molecule itself spontaneously degrades, creating a highly toxic arsine gas. This was Cezanne's favorite pigment, and it dominates many of his paintings. In his watercolors, thin washes have turned brown but thicker applications have remained bright green. The pigment was also used heavily by other artists of his era, such as Van Gogh. Cezanne developed severe diabetes, which is a symptom of chronic arsenic poisoning. Monet's blindness and Van Gogh's neurological disorders are likely directly related to their use of Emerald Green, as well as lead pigments, mercury-based Vermilion, and solvents such as turpentine.

The permanent purple pigment "Cobalt Violet", also used by the Impressionists, was once formulated with arsenic. It was reformulated successfully without arsenic in two varieties, "light" with ammonium or lithium and "deep" without. It would be useful to artists and others in need of a brilliant green pigment for a compound similar to Emerald Green to be synthesized without arsenic.

According to the book Wisconsin Death Trip, paris green was popularly used in domestic poisonings, especially by servants and children seeking freedom from the man or lady of the house.