Scheele's Green

Scheele's Green, also called Schloss Green, is chemically a cupric hydrogen arsenite (also called copper arsenite or acidic copper arsenite), CuHAsO₃. It is a compound similar to Paris Green. It is a green pigment, of yellowish hue and was used in the past in some paints but has since fallen out of use due to its toxicity.

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Scheele's Green was invented in 1775 by Carl Wilhelm Scheele. ^[1] By the end of the 19th century, it virtually replaced the older green pigments based on copper carbonate.

Scheele's original method of preparing the compound was dissolving arsenic sulfide and potash in water and heating it, then slowly adding the alkaline solution to a warm solution of copper sulfate, then letting it stand. The pigment precipitated, the liquid was poured off, the pigment was washed and dried on gentle heat.

Scheele's Green was more brilliant and durable than the then-used copper carbonate pigments. However it tended to fade and blacken when subjected to hydrogen sulfide containing atmosphere. It also can not be mixed with pigments based on sulfides or containing sulfur.

Emerald green, also known as Paris Green, was developed later in attempt to improve Scheele's Green. It had the same tendency to blacken, but was even more durable.

By the end of 19th century, both greens were made obsolete by zinc oxide and cobalt green, also known as zinc green.

Scheele's Green was used as an insecticide in 1930's, together with Paris Green.

Scheele's Green can be used also to color wax candles. There is one example of an acute poisoning of children attending a Christmas party where such candles were burned.^[2]

Scheele's Green was used as a color for paper, eg. for wallpapers and paper hangings, and in paints, even on some children toys.^[3] It was also used to dye cotton and linen. The wallpapers containing Scheele's Green are implicated in the arsenic poisoning of Napoleon Bonaparte. Tiny particles of the pigment tend to flake off and become airborne, and then are absorbed by the lungs. Also, when the wallpaper becomes damp and moldy, the pigment hydrolyzes and releases poisonous arsine gas.

Despite its high toxicity, Scheele's Green was also used as a food dye for sweets, drink: the green in absinthe is now thought to be the source of the problems with the liquor instead of the ingredient thujone (leading to its banishment in most European countries and the US by 1915) eg. green blancmange and others,^[4] a fondness of traders in 19th century Greenock, leading to a long-standing Scottish prejudice against green sweets. Another popular poisonous colorant was chrome yellow, used for sweets, snuff and custard powder.

References

1. ^ http://www.lilinks.com/mara/history.html
2. ^ http://www.harvestfields.ca/HerbBooks/01/01/047.htm
3. ^ http://www.gutenberg.org/dirs/etext04/dvmth10.txt
4. ^ http://www.chemistry.ucsc.edu/teaching/Winter97/BMB150C/lecture3.html