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 chemically related to Paris Green. Scheele's Green was invented in 1775 by Carl Wilhelm Scheele. By the end of the 19th century, it had virtually replaced the older green pigments based on copper carbonate. It is a yellowish-green pigment commonly used during the early to mid-19th century in paints as well as being directly incorporated into a variety of products as a colorant. It began to fall out of favor after the 1860s because of its toxicity and the instability of its color in the presence of sulfides and various chemical pollutants. The acutely toxic nature of Scheele's green as well as other arsenic-containing green pigments such as Paris Green may have contributed to the sharp decline in the popularity of the color green in late Victorian society. By the dawn of the 20th century, Scheele's green had completely fallen out of use as a pigment but was still in use as an insecticide into the 1930s. At least two modern reproductions of Scheele's green hue with modern non-toxic pigments have been made, with similar but non-identical color coordinates: one with hex#3c7a18 (RGB 60, 122, 24) and another with hex#478800 (RGB 71, 136, 0). The latter is the more typically reported color coordinate for Scheele's green. The pigment was originally prepared by making a solution of sodium carbonate at a temperature of around , then slowly adding arsenious oxide, while constantly stirring until everything had dissolved. This produced a sodium arsenite solution. Added to a copper sulfate solution, it produced a green precipitate of effectively insoluble copper arsenite. After filtration the product was dried at about . To enhance the color, the salt was subsequently heated to . The intensity of the color depends on the copper : arsenic ratio, which in turn was affected by the ratio of the starting materials, as well as the temperature.