Making Scheele's Green — The Copper Arsenite Pigment That Poisoned the 19th Century

Scheele's Green (copper arsenite, CuHAsO₃) was discovered by the Swedish chemist Carl Wilhelm Scheele in 1775. It was the first bright, lightfast green pigment that did not fade or darken like verdigris — and it became enormously popular for wallpapers, fabrics, toys, artificial flowers, and confectionery colouring throughout Europe.

The chemistry is a wet precipitation: arsenic trioxide dissolves in hot potassium carbonate solution to form potassium arsenite, which then reacts with copper sulfate to precipitate a vivid yellowish-green powder of copper arsenite. The reaction is simple, the materials were cheap, and the colour was brilliant — a combination that made it irresistible to manufacturers.

The pigment was also a slow-acting poison. In damp conditions, moulds growing on Scheele's Green wallpaper converted the copper arsenite into trimethylarsine gas — a volatile organic arsenic compound. Entire families suffered chronic arsenic poisoning from their own wallpaper. Children died from playing with green-dyed toys. Napoleon Bonaparte may have been slowly poisoned by Scheele's Green wallpaper in his exile on St. Helena. The pigment was eventually replaced by its more vivid cousin Emerald Green (Paris Green), which was itself equally toxic, and both were finally displaced by non-toxic chromium oxide greens in the late 19th century.

SAFETY WARNING: This process uses arsenic trioxide, a deadly poison. Copper arsenite dust is toxic by inhalation, ingestion, and skin contact. ALL work must be performed in a ducted fume hood. Wear a P100 respirator with acid gas cartridge, chemical splash goggles, thick nitrile gloves, and a lab coat at all times. All waste solutions and wash water must be collected and disposed of as hazardous chemical waste — never pour down the drain. This is an advanced laboratory procedure for trained chemists only.