Emerald Green pigment

This deadly green pigment contains copper and arsenic.
Discovered in 1775 and named after its inventor, the Swedish chemist Carl Wilhelm Scheele, Scheele’s green is copper hydrogen arsenite, with the approximate chemical composition CuHAsO₃.

It is an opaque yellow-green pigment developed in an attempt to replace the historic copper-based greens of verdigris and malachite, offering greater opacity and ease of manufacture. Owing to the scarcity of reliable green pigments in the late 18th century, Scheele’s green was initially welcomed by artists, dyers, and decorators.

However, it soon fell out of favour in fine art due to its extreme toxicity and its chemical instability: in the presence of acids, atmospheric sulphur compounds, or sulphide-containing pigments, it readily discolours, darkening or shifting towards brown and grey.

Emerald green, chemically copper aceto-arsenite (Cu(C₂H₃O₂)₂·3Cu(AsO₂)₂), was developed in 1808 in an effort to improve upon Scheele’s green. It is sometimes described as a double salt of copper acetate and copper arsenite. While more durable than Scheele’s green, emerald green still reacts with sulphur-containing pigment (such as cadmium yellows or ultramarine) forming dark copper sulphides and arsenic compounds that cause progressive browning. Its relative stability is highly dependent on binding medium and environmental conditions.

Despite these drawbacks, emerald green possessed a brilliance and saturation unmatched by any previous green pigment. Its high refractive index and strong opacity gave it an intense, almost enamel-like colour that made it immediately popular with painters, textile dyers, printers, and manufacturers of decorative papers. Made by reacting verdigris with arsenic compounds, emerald green inherited the dangers of both copper and arsenic chemistry. In powdered form it is readily absorbed through inhalation or ingestion, and chronic exposure leads to arsenic poisoning.

The pigment’s most notorious use was in printed wallpaper and domestic decoration during the 19th century. In damp or poorly ventilated interiors, emerald green can react with moisture, mould, or organic binders to release volatile arsenic compounds, including arsine and trimethylarsine gases. These colourless, garlic-smelling vapours accumulated in enclosed spaces and were responsible for numerous cases of chronic illness and death, particularly among children confined to nurseries.

By 1815, medical suspicion had already fallen on green wallpapers, but the commercial popularity of the colour delayed meaningful regulation for decades. Remarkably, emerald green was also used in clothing dyes, artificial flowers, candles, and even food colouring.

Emerald green was known by a bewildering array of names, including Schweinfurt green, Veronese green, Vienna green, and Mitis green; in total, more than eighty different names have been recorded. This proliferation of terminology likely served to obscure the pigment’s growing reputation for toxicity and to preserve its marketability.

Even after scientific proof of its highly poisonous nature was established in the mid-19th century, production continued. The manufacture of emerald green paint was not fully prohibited until the 1960s, and today the pigment survives primarily as a subject of historical, toxicological, and conservation concern rather than as a viable artists’ material.