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Paris green (copper(II) acetate triarsenite or copper(II) acetoarsenite) is an arsenic-based artificial pigment, first invented in 1814. As a green pigment it is also known as Schweinfurt green, emerald or Vienna green. It is a highly toxic emerald-green crystalline powder that has been used as a rodenticide and insecticide, and also as a pigment, despite its toxicity. It is also used as a blue colorant for fireworks. The color of Paris green is said to range from a pale blue green when very finely ground, to a deeper green when coarsely ground.

Preparation and structure
A subunit of the Cu2As3O6(OAc) framework, highlighting the [As3O6]3- ligand. Color code: Cu = blue, As = large gray, C = gray, O = red.

Paris green may be prepared by combining copper(II) acetate and arsenic trioxide. The structure was confirmed by X-ray crystallography.

History
In 1814, Paris green was invented by paint manufacturers, Wilhelm Sattler and Friedrich Russ, in Schweinfurt, Germany. They were attempting to produce an improved pigment over Scheele's green, particularly so that it was longer-lasting and less susceptible to darkening around sulfides. In 1822, the recipe for Emerald green was published by Justus von Liebig and André Braconnot. In 1867, the pigment was named Paris green and was officially recognized as the first chemical insecticide in the world. Because of the arsenic contents, the pigment was dangerous and toxic to manufacture, often resulting in factory poisonings. At the time, Emerald green was praised as a more durable and vibrant substitute to Scheele’s green, even though it would later prove to degrade quickly and react with other manufactured paints.

Visual Characteristics
The powder that is a result from chemical mixing, can come out as a light turquoise. However, since manufacturing of this pigment was not uniform between distributors in the 19th century, the powder could vary and be paler or darker, depending on the formula the factory used: more arsenic led to a lighter shade. In comparison, Scheele's green's powder is uniquely more yellow, and therefore, more lime-green.

In paintings, the color can be described as a rich dark green with an undertone of blue. It received popularity after its release into the art market in the 19th century because of its brilliant color. Its other name, Emerald green, originates from the paint's resemblance to the gemstone's deep color.

Permanence
Paris or Emerald green is one of the few paints that contain arsenic inside them. In some cases, the paint has darkened significantly over time and in some cases, it has turned brown. Within the same century of Emerald green being invented, artists and scientists were able to see the instability of the pigment. In a 1888 study, the pigment was used as a watercolor and exposed to natural light and air, resulting in the pigment darkening and browning. The pigment was then experimented with by many different scientists at the turn of the 20th century, and the results varied. Some found that the Paris green degraded slightly while others said the pigment was weatherproof. This discrepancy could be due to the fact that each experiment used a different brand of Paris green.

More recently, a study from 2013 examined Descente Des Vaches by Théodore Rousseau since the green has tarnished significantly since its creation. The study offered that the oxidation of high amounts of arsenic can cause it to react with the fatty acids in the binding medium in the paint. Because of this reaction, arsenic and copper molecules are destabilized and create copper soaps. This creates small dark particles in the paint and reduces the amount of Emerald green particles present, making the appearance darker.

Overall, conservators in the twenty-first century have determined that, depending on the chemical makeup of the pigment, oxidation overtime or exposure to hydrogen sulfide causes rapid deterioration of the vibrancy of the green paint.

Notable Occurrences
Since the pigment was invented during the 19th century, many of the most famous occurrences of the green color are in the most famous pieces of that time. While artists did adapt to using the industrially manufactured pigments, they were also wary of them because of the inconsistencies between production companies. However, industrial pigments came in collapsible metal paint tubes that were easy to transport and reseal– which was helpful for painting outdoors– and it saved time for the artist, who no longer had to grind their own pigments. While there is no direct correlation between manufactured pigments and the rise of Impressionists and neo-Impressionists, many prominent artists adopted Paris green into their practice. For example, Emerald green is present in various paintings by Claude Monet and Paul Gauguin. Artists were drawn to the vibrant pure shade of Paris green, which was hard to replicate with natural materials.

Insecticide
In 1867, farmers in Illinois and Indiana found that Paris green was effective against the Colorado potato beetle, an aggressive agricultural pest. Despite concerns regarding the safety of using arsenic compounds on food crops, Paris green became the preferred method for controlling the beetle. By the 1880s, Paris green had become the first widespread use of a chemical insecticide in the world. It was also used widely in the Americas to control the tobacco budworm, Heliothis virescens.

However, in the early 20th century, labor laws were emplaced in New York because of the health risks for laborers.

Paris green was heavily sprayed by airplane in Italy, Sardinia, and Corsica during 1944 and in Italy in 1945 to control malaria. It was once used to kill rats in Parisian sewers, which is how it acquired its common name.

Related pigments
Similar natural compounds are the minerals chalcophyllite Cu

18Al2(AsO4)3(SO4)3(OH)27·36H2O, conichalcite CaCu(AsO4)(OH), cornubite Cu5(AsO4)2(OH)4·H2O, cornwallite Cu5(AsO4)2(OH)4·H2O, and liroconite Cu2Al(AsO4)(OH)4·4H2O.

These vivid minerals range from greenish blue to slightly yellowish green.[citation needed]

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

The ancient Romans used one of them, possibly conichalcite, as a green pigment. The Paris 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.