Cobalt violet
The first true violet pigment in the history of painting. Before 1859 a painter who wanted purple had to mix it, and the mixture was always disappointing — violet is the hardest colour to make from other colours, because every route to it loses saturation.
Origin
Discovered by Salvetat in France in 1859 as cobalt arsenate, which was magnificent and extremely poisonous, and reformulated soon afterwards as cobalt phosphate, which is slightly less brilliant and merely expensive. Both were made by precipitation from cobalt salts. The arsenate version was in use into the twentieth century despite being one of the more dangerous materials ever sold in a tube; the phosphate is what is sold today. A related pigment, manganese violet, appeared in 1868 and is cheaper, stronger and more purple, and it took over much of the same work.
In art history
Its arrival is one of the clean cases where a new pigment visibly changes what painting looks like. The Impressionists were preoccupied with shadow, and specifically with the observation that shadows on a sunlit day are not grey or brown but the complement of the light — violet. Monet is supposed to have said that he had finally discovered the true colour of the atmosphere and it was violet, and the critical mockery of "violettomania" in the 1880s was aimed at exactly this. Cobalt violet made those shadows possible, and it appears in six eras of this atlas, all of them from the second half of the nineteenth century onwards. Signac and the Neo-Impressionists used it, and Monet's late water lilies are unimaginable without it.
In the studio
It is semi-transparent, weak in tinting strength and very expensive, and painters use it sparingly for those reasons rather than because it fails in any way. It is completely lightfast, stable in every mixture, and dries at a moderate rate. Its particular value is that it is a genuinely single-pigment violet: mixed violets, made from red and blue, are always duller than either parent because the pigments absorb each other's light, whereas cobalt violet is as saturated as the material allows. It is granular in watercolour and produces a distinctive mottled wash.
Afterlife
Still manufactured, still one of the most expensive artists' pigments, and still without a true equivalent. Manganese violet and the synthetic dioxazine violets are cheaper and stronger; dioxazine in particular is enormously powerful and transparent, and has taken over general use. Neither has the pale, chalky, slightly cool delicacy of cobalt, which is why it survives in professional ranges despite the price.
Where it appears
Every era and artist in this atlas whose palette includes this pigment.