In German folk legend Kobald was the name of a vicious sprite, who lived in the earth to haunt the mines and torment the miners.

Cobalt occurs naturally in the ore smaltite (a mixture of cobalt and nickel arsenides). When exposed to air, this forms a brilliant blue brous crystal known to miners as ‘cobalt bloom’. It is a dangerous ower, for the arsenic compounds in the mineral dust are toxic.

Smaltite was found in silver mines and the miners who came across it threw it away before it ate their feet and attacked their lungs.

The use of cobalt as colouring material in pigments such as smalt had been known at least since the Middle Ages, but the purer cobalt colours created in the 19th century were far more intense and stable.

In 1802 Thénard synthesized a blue solid by mixing cobalt salts with alumina. Thénard’s blue (cobalt alu- minate) had a purer tint than azurite, Prussian blue or indigo, and was taken up immediately as a pigment. It was expensive but popular none the less, its only serious competition came from the synthetic version of ultramarine.

Yet there was more to be had from the chameleon-like qualities of cobalt:

In the mid-19th century, three more cobalt colours appeared on the market: green, violet and yellow.

Cobalt green had a composition similar to cobalt blue but with some or all of the alumina replaced with zinc oxide. It had been originally discovered before cobalt blue in 1780. However, only when zinc oxide became readily available was the manufacture of cobalt green viable. It does not have particularly good opacity and like all cobalt pigments, it remained expensive.

Cobalt Violet was manufactured in France from 1859 but, not unlike its older sister Cobalt Green, its high cost and low tinting power limited its use among painters.

The most complex of the cobalt colours was a yellow that painters knew as Aureolin. First synthesized in 1831 it was not sold as a pigment until its independent rediscovery in the early 1850s and released onto the market in 1861.

These new pigments had only a modest tinting strength, but gave a stronger hue than could be attained by mixtures. The Impressionists and Post-Impression- ists entusiastically adopted their use: Monet favoured them more than most to dazzling effect.