Cobalt blue

Origin

Cobalt compounds had coloured glass and ceramic glazes for thousands of years — the blue of Egyptian and Persian glass, of Chinese blue-and-white porcelain, of the smalt that Renaissance painters ground from blue glass. But smalt is a poor paint: coarse, weak and prone to turning grey in oil. The modern pigment was isolated by the French chemist Louis Jacques Thénard in 1802, working at Sèvres on a commission to find a replacement for expensive blues, by calcining cobalt salts with alumina. The result was a true, stable, permanent blue, and it went into commercial production almost immediately. The name comes from the German Kobold, a malicious goblin: miners blamed underground spirits for the useless ore that gave off poisonous arsenic fumes when smelted and yielded no silver.^{[1, 2]}

In art history

Cobalt arrived in the same decades as the collapsible metal paint tube and the portable easel, and the three together produced Impressionism. It was the sky blue of the nineteenth century: Monet, Renoir, Pissarro and Sisley all used it constantly, and Van Gogh wrote to Theo about it in terms that read like devotion — "cobalt is a divine colour and there is nothing so beautiful for putting atmosphere around things". Turner used it in his late skies. It is also the blue in a great deal of Post-Impressionist and Fauve painting, where its slight coolness lets it sit against orange without either colour dulling. In this collection it spans fifteen separate eras, from early nineteenth-century landscape to mid-twentieth-century abstraction.^[3]

In the studio

Cobalt blue is semi-opaque and comparatively weak in tinting strength, which sounds like a defect and is in fact why painters love it: it does not bully a mixture the way Prussian or phthalo blue does, so it can be modulated in small steps. It is the most reliable blue for skies because it stays blue when lightened with white, where ultramarine turns slightly purple and Prussian turns slightly green. It dries fast in oil — cobalt is itself a drier, which is why cobalt salts are added to commercial oil paints for exactly that purpose — and this makes it prone to cracking if laid over slower-drying layers. It is completely lightfast and stable with every other pigment.

Afterlife

It is still made by the same calcination process and remains expensive, since cobalt metal is costly and increasingly contested — the same element that colours the sky in a Monet now goes overwhelmingly into lithium-ion batteries, much of it mined in the Democratic Republic of the Congo under conditions that have drawn sustained criticism. Cheaper "cobalt blue hue" paints substitute ultramarine and phthalo mixtures, which match the hue but not the handling: they are stronger, more transparent, and they do not hold their colour in tints the same way.

In different media

Oil

Semi-transparent in most oil paints, and rated extremely permanent by its makers.^[4]

Fresco

Unaffected by acids and alkalis, so it survives lime; Gettens and Stout list it as usable in all painting techniques.^{[1, 2]}

Cerulean or cobalt?