I’m repeatedly drawn to decorating only in cobalt blue. It’s not exactly monochrome decoration because of the variation in tone you can get with it, but it’s mono-pigment.
Cobalt is absolutely reliable, performing well at all temperatures and in all kiln atmospheres. It’s one of the most versatile of oxides, usable on raw clay, as an underglaze colour or painted over glaze. What makes it valuable to the decorator is its range, from a pale wash to deepest midnight blue. Its versatility was exploited by the maiolica and Delft painters. On Delft chargers, for example, pale washes could be applied with a big soft brush for clouds and sky, and outlines with a fine pointed brush, as in the accomplished piece below depicting Perseus and Andromeda.
Other oxides don’t perform in this way. I find it difficult to control the tone of iron oxide, for example, a thin wash of which often disappears completely. Copper oxide has a fuzzy edge, which is good if you want that that sort of thing, but if you put it on thickly it turns from green to an ugly metallic black.
Cobalt was one of the five pigments described by Piccolpasso in The Three Books of the Potter’s Art (1557). It came as zaffre, an ore with traces of sulphur, arsenic, nickel and manganese. Piccolpasso says it came from Venice, but Venice was only the entrepôt. According to Ahmad Yousef al-Hassan Gabarin (of the Institute for the History of Arabic Science at Aleppo), it was mined in Persia, Oman and the northern Hijaz and sent to glass and ceramics centres in the Islamic world, from where it made its way to Europe.
The high cost made extraction in Europe worthwhile, and by the eighteenth century it was being mined in Saxony, the Mendip Hills and in Cornwall. Luke Hebert, in The Engineer’s and Mechanic’s Encyclopaedia (1836) says that in the North Staffordshire Potteries, the best zaffre cost about four pounds a kilo – equivalent to seven weeks’ average earnings, or £3,000 at modern values. The best price I can get today is £65 a kilo, but cobalt oxide is still the most expensive pigment used by the potter. It costs about 25 times as much as the cheapest, iron oxide. But you don’t have to use much: a quarter of one per cent will make a good blue in a white or transparent glaze.
Its strength means it has to handled with care. One stray speck, invisible before firing, will shows up in a tin glaze and it’s advisable to wash your hands often in order to prevent cobalt fingerprints. Stray cobalt is one of the hazards of pottery evening classes, where students and technicians are careless. It’s so strong that white clays can be made to look even whiter by the addition of one part of cobalt to 20,000 parts of clay.
Cobalt oxide is now is so pure that it produces a colour too bright for my taste. The popular tin-glazed plates from Spain and Italy use this garish blue, but I tone it down by mixing it with copper oxide. Cobalt is a flux but copper is more powerful and I like to fire my work to the point at which the copper makes the blue run. Where the colour is thick, it runs more than it does where it’s thin, so I can create contrasts in the performance of the stain as well as in its tone. The copper and cobalt also separate, which gives the blue a green halo. You might call these effects imperfections, but they’re perfect for me because I want this sort of glaze-colour reaction, often seen in stoneware but unusual in earthenware.
See also: Cobalt Blue: Letter to an Admirer