![]() Its strength means it has to handled with care. 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. It costs about 25 times as much as the cheapest, iron oxide. The best price I can get today is £65 a kilo, but cobalt oxide is still the most expensive pigment used by the potter. 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 high cost made extraction in Europe worthwhile, and by the eighteenth century it was being mined in Saxony, the Mendip Hills and in Cornwall. 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. Piccolpasso says it came from Venice, but Venice was only the entrepôt. It came as zaffre, an ore with traces of sulphur, arsenic, nickel and manganese. 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.Ĭobalt was one of the five pigments described by Piccolpasso in The Three Books of the Potter’s Art (1557). I find it difficult to control the tone of iron oxide, for example, a thin wash of which often disappears completely. 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. Its versatility was exploited by the maiolica and Delft painters. ![]() What makes it valuable to the decorator is its range, from a pale wash to deepest midnight blue. It’s one of the most versatile of oxides, usable on raw clay, as an underglaze colour or painted over glaze. It’s not exactly monochrome decoration because of the variation in tone you can get with it, but it’s mono-pigment.Ĭobalt is absolutely reliable, performing well at all temperatures and in all kiln atmospheres. I’m repeatedly drawn to decorating only in cobalt blue.
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