Cobalt blue is a deep, intense blue colour produced from compounds of the metal cobalt, used as a colourant in ceramics, glass and enamels. Cobalt oxide is one of the few colourants able to withstand the very high temperatures of porcelain firing, which made it ideal for underglaze decoration.
Cobalt has been used to colour glass and ceramics since antiquity, but its most celebrated use is in Chinese blue and white porcelain, which reached a high level of refinement at Jingdezhen from the fourteenth century. Exported Chinese blue and white inspired imitations around the world, including Dutch Delftware, English tin-glazed earthenware, and the underglaze blue painted and transfer-printed porcelain and earthenware of factories such as Worcester, Caughley and Spode. Cobalt was also used for rich ground colours, such as the dark blues of Sèvres and later English porcelain. In glass, cobalt gives the vivid blue associated with Bristol Blue glass, used for decanters, finger bowls and scent bottles. Collectors note that the tone of the blue varies with the source and purity of the cobalt and with firing conditions, from soft greyish shades to brilliant sapphire. Groupings of cobalt-decorated wares create striking displays on dressers and shelves.
