Cobalt blue is a blue pigment made from one of a number of combinations of cobalt, aluminum and oxygen. Cobalt pigments produce a cool blue color, and were favored by painters such as Maxfield Parrish for painting skies. It is also an important traditional pigment in Chinese porcelain.
Cobalt blue pigment consists either of cobalt(II) oxide-aluminum oxide or cobalt(II) aluminate. These pigments are slightly different in their chemical composition, but both are the product of finely ground cobalt oxide and aluminum oxide, or alumina, being joined by a process known as "sintering." Pigment manufacturers grind the two substances, mix them together, and subject them to intense heat to bond them.
Cobalt blue is the main pigment used in distinctive Chinese blue and white porcelain, known as qing-hua or "blue flower" porcelain. The earliest known examples of this type of pigment date from the seventh century in China, although examples of cobalt-based blue pigments are known from ancient Greece, Egypt and the middle east. Porcelain manufacturers imported cobalt from the middle east to make pigment. To decorate porcelain, potters first made the vessels themselves, then applied decoration by hand before glazing. Chinese blue-and-white pottery pigments are made from smaltite, a form of cobalt oxide, whereas most modern cobalt-based pigment uses cobalt aluminate.
Although Chinese porcelain had been made using cobalt blue pigment for centuries, cobalt blue emerged in Europe independently. Medieval European glassmakers added small amounts of smaltite to quartz and potassium carbonate to make the ingredients for a dark blue glass known as smalt. Although it produced a deep blue color in glass, smalt-based pigments were unsuitable for painting because of their tendency to fade over time.
In 1802, French chemist Louis Jacques Thénard discovered a slightly different form of the pigment, which became popular among painters. Mines in Norway and Germany produced the majority of Europe's cobalt, and these countries were known for their cobalt pigments. This pigment, based on cobalt aluminate, is the pigment most commonly referred to as "cobalt blue."
European miners had been aware of the existence of cobalt for some time. The name "cobalt" comes from the German word "kobold," which is the name of a type of mischievous goblin. Cobaltite and smaltite are common cobalt ores which are very difficult to work with. They contaminate other ores, can be difficult to smelt and can release highly poisonous dust. Due to the "mischievous" natures of these ores, they were named after the pesky underground goblins.