# Cobalt blue

Cobalt blue is a blue pigment made by sintering cobalt(II) oxide with aluminium(III) oxide (alumina) at 1200 °C. Chemically, cobalt blue pigment is cobalt(II) oxide-aluminium oxide, or cobalt(II) aluminate, CoAl$_2$O$_4$. Cobalt blue is lighter and less intense than the (iron-cyanide based) pigment Prussian blue. It is extremely stable, and has historically been used as a coloring agent in ceramics (especially Chinese porcelain), jewelry, and paint. Transparent glasses are tinted with the silica-based cobalt pigment "smalt".

## Historical uses and production

Ores containing cobalt have been used since antiquity as pigments to give a blue color to porcelain and glass. Cobalt blue in impure forms had long been used in Chinese porcelain. In 1742, Swedish chemist Georg Brandt showed that the blue color was due to a previously unidentified metal, cobalt. The first recorded use of cobalt blue as a color name in English was in 1777. It was independently discovered as an alumina-based pigment by Louis Jacques Thénard in 1802. Commercial production began in France in 1807. The leading world manufacturer of cobalt blue in the nineteenth century was Benjamin Wegner's Norwegian company Blaafarveværket ("blue colour works" in Dano-Norwegian). Germany was also famous for production of it, especially the blue colour works (Blaufarbenwerke) in the Ore Mountains of Saxony.
Cobalt glass is used decoratively, and also as an optical filter to remove or hide certain visible colors.

## In human culture

Art
* Cobalt blue was the primary blue pigment used for centuries in Chinese blue and white porcelain, beginning in the late eighth or early ninth century.
* Traditional Bunzlauer pottery made by Germans from Bunzlau also made an extensive use of cobalt blue glaze. The pigment was used along with white in classic patterns of blue and white dots before the synthetic production of more variously colored pigments (yielding such colors as green, red, orange).
* Watercolorist John Varley suggested cobalt blue as a good substitution for ultramarine for painting skies, writing in his "List of Colours" from 1818: "Used as a substitute for ultramarine in its brightness of colour, and superior when used in skies and other objects, which require even tints; used occasionally in retrieving the brightness of those tints when too heavy, and for tints in drapery, etc. Capable, by its superior brilliancy and contrast, to subdue the brightness of other blues."
* Cobalt blue has been used in paintings since its discovery by Thénard, by painters such as J. M. W. Turner, Impressionists such as Pierre-Auguste Renoir and Claude Monet, and Post-Impressionists such as Vincent van Gogh. It is stable and lightfast and also compatible with all other pigments.
* Maxfield Parrish, known for the intensity of his skyscapes, frequently used cobalt blue, and as a result cobalt blue is sometimes called Parrish blue.
* Cobalt blue is a commonly used color for interior decorating.
Automobiles
* Several car manufacturers including Jeep and Bugatti have cobalt blue as paint options.
Construction
* Because of its chemical stability in the presence of alkali, cobalt blue is used as a pigment in blue concrete.
Sports
* Two Major League Soccer teams have cobalt blue as a secondary color: Real Salt Lake from its inception, and Sporting Kansas City on its home uniforms since 2008.
Vexillology
* Several countries including the Netherlands and Romania, and a U.S. state – Nevada – have cobalt blue as one of three shades of their flags.
Video games
* Sega's official logo color is cobalt blue. Sonic the Hedgehog, Sega's current mascot, was colored to match.
* An example of cobalt blue hue (not pure cobalt blue)
* Pierre-Auguste Renoir, Boating on the Seine (La Yole), c. 1879

## Toxicity

Cobalt blue is toxic when ingested or inhaled. Its use requires appropriate precautions to avoid internal contamination and to prevent cobalt poisoning.