Aureolin (sometimes called cobalt yellow) is a pigment used sparingly in oil and watercolor paint. Its color index name is PY40 (entry 40 in the list of yellow pigments). It was first produced in 1831 by Nikolaus Wolfgang Fischer in Breslau under the name of “Doppelsalze” or double salts and its chemical composition is potassium cobalt nitrite. He recharacterized it and wrote about it in more detail in 1842, calling it “cobalt oxide nitrosate of potash”. In 1851-1852, Edouard Saint-Evre independently synthesized cobalt yellow. He is credited with introducing cobalt yellow as a pigment to artists. Gates’ study provides the exact modern processes used to produce halo, as well as methods for identifying it in paints. Halo is considered permanent in some reports, but there are other sources that consider it unstable in oil but stable in watercolors. Others find it unstable in water colors, fading towards grayish or brown tones. It is a medium-intensity yellow pigment, transparent, slightly colored, with a luminous value. It is quite an expensive pigment and is sold by various oil color manufacturers such as Grumbacher, Michael Harding and Holbein. However, pigment has never been as popular as oil paint and is far more widely available than watercolor from manufacturers such as: Winsor & Newton, Talens Rembrandt, Rowney Artists, Sennelier, Art Spectrum and Daniel Smith.