Cobalt Yellow (Aureolin) Pigment

Cobalt yellow (commonly called aureolin) is a potassium or sodium-substituted potassium cobalt nitrite. The compound was discovered in 1831 but not introduced as a pigment until after 1851. However, the pigment never gained popularity as an oil color and was much more widely available as a watercolor.

Water-Based Paint
In watercolors, cobalt yellow darkens and fades. It is a transparent, lightly staining, light-valued, intense yellow pigment.

Oil Paint
In oil paint, cobalt yellow is a transparent color with good tinting strength and is considered permanent.

Origin and History

The history of cobalt yellow pigment has been well documented by M. Cornman in 'Cobalt Yellow (Aureolin),' Artists' Pigments. A Handbook of their History and Characteristics, Volume 1, National Gallery of Art (1986). It is generally understood that cobalt yellow was first synthesized by N.W. Fischer in 1831 as part of his study of nitrite salts. However, Fischer did not describe the compound specifically until 1848.

Meanwhile, Saint-Evre, working in Paris, independently rediscovered the compound around 1851 and introduced it as a pigment. It first appeared in Windsor & Newton's catalogs in 1861. According to Gettens and Stout, the pigment was reputedly made popular by the American watercolorist Aaron Penly and the German artist Arthur Mühlberg.

Source

According to Cornman, the principal method of preparing cobalt yellow was to mix solutions of cobalt salts with acetic acid, followed by the addition of a concentrated potassium nitrite solution. Cobalt yellow would then slowly precipitate as a yellow crystalline mass.

Permanence and Compatibility

Cobalt yellow is slightly soluble in water and darkens when mixed with sodium sulfide and caustic soda. In admixtures with organic pigments, such as lakes of cochineal, indigo, etc., it causes the decomposition of these; the organic colors become altered in hue, and cobalt yellow becomes a brownish hue.

Oil Absorption and Grinding

Cobalt yellow absorbs a high amount of oil, about 55 grams of linseed oil per 100 grams of pigment, to make a paste. Some manuals note that the pigment works better as a watercolor than it does in oil, and it is highly valued among moist colors used by artists. According to Uebele, mixing and grinding requires six parts of oil to four parts of the dry color.

Toxicity

Cobalt yellow is considered slightly toxic, and hence, care should be taken when handling the dry powder pigment to avoid inhaling the dust. Soluble cobalt compounds and cobalt metals may have a sensitizing effect. However, none have been reported during many years of handling cobalt yellow pigments.

Rublev Colours Pigment: Cobalt Yellow (Aureolin)