Cobalt yellow
Composition and Properties of Cobalt Yellow
Cobalt yellow is potassium cobaltinitrite K3[Co(NO2)6].The stability of this pigment was subject to discussion since its first use. It has been reported to be fairly stable in watercolor use and not suitable for oil painting (1). The pigment is lightfast in watercolor paintings and is compatible with all inorganic pigments but should not under any circumstances be mixed with organic pigments.
Names of Cobalt Yellow
AureolinPY 40, CI 77357Cobalt: From the German word Kobold meaning evil house spirit. The name was originated by medieval miners who believed, that cobalt ores made it difficult to extract silver from silver ore.Aureolin: from Latin auereus = golden
Preparation of Cobalt Yellow
The pigment can be prepared by mixing an acidic solution of a cobalt salt with a concentrated solution of potassium nitrite. The precipitate is then filtered, thoroughly washed and dried (1). The description of the first synthesis of this pigment by N.W. Fischer can be found in his original article in German (3). A thorough investigation of the methods of preparation was conducted more recently by G. Gates (2).
History of Use
The pigment was discovered by N.W. Fischer in 1831 (2) and again independently rediscovered by Saint-Evre in 1851. It was not used broadly due to its unsatisfactory properties and high price (1).
