Cobalt blue
Composition and Properties of Cobalt Blue
Cobalt blue (Thénard’s blue) is a mixed oxide of cobalt and aluminum CoO • Al2O3. It can also be considered to be cobalt (II)-aluminate with the formula of CoAl2O4.It is very stable and is unaffected by heating and by concentrated acids and alkalis. The pigment is practically unaffected by light and is compatible with all other pigments.
Names of Cobalt Blue
Thénard’s bluePB 28, CI 77346From 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.
Preparation of Cobalt Blue
Attention: Cobalt compounds are highly toxic and should not be handled by people not trained to do so.Cobalt blue can be prepared by heating a mixture of cobalt (II)-chloride CoCl2 · 6H2O and aluminum oxide Al2O3.
History of Use
Cobalt blue has been used since its first preparation by Thénard in 1803-04 and it is still one of the most popular blue pigments. The following graph gives the frequency of its use in the paintings of the Schack Collection in the Bavarian State Art Collections in Munich (1).An extensive collection of occurrences of this pigment in paintings from several historical periods can be found in the blog post ‘Pigment: Cobalt Blue, the 19th-century sky‘ by The Eclectic Light Company.References(1) Kühn, H., Die Pigmente in den Gemälden der Schack-Galerie, in: Bayerische Staatsgemäldesammlungen (Ed.) Schack-Galerie (Gemäldekataloge Bd. II), München 1969.Examples of use
