Colour Number: 20
Pigment Details: Cobalt Aluminium Oxide / Colour Index Pigment Blue 28 (C.I. PB28)
Cobalt Blue Professional Watercolour is prepared using the mixed metal oxide inorganic pigment, cobalt (II) aluminate. The pigment has been selected for its purity, light fastness and intense blue hue, making it ideal for use in A J Ludlow’s Professional watercolour range.
Blue pigments derived from cobalt (II) aluminate have been known for centuries; the Chinese used an impure pigment, possibly formed during the firing process, in their “blue and white” porcelain, dating from 14th Century (Yuan dynasty) and on earthenware produced even earlier during the 1st Century (Tang dynasty). The discovery of cobalt blue pigment however, was attributed to Joseph Leithner in Vienna, who developed the basic process of calcining cobalt oxide and alumina (Leithner’s Blue) in 1775 (Pigment Compendium, page 241). Subsequently in 1803, the French Chemist, Louis Jacques Thénard used different cobalt salts in the precipitation process making it easier to obtain the cobalt aluminate pigment. Two processes were developed, one yielded pigment with a deeper blue hue, the other, produced a darker blue. Both shades were known as Thénard’s Blue; a term that was later used to mean simply cobalt blue (Pigment Compendium, page 367). Other names for cobalt blue which reference an inventor, production method or a corruption of the same, include; Gahn’s Ultramarine or Gahn’s Blue, Wenzel’s Blue, Binder’s cobalt blue, Kӧnigsblau (corrupted to Kings Blue*), Leidner and Leyden Blues (corruptions of Leithner’s Blue), New Blue and Dresden Blue.
The crystal structure of cobalt (II) aluminate (and other cobalt mixed metal oxide pigments) form a spinel lattice corresponding to the formula CoO-Al2O3, where the four oxygen anions (4O2-) arrange in a cubic close packed lattice and the metals occupy the octahedral (Co2+) and tetrahedral(2Al3+) sites in the lattice. Much of cobalt blue’s outstanding heat stability and high colour fastness are due to this tightly packed spinel lattice.
Cobalt blue is considered one of the traditional pigments for watercolour (Mayer 1991, page 136). With respect to colour “temperature”, cobalt blue is generally regarded as a neutral blue (where cerulean blue is for example, regarded as cool and ultramarine blue is warm). The brightness and cleanness of hue makes this pigment the best choice for Cobalt Blue in A J Ludlow’s Professional watercolour range.
Eastaugh N, Chaplin T, Siddall R, Walsh V, “Pigment Compendium: A Dictionary and Optical Microscopy of Historic Pigments”, Routledge, Abingdon 2013
Mayer, R “The Artist’s Handbook of Materials and Techniques”, 5th Ed, Viking, New York, 1991.
* It should be noted that although King’s Blue was used to describe cobalt blue and more recently has been classified by Colour Index International as a term for both smalt (which is a cobalt-doped glass) and cobalt blue, the name is also associated with the oil colour that is obtained when cobalt blue or ultramarine blue is mixed with cremnitz white (a white oil colour based on either bismuth oxide or lead oxide).
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