Cobalt Blue

Synonyms:

Cobalt, Cobalt Blue, Bleu De Thénard, Kobaltblau

Excluding smalt, which owes its colour to a cobalt silicate, there are at least three pigments which go under the name of 'cobalt' or 'cobalt blue.' The best known of these is a combination of alumina and cobalt oxide; then comes Leithner's or Thénard's blue, a cobalt phosphate on an aluminous base; lastly, there is an aluminous cobalt arseniate very much like the phosphate.

The original and simplest form of cobalt blue, or Wenzel's blue, may be made by calcining strongly an intimate mixture of aluminium hydrate and cobalt oxide. A better way consists in moistening freshly-precipitated aluminium hydrate with a solution of cobalt nitrate, drying and then strongly igniting the mass. It may likewise be prepared by precipitating a solution of sodium aluminate by means of cobalt chloride solution. Thénard's blue may be prepared by mixing about 8 parts of aluminium hydrate with 1 part of cobalt phosphate, both in the moist condition, then drying and strongly calcining the mixture; cobalt arseniate may replace the phosphate. Another variety of Thénard's blue is obtained by adding sodium phosphate solution to a solution of alum containing a little cobalt sulphate. In all the above methods, the complete freedom from iron and nickel of the materials used is essential to the purity and beauty of the blue pigment formed.

The cobalt blues work well in all media. They are unaffected by light, moisture, and oxygen. The best samples of them are practically permanent even in impure air, but ammonium sulphide tends to discolour them. If they appear changed in hue in any oil-paintings, the yellowing of the admixed or overlying oil or varnish must be regarded as the cause. Cobalt blues may be used in fresco-painting; they are unaffected by commixture with other pigments. Cobalt blues do not lose their colour when boiled with alum solution, nor when treated with moderately strong acids.

Cobalt blue as an oil-colour is usually ground with about three-fourths its weight of linseed or poppy oil.

Cobalt blue examined optically is found to reflect much green and violet light as well as blue. Viewed by candle or gas light it acquires a very marked purplish hue. Burton's cobalt, originally prepared for the use of potters, is of fine quality and of more than usual stability.

The introduction of cobalt blue to the palette of the artist may be said to have created a revolution in the style of painting, especially obvious in water-colour landscapes.