Cobalt Blue, Thénard’s Blue, Cobalt Ultramarine, King’s Blue, Leyden Blue.—The pigment known under these names is a compound of cobalt oxide with alumina, and is prepared in a similar manner to Rinmann’s green. Of all the blue pigments used in painting, cobalt blue is the most handsome and most durable. It is completely unaltered by the atmosphere. The most powerful chemical reagents have but a slight action upon it, and their action is less the higher the temperature used in the manufacture. Cobalt blue is made by precipitating mixed solutions of a cobalt salt and alum, washing, drying, and igniting the precipitate. A product of pure blue colour is only obtained when alum absolutely free from iron is used. If but very small quantities of iron are present, the red ferric oxide has a very considerable influence on the shade of the product.

The depth of colour varies according to the proportion of cobalt salt used. The variation is not so considerable as in the case of cobalt green; as a rule, 50 to 100 parts of cobalt sulphate or nitrate are used to 100 parts of alum. The temperature at which the precipitate is ignited must be much higher than that used for cobalt green; it should be raised to a white heat. The crucible should be provided with a well-fitting lid—the fire gases damage the shade of the colour.

When cobalt sulphate is ignited with ammonia alum, cobalt blue is obtained; but this method is not to be recommended. The ignition must be of long duration to completely drive off the sulphuric acid. The author has made a cobalt blue of remarkable beauty in the following manner: Alumina obtained by precipitating electrolytically an alum solution (page 49) was mixed with a solution of pure cobalt chloride to a thin pulp, which was quickly dried in a shallow porcelain dish. The mixture, whilst still hot, was pressed into a crucible placed in a larger one filled with magnesia; the crucible was heated to a good white heat, at which it was maintained for an hour, when it was allowed to cool. The cobalt blue made by this method has the purest blue colour, being without that admixture of red which is not rarely to be perceived in this pigment. To avoid the appearance of this red tint, certain additions are made; a small quantity of a zinc salt is most frequently employed, according to the quantity of which more or less pale shades are produced.

Cæruleum is a pale blue pigment consisting of a compound of the oxides of cobalt and tin. It is made by converting tin by the action of nitric acid into white stannic oxide, adding a solution of cobalt nitrate and evaporating to dryness in a porcelain dish whilst stirring; whilst still warm the powder is strongly ignited. In cæruleum the chemical combination is less firm than in the compounds of cobalt oxide with zinc oxide or alumina, strong hydrochloric and dilute sulphuric acids decompose it, continued treatment with nitric acid extracts all the cobalt. Cæruleum resists completely all atmospheric influences.

The circumstance that all the cobalt in this compound may be removed by treatment with nitric acid leads to the conclusion that the pigments composed of cobalt oxide and another oxide are not chemical compounds of the two oxides, but that the alumina, zinc oxide and tin oxide serve simply as bases for the blue modification of cobalt oxide. That a chemical compound cannot be present follows from the fact that the quantities of the oxides uniting together may be very considerably altered and a coloured product still obtained. The author has shown in 1868 that the blue modification of cobalt oxide is obtained by heating the red. The change of the red colour of cobalt chloride into the blue is not caused, as was formerly thought, by loss of water, but by the transformation of the cobalt oxide contained in the compound from the red into the blue modification. Strong bases are able to retain the cobalt oxide in the blue form, and strongly basic oxides are therefore used in the manufacture of this series of pigments.

Cobalt Zinc Phosphate.—This pigment is made by precipitating a solution of zinc sulphate, free from iron, by sodium phosphate, adding a solution of cobalt sulphate and again sodium phosphate so long as a precipitate is produced. Excess of sodium phosphate is to be avoided, otherwise a considerable quantity of cobalt remains unprecipitated. In the directions given by Gentele it is stated that the sodium phosphate must be in excess, and that the liquid above the precipitate must possess a reddish colour. The author’s experiments have shown that a pigment in every respect equal to that produced by Gentele’s process is obtained when all the cobalt is precipitated. The precipitate, which consists of variable quantities of zinc and cobalt phosphates, is dried and ignited, when it acquires a very deep blue colour.