BLUE, as a pigment. The substances used for this purpose are of different natures, and derived from various sources : they are all com pound bodies,some natural and others artificial.
Prussian Blue.—This beautiful pigment was discovered in 1710 by Diesbach, a manufac turer of Berlin. It is a compound of cyanogen and iron, is inodorous, tasteless, insoluble in water, alcohol, tether, and oils. It is hygro.. metric, attracting water strongly from the air, which it retains until heated to nearly 280°. Prussian blue is employed both as a water colour and in oil ; in the latter case, on ac count of the deficiency of what is termed body, it is usually mixed with white lead, and it will bear admixture with a large portion of this on account of the intensity of its colour. Its. stability is very considerable, and it is not only used as a pigment, but also as a dye.
Indigo.—This fine blue is extracted from different species of indigofera in the East Indies, and Guatimala in North America, of which the latter is most esteemed. [INDIGO.] Blue Verdiler.—This pigment is used as a water-colour, and chiefly in the manufacture of paper-hanging. It is a gritty powder of a very fine light blue. It is a carbonate of copper.
Ultranutrine.—This splendid and permanent blue pigment was originally, and indeed until within a few years exclusively, prepared from a mineral called Azure Stone, or Lapis Lazuli ; but it is now prepared by artificial means. It has hitherto, on account of its high price, been used almost exclusively by artists, both as a water-colour and in oil ; but on account of the reduced charge at which it will pro bably he hereafter obtained, it will doubtless be rendered much more extensively useful.
Cobalt Blue.—This was proposed as a sub stitute for ultra-marine before the invention above described had rendered this latter colour easily obtainable at a moderate price ; it is of great permanence, but is not so fine as the ultra-marine, and will hereafter be probably little employed.
Small is a blue colour also prepared from cobalt, buds generally used rather to diminish the yellow tint of paper and of linen, and to give a bluish colour to starch, than strictly speaking as a pigment; it is merely glass rendered blue by oxide of cobalt, and this, when reduced to a very fine powder, is commonly called powder blue.