ULTRAMARINE, is a well-known blue pigment of extraordinary beauty and great permanence.
Ultramarine was originally prepared from the lapis lazuli or laznlite. This mineral presents itself in small masses of granular structure, in a rock of heterogeneous struc ture ; this rock, which in commerce is called lapis, sells for a higher or lower price for pigments according to the proportion of lazulite which it is found to contain. The most perfect specimens of lazulite are used as gems ; while the less perfect are used in the preparation of ultramarine. The lazulite is made red hot, quenched, pounded to powder, washed, dried, made into a paste with pure linseed oil and certain resinous substances, kneaded, diffused in hot water, and allowed to settle until the ultramarine (leaving all the other ingredients) falls to the bottom. The whole of these processes require great care.
This colour is now prepared at a very moderato price, and equal in beauty to that obtained from the lazulite. M. Gmelin of Tubingen considers that sulphuret of sodium is the colouring principle both of the natural mineral and the artificial products. The arti ficial ultramarine is stated to be prepared by adding freshly-precipitated silica and alumina, mixed with sulphur, to a solution of caustic soda, and the mixture is to be evaporated to dryness: the residue is put into a covered crucible and exposed to a white heat, by which, when the air has partial access to it, a dark pure bluo mass is obtained. The pro-1 duct is then reduced to impalpable powder. The proportions of materials to be used are, about 30 silica, 3d alumina, 21 soda, and 3 sulphur.