In the whole range of pigments there is no more important class than those to be described in this chapter. Not only are the white pigments largely employed for the sake of their distinctive colour, but they are probably even more extensively applied as a basis of other pigments, both as an ingredient in the composition of the other coloured paints and for ground coats where the final coat is to be of a delicate shade. They are among the cheapest and most permanent pigments, and possess as a whole remarkably good covering powers. BARYTA WHITE.--Barytes or sulphate of baryta, the most important of the salts of barium, is found native in large quantities, forming the species of mineral termed barites or barytes, and commonly known as heavy-spar, on account of its weight (sp. gr. from 4·3 to 4·7). It is found in Derbyshire and Shropshire, and often occurs in fine tabular crystals. The massive variety found in the mountain limestone of the above counties is sometimes called “cawk”; it is more frequently found in white or reddish-white masses. In Saxony it occurs as the mineral _stangen-spath_, in a columnar form; and at Bologna, a nodular variety is found, called Bologna stone, which is notable for its phosphorescent powers when heated. The pure salt may be prepared artificially for use as a pigment, by adding dilute sulphuric acid to a solution of chloride of baryta, when a white precipitate is formed; this is well washed and dried. It is a heavy, white powder, insoluble in water and nearly insoluble in all other menstrua. It may also be prepared by heating the native mineral, grinding it to powder, and well washing it, first in dilute sulphuric acid, in order to remove any traces of iron, and afterwards in water; the white powder is afterwards thoroughly dried. This process is employed at several works in the neighbourhood of Matlock Bath, in Derbyshire, but much larger quantities could be produced in different parts of the country if the demand for the article rendered its production more profitable. The principal use of sulphate of baryta is to adulterate white lead, and to form the pigment known as _blanc fixe_, or permanent white. For these purposes, the native mineral, ground and washed as described above, is commonly employed.