_Characters._--Being virtually a hydrated silicate of alumina, china clay is a remarkably stable pigment. Not only is it unaffected by prolonged exposure to strong light and impure air, but is insoluble in water, weak acids, and alkalies. It is moreover very much lighter in weight than any other white pigment, an advantage on the score of cost when buying by weight. Its covering power in distemper work and as a water colour is good; but the addition of oil reduces its capacity. The best qualities are exceedingly fine in grain and pure in tint, but inferior samples sometimes have their yellowness “corrected” by the addition of a little ultramarine. ENAMELLED WHITE.--Another name for the finest kinds of baryta white, see p. 170. ENGLISH WHITE.--A synonym for whiting, see p. 246. GYPSUM.--This very common and abundant mineral is a hydrated sulphate of lime, occurring in several forms, of which only the opaque white variety is useful as a pigment. The native mineral is quarried, dressed, ground, and levigated, in all which operations there is nothing special to be noted. Whether obtained in this way, or prepared artificially, or formed as a bye product in other industries, gypsum affords a permanent and neutral white pigment, mixing well with oil or water, and possessing a covering power which ranks between white lead and zinc white. It has a bluish tint, but less so than ordinary white lead. KAOLIN.--One of the names applied to china clay, see p. 172. LEAD WHITES, OR WHITE LEADS.--On the grounds of the quantity in which it is produced and the extent to which it is applied, probably no pigment can compare with white lead, including in that term the various white pigments having lead as a basis. In its commonest form white lead is lead carbonate. There are many ways in which it is made commercially, all dependent upon certain chemical reactions. When a solution of normal plumbic acetate is attacked by carbonic acid, no precipitate is produced. That normal solution is formed by the action of acetic acid or hydric acetate upon oxide of lead. It consists of a certain weight of lead to a certain weight of acid, which converts it into the acetate. Carbonic acid has no power to separate out from it the lead, and form carbonate of lead. But this acetate of lead has the power of dissolving a considerable quantity of oxide of lead in addition to that which was used in its first formation, and when this additional quantity of oxide of lead is dissolved by the acetate, a substance is formed which is termed a basic acetate, that means an acetate which contains more of the base (the lead oxide) than the normal acetate itself does. From such a solution we are able to precipitate, by means of carbonic acid, a white substance, which white substance is a carbonate of lead.