Its preparation consists in hand selection to exclude the silica which, occurs in the more pronounced form of flints, then grinding in several stages, levigation and drying. The levigation is effected by having a series of settling pits into which the ground material flows with water, and deposits according to its degree of fineness. The drying is performed in chambers provided either with pipes carrying steam or heated air, or by fires beneath the floor, and thoroughly ventilated so that the moist air can escape as fast as it is saturated. Finally the dried whiting is again ground very fine. The drying must be done with great care and at a low temperature, so as to ensure avoiding calcination, whereby the carbonate of lime is changed into oxide (quicklime). Whiting is a permanent and useful pigment mixed with water in distempers, but is not applicable as an oil colour. ZINC WHITES.--Originally and properly the term zinc white was reserved for the white pigment consisting of zinc oxide; but latterly many kinds of white pigment have been introduced containing a large proportion of sulphide of zinc, sometimes associated with more or less oxide, and sometimes without any oxide, and these are also by many people called zinc whites, to which name they are perhaps as well entitled as the original zinc oxide. It will therefore be convenient to arrange them all under the same general heading of zinc whites. (1) _Oxide._--Under the influence of a white heat metallic zinc is volatilised, and if the vapour is thus brought into contact with oxygen, either in the pure state or as air, combustion takes place, and the oxygen unites with the metal to form zinc oxide. On this very simple principle is based the manufacture of zinc oxide white. The operation is conducted in plant similar to that shown in Fig. 28, which consists essentially of two departments, that in which the zinc is volatilised and that in which the oxidised vapour is deposited for collection. The volatilising process takes place in a series of oblong fire-clay retorts _a_, varying somewhat in form but always with a contracted and rising neck. Ordinary dimensions are about 2 feet long and 9 inches in diameter each way, with walls about 1½ inches thick. These are heated to whiteness and then charged with ingots of metallic zinc. The retorts are arranged in double rows in reverberatory furnaces _b_, two furnaces being arranged back to back so as to economise heat. The furnaces are fired at the side, and the heat is conveyed around the retorts by means of the flues _c_, the products of combustion of the fuel finally escaping by the chimney stack _d_. [Illustration: Fig. 28.--APPARATUS FOR MAKING ZINC OXIDE.]