C. Freitag recommends for the production of zinc white the use of retorts of oval section, _A_, Fig. 24. These retorts, containing crude zinc, are heated to a white heat, and then a mixture of generator gas from coke and air is introduced by _B_ and the pipe running through _B_. The zinc burns completely in the flame of the generator gas, which contains excess of oxygen. A product of faultless nature is said to be obtained in this way. In zinc works zinc white is always made in the manner which has been described. It may also be obtained as a by-product in another metallurgical operation, the desilverisation of lead by Parkes’ process. In this process an alloy of silver and zinc is obtained. By sending a current of superheated steam over the molten alloy the zinc decomposes the steam, hydrogen and zinc oxide being formed. The zinc white is carried by the current of gas into chambers, in which it deposits. The zinc white obtained by burning zinc is, as has been said, a very fine pure white powder, which can at once be used for paint without further preparation. The price of zinc white is rather higher than that of white lead, but the difference is counterbalanced by the greater covering power of the zinc white. Ten parts by weight of zinc white completely cover a surface for which 13 parts of white lead are required. Whilst white lead cannot be mixed with many pigments, such as those which contain sulphur, zinc white may be mixed with all without fear of alteration. Zinc white is even better than enamel white for producing pale pigments from lakes; it has a lower specific gravity than enamel white, so that the mixture with the light lake can be more easily made. Zinc grey, which is produced by some works, is zinc oxide discoloured by metallic zinc. Pure zinc white always has a pure white colour; if it is tinged with grey it is contaminated by metallic zinc, whilst a brownish hue denotes the presence of cadmium oxide. The latter impurity will be rarely met with in commercial samples, since cadmium is worth much more than zinc. Zinc oxide is used by the colour maker in the preparation of Rinmann’s green; it is also used, as stated above, as an addition to other pigments. =Griffith’s Zinc White.=—This pigment, which is equal in covering power to white lead, consists of zinc oxysulphide. It is obtained by precipitating a zinc solution with a solution of barium sulphide, washing, drying, igniting and grinding the precipitate. As it contains sulphur, it should not be mixed with copper or lead pigments. =Tungsten White= (Lead Tungstate), PbWO₄, is obtained as a heavy powder by precipitating a solution of sodium tungstate with lead acetate and treating the precipitate, which consists of basic lead tungstate, with dilute acetic acid, by which lead oxide is dissolved and a salt of the above composition left. This white pigment is dearer than other lead pigments, and has no special advantages over them; it is, therefore, seldom used.