When sulphur yellow chrome is to be made, the process is substantially the same as for the preparation of the lemon shade, but with the difference that everything is done to prevent the swelling of the precipitate. The precipitation and washing of the precipitate are done as quickly as possible: the washed precipitate is filled into press bags and strongly pressed. Care must be taken in the pressing that the pressure is only gradually increased; if powerful pressure is applied at once, even the strongest cloths will be burst. The more thoroughly the precipitate is pressed, the closer will be the fracture of the chrome, a property which is regarded as a sign of good quality in this species of chrome yellow. The manufacture of chrome yellow is intimately connected with that of a number of colours, varying from orange to dark red, which are known under the names of chrome orange or chrome red. In accordance with the division adopted of pigments according to their colour, chrome orange and chrome red will be considered among the red mineral pigments. CHAPTER XIII. LEAD OXIDE PIGMENTS. Lead monoxide, PbO, exists in two different modifications; in the crystalline form, as litharge, which is generally pale yellow with a reddish tinge, and amorphous, as massicot, which is yellowish red. Lead monoxide is a product of metallurgical works rather than of colour works; still the preparation of the crude oxide falls in the domain of the colour maker, since from litharge several pigments may be prepared by a simple treatment. Massicot is obtained by heating white lead, lead nitrate or red lead, and also by heating melted lead in the air, with the precaution that the oxide formed does not itself melt. Litharge is obtained as a by-product in several metallurgical processes, such as the cupellation of lead containing silver, in which the lead is melted in a furnace with a shallow hearth and a powerful current of air blown over the melted metal. The lead is oxidised, the oxide melts at the high temperature approaching 1000° C., and flows through an orifice in the wall of the furnace, whilst the silver remains on the cupel. The litharge is ground and levigated and, according as it is pale yellow or reddish, sold under the name of silver or gold litharge. Both massicot and litharge have no particularly striking colour; they are seldom used as pigments. Litharge has an extensive use in oil boiling and for the manufacture of lead peroxide, which is used for matches. =Red Lead, Minium.=—Lead forms a number of other oxygen compounds, one of which, red lead, has the composition, Pb₃O₄. It is a bright red powder, used as a pigment and as a constituent of certain cements (for gas and water pipes). Red lead is, like litharge and massicot, a metallurgical product, but, by working on a small scale, products can be obtained of a much brighter colour than the produce of the large scale. [Illustration: FIG. 25.]