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.
