So far as the demand by painters is concerned, this might be as well omitted from the lists of oil colors. But there is quite a demand for it from industrial concerns, where a great deal of metal is to be coated. However, there is no established standard for the material sold under this name, with the exception of specifications issued years ago by the United States Navy Department, which call for a black paint to be composed of natural mineral containing not less than 15 per cent carbon, 5 per cent lead oxide in the pigment, balance to be inert material as occurs in nature with bituminous shale, etc., to be ground in boiled linseed oil. The black fillers mined in Pennsylvania near Muncy form a basis for blacks of this class and also for iron fillers, etc.
What is known and sold to the trade as dry mineral black differs however, from the fillers in question, as well as from that which is specified by the Navy Department. As a rule, it is not a safe pigment to use with oil, as it appears to contain a great percentage of soluble salts, principally iron sulphate, and we would caution grinders to make very close examinations of the material offered before adopting same for use in oil paint.
There are a few other black pigments to which we did not refer because they are practically obsolete, there being no demand for them. These comprise candle black, lead black, manganese black, Prussian black, and prussiate black, and black lake, which latter fades rapid on exposure to light and air. Prussian black is simply Prussian blue blackened by calcination. Prussiate black can only be used as a decolorizing agent, being a residuum from yellow prussiate of potash. Lead black is the black sulphite of lead and has no place in color grinding. Manganese black is simply black manganese oxide.