Derby red possesses great covering power and considerable brilliancy; but if not very carefully washed it is liable to retain a little alkali, which renders it unstable. Otherwise, it well resists damp, strong light, and impure air so long as sulphuretted hydrogen is absent. Taken altogether it is not one of the best red pigments, and its consumption is declining.
Indian Red.—This is one of the names for the red pigments due to oxide of iron, and is described under oxide red, p. 150.
Lead Orange.—Equally well known as chrome orange, this pigment may be regarded as a Derby red in which the reactions have been curtailed. That is to say, the yellow normal lead chromate being in excess, the red chromate formed by the action of the alkali combines with that excess of the yellow salt and forms a yellow-red, i.e. orange. Obviously, therefore, a great variety of tints can be produced by altering the proportions of the alkali, and this is further regulated by the duration of the boiling, while the tint can also be weakened by admixture of barytes or gypsum. The better kinds of lead orange are prepared with the aid of caustic potash or soda as the alkali, while the cheaper sorts depend on lime. The operations are practically identical with those adopted in the case of Derby red (see [p. 145]), the chief differences lying in the proportions of the ingredients. Thus:—
(1) Pale.—Add a thin cream made from 10 lb. of quicklime to a chrome yellow made from 100 lb. of lead acetate, 30 lb. of soda or potash bichromate, and 21 lb. of soda sulphate. Boil.
(2) Pale.—Add a thin cream of 10 lb. of quicklime to a chrome yellow made from 200 lb. of baryta sulphate, 100 lb. of lead acetate, and 35 lb. of potash bichromate. Boil.
(3) Deep.—Precipitate a chrome yellow by adding 35 lb. of soda or potash bichromate to 100 lb. of lead acetate; settle. Draw off supernatant liquor and admit solution of 9 lb. of caustic soda at 77 per cent.
(4) Deep.—Add a cream of 10 lb. of quicklime to a chrome yellow made from 100 lb. of lead acetate, 75 lb. of baryta sulphate, and 35 lb. of potash bichromate. Boil.
In characters the lead oranges resemble Derby red (see [p. 145].)
Minium.—The important red pigment known as minium or red lead is composed of two oxides of lead in combination, viz. about 65 per cent. of protoxide and 35 per cent. of binoxide. In its preparation, metallic lead is first converted by roasting into protoxide (termed “massicot,” “dross,” or “casing”) and this protoxide is further subjected to heat in a reverberatory furnace whereby a portion of it is changed into binoxide. It is also possible to produce red lead by the decomposition of the carbonate of lead (white lead) at a high temperature, but this does not seem to be an industrial process. The following methods are recognised:—
(1) The practice in France, as carried on near Tours, at the white lead works using the Thénard process, is to calcine the best metallic lead in reverberatory furnaces built in the rock. These furnaces are five in number, with double fireplaces, four being constantly in operation, dealing with about 4000 lb. at a charge, and using bituminous coal as fuel. Each furnace is nearly circular in shape and about 11 feet in diameter, with a fire-place on each side of the hearth. The latter is constructed of fire-brick containing as little silica as possible, and is made hollow so as to retain the metallic lead when the heat has rendered it fluid.