Pigments.
(A) Added to the pulp in the form of mineral in a finely divided state.
Yellow.—This colour is obtained by the use of ochres, which are natural earth colours of varying shades, from bright yellow to brown.
Red.—Ordinary red lead.
Various oxides of iron, such as Indian red, Venetian red, red ochre, rouge.
Blue.—Smalts—An expensive pigment prepared by grinding cobalt glass.
Ultramarine—A substance of complex composition prepared by heating a mixture of china clay, carbonate of soda, sulphate of soda, sulphur, charcoal, and sometimes quartz, rosin and infusorial earth.
Prussian Blue—A compound prepared by adding potassium ferrocyanide to a solution of ferrous sulphate.
Brown.—Natural earth colours, such as sienna, umber, Vandyke brown.
Black.—Lamp-black, bone-black, Frankfort black.
(B) Produced by the reaction of soluble salts upon one another when added to the pulp in the beating engine.
Yellow.—Chrome Yellow—The paper pulp is first impregnated with acetate of lead, and potassium or sodium bichromate added. This precipitates the chromate of lead as a yellow pigment.
Chrome Orange—The addition of caustic alkali to the bichromate solution converts the chrome yellow into an orange.
Blue.—Prussian Blue—The paper pulp impregnated with iron salts is treated with potassium ferrocyanide. The blue colour is at once obtained.
Brown.—Iron Buff—A light yellow-brown colour due to the precipitation of ferrous sulphate by means of an alkali.
Bronze.—Manganese chloride followed by caustic soda.