1. By blending with the pulp in the beating-engine some insoluble substance, such as smalts blue—a kind of glass coloured by oxide of cobalt—ultramarine, yellow ochre, etc.
2. By adding a coloured liquid, which simply dyes or stains the fibre.
3. By using rags which are already coloured, in proportions to give the required shade, in which case of course the process of bleaching must be omitted.
4. By employing two substances, as yellow prussiate of potash (ferrocyanide of potassium) and a persalt of iron, for example, which, when combined, yield the requisite blue tint—Prussian blue.
By this latter method the buff shade given to what is termed toned paper is effected, by using a solution of copperas (sulphate of iron) and an alkaline solution, or by using a solution of pernitrate of iron. In experimenting in this direction we have found that a mixture of solutions of sulphate of iron and bichromate of potassa produce an agreeable and permanent buff tint. The solutions may be added to the pulp alternately, or may be first mixed and then at once put into the beater. From 2 to 3 ozs. of each salt for each gallon of water may be used if the solutions are to be mixed before using; but when applied separately the solutions may be used in a more concentrated condition.
Colouring Matters used in Paper-Making.—The following substances, used either alone or mixed in suitable proportions, are employed in colouring pulp for paper-making:—
Smalts blue.
Prussian blue.
Indigo blue.
Aniline blues.