Gum Arabic and tragacanth are used as binding materials for the pigments. Dextrine is also much used in place of the expensive gums; for this purpose only pure white dextrine should be used, since the brown colour of ordinary dextrine would injure the shade of the colours, especially of pale colours. Gum Arabic and dextrine, which are readily soluble in water, require no special preparation before their solutions are mixed with the pigments. The solutions are made by treating with water; they are then allowed to stand for several days in a tall vessel so that impurities may settle. If the solutions are very turbid, they are filtered through closely woven linen. Gum tragacanth requires rather different treatment. It is not completely soluble in water, in which it only swells up to a great extent. It is prepared by leaving it for several days in water, and when it has swollen, rubbing the slimy mass in a mortar until it is completely uniform. The levigated pigments are allowed to dry in the air to a soft paste, which is mixed with the proper quantity of gum Arabic and tragacanth solutions. As a rule the two gums are used together. The colouring matter and binding medium may simply be ground together, but long grinding would be required to produce a completely homogeneous mixture. The costly manual labour is therefore as far as possible replaced by machinery, by which means a cheaper and also more uniform product is obtained. The machines by which the pigment is ground with the binding material are of simple construction. They consist of rollers placed in pairs one above the other and moving in opposite directions. The two rollers of each pair are connected by cog-wheels, in such a way that the lower moves more slowly than the upper. In consequence of this arrangement, the rollers, in addition to crushing, exert a grinding action upon the viscous mass passing between them. As a rule, after the colour has passed through one pair of rollers, it goes through a second and third, from the last of which it is taken off by a scraper. It is now completely uniform. The binding medium must be of such consistency that, after grinding with the pigment, a fairly stiff paste results which is suitable for pressing. The cakes of colour are pressed out in an ordinary spindle press, which should be so constructed that the stamp in the down stroke comes upon the paste beneath it and stamps out cakes upon which the engraving of the die is clearly shown. If the cakes crack when slowly dried, the medium contains too much gum Arabic. If the impression of the die upon the cakes is not sharp, and if they remain rather elastic when completely dry, too much tragacanth has been used. It is rather difficult to grind a stiff paste, many manufacturers prefer to grind the colour in a rather more fluid condition, and then to allow the paste to thicken by drying to the consistency necessary for the production of good cakes.