The colours thus called are so prepared that they easily mix with water to such a condition that they can be applied with the brush. Special pigments are not required for this purpose, ordinary dry colours are simply mixed with a binding medium soluble in water, and the paste is generally pressed into cakes. In commerce very different qualities of water colours are found, ranging from cheap children’s playthings to the most costly colours used by painters in water colour. Those pigments which are not already in very fine powder, in consequence of the process by which they are made, such as chrome yellow, must be subjected to a very careful process of levigation. To simplify as far as possible this laborious process, it should be conducted by grinding the pigment as finely as possible, well stirring it in a tub with water, allowing to rest for several minutes, and then running off the liquid into a second tub, from which it is again drawn off into a third after several, minutes. The liquid remains in the last vessel until it is quite clear, when the deposit is collected. This is now so fine a powder that a powerful microscope is required to distinguish the separate particles. The residue in the first and second vessels consists of the coarser particles; it is again ground with fresh material. In this process regard must be given to the specific gravity of the material: the higher it is the shorter is the time during which the liquid is allowed to remain in the tubs, since the coarser particles of heavy substances settle very rapidly. In dealing with the light lakes, such as the alumina lakes, the liquid in which the lake is suspended must remain much longer at rest. Two vessels may be used instead of three, without danger of the precipitate containing coarse particles.

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.

The moulds in which the cakes are made must be very carefully worked in metal, so that the description and trade mark print clearly upon the cakes. The cakes should be dried at the ordinary temperature or but little higher. They are placed upon smooth boards, and care is taken that the temperature of the drying room remains uniform, which is the condition requisite for the production of the fewest cracks. When it is intended to place on the market only faultless and well-stamped cakes they must be sorted when dry, and the cracked ones rejected; these can be worked up in the next operation.

The dry cakes are given a good appearance by coating them with a weak solution of gum, and then drying. According to the price at which these colours are to be sold the cakes are given a different character. The finest colours are generally made into larger cakes and packed in handsome boxes, whilst ordinary cheap colours are made into small lumps or circular plates, flat on one side and somewhat convex on the other, and packed in boxes of soft wood.

Moist Water Colours.—Instead of grinding water colours with gum Arabic or tragacanth and bringing them on the market in the dry state, they may also be sold in a condition resembling that of oil paints. This may be accomplished by using very viscid glucose syrup instead of gum Arabic, and grinding the pigments in this exactly as in oil. Glucose is very hygroscopic, the colours prepared with it remain moist, and may be spread out upon the palette like oil colours. It is then only necessary to wet the brush in water and mix the mass with it in order to obtain colour of the proper consistency.

Moist water colours have also been known as ”honey colours,” since this mixture of sugars was formerly used for their preparation; it also is hygroscopic. Honey is no longer used; the much cheaper glucose answers the same purpose. On account of their semi-fluid nature, moist water colours are put up in tubes just as oil colours. They are thus rather dear, but are little used.