When a tin lake is employed in the preparation of a sap colour, the precipitate obtained by adding stannic chloride to the solution of the colouring matter is washed, and, without drying, treated with a small quantity of strong ammonia; generally about 10 per cent. of the volume of the precipitate is sufficient to dissolve the lake. It is always better to leave a portion of the precipitate undissolved than to modify the colour by the addition of too much ammonia. The precipitate should be dissolved in a glass vessel; the liquid is well stirred, covered over, and allowed to stand until the undissolved portion of the precipitate has settled. The ammoniacal solution of the colouring matter cannot be concentrated by evaporation. Thus, before the precipitate is dissolved it must be well freed from water by long draining. To give the solution the proper consistency about an equal quantity of thick gum solution is added, and then sufficient starch to form a paste which can be rolled into thin sticks, which are then dried upon boards at a gentle heat. Sap colours are made in exactly the same way from any other colouring matter adapted to the purpose. In using these colours it is simply necessary to bring them into water; if they contain only gum and sugar in addition to colouring matter they completely dissolve, but if they contain starch the solution is incomplete. As a rule starch is only added to give the colour the consistency requisite for printing.
Red sap colours can be made either from red wood lake or from cochineal carmine; the former is used in ammoniacal solution, the latter as a solution of the pure carmine in ammonia. The colour is prepared from red wood by allowing a decoction to stand for several days, adding 2 to 3 per cent. of alum, evaporating and thickening with gum solution. This colour is cheap, but cannot compare in beauty with that obtained by treating red wood tin lake with ammonia. The solution of genuine carmine in ammonia produces a sap colour which leaves nothing to be desired in regard to shade, but on account of the cost of carmine it can but rarely be used. A solution of pure indigo carmine mixed with gum solution is used as a blue sap colour; it can be shaded by additions of red or yellow colours. Chinese blue may be used instead of indigo carmine; it dissolves in a solution of oxalic acid. When Chinese blue, which has been thoroughly washed, is mixed whilst still moist with a little saturated oxalic acid solution, the blue dissolves, and the solution may then be made into a paste by the addition of thickening materials.
Green sap colours are made, as a rule, by mixing yellow and blue. All shades can be thus obtained. A green colour of different composition is obtained by boiling the violet solution of chrome alum with gum solution until the colour changes to green. Violet colours are obtained by mixing red and blue. Sepia produces a brown sap colour.
It has been proposed to use glucose in place of the much dearer gum Arabic, and also to use malt extract as thickener. These substances absorb water from the air, and thus the colours prepared with them never become quite hard, but always remain pasty. For some purposes this is a decided advantage, since such colours can be readily taken up by the brush and easily rubbed up with water, but they cannot be formed into cakes as is usually done with water colours. When the colours are to be made into cakes they must be thickened with gum Arabic alone, without glucose or malt extract. The cakes must be dried with great care; if they are dried too quickly they will crack or even fall to pieces.
CHAPTER LXVI.
WATER COLOURS.
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.