Moulding
The colour pulp can be made into tablets by moulding it in dry boxes divided into a number of compartments. The colour shrinks in drying, and the tablets will then easily fall out of the moulds. Cones are obtained by placing the pulp in a box, the bottom of which is perforated with numerous holes of uniform size, the box being then tapped against the surface of a stone table. At each stroke, a certain amount of colour is forced, in the shape of small cones, through the perforations, on to a sheet of paper underneath. The cones are then dried.
Some colours are moulded into blocks by forcing the partly dried paste into suitable moulds—preferably of metal, so that they may be engraved with the maker’s name, or other imprint—and left to dry slowly and without cracking. The cakes may be prevented from crumbling by incorporating a small quantity of adhesive, such as a weak solution of dextrin, with the water in which the colour is suspended.
CHAPTER IV
WHITE EARTH COLOURS
The white earth colours are important for the purposes of the colour-maker, because not only are they used by themselves as paints, but also serve in the production of light shades of other colours.
The white colours containing clay or lime are the most abundant and important of all, and will therefore be described first. The lime colours comprise caustic lime, carbonate of lime (chalk or powdered limestone), gypsum and bone ash.