The deposit in the settling-tanks is shovelled into wooden boxes, perforated at the sides to enable the water to drain away, the chalk being prevented from escaping by lining the boxes with linen cloths. The pulp soon loses its liquid character and shrinks considerably, the boxes being then filled up with more sludge, and so on until the contents have ceased to shrink. When the mass is so far dry that it will no longer run when lifted, the boxes are covered with boards and inverted, discharging the contents on to the boards, on which the mass is left to become quite dry. Filter-presses are also used.
Large prismatic masses of chalk never dry so uniformly as to prevent the formation of cracks, and if the chalk is to be sold in this form the cracks are plastered up with thick pulp; this operation, however, being superfluous when the chalk is to be sold as powder.
In order to obtain a more compact product and accelerate the drying of the moulded lumps, some makers use presses, in which the fairly dry chalk is subjected to progressive heavy pressure.
Owing to the fineness of the component particles of chalk, they adhere so firmly together, without any bind, that a fair amount of force is necessary to break down a piece of perfectly dry levigated chalk. Sometimes, however, chalk exhibits the unpleasant property of losing its cohesion almost completely when dry, and in such cases it can only be shaped into prisms with great trouble. This peculiarity is specially accentuated when the chalk contains magnesia; and in order to mould chalk of this kind into blocks, a binding agent, such as ordinary glue, must be added to the water used in grinding, care being taken not to use too much, or the chalk will become too hard, when dry, for certain purposes, e. g. as drawing or writing chalk.
For some purposes, chalk is sold in powder form, and very high purity is not then essential, an admixture of magnesia or clay being harmless. Gilders, for instance, use large quantities of chalk for priming picture frames, and stir the chalk up with a certain amount of bind (mostly size), to give the particles the desired cohesion.
The chief requirement exacted of a good quality chalk is a handsome white colour; and this depends entirely on the quality of the raw material, not on the method of preparation. It is known that a substance quite devoid of colour will furnish a perfectly white powder, because the colourless particles reflect the light in all directions without breaking it up into its constituent yellow, red and blue rays. Chalk, too, is in reality a colourless substance, and reflects light with greater uniformity in proportion as the fineness of the particles increases. Consequently, when one has a chalk that is not perfectly white, it can, nevertheless, be made to furnish a very handsome product by bestowing great care on grinding and levigation. Properly prepared chalk should be as fine as the finest flour.
When the colour of the best grades of chalk are compared with what may be termed pure white—such as that of white lead, zinc white, permanent white—a skilled eye will always detect a greyish or yellowish tinge in the former, even if obtained from the whitest Carrara marble.
The grey tinge is due to the presence of organic matter, which cannot be eliminated by any known means, but which can be shown to exist by the fact that when such chalk is heated to incandescence in the air for a short time, the resulting burnt lime is pure white, the organic matter having been burned off. A yellow tinge is caused by minute traces of ferric oxide, which—as also ferrous oxide—almost invariably accompanies calcium carbonate; and limestone free from determinable quantities of these oxides is of rare occurrence. Ferrous oxide does not reveal its presence in limestone unless in large proportion, its pale green colour being of low tinctorial power, whereas ferric oxide, which is a very strong colouring agent, can be more readily detected.
To those who are engaged in the manufacture of white earth colours, however, it is quite immaterial whether a limestone or chalk contains ferrous oxide, because that oxide quickly changes into ferric oxide in the finely divided product, and a chalk which was originally pure white will become decidedly yellow in a short time.
Fortunately, such a yellow-tinged product can be rendered perfectly white by simple means and at small cost, all that is necessary being to add a suitable quantity of a blue colouring matter. When this has been done, the chalk will seem pure white to even the most skilled eye.