In chemical composition, true chalk is calcium carbonate, but of a fossil character, for if chalk dust be examined under a high-power microscope, it will be seen to consist of the shells of minute animals, and is therefore to be regarded as fossil. The organic matter of the animals has long disappeared, leaving the inorganic material, a very pure calcium carbonate, behind.
Fig. 27.
Such progress has been made that the zoological status of the animals which inhabited the shells—many thousands of which are present in a lump of chalk—has been identified; and it is known that these animals were of marine type. [Fig. 27] shows the appearance of the animal remains in Meudon chalk when highly magnified, the upper half being viewed by transmitted light and the lower by reflected light.
Notwithstanding the extremely minute dimensions of the chalk animalculæ, their remains form rocks of great thickness in all parts of the world. In Europe we find, for example, extensive chalk formations in England, whose Latin name Albion was bestowed on account of the white chalk cliffs occupying long stretches of the coast. The hills of Champagne consist almost entirely of chalk; and Rügen, together with many other islands, is nearly all chalk cliffs.
It is only in very rare cases, however, that chalk occurs in sufficient purity to be immediately suitable for use as a pigment or writing-material. For the most part it contains other minerals, or large fossils, from which it has to be separated by mechanical treatment. Nodular flints are often met with in chalk, and many deposits contain such large numbers of the petrified shells of the sea urchin that the chalk really cannot be used as a pigment at all, by reason of the high cost of purification. The only places where chalk can be advantageously worked for the preparation of pigment is where the mineral is in a high state of purity, and also contains only very few sandy particles. Such chalk deposits are worked on a mining scale, and, as a rule, in the state in which the chalk comes from the quarry; it is in the form of a soft mass, easily scratched with the finger-nail and of fairly high density, owing to the considerable quantity of water with which it is ordinarily impregnated.
In order to convert this crude chalk into a product that can be used as a pigment, it is first left to dry until the lumps can be easily broken, and then crushed into small pieces, from which all the extraneous minerals, which occur as large lumps, are sorted out and removed. This picking process is important, especially when the chalk contains flints, because these latter are very hard and would injure the millstones in the subsequent grinding.
The lumps of chalk are reduced by mechanical means, such as a stamp-mill, or, more frequently, in a mill of the same type as for grinding flour, since it is impossible to get the lumps so dry as to produce the degree of brittleness necessary for a thorough reduction in a stamp-mill. The millstones are enclosed in a wooden casing, and the chalk is ground in admixture with water, the ground mass escaping, through an opening in the casing, as a thick pulp which is stored for a considerable time in large tanks.
Experience has shown that this method of prolonged storage in contact with water greatly improves the colour. The only explanation of this fact is that the chalk still contains a very small amount of organic matter, which gradually decomposes in presence of water. The evidence in favour of this is the peculiar smell given off during storage.
Even with the most careful grinding, chalk cannot be transformed into such a fine powder that is directly fit for all purposes; and the only way to obtain the requisite fineness is by levigation. Owing to the large quantities that are usually handled in this process, the milky liquid coming from the mill is mostly run into large brick tanks, where it is left to settle until all the chalk has deposited and the supernatant water is perfectly clear. Tapping-off being usually impracticable, the water is generally drawn off by careful syphoning, so as not to disturb the fine sludge at the bottom of the tank.