Chalk
The name chalk is used for a number of commercial substances which differ considerably in both the mineralogical and chemical sense. French chalk, for instance, is a mineral belonging to the steatite group and, apart from its name, has nothing in common with true chalk, except the white colour, and even this differs altogether from that of chalk properly so called. It is therefore necessary, in the interests of proper nomenclature, to differentiate the various kinds of chalk, commencing with the mineral known by that name to the chemist and mineralogist.
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.
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.
This result of adding a blue pigment is based on the well-known physical fact that certain kinds of coloured light produce white light when combined, the colours that give this effect being termed “complementary.” A pure blue is complementary to a yellow with a reddish cast—e. g. ferric oxide—and therefore a chalk that is tinged yellow by a small quantity of ferric oxide can be changed into a seemingly pure white substance by the addition of a blue pigment.
The only pigments of use in this connection to the colour-maker are such as have very intensive colouring power and at the same time are low enough in price. Such substances are ultramarine, smalt and coal-tar dyes. Smalt is the best because its colour is unalterable. In point of chemical composition, this substance is a very hard glass coloured blue by cobaltous oxide. For improving the colour of chalk or any other white, the smalt must be in an extreme state of fine division, and levigated to an impalpable powder. Ultramarine can be used for the same purpose, but is not so permanent.
To ascertain the correct proportion of blue pigment, it is advisable to make a systematic experiment, which is easily performed. Exactly 90 parts of the chalk in question are triturated with 10 parts of blue pigment in a mortar until the entire mass has become a perfectly uniform pale blue powder, which contains 10% of the blue ingredient.
Several samples, each representing one hundred parts of the white pigment to be corrected are carefully weighed out, 1 part of the blue powder being added to the first sample, 2 parts to the second, 3 to the third, and so on, and the mixtures are compared with a standard white substance, such as best white lead or zinc white, to see which most nearly approaches the standard colour. It is then easy to calculate how much of the blue requires to be added to 100 or 1000 lb. of the material to be corrected.
The correction can be effected in several ways; for instance, by grinding the blue pigment directly with the bulk, by adding it at the levigation stage, or mixing it with the dry, finished product. The first two methods are attended with certain drawbacks which render it difficult to obtain a perfectly uniform product, owing to the specific gravity of the blue pigments being higher than that of the whites. Consequently, when the two are mixed in presence of water—as is always the case in grinding and levigation—the heavier blue pigment settles down more quickly, and several strata can be clearly distinguished in the sediment. The upper layers will still have a decided yellow tinge—the proportion of blue being too small for proper correction—whilst the next in order will be pure white—accurately corrected—and those at the very bottom will be decidedly blue, because they contain the largest proportion of the blue substance.
The most satisfactory results are obtained by dry mixing; and this can be successfully practised when the colour-maker has a cheap source of power (such as water power) available. Where, however, costly power plant has to be provided, only the finest grades of white pigments can be improved in this way, the expense of labour being too high for cheap materials.
As a pigment, chalk possesses many valuable properties. The organic structure of chalk gives it high covering power as a wash, a thin layer applied to a surface sufficing to mask the colour of the underlying ground completely. The lime in chalk being combined with carbonic acid, its basic properties are so extensively weakened that chalk can be mixed with even the most delicate colours without fear of their shade being affected. A coating of pure chalk paint on any surface will never change colour in the air; and on this account, chalk is extensively used both as an indoor wash and by wall-paper manufacturers.