Kaolin, Pipeclay
Large areas of the earth’s surface are covered with clay, which often attains a considerable thickness. Nevertheless, the kind of clay that is suitable for use as pigment is comparatively scarce. The principal requirement for this purpose is a pure white colour, but by far the great majority of clays are either yellow or of a shade between blue and grey (for example the clay of the Vienna basin).
The character of clay is just as varied as its colour. In some places, large deposits of extremely fine clay are found, the material, when mixed with water, forming a highly plastic mass which, when dried and subjected to slight pressure, furnishes a very soft powder. On the other hand, some clays are so interspersed with large quantities of sand, large stones and the debris of mussels, that they cannot be used until they have been put through very careful mechanical treatment.
This great divergence in the physical character of clays is due to their method of formation. Clay originated in the weathering of felspar, which chiefly consists of a double salt, a compound of the silicates of alumina and potash. Under the influence of air and water, this compound is decomposed, the potassium silicate passing into solution, whilst the aluminium silicate, being insoluble in water, is carried away by that medium. When the water can no longer carry the particles of aluminium silicate in suspension—for example when it reaches a sea or lake—the silicate settles down to the bottom, and a deposit of clay is formed.
If the original felspar was very pure, and in particular very low in iron, the resulting clay will be of a handsome white colour. An example of this is afforded by kaolin, or porcelain earth, which is preferably used for making china. If, however, the felspar contained a considerable proportion of ferric oxide, the resulting clay is yellow; and if stones or mussel shells became incorporated with the clay prior to deposition, these bodies will be found as inclusions in the deposit, and such clay will require much troublesome preparation—grinding and levigation—before it is fit for use.
For the purposes of the colour-maker, the most suitable clay is one that is pure white, free from inclusions, and does not change colour when exposed, in a finely divided state, to the action of the air. Many clays that were originally white gradually assume a yellow tinge on prolonged exposure to air and moisture, because the clay contained ferrous oxide, which changes, in the air, to the stronger pigment, ferric oxide.
Many kinds of clay merely require a simple levigation to fit them for use as pigment. The lumps of freshly dug clay are placed in large tanks, etc., filled with water and stirred up continuously in order that, instead of forming a plastic mass which is very difficult to distribute in water, the particles detached from the lumps may pass at once into suspension. This turbid water is then transferred to another tank, etc., where the minute particles of clay are allowed to settle down, and the water becomes quite clear.
Where this work is carried on on a large scale, it is advisable to put the freshly won clay into large pits close to the clay deposit, and to leave it there, covered with water, during the winter season. The freezing of the water breaks down the larger lumps of clay, by the resulting expansion, and this facilitates the subsequent levigation, the cohesion between the particles being destroyed.
If the clay contains larger proportions of lime or magnesia, a little experience will enable their presence to be detected at once by the way the clay behaves on being placed in contact with water. Pure clay quickly forms a fatty and extremely plastic paste, and sticks closely to the tongue when applied in the dry state. On the other hand, clay containing much lime or magnesia is far less plastic when mixed with water, and the dry clay hardly adheres to the tongue at all.
These latter clays are classed as poor or lean, in contrast to the fat, plastic kinds. For certain purposes for which clay is used as pigment, these admixtures are not harmful; whereas others, especially quartz sand and mica, not infrequently present in white clays, constitute a serious drawback.
As already mentioned, clay is formed by the weathering of felspar, which is a constituent of granite and gneiss, both rocks composed of quartz, mica and felspar. When the clay has been derived from the weathering of such rocks, it is easy to understand that it may contain admixtures of quartz and mica, which are frequently visible to the naked eye, or at any rate under the microscope. Whereas clay forms a white, amorphous mass, the grains of quartz sand are decidedly crystalline, transparent and of vitreous lustre; the scales of mica, on the other hand, appearing as thin tabular crystals, mostly of a green or brown colour and exhibiting, when viewed at certain angles, a brilliant metallic sheen.
Quartz sand can be eliminated from clay without any special difficulty, quartz being of higher specific gravity and therefore settling down quickly, leaving the delicate particles of clay in suspension in the liquid. The scales of mica are harder to get rid of, their tabular form retarding deposition from the suspending liquid; and on this account, several washings are often required to separate them completely.
In all cases where clay is to be used as a white distemper, the presence or absence of lime is immaterial; but where it is to be employed for removing grease, lime is a drawback. This is also sometimes the case when the clay is wanted for the purposes of the colour manufacturer. The author has found, by experience, that perfectly pure, white clay forms a good paint, in a vehicle of oil or varnish—a purpose to which it has, so far, been seldom applied, if at all. Such paint is of good covering power, and possesses the valuable property of remaining quite unaffected by atmospheric influences.
If, however, the clay contains even but a small quantity of lime, it cannot possibly be used as an oil or varnish paint, for though the freshly made paint has a very good appearance, its character soon changes, turning viscous and suffering a considerable diminution of covering power. Thinning with turps or boiled oil results in the formation of small lumps, so that it is quite impossible to obtain a uniform coating on even a small surface.
This behaviour is apparently due to the presence of the lime, the explanation being that the fatty acids always present in the oils and varnishes used for the paint combine with the lime to form compounds which, from the standpoint of the chemist, must be regarded as soaps. The small lumps already mentioned really consist of lime soap, and the formation of these colourless compounds accounts for the lessened covering power.
Given a fine white clay, otherwise capable of forming a valuable pigment, it is sometimes possible, by simple means, to eliminate accompanying lime, provided the amount of the latter is not too great, and also provided that very cheap hydrochloric or acetic acid is available. The acid need not be pure, and the impure but very strong pyroligneous acid, which is very cheap on account of its empyreumatic smell, may be used.
To eliminate lime from the clay, the still moist levigated mass is introduced, in small quantities, into a vat containing the requisite quantity (see later) of hydrochloric or acetic acid, the addition being continued until the liquid gives only a faintly acid reaction with blue litmus paper. When the clay is run in, effervescence is produced by the liberation of the carbon dioxide displaced by the stronger acid employed.
The amount of lime present in a clay may be determined by very simple means. A small sample of the clay is dried by artificial heat, until of constant weight, and exactly 100 parts by weight of the dry mass are placed in a glass and suffused with hydrochloric acid, sufficient of the latter being used to make the liquid still strongly acid after effervescence has ceased.
The contents of the glass are transferred to a filter, and washed with pure water so long as the washings continue to redden blue litmus paper. The residue is then dried until of constant weight, and the difference between the initial and final weights will give the percentage of substances soluble in hydrochloric acid.
After performing this simple test on a clay, it is easy to calculate the quantity of acid needed to extract all the soluble constituents from a given weight of the material. All that is necessary is to measure the volume of acid required to extract a small quantity of the clay completely. Thus, if one pint of the acid at disposal is sufficient to treat one pound of the clay, the amount needed for a given quantity of clay is a simple matter of calculation.
Since, on account of the cost of pure hydrochloric acid, crude acid will always be used, it will be necessary to remember that this crude acid always contains ferric oxide in solution—this being the cause of its yellow colour. If the amount of acid taken is barely sufficient to combine the whole of the lime, leaving the latter slightly in excess, the ferric oxide—which would otherwise tinge the clay yellow—will be precipitated.
If, on the other hand, the acid is in excess, the clay is obtained free from all constituents soluble in the acid. The purified clay must then be freed from the calcium chloride, formed by dissolving the lime, by a thorough washing, since the clay would otherwise always remain moist on account of the hygroscopic properties of the chloride in question. Moreover, any small residuum of free acid would constitute a drawback on the clay being mixed with other colours.
Calcium chloride is very soluble in water, and therefore can be completely removed from the clay by washing. The purified clay is left to settle down as completely as possible, and after drawing the liquid off from the sediment, the latter is suffused with pure water and left to settle once more. As a rule, two such washings will cleanse the clay of calcium chloride and free acid sufficiently to render the product suitable for any purpose.
When large quantities of clay have to be treated in this manner, considerable amounts of calcium chloride solution will be obtained, which can be advantageously utilised for the production of precipitated chalk, all that is necessary being to collect the liquor in a large tank and treat it with a small quantity of slaked lime, to transform the surplus free acid into calcium chloride and precipitate the ferric oxide present in solution. At the end of a few days the liquor in the tank will consist of a very pure solution of calcium chloride which will furnish an excellent precipitated chalk when treated in the manner already described under that heading.