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.