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.