Some boulder clays contain limestone in the form of gravel or as a coarse powder produced by the crushing of larger fragments. These are less suitable for manufacturing purposes as the lime produced when the articles are burned in the kilns is liable to swell and to disintegrate them on exposure.
Owing to their origin and the nature of the impurities they contain, boulder clays are never pure and when burned are irregular in colour and somewhat fusible unless subjected to some process of purification.
SOME CLAYS OF COMMERCIAL IMPORTANCE
Although clays occur in deposits of almost all geological periods, many of them are of little or no commercial value. This may be due to their situation or to their composition and other characteristics. Thus, a Coal Measure clay is ordinarily quite inaccessible, and to sink a shaft specially to obtain it may be an unprofitable undertaking; if, however, a shaft is sunk for coal the clays in the neighbourhood of the coal seams are rendered accessible and, usually, a certain amount of such clays is brought to the surface in order to remove it out of the way of the coal miners.
Again, a clay deposit may be so far removed from human habitations as to make it practically valueless, but if, for any reason, the population of the district in which the clay is situated grows sufficiently, the clay may become of considerable value. It not infrequently happens, therefore, that the commercial importance of a clay deposit is one which fluctuates considerably, yet, in spite of this fact, there are certain kinds of clay which are nearly always of some commercial value. The most important of these are the kaolins (china clays), the pottery and stoneware clays, the refractory clays (fireclays), the brick and terra-cotta clays and shales, and the clays used in the manufacture of Portland cement. The origin and manner in which these clays have been accumulated have been described in the previous chapters; it now remains to indicate their characteristics from the point of view of their commercial value.
Commercial china clays and kaolins in the United Kingdom are not simple natural products but, in the state in which they are sold commercially, have all been subjected to a careful treatment with water, followed by a process of sedimentation whereby the bulk of the impurities have been removed. According to the extent to which this treatment has been carried out, they will contain 10 per cent. or more mica and quartz, with little or no tourmaline, felspar and undecomposed granite. In some parts of Europe and America, kaolins are found in a state of sufficient purity to need no treatment of this kind unless they are to be used for the very highest class of wares.