On raising the temperature of firing or on prolonging the heating at the previous maximum temperature the viscosity of the fused portion is diminished and crystallization may then occur. The facility with which crystallization occurs varies greatly with the composition of the fused material, those silicates which are rich in lime and magnesia crystallizing more readily than those containing potash or soda. Vogt has stated that small quantities of alumina promote the formation of a glassy structure, and Morozewicz has shown that a large excess of this substance must be present if crystallization is to occur.
The study of the reactions which occur when clays are heated is, however, extremely complex, not only on account of the variety of substances present, but also on account of the high temperatures at which it is necessary to work, so that for a further consideration of it the reader should consult special treatises on the fusion of silicates. This subject has now become an important branch of physical chemistry.
CLAY AND ASSOCIATED ROCKS
Clay, as already mentioned, is geologically a rock and not a mineral, and belongs to the important group of sedimentary rocks which have been derived from the igneous or primary ones by processes of weathering, suspension in water and subsequent deposition or sedimentation.
Whatever may be the primary origin of clay, its chief occurrence is in geological formations which have undoubtedly been formed by aqueous action. The materials resulting from the exposure of primary rocks to the action of the elements have been carried away by water—often for long distances—and after undergoing various purifications have been deposited where the speed of the water has been sufficiently reduced.
In some cases they have again been transported and re-deposited and not infrequently clay deposits are found which show signs of subsequent immersion at considerable depths and have every appearance of having been subjected to enormous pressures and possibly to high temperatures.
Some clays have only been carried by small streams and for short distances; these are seldom highly plastic and resemble the lean china clays and kaolins. Others have been carried by rapidly moving rivers and have been discharged into lakes or into the sea; they have thus undergone a process of gradual purification by elutriation, the sand and other heavier particles being first deposited and the far smaller particles of clay being carried a greater distance towards the centre of the lake or the quieter portions of the ocean. The nature of such deposits will, naturally, differ greatly from each other, the materials at first associated with the clay, or becoming mixed with it at a later stage, exercising an important influence on its texture, composition and properties. If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without destroying the nature of the clay. In addition to this it must be remembered that the land is continually rising or sinking owing to internal changes in the interior of the earth, and that these subterranean changes bring about tilting, folding, overturning and other secondary changes, which, later, cause a fresh set of materials to be mixed with the clays. Further than this, the action of the weather, of rivers and of the sea never ceases, so that a process of re-mixing and re-sorting of materials is continuously taking place, and has been doing so for countless ages. It is, therefore, a legitimate cause for wonder that such enormous deposits of clays of so uniform a character should occur throughout the length and breadth of Europe, and practically throughout the world. For although the composition of many of these beds is of a most highly complex nature, the general properties such as plasticity, behaviour on heating, etc., remain remarkably constant over large areas of country, and the clays of each geological formation are so much alike in different parts of the world as to be readily recognized by anyone familiar with the material of the same formations in this country. Considerable differences undoubtedly exist, but these are insignificant in comparison with the vastly different circumstances under which the deposits were accumulated.