DEHYDRATION.
Nature of process—Pure kaolin, the basic clay substance, contains in round numbers 14 per cent of water, chemically combined. At ordin ary drying heats the amount of this chemically combined water in the kaolin is supposed to be unaltered. In fact, there is experimental evi dence to support the belief that there is some water mechanically re tained by the clay even at the highest heat ordinarily attained in any dryer, but this has no relation to the chemically combined water.
Since, however, in the ordinary clay or shale but a fractional part of the whole is kaolin, ranging from a possible 100 per cent in the purest varieties down to 25 per cent or less in the more impure clays, it is not surprising that the amount of chemically combined water varies greatly in the different clays. Even in the_ purest it varies to some extent, amounting in some cases to more than 14 per cent. In these not rare cases some other hydrous minerals are supposed to be present that carry a higher percentage of combined water. It is aside from our purpose to dwell upon the kind and nature of the hydrous minerals that may occur in clay except to note that, if they occur in the purest types of clays, and especially those which have not been moved from their place of formation, it is reasonable to suppose that in a heterogeneous mixture of minerals such as shales seem to be, these highly hydrous minerals may in some cases be present in considerable quantities. Since each hydrous mineral substance retains its chemically combined water with a tenacity peculiar to itself, it follows that the period of dehydration of clays will vary with each variation in quan tity and kind of hydrous minerals present. Likewise the physical alteration in the mass at this period will vary with each variation in kind and quantity of hydrous minerals present. Since, however, it is impossible to gather reliable data as to the mineralogical constitution of the impure clays, the quantities of these hydrous minerals present must be mere speculation. The varying effects produced during the period of dehydration, which probably originate in variable mineralog ical composition, are the only known or determined facts in the case.
From the foregoing considerations it is not surprising that the tem perature of dehydration has been considered as ranging from 550 to 650° centigrade (990 to 1170° Fahrenheit), and that there are clays which can withstand a heat treatment of 16 hours duration at a tem perature which will average during this period at least 650° C. without entire loss of plasticity.
Six clays (K 5, H 16, K 8, K 13, K 14, K 15) after subjection to a heat treatment 'supposedly sufficient to affect complete dehydration, slaked down in water to a red plastic mass similar to that produced from hard shale on weathering. If it is true that on dehydration clay loses the properties that cause the mass to exhibit plasticity then these clays were not dehydrated. If clays that have been subjected to just sufficient heat treatment to cause their complete dehydration still retain consid erable plasticity, then many will have to change their conception as to the cause of plasticity, for surely nearly, if not all, of the physical properties of the kaolin particles must be altered by dehydration. These six clays tested continued to lose weight after this period. This loss may possibly be accounted for by the loss of volatile matter other than chemically combined water. In the absence of analytical data, however, it was fair to assume that this additional loss was in part at least due to the further expulsion of the chemically combined water. If this as sumption is .correct, these cases would indicate that the usually allotted range in temperature for this period is altogether too limited. If a clay can withstand heat treatment for 16 hours at a temperature that ranges from 500° to 740° C., with an average equal to 650, without complete loss of its combined water, it is fair to conclude that the max imum temperature limit for the dehydration period is above 700° C.