Loss due to Constituents other than Combined Water—The actual loss in weight of a clay, aside from loss of the chemical water, up to this temperature may in part, according to Prof. Orton' be, accounted for as follows: Vegetable tissues, such as roots, leaves, etc., ignite and burn at about 300°C.
Bituminous matter, common to 'shales, ignites and burns between 300 and 400°C.
Graphitic carbon, does not ignite much before 500°C.
Sulphur distils from iron pyrites between 400 and 600°C.
Calcium carbonate decarbonizes between 600 and 1000°C.
Ferrous carbonate decarbonizes between 350 and 430°C.
The loss of any or all these constituents would not materially affect the plasticity of clay, and in the main these reactions would be com pleted before or at the same time as dehydration. In saes of K 14 be fore referred to, they had all evidently been completed before the com pletion of dehydration, except perhaps the decarbonation of the small amount of contained calcium carbonate. The bricks were thoroughly oxidized and normal salmon-colored throughout. In this case the only possible conclusion seems to be that dehydration of clay requires more heat than heretofore supposed.
It has been demonstrated by Hopewood' that, aside from the loss of combined water, solid carbon, carbonic acid gas, sulphur, etc., quite a large variety of acids and basest are expelled from the clay by vola tilization at temperatures below the maximum required for complete dehydration. The evidence given to Hopewood's experiments, together with the vast accumulation of data by agricultural chemists, makes it appear as though the adsorbed as well as the absorbed salts are seriously affected during this period. Direct evidence is not at hand that would throw light on this question, but the value of such evidence is con sidered by the writer to be of such importance that an extensive re search dealing with this subject has been outlined. It is anticipated that the manner in which this period of burning (dehydration) is conducted will be found to play a very significant role in the character of the ware developed in "the finishing heats."