THER3IO-CHEMICAL AND PHYSICAL CHANGES DURING FUSION.
It is indeed very difficult, if not impossible, to determine what the actual thermo-chemical reactions really are, which take place in the fusion of the clay particles, first between themselves, and, secondly, when the whole mass becomes a more or less homogeneous solution By the aid of the microscope, as will be seen later, more can he told con cerning these changes in an unknown mixture of minerals than by any other means ; inferences from artificial and known mixtures being of no avail. The effect of thermo-chemical reactions, however, can be detected by the changes in porosity and specific gravity. Because of our present inability to ascertain in full the reactions that take place, it seems best to refer to the chemical phases of fusion as "changes" instead of "reactions." The greater portion of the constituents of our clays being mineral substances, many of which do not entirely lose their identity in the burning of clay wares, it is most natural that these should exhibit in nature the same changes when treated separately that they do when heated together in clays. Botha gives the following description of the physical changes in minerals on melting: - The alterations in the minerals and rocks above cited are those in duced when they are changed by melting, from a crystalline to an amorphous condition. Such complete changes as this cannot be per mitted to take place in the whole mass of clay ware during burning, and vet, as will be shown, the percentage of decrease in specific gravity of many of our clays from the unburned to the vitreous stage is greater than that given in the above data. This being true, it is evident that .
there are factors other than the alteration of minerals from the crystal line to the amorphous condition that affect decrease in the specific grav ity of clays.
In the following fable are given data which show the effect of heat on physical structure of briquettes made from various clays: It was a surprise to learn that bricks will decrease in volume with out loss of weight, and at the same time decrease in specific gravity. Had the clay been carried to complete fusion, i. e., to a glass, the de crease in specific gravity would have been credited to the phenomenon as in the case of minerals, i. e., the changing of its constituents from crystalline to amorphous forms. But in the case of a clay briquette, a small portion of which enters into the • fusion, decreasing in specific gravity before the minerals have been rendered amorphous, i. e., fused to a glass or even before vitrification has been completed, cannot be explained wholly on this basis. Mr. C. H. Wegemann, of the geological department, was, therefore, requested to make a microscopic study of briquettes of two different clays burned at different temperatures. His report follows: