THE NATURE OF THE GLASS PHASE IN HEATED CLAY MATERIALS* I. COMMON CLAYS†

Abstract

ABSTRACTThe progress of fusion in an alluvial clay, a glacial clay, and a shale, when heated at various temperatures, was determined with the aid of the petrographic microscope. Thin sections and powdered samples obtained from the heated clay specimens were used in these tests. The relationships between the amounts and kinds of glasses in the specimens and the properties of the glasses were determined. Shrinkage and porosity of the specimens were found to depend not only on the temperature at which glass formation started but also on that at which the glass became sufficiently fluid to allow contraction of the specimen to take place. The colors of the heated clay specimens were governed largely by the crystallization of the glass bonds. Solution of the crystals formed by devitrification also profoundly affected the colors of the specimens. The increase in intensity of the red color of the specimens of alluvial clay and shale was directly related to the increase in the number of red crystals produced in the glass phases. The loss of red color by these specimens heated at very high temperatures in an oxidizing atmosphere and also at lower temperatures in a reducing atmosphere was caused by solution of the red crystals. The glassy bond in the specimens of glacial clay differed in composition from that in the alluvial clay and shale, which prevented the development of a red color of the specimens. A black glass, formed on solution of the red crystals, produced a dark gray color in the overheated specimens and in those heated in a reducing atmosphere. The growth of fine‐grained green crystals in the glacial clay specimens was a factor in neutralizing the small amount of red color originally in these test pieces.