NOTES ON TIIE MICROSCOPIC STIIUCTURE OF CERTAIN PAVING BRICK CLAYS. AT VARIOUS STAGES OF FUSION.
In the hope of explaining some of the phenomena of simultaneous decrease in volume, porosity and specific gravity without loss in weight and to obtain some idea of the manner in which fusion takes place in a vitrifying brick, microscopic sections were prepared from briquettes of two paving brick clays.
Thin sections of the briquettes burned at a low temperature exhibit under the microscope a very fine-grained fragmental ground mass, or matrix, in which are imbedded crystalline and other fragments which were present in the original clay. From these materials are developed, at high temperature, amorphous glasses and crystals.
The cavities between the particles of a brick may be divided into two classes: (1) Pores, which are present in pieces fired at low temperatures, due to the incomplete consolidation of the clay. These are the original interstitial spaces of the unburnt clay.
(2) Blebs or bubbles, which are formed in the glass at higher temperature by the liberation and expansion of gases.
Pores of the first sort are of small size and irregular outline. As the temperature increases, and the material of the matrix gradually fuses into glass, these interstitial spaces tend to disappear.
Cavities of the second sort, which we may for convenience designate as blebs. are simply gas bubbles in glass. They are circular in outline and vary greatly in size. They are not present in the bricks burned at lower temperatures. but appear only after the formation of considerable glass.
R3-14—This briquette was drawn at cone 3 or about 1190°C. The color is red. Under the microscope, the earthy matrix or ground mass is dark brown, the color being due to the presence of iron oxides.
The mineral fragments are quartz, feldspar and mica, named in the order of their abundance. They are angular in outline, the thin edges being sharply defined.
Glass has formed to some extent throughout the ground mass and in a few instances it has separated out into clear transparent masses, in several of which blebs appear. The blebs. however, are so few and so small that the
cavities may be considered as made up almost entirely of pores of the first class. As estimated under the microscope. the porosity is 1.9 per cent.
R3-16—Drawn at cone 5, or approximately 1230°C; color dark brown. Under the microscope the ground mass appears somewhat denser and darker than in R 3-14. The quartz fragments are apparently unchanged. The feldspar fragments, however, have disappeared). Mica is present, but in very small quantity.
Glass has been formed in considerable amount. It appears in clear trans parent areas, often 0.1mm. in diameter. In some of the glass, needle-like Crystals have begun to form, but where free from these the glass is color less. This fact would seem to indicate that but little iron has entered into its composition.
As stated above, fine needle-like crystals are often present, imbedded in the glass. They do not appear to have any definite arrangement with re spect to each other, but occur singly or in dense masses. When viewed singly they are colorless, but when seen in masses, they possess a greenish yellow tint, which they impart to the glass in which they are imbedded. What the crystals are was not determined.
The iron oxides• present in the matrix have become segregated into dense masses, which, where they transmit light at all, show the red of hematite, but no definite crystals are to be seen. Pores of the first class have .dis appeared, and blebs In the glass have become numerous and large, their average diameter being 0.066 mm. The estimated pore space has increased to 4.2 per cent.
R 3-18—Drawn at cone 7, or 1270°C. The fragments of quartz appear un changed. The earthy ground mass is rapidly fusing into glass, which has increased greatly in amount over that in the preceding slide. The fine needle like crystals are also present in greater number.