The characteristics of glazes are too frequently regarded as the only important factor governing the resistance of glazed ware to thermal shock crazing. I n this investigation it was found that thermal shock crazing was caused, in some cases, by craze-like cracks which developed in the bodies. The importance of selecting suitable bodies, as a means of controlling crazing, became evident a t once when it was found that bodies with a high resistance to thermal shock were not so liable t o craze when glazed and subjected to sudden temperature changes as those with a low resistance to thermal shock. I n general, glazed ware on which the glazes are under fairly high compression are more resistant to thermal shock crazing than those on which the glaze is under either low compression or tension. Glazed ware which is susceptible to thermal shock crazing may be resistant to moisture crazing and vice versa.
The procedure of casting glass pots has been described by several contributors to THIS JOURNAF.~ It is the pqrpose of the present paper to call attention to several points of practical importance. The characteristics of a casting slip as well as of the fired body depend, first, upon the composition-the content of grog, kaolin, ball clay, and feldspar; second, the sizing of the grog; third, the peculiar physical and chemical qualities inherent in primary kaolins as differentiated from the secondary ones, and in ball clays against the fire clays; fourth, the amounts and proportions of sodium silicate and carbonate of soda. The following mixtures have been used successfully in the Pittsburgh Laboratory of the Bureau of Standards for the casting Mixture No. I .
I n order to determine whether as good results could be obtained from mixtures of American clays as from the Gross Almerode clay in glass pot bodies, two American ball clays were combined in various proportions with two silicious American bond clays and compared with the Gross Almerode clay in bodies containing 50% grog. Physical properties and resistance to glass batch corrosion were determined. I t was not found possible to duplicate the physical properties, especially the low water of plasticity and low drying shrinkage of the Gross Almerode body but several of the bodies gave better results as regards resistance to corrosion than the one containing the Gross Almerode clay. In general it was concluded that it is possible by blending American clays to produce a body equal or superior to the Gross Almerode body. More thorough heat treatment of pots or tank blocks in the arches and greater uniformity in temperature during the burning is required.
Microscopic examination has shown that the bubbles always expand downward into the iron oxide layer. It has been found that iron oxide lowers the softening point of an enamel and it is believed that the viscosity of this layer is much lower than that of the outer layer. Rust spots will cause dark spots in a ground-coat enamel when present in small amounts.Microscopic examination of these spots shows that they are free of bubbles. This is evidence that the viscosity of enamel containing iron oxide is lower than the iron oxide free enamel. I t is also true that examination of many enameled specimens has shown that bubbles rarely occur in the iron oxide layer of a normally fired ground coat. The reboiling bubble invariably touches the iron oxide layer at the beginning of the expansion and always expands downward into the iron oxide layer.
V. SummaryA mechanism for the formation of black specks in enamels from reboiling of the ground coat has been shown to be a fracture with the subsequent formation of large bubbles in the enamel glass.No evidence has been found that gas evolution from the metal causes reboiling. A consideration of all the experimental evidence and microscopic examinations leads to the conclusion that gas evolved from the metal is not a major cause of reboiling.The prevention of reboiling will depend on the control of the factors which produce the stresses in the enamel coat.
AMERICAN ROLLING MILL Co
MIDDLETOWN. OSIIO
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