Discussion* on “A Mechanism of Black Speck Formation in Enamel Resulting From Reboiling of the Ground Coat”

Abstract

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