Factors Influencing Oxidation of Iron During Firing of Ground‐Coat Enamels

Abstract

The oxidation of the iron beneath a ground‐coat enamel was measured by recording the progressive weight change with a sensitive balance on heating a specimen from room temperature to 1550° F. and correcting for the volatile constituents. The permeability of the dry, unfired enamel films was correlated with the variables characteristic of ground‐coat enamel slips. Clays caused the greatest change in the permeability of the unfired films; the ball clays gave lower permeability than the kaolin clays. The results showed that the permeability of the unfired coating was not an index of the amount of oxidation that occurred when the coating was fired on iron. The oxidation studies indicated that (1) a kaolin clay in an enamel slip allowed less total iron oxidation than a ball clay, since with the latter a more permeable coating developed between 900 °F. and the fusion temperature, which is believed to be due to either the organic material or the free‐silica content of the ball clay, (2) the nickel deposited on the iron by nickel flashing caused a decrease in the total amount of iron oxidation, and (3) with low‐temperature ground coats the rate of metal oxidation was the same as with conventional ground coats up to the fusion temperature of the low‐temperature enamel, but owing to the lower fusion temperature the total amount of oxidation was considerably less than with regular ground‐coat enamels.