ABSTRACTPhysical defects in the castings, especially “sponginess,” will cause blisters, as will also fanlty composition or application of enamels. There are, however, differences in the tendencies of different sound castings to give blisters when enameled under identical conditions. The gases forming these blisters are CO and CO2.A gray iron casting acquires in freezing and cooling a very thin surface skin, or “microchill,” of varying thickness and hardness. The data indicate that removal of this skin from sound castings eliminates blistering. During the enameling process the combined carbon of this skin tends to break down to a nascent, readily oxidizable form of carbon, which evolves CO and CO2. There are probably two kinds of nonblistering iron, one in which little combined carbon is present at the surface, and another in which it is stabilized. Some irons are more prone to give the “microchilled” layer than others. During the early stages of enameling both blistering and nonblistering irons evolve gas, which is attributed to quick oxidation of submicroscopic graphite and which escapes bcfore the enamel has fused to a retentive condition. Addition of graphitizing agents, such as silicon, may be beneficial, but the microchill is harder to avoid than the ordinary, or macrochill. Removal of the surface layer by deep sandblasting or “burning out” appears to be the most practical remedy for blistering of