Gas evolved from all delayed enamel defects was found to be substantially pure hydrogen. By the method of microscopic‐gas‐bubble analysis, gas from individual reboiling blisters in ground‐coat enamel was shown to contain more than 84% of hydrogen. The water‐iron reaction is an ever present source of hydrogen in enameling. The water is supplied by the dried enamel slip, not only as structural water in clay, but also as part of the glass structure of the frit. Clays containing high percentages of structural water and having high decomposition temperatures tend to increase the production of gas‐formed defects. Enclosed‐system experiments revealed that hydrogen is absorbed by steel‐enamel interfacial material at 500°C. Similarly an enamel containing 3.0% of Sb 2 O 3 will absorb hydrogen very readily at 500° to 700°C. A study was made of gas evolution in the heating of enamel slip or frit separately or with steel. Heating the dried frit with steel produced hydrogen. The roles of water and hydrogen in the enameling process and the mechanism of the formation of reboiling blisters and delayed defects are discussed.
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