An examination was conducted to determine the mechanism of peeling of fire‐clay brick in the low‐temperature region of a blast furnace where 3 to 10% K2O is the principal contaminant. In laboratory tests, as‐received high‐duty and superduty fire‐clay brick and 70% alumina brick treated with KCl‐K2CO3 mixtures showed no peeling at a temperature of 1600°F. Cracks were found in high‐duty brick that were treated with KCN at 1500°F. under partially reducing conditions. X‐ray diffraction studies of mixtures of crushed brick and K2CO3 indicated the formation of leucite (K2O.Al2O3.4SiO2) and kaliophilite (K2O.‐Al2O3.2SiO2) at temperatures below 1700°F. These latter data, confirmed by specimens from used blast‐furnace linings, showed that silica is the first constituent attacked by alkali. Since the formation of leucite and kaliophilite in fire‐clay brick is the probable cause of peeling, the increased reaction of silica, in a dense Al2O3.SiO2 refractory of higher silica content than fire‐clay brick, should confine the alkali attack to the surface of the brick in low‐temperature applications.