Oxygen evolution and chemical state changes of iron during firing of red clay bodies at low oxygen pressure

A red clay body containing 7 mass% Fe2O3 was fired in a mildly reducing atm osphere (Ar-O-2, P-o2= 0.1 kPa) and its blackening and bloating mechanisms were investigated. Reduction of Fe2O3 started at around 1100 degrees C, acc ompanied by the blackening of the clay body. Bloating was caused by the ent rapment of oxygen that was released by iron oxides in closed pores, the for mation of which started at about 1200 degrees C. Fe2+ that was formed in th e clay body during the heating process migrated toward the surface layer du ring the cooling process, resulting in the enrichment of Fe in the surface layer. The results of thermodynamic calculations suggested that oxygen evol ution was caused or at least assisted by the reaction of iron oxides with A l2O3 to form FeAl2O4, a stable spinel.