A57Fe Mössbauer study of black coring phenomena in clay-based ceramic materials

The black discoloration sometimes found in the interior cores of iron-containing clay-based ceramic materials was studied in three different samples by X-ray diffraction, elemental analysis and⁵⁷Fe Mössbauer spectroscopy. The primary cause of black coring (locally reducing conditions in the interior of the body resulting from incomplete oxidation of carbonaceous impurities) results in the formation of Fe²⁺, but, contrary to previous suggestions, the presence of discrete lower iron oxides or iron metal was not confirmed in the present samples. Instead, the Fe²⁺) was found to occur as a lattice substituent of the normal products of the high-temperature ceramic reactions (as ferroan pyroxenes, olivines or feldspars, depending on the chemical composition and reaction conditions). Thus, in the present materials, the dark core reflects the deeper colour of these ferrous minerals, which are not present in the oxidized material of the outer rim.