Iron-manganese crystalline phases developed in high lead glazes during firing

Abstract

Lead glazes have been coloured and decorated with iron and manganese oxides since ancient times. During firing, these pigments react and form new crystalline compounds such as haematite, andradite, melanotekite, kentrolite, braunite, jacobsite and magnetoplumbite which have been identified in lead-glazed ceramics from the 11th to 17th centuries. To determine the sequence of formation of these pigments, an X-ray powder diffraction experiment was conducted at the ALBA synchrotron up to 928°C with various mixtures. The influence of Al₂O₃, CaO, and MgO was also studied. The sequence of crystalline phase formation in different systems was established, and laboratory recreations of glazed ceramics at temperatures from 850°C to 1020°C were analysed using optical and electronic microscopy. Understanding the sequence of formation of iron and manganese crystals in lead glazes helps to elucidate ancient production techniques and the final appearance of the glazes.