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. Duringfiring, 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 the11th to 17th centuries. To determine the sequence of formation of these pigments, an X-ray powder diffractionexperiment 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 estab-lished, and laboratory recreations of glazed ceramics at temperatures from 850◦ C to 1020◦ C were analysed usingoptical and electronic microscopy. Understanding the sequence of formation of iron and manganese crystals inlead glazes helps to elucidate ancient production techniques and the final appearance of the glazes.