Microscopy and microanalysis of crystalline glazes

Crystalline glazes on ceramic plates produced commercially in the UK and on ceramic pots produced commercially in Taiwan and Spain have been examined by X-ray diffraction, conventional and polarized light microscopy, and scanning electron microscopy in order to identify the crystalline phases present in the glazes and to ascertain through X-ray microanalysis the partitioning behaviour of the transition metal ions used to colour the glazes and the crystals within them. In each case examined, the macroscopic two-dimensional spherulites within the glazes clearly seen by the naked eye were found to consist of large numbers of radially orientated acicular crystals each 5 micro m or less in width embedded within the silica-rich glaze. Energy dispersive X-ray microanalysis and X-ray diffraction of these crystals identified these crystals as willemite, alpha-Zn2SiO4. The strong [001] texture of these crystals within the glaze evident from the X-ray diffraction patterns was consistent with polarized light microscopy observations of the willemite crystals. In addition to willemite, small iron-doped gahnite (ZnAl2O4) crystals were found in a honey-coloured crystalline glaze and acicular rutile (TiO2) crystals were found in the Portmeirion Pottery plates examined. Transition metal ions with a preference for tetrahedral coordination were observed to substitute for Zn2+ ions in willemite and to partition preferentially to the willemite crystals, whereas ions preferring octahedral coordination preferred to remain in the glaze.