Thin-section petrography and SR-µXRD for the identification of micro-crystallites in the brown decorations of ceramic lead glazes

The Mn-and Fe-rich micro-crystalline precipitates formed in the decorations of lead glazes during ceramic firing are responsible for colour, shine and opacity, among other visual characteristics, besides being the most important relics of the technology. Due to their small size and growth habits, the micro-crystalline precipitates may remain unidentified on polished cross sections of the glazes. Thin-section petrography, combined with synchrotron-radiation X-ray micro-diffraction, appears to be a very powerful tool to reveal the presence, growth habits, distribution and nature of the micro-crystallites. This is illustrated here by two case studies directly related to the brown decorations technology: the 19th century French imitations of the Ligurian Taches Noires pottery from the Joques workshop and the 17th century Catalan-decorated tablewares. Melanotekite, cristobalite and hematite micro-crystallites were found in the glazes of the French imitations. Hematite acted as nucleating agent effective in promoting both the crystallization of melanotekite and cristobalite. The presence of cristobalite is not related to alkali ions, which are normally responsible for its formation far from equilibrium conditions. Melanotekite and cristobalite can be considered as technological markers associated to the Joques production and then be used to identify imitations of this workshop. Braunite and kentrolite were identified in the brown decoration of the Catalan tablewares. The manganese pigment was probably mixed with a lead oxide compound and applied directly over the raw glaze. The presence of braunite and kentrolite indicates a firing temperature of about 950 °C. Thin-section petrography reveals that the crystallite growth has taken place under two different conditions. Euhedral braunite crystallites were produced during the heating. A drop in temperature during the firing process gave rise to the crystallization of dendritic braunite and kentrolite.