Cuprite crystallization in glasses: Redox changes and the impact of copper and lead in reducing atmospheres

The production of red and orange cuprite glass and glazes depends on a complex interplay of glass composition, redox states, and thermal treatments. This study investigates how lead and copper concentrations impact the copper redox state and cuprite crystal formation. Spectroscopy (EPR, XANES at Cu L₃ and K edges) shows that high copper content leads to slight oxidation, while high lead levels induce mild reduction, neither of which directly initiating cuprite crystallization. In situ XANES analysis under reducing conditions reveals that strong undercooling is essential for cuprite formation, with lead primarily lowering the glass transition temperature, allowing large dendritic cuprite crystals to produce red colors while smaller crystals yield orange. These findings shed light on ancient glassmaking techniques, such as those in Roman glass tesserae.