Nondestructive Redox Quantification Reveals Glassmaking of Rare French Gothic Stained Glasses

The sophisticated colors of medieval glasses arise from their transition metal (TM) impurities and capture information about ancient glassmaking techniques. Beyond the glass chemical composition, the TM redox is also a key factor in the glass color, but its quantification without any sampling is a challenge. We report a combination of nondestructive and noninvasive quantitative analyses of the chemical composition by particle-induced X-ray emission-particle-induced γ-ray emission mappings and of the color and TM element speciation by optical absorption spectroscopy performed on a red-blue-purple striped glass from the stained glass windows of the Sainte-Chapelle in Paris, France, during its restoration. These particular glass pieces must have been produced as a single shot, which guarantees that the chemical variations reflect the recipe in use in a specific medieval workshop. The quantitative elemental mappings demonstrate that the colored glass parts are derived from the same base glass, to which TMs were deliberately added. Optical absorption spectra reveal the origin of the colors: blue from Co^II, red from copper nanoparticles, and purple from Mn^III. Furthermore, the derivation of the quantitative redox state of each TM in each color shows that the contents of Fe, Cu, and Mn were adjusted to ensure a reducing glass matrix in the red stripe or a metastable overoxidized glass in the purple stripe. We infer that the agility of the medieval glassmaker allowed him to master the redox kinetics in the glass by rapid shaping and cooling to obtain a snapshot of the thermodynamically unstable glass colors.