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 CoII, red from copper nanoparticles, and purple from MnIII. 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.

The restoration of the four northern windows of the 13th century Sainte-Chapelle in Paris from 2011 to 2014 has offered a unique opportunity to investigate the chemical composition and color of medieval glasses. This impressive corpus, covering a total surface of 660m 2 , was created in a record time of a few years. The glasses from ten selected panels were analyzed using non-destructive and non-invasive techniques, with a specific consideration for the color of the glasses. Ion beam analyses performed at the New AGLAE facility enabled revealing that all ancient glasses are potash type glasses made from plant ashes, likely beech, in agreement with previous results on off-site panels. The multivariate analysis of major and minor elements demonstrates the presence of compositional clusters with a small variability suggesting the identification of bundles of glasses. The coloration of the glasses was measured by optical absorption spectroscopy, using a mobile spectrophotometer over the entire UV-visible-NIR energy range. The color palette is made of six colors assigned to typical medieval recipes. The chromophores of the different glasses are identified by combining the chemical composition, optical absorption spectroscopy and colorimetry. Colorless, yellow and purple glasses arise from the subtle redox equilibrium between manganese and iron. Their reduced usage shows their uncertain production. Blue glasses are colored by Co 2+ using saffre from the contemporary German mines, green glasses are colored by Cu 2+ and Fe 3+ using high concentrations of copper and red glasses are striated glass colored by metallic copper nanoparticles. Glass matrix and chromophores form compositional clusters, which are spread among the panels of the four windows suggesting that the glazing of these four windows was run simultaneously by the same atelier using the same supply of glass.

The restoration of the rose (15th century) of the Sainte-Chapelle in Paris, France, offered a unique opportunity to investigate the color and chemical composition of these emblematic medieval French stained glasses with non-destructive analyses. The obtained results are aimed at complementing the knowledge from art historians and thus together trying to compensate for the total absence of archives on the construction of the rose. Comparison with the glasses of the nave (13th century) reveals an important evolution of the aesthetics based on new types of glasses: new colors and extensive use of flashed glass. The systematic study of the chemical composition of both sides of each glass piece revealed that about half of the studied glasses were flashed. For non-flashed glasses, this comparison allowed evaluating the influence of glass surface weathering, although very moderate, on the composition variability. In light of the variability criteria, the multivariate analysis of the chemical composition allowed inferring that most glasses originate from the same production glasshouse.

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