Looking through model medieval green glasses: From color to recipe

Abstract

The colors of medieval stained glasses depend on their chemical composition. The redox interactions in glass between iron and copper and their influence on the color were investigated in two soda-lime silicate model glasses. Both glasses have close iron and copper redox ratios and copper oxidizes iron when the iron content is smaller than the copper content. The redox speciation of iron and copper was determined using combining x-ray and optical absorption spectroscopies in relation with the resulting color. The molar extinction coefficient of Cu 2+ was estimated from the combined analysis to be 32 mol/L/cm. The glass color change from green to blue-green color is assigned to the decrease of the total iron content, confirming that ancient glassmakers could control the final color with the glass recipe. These results provide reference data for further spectroscopic investigations of medieval stained glasses.Comparison with ancient copper-colored green glass compositions suggests that the higher ratio of copper to iron total contents was imposed by the more reducing melting conditions in medieval furnaces.How to cite this article: Hunault MOJY, Loisel C. Looking through model medieval green glasses: From color to recipe. Int J Appl Glass Sci. 2020;11:463-470. https://doi.

The rose of the Sainte-Chapelle in Paris: sophisticated stained glasses for late medieval painters

“…The PIXE analyses reveal that the total CuO content in the bulk green glass of panel S1 is 1.3 wt% (Figure 15b), which agrees with typical CuO concentrations in medieval glasses [Hunault et al, 2021]. The high copper content is needed to compensate the oxidation of Fe 2+ by Cu 2+ , which are incompatible [Hunault and Loisel, 2020] as confirmed by the absence of absorption band from Fe 2+ . Some green glasses show CuO contents equivalent to other colors (Figure 15b).…”

“…Green glasses are colored by Cu 2+ as revealed by their optical absorption spectra (Figure 15a) showing an intense and large absorption band near 13,000 cm −1 [Hunault and Loisel, 2020]. Some green glasses show an additional contribution from the Co 2+ triple absorption bands in agreement with the relatively high CoO content (250 ppm for the S1 sample presented in Figure 15a).…”

Thirteenth-century stained glass windows of the Sainte-Chapelle in Paris: An insight into medieval glazing work practices

“…Copper can be present in the glass in 3 different oxidation states: Cu 0 , forming metallic nano-particles (Kunicki-Goldfinger et al, 2014) (see the red glass section), Cu + which is colorless (Hunault et al, 2016a(Hunault et al, , 2017a and Cu 2+ which gives a blueish or greenish hue. (Hunault and Loisel, 2020) The possible contribution of Cu 2+ absorption, which is expected around 13000cm -1 (see green glass section), is difficult to assess since it overlaps with Fe 2+ (Figure 8c). According to previous studies, (Hunault et al, 2016a) the redox conditions of blue medieval glasses favor Cu + oxidation state and a negligible contribution of Cu 2+ .…”

“…The high energy absorption is assigned to Fe 3+ . (Hunault and Loisel, 2020) Compared to blue glasses, the absorption from Fe 3+ is more intense. This is assigned to the higher Fe 3+ concentration in green glass in agreement with the redox interaction between iron and copper.…”

“…This assignment agrees with the high concentration of copper found in the green glasses (between 1.5-2 wt.% of total CuO) (Figure 10). Although Cu 2+ alone gives a blue color to glass, (Bamford, 1977;Hunault and Loisel, 2020;Smirniou and Rehren, 2013) the combination with the absorption of the blue wavelengths revealed by the optical spectrum results creates a light transmission window in the green around 18000cm -1 (550nm). The high energy absorption is assigned to Fe 3+ .…”