Spectroscopic Investigation of the Coloration and Fabrication Conditions of Medieval Blue Glasses
Résumé
Potash- and soda-lime-stained glasses from the 12th–13th cen- turies, blue-colored by cobalt, have been investigated by Mn, Fe, and Cu K-edge X-ray and optical absorption spectroscopies in order to determine the oxidation state of these elements and their impact on the blue color. Remelting these historical glasses in air at 1200°C, the estimated temperature of medieval furnaces, revealed that these four glasses are more reduced before remelting. This favors Mn as weakly absorbing Mn2+, Fe as Fe2+ and Cu as colorless Cu+. Therefore Fe2+ is the second blue chromophore and copper was not intentionally used by glassmakers to obtain a blue color. A colorimetric analysis indicates that these specific melting conditions have a limited effect on the blue color of these glasses. Based on the spectro- scopic determination of the redox state of Fe, Mn, and Cu, we estimate the oxygen partial pressure in medieval furnaces to be 10 – 7–10 – 9 and 10 – 5 bar for the potash- and soda-lime sam- ples, respectively. The comparison with previous results enables to prove the evolution of furnace technology over centuries.