Coloration Process of Model Medieval Glasses: Experimental Study of the Role of Fe and Mn Ions

ABSTRACT Glasses containing simultaneously iron and manganese have been widely used in medieval times to produce a wide range of colorations from green to purple. Technical recipes are, however, not known due to the lack of historical documents. A large panel of medieval‐like glasses containing variable amounts of iron and manganese oxides has been produced under variable atmosphere (air, H 2 , O 2 ) and at different melting temperatures (1200°C, 1350°C, and 1500°C). Using optical absorption spectroscopy to probe the glass coloration, information has been derived on the interactions between Fe 2+ , Fe 3+ , Mn 2+ , and Mn 3+ ions during the glass synthesis. An abacus relating temperature and the MnO to (MnO+FeO) ratio has been set up and used to deduce the melting temperatures of a set of historical glasses from the literature. The derived temperatures were, however, too high for medieval furnaces, quantitatively confirming the common knowledge that the synthesis atmosphere in the medieval furnaces had to be more reduced than pure air. The relationship between glass coloration and synthesis parameters, the extensive set of glasses studied in this work, quantitatively allowed to enlighten the effect of the so‐called “glassmaker's soap” for the first time and the conditions to obtain an uncolored glass at any temperature.