Glass colourations caused by Mn-Fe redox pair : Application to ancient glass technology

•Medieval-like glass synthesis at variable temperature and atmosphere.•Optical absorption spectroscopy as a sensor of Fe /Mn redox reactions in glass.•Characterization of colourless composition using chromatic coordinates.•Variations of Mn/Fe ratio in colourless glasses depending on synthesis conditions.•Glass composition as an indicator of ancient technological processes. Manganese is a chemical element used as a colourizer in glass industry since antiquity. Combined with iron, manganese often plays the role of a decolourizer leading to uncoloured glasses used to represent hands and faces in medieval stained glass windows. A series of medieval-like glasses has been synthesized in order to relate the colouration of glasses with the conditions of synthesis (temperature, atmosphere, glass composition). Optical absorption spectra in the UV–Vis-NIR range have been measured to follow and characterize the changes in colouration. For a given temperature, the addition of Mn in a Fe-rich glass composition implies a decrease of the concentration in Fe2+and an increase of the concentration in Fe3+ as compared to the Mn-free glass composition. The uncoloured glass composition is obtained for different contents of Mn when melting temperature varies. These results might help deciphering the complexity of colour making in ancient times.