THE COLOR OF STAINED GLASSES

Stained glass windows are one of the most striking characteristics of Gothic architecture. Their manufacture began during the 12th and 13th centuries in response to the increasing size of windows in medieval monuments, to let light inside. Stained glasses serve as iconographic supports and fill the space with light and bright colors that make them one of the most well-known heritage objects.

However, the scarcity of quantitative data on the color of stained glasses limits our knowledge of the control and choice of dyes used over the centuries. An original color measurement device (a new type of portable optical spectrophotometer) perfectly suitable to quickly identify the chromophore responsible for the color without any direct contact with the artwork was used. Thanks to the coupling between spectroscopic measurements and authenticity studies, we were able to obtain information on both the way in which medieval artists managed the color of glasses but also on how the glass artists managed their production, during recent restoration campaigns, to integrate modern glasses respecting the harmony of the medieval building (e.g. Reims Cathedral, Sainte Chapelle of Paris). These spectroscopic measurements make it possible to know in what chemical form elements used in the coloring of the stained-glass window are incorporated into the structure of the glass e.g. their redox state and therefore the synthesis conditions for producing these glasses. One typical example is the yellow color of stained glasses. A method used for a long time to color the stained glass in yellow is the use of a chemical complex of iron and sulfur, also called “amber chromophore”. Studying this amber chromophore allows a better understanding of the color of stained glass but also a better knowledge and control of the amber coloring for UV protecting industrial bottles such as beer bottles. This collaboration between mineralogists, chemists and art historians shows that the color of stained glass provides information on the evolution of glass manufacturing and restoration techniques over the centuries.