Chemical composition and properties of stained glasses: rediscovering the glassmaking processes

The production of stained glass windows is emblematic of the art of medieval France. As there are few historical documents, manuscripts or iconographic data, the stained glass windows themselves remain the main source of information, especially for the early Middle Ages.
In this PhD project, we propose to study the stained glass windows housed at the Musée de Cluny that cover a large period (12-15th centuries) that has seen great evolutions in the composition and technology of glass as well as in the graphic arts.
Using a multi-technical approach with portable instruments (XRF, optical and Raman spectroscopy), we will measure the composition, optical properties and structure of these stained glass windows. This will provide a dataset of changes in chemical composition over time which would help to trace the origin of raw materials.
The variation of viscosity with the variation of temperature is the main constraint for the manufacture of glass objects, but it is not possible to melt stained glasses to measure their viscosity. The chemical composition database that we will have established will allow us to reproduce glasses in the laboratory to measure their viscositiy. This property has not yet been systematically studied for stained glass windows even though it is essential for their shaping.
In addition, the role of colouring agents will be studied, in particular how the coupling of elements (Co, Cu, Fe) can affect color and how these elements modify the rheology of glass, an aspect never studied before.

The project is being led by a multidisciplinary consortium of historians specializing in ancient glass and experts in analytical chemistry and glass physics. The PhD project will provide a comprehensive and unique overview of stained glass production processes and the reasons behind technological and compositional changes.