The Grande Rose of the Reims Cathedral: an eight-century perspective on the colour management of medieval stained glass.

The Grande Rose of Reims Cathedral (France), a UNESCO Cultural Heritage Monument from the 13 ^th century, underwent several restoration works during the 20 ^th century. Its colours result from centuries of colour management from which little information remain. We used non-destructive and portable optical absorption spectroscopy to quantify glass colour and determine the colouring species on a large-scale study of this monumental window. We found six distinct colour groups, each containing both medieval and modern glasses, with colouring processes specific to each colour. This illustrates medieval glassmakers' mastering of glass colouring and modern glassmakers' management to reproduce medieval glasses colours. Full UV-visible-NIR energy range is necessary for determining the contribution of colouring elements as Fe ²⁺ and Cu ²⁺ . Systematic thickness measurements reveal an average glass thickness of 3 mm and demonstrate the major control of chromophore concentration on glass colour. Yellow, red and purple colours arise from a single chromophore each, suggesting the use of well-defined glassmaking techniques leading to robust colour reproducibility. By contrast, blue and green glasses show different chromophore combinations depending on production time, which suggests more diversity in glassmaking techniques.