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Journal Articles Journal of Archaeological Science: Reports Year : 2021

The representation of skin colour in medieval stained glasses: The role of manganese

Abstract

The present study concerns a corpus of 17 glass pieces representing character heads, originating from 11 major cathedrals and churches from North Western France and which constitute the masterpiece of stained glass windows. Skin complexion ranges from colourless to flesh-tone and purple. These glasses have been investigated using non-destructive and non-invasive techniques. Particle Induced X-ray Emission and Particle Induced Gamma Emission analyses at AGLAE show that these glass pieces show potassic plant ash glass compositions, typical to 12th −15th centuries. The compositional variability of the major glass components remains limited, similar to that found for single monuments, despite the diversity of geographic origins. The Mn and Ba concentrations follow a different trend in flesh-coloured or purple glasses and in colourless glasses, suggesting that the sources of Ba and Mn in these two kinds of glasses originated from different raw materials. Purple and flesh tone glasses contain more manganese than colourless glasses with an almost similar iron content. Synchrotron X-ray absorption near-edge structure spectroscopy (XANES) and portable UV–visible-NIR optical absorption spectroscopy indicate that glass colour varies with results from Fe2+, Fe3+, Mn2+ and Mn3+. The thickness of the glasses, about 3 mm, has been measured using ultrasound techniques. Its dispersion is similar in the various coloured glasses. This parameter mostly influences the colour saturation but not the hue. Obtaining flesh-coloured or purple glasses requires some control of the oxidation state of manganese during glass making. Divalent manganese is largely prevalent and the colour change from flesh-tone to purple hues is driven by small variations in the Mn3+ content. As these variations are not related to the Fe/Mn ratio, the resulting glass colour is difficult to predict on the only basis of glass composition. Due to the low kinetics for obtaining a redox equilibrium state between furnace atmosphere and the silicate melt, using Mn4+- and Mn3+-oxide minerals made it possible to favour oxidized melts, provided the glasses be taken out of the oven before redox equilibrium be reached: the shorter the melting time, the more oxidized will be resulting glass. This shows that medieval glassmakers were able to overcome the challenge of making glasses under highly oxidizing conditions to retaining enough oxidized manganese to favour flesh-tone and purple colours in wood-fuelled furnaces.
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Dates and versions

hal-03356047 , version 1 (27-09-2021)

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Natan Capobianco, Myrtille O.J.Y. Hunault, Claudine Loisel, Barbara Trichereau, Fanny Bauchau, et al.. The representation of skin colour in medieval stained glasses: The role of manganese. Journal of Archaeological Science: Reports, 2021, 38, pp.103082. ⟨10.1016/j.jasrep.2021.103082⟩. ⟨hal-03356047⟩
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