To be purple or not to be purple? How different production parameters influence colour and redox in manganese containing glass

Abstract

The purpose of this paper is to understand why manganese containing Roman glass could be purple or colourless in spite of having very similar chemical compositions. The strategy followed to tackle this question consists in the production of glass with the same chemical composition as Roman glass whereby various production parameters were controlled and systematically analysed. It is shown that redox and colour of glass is more likely to have been managed through internal control through the choice of raw materials and the addition of organic matter. The main difference between ancient and modern glass production relies on the lower melting temperature of Roman furnaces, so that sulphate would have played a less important role in the redox determination. 1 Secondly, a colour compensation occurs between the blue reduced iron and the purple oxidised manganese. This causes a general 'greying' of the glass where no wavelength is absorbed more than others, but the

Thirteenth-century stained glass windows of the Sainte-Chapelle in Paris: An insight into medieval glazing work practices

“…The optical absorption spectrum of purple glass from the Sainte-Chapelle agrees with typical spectra from Mn 3+ colored glass. (Bidegaray et al, 2019;Capobianco et al, 2019;Hunault et al, 2017a). Compared to the colourless glasses, we observe the disappearance of the absorption band of Fe 2+ , and the presence of Fe 3+ and Mn 2+ , which only give weak absorption bands in the blue and UV range (Figure 6).…”

“…These values agrees with other reported values for colorless glasses. (Bidegaray et al, 2020(Bidegaray et al, , 2019Capobianco et al, 2019) Yet they show a greenish color (negative CIE a*). The optical absorption spectrum of a typical colorless glass (Figure 6) from the Sainte-Chapelle in Paris agrees with previously published data.…”

“…The optical absorption spectrum of a typical colorless glass (Figure 6) from the Sainte-Chapelle in Paris agrees with previously published data. (Bidegaray et al, 2020(Bidegaray et al, , 2019Capobianco et al, 2019;Hunault et al, 2017a) The greenish color is due to Fe 2+ that gives a broad absorption band in the NIR centered at 9000cm -1 (1100nm) (Figure 6) and absorbs the red, orange and yellow wavelengths more than the green and blue side of the light spectrum. In glass, iron is present as Fe 2+ and Fe 3+ in various relative amounts depending on the glass chemical composition and redox conditions of fabrication.…”

Obsidian and Obsidian-like Glass Tesserae: A Multidisciplinary Approach to Study the Dedication Wall Mosaic in the Church of St. Mary of the Admiral in Palermo (12th Century)

“…The obsidian naturally shows different shades of color varying from grey-green to black due to variable iron contents (Barca et al, 2019). The color of our tessera is due to the very high manganese content, which, when present as Mn 3+ , imparts hues variable from pink to purple, until to the black-appearing one in the function of its increasing concentration and thickness (Bamford, 1977;Cagno et al, 2014;Bidegaray et al, 2019Bidegaray et al, , 2020. The chemical and micro-textural characteristics of the analyzed sample allow us to hypothesize a substitution of original tesserae with obsidian-like ones during restoration in place of damaged or lost "natural" obsidian tesserae, and to confirm results of the archive research on the discontinuity involving the abbreviation "MP", where the black tessera sampled in this study was located.…”

Production and provenance of architectural glass from the Umayyad period

“…The purple tesserae are coloured with added manganese in the range of 0.8-1.7% MnO. Mn:Fe ratios for the purple tesserae are approximately 1:1, which is surprising in light of recent experiments that suggested the most important factor for the successful generation of purple was that manganese concentrations must exceed iron contents [72]. Moreover, the manganese concentrations of most of the gold leaf tesserae exceed those of the purple glass.…”