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

Unravelling the role of iron and manganese oxides in colouring Late Antique glass by micro-XANES and micro-XRF spectroscopies

“…Besides the pre-edge intensity varies as a function of the site symmetry. A shift to higher energy for the pre-edge and the main-edge is also observed at the Mn K-edge with increasing Mn oxidation state [23,24,[26][27][28][29]. However, determining a link between glass colour and Mn speciation (2+, 3+ and 4+) in ancient glass has proved more complex than for iron; whilst distinctions are clear in mineral samples with different oxidation states [30][31][32], studies of ancient and reproduction glasses have consistently failed to detect much variation in Mn K-edge spectra using XANES in glasses of different colours [23,28].…”

“…A shift to higher energy for the pre-edge and the main-edge is also observed at the Mn K-edge with increasing Mn oxidation state [23,24,[26][27][28][29]. However, determining a link between glass colour and Mn speciation (2+, 3+ and 4+) in ancient glass has proved more complex than for iron; whilst distinctions are clear in mineral samples with different oxidation states [30][31][32], studies of ancient and reproduction glasses have consistently failed to detect much variation in Mn K-edge spectra using XANES in glasses of different colours [23,28]. Analyses suggest a prevalent presence of the almost colourless Mn 2+ dominating the XANES spectrum, and possibly hiding the additional presence of the pink Mn 3+ form, even in strongly purple glass.…”

“…The same observations were made by Schalm et al [34] and Arletti et al [24], who emphasised that the higher oxidation states of Mn are likely unstable at the temperatures used in glass production. Capobianco et al [28] investigated a wide range of flesh-tone to purple glass in medieval stained glass and confirmed that the vast majority (at least 95% of the total Mn) of manganese in the glass is Mn 2+ and that the remaining small amount of Mn 3+ shifts the colour from light pink to purple. It was also underlined that purple glass can be produced only in unusually oxidising conditions.…”

“…However, considering the limitations of XANES with glass samples discussed above, the presence of a small and subordinate amount of Mn 3+ , which would be responsible for the purple colour of the streaks, could not be discounted [23,28]. Previous studies indicate that even a small amount of Mn 3+ can produce purple to light pink colours [43].…”

Comparison of Hyperspectral Imaging and Fiber-Optic Reflectance Spectroscopy for Reflectance and Transmittance Measurements of Colored Glass

“…This observation is confirmed by the calculated values of b*, which are positive for the first sample and negative for the other (see Colorimetry subsection). From a chemical point of view, the different color is due to the fact that for sample Purple 1 the main chromophore is manganese (Mn 3+ ), characterized by a broad band at around 500-490 nm and a shoulder at about 670 nm, with a small contribution of iron (as Fe 3+ ) [4,9,11,16,20,22,24,42,49], while in Purple 2 the three absorption bands at around 525, 590, and 650 nm suggest that cobalt (Co 2+ ) has been added to give a more bluish hue [4,50] The color difference values calculated between the FORS and HSI transmittance and reflectance measurements and between transmittance and reflectance measurements from the same instrument are reported in Table 14. Values above 10 are shown in bold.…”