A Fragile Heritage
Aspects of Historic Glass
Cambridge - Wednesday 6th September 2017



Natan Capobianco
<natan.capobianco@impmc.upmc.fr>

article posted 12 Apr 2017

Natan Capobianco

2014 - Graduation (Chemistry) at Ecole Normale Supérieure, Paris and graduated in Materials Chemistry, University P&M Curie, Paris

2015-present. PhD Student at Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Paris


Oxidation states of manganese and iron
in medieval purple stained glasses
Natan Capobianco1, Claudine Loisel2, Myrtille Hunault1,2,3,
Laurence Galoisy1, Georges Calas1

In 2009, French authorities retrieved 67 glass pieces originating from medieval stained-glass windows from major French churches and cathedrals representing characters' heads, before auction selling. Since the Middle Age purple manganese (III) ions have been used in stained-glass windows for the skin of the characters, especially for the heads. The characters' faces are among the most beautiful pieces as they show the glazier's painting skills, and as a consequence were often stolen and replaced with copies by indelicate restorers. This exceptional set of historical pieces, which will be exhibited to the public, shows the variety of skin colour hues that were obtained by medieval glassmakers and therefore offers a unique insight into the medieval glassmaking techniques, and their evolution from the 12th century to the Renaissance.

The chemical composition was analysed using non-destructive techniques at the AGLAE facility, but it is not able to explain alone the observed variations of colours. Optical absorption spectroscopy qualitatively reveals that iron and manganese are the main colorant in these glasses, and three main colour hues were identified, but it does not allow quantitative analysis because of the superimposing contributions and complex shape of the optical absorption bands.


Iron, naturally present in the sand, occurs in glasses as a multivalent (Fe2+/Fe3+) ion and is responsible for its typical greenish colour. Manganese is another ion commonly present in glasses as an impurity but it has been also added on purpose by glassmakers to adjust glass colour using the following equilibrium: Fe2+ + Mn3+=> Fe3+ + Mn2+. Therefore small amounts of manganese were used to oxidize Fe2+ and remove the greenish colour, and it was known as the "glassmaker's soap". At higher concentration, when all iron is oxidized, the excess of Mn3+ enables to produce "purple" glasses, extensively used for skin colour in stained glass windows. The glass matrix composition and the melting conditions (furnace temperature and atmosphere) are further crucial parameters that govern the redox ratio of these ions and thus the resulting colour. It was recently demonstrated using XANES that medieval blue glasses were made in reducing conditions, which is incompatible with the presence of Mn3+ in the glass. This would explain why the purple colour is unstable and tends to disappear. Therefore it results from a smart mastering by the glassmaker of the kinetic vs. thermodynamic competition of the Mn3+ reduction in the glass.

The use of XANES at K-edge of both iron and manganese has allowed us to determine their coordination number and the redox ratios Mn3+/Mn2+ and Fe3+/Fe2+, which enable to assess the furnace partial pressure of oxygen. The comparison with previous results observed in glasses of other colours provides exceptional insights into the medieval glassmaking technology.

Reference:

Hunault, M., Bauchau, F., Loisel, C., Herold, M., Galoisy, L., Newville, M., & Calas, G. (2016). Spectroscopic Investigation of the Coloration and Fabrication Conditions of Medieval Blue Glasses. Journal of the American Ceramic Society, 99(1), 89 97. https://doi.org/10.1111/jace.13783

Institutions:

1: Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Universités, Université P&M Curie and CNRS, Paris, France

2: Laboratoire de Recherche des Monuments Historiques, Champs-sur-Marne, France

3: SOLEIL, Saint-Aubin, France