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European Journal of Mineralogy; December 2007; v. 19; no. 6; p. 883-890; DOI: 10.1127/0935-1221/2007/0019-1771
© 2007 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Red lead darkening in wall paintings: natural ageing of experimental wall paintings versus artificial ageing tests

Sébastien Aze1,*, Jean-Marc Vallet1, Monique Pomey1, Alain Baronnet2 and Olivier Grauby2

1 Centre Interrégional de Conservation et Restauration du Patrimoine (CICRP), 21 rue Guibal, 13003 Marseille, France
2 Centre de Recherche sur la Matière Condensée et les Nanomatériaux (CRMCN-CNRS), Campus de Luminy, 13288 Marseille Cedex 9, France

* Corresponding author, e-mail: sebastien.aze{at}cicrp.fr

The mechanisms of red lead pigment darkening were investigated through complementary observations and analyses of naturally aged experimental wall paintings. Red lead transformation into mainly plattnerite [β-PbO2] and also anglesite [PbSO4] was established by mineralogical analyses of a micro-sample using focused X-ray diffraction and confirmed by micro-Raman spectroscopy. According to both optical and electron microscopy observations, β-PbO2 appears to have formed in the outer part of the paint surface, while PbSO4 is only located in a thin superficial layer. Structural features of plattnerite crystals were investigated using both scanning and transmission electron microscopy. Besides, red lead stability in various conditions was assessed by means of a set of physico-chemical tests applied on both raw pigment and experimental paintings. The pigment alteration is assumed to occur through the sulfation into anglesite of lead monoxide initially present in the pigment, followed by the so-called "solvolytic disproportionation" of Pb3O4. Such an alteration process is controlled by environmental parameters, namely humidity, temperature and sulfate concentration, which depend on the oxidative dissolution of atmospheric SO2 pollutant.

Key-words: chromatic alteration, red lead darkening, experimental fresco painting, natural ageing, solvolytic disproportionation, lead oxide.






JOURNAL HOME HELP FEEDBACK/COMMNET SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2008 by E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers