Blackening of Pompeian cinnabar paintings: X-ray microspectroscopy analysis.
Red Pompeian paintings, very famous for their deep intensity, are currently suffering from darkening. The origins of this darkening degradation are not clearly identified yet and remain a major issue for curators. In the specific case of cinnabar (HgS)-based red pigment, a photoinduced conversion into black metacinnabar is usually suspected. This work is focused on the blackening of red cinnabar paintings coated on a sparry calcite mortar. Different samples exhibiting different levels of degradation were selected upon visual observations and analyzed by synchrotron-based microanalytical techniques. Atomic and molecular compositions of the different debased regions revealed two possible degradation mechanisms. On one hand, micro X-ray fluorescence elemental maps show peculiar distributions of chlorine and sulfur. On the other hand, X-ray absorption spectroscopy performed at both Cl and S K-edges confirms the presence of characteristic degradation products: (i) Hg-Cl compounds (e.g., corderoite, calomel, and terlinguaite), which may result from the reaction with exogenous NaCl, in gray areas; (ii) gypsum, produced by the calcite sulfation, in black coatings. Metacinnabar is never detected. Finally, a cross section was analyzed to map the in-depth alteration gradient. Reduced and oxidized sulfur distributions reveal that the sulfated black coating consists of a approximately 5-mum-thick layer covering intact cinnabar.
Cotte
Marine
M
European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France, Laboratoire Pierre Süe, CEA-CNRS, CE-Saclay, 91191 Gif sur Yvette, France. cotte@esrf.fr
Susini
Jean
J
Metrich
Nicole
N
Moscato
Alessandra
A
Gratziu
Corrado
C
Bertagnini
Antonella
A
Pagano
Mario
M