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Authors: Cotte, M.*
Susini, J.*
Métrich, N.*
Moscato, A.*
Gratziu, C.*
Bertagnini, A.*
Pagano, M.*
Title: Blackening of Pompeian Cinnabar paintings: X-ray micro-spectroscopy analysis
Title of journal: Analytical Chemistry
Series/Report no.: 21/78 (2006)
Publisher: American Chemical Society
Issue Date: 2006
DOI: 10.1021/ac0612224
Keywords: Microspectroscopy Analysis
Abstract: 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 5-ím-thick layer covering intact cinnabar.
Appears in Collections:05.09.99. General or miscellaneous
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