Marine Cotte and co-authors studied samples of those unique red pigments from wall paintings in a house near Pompeii that was buried under ash during the infamous eruption of Mount Vesuvius in 79 A. D. The paint, which used pigment made from red mercuric sulfide (called Cinnabar, HgS), was preserved under ash until excavations began in 1988. Since the 1990s, however, the brilliant red paintings have darkened and deteriorated.

In a report scheduled for the Nov. 1 issue of the ACS semi-monthly journal Analytical Chemistry, the authors describe how they used micro x-ray fluorescence and x-ray absorption spectroscopy at the European Synchrotron Radiation Facility to determine how the darkening could happen. The findings will help curators and restorers to develop better methods for preserving the brilliant artwork from ancient Rome, the report states.

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-m-thick layer covering intact cinnabar.