14 DAY TRIAL // A catastrophic eruption of the volcano Mount Vesuvius buried the city of Pompeii under many feet of ash and pumice stone.
The Villa Sora, in Torre del Greco, went underground with the city until twenty years ago, when archaeologists dug it up. The house's walls, like in many buildings from the unearthed Pompeii, were painted with a brilliant crimson pigment, made of cinnabar (mercury sulfide HgS). But the gorgeous red color of the wall frescoes has mysteriously shifted to black in many ancient buildings since their unearthing.
Even antique authors, like Vitruvius, in his treatise "De Architectura", signal the problem, prevented at that time with a protective varnish made of "punic wax". Until now, the causes of the color shift have remained unknown. The most accepted concept is that exposure to the sun turn cinnabar into another phase, metacinnabar, of black color.
A Franco-Italian team has verified this theory probing four wall samples from Villa Sora with synchrotron light at European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The results has showed that the chemical composition in the altered pieces did not contain, as previously thought, metacinnabar, and the chemical composition of the samples was more complex. First, cinnabar had reacted with chlorine from sea water or maybe from "punic wax", forming grey chlorine-mercury salts. Sulfur from cinnabar reacted with calcium from the wall, developing a black coating on the painting surface.
The team studied cross-sections of the paintings to see the depth of paintings' alteration. The black layer was only 5 microns thick and underneath the cinnabar remained untouched. "The chemical distribution of the samples is not stable, which means that atmospheric conditions probably play a role in this change of colors", explains Marine Cotte, first author of the study. "The sun surely influences this process, but the rain may possibly do too", she adds.
Physical environmental factors, but also bacterial activities, can increase the reaction between cinnabar and the walls. "The research carried out at the ESRF has an extraordinary importance not only for conservation of wall paintings of Villa Sora, but in general for preservation of roman wall paintings discovered in the most important Roman archeological sites (such as Pompeii and Herculaneum)", explains Corrado Gratziu, geology professor at the University of Pisa.
The team performed on the x-ray microscopy beamline (ID21) by combining the techniques of micro X-ray fluorescence, to detect chlorine and sulfur presence, and micro x-ray absorption spectroscopy, to determine their chemical combinations. "The next step is to examine more samples and not only from frescoes in the archaeological site but also from those in museums. In this way, we will be able to compare the results and better establish the causes for their degradation", explains Cotte.
Pompeii ancient city is the most visited archaeological site in Europe and it is also a UNESCO World Heritage Site.
Photo credit: Mario Pagano