CHEMISTRY, CULTURAL HERITAGE
European researchers combating discolouration of Pompeii
The Italian city of Pompeii offers a unique window into the past. An entire city frozen in time as it was engulfed by tonnes of ash rained down upon it from nearby Mt Vesuvius. It lay undisturbed for 1500 years before being discovered, providing a glimpse into a typical city at the height of the Roman Empire. In its day, Pompeii’s walls were painted a brilliant red that has turned black since excavation, leaving conservationists baffled as to the cause. A Franco-Italian research team have investigated the chemical origins of the phenomenon afflicting Europe’s most popular archaeological site.
Scientists at the European Synchrotron Radiation Facility (ESRF)
in Grenoble, France have used synchrotron light to examine the
chemical composition of the discoloured frescos adorning the
facades of Pompeii’s buildings.
|Darkening Pompeii frescos feel the heat of ESRF synchrotron light.|
© Mario Pagano
The frescos’ red colour comes from cinnabar, originating
in a type of mercury ore. The discolouration of cinnabar is
well-known, the ancient Roman writer Vitruvius wrote about it
in the 1st century BC, but poorly understood. In ancient times
artisans applied a protective varnish made from punic wax to
prevent colour damage. A common modern theory holds that exposure
to the sun causes cinnabar to transform into metacinnabar turning
it black. The ESRF researchers sampled specimens from Pompeii’s
Villa Sora in an attempt to prove the theory.
They determined that the cinnabar had indeed undergone significant chemical transformation due to solar radiation, though it had not been transformed into metacinnabar. They discovered that the mercury in cinnabar had reacted with chlorine from the sea to form a greyish chlorine-mercury compound. Though Pompeii currently stands some distance from the sea, in ancient times the shoreline was much closer. The punic wax applied to the walls may also have been a source of the chlorine.
The research team was able to establish that only a relatively thin layer of the surface - around 5 microns - had been affected. They report that weather conditions are most likely the root of the problem.
“The chemical distribution of the samples is not stable,
which means that atmospheric conditions probably play a role
in this change of colours,” explains Marine Cotte, the
first author of the paper recently published in Analytical
Chemistry. “The sun surely influences this process,
but the rain may possibly do so too,” she adds.
The research project is still in its early stages and is expected to provide conservationists with vital tools to add to the repertoire of preservation practices used in Roman archaeology.
“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 Ms Cotte.
“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 archaeological sites
(such as Pompeii and Herculaneum),” explains Corrado Gratziu,
Professor Emeritus in geology, with a specialisation in petrology
of sedimentary rocks, at the University of Pisa, and who is
also part of the team.
European Synchrotron Radiation Facility homepage
Analytical Chemistry article