Volcanic spectroscopy

Italian researchers have used a salver of techniques, including microscopic Raman and Fourier transform infrared (FTIR) spectroscopies, X-ray diffraction and scanning electron microscopy-energy dispersive X-ray to study powdered pigments found in bowls from the Pompeii archaeological site as well as wall-painting fragments from the Vesuvian region.

The Roman town of Pompeii, close to the present-day Naples and its civic sibling Herculaneum, were buried under 22 metres of ash and pumice over a two-day period in 79 AD by the eruption of the volcano Mount Vesuvius. It was entirely lost from history for almost 1,600 years until its serendipitous rediscovery in 1592. Subsequent excavations and studies of the sites and the volcano's countless victims have given archaeologists a bitter-sweet insight into city life at the height of the Roman Empire and continues to do so.

Irene Aliatis, Danilo Bersani, Pier Paolo Lottici, Silvia Mantovan, and Iari-Gabriel Marino of the Department of Physics, working with Elisa Campani and Antonella Casoli in Chemistry, at the University of Parma, Italy, have used various analytical tools to study powdered pigments found in bowls from the Pompeii archaeological site and wall-painting fragments from the region.

"The extraordinary importance of Pompeii in studies on the Roman age is essentially related to the eruption of Mt. Vesuvius and to the excellent preservation state of the ruins," the team says. "The natural disaster was [pre-dated] by a violent earthquake in 62 AD, which damaged many houses with their decorative plasters: at the time of the eruption restorations were still going on."

The team points out that the lava flow on the ancient Pompeian yard preserved frescoes and raw pigment bowls from loss. "The wall paintings and the raw pigments discovered in the Vesuvian area are therefore of extraordinary importance for the identi?cation of the pigments and the techniques employed by the Roman artists," the team says. Chaptal, Davy and others examined these in the nineteenth century and Augusti in the twentieth century.

Of course, the technological explosion of recent years has led to many new high-precision analytical techniques with which ancient artefacts might be examined. Micro-Raman spectroscopy is one that has been used widely but only rarely in this particular field.

The team has now demonstrated that brown, red and yellow pigments are composed of well-known ochres based on goethite and haematite. Blue pigments are Egyptian blue (the only entirely synthetic colour used) and the presence of tridymite and cristobalite in these indicates that the bowls were fired in a kiln at between 1000 and 1100 Celsius. The team also found that pink pigments were apparently made from purely inorganic materials, by mixing haematite and Egyptian blue (which has a violet hue), or by adding an organic dye to a matrix of aluminium-silica. A white powder also revealed itself to contain an unusual pigment huntite, calcium magnesium carbonate. The team adds that a green, bowl pigment contains malachite mixed with goethite, Egyptian blue, haematite, carbon, cerussite and quartz.

Some of the same pigments were found in the fragments from wall paintings. Celadonite is found in the green samples, they say. Egyptian blue and basic lead carbonate were also found, the team adds.

The team adds that IR and Raman revealed that all fragments investigated carried a layer of beeswax layer, which was most likely added during restoration in the 1800s in order to add a sheen to the wall paintings.

"Pompeian artists used a wide range of colours, both natural and artificial. The analyses on raw pigments make it possible to extend the previous studies based only on pigments sampled from wall paintings, where in some cases binding agents could modify the composition by chemical/physical reactions," the team concludes.