The ruins of the ancient Roman city of Pompeii are full of morbid mystery. In 79 CE, a volcanic eruption wiped out the city of between 10,000 and 20,000 inhabitants. Massive plumes of volcanic ash and pumice shot out of Mount Vesuvius, covering and suffocating Pompeii’s doomed residents. Archaeologists have found the remains of over 1,300 victims in the site southeast of the city of Naples over the last 250 years.

Some of the bodies of Pompeii were also preserved in plaster, but not from Mount Vesuvius and not in 79 CE. In the 1860s and 1870s, archaeologists led by Giuseppe Fiorelli poured plaster into the voids left behind by the bodies that had decomposed. These casts typically have the skeletal remains embedded in the plaster that retain the body shape and give a realistic image of victims of the eruption.

“Pompeii is one of the most important places from an archaeological point of view,” Gianni Gallello, an archeological scientist at the University of Valencia in Spain tells PopSci.  “All of Roman society is imprinted at the moment after the eruption, all stuck in time.”

However, the plaster may have contaminated the chemical composition of the bones, according to a study published August 23 in the journal PLOS One. While the plaster may have altered the chemical makeup of the bones, bio archaeological analysis still supports the theory that these specific victims died from asphyxiation and not from blunt force trauma from rocks or burning.

Gallello is one of the co-authors of the study who specializes in applying analytical chemistry to archeological finds. He brought a technique called portable X-ray fluorescence as a way to noninvasively examine the elemental composition of the bones and plaster for the first time.

“It’s a portable device that takes the material profile invisibly,” Gallello explains. “Everything was in contact with the plaster, so you can get contamination. Plaster also has high levels of compounds similar to the bones.”

In this study, Gallello and his colleagues looked at six plaster casts from the Porta Nola (gate) area of Pompeii and one cast from the city’s Terme Surbane (or frescoed bath house) for anthropological and multielemental analysis. They also compared these bones to cremated bones from a Roman necropolis and ones found in a Spanish Islamic necropolis.

“Cross referencing is important for volcanologists and anthropologists. It provides complimentary data [for the] reconstruction of the evidence. Anthropological work can say that the position of the bones of the people who died while they were escaping is telling that they probably died from asphyxiation, while archaeological data can say if it was during the second part of the eruption,” says Gallello.

Gianni Gallello (in the front) measuring Cast #57 by pXRF, together with Llorenç Alapon (in the back) at Pompeii Archaeological Park. CREDIT: Alapont et al.

Using portable X-ray fluorescence, they found that the plaster from Pompeii was completely different from the burned and unburned bones from the collection. Testing out this method for the first time on the Pompeii casts also helped add to the prevailing theories of what killed these specific residents of Pompeii during the eruption. While the plaster contamination makes it more difficult to study, the chemical analysis supports the theory that the victims suffocated from the volcanic ash.

“We don’t pretend to say how they died. What we do is provide more evidence and data to complement and allow the volcanologists who are very active in Pompeii to study,” says Gallello.

The team hopes that using noninvasive techniques like this on other archeological finds and cast skeletons will help find better evidence to draw stronger conclusions on the causes of death.

“It’s an honor to work in Pompeii,” says Gallello. “We do work that we love, and for us, it’s not work.”