How a volcanic eruption turned a human brain into glass

“It’s an extraordinary finding,” says Matteo Borrini, a forensic anthropologist at Liverpool John Moores University in the UK, who was not involved in the research. “It tells us how preservation can work … extreme conditions can produce extreme results.”

Glittering remains

The Roman city of Herculaneum has been covered in ash for many hundreds of years. Excavations over the last few centuries have revealed amazing discoveries of preserved bodies, buildings, furniture, artworks, and even food. They’ve helped archaeologists piece together a picture of what life was like for people living in ancient Rome. But they are still yielding surprises.

Around five years ago, Pier Paolo Petrone, a forensic archaeologist at the University of Naples Federico II, was studying remains first excavated in the 1960s of what is believed to be a 20-year-old man. The man was found inside a building thought to have been a place of worship. Archaeologists believe he may have been guarding the building. He was found lying face down on a wooden bed.

Petrone was documenting the man’s charred bones under a lamp when he noticed something unusual. “I suddenly saw small glassy remains glittering in the volcanic ash that filled the skull,” he tells MIT Technology Review via email. “It had a black appearance and shiny surfaces quite similar to obsidian.” But, he adds, “unlike obsidian, the glassy remains were extremely brittle and easy to crumble.”

An analysis of the proteins in the sample suggested that the glassy remains were preserved brain tissue. And when Petrone and his colleagues studied bits of the material with microscopes, they were even able to see neurons. “I very excited because I understood that was something very unique, never seen before in any other archaeological or forensic context,” he says.

The next question was how the man’s brain turned to glass in the first place, says Guido Giordano, a volcanologist at Roma Tre University in Rome, who was also involved in the research. To find out, he and his colleagues subjected tiny pieces of the glass brain fragments—measuring millimeters wide—to extreme temperatures in the lab. The goal was to identify its “glass transition state”—the temperature at which the material changed from brittle to soft.

These experiments suggest that the material is a glass, and that it formed when the temperature dropped from above 510 °C to room temperature, says Giordano. “The heating stage would not have been long. Otherwise the material would have been … cooked, and disappeared,” he says. This, he adds, is probably what happened to the brains of the other people whose remains were found at Herculaneum, which were not preserved.

The short periods of extremely high temperature might have resulted from super-hot volcanic gases and a few centimeters’ worth of ash, which enveloped the city shortly after the eruption and settled. Denser pyroclastic flows from the volcano would have hit the building hours later, possibly after the brain had a chance to rapidly cool down.