Vesuvius Turned a Roman Man’s Brain Into Glass. Now, Scientists Reveal How the Extremely Rare Preservation Happened

In 79 C.E., Mount Vesuvius erupted, burying the nearby ancient Roman city of Pompeii and the smaller town of Herculaneum under deadly layers of volcanic ash, pumice and pyroclastic flows. But the disaster that demolished the two settlements also immortalized them, preserving everything from the shape of victims’ bodies to frescoes in private villas and library scrolls.

Archaeologists even discovered the remains of a young man whose brain and parts of his spinal cord had turned into glass. Scientists had never seen a glassy soft tissue in nature before—and no one has found anything like it since.

“When we realized that there was really a glassy brain, the scientific question was: How is it possible?” Guido Giordano, a geologist and volcanologist at Roma Tre University in Italy, tells Science News’ Alex Viveros.

Now, Giordano and other researchers have put forth an explanation for how this incredibly rare phenomenon—known as vitrification—might have happened, as detailed in a study published last week in the journal Scientific Reports. The glass, they suggest, was formed by a super-hot ash cloud that rapidly heated and cooled the brain in the wake of the eruption.

The unique remains were unearthed in the 1960s within a building called the College of the Augustales in Herculaneum, reports Reuters’ Will Dunham. They likely belonged to the approximately 20-year-old college custodian, who was found in his bed. But in 2020, researchers examining the remains discovered that parts of his brain were a shiny material that seemed to be a glass. This sparked heated debates, not just about how his brain may have turned to glass, but whether the finding really was glass in the first place.

The recent study, however, supports the 2020 research claiming that the remains are indeed a brain—they found preserved neurons and axons, as well as proteins known to be common in brain tissue. They conclude the vitrification was caused by an ash cloud that arrived in Herculaneum before the pyroclastic flows from the volcano.

Pyroclastic flows are extremely hot and fast currents of volcanic debris, gas and air that travel close to the ground. An ash cloud is a similar phenomenon, but it travels at a higher altitude.

“[An ash cloud is] a dilute part of the pyroclastic flow. It’s usually formed at the edges, above and laterally, where most of the material is like an avalanche or landslide, but the peripheral part is of finer particle ash,” Giordano explains to CNN’s Katie Hunt. “These clouds can be hot enough to kill you.”

Part of the contention regarding the glass brain comes from the fact that Herculaneum’s pyroclastic flows would not have been hot enough—or allowed for a rapid enough cooling—to enable the rare vitrification process. Even in nature, glass only forms when, in addition to other factors, a liquid material can cool quickly enough to avoid the formation of crystals.

“It’s not the heating process that transforms things into glass,” Giordano tells New Scientist’s Matthew Sparkes. “The heating process would bake it, would transform it into charcoal, would vaporize it. But it’s not going to make it glass. In order to make a glass, you need a fast cooling.”

Giordano and his colleagues calculated that the man’s brain would have had to reach over 950 degrees Fahrenheit before cooling quickly, but the known pyroclastic flows were at most 869 degrees Fahrenheit—so the cause must have been something else.

The ash cloud from the eruption would have been “short-lived, dilute and much hotter,” the researchers write in the study. Since such a cloud would have appeared suddenly and dissipated quickly, it could have created the conditions necessary for vitrification.

But there’s a catch: “Conditions must have been very, very specific, because the organic tissue must have experienced a heating fast enough not to entirely destroy it (which is instead the most common occurrence) and then fast-cool to turn into glass,” Giordano says to Popular Science’s Andrew Paul.

To achieve those specific circumstances, the skull and spine acted as protective layers, shielding the soft tissue within from a bit of the heat. “The glass that formed as a result of such a unique process attained a perfect state of preservation of the brain and its microstructures,” the team writes in the paper.

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Giordano tells Live Science’s Tom Metcalfe that they found charcoal fragments in Herculaneum that supported their idea. These fragments “experienced multiple [heating] events, and the highest temperatures were associated with the early, super-hot ash cloud.”

Benjamin Andrews, a volcanologist at the Smithsonian’s National Museum of Natural History who was not involved in the study, tells Science’s Collin Blinder that the team’s findings are “remarkable.”

“There’s a huge story, a huge wealth of information, contained in these little particles,” he adds.

Not everyone agrees with the new conclusions, however. For example, Alexandra Morton-Hayward, a forensic anthropologist at the University of Oxford in England who was also not involved in the research, is not convinced that the glassy material is indeed parts of the man’s brain. She maintains that soft tissue vitrification is “incredibly unlikely,” per CNN.

Regardless of how the glass formed, the young custodian’s story begs the question—what does it feel like to have your brain flash-glassified? Luckily, no one—not even the victim—knows, since “the custodian of the college died instantly from the impact with the hot volcanic ash surge,” study co-author Pier Paolo Petrone, a forensic anthropologist from the University of Naples Federico II in Italy, tells Reuters.