Brain turned to glass reveals a city’s catastrophic final moments
New analysis of Mount Vesuvius victim supports claim that an ash cloud was the stealthy destroyer of Roman city
While his neighbors frantically fled from their oncoming doom, one man stayed in bed. For whatever reason, he didn’t join the other 2000 residents of Herculaneum—the ancient Roman city just north of Pompeii—as they ran from erupting Mount Vesuvius. The first scorching cloud of ash passed through the city so quickly, it turned his brain into black, glasslike shards. Now, a new analysis of these shards, published today in Scientific Reports, offers clues about how the man and his neighbors perished in 79 C.E.
Until recently, scientists believed the people of Herculaneum were annihilated by Mount Vesuvius’s avalanche of hot rock, ash, and gas that buried their city, along with Pompeii. But when researchers announced the discovery of those black, glossy chips in 2020, a new culprit emerged: a swift ash cloud preceding this flood of debris. This rethink, however, hinged on whether the brain had indeed turned to glass.
For glass to form, a liquid needs to be cooled so rapidly that its molecules suddenly “freeze” into a rigid structure rather than forming crystals. For this reason, glass is sometimes referred to as a “liquid solid,” says Brian Wowk, a cryobiologist at 21st Century Medicine who was not involved with the work. Thick blankets of pyroclastic flow–the torrent of rocks, ash, and gas expelled by volcanos–cool off far too slowly for glass to form, says study co-author Guido Giordano, a volcanologist at the Roma Tre University. “Once they’re in place, they can take years to cool down.”
Hence the alternative hypothesis: The first flood of hot debris stopped short of Herculaneum or got diverted elsewhere, delivering a lighter cloud of the ash and gas within reach of the city, Giordano explains. The cloud hastily heated the city before dissipating. The rapid heating and cooling allowed the liquefied brain to harden into a glass before the next wave of pyroclastic flow from the ongoing eruption poured in.
This theory hinged on proving the remains were not a crystalized structure that could have formed through a slower cooling process. To show this, Giordano and colleagues took a closer look at the brain, which belonged to a man believed to have been a guard of the Collegium Augustalium—a center for the cult of Emperor Augustus. The team exposed small samples of the supposed brain glass to steadily increasing heat while measuring their temperature. When it got hot enough, the “liquid solid” was finally able to relax, releasing its stored potential energy and producing a nonlinear spike in temperature. Had the samples been crystalized, this characteristic relaxation and spike would not have occurred.
The glassy brain uncovered at Herculaneum is the only known example of a brain being naturally preserved as glass, the researchers report. Therefore, they took care to exhaustively test the samples. Repeating the experiment at a different rate of heating revealed the same results. And when the scientists used spectroscopy to examine the molecular structure of the samples, they found other telltale signs of a relaxed glass.
The finding suggests the event “that impacted the city and killed this individual was very short lived, and very, almost, impulsive,” says Benjamin Andrews, a volcanologist at the Smithsonian Institution's National Museum of Natural History who was not involved with the work.
The Herculanean man’s choice to stay in his bedroom while his neighbors fled may have produced the very specific conditions necessary for the glass to form, Giordano says. Without those glass shards, researchers might never have discovered the deadly effect of the ash cloud. That’s because such a cloud would have only left a few centimeters of ash behind, buried under 40 meters of pyroclastic flow.
“It’s a remarkable finding,” Andrews says. The research, he says, is an important example of how even tiny discoveries can reveal a lot about these events. “There’s a huge story, a huge wealth of information, contained in these little particles.”
