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Glass rarely forms naturally from organic materials. However, in 2020, researchers discovered a black, glassy substance inside the skull of a person killed during the eruption of Italy’s Mount Vesuvius in AD 79.
Now, the scientists say they have worked out the sequence of events that likely killed the victim and led to the formation of the unique and puzzling glass, thought essentially to constitute fossilized brain tissue.
Recovered from the coastal town of Herculaneum, which along with Pompeii was wiped out by the eruption, the remains belonged to an individual, thought to be a young man, who was found lying face down on a bed buried by volcanic ash.
A new analysis of samples of the glass found inside the skull and spinal cord suggests that the person’s body tissue must have been heated to above 510 degrees Celsius (950 degrees Fahrenheit) before cooling rapidly to allow the glass to form in a process known as vitrification.
“The process of transformation of anything liquid into glass is the fast cooling, not the fast heating,” said Guido Giordano, a volcanologist at Roma Tre University in Rome and lead author of the study published Thursday in the journal Scientific Reports.
“Obsidian glass, that is a volcanic glass, forms when lava is very quickly cooled, for example, where it enters into water,” Giordano added.
However, the pyroclastic flows, composed of fast-moving volcanic material and toxic gas, that charged out of Vesuvius and buried Herculaneum could not have caused the brain tissues of this young man to turn into glass, he said. The temperatures of these flows did not reach higher than 465 C (869 F), the study said. Plus, they would have cooled slowly.
Instead, based on observations of more recent eruptions, an extremely hot ash cloud that dissipated quickly could have created the conditions necessary for the vitrification of human brain tissue to occur, the study concluded. But the specific set of circumstances needed to vitrify soft tissue has raised some skepticism in the scientific community.
The young man’s skull and spine likely protected the brain from “complete thermal breakdown,” allowing fragments of the unique organic glass to form.
Unlike pyroclastic flows, which hug the ground, an ash cloud is airborne. However, the two are linked, Giordano said.
“What is an ash cloud? It’s 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,” he explained. “These clouds can be hot enough to kill you.”
To reach the findings, Giordano and his colleagues systematically cooled and heated fragments of the glass sampled from inside the skull and spine to understand what degree of heat and subsequent cooling was necessary. They found that the brain tissue transformed into glass at a temperature of at least 510 C (950 F).
“The ash cloud basically instantly killed the people, because they were engulfed in a cloud that was probably about 510, maybe 600 degrees (Celsius),” he added.
At the bottom of the layers of ash and rubble that buried Herculaneum is a layer of fine volcanic ash that may have been deposited by the ash cloud, Giordano said.
Vitrification of soft tissue was “incredibly unlikely,” said Alexandra Morton-Hayward, a forensic anthropologist at the UK’s University of Oxford, adding that she was not convinced that the glassy substance was brain tissue. Morton-Hayward has compiled a unique archive of information about 4,405 brains unearthed by archaeologists. She was not involved in the research.
She said organic tissues, which are mostly water, can only be vitrified by rapid cooling to extremely low temperatures, well below zero C (32 F) — a process known as cryopreservation.
Cryopreservation involves cooling organs in liquid nitrogen to prevent ice formation, stabilizing them in a solid-like state while maintaining their molecular structure. “I’m not convinced this (artifact) is the one and only exception to this rule.”
Giordano said there was no doubt that the glass was organic in origin. Previous research has shown that neurons and proteins in the individual’s brain were preserved, he noted.