Five years ago Italian researchers published a study on the eruption of Mount Vesuvius in A.D. 79. that detailed how one victim of the blast, a male presumed to be in his mid 20s, had been found nearby in the seaside settlement of Herculaneum. He was lying facedown and buried by ash on a wooden bed in the College of the Augustales, a public building dedicated to the worship of Emperor Augustus. Some scholars believe that the man was the center’s caretaker and was asleep at the time of the disaster.
In 2018, one researcher discovered black, glossy shards embedded inside the caretaker’s skull. The paper, published in 2020, speculated that the heat of the explosion was so immense that it had fused the victim’s brain tissue into glass.
Forensic analysis of the obsidian-like chips revealed proteins common in brain tissue and fatty acids found in human hair, while a chunk of charred wood unearthed near the skeleton indicated a thermal reading as high as 968 degrees Fahrenheit, roughly the dome temperature of a wood-fired Neapolitan pizza oven. It was the only known instance of soft tissue — much less any organic material — being naturally preserved as glass.
On Thursday, a paper published in Nature verified that the fragments are indeed glassified brain. Using techniques such as electron microscopy, energy dispersive X-ray spectroscopy and differential scanning calorimetry, scientists examined the physical properties of samples taken from the glassy fragments and demonstrated how they were formed and preserved. “The unique finding implies unique processes,” said Guido Giordano, a volcanologist at the Roma Tre University and lead author of the new study.
Foremost among those processes is vitrification, by which material is burned at a high heat until it liquefies. To harden into glass, the substance requires rapid cooling, solidifying at a temperature higher than its surroundings. This makes organic glass formation challenging, Dr. Giordano said, as vitrification entails very specific temperature conditions and the liquid form must cool fast enough to avoid being crystallized as it congeals.
Dr. Giordano and his colleagues deduced that shortly after Vesuvius began belching up debris, a steaming toxic cloud of ash and white pumice flashed through Herculaneum, instantly killing its inhabitants. Claudio Scarpati, a volcanologist at the University of Naples Federico II, has proposed that this so-called pyroclastic density current was the third of 17 that spewed from Vesuvius.
Pulses of colder volcanic debris followed, engulfing the area. “The residents of Herculaneum were already dead by the time they were buried,” Dr. Giordano said.
Although the short-lived ash cloud left only an inch or two of debris and little if any structural damage, it is said to have heated the caretaker’s brain to well above 950 degrees, the glass transition temperature; this broke the soft tissue into smaller pieces without destroying it. Dr. Giordano said that the bones of the man’s skull and spine probably gave some protection to the brain.
As the ash cloud dissipated, temperatures quickly returned to normal. In the open air, at 950 degrees, the caretaker’s brain fossilized into glass. Only body parts containing some liquid can vitrify, Dr. Giordano said, which is why the caretaker’s bones remained intact.
The 2020 study was met with some skepticism by other scientists, largely because the raw data was not available. Tim Thompson, a forensic anthropologist at Maynooth University in Ireland, was perhaps the most vocal doubter. This time around, the results excited him. “I very much enjoy seeing new scientific methods applied to the archaeological context,” he said.
But Dr. Thompson would like to see more evidence and more of the original data: “The heating and cooling within Herculaneum following the eruption is likely to be complicated, and the results of the investigation certainly support their conclusions. It just depends on whether the material is brain.”
