Shards of solid black material found on his skull, they wrote in Thursday’s edition of the New England Journal of Medicine, underwent a process called vitrification: Extreme high temperatures from the volcano liquefied the man’s brain, which then cooled quickly and turned into bits of glass.

“It’s the very first time that vitrified brain remains have been found,” Pier Paolo Petrone, a forensic anthropologist at the University of Naples Federico II, told The Washington Post on Thursday.

That may be because of the man’s lonely death.

Most of his neighbors in the ancient seaside town of Herculaneum, about 11 miles north of Pompeii, had succumbed to the volcano’s eruption on the beach. In waterfront chambers along the Gulf of Naples, hundreds of victims were buried and killed by an initial surge of fine ash, Petrone said.

The caretaker, however, was one of the few who appear to have stayed about 550 yards further inland, closer to Vesuvius. The researchers say he was killed in the volcano’s first pyroclastic surge, which made the town reach temperatures of 968 degrees Fahrenheit, but wasn’t buried until later waves of crushed volcanic rock flowed through Herculaneum.

(Most of Vesuvius’s victims in A.D. 79 had lived in the much larger city of Pompeii, where 2,000 out of 20,000 residents died by asphyxiation from the smoke and spewing ash, scientists say.)

In the 1960s, archaeologists found the man’s bones while digging through a structure belonging to the College of the Augustales, an imperial order devoted to the Roman emperor Augustus. He was thought to be 25 years old and working as a guard for the college, the only victim who died inside.

His skeleton was charred and cracked into many pieces, and decades would go by until researchers found anything more.

In October 2018, during one of Petrone’s frequent trips to the Herculaneum ruins to study the college and preserve the man’s bones, the victim’s skull grabbed his attention.

“I saw something shining inside the head,” he said, “and it was these small, glassy black fragments that were just attached inside the skull.”

Resembling obsidian stone, they were unlike anything else he had seen, even after studying dozens of other Vesuvius victims.

“This must be the brain,” he thought to himself.

Piero Pucci, a biochemist at the Center for Genetic Engineering in Naples, tested the unknown material and found the presence of fatty acids matching those found in human hair. But animals and vegetables also contain that substance, so it wasn’t enough to confirm the brain theory.

In their Thursday journal article, Petrone, Pucci and their colleagues said they could now confirm it. The fragments also contained proteins that are common in brain tissue, they wrote, and, crucially, those proteins were found only near his skull.

Solid black shards encrusted on the man’s skull contained proteins common in brain tissue, researchers found, and had undergone vitrification and transformed into glass.

Archaeologists rarely encounter preserved brain tissue, Petrone said, and when they do, the brain matter is only preserved as a soaplike substance. His team’s discovery marks the first time brains from any human or animal have been found fossilized as glass.

Petrone said the finding confirms his theory about how the extreme heat coming from Vesuvius killed people. Steam coming from their boiling blood created extreme pressure in their skulls, causing their heads to explode.

Not all scientists agree on that theory. Some think fractured skulls are a result of falling debris, and others now say that the volcano victims in Herculaneum were “baked” through longer exposure to heat, The Post’s Michael E. Ruane reported.

Petrone said he also discovered a spongy mass around the ancient caretaker’s chest bones, showing that the first surge of lava from Vesuvius was so hot that it probably burned the man’s body fat.

There may be more to uncover through the glass brains, though. In nature, the process of vitrification has only been found on vegetable remains, and it’s considered the most effective way to preserve human cells and tissues.

“It can perfectly preserve structures,” he said, “so it’s possible to imagine that we’re going to find more than just proteins.”