It was a surprising discovery when scientists examining the remains of a man who died in bed in the ancient city of Herculaneum after Italy's Mount Vesuvius erupted in 79 AD found dark fragments resembling obsidian inside his skull. It turns out the eruption had somehow turned his brain into glass.
It is the only case on record of such a phenomenon, and researchers now have answers for why and how it happened.
They say the vitrification - transformation into glass - of this victim's brain was the apparent effect of a scorching ash cloud that suddenly descended upon his city along the Bay of Naples, instantly killing all the inhabitants.
They concluded vitrification took place through a unique process of rapid exposure of the brain's organic material to a very high temperature - at least 510 degrees Celsius (950°F) - and its subsequent rapid cooling.
The researchers conducted an extensive analysis that confirmed the glass nature of the fragments and revealed their physical properties.
"The glass formed as a result of this process allowed for an integral preservation of the biological brain material and its microstructures," said forensic anthropologist Pier Paolo Petrone of Università di Napoli Federico II in Italy, one of the leaders of the research published in the journal Scientific Reports.
The eruption obliterated the thriving ancient Roman cities of Pompeii and Herculaneum.
"The only other type of organic glass we have evidence of is that produced in some rare cases of vitrification of wood, sporadic cases of which have been found at Herculaneum and Pompeii. However, in no other case in the world have vitrified organic human or animal remains ever been found," Petrone added.
Ancient destruction
Thousands of people were killed and the two cities were buried under a thick layer of volcanic material and mud, sitting untouched until their rediscovery in the 18th century. Victims were preserved in sudden death, as was the case with the one whose brain was turned to glass.
His body was first discovered in the 1960s inside a building called the College of the Augustales dedicated to the cult of Emperor Augustus, who had died in 14 AD. The individual was identified as a young man believed to have been the college's custodian. The remains were re-examined in 2018.
"I was in the room where the college's custodian was lying in his bed to document his charred bones. Under the lamp, I suddenly saw small glassy remains glittering in the volcanic ash that filled the skull," Petrone said.
"Taking one of these fragments, it had a black appearance and shiny surfaces quite similar to obsidian, a natural glass of volcanic origin - black and shiny, whose formation is due to the very rapid cooling of the lava. But, unlike obsidian, the glassy remains were extremely brittle and easy to crumble," Petrone said.
Analyses of this material, previously published in other scientific journals, revealed the presence of proteins and fatty acids common in human brain tissues, with the entire central nervous system exceptionally well preserved, represented by nerve cells interconnected by a dense network of fibers called axons.
The research is forging a deeper understanding of how the tragic events unfolded after the eruption.
"The study shows that the 'killer' at Herculaneum was the arrival in town of an early hot ash cloud. This highlights the importance of understanding the behavior of ash clouds, as they are very hazardous and still very poorly studied and understood," said volcanologist and study first author Guido Giordano of Roma Tre University in Italy.
The actual burial of the city occurred in later stages of the eruption.
The custodian at apparently around midnight was surprised while sleeping in his bed by the first effects of the eruption.
"As the postures of the victims' bodies show, the custodian of the college died instantly from the impact with the hot volcanic ash surge, as did all the rest of the inhabitants of Herculaneum," Petrone said. "The body of evidence found for the victims at Herculaneum shows that all people died instantly, so they did not have time to notice or suffer."