Top image: A piece of organic glass discovered within the skull of a man from ancient Herculaneum.

A young Roman man was sleeping peacefully in his bed one night nearly 2,000 years ago, when a cloud of scorching-hot ash descended from the top of the erupting Mount Vesuvius to smother him and turn his brain to glass. This happened in the city of Herculaneum, just down the road from Pompeii, in October of 79 AD.

This is the conclusion of a team of Italian scientists, who recently finished their study of an ancient Roman brain extracted during excavations at Herculaneum, a neighbor of Pompeii that was destroyed by the same eruption that doomed the latter city. Their analysis showed that this young man’s brain had indeed been converted into glass, a remarkable outcome that had never been seen in the archaeological record before.

 “Although human brain preservation is documented in the archaeological record, it is a relatively infrequent phenomenon,” the study authors noted in a Scientific Reports article about their amazing discovery. “The known mechanisms responsible for such occurrences are dehydration, saponification, tanning, and freezing, but not vitrification [the process of conversion into glass].”

And yet, incredibly, it was vitrification that occurred in this case.

“Our comprehensive chemical and physical characterization of the material sampled from the skull of a human body buried at Herculaneum by the 79 AD eruption of Mount Vesuvius shows compelling evidence that these are human brain remains, composed of organic glass formed at high temperatures,” the Italian scientists confirmed, “a process of preservation never previously documented for human or animal tissue, neither brain nor any other kind.”

This one-of-a-kind brain is forcing a reevaluation of what exactly happened on that fateful day in October in the year 79, and what is learned could change the way the threat from future volcanic eruptions is perceived.

Frozen in Time, with a Brain Made of Glass

When Mount Vesuvius, which is near the modern-day Italian city of Naples, experienced its most catastrophic eruption 1,946 years ago, the cities of Pompeii and Herculaneum were buried in by a fast-moving, thick river of rock and ash known as pyroclastic flow. Thousands of bodies have been discovered at both sites, preserved in their death states and providing insights into how people responded to this disaster, and about how what they’d been doing just before the end arrived.

In the 1960s, the toasted remains of a young adult male were found on a wooden bed in a Herculaneum building dedicated to the worship of the first Roman Emperor, Augustus. While taking a closer look at his remains in 2018, Italian anthropologist Pier Paolo Petrone, a co-author of a new study, noticed something peculiar. It seems the man’s brain was in a most unusual condition, as it was shimmery and shiny just like glass – which is exactly what it was. In fact there was nothing left of the man’s brain except glass, which was something Petrone had never seen (or heard of) before.

(a) The charred remains of the guardian were found in his wooden bed at the Collegium Augustalium, with vitrified brain matter preserved inside his skull. (b) A panoramic eastward view of the Herculaneum ruins, showcasing Mount Vesuvius in the background

The glass “chips” found in the man’s skull were about one centimeter wide. When the researchers looked at them under an electron microscope, they were stunned to see that complex networks of neurons, axons and other identifiable parts of the man's brain and spinal cord had been preserved.

Glass is rare in nature, because it can only be produced when a substance is heated to an extremely hot temperature before cooling rapidly, preventing its crystallization. Only meteorites strikes, lightning, or exposure to lava are likely to produce such a result, although turning human tissue to glass is extremely difficult because it is made mostly from water.

As of now, the vitrification of the Herculaneum resident’s brain is the only example of this phenomenon known from nature. For this to happen, the Italian scientists say, his brain must have been exposed to temperatures higher than 510 degrees Celsius (950 degrees Fahrenheit). The scientists determined this by repeatedly heating and rapidly cooling small samples of the man’s glassy brain, to see what temperatures were required to produce the observed effect.

Notably, this is hotter than the pyroclastic flow that buried the city, which never surpassed 465C (869 degrees Fahrenheit). The "only possible scenario" for what occurred is that the ash cloud emitted by Vesuvius delivered an initial blast of incredibly intense heat before quickly dissipating, the study said. This theory is supported by the existence of a thin layer of ash that settled in the city shortly before it was smothered, and it means the people of Herculaneum were killed by the ash cloud, and not the pyroclastic flow as had previously been believed.

“What is an ash cloud? It’s a dilute part of the pyroclastic flow,” explained lead study author Guido Giordano, a volcanologist at Roma Tre University in Rome, in an interview with CNN. “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. The ash cloud basically instantly killed the people, because they were engulfed in a cloud that was probably about 510, maybe 600 degrees (Celsius)( 1112 degrees Fahrenheit).”

SEM images (scale bar magnification) of sample C1 following C-DSC experiments at increasingly higher peak temperatures.

Recognizing the Threat

Moving forward, Giordano and his colleagues hope their research will increase knowledge about the threat posed by hot ash clouds following volcanic eruptions. This phenomenon is not easy to study because it leaves only traces behind, although researchers who study volcanoes do known that 215 people were killed in the 2018 eruption of Guatemala's Fuego volcano by their exposure to such a cloud.

In the case of the man from Herculaneum, the scientists theorize that he had been sleeping at the time of the eruption, in a building (the Collegium Augustalium ) located in the first region of the city to be hit by the scorching ash cloud. Most people were awake and had been able to escape from the Mount Vesuvius fallout , which arrived later than in Pompeii. But this one individual was not so fortunate and ended up as a historical anomaly as a result of his bad luck.