Nearly 2,000 years after a young man died in the Vesuvius volcanic eruption, scientists have discovered that his brain was preserved when it turned to glass in an extremely hot cloud of ash.

Researchers found the glass in 2020 and speculated that it was a fossilised brain but did not know how it had formed.

The pea-sized chunks of black glass were found inside the skull of the victim, aged about 20, who died when the volcano erupted in 79 AD near modern-day Naples.

Scientists now believe a cloud of ash as hot as 510C enveloped the brain then very quickly cooled down, transforming the organ into glass.

It is the only known case of human tissue – or any organic material – turning to glass naturally.

“We believe that the very specific conditions that we have reconstructed for the vitrification [the process of something turning into glass] of the brain make it very difficult for there to be other similar remains, although it is not impossible,” Prof Guido Giordano from Università Roma Tre told BBC News.

“This is a unique finding,” he said.

The brain belonged to a man killed in his bed inside a building called the Collegium on the main street of the Roman city Herculaneum.

The fragments of glass found by the scientists range from 1-2 cm to just few millimetres in size.

The massive eruption of Vesuvius engulfed Herculaneum and nearby Pompeii where up to 20,000 people lived. The remains of about 1,500 people have been found.

Scientists now think the hot ash cloud descended from Vesuvius first, probably causing most of the deaths.

A fast-moving current of hot gas and volcanic matter, also called a pyroclastic flow, followed, burying the area.

Experts believe the ash cloud turned the man’s brain into glass because the pyroclastic flow would not have reached high enough temperatures or cooled quickly enough.

The process of glass formation requires very specific temperature conditions and rarely occurs naturally.

For a substance to turn to glass, there must be a huge temperature difference between the substance and its surrounding.

Its liquid form has to cool fast enough not to crystallise when it becomes solid, and it must be at a much higher temperature than its surroundings.

The team used imaging with x-rays and electron microscopy to conclude that the brain must have been heated to at least 510C before cooling rapidly.

No other parts of the man’s body are believed to have turned to glass.

Only material containing some liquid can turn to glass, meaning that the bones could not have vitrified.

Other soft tissues, like organs, were likely destroyed by the heat before they could cool down enough to turn to glass.

The scientists believe the skull gave some protection to the brain.

The research is published in the scientific journal – a publication where researchers report their work to other experts – Scientific Reports.