A rare organic glass was found inside a skull from Herculaneum’s 79 CE Vesuvius eruption. Researchers determined that a super-heated ash cloud briefly raised the brain’s temperature above 510°C, then cooled rapidly, forming the glass.

A unique, dark-colored organic glass found inside the skull of an individual who died in Herculaneum during the 79 CE Mount Vesuvius eruption likely formed when the person was killed by an extremely hot but short-lived ash cloud. This conclusion, based on research published in Scientific Reports, stems from an analysis of the physical properties of the glass, which is believed to comprise the fossilized brain of the individual.

Glass rarely forms naturally due to the specific conditions required for its creation. For a substance to become glass, its liquid form must cool rapidly enough to prevent crystallization as it solidifies—requiring a significant temperature difference between the substance and its surroundings—and it must solidify at a temperature well above that of its environment.

As a result, it is extremely difficult for an organic glass to form, as ambient temperatures are rarely low enough for water — a key component of organic matter — to solidify. The only suspected natural organic glass was identified in 2020 in Herculaneum, Italy, but it was not clear how this glass formed.

Analysis of the Herculaneum Skull Fragments

Guido Giordano and colleagues analyzed fragments of glass sampled from inside the skull and spinal cord of a deceased individual from Herculaneum, found lying in their bed in the Collegium Augustalium. The results of the analysis — which included imaging using X-rays and electron microscopy — indicated that, for the brain to become glass, it must have been heated above at least 510 degrees Celsius before cooling rapidly.

The authors note that this could not have occurred if the individual was heated solely by the pyroclastic flows which buried Herculaneum, as the temperatures of these flows did not reach higher than 465 degrees Celsius and would have cooled slowly. The authors therefore conclude, based on modern volcanic eruption observations, that a super-heated ash cloud which dissipated quickly was the first deadly event during Vesuvius’s eruption. They theorize that such an event would have raised the individual’s temperature above 510 degrees Celsius, before it rapidly cooled to ambient temperatures as the cloud dissipated. The bones of the individual’s skull and spine likely protected the brain from complete thermal breakdown, allowing fragments to form this unique organic glass.

Reference: “Unique formation of organic glass from a human brain in the Vesuvius eruption of 79 CE” by Guido Giordano, Alessandra Pensa, Alessandro Vona, Danilo Di Genova, Raschid Al-Mukadam, Claudia Romano, Joachim Deubener, Alessandro Frontoni and PierPaolo Petrone, 27 February 2025, Scientific Reports.
DOI: 10.1038/s41598-025-88894-5