Vitrified fortresses found in Europe

About 200 ancient stone fortresses in Europe have been vitrified by heating, puzzling archaeologists for the past 250 years.

During the Bronze and Iron Ages, Europeans built many stone hill forts and ramparts. About 200 of these structures show signs of being exposed to intense heat. The walls were burned at such high temperatures that the rocks partially melted and fused together, a phenomenon known as vitrification. For 250 years, the vitrified forts have remained a mystery to archaeologists.

Illustration of the vitrified fortress of Dun Deardail in Glen Nevis, Scotland. (Photo: Scotsman).

Initially, the vitrification process was thought to be a result of past battles. But strangely, vitrification is the only thing holding the stone walls together. These forts do not contain any binding materials such as mortar or lime. Some evidence suggests that the stones were stacked in a dry state, then intentionally burned, causing them to fuse into solid blocks.

Currently, scientists have proposed two hypotheses. The first hypothesis suggests that the melting of the stone wall was the unintended consequence of another activity, such as metal forging, furnaces, or signaling fires. The second hypothesis suggests that it was the result of an incredible construction effort.

In a pioneering experiment conducted in the 1930s by engineer Wallace Thorneycroft and archaeologist Vere Gordon Childe, a 6-foot by 6-foot wall was built of stones interspersed with horizontal wooden slats, then set on fire. The fire burned for three hours, after which the wall collapsed. Thorneycroft and Childe found that the rubble had vitrified, with fragments of wood embedded in it. They estimated the fire to be about 2,500 degrees Fahrenheit.

According to another study by E. Youngblood of the Smithsonian Institution and his colleagues, published in the journal Archaeological Science in 1978, the burning of a wall with wood is not enough to explain the intense vitrification of ancient forts. The research team believes that the fire that caused this vitrification could have burned for many days at temperatures above 1,000 degrees Celsius.

This could only have happened if the fire had been controlled, for example by filling the spaces between the stones in a wooden frame with soil, clay, and combustible materials such as peat. It is unlikely that the walls were set ablaze by accident or by enemies, which would mean that the burning was intentional. But why did the ancients do it?

The vitrified fortress at Sainte-Suzanne, France. (Photo: jp.morteveille/Wikimedia).

One possible explanation is that it was done to strengthen the stonework. This explanation has been dismissed by many researchers, since heating weakens the stone structure by creating microscopic cracks as the stone expands at different rates. However, a 2016 study published in the journal Archaeological Science suggests that this is not true of sandstone, a common fortification material. The stone gains strength under heat because the small particles within the stone fuse together to form a solid mass of glass. If true, ancient vitrified forts were truly ingenious constructions.

Initially, scientists thought that vitrified forts existed only in Scotland. But they have since been found in several other areas of Western and Northern Europe. There are more than 200 such structures across Europe, 70 of which are in Scotland.