Throughout the Bronze and the Iron Ages, Europeans have constructed hilltop forts and enclosures made of stone. About two hundred examples of these show signs of intense heat damage. These stone walls were burned at such high temperature that the rocks have partially melted and fused with each other. They are known as vitrified forts, and for the past 250 years they have been a source of mystery for archeologists.

At first, the vitrification was thought to be scars from past battles, except for the curious fact that the vitrification was the only thing holding the stone walls together. None of these vitrified forts contain any cementing material such as mortar or lime. Evidence suggests that the rocks were stacked dry and then set on fire deliberately to fuse them together to one solid block —an extraordinary method of construction.

An artists impression of Dun Deardail vitrified fort in Glen Nevis. Photo credit: www.scotsman.com

Currently, there are two schools of thought. One believe that the vitrification of the stone walls was the unintended consequence of another operation, such as metal forging, hearths and signal fires. The other see these forts as a result of constructive efforts. In a pioneering experiment conducted by Wallace Thorneycroft and Vere Gordon Childe in the 1930s, a wall six feet by six feet was constructed with stone slabs and interlaced with horizontal timber, and then set to fire. The fire burned for three hours after which the wall collapsed. Thorneycroft and Childe found the rubble vitrified with pieces of timber embedded in the vitrified glass. They estimated the fires to be about 1200°C hot.

Another study conducted by Youngblood and co-workers and published in the Journal of Archaeological Science in 1978 found that simply burning an ordinary timber-laced wall cannot account for the strong vitrification found, and that the fires probably razed for days with sustained high temperatures of over a thousand degrees. This is only possible when the fires are contained, such as by filling the space between the rocks in the timber framework with soil, clay and combustible materials such as peat. It seems unlikely that the walls were burned accidentally or set on fire by enemies, which means the act was deliberate. This leaves the question: why?

Vitrified rock at Dunagoil fort, Scotland. Photo credit: Rod Collier

Vitrified Fort at Sainte-Suzanne, France. Photo credit: jp.morteveille/Wikimedia

One possible explanation is to strengthen the stonework. This theory has been dismissed by many researchers because heating rocks generally weakens them by creating microcracks caused by differential expansion of the rocks. However, a new study suggests that this is not true of sandstone—a material commonly used to construct forts. Sandstone increases in strength when subjected to heat by causing the small particles within the rock to fuse together into a dense mass of glass. If this is true, these vitrified forts were truly ingenious constructions.

Vitrified forts were originally thought to exist only in Scotland, but they have since been identified in several other parts of western and northern Europe. More than two hundred such examples exist across Europe, of which seventy are in Scotland alone.