Vitrified fortresses in Europe

During the Bronze and Iron Ages, Europeans built many stone forts and citadels on the hills. About 200 such structures show signs of being affected by high temperatures. The walls were burned at such high temperatures that the rocks partially melted and fused together, a phenomenon called vitrification. For the past 250 years, the vitrified fortress has remained a mystery to archaeologists.

Initially, the vitrification process was thought to be a consequence of past battles. But strangely, vitrification is the only thing keeping the stone walls together. These forts do not contain any binding materials such as mortar or lime. Some evidence shows that the rocks were stacked on top of each other in a dry state, then intentionally burned, causing them to merge into solid blocks.

Currently, the scientific community offers two hypotheses. The first hypothesis is that the melting of the stone wall was an unintended consequence of another activity, such as metal forging, furnace lighting or signaling fires. The second theory is that this is the result of an incredible construction effort.

In a pioneering experiment conducted by engineer Wallace Thorneycroft and archaeologist Vere Gordon Childe in the 1930s, a wall measuring 1.8 m x 1.8 m was built of stone interspersed with horizontal wooden slats, then burned. The fire burned for 3 hours, then the wall collapsed. Thorneycroft and Childe found the rubble had been vitrified with pieces of wood mixed in. They estimated the fire was about 1,200 degrees Celsius.

According to another study conducted by expert E. Youngblood at the Smithsonian Institution and his colleagues, published in the journal Archaeological Science in 1978, the burning of a wall mixed with wood was not enough to explain the strong vitrification of ancient fortresses. The research team believes that the fire that caused this vitrification could have burned for many days with a sustained temperature of over 1,000 degrees Celsius.

This can only happen when the fire is controlled, for example, by filling the spaces between the rocks in a wooden frame with soil, clay and combustible materials such as peat. It is unlikely that the walls were burned by accident or by enemies, meaning the act of burning was intentional. But why did ancient people do that?

One possible explanation is to increase the durability of stone masonry. This explanation has been rejected by many researchers because heating often weakens the rock structure by creating small cracks due to different expansion of the rock. However, a study published in the journal Archaeological Science in 2016 suggested that this was not true of sandstone – a popular fortress building material. This type of stone increases its durability under heat because the small particles inside the stone combine together to form a solid glass mass. If true, ancient vitrified fortresses were truly works of genius.

Initially, scientists believed that vitrified fortresses only existed in Scotland. But later, they were also found in some other areas of Western Europe and Northern Europe. There are more than 200 such structures across Europe, of which 70 are in Scotland.