However, one Scottish archaeologist, Helen Nisbet, believes that the vitrification was not done on purpose by the builders of the forts. In a thorough analysis of rock types used, she reveals that most of the forts were built of stone easily available at the chosen site and not chosen for their property of vitrification.

The vitrification process itself, even if purposely set, is quite a mystery. A team of chemists on Arthur C. Clarke's Mysterious World subjected rock samples from 11 forts to rigorous chemical analysis, and stated that the temperatures needed to produce the vitrification were so intense--up to 1,100°C--that a simple burning of walls with wood interlaced with stone could not have achieved such temperatures.

Nevertheless, experiments carried out in the 1930s by the famous archaeologist V. Gordon Childe and his colleague Wallace Thorneycroft showed that forts could be set on fire and generate enough heat to vitrify the stone. In 1934, these two designed a test wall that was 12 feet long, six feet wide and six feet high, which was built for them at Plean Colliery in Stirlingshire. They used old fireclay bricks for the faces and pit props as timber, and filled the cavity between the walls with small cubes of basalt rubble. They covered the top with turf and then piled about four tons of scrap timber and brushwood against the walls and set fire to them. Because of a snowstorm in progress, a strong wind fanned the blazing mixture of wood and stone so that the inner core did attain some vitrification of the rock.

In June 1937, Childe and Thorneycroft duplicated their test vitrification at the ancient fort of Rahoy, in Argyllshire, using rocks found at the site. Their experiments did not resolve any of the questions surrounding vitrified forts, however, because they had only proven that it was theoretically possible to pile enough wood and brush on top of a mixture of wood and stone to vitrify the mass of stone. One criticism of Childe is that he seems to have used a larger proportion of wood to stone than many historians believe made up the ancient wood and stone fortresses.

An important part of Childe's theory was that it was invaders, not the builders, who were assaulting the forts and then setting fire to the walls with piles of brush and wood; however, it is hard to understand why people would have repeatedly built defences that invaders could destroy with fire, when great ramparts of solid stone would have survived unscathed.

Critics of the assault theory point out that in order to generate enough heat by a natural fire, the walls would have to have been specially constructed to create the heat necessary. It seems unreasonable to suggest the builders would specifically create forts to be burned or that such a great effort would be made by invaders to create the kind of fire it would take to vitrify the walls--at least with traditional techniques.

One problem with all the many theories is their assumption of a primitive state of culture associated with ancient Scotland. "

Looking at the amount of wood required to vitrify a fort, in addition to the wood contained in the rampart and coupled with the proposal that the process mahes makes the fort stronger. One would have to ask why would an enemy do this?

Therefore, I propose that vitrification was the result of a deliberate act by the builders as part of construction, and was very probably linked with a lengthy religious dedication ceremony and festival.