This presentation examines whether the vitrified forts of Scotland could have been created by a lost ancient technology, by accidental or enemy fire, or by a coronal mass ejection of the Sun.
Transcript:
Vitrified Forts: Ancient technology? Destructive burning? Or solar plasma?
The construction of hillforts, such as Dun Deardail in Scotland, was prevalent in what archaeologists traditionally call the Iron Age - the time between roughly 700 BC and 700 AD. There are more than 50 vitrified forts in Scotland, and they are also found throughout Europe.
Hillforts are hilltop settlements that take a range of shapes and sizes, but all include fortifications, usually in the form of ditches and ramparts. At one time, they might have had walkways and watch towers around the circumference of the site to monitor the surrounding area.
One peculiarity of these forts is that the stone walls are fused as if melted by great heat. This vitrification has been attributed to intentional fires, accidental fires, and even to coronal plasma mass ejections from the Sun.
The Tap o'Noth is a hill and fort, 20 miles west of Inverurie in Aberdeenshire, Scotland. It is the second highest fort in Scotland and it has a well-preserved vitrified wall that encloses an area of approximately 100 meters by 30 meters.
In 1980, the TV program "Arthur C. Clarke's Mysterious World" featured Scottish vitrified forts. In this episode, Professor Ian Ralston, who is now at the department of archeology at the University of Edinburg, examined the walls of the Tap o'Noth Fort. He noted that "It's absolutely solid. In places one can kick it and it won't disintegrate. It's as if it's cemented together. In fact, what appears to have happened, is that at some stage, presumably with great heat, the rocks have melted or at least partly melted."
As part of the TV program, a wall was built by professional dry stone wallers in Aberdeen, Scotland. The wall contained wooden beams like those documented in the vitrified walls, and a large quantity of wood was piled against the wall and then it was set on fire. After twenty-two hours of burning, there were no ramparts of fused stone, but a very small quantity of the rocks indeed melted and then resolidified. Most of the fused rocks were little pieces with no great chunks of vitrified material. Considering the large amount of lumber used, the program concluded that it would have taken half the trees in Scotland to vitrify all the forts.
The vitrified hillfort of Dun Deardail sits high above Glen Nevis, overshadowed by a higher adjacent hill called Ben Nevis. It was built in the middle of the first millennium BC, around 2500 years ago, and it was eventually destroyed in a catastrophic fire.
The Dun Deardail project was established to study the vitrified hillfort with funding from the Forestry Commission Scotland and the Heritage Lottery Fund. Technical support was provided by AOC Archaeology Group and the University of Stirling. The project recruited and trained many volunteers from the community who were interested in the local archaeology, its historic environment, and learning opportunities for local schools. The three-year project produced a booklet that gives a glimpse at the Iron Age from the first ever excavation of Dun Deardail. A link to the booklet is given in the description of this video.
The archaeological excavation explored the terraced interior of the hillfort and its enclosing walls. Professional archaeologists and volunteers investigated the construction of the rampart and dug down looking for evidence of houses, hearths and workshops. The hillfort of Dun Deardail measures about 46 meters by 28 meters. The wall is well-preserved on the southwest side, but it is not clear where the original entrance would have been located. Much of the vitrified material appears to have been displaced from its original location, with many blocks having collapsed downslope.
Dun Deardail was clearly built with a significant timber component. Many of the trenches showed evidence for the complexity and skill involved in the drystone construction of the wall, with differing stones being laid down both horizontally and vertically around the interlacing timbers. The large amount of collapsed material found across all of the trenches suggests that the wall itself was massive, and perhaps very high. The upper areas of the wall were the most vitrified.
The hillfort is surrounded by several areas of natural peatland, which is waterlogged bog made up of layer-upon-layer of partly decomposed plant matter. Peat preserves the organic materials embedded within it, and the layers act as a stratigraphic chronological record. Results of radiocarbon dating demonstrate that Iron Age activity at Dun Deardail began in the 5th century BC and ended sometime in the 4th to 2nd centuries BC.
Analysis of a peat core identified four significant fire events. The first occurred around 400 BC and could represent fire being used to clear trees and scrub during the construction of the hillfort. Following this first event, fire activity decreases slightly but remains significant, likely representing local hearths and fires associated with daily life at the hillfort. The largest of the fire events dates to between 347 and 284 BC. This probably represents the destruction of the hillfort. The date was refined further using a technique called fire event analysis that examines tiny flecks of charcoal expelled by the fire which become trapped in a near-by peat bog. This dates the vitrification event more precisely, suggesting that it occurred around 310 BC. After this peak burning event, the abundance of charcoal decreases in the core and ultimately disappears from the record by 200 BC, possibly signifying the final abandonment of the site.
The dominant material recovered during the excavation was fragments of vitrified stone which had collapsed from the heavily burnt ramparts. The process of vitrification occurs when a timber-framed drystone rampart is destroyed by fire. With temperatures reaching over 1000 degres Celsius, the heat from the blaze begins to melt the rubble core of the rampart. As the burning rampart collapses, the rocks first fracture and then become liquid. Gas bubbles form inside the rocks as the extreme temperatures change their mineral composition. When the fire burns out and the rampart finally cools, the burnt and molten rocks form large blocks of conglomerated stone.
The Dun Deardail project report concludes that vitrification is not a deliberate construction method as the original timber-framed drystone rampart would have been more stable, and that it is much more likely that the vitrification was the result of accidental fire or deliberate destruction. Since the vitrified core is only found in the upper section of the rampart, it may have been caused by the collapse and burning of a substantial timber and thatch superstructure. The vitrified rubble upper core of the rampart has three voids that are roughly spaced about half a meter apart. These represent the locations of horizontal timber beams that once lay within the rampart wall.
This image shows a clast from the ramparts of the Dun Deardail hillfort with the glassy appearance and bubble cavities characteristic of vitrification. When the composition of this clast was compared to samples of the local rocks surrounding the hillfort, the results were very similar. The rocks in the clast were most likely local rocks, chosen for their ready availability, rather than rocks that had been brought from elsewhere.
As part of the Highland Archaeology Festival in October 2001, local archaeologist and historian Roddy Mainland attempted to recreate the process of vitrification by constructing a short section of timber-laced rampart and burning it. The fire burned fiercely for about three hours until the structure began to collapse. A safety watch was maintained through the night, during which time 90% of the wood was burnt away, leaving a smoking core. At midday the next day, light rain had quenched the fire completely, but there was still heat within the core. Inspection of this material failed to provide any evidence of vitrification. Six days later, a trench was dug through the still smoking core, which was still too hot to touch, but again there was no vitrified material.
A paper published in 2016 reported that the firing of Iron Age fort walls to temperatures where significant vitrification is achieved through sintering is likely to result in significant strengthening. The paper provides experimental evidence that firing of Iron age forts may have led to their strengthening. The new evidence is based on controlled experiments on heat-treated samples of sandstone. This conclusion completely contradicts the report of the Dun Deardail project, which proposes that the vitrification was the result of accidental or intentional fire, rather than an ancient technology of fort wall construction.
Dr. Robert Schoch proposed a hypothesis that the onset of the Younger Dryas was due to reduced solar activity, and that the end of the Younger Dryas was brought about by solar outbursts and coronal mass ejections from the Sun. In his words: "Plasma hitting the surface of the Earth could heat and fuse rock, incinerate flammable materials, melt ice caps, vaporize shallow bodies of water creating an extended deluge of rain, and send the climate into a warming spell." Dr. Schoch writes that "Plasma and electrical discharges hitting the surface of Earth may have been responsible for the vitrification (melting into crude glass) of ancient stone structures seen in some parts of the world, such as various hill forts in Scotland.
The suggestion that solar outbursts caused vitrification has several problems. The Dun Deardail hillfort is not as high as the adjacent Ben Nevis hill, so why isn't Ben Nevis vitrified? It seems that a solar flare would have vitrified the taller hills that are closer to the sun before melting the lower hillfort. Also, the fact that the fires that caused the vitrification can be precisely radiocarbon dated is an argument against a world-wide coronal mass ejection catastrophe since no large-scale extinctions are known for those dates.
The difficulty of creating vitrified walls experimentally, makes it necessary to consider the chemistry of rocks and minerals. Eutectic mixtures of minerals melt at hundreds of degrees lower than the individual constituents. The alkali metals, sodium and potassium, are used in pottery glazing. Sodium and potassium act as fluxes that produce chemical compounds with lower melting points. This chart illustrates that a mixture of approximately equal parts of potassium aluminum-silicate and silicon dioxide, which is sand, melts at a temperature 700 degrees lower than either component.
If vitrification of the fort walls was intentional during their construction, we need to consider that perhaps salt, which is readily available, was used as a fluxing agent. The sodium in the salt reacts to form a sodium alumino-silicate glaze. If the vitrification of the walls was caused by accidental or enemy fire, it is important to know that a wood fire can provide the chemical components for vitrification, particularly when burning timbers and a thatch roof fall on top of the stone ramparts.
Wood ash contains 25 to 45 percent of calcium carbonate, about 10 percent of potash, and less than one percent phosphate and other trace elements. Potash is mainly potassium carbonate. Ash glazing began initially by accident around 1500 BC in China during the Shang Dynasty as ash from the burnt wood in the kiln landed on pots. Around 1000 BC, the Chinese started adding the ash before the pot went into the kiln. Ash glaze was the first glaze used in East Asia, and contained only ash, clay, and water. In Korea, the traditional ash glaze composed of only ash and water is currently used to make functional pottery such as bowls, cups, and teapots.
I wanted to test the glazing hypothesis. My experimental setup consisted of clay, salt, brushes, stones, a propane torch, paper towels and goggles for eye protection. I had some dried-up clay left over from my Carolina Bay impact experiments. So, I had to rehydrate it to get a thin slurry. I labeled two containers and divided the clay slurry. I then added two tablespoons of salt to one of the containers.
My experiment assumed that the hillforts had been intentionally set on fire and that salt might have been used as a flux. So, I coated the stones with the clay slurries. One pile of stones was covered with plain clay, and the other pile was covered with the mixture of clay and salt. I used different brushes to avoid contamination.
A propane torch was used to heat the stones covered with the clay slurry. The torch can achieve a temperature of about 1,980 degrees Celsius, which is hotter than a wood fire. The flame of the torch turned yellow when the stones covered with clay and salt were heated This is the usual spectral light given by sodium vapor. The clay layer quickly turned brown as evidence of a chemical reaction, and a portion of the clay layer became incandescent and melted after five minutes of firing. This is a sharp contrast with the clay without salt, which did not change color when heated and did not show vitrification.
My results show that vitrification can be achieved with salt at the temperature produced by a propane torch in the open air. The sample without salt was not vitrified even though it became incandescent when heated.
I contacted Professor Ian Ralston, who conducted the 1980 experiment, and I asked whether the Scots might have added a flux to create vitrified walls intentionally. Professor Ralston replied: "Forty years ago we produced small quantities of vitrified material from a wall which was still alight when it had to be demolished to fit the TV company's schedule. It has sometimes been argued that a fluxing agent would assist the process of vitrification in walls of this type, but to my knowledge no evidence has ever been adduced from a vitrified wall for the deliberate addition of such a flux."
The Dun Deardail project has provided valuable insights. Vitrification is not a deliberate construction method because the original timber-framed drystone rampart would have been more stable. it is much more likely that the vitrification was the result of accidental fire or deliberate destruction.
The chemistry of eutectic mixtures indicates that ashes from burning timbers and thatched roofs falling on the stone walls could have caused vitrification. The reason why the experiments by Ian Ralston and Roddy Mainland were not successful could be because the stone walls were not covered by potassium-rich wood ashes as would have happened in a real hillfort fire.
Finally, the accurate radiocarbon dates for the fires are not associated with widespread fires at other locations or with any extinction events during the Iron Age, so melting of the fort walls by a coronal mass ejection or solar plasma is highly unlikely since such an event would have caused a world-wide catastrophe. I have provided a link to the Dun Deardail Project in the description of this video. It offers a fascinating view of life during the Iron Age based on the archeological record.
