Why Vitrify a Fort?

Vitrified Forts Distribution

One of the great mysteries of classical archaeology is the spartan worldwide distribution of vitrified forts, except for Scotland and Read more

How to Vitrify a Fort

Castle Hill and Almondbury from Kirkheaton Vitrification of Hill Forts The Vitrification process Vitrification as seen Read more

Tap O’Noth Hill Fort

This is one of the best examples of a vitrified fort, it is near the village of Rhynie in northeastern Read more

Mystery of Vitrified Forts

It was during a trip to.Scotland, in the Summer of 1997, that I first heard of vitrified hillforts. It was Read more

Barry Hill Fort

Barry Hill is a textbook “big fort waiting for a small trench”: its spectacular vitrified rampart, enigmatic outer works and Read more

Mote of Mark, Dumfries

  "Mote of Mark from sea shore path - geograph.org.uk - 6273954" by Andrew Curtis is Read more

Eileen na Goar Fort

This island, locally termed Eilean na Goar, is the most eastern and is bounded on all sides by precipitous gneiss Read more

Guide – Landscape Archaeology Introduction

Landscape archaeology is a vital tool for understanding how ancient peoples interacted with their environment, shaping and being shaped by Read more

Castercliff Hill Fort

This denuded hillfort is oval and encloses almost two acres. The defences comprise triple circuits of bank and ditch, with Read more

Classification of Vitrified Forts

Archaeologists have suggested ways to organise the very varied “vitrified forts” into formal classes – especially schemes that distinguish forts Read more

Vitrified Fort Sites

This page is a master placeholder for all vitrified fort sites in the Vitrified Forts section of Brigantes Nation.

Vitrified Forts: Glossary of Terms

This page holds our Glossary of Terms for Vitrified Forts.

Why Vitrify a Fort?

Why was it done and by whom?

Practical reasons?

Originally, it was thought that the forts had become vitrified due to an enemy attack. A theory proposed by Childe in the 1930 thought it 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. Also, this theory does not stand up to the geographic distribution of hill forts versus the known warring area where hill forts were in use. For example the south of England suffered wave after wave of hostile invasion from other Gaulish tribes, yet no vitrification has been noted – surely if it was a natural effect of a battle then these forts would be more likely to occur in the south of England (given the large concentration of timber laced ramparts and the frequency of fighting in the area).

This idea was amended with the theory that the builders of the walls had designed the forts in such a way that the vitrification was purposeful in order to strengthen the walls. This theory postulated that fires had been lit, and flammable material added to produce walls strong enough to resist the invading armies of the enemy. It is an interesting theory, but one that presents several problems. The main problem with this theory is there is no indication that such vitrification actually strengthens the walls of the fortress; rather, it seems to weaken them. In some cases, the walls of the forts seem to have collapsed because of the fires, however this may show an error in the calculations of the builders.

To further illustrate this point, Julius Caesar described a type of wood and stone fortress, known as a murus gallicus, in his account of the Gallic Wars. This was interesting to those seeking solutions to the vitrified fort mystery because these forts were made of a stone wall filled with rubble, with wooden logs inside for stability. Caesar notes how the flexibility of the wood adds to the strength of the fort in case of battering ram attack.

Some researchers are sure that the builders of the forts caused the vitrification. Arthur C. Clarke quotes one team of chemists from the Natural History Museum in London who were studying the many forts:

“Considering the high temperatures which have to be produced, and the fact that possibly sixty or so vitrified forts are to be seen in a limited geographical area of Scotland, we do not believe that this type of structure is the result of accidental fires. Careful planning and construction were needed.”

Our own research into how forts were vitrified does indeed suggest a deliberate and planned action on behalf of the builders. Looking at the hill forts in evidence today, it appears that although many seem to be specifically built in strategic locations, some do not take full advantage of the natural defences available. Another common feature is that many vitrified forts have two rings of ramparts, but only the inner is vitrified. This suggests that the vitrified rampart was for the benefit of the fort’s users (not visible outside the fort) and that although linked with battle, the vitrification served a purpose other than strengthening the fort.

Beyond this reasoning, any further comment is pure speculation, but our research does indicate that the vitrification process could have been part of a lengthy ‘ceremony’ and will have been directed by the most powerful members of the community. Although it is strongly felt that vitrification of forts represents a cultural or religious element, further comment is reserved until further investigative work can be performed.

Experimental attempts to replicate the vitrification seen at Tap o’ Noth

The rampart of Tap o’ Noth hill-fort is one of the best-preserved examples of a stone wall whose core has melted and fused into green-black glass. Because the temperatures required (> 1 000 °C) far exceed what an uncontrolled brush-fire can achieve, archaeologists have tried—twice and at full scale—to discover how such heat might have been generated and whether the melting was accidental, structural or deliberate.

Key technical findings

Fuel-to-stone ratio – Both experiments needed c. 1 part dry timber to 2–3 parts stone by volume. That equates to many thousands of mature trees for a full hill-fort wall: mass felling and organised labour are implied.

Airflow control – Heat concentrated where through-drafts fed the core (beam sockets, gaps between facing stones). Stagnant pockets never melted.

Glass chemistry – Melted granite and gabbro at 1 050 °C produced the same green-black glass and vesicles seen in Tap o’ Noth samples, vindicating the experimental design.

Behavioural implications for Tap o’ Noth

Deliberate conflagration is feasible: a well-planned firing could vitrify selected stretches in one episode, whether as an act of destruction or as a dramatic closure rite.

Structural strengthening unlikely: both replica walls became unstable once timber burned out; vitrification weakens rather than “welds” a rampart.

Labour & resource cost: harvesting and hauling the timber needed for a 100 m-long, 3 m-thick rampart would require a supra-household workforce—fitting a scenario of elite display or punitive destruction, not accidental fire.

Unresolved questions

Who lit the fire? —No siege debris or arrowheads were found in Tap o’ Noth’s vitrified tumble; ritualised self-burning remains as plausible as enemy attack.

Why re-occupy a melted wall seven centuries later? —Pictish re-occupation (7th c. AD) suggests the ruined, glass-fused rampart still carried prestige or ancestral authority despite its weakened fabric.

Regional pattern: comparable vitrification and radiocarbon brackets at Craig Phadrig, Dunnideer and Dun Deardail hint at a shared cultural practice of “fire-finishing” forts in north-east and Highland Scotland.

Take-away: the Tap o’ Noth experiments demonstrated that Iron-Age builders could intentionally melt a timber-laced wall using only locally available fuel and simple draught control. They did not prove whether the goal was tactical, structural or symbolic—but they push the balance of probability toward a planned, labour-intensive fire that turned a granite rampart into a smoking, glassy monument of power.

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Vitrified Fort References

Cook, M., Watson, F., and Cook., G. (2016). Burning Questions: New Insights into Vitrified Forts. In Erskine, G., P. Jacobsson, P. Miller, and S. Stetkiewicz (EDS.).

Proceedings of the 17th Iron Age Research Student Symposium, Edinburgh. Oxford: Archaeopress Publishing.

Friend, C.R., Kirby, J.E., Charnley, N.R. and Dye, J., 2016. New field, analytical data and melting temperature determinations from three vitrified forts in Lochaber, Western Highlands, Scotland. Journal of Archaeological Science: Reports, 10, pp.237-252.

Horn, J. A. (2016). An approach to re-examining the chronology of hillforts and other prehistoric monuments Jonathan A. Horn University of Edinburgh. In Erskine, G., P. Jacobsson, P. Miller, and S. Stetkiewicz (eds.). Proceedings of the 17th Iron Age Research Student Symposium, Edinburgh. Oxford: Archaeopress Publishing.

Kresten, P., 2004. The vitrified forts of Europe: saga, archaeology, and geology. International Council for Applied Mineralogy: development in Science and Technology, pp.355-357.

McCloy, J.S., Marcial, J., Clarke, J.S. et al. Reproduction of melting behavior for vitrified hillforts based on amphibolite, granite, and basalt lithologies. Sci Rep 11, 1272 (2021). https://doi.org/10.1038/s41598-020-80…

Ralston, I., 1987, November. The Yorkshire television vitrified wall experiment at East Tullos, city of Aberdeen District. In Proceedings of the Society of Antiquaries of Scotland (Vol. 116, pp. 17-40).

ScARF, I.A.P., 2010. Iron Age Scotland: ScARF Panel Report. Scottish Archaeological Research Framework. Sanderson, D.C.W.,

Placido, F. and Tate, J.O., 1988. Scottish vitrified forts: TL results from six study sites. International Journal of Radiation Applications and Instrumentation. Part D. Nuclear Tracks and Radiation Measurements, 14(1-2), pp.307-316.

Wadsworth, F.B., Heap, M.J., Damby, D.E., Hess, K.U., Najorka, J., Vasseur, J., Fahrner, D. and Dingwell, D.B., 2017. Local geology controlled the feasibility of vitrifying Iron Age buildings. Scientific Reports, 7(1), pp.1-7.

Lock, G. And Ralston, I. (2017). Atlas of Hillforts of Britain and Ireland. [ONLINE] Available at: https://hillforts.arch.ox.ac.uk