Vitrification of Hill Forts
Gazeteer and Research Guide
This is the output from an ongoing research discussion aimed at understanding vitrified hill forts in the British Isles, this project is still in its early stages of development. If after reading this document you have additonal information not already covered or would like to help contribute to this research by visiting and taking pictures please email our vitrified forts research team – email@example.com Your input would be most welcome.
Gazeteer of Vitrified Forts
There are at least 60 vitrified hill forts, so far we have collected only a small sample, additional suggestions would be welcome.
Scope of this document
This document is intended to be a comprehensive guide to vitrified forts. It attempts to explain the outcome of research to date and to help take these investigations forward. A significant part of this is an attempt on our behalf to clearly state what we believe to be the most likely way that vitrified forts came into existence. This is because to date the subject has suffered from a myriad of partial explanations which in many cases serve to confuse rather than enlighten.
John Williams, one of the earliest of British geologists, and author of The Mineral Kingdom first described the phenomenom of vitrified forts 1777. Further reports on vitrified forts were made in 1880 when Edward Hamilton wrote an article entitled “Vitrified Forts on the West Coast of Scotland” in the Archaeological Journal. Since then vitrified forts have received varying levels of attention, there enigmatic and semmingly unexplainable existence has attracted a great many ‘odd-ball’ theories which have had the effect of stifling serious scientific debate.In his article, Hamilton describes several sites in detail, including Arka-Unskel, which he found that the rampart of local Gneiss was covered with imported feldspatic sanstone in order to create the vitrified effect. This method found also in the vitrified fort of Dun Mac Snuichan, on Loch Etive.
Hamilton and Williams naturally asked a few obvious questions about the forts. Were these a means of defence? Was the vitrification the result of design or accident? How was the vitrification produced? Since that time the subject has been a debating point for Archaeologists around the globe.
Within this research note we have endevoured to present the most likely explanation of the vitrification process, rather than confuse with a full breakdown of all the possible hows and why’s which have been suggested. In the past archaeologists have felt themselves duty bound to present all the cases since no single explanation has been accepted by all. However this has had the effect of confusing the ameteur enthusiast and has meant proffessional research has become bogged down with an over large set of conflicting theories.
In order to cut to the salient information, it is usefull to separate out particular features of vitrified forts into common theme’s. By doing this we are able to isolate and therefore removed isolated features which would otherwise cloud our analysis.
Type of Vitrified Fort
The first thing that must be said is that the current classification seems to include just about any fort from any period which has evidence of burning. This is not a helpful state of affairs. As will be shown, although it is possible for a fire either by accident or from malicious intent to cause limited vitrification to a forts ramparts, it is clear that common factors tie together a number forts which show a significant and consistent vitrification over a large portion of their ramparts. It is this collection of similar fort types that are of interest for the purposes of this article, it is not that others forts are uninteresting, but that treating all forts the same has weakened the overall discussion.
for the purpose of this discussion we are solely interested in forts which we have categorised as since treating all forts the same when there may be a number of origins to the vitrification does not help to add clarity to the discussion.
How do you vitrify a fort?
There have been many theories as to how vitrification has occurred in some ancient forts, ranging from the use of special chemicals to the composition of the rocks used for the forts. As a result of our research we have concluded that the in order to create a “true” vitrified rampart, the heat required is in excess of that that which would normally be the case if the rampart alone was the only fuel. Temperatures in excess of 1000 degrees Centigrade would need to be applied consistently over large areas of the wall, in close proximity for a significant period of time.
To build hot fires, one has to remember two things, it is the combination of the fuel and oxygen that is creating the fire, the best fuels present the maximum surface area to the air. We can understand the difficulty of reaching consistently high temperatures what we consider the effort required to turn iron molten – 1050 degrees, here bellows need to pump gusts of “wind” into a kiln, which is constructed to reflect the heat back into the furnace, the hottest kilns, use coal or coke, their granular composition combined with the forced air flow serve to maximize the air to fuel ratio. Clearly, the concept of a wall constructed with heavy timber beams, separated by layers of rock, reaching such extreme temperatures is difficult to imagine. It has been suggested that the correct wind conditions may serve to “fan” a fire, perhaps after the rampart had been set on fire during an attack, but it must be remember that for a furnace to reach these sorts of temperatures the fire is enclosed and the heat reflected in, an open fire stands little chance of reaching such temperatures without much greater amounts of fuel.
Given this, it is clear that vitrified ramparts are the result of a deliberate and continued effort.information, it has been suggested that some forts used imported stone vitrified forts involved a specially constructed rampart, its stone is not easily vitrified would be covered with more suitable rock which would melt onto the base rock in a similar way that enamel melts into clay to form pottery. Additional chemicals such as sea salt may have also been used to increase the temperature of the process and to act as a ‘flux’ to help the rock join. Wood was used as the main fuel, perhaps Oak or Yew which were widely available and have a high natural burning temperature.
However, these two factors of there own will not produce vitrification to a satisfactory level. In order to consistently achieve a satisfactory level of vitrification the heat produced by the fire must be concentrated and directed towards the ramparts. It is therefore suggested that during their construction/modification the rampart will have been shrouded by a wood and turf structure, which turned the rampart into the interior of a kiln. To date no evidence of such a structure exists but the limited excavations performed on vitrified sites and the nature of vitrification mean that this is not surprising. In all, the following factors have lead to our conclusion:
(1) Many of the Primary rocks, particularly the schists, gneisses and traps, which contain large quantities of potash and soda, can be readily fused in the open air by means of wood fires—the alkali of the wood serving in some measure as a flux. (2) The walls are chiefly vitrified at the weakest points, the naturally inaccessible parts being unvitrified. (3) When the forts have been placed on materials practically in-fusible, as on the quartzose conglomerates of the Old Red Sandstone, as at Craig Phadraic, and on the limestones of Dun Mac Uisneachain, pieces of fusible rocks have been selected and carried to the top from a considerable distance. (4) The vitrified walls of the Scottish forts are invariably formed of small stones which could be easily acted upon by fire,whereas the outer ramparts where used, are not vitrified and are built of large blocks. (5) Many of the continental forts are so constructed that the fire must have been applied internally, and at the time when the structure was being erected. (6) Daubrée, in an analysis which he made on vitrified materials taken from four French forts, and which he submitted to the Academy of Paris’ in February 1881, found the presence of natron in such great abundance that he inferred that sea-salt was used to facilitate fusion.
Why virtify a fort?
As we have already seen, vitrification was a deliberate and planned act on behalf of the forts builders, Why did they do it? Fusing rock in this way makes a rampart brittle rather than stronger and we must conclude that vitrification was done for reasons other than to create a more defensible structure.
There have been many theories as to why vitrification has occured. this will probably be a question which can never be totally resolved, however the understanding on how vitrification was achieved enables use to propose the most likely reasons as to why it was done.
Based on the evidence to hand the most likely reason for vitrification must therefore be that it was part of a ceremonial or religious act, perhaps as dedicating the fort to a deity or to inspire the natives in time of war. A further reason could be to commemorate a significant event in the tribes history pertinent to the fort, such as a major defeat or victory.
Further work needs to be done on the classification and understanding of the features of vitrified forts and also the culture and lifestyle of the Scottish pre-historic peoples before a more clear understanding of this aspect can be achieved.
One of the great mysteries of classical archaeology is the distribution of vitrified and their apparent concentration in Scotland. Although very limited instances of vitrified forts have been discovered in various locations throughout the world the sheer number of vitrified forts in Scotland indicates the presence of a specific cultural phenomenon rather than a random distribution.
Vitrified ruins have been found Scotland, England, Ireland, France, Turkey, Iran, Germany and elsewhere, however, out of some 100 forts identified throughout the world, more than half are located in Scotland, and most of those are north of the Forth – clearly a highly concentrated feature of sufficient units to indicate a cultural difference rather than an occurrence due to happenstance. Many feel this broadly means that the occurrence of vitrified forts outside of Scotland simply shows evidence of links between Scotland and other area of the world. Another thought is that vitrification was a technique imported to Scotland and adopted on a wide scale.
Attempts to re-create vitrification
So far only limited experimental research has been done to re-create the vitrification process, usually in order to prove a particular theory and often with dubious results. However these albeit limited experiments can give further evidence of the determined effort required to produce a truely vitrified fort.
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. Although this experiment was intended to prove that vitrification was a result of the ramparts catching fire during an attack, in fact it proved the opposite, for it was pointed out that in order to achieve the very limited vitrification in evidence they had to specially design their rampart, used more internal wood than was known to be the case for hill forts and use much more wood than would be normally attached to the ramparts and surrounding area.
Dating of Vitrified Forts
The dating evidence available for vitrified forts covers a very wide spread – 700BC – 900AD. This is not very helpfull in attempting to understand the environment in which vitrification has taken place.
Clearly the practice of classifying all forts with any extent of burning does not help, since a fort could feasibly be subjected to all manner of accidental fire damage at any time after it was first built and does not truly represent the vitrified forts of interest here. In a lot of cases the life of the fort has been much more complex than the single event of vitrification, with occupation envidence occuring possibly many years prior to and/or after the vitrification took place.
In many cases it may be that the act of vitrification itself did much to remove dating evidence at the site and it is likely that a number of dates currently available may be due to mis-interpretation of earlier or later use of the fort.
It is doubtful that a full explanation of vitrified forts will be obtained, In prehistoric northern Scotland, either invented or imported from elsewhere a way of life developed which meant that the local tribes increasingly saw vitrification as the desirable status for the internal ramparts of some of their fort-like structures. Vitrification took a great degree of planning and involved the collection of a large quantity of wood and other materials in order to create the desired effect. It is likely that a super-structure will have been erected around the rampart which had the effect of concentrating the heat onto the rampart so as to ensure an even application of the extreme temperatures involved. In some cases, where the local rock would not easily vitrify imported rock such as sandstone was brought in and used to provide a melted frontage to the rampart.
Vitrification could have many reasons for being, it is suggested that these forts were created either for ritual or ceremonial purposes rather than for any defensive measure, it may be that a fort could be ‘dressed’ by vitrification ready for battle, but this vitrification was not specifically intended to strengthen the defences of the fort.
The culture which began vitrification in Scotland seems to have spread southwords and it is likely that the few examples in England and Ireland belong to Scottish tribes who either invaded or ‘helped out’ their distant neighbours in times of conflict.
The date that vitrification existed is currently thought to span from 600BC to 100AD however it may be that a narrower timescale can be determined in future.
As part of this research exercise, it has been noted that a full classification of the features of vitrified forts has not so far been attempted, this has reduced the extent of analysis that can be done with the information currently available. Here we have begun to identify those features which can be usefully classified. It is hoped that ultimately these can be applied to all vitrified forts.
Many thanks for input from TMA and the other Internet based archaeology forums and sites.
AUTHORITIES.—John Williams, An Account of some Remarkable Ancient Ruins (1877); A. Fraser Tytler, Edin. Phil. Trans. vol. ii.; Sir George Mackenzie, Observations on Vitrified Forts; Hibbert, Arch Scot. vol. iv.; J. MacCulloch, Highlands and Western Islands (1824), vol. i.; Hugh Miller, Rambles of a Geologist (1858), chap. ix.; Sir Daniel Wilson, Archaeology and Prehistoric Annals of Scotland (1851), vol. ii.; J. H. Burton, History of Scotland (1867), vol. i.; R. Angus Smith, Loch Etive and the Sons of Uisneach (1879); J. Anderson, Scotland in Pagan Times (1886); C. MacLagan, The Hill Forts of Ancient Scotland; Thomas Aitken, Trans. Inverness Scientific Soc. vol. i.; Charles Proctor, ‘Chemical Analysis of Vitrified Stones from Tap o’ Noth and Dunideer (Huntly Field Club); various papers in Proceedings of Soc. A ntiq. Scot. (since 1903 The Scottish Historical Review) and Proceedings of Royal Irish A cademy;
R. Munro, Prehistoric Scotland (1899); G. Chalmers, Caledonia (new ed., 7 vols., Paisley, 1887—94); Murray’s Handbook to Scotland (1903 ed.); Leonhard, Archiv für Mineralogie, vol. i.; Virchow, Ztschr. für Ethnologie, vols. iii. and iv.; Schaaffhausen, Verhandlungen der deutsch. anthrop. Gesellschaft (1881); Kohl, Verhand. d. deutsch. anthrop. Gesel/schaft (I883); Thuot, La Forteresse vitrifiée du Puy de Gaudy, &c.; De Nadaillac, Les Premiers Hommes, vol. i.; Mimoires de la Soc. Antiq. de France, vol. xxxviii. Hildebrand, De forhistoriska folken i Europa (Stockholm, 1880); Behla,. Die vorgesch-ichtlichen Rundwalle im östlichen Deutschland (Berlin, 1888) Oppermann and Sehuchhardt, Atlas vorgeschichtlicher Befesisgungen in Niedersachen (Hanover, 1888—98); Zschiesche, Die vorgeschichtlichen Burgen und WaIte im Thuringer Zeniralbecken (Halle, 1889); Bug, Schlesische Heidenschanzen (Grottkau, 1890); Gohausen, Die Befestigungswe-isen der Vorzeit und des Mittelalters (Wiesbaden, 1898).