Processes of Vitrification in Scottish Iron Age Hillforts

Abstract

Iron Age Vitrified hillforts are a relatively common feature in the Scottish landscape. There are at least sixty confirmed examples spread across Scotland. How and why these hillforts were vitrified are unanswered questions to modern archaeologists. There has long been debate as to whether this vitrification was an act of construction or destruction and if vitrification was a destructive process, was this vitrification event intentional or accidental. This research sets out to firstly determine the provenance and variability of the building lithology and secondly, to understand the conditions of vitrification in Scottish Iron Age hillforts. Research was conducted on nine hillforts across Scotland taking in a variety of local geologies, but work focussed on Dun Deardail in Glen Nevis. Dun Deardail was excavated for three seasons between 2015 and 2017. These three seasons of excavation produced samples of vitrified, burnt and unburnt rock, soil and charcoal. The provenance of the building materials at all of the hillforts included in this research has been found to have been local, with the building materials found to be mainly found from within one kilometre of the hillfort site. Variability across each hillfort was shown to be low, with each of the hillforts being constructed using only a few lithologies. Conditions of vitrification were determined using a variety of geochemical and petrological techniques. Experimental melts were carried out to study the effect of vitrification conditions on the melting process and vitrified material and compared to the evidence seen at the hillforts themselves. These melts allowed an exploration of the processes of the vitrification under controlled conditions, allowing the different variables and theories to be examined. The minimum melting temperature of the rubble core at Dun Deardail was found to be approximately 1140˚C, and this result was mirrored at Craig Phadrig, The Torr and Knockfarrel, where similar temperatures were also calculated. This suggests that only partial melting took place, with this temperature being in between the solidus and liquidus temperatures of pelitic material. Each of the hillforts researched was timberlaced in construction. This timberlacing allowed the transportation of the heat from the outside of the hillfort into the rubble core. This allowed the core to remain at a temperature high enough to allow mineral melting, even when the outer face of the walls had cooled to below the solidus temperature. Excavation of Dun Deardail identified very few items, and this suggests that the hillfort was cleared of goods before the fire. This may suggest that the fire was a deliberate act at the end of the hillfort’s life. Perhaps a sort of ritual closure. Compared with modern glass, the glasses formed during vitrification have shown a much lower degree of erosion over time due to an enrichment of calcium in the glass. This retardation of typical erosional properties of the vitrified material gives strength to the vitrified walls and prevents erosion, preserving these vitrified hillforts for future generations. Further study into this erosion should be undertaken to further understand the conservation of these, and similar, monuments.