The feasibility of vitrifying a sandstone enclosure in the British Iron Age

Abstract

Iron Age structures with evidence for having been subjected to high temperatures have been identified throughout Europe. The thermal conditions that must have yielded such evidence of alteration remain enigmatic, especially for the case of high-silica, quartz-rich building materials such as sandstones. Here, we conduct an experimental investigation of thermal treatment using the Wincobank Iron Age hill fort site in Sheffield, South Yorkshire (U.K.) as a test case. We have selected samples of the unaltered protolithic sandstone from which the fort was constructed as starting material as well as material from the vitrified wall core. An experimental suite of thermally treated protolith samples has been analysed using a combined approach involving X-ray diffraction and thermal analysis (simultaneous differential scanning calorimetry with thermogravimetric analysis). Comparison between our experimental products and the variably vitrified samples found in the wall of the Wincobank hill fort helps to constrain firing temperatures and timescales. For mineralogical markers, we employ the high-temperature conversion of quartz to cristobalite and the melting of feldspar to compare the relative abundance of these phases before and after thermal treatment. We find that the Iron Age wall samples have mineralogical abundances most consistent with a minimum firing temperature range <1100–1250°C and a firing timescale of >10h. These first quantitative constraints for a fort constructed of sandstone are consistent with those found for forts constructed of granitic material. Finally, we explore the reasons for thermal disequilibrium during firing and invoke this mechanism to explain the differential vitrification found at some Iron Age stone-built enclosures.