Thermoluminescence dating of Scottish vitrified forts : development, evaluation and demonstration of the potential of thermoluminescence dating techniques to resolve outstanding chronological problems associated with Scottish vitrified forts

• Main Author: Sanderson, D. C. W.
• Published: University of the West of Scotland 1987
• Subjects: 930.1; Archaeological dating methods
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376105

The Scottish vitrified forts form a unique assemblage of archaeological monuments exhibiting the common feature of partly melted masonry generally found in the vicinity of a ruined wall. Although they have received considerable attention from scholars since their discovery in the late eighteenth century many questions relating to origins, purpose and chronology remain unanswered. This work represents the first coherent attempt to develop and apply direct dating to vitrified rocks _ using thermoluminescence (TL) techniques to establish the time elapsed since the last heating of the sample. The technique is based on equating the thermoluminescence acquired by minerals within vitrified rocks, during the period following vitrification, to the levels of natural ionising radiation within and around the samples. Study sites were selected on the basis of a mineralogical, analytical and dosimetric survey of material from museum collections, and sampled from secure contexts using a portable diamond coring tool. Radiation dose rates to the samples were determined using a combination of thermoluminescence and low level counting methods coupled to standard microdosimetric models for specific mineral phases. A promising new method of rapid beta dose rate measurement was developed during this work. Thermoluminescence measurements of separated mineral extracts were performed using computerised eqUipment to determine the total radiation dose experienced since firing. Particular attention was paid to the form of the radiation dose dependence of samples from different Sites, and to the stability and reproducibility of TL signals. Coherent thermoluminescence ages were obtained from the sites of Finavon,Craig Phadrig, Dun Lagaidh, Langwell, Knockfarrel and Tap 0 Noth, using feldspar fractions extracted from the samples, and indicate a long time span for the monuments stretching from the 2nd millenium Be to the first millenium AD. This evidence considerably extends the timespan attributable to the monuments and paves the way for further work to establish the relationship between the morphology and chronology of a wider range of sites.