A mineral magnetic investigation into fuel derived deposits from Old Scatness Broch, Shetland

Abstract

There is increasing interest in the use of measurements of magnetic mineralogy to answer archaeological questions (e.g. R. Thompson, F. Oldfield, Environmental Magnetism, Allen & Unwin, London, 1986, p. 83; The IRM Quarterly 9(4) (2000) 2; Physics and Chemistry of the Earth (A) 25(5) (2000) 455; Archaeological Prospection 8(4) (2001) 227). The aim of the research presented here is to explain the distinctive magnetic signatures which characterise the fuel ash deposits encountered during excavation at Old Scatness Broch, Shetland, and to attempt to identify fuel sources used at the site by comparison with modern analogues. Archaeological deposits thought to contain fuel ash, from a variety of contexts and periods within the site were sampled and were supplemented by samples of modern fuel sources, including a variety of forms of peat, turf and wood. Magnetic investigations included mass specific magnetic susceptibility, high temperature variation in susceptibility, fractional conversion and laboratory imparted remanences. The archaeological deposits are shown to comprise complex mixtures of minerals with different thermal histories. Whilst most of the archaeological ash deposits were closest in magnetic characteristics to modern turf ash, some appeared to have close parallels with furnace residues obtained from reconstruction iron-smelting. The research highlights the potential benefits of these techniques to archaeological investigations, but also demonstrates the complexity of the processes of formation of such archaeological deposits.