Investigation of fire ash residues using mineral magnetism

Abstract

As part of a wider research programme of experimental archaeology at Calanais Farm, Isle of Lewis, Scotland, a number of experimental hearths were constructed, based on excavated evidence from the Late Iron Age houses at Bostadh, Lewis. Controlled and repeated burning of different fuel sources (well-humified peat, fibrous upper peat, peat turf and wood) was carried out over a number of burning episodes, each of three days duration. A range of mineral magnetic measurements, including remanences and the variation of susceptibility with high temperature, were taken from the resulting ash samples. The high temperature susceptibility measurements show that the fibrous upper peat and peat turf have a single magnetic component, with a drop in magnetic susceptibility at ca. 600 °C. In comparison the well-humified peat and wood have one, sometimes two, distinct magnetic components characterized by drops in susceptibility at ca. 330 and ca. 550 °C. Stepwise discriminant analysis was performed on the room temperature magnetic data. A biplot of the resulting two main variables distinguishes the well-humified peat and wood. Some overlap is observed between the fibrous upper peat and peat turf. Magnetic measurements also were carried out on Iron Age and Medieval hearth, floor and ash spread samples from the multiperiod archaeological site of Guinnerso, on the Isle of Lewis. Comparison was made with the modern ash samples in order to determine if fuel sources could be identified. The high temperature susceptibility curves and the discriminant analysis biplot suggest that for the selected archaeological samples the predominant fuel source was well-humified peat.