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.

Sourcing fire ash on archaeological sites in the Western and Northern Isles of Scotland, using mineral magnetism

“…Post-depositional processes and pedogenesis of archaeological deposits could also alter the magnetic properties of the samples. For example, floor levels and middens from Galson and Guinnerso displayed evidence of a possible bacterial magnetosome component in the samples (Peters et al, , 2001. This was attributed to the higher levels of organic material within the floor levels and middens compared to the less organic, ash-rich samples chosen for this analysis.…”

“…The detailed description of the experimental methodology and the results are described by Peters et al (2001Peters et al ( , 2004. A summary of the technique is presented here.…”

“…This allowed the ash from all the hearth and ash spreads from each of the sites to be sourced, using material from the routine soil samples. More detailed column sampling at 2 cm intervals through hearth deposits were undertaken at Guinnerso and Galson (see Peters et al, 2000Peters et al, , 2001, Cladh Hallan , and Old Scatness (Dewar et al, 2002). Only hearth material and ash spreads were analyzed using the detailed mineral magnetic work described above, as past research has shown that these contexts allowed more coherent interpretation than mixed deposits, such as floor levels and middens.…”

“…Only hearth material and ash spreads were analyzed using the detailed mineral magnetic work described above, as past research has shown that these contexts allowed more coherent interpretation than mixed deposits, such as floor levels and middens. These latter context types were more likely to contain other magnetic material of unknown origin, possibly of a bacterial magnetosome component (Peters et al, , 2001.…”

Archaeological Investigations at the Ice House Site, 41HY161: Early Archaic Technology, Subsistence, and Settlement Along the Balcones Escarpment, Hays County, Texas

“…The magnetic enhancement of soils is used as a proxy indicator for long term patterns of rainfall and drought such as the 2.5 million year record of the Chinese Loess Plateau, and a 500,000 year record in southern Illinois (Grimley, et al 2003;Maher and Thompson 1995). Magnetic susceptibility values can indicate the presence of buried paleosols and anthropogenic activity, contributing to the geomorphic interpretation used to identify archaeological site formation processes (Batt and Dockrill 1998;Crowther 2003;Macphail, et al 2004;Marwick 2005;Peters, et al 2001;Peters and Thompson 1999).…”