Contributions of wood ash magnetism to archaeomagnetic properties of fire pits and hearths

Abstract

Magnetic studies were conducted on a variety of plant ash and ash material from fire pits to investigate the possible contribution of ash to the magnetic signature of hearths. The measurements included magnetic susceptibility, remanence acquisition to saturation (SIRM), alternating field demagnetization of SIRM and remanent coercivity ( B rc). We conclude from these studies that wood ash produces fine-grained magnetic iron oxides. These oxides are probably magnetic that is concentrated enough to add to the magnetic signature of hearths and fire pits. At one site (Ellicottville, New York), the ash layer exhibits a magnetic susceptibility that is 22 times greater than a control soil sample from the site. We attribute this large enhancement to repeated burns at the site which continued to add ferromagnetic material to the fire pit. The source of the magnetic material is probably phytoferritin from plants. The variation in the intensity of magnetic anomalies over ancient hearths and fire pits may in part be caused by the magnetic contribution from ash.