Different mechanisms of magnetisation recorded in experimental fires: Archaeomagnetic implications

We present here the archaeomagnetic and rock-magnetic results obtained from the experimental recreation of prehistoric fires on a clayish soil substratum. By using a system of thermocouples that allows the control of temperatures and heat penetration with depth, the magneto-mineralogical transformations as well as the ability to acquire magnetic remanences are studied. A detailed set of rock-magnetic analyses – carried out both on surface and depth with a millimetre scale resolution– shows the creation of post-burn magnetite over the entire surface of the hearth, thus promoting a distinguishable magnetic enhancement. On the basis of a detailed control of temperatures, interesting differences between areas (centre vs. periphery) were observed and two different mechanisms of magnetisation have been recorded. The central area which exceeded 600 °C acquired a thermal remanent magnetisation (TRM) and the periphery, which was heated up to 300 °C, recorded a thermo-chemical (TCRM) remanent magnetisation. The most noticeable mineralogical changes are focused to the first centimetre of the central area, where single-domain magnetite has been created. The implications of these results for archaeomagnetic and palaeointensity studies as well as their utility for the detection of fire in archaeological sites are discussed. ► Experimental fires to study mechanisms of magnetisation on a clayish substratum. ► Control of temperatures on surface and depth, fuel and previous magnetic mineralogy. ► Neoformation of magnetite on the whole surface in single-domain state at the centre. ► Two mechanisms of magnetisation recorded: TRM (centre) vs. TCRM (periphery). ► Superficial central samples the most suitable for archaeomagnetism in similar contexts.