Magnetic measurements as indicator of the equivalent firing temperature of ancient baked clays: New results, limits and cautions

Abstract

Abstract We present new experimental results on the variation of the magnetic properties of baked clays as a function of the temperature reached during laboratory treatments. Such experiments, including continuous monitoring of the magnetic susceptibility and magnetic moment versus temperature, were applied to a set of natural clays experimentally heated in the laboratory at 200 °C, 400 °C and 600 °C as well as to archaeological baked clays collected from two archaeological sites in Northern Italy (Santhia and Carbonara Scrivia). The aim of this study is to investigate the reliability of the magnetic properties to identify the equivalent firing temperatures of ancient baked clay artefacts based on the reversible behavior of thermomagnetic diagrams. The results obtained indicate that the magnetic properties do not always succeed in estimating the firing temperature of the baked clays, mainly when clays have been heated only once and at relatively low temperatures, e.g. less than 300–400 °C. On the contrary, magnetic properties of ancient clays that have been repeatedly heated at the past at temperatures higher than 400 °C appear to be more stable and representative of the equivalent firing temperature. This study confirms that the reversibility of thermomagnetic curves can be a useful indicator of ancient firing temperatures in the case of baked clays that have experienced multiple heatings at the past while caution should be paid on its general use as archaeo-temperature marker.