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

We present new experimental results on the variation of the magnetic properties of baked clays as afunction of the temperature reached during laboratory treatments. Such experiments, including contin-uous monitoring of the magnetic susceptibility and magnetic moment versus temperature, were appliedto a set of natural clays experimentally heated in the laboratory at 200◦C, 400◦C and 600◦C as well asto archaeological baked clays collected from two archaeological sites in Northern Italy (Santhià and Car-bonara Scrivia). The aim of this study is to investigate the reliability of the magnetic properties to identifythe equivalent firing temperatures of ancient baked clay artefacts based on the reversible behavior ofthermomagnetic diagrams. The results obtained indicate that the magnetic properties do not alwayssucceed in estimating the firing temperature of the baked clays, mainly when clays have been heatedonly once and at relatively low temperatures, e.g. less than 300–400◦C. On the contrary, magnetic prop-erties of ancient clays that have been repeatedly heated at the past at temperatures higher than 400◦Cappear to be more stable and representative of the equivalent firing temperature. This study confirmsthat the reversibility of thermomagnetic curves can be a useful indicator of ancient firing temperaturesin the case of baked clays that have experienced multiple heatings at the past while caution should bepaid on its general use as archaeo-temperature marker.