Relevance of magnetic properties for the characterisation of burnt clays and archaeological tiles

The archaeomagnetism of pottery, bricks and tiles is typically employed for dating inferences, yet the magnetic properties of ancient ceramics can also be convenient for their characterisation, to evaluate the technological conditions applied for their production (temperature, atmosphere, and duration of firing), as well as to distinguish groups of sherds having different provenance. In this work, the measurement of hysteresis loops has been applied and combined with colour survey to characterise the magnetic properties of burnt clays and archaeological tiles. Four calcareous and non-calcareous clays, along with seventeen tile fragments excavated from the sites of the ancient Roman towns of Pompeii and Gravina di Puglia, in Southern Italy, are examined. The ferrimagnetic character of the clays, in general, enhances with increasing firing temperatures until vitrification processes occur (900–1000 °C) dissolving iron oxides and dispersing the colour and magnetic properties they provide. High values of saturation magnetization are observed in clays with relevant calcareous content after firing above 900 °C, which results in the formation of Ca-silicates able to delay the onset of the vitrification processes. Magnetic properties of the tiles have been evaluated in terms of the high coercivity (i.e. mainly ferrimagnetic) or low coercivity behaviour (i.e. including relevant paramagnetic and superparamagnetic contributions). Enhanced ferrimagnetic character, mostly depending on the growth in number and volume of iron oxide particles, is associated with the development of an intense reddish hue.