Factors Determining Magnetic Enhancement of Burnt Clay from Archaeological Sites

Abstract

The magnetic susceptibility of a collection of 212 archaeological samples of burnt clay of different origin (burnt soil, ovens and bricks) taken from 60 archaeological sites from Bulgaria, were studied in order to determine factors which may have affected their magnetic enhancement. The highest magentic susceptibility values were obtained for bricks, and the lowest for burnt soils, suggesting that the firing temperature and the duration of burning are important conditions for determining magnetic enhancement. Specific spatial pattern of the low-field magnetic susceptibility was also observed, characterized by separation of sites situated in northern and southern parts of the country, divided by the west-east trending mountain chain Stara Planina. Maximum susceptibility values are obtained for burnt soils in southern Bulgaria and minima in northern Bulgaria. In contrast, burnt clay plasters and bricks show the opposite picture. It is supposed that the observed pattern is due to differences in the outcropped geological formations and variable climate conditions between the two regions which determine different iron-supply during burning of initial clay material. Results from the thermomagnetic analyses show that the dominant ferrimagnetic mineral in all burnt clay materials studied, is magnetite/titanomagnetite with low Ti content and different grain-sizes. High values of frequency-dependent magnetic susceptibility indicate the presence of significant amount of fine-grained (d<0·015μm) superparamagnetic magnetite.