On the suitability of baked clay for archaeomagnetic studies as deduced from detailed rock-magnetic studies

• Published in: Geophysical journal international Vol. 153; no. 1; pp. 146 - 158
• Main Authors: Jordanova, N., Kovacheva, M., Hedley, I., Kostadinova, M.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.04.2003; Blackwell Science Ltd
• Subjects: archaeomagnetism; magneto-diagnostic methods; rock magnetism; suitability for archaeomagnetism
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1046/j.1365-246X.2003.01889.x

Extensive rock-magnetic investigations have been carried out on baked clays from four kilns (two from Bulgaria and two from Switzerland) found in archaeological sites of different age. Knowledge of the magnetic characteristics of the grains responsible for the archaeomagnetic signal enables us to determine which baked clays have the stablest magnetization and why this is so. This is important in directional studies, but even more so in painstaking palaeointensity studies that require a very careful evaluation of the suitability of the burnt clay material. The proposed rock-magnetic experiments enable the identification of the carriers responsible for the remanence and an adequate interpretation of the experimental results connected with the palaeointensity evaluation. The experimental methods employed are illustrated with the particular results obtained from each of the four kilns studied. The preliminary elucidation of the magnetic mineralogy of the archaeological samples helps first by obtaining a more reliable palaeointensity result, and secondly by explaining some of the discrepancies in the palaeodirectional results. Examples of successful and failed palaeointensity experiments are given in relation to the magnetic properties previously established for each oven. The burnt-clay materials in this present study satisfy the essential condition of carrying a thermoremanence. In spite of that, it is shown that there are many factors that can produce undesirable magnetic properties and thus restrict the suitability of these materials for archaeomagnetic analysis. The most important factors influencing the magnetic behaviour during magneto-diagnostic experiments are: the degree of heating in antiquity, the initial composition of the unbaked material and the burial conditions. The large difference in heating temperatures within a particular archaeological feature is a major cause of variation in magnetic behaviour amongst individual specimens, and so preventing a successful pre-selection of specimens for palaeointensity experiments. Nevertheless, the study has shown a very good coincidence between the determined rock-magnetic characteristics and the success rate in palaeointensity evaluation.