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

Abstract

SUMMARY 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.

An Archaeometric and Archaeological Approach to Hellenistic-Early Roman Ceramic Workshops in Greece: Contribution to Dating

“…A combination of a smooth, sticky clay with another one containing more sand and coarse particles is often ideal, because such a mixture will resist cracking during firing (Jones 1993). The use of baked clays in archaeomagnetism has been thoroughly investigated and procedures for optimum selection of samples have been proposed (Jordanova et al 2003). The firing phase was subdivided into three equally important stages: prefiring, firing, and cooling.…”

“…From the experiments conducted, the majority of samples proved to be suitable for the archaeomagnetic study, fulfilling two prerequisites: the Koenisberger ratio (Q) values, measured on 435 samples, mostly lie in the 5<Q<80 interval, and are characteristic for a stable (thermoremanent) origin of the NRM. The magnetic viscosity coefficient, S v , was calculated after storing the samples in a field-free space for 3 weeks, following Jordanova et al (2003). Obtained values S v < 6% indicate sufficient heating of the material.…”

Remanence anisotropy effect on the palaeointensity results obtained from various archaeological materials, excluding pottery

“…The anisotropy effect on the palaeointensity result estimated through the anisotropy of induced isothermal remanent magnetisation (IRM), as recommended in Stephenson et al (1986), has been previously performed for samples of baked clay of prehistoric ovens. This effect was found to be negligible, and the correction factor " f " of the absolute palaeointensity values is usually lower than 5% (Jordanova et al, 1995;Jordanova, 1996;Kovacheva et al, 1998). The idea of substituting the determination of the TRM anisotropy tensor by the determination of the IRM anisotropy tensor without heating has practical advantages, but needs confirmation.…”

“…The palaeointensity study was performed using the classical Thellier method (Thellier and Thellier, 1959) in two laboratories (the Geosciences Laboratory, University of Rennes 1, France and the Palaeomagnetic laboratory of the Geophysical Institute, Bulgarian Academy of Sciences, Sofia). In addition to other factors involved in the palaeointensity determination, such as the non-linearity of the Arai diagram, mineralogical changes, non-suitable magnetic characteristics, among others (Chauvin et al, 1991;Jordanova et al, 1997;Kovacheva et al, 1998;Jordanova et al, 2003), a detailed examination of the magnetic anisotropy effect on the palaeointensity results obtained from materials other than pottery is still required.…”

Mineralogía magnética de suelos volcánicos en una toposecuencia del valle de Teotihuacán

“…Estos estudios han sido particularmente prometedores en las secuencias de loess-paleosuelos de China (Liu et al, 2007, Lu et al, 2008, Europa (Oaches y Banerjee, 1996;Jordanova et al, 2003Jordanova et al, , 2004Liu et al, 2010), Argentina (Orgeira et al, 2003;Bidegain et al, 2009) y el noroeste de Estados Unidos (Geiss et al, 2008). En otros trabajos se ha establecido la relación entre la mineralogía magnética de suelos y cambios ambientales (Grimley et al, 2004;Geiss y Zanner, 2006, 2007 y contaminación (Chaparro, 2006;Orgeira et al, 2008), entre otros.…”