Study of cooling rate effect on baked clay materials and its importance for archaeointensity determinations

Abstract

The effect of cooling rate was investigated for various baked clay materials as an attempt to follow how different factors (clay type, the experimental temperature chosen and material‘ heterogeneity) affect the corrections obtained and to estimate the importance of correction factors on the final archaeointensity determinations. The collection analyzed includes archaeological samples taken from eight archaeomagnetically studied Bulgarian archaeological sites and artificial samples with identical heating/cooling history prepared in the laboratory from different clay types. The results from the rock-magnetic analyses carried out indicate stability of magnetic mineralogy during heating. The identified magnetic minerals are mainly of magnetite type but hematite is also detected in certain cases. The cooling rate experiment was performed at several temperatures – 450 and 520 °C for the archaeological materials (taking into account the temperature intervals used for archaeointensity determination) and 450, 520 and 620 °C for the artificial samples. The results obtained point out that the cooling rate corrections can vary significantly not only within the same archaeological structure but also within the same sample. Major part of applicable corrections is obtained using lower experimental temperature. The experiment with the artificial samples illustrates that correction factors could differ even for materials subjected to the identical heating/cooling conditions. After applying individual cooling rate corrections the mean archaeointensities are reduced by 2–16%. For the half of the archaeological collections tested, the corrected mean values are within the experimental errors. Generally, the application of cooling rate correction does not improve the scatter of the AI results but can considerably diminish the number of the results used for the final AIs determination. It is suggested that this problem could be overcome using calculated median values for the cooling rate correction per site. The applicability of the proposed approach was tested and verified using published by other authors archaeointensity data obtained from pottery fragments (coming from three archaeological sites in Greece) and baked clay samples (taken from seven Spanish kilns).