Magnetic and Optical Methods for Detecting the Heat Treatment of Chert

Magnetic Properties of Cherts from the Basque-Cantabrian Basin and Surrounding Regions: Archeological Implications

“…Environmental magnetic parameters have been proposed to be useful for assessing chert quality, distinguishing source areas, and identifying heated samples in the archeological record (Borradaile et al, 1993(Borradaile et al, , 1998Thacker and Ellwood, 2002;Sternberg, 2008). We discuss our results from the BCB chert types in light of these three main issues, and consider first an additional aspect (type and origin of magnetic minerals in chert) that has not been addressed in previous archeo-magnetic studies.…”

“…We attribute the modest magnetic enhancement of BCB flints upon heating to the low amount of non-silica (e.g., Fe-bearing) impurities. Given such slight magnetic enhancement, it is unlikely that other magnetic properties that can be eventually measured as proxies for the concentration of magnetic minerals (e.g., IRM@1.2T) will experience the dramatic (e.g., at least ten-fold) increase observed in other heated cherts (Borradaile et al, 1993(Borradaile et al, , 1998. In any case, the negative magnetic susceptibility values of most BCB natural flints, along with the local use only of the Artxilondo type (the only natural flint with positive magnetic susceptibilities), suggests that any flint tool within the core of the BCB with positive magnetic susceptibility values is likely to have been subjected to heating for improving its knapping properties.…”

“…In order to simulate conditions likely available for ancient humans, samples were heated with a rate of 30 • C per min to a maximum temperature of 450 • C (Schindler et al, 1982;Borradaile et al, 1993). After 2 h at this temperature, samples were left to cool down freely.…”

“…They have been also applied in archeology for discriminating obsidian and other raw material of archeological interest such as chert and flint (McDougall et al, 1983;Tarling, 1990;Church and Caraveo, 1996;Borradaile et al, 1998;Thacker and Ellwood, 2002;Sternberg, 2008). Studies focused on chert samples have already shown the usefulness of rock magnetic properties to establish the degree of chert purity and to assess the provenance of some chert types (see Borradaile et al, 1993Borradaile et al, , 1998Thacker and Ellwood, 2002;Sternberg, 2008). Some of these papers have also noticed the important changes in rock magnetic properties of chert samples upon heating (Borradaile et al, 1993(Borradaile et al, , 1998.…”

“…Studies focused on chert samples have already shown the usefulness of rock magnetic properties to establish the degree of chert purity and to assess the provenance of some chert types (see Borradaile et al, 1993Borradaile et al, , 1998Thacker and Ellwood, 2002;Sternberg, 2008). Some of these papers have also noticed the important changes in rock magnetic properties of chert samples upon heating (Borradaile et al, 1993(Borradaile et al, , 1998. This has important implications, because thermal treatment of chert improves its flaking properties (by helping fractures to propagate within the material, Domanski and Webb, 1992) and its identification can be used to constrain variable technological capabilities.…”