Magnetic refraction studied on two experimental kilns

Abstract

SUMMARY experimental kilns in the form of shells (half-spheres) and circular bottom plates (inner radius: cm, outer radius 17 cm) were made from clay, fire-clay and less than 5 per cent by volume of fine grained haematite powder for the study of the effect of magnetic refraction in archaeological structures. The haematite in the clay was reduced to a strongly magnetic phase (maghematized magnetite). The acquisition of TRM was performed in a non-magnetic gas-heated furnace in the local geomagnetic field. The kiln material has a maghemite ore content of about 2 per cent by volume and an apparent magnetic susceptibility at room temperature of about 4 x lo-* SI units. The deviations of the declination and inclination of the TRM at different 'latitudes' of the shells were, respectively, up to 40 and 15 and systematically dependent from the position of the specimens in the shell or bottom plate in agreement with model calculations. However, the classical approach for the magnetic refraction in materials with ferrimagnetic ore grains dispersed in a nonmagnetic rock matrix with simple assumptions for the demagnetizing fields of the ore grains and of the macroscopic sample was not able to explain the large refraction effects which have been observed. An adequate theory for the magnetic refraction in rocks is still not available. For archaeomagnetic studies, some advice is offered for sampling and demagnetization treatment of the material in order to minimize refraction effects in archaeomagnetic data. Some consequences for palaeomagnetic studies are also discussed.