Aligned submicron grains in archeological potteries with high TRM anisotropy

Potteries have been often used to obtain archeointensity data because of the extremely high success rates in Thellier experiments. Since high anisotropy of thermoremanent magnetization (TRM) are commonly observed for potteries, anisotropy correction is routinely applied to make archeointensity data reliable. Such a TRM anisotropy is characterized by the foliated structure parallel to the surface and has been interpreted to reflect aligned magnetic minerals during molding. However, the high thermal stability and the higher degrees of TRM anisotropies than those of low-field susceptibility suggest that pottery TRM resides in fine-grained magnetic minerals formed during firing. Molding is not sufficient to explain highly anisotropic TRM of potteries. We measured TRM susceptibility tensors on potteries manufactured from 14th to 17th centuries in Japan. These potteries were fired up to about 1200°C resulting in partial vitrification. The ratios of maximum and minimum eigenvalues of the TRM susceptibility tensors are well distributed and reaches 1.8 for most anisotropic samples. The Curie temperature ranges from 500°C to 550°C implying titanium-poor titanomagnetite as TRM carriers, and the narrow unblocking temperature indicates that the low-titanium titanomagnetites are in or close to the single-domain size range. Submicron titanomagnetite grains were commonly observed on the polished sections under a FE-SEM. Some of the grains occur as inclusions in iron-bearing silicates such as pyroxene or hornblende, and other grains reside in glass matrix. In addition, vesiculated clay minerals contain highly elongated titanomagnetite grains. Such clay minerals seem stretched parallel to the pottery surface and inside titanomagnetite grains are also elongated along the surface. Relative abundance of titanomagnetite inclusions in silicates or glass against in clay minerals should control the degree of TRM anisotropy. Foliated TRM anisotropies originate from both alignment of clay minerals during fabrication and formation of elongated titanomagnetite during firing.