論文の公開元へ加速器質量分析法による日本刀や古い鉄器類の放射性炭素年代測定
概要
The age of ancient iron artifacts can be measured by using carbon component contained in metal iron that was included during the iron producing or iron reprocessing stages. To extract carbon from iron artifacts in a form of CO2, a quartz tube has been used for metal iron combustion at the temperature of 1000°C for about 15 hours. We tested the quartz tube combustion method successfully of metal iron in a fine flake condition of almost 1mm in size. Almost 87 % recovery in average for 24 samples from a standard iron material was attained when metal iron weight is about 25% of CuO used as an oxidizer. The CO2 of about 1mg carbon was changed to graphite and 14C dated with the HVE 14C-AMS system at Nagoya University as well as NEC 0.5MV compact AMS at Paleo-Lab Co. Ltd. One of the common iron artifacts in Japan is Japanese sword, owned traditionally by samurai, a Japanese warrior, and used during battles. At the present time, Japanese swords are one of excellent art collections, and many fake swords exist in the market. Traditionally, Japanese sword has been authenticated by experts based on visual outlook of them and judged real ones out of fakes. A more scientific method was required for judging the original sword. We have analyzed several iron flakes of about 1mm in size collected from Japanese swords in the course of grinding and re-sharpening them. The analyzed swords were believed to have produced in the 5th century and after, by the name of the manufacturer inscribed on the surface of the sword, and/or by the traditional judgment by experts. The obtained 14C dates for the sword samples were calibrated with IntCal20 and compared with the authorized production dates by visual analysis. The 14C ages were almost consistent with the traditional judgments. We consider that 14C dating is a useful tool to certificate the visual judgment of the swords by experts and to accept existing historical documents about them. In addition, iron rust samples are commonly excavated from archeological sites in Japan and in foreign countries. We have tested four rust nail samples from archeological sites in India, to collect carbon component successfully by the quartz tube in the same way as pure metal iron. The nail samples were covered by rust on the surface, but the samples should contain metal portion, because they reacted on the magnetic filed. The carbon content of the rust nails was from 0.2 to 0.5 % in weight ratio, similar to that of pure metal iron samples. The obtained 14C ages were consistent with their archeological chronology.
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