青銅器の炭素14年代測定の可能性と道成寺鐘巻銅鐸への適用  [in Japanese] Potential application of radiocarbon dating to bronze implements and application to the Dojoji Kanemaki Doutaku bronze bell  [in Japanese]

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Abstract

Radiocarbon dating is a useful method to determine the ages of wood, charcoal, and paper. All three of these materials contain carbon derived from carbon dioxide in the atmosphere. Verdigris, the rust produced on the surfaces of bronze implements, is basically composed of copper carbonate CuCO3・Cu(OH)2. The Cu contained in thebronze reacts with the CO2, H2O, O2 in the atmosphere. Once verdigris is formed, it seals the bronze as a close film, restraining the generation of new rust. Verdigris can thus be assumed to capture and preserve carbon from the atmosphere at the time it forms. The extraction of carbon from verdigris enables the radiocarbon dating of bronze implements. This study was conducted to prove that the radiocarbon dating of bronze is possible. Preparatory experiments confirmed that CuCO3・Cu(OH)2 heated at 250ºC for 1 hour releases CO2. The extraction method for archaeological samples of known ages was then applied to measure their radiocarbon ages. The archaeological ages of two Chinese bronze goblet, Jue, date between BC2070 and BC256. One goblet had a radiocarbon age of 2153±21 BP, which corresponds to 338–169 cal AD. The other goblet was dated at 3623±25 BP and 2027–1940 cal AD. The archaeological age includes their calibrated radiocarbon ages. Kanemaki Doutaku, a Japanese bronze bell at Dojoji temple, Wakayama Japan, was also measured. The bell has an archaeological age from the latter half of the 1st century to the first half of the 3rd century. The radiocarbon age was 1759±19 BP and calibrated to 241–324 cal AD. The result of archaeological samples shows that verdigris captures and preserves the carbon in the atmosphere when it forms, and thus can be used as a suitable sample material for radiocarbon dating.青銅器に含まれるCuは大気中のCO2と反応し,緑青(塩基性炭酸銅,CuCO3・Cu(OH)2)を生じる.保存科学の分野では,緑青は一旦形成されるとそれ以降の新たな緑青の形成を阻止する性質をもつ「良いさび」といわれている.したがって,緑青に含まれる炭素の抽出と,その14C年代測定が可能となれば,そのさびの形成年代が得られ,さらに青銅器の使用年代を求めることが実現するはずである.そこで本研究では,まず14C年代測定のための緑青の調製法を開発することを行った.その上で,考古学的な視点から年代の判明している道成寺鐘巻銅鐸の緑青について14C年代測定をし,3世紀中頃もしくは4世紀初頭に相当する結果を得た.この結果から,緑青が形成当時の大気中CO2を保持していることが実証された.本研究の一部には,平成27年度~平成29年度日本学術振興会科学研究費補助金挑戦萌芽的研究(課題番号:15K12443,研究代表者; 小田寛貴)を使用しました.

Journal

  • 名古屋大学年代測定研究

    名古屋大学年代測定研究 (1), 118-121, 2017-03-31

    名古屋大学宇宙地球環境研究所年代測定研究部

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