江戸時代貨幣『豆板銀』の表面層の構造

抄録

  The surface structure of Mameitagin, which is a silver coin used in the Edo period, was investigated by spectrophotoscopy, X-ray diffraction (XRD), electron diffraction, electron dispersive X-ray analysis (EDS), X-ray fluorescence analysis (XRF), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The EDS analysis confirmed that Ansei Mameitagin is composed of only 13% silver and 87% copper. However, an absorption edge at 600 nm that is characteristic of copper was not observed in the reflectance spectra. A layer with high silver concentration was detected on the surfaces of Ansei and Bunsei Mameitagin. After polishing, the surface color of Ansei Mameitagin changed from silver to copper, and an absorption edge was detected at 600 nm. The microstructure analyzed by TEM revealed that Ansei Mameitagin has two surface layers. One is a 0.6-µm-thick Ag-rich layer that forms on the ground metal while the other is a 0.8-µm-thick oxide layer composed of Cu₂O. These results suggest that the surface of Mameitagin had been chemically treated to form Ag-rich layer called Iroage.