Analysis of the corroded layers of ancient bronze mirrors found in clay

Excavated bronze objects are, depending on how long they were in the ground and under what conditions, generally corroded externally as well as having intricately corroded layers inside. To date, our group has performed metallurgical investigations on 18 ancient bronze mirrors and confirmed that pure copper lumps and several varieties of unidentified corrosion products are formed in and on the corroded layers of the mirrors. Accordingly, we investigated the corrosion products by using EPMA, SEM, μ-XRD and XPS, and discussed the possible causes of corrosion. The results are as follows; (1) Amorphous SiO₂ had formed in the outermost corroded layer I. (2) Cu₂O (cuprite) and Cu (OH)₂ had formed in the layers II and III, which comprise the first two internal layers adjacent to layer I. (3) Cu-Sn-Pb- complex oxide had formed in layer IV, the next internal layer. Small amounts of PbSO₄ and an Sn-organic compound were also detected. (4) We observed that Cu₂S (chalcocite) forms like mold on the polished surface of the corroded layers, especially notable on layer IV, after being mechanically polished and left to stand for about one month. Cu₂S is presumed to be the resultant of sulfate reducing bacteria. (5) Numerous pure copper lumps appeared in layers or particles and tended to form in the vicinity of layer IV. (6) The evidence from these analyses suggest that microorganisms could have been the cause of the corrosion found in ancient bronze mirrors excavated from clay.