Corrosion of ancient Chinese bronze fragments from different periods and protective effect of menthol coating

Chinese ancient bronze fragments from three periods of history were investigated in terms of the characteristics of substrate and patina. The non-destructive protective effect of menthol coating was verified. Patinas on the fragments of the Spring and Autumn period, Warring State period, and Han Dynasty all contain the major components CuCl2·3Cu(OH)2 and Cu2(OH)2CO3. The residual components, some oxides and chlorides of Pb and Sn, show no evident distribution trend in the patinas except PbCl2 is segregated beneath the patina of the Han dynasty fragment. Menthol coating shows a consistently protective ability for these patinated fragments. However, its sublimation is strongly affected by ambient temperature and relative humidity (RH). Menthol coating with an initial thickness of ∼500 µm could effectively protect the fragments for 328 h under 23 ℃ with 54% RH while for only 42 and 2 h respectively under 25 ℃with 100% RH and 35 ℃ with 100% RH.