What kind of corrosion products are “black spots”? —Effects of reduced sulfur compounds on corrosion of bronze artefacts

While deterioration due to the generation of “black spots” is reported, consensus on the specific corrosion products comprising black spots and underlying deterioration mechanisms is lacking. This study investigated the deterioration of copper objects by analysing the black spots that appeared on them and proposes environmental conditions to suppress the appearance of black spots. Bronze artefacts excavated from a marine archaeological site, on which black spots were generated, were closely observed, and investigated. First, X-ray fluorescence and micro-X-ray diffraction analyses were performed on a bronze artefact, from which the black spots were removed, to determine the composition of the artefact. Second, to investigate the composition of the black spots, fine structure analysis of the black spots on the fine fragments of the bronze artefacts was conducted using scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron diffraction (ED) analysis. The results indicate that the black spots were partially composed of metallic copper and a fine particle mixture of Cu₂S and CuSO₄. These results imply that the transformation of copper sulfide to metallic copper may have played an important role in the initiation and propagation of black spots. In this study, ED analysis was performed on the microscopic area of the black spots; no amorphous phases were detected, and all observed phases were identified as crystalline materials. To determine the sources of gas-phase sulfur compounds that cause black spots, H₂S and carbonyl sulfide (COS) emissions from a spherical clay artefact (a cannon ball with gunpowder excavated from an underwater archaeological site) which was exhibited in the same showcase with the bronze artefacts were analysed via gas chromatography. The concentrations of H₂S and COS were 0.462 and 8.636 ppb, respectively; these are significantly higher than those in the lower troposphere. These results indicate that deterioration by black spots occurred because of H₂S and COS, which were emitted from the spherical clay artefact excavated from an underwater archaeological site in the exhibition case. In addition, it was confirmed that the inclusion of deoxygenating and dehumidifying agents (RP-AN) in the plastic bag in which the spherical clay artefact was inserted resulted in a significant decrease in the concentration of H₂S and COS.