Surface investigation of naturally corroded gilded copper-based objects

• Published in: Applied surface science 2016-11, Vol.387, p.244-251
• Main Authors: Ingo, G.M., Riccucci, C., Lavorgna, M., Salzano de Luna, M., Pascucci, M., Di Carlo, G.
• Format: Article
• Language: English
• Subjects: Ancient coating methods; Artefacts; Copper alloys; Corrosion; Corrosion products; Degradation; Floating structures; Gilding and silvering; Gold; SEM + EDS; Silver; Sulfur; XPS
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2016.06.082

•Naturally corroded Au and Ag coated Cu-based objects studied by XPS, SEM+EDS and OM.•The main degrading agents are Cl, S and P species from surrounding environment.•Metal galvanic coupling enhances corrosion phenomena.•Corrosion forms a layered patina of noble metal remains, soil components and Cu2O.•Useful information to tailor safe cleaning and reliable conservation strategies. Gold and silver coated copper-based artefacts subjected to long-term natural corrosion phenomena were studied by means of the combined use of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM+EDS), and optical microscopy (OM). The results allowed the identification of the chemistry and structure of the Au or Ag layers deposited by fire-gilding or mercury-silvering and the determination of the corrosion products formed due to interaction with the surrounding environment. Different degradation phenomena of the noble metal layer and copper substrate are induced by the presence of chlorine, sulphur and phosphorous and they are boosted by the metal galvanic coupling which makes gilded-metal art works unstable from a chemico-physical point of view. The SEM+EDS and OM results also suggest that particular care must be used during the removal of the encrustations and of the external corrosion products to avoid the loss of the remains of the noble layer often floating or embedded in the corrosion products. Furthermore, in order to avoid the reaction between nantokite (CuCl) and moisture the use no or low toxic inhibitors is suggested to avoid further severe degradation phenomena enhancing the long-lasting chemico-physical stability of these precious artefacts and giving them a greater chance of survival.