Mechanism of corrosion due to unalloyed copper inclusion in ancient bronzes

Abstract

Ancient bronze objects buried in the soil for a long time corrode because of various environmental conditions. This can also cause different corrosion morphologies. One of the features of ancient bronzes is the presence of unalloyed copper with copper inclusions (UCI); when in the alloying process, the copper-tin alloy did not form properly. Because of the phenomenon of preferential corrosion, the tin content decreases in an environment with low oxygen and high chloride and sulphide content. The results are various corrosion products. A decorative plaque belonging to the National Museum of Iran has been investigated by X-ray diffraction, SEM-EDX and reflected polarised light optical microscopy. The investigations showed that the UCI effect appeared as type B, i.e. with copper sulphide in the metallic core. The presence of UCI (type B) in ancient bronzes proves that the objects corroded after deposition in the soil.

New insights on Late Bronze Age Cu-metallurgy from Coles de Samuel hoard (Central Portugal): A combined multi-analytical approach

“…The formation of this kind of structure is generally associated with corrosion mechanisms of the alloy (Bosi et al, 2002;Emami and Bigham, 2013). As has been already observed in the rest of the collection from Coles de Samuel, Pb-rich inclusions are also visible (Fig.…”

“…13). The occurrence of this particular microconstituent is a rather common feature of ancient bronzes (Silva et al, 2008;Wang and Merkel, 2001), and its formation is generally considered as the result of a copper redeposition process resulting from corrosion phenomenon (Bosi et al, 2002;Emami and Bigham, 2013).…”

Towards a deeper understanding of the third millennium BC Iranian metallurgy: Use of synchrotron light for characterizing arsenic‐bearing minerals in metal objects from Espidej

“…The mineralogy of the objects examined through the light microscopy provides information on the texture and corrosion behaviour of the metals. 41 Based on the visual inspection, the surfaces of the objects (axes) appear to be very smooth with bright colour with respect to its alloying agents. [42][43][44] Considering the influence of arid circumstances on corrosion performance, 45 the corrosion phenomenon on the objects was explained briefly.…”

“…The formation of an unalloyed copper inclusion as a corrosion/redeposition reaction can also be considered. 41 This suggests that the redeposition of unalloyed copper is kinetically slow. This can only happen in humid environmental conditions with high oxygen fugacity and in the presence of high chloride (which is responsible of heavy corrosion layers) and sulfide (S 2À ) ions.…”

Surface and interface investigation of electrochemically induced corrosion on a quaternary bronze

“…Understanding the complex and heterogeneous corrosion processes, which threaten metallic cultural heritage artefacts, is a prerequisite for the design of tailored conservation protocols . Copper‐based alloys are more vulnerable to chloride attack compared to other corrosive species, especially when they are exposed to marine or coastal environment or during burial in Cl − enriched soils.…”