A study on metallurgy and corrosion of ancient copper-based artefacts from the portuguese territory

PhD thesis in Conservation and Restoration by Elin Figueiredo

(Thesis) 2010

In the present thesis metallic artefacts of various typologies and diverse materials related to metallurgical operations were studied. The items are from various sites in the Portuguese territory,covering a period of circa 3 millennia, from Chalcolithic to Iron Age. A large part of the studied items belong to emblematic museum collections, as the Castro de Pragança and the Baiões ones. The aim of the present study is to provide a general view of the ancient metallurgy in the Western territory of the Iberian Peninsula, as well as detailed information on the metallurgy of each archaeological site. It was also aimed an evaluation of the corrosion of archaeological metals, namely bronzes (Cu-Sn alloys). Accordingly, elemental analysis and microstructural examinations were made, combining diverse analytical techniques as energy dispersive X-ray fluorescence spectrometry (EDXRF), micro-EDXRF,optical microscopy and scanning electron microscopy. The main results of the corrosion study showed that decuprification is the main corrosion phenomena among the bronzes, but that destannification can also occur in particular cases. It was found that these phenomena can have a major influence in the results of superficial EDXRF elemental analysis, mainly due to the thickness of the corrosion layers that can reach 500 μm. It was also found that the most internal corrosion, namely the intergranular corrosion, can be very pronounced among the artefacts with a heterogeneous microstructure, i.e. mainly among those that were less subjected to thermomechanical processing. Additionally, particular long-term corrosion phenomena were described, as the preferential corrosion of α or δ phase and the presence of redeposited metallic copper in the most internal corrosion regions. The main results of the archaeometallurgical study showed that during the Chalcolithic period relatively pure coppers were used (with exception for the presence of As), and that during Late Bronze Age binary bronze with relatively constant tin contents (average of ~13 wt.% Sn) and impurities as Pb,As and Sb was the main material used, being unalloyed copper only used sporadically to produce particular items where the properties of this metal were an advantage. Differing from other Western European regions, ternary bronzes seem to have a later appearance, i.e. during Iron Age. The shaping of large and more complex artefacts (e.g. spear heads, axes, closed rings) was done in the mould,being needed just some final thermo-mechanical processing. On the other hand, smaller and simpler items (e.g. chisels, awls, simple fibulae, open rings) were produced by shaping pre-defined forms, as cast bars, through thermo-mechanical processes that could be very intense, as those that include various cycles of deformation and annealing.