Iron and oxygen isotope systematics during corrosion of iron objects: a first approach

Iron objects are among the most abundant type of metal artefacts in the archaeological record and help to deepen our understanding of past societies and their technologies. However, sampling of them is often problematic due to the destructive character of most analytical methods. In this study, iron and oxygen isotope compositions of iron artefacts from marine and water under-saturated oxidising environments were analysed in a first attempt to gather artefact information from the corrosion layer without sampling the object directly. No Fe isotope fractionation between artefact and its corrosion products was recognised for both environments but cannot be excluded for marine environments. Hence, the artefact’s Fe isotope composition can be determined from the corrosion layer. This allows the characterisation of artefacts which cannot be sampled directly. Because the available data precluded a clear identification of the underlying processes, possible fractionation mechanisms resulting in this situation are presented. Furthermore, the results of this study indicate that corrosion products have the same oxygen isotope composition as their source water. As for marine corrosion, general absence of oxygen isotope fractionation could not be manifested here, because only a small sample size was available for the present study. However, a complex interplay of many parameters governs the oxygen isotope compositions of corrosion products on metallic iron. Especially oxidising environments above the water table have a strong impact on the oxygen isotope composition of the corrosion layer. The first-order controlling mechanisms, such as evaporation, are set by the local environment and cannot be reconstructed. Therefore, the oxygen isotope composition of corrosion products seems to bear no valuable potential for archaeometallurgical research.