PNNL

Abstract

We have conducted a scanning Auger microscopy (SAM) analysis on a Syrian bronze of the early I millennium BC. The objective was to derive a picture of modifications of the object's surface composition, induced by chemical attack by the ambient. Understanding degradation of ancient materials dating from a well-defined historical context provides information about long-term corrosion phenomena that is not possible from laboratory data. Such information is also useful for restoration and conservation efforts. While corrosion processes in the bulk are understood, this is not true for localized intergranular and transgranular surface corrosion in these materials. This requires information about local chemical composition and movement of ionic species that has not generally been available, and we find AES useful in accomplishing this task. SAM images of Cu, Sn and O, ~ 100-nm spatial resolution, and Auger point spectra show that Sn acts as a barrier against O2 attack, as it entirely traps this gas into a SnO2-like oxide, thus minimizing copper oxidation. Sub-micron spectromicroscopic evidence is given that S-induced corrosion occurs mainly via chemical attack along grain boundaries, where Sn has a low abundance and copper sulfides are detected. Lateral segregation of Sn and Cu domains is imaged with a spatial resolution of 15 nm. This result marks the best spatial resolution any analytical method has yet achieved in highlighting lateral chemical heterogeneities of ancient bronzes. Although archaeomaterials lie outside the mainstream applications of Auger techniques, this study provides convincing evidence that SAM can greatly advance our understanding of these materials.