Pure gold is very resistant to corrosion but most gold objects produced at any date are not pure gold and are made from an alloy of gold with silver and copper. For archaeological materials, the base metals present (i.e. copper and silver) can be affected by the burial environment. Copper is often lost from the surface of gold alloys because of corrosion. Silver on the other hand is more corrosion resistant, but researchers agree that small proportions can be lost from the surface of objects due to natural corrosion processes. All such losses leave the surface of objects richer in gold than the core.

This so-called surface enrichment can be caused accidentally during use and burial, by natural corrosion mechanisms or during manufacture (either accidentally or deliberately). There is documentary evidence for several ancient methods for removing copper and silver from gold, and analysis of gold alloy objects from other periods and cultures have shown that deliberate surface enrichment was sometimes being undertaken.

At the British Museum a combination of X-ray fluorescence analysis (XRF) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was carried out on a small number of objects from the Staffordshire Hoard, most of which are hilt plates. The surface and sub-surface of the objects were analysed to investigate evidence for surface enrichment and to determine whether surface analysis could be used to provide a reliable indicator of the original bulk alloy composition for these archaeological objects.

Surface XRF analysis revealed that most of the objects examined are of similar composition. However there were two outliers within this group: K95 which is almost pure gold, and K133 which had less gold and more silver than the other objects analysed.

Figure 1

Figure 2

The graphs in Figure 1 show SEM-EDX data for K79. When the silver content is plotted against the copper content, the surface and sub-surface analyses form two distinct groups indicating a decrease in copper at the surface (Figure 1, top). For the same object, plotting silver against gold indicates that the sub-surface and surface analyses are very similar. Surface depletion in copper was noted in three of the sixteen objects analysed while two objects showed very little evidence for surface depletion of any kind.

By comparison, the remaining eleven objects revealed extensive loss of silver at the surface, and in six cases this was particularly striking. For example the analysis of K3 revealed that along with the loss of copper at the surface (likely due to corrosion) there was also over 40% loss of silver at the surface compared to the core.

This degree of silver depletion is very unlikely to be due to corrosion during burial alone. This was confirmed by the analysis of the metal revealed within deep scrapes on K1221. These scratches were probably made by those dismantling the swords before burial. The scrapes showed the typical surface loss associated with corrosion during burial, i.e. loss of copper and no loss of silver (Figure 2). However, the undamaged surfaces of the same object revealed a different picture, with significant loss of both silver and copper from the surface. This suggests that some form of deliberate “depletion gilding” had taken place (e.g. deliberate removal of both silver and copper from the surface of the gold alloy, probably to enhance the colour of the object). This would have been followed by burnishing of the treated surfaces by the maker to create the shiny surfaces seen.

This evidence suggests that in the Anglo-Saxon period there was deliberate surface treatment of at least some gold alloys. This has implications not only for the Staffordshire Hoard Project but also for other research projects on Anglo-Saxon gold, and potentially for other gold objects from other periods and our understanding of metalworking technology. This study has also shown that any future quantitative or semi-quantitative analysis by SEM-EDX or XRF of objects from the Staffordshire Hoard would require tiny areas to be cleaned down to the sub-surface region to allow analysis of the core alloy.

References

Blakelock, E.S., 2013. Pilot study of surface enrichment in a selection of gold objects from the Staffordshire Hoard, British Museum Department of Conservation and Scientific Research, Science Report PR07444-10.

Blakelock, E.S., 2016. Never judge a gold object by its surface analysis: A study of surface phenomena in a selection of gold objects from the Staffordshire Hoard, Archaeometry 58 (6). 912-929.