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.