Analysis of Swedish Silver Coins: Silver Surface Enrichment, Dirt, Corrosion, Polishing, and Origins

摘要

Fifty-eight Swedish silver coins from the last five centuries were analysed by SEM-EDX and other techniques. All coins, even those from the twentieth century, showed a surface enrichment of silver. Accordingly, a SEM-EDX analysis of a silver coin gives an erroneous result for the bulk composition. On the other hand, a silver coin assiduously polished with a rough emery cloth usually gave the same silver concentration as the official bulk composition, at least for coins minted after AD 1600. Cross-section analyses indicated that the enriched surface layer had a thickness varying from 15 to 100 mu m. The corrosion and dirt contained light elements like carbon, oxygen, chlorine, sulphur, phosphorus, and silicon. Treatment with silver polish removed most of the dirt, but also a small part of the silver. Analyses by XPS (ESCA) showed that the outermost surface was principally enriched in carbon, and that most contaminants decreased after an argon ion sputtering. Eight coins were analysed for their lead isotope composition, which indicated that the silver mainly originated from mines in Sweden and Germany. Similar studies of silver coins are discussed, as well as the cause of the surface enrichment of silver.