Microstructural and Compositional Characterization of Roman Bronze Coins from Khirbat Edh-Dharih in Jordan

This study aimed at investigating the chemical and mineralogical compositions of five Roman coins (four copper-based and one silver-based alloys) corrosion products, and explore the topographic and morphological microscopic features of the patinas formed on the surface of the copper-based coins. For this purpose, an interdisciplinary approach to micro-destructive methods—microscopic (OM and SEM), mineralogical (XRD), elemental (XRF and SEM-EDX), and molecular (ATR-FTIR)—was conducted. The results showed that cuprite is the principle patina initially formed on the surface of the copper-based alloys by the redundant interaction with the surrounding environmental burial conditions, which is most likely an oxygenated and moisturized soil. This interaction was also observed in the formation of a secondary patina composed of malachite and azurite, which lately was invaded by the corrosive cycle process (bronze disease) represented by the formation of nantokite, atacamite and paratacamite that affected the cuprite primary patina of the copper-based coins during burial. The silver-based coin also suffered an aggressive attack by oxygen, sulfur and chloride ions during burial and formed oxide, sulfide, and chloride of silver, in addition to the corrosion products of cuprite, atacamite, and carbonate of copper, which is one of the alloying elements of this coin. The findings of this study also show that the copper-based coins were made of quaternary Cu-Sn-Zn-Pb alloy, and the silver-based coin was made of ternary Ag-Cu-Sn alloy. Therefore, the study points out that these coins were suffering from the corrosion phenomenon by the reaction with oxide, sulfide, carbonate, hydroxyl, and chloride ions, which are most likely found in the burial soil and incorporated within the alloy corrosion products. Contamination with Si, Fe, Al, and Ca elements present in the soil was also seen. We recommend protecting these alloys to prevent further degradation that may occur during storage and exposure to the atmosphere after excavation.