We studied the corrosion of Roman copper alloy coins that experienced alternations or progressive changes in their burial environment. We used coins that were still embedded in soil or in a concretion selected from three professional excataved sites - Berlicum and Krommenie in the Netherlands and Kempraten in Switserland. mCT scanning and neutron scanning were used to record the 3-D properties of these coins prior to (destructive) analyses. It proved possible to tentatively identify the coins. Microscope observations and SEM-EDX analyses revealed complex corrosion processes, related to changing burial environments. In soil horizon with fluctuating groundwater levels in a region with upwelling reducing, iron-rich groundwater, the copper in a gunmetal coin is essentially replaced by iron oxides while tin remains and forms tin-oxide bands. Fluctuating redox conditions in marine-influenced environments was shown to transform a copper-alloy coin into strongly laminated copper sulphides with embedded gypsum crystals, with an outer surface of copper and copper-iron sulphides. Burial of bronze in a charcoal rich layer probably caused temporary highly alkaline soil conditions. This caused most of the copper to leach from this coin, leaving behind a laminated tin-dominated mass, with only a limited amount of (malachite) corrosion products remaining in the surrounding groundmass. In all three cases, corrosion processes tend to be anisotropic, probably because of cold-hammering of the coins during their manufacture. Such corrosion processes on massive copper alloy coins may produce features that may lead to their incorrect classification as subferrati, i.e. copper alloy coins with an iron core. Our results may help in future to distinguish strongly corroded massive coins from subferrati.
We would like to thank Nico and Wim van Schaijk, whose kind cooperation made the research at Berlicum possible. Piet Kleij, municipal archaeologist of Zaanstad, is thanked for making the research on the Krommenie coin possible. Mario van IJzendoorn and the late Thomas Beckman are thanked for preparing the impregnated blocks and thin sections of Berlicum and Krommenie, and of Kempraten respectively. Paul Beliën is thanked for his help in querying the NUMIS database for subferrati from the Netherlands. Andreas Pangerl helped with finding suitable reference images for the Krommenie coin in Fig. 6, and gave permission to use image 6I from www.romancoins.info
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