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.

Summary During rescue excavations carried out near the vicus at Kempraten (municipality of Rapperswil-Jona, St. Gallen, Switzerland) in advance of a private construction project, a Mithraeum measuring approximately 8 by 10 m was unexpectedly discovered in the summer of 2015 and subsequently excavated and investigated in detail. This paper presents the preliminary results of the excavation, which was completed less than a year ago, and pays particular attention to the interdisciplinary approach used in the excavation. These included intense sampling of the features for the purposes of micromorphology and archaeobiology. Three construction phases with intermittent conflagrations were identified. The question as to whether there was an ante-chamber remains unanswered. The external areas are also quite difficult to interpret, at least for the time being. The rich assemblage of finds, which included numerous coins, pottery, animal bones and a range of religious artefacts (e.g. altars and a half relief), will only be dealt with in a cursory manner here. According to the range of coins, the Mithraeum undoubtedly dated from the late 3rd to the late 4th or early 5th centuries. The site will be analysed by an interdisciplinary team and preliminary work is already underway.