Surface and microstructural analyses of a Roman quadrans dating back to first century ad

The paper reports the results of the analyses performed on a Roman quadrans dating back to the first century ad, belonging to a group of more than 8.000 coins found in the bed of river Tiber during an archaeological research in 1987-1988. The quadrans was examined by means of X-ray diffraction, scanning electron microscopy, energy dispersion spectroscopy, and X-ray photoelectron spectroscopy. Energy dispersion spectroscopy analytical results confirmed that the coin is made of pure copper. The compounds present in the patina have been identified through X-ray diffraction. The spectrum is the overlapping of several peaks, and the results show that the patina substantially consists of malachite Cu2CO3(OH)2, cupric chloride CuCl2, and nantokite CuCl. X-ray photoelectron spectroscopy only evidences nantokite because of the different analysis depths of the two techniques. This means that nantokite forms a thin outer layer in the concretion covering the quadrans.