Exploring microstructure and surface features of Chinese coins using non-invasive approaches

Despite the apparent significance of Chinese coins, the knowledge about the surface properties of the coins is still largely unknown. To date, most analytical techniques (e.g., cross-section analysis, inductively coupled plasma-mass spectrometry, thermal analysis) require the partial or total destruction of the investigated sample, which is fatal to precious objects (e.g., artefacts and monuments). Herein, we systematically investigate the surface of a series of one yuan Chinese coins to disclose their chemical composition, morphology, and microstructure features using non-invasive techniques. Investigations were performed with scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy, and X-ray diffraction. The application of these approaches enables unambiguous explorations of the component, morphology, microstructure and physical properties of the samples without destroying them. The identification of the coins was achieved in light of the name of issuing authority and floral pattern. The morphology observations of the samples display that these coins possess mostly homogeneous surfaces; hence such a finding allows the formulation of a possible minting technology. Besides, the energy dispersive X-ray spectroscopy has proved of great role in exploring these coins, mainly because of its detectability to easily probe the presence of certain minor elements, which is critical in understanding surface finishing technologies, and production processes. The findings manifest that the coins were made of high purity nickel and a good refining process was applied in general. The detectable amounts of carbon measured in some coins suggest that the refining process was not exactly alike. These coin samples are found to be highly crystalline in nature with face-centred cubic crystallographic structure. Furthermore, to shed more light on the surface features of the coins, their physical properties (e.g., interplanar spacing, lattice parameter, lattice microstrain, X-ray density, dislocation density, porosity) were investigated in detail. The results demonstrate that our protocol may be a viable adjunct or alternative to conventional analytical approaches. More importantly, unlike traditional approaches, our protocol is totally non-destructive, a highly significant factor when probing irreplaceable artefacts.