Novel methods for the non-invasive study of archaeological coins with electrochemical impedance spectroscopy

Archaeological copper-based coins are usually covered by insulating patina layers, which prevent the establishment of effective electrical contacts. This hinders the use of electrochemical techniques, including the electrochemical impedance spectroscopy, as the undesirable removal of the patina irreversibly damages the artifacts. To overcome this and other constrains imposed by Heritage samples, we designed two novel experimental approaches.

In the first approach, the electrical contact problem is solved by exploiting the unique capabilities of laser ablation in producing a micro-dig that is deep enough to reach the underlying alloy, and small enough to be invisible to the naked eye. The insertion of graphite micro-fibres in the micro-dig granted the required electrical contact. Following the best practice in the field, the three-electrode cell was completed by a gel electrolyte pressed against the opposite side of the coin and containing reference and counter electrodes.

The second approach is completely non-invasive and exploits a symmetric four-electrode cell design that has never been applied to Cultural Heritage objects before. The symmetric cell was realised by putting each side of the coin in contact with a gel electrolyte, containing a reference and a working/counter electrode.

In this paper, the results obtained on real archaeological samples with each approach are compared and thoroughly discussed, with reference to experimental setups proposed in literature. Both novel approaches were able to achieve an unprecedented combination of high-quality data and non-invasiveness level. In addition, preliminary data fitting suggested a role for a diffusion phenomenon in porous media in the low-frequency regime.