Comparison of Different Electrochemical Setups for Noninvasive Impedance Measurements on Archeological Coins

The electrochemical characterization of cultural heritage objects requires noninvasive approaches. In the case of patina-covered bronze objects, two essential features of the setups recently proposed to satisfy this requirement are (i) the use of gel electrolytes and (ii) the absence of metal contacts that might damage the patina (or, at least, the control of the size of the damaged area to make it invisible to the naked eye). In the present work we show how the impedance of model coins is most conveniently measured using a four-electrode setup where the sample is mounted in a symmetrical two-compartment cell filled with a gel electrolyte which contacts both coin faces. Results are presented for model coins with polished surfaces, patina-covered coins, coins pretreated with a corrosion inhibitor (benzotriazole), and coins coated with a commercial acrylate/methacrylate copolymer (Paraloid B-72^(R)). Compared with alternative approaches involving two-electrode or three-electrode setups, our approach is the only one which simultaneously warrants a noninvasive character and the direct determination of the sample impedance without needing any correction. Although only model coins were used in this study, the above conclusion applies as well to true archaeological coins.