The colossus of San Carlo Borromeo in Arona (NO, Italy). On site characterization of the metallic materials

The so-called San Carlone, a colossal statue dedicated to San Carlo Borromeo, built in Arona (NO, Italy) during the XVII century, is a unique monument. With its 23.5 meters of height, it is constituted by an internal structure of iron bars, fixed on a massive central stone tower, and by a "skin" of hammered and embossed copper sheets. After 40 years from the last restoration, a conservation intervention may be planned in the near future. It is therefore important to investigate and characterize the materials of the monument, in order to evaluate their state of conservation. One of the main concerns is related to the fact that copper and iron are very likely to create galvanic coupling, and thus the iron could be corroded very quickly. However, the iron bars appear in quite good conditions in most parts of the statue. It is therefore interesting to study the materials in order to enrich the knowledge of the know-how of the XVII century, and to support the ongoing conservation project. The studies and characterisation of San Carlo started in October 2017, with a multi analytical approach, that includes a large variety of non-invasive in situ measurements to be associated with laboratory analysis of micro-samples. The following in-situ analysis were performed on the statue, both inside and outside: portable digital microscopy, spectrophotometry in visible light, electrochemical measurements (LPR and EIS), US thickness measurements of the copper sheets, thickness measurements of the surface protection coating and corrosion layers with eddy currents, electrical conductivity measurements, infrared spectroscopy and x-rays fluorescence analysis. Aknowledgments This research activity has been financially supported by the European Copper Institute (ECI) and the Helmut-Fischer-Stiftung