IMAGING AND CHARACTERIZATION OF CULTURAL HERITAGE MATERIALS BY ADVANCED ELECTROANALYTICAL TECHNIQUES

The common thread of the PhD project is the development of strategies, based, primarily, on dynamic electroanalytical techniques, for investigating and characterizing different types of substrates of historic/artistic interest. The use of electrochemical apparatuses and approaches was done, where possible, in the least invasive way, to characterize metal-based materials; from the other side to propose new systems and approaches to study materials, which can suffer from contact with liquid electrolytes. The research on copper natural patinas was applied, in a first part, to relatively easy systems composed by primary patina, essentially made of cuprite. Here, ancient coins were studied by the voltammetry of immobilized particles (VIMP) technique simply by pressing a graphite pencil on their surface, thus resulting in an essentially non-destructive sampling, useful characteristic for the study of cultural heritage materials. The new approach developed has revealed the existence of different corrosion patterns which are sensitive to the composition of the base metal, in particular to the presence of Zn. Voltammetric data suggests the possibility of establishing a chronological ordering of the coins and hence to add this multiple scan strategy to the existing electrochemical dating methods. A further step in the research activity was was to investigate copper-based materials in a more complex corrosion status (presence of primary cuprite together with brochantite). The goal was to study the effect of alternative, to classical and poorly environmentally safe, corrosion inhibitors. Specifically, decanoic acid (HC10) and its sodium salt (NaC10) have been considered here, as potential eco-friendly inhibitors. In a first part of the research activity, the interactions between Cu2+ and HC10 in water and 50% (v/v) E/W mixture was elucidated by a detailed analysis of the voltammetric responses. The results obtained proved useful to clarify some aspects of the mechanisms which could be involved when the corroded roof claddings are treated with the inhibitors and the stability of the coatings when the samples are immersed in synthetic acid rain. A second section of the study aimed at comparing the protective effects of HC10, NaC10 and the poorly environmentally safe benzotriazole (BTA) on bare copper and roof cladding samples. A third section of the study examines in detail the reactivity of the top layer and cross section of the roof cladding samples untreated and treated with HC10 and BTA. This study has been performed by using SECM to get information on the reactivity with high spatial resolution. The ability of the inhibitors to prevent further corrosion of the various layers of patina was assessed in aqueous media containing NO3-, SO42- and Cl-, which are characterized by different complexing ability towards copper ions. The research activity continued in the direction of investigating innovative strategies based on the use of microcapillary (MC) techniques. In particular, MCs filled with conductive hydrogels have been developed with the objective of applying it to study materials that suffer the contact with liquid solutions and metallic objects. Here, the properties of agarose hydrogel as medium to perform voltammetric measurements, using the electroactive redox probes and a Pt microdisk electrode were examined. The hydrogel containing the redox probe and various electrolytes are employed for MC contact measurements above platinum and gold substrates. To demonstrate the suitability of the above microprobe for more practical applications, proof-of-concept measurements were performed to characterize micro areas of metallic silver deposited on to the surface of a pencil graphite line deposited on a paper material. With the purpose to tailor the electrode surface or the content of MCs with more selective compounds, condconducting polymers, presenting intrinsic chiral activity, have been studied.