Metals and Organic Materials in the Cultural Heritage Field : An Approach Through Secondary Ion Mass Spectrometry

Abstract

The research was focused on the study of the chemical interactions between metals and organic compounds, which often coexist side by side in artifacts. Thanks to the high mass resolution, selectivity and the excellent lateral resolution of secondary ion mass spectrometry technique is possible the identification and the interpretation of mass spectral data, and localize of these characteristic fragments in the samples studied. In particular, the research has developed the case of reaction of metallic foils and/or inorganic metallic pigments in contact with oil-resinous compounds, commonly present in pictorial binders. One of the most diffused examples of metallic-organic compound interaction in cultural heritage studies is represented by the gilding decoration technique, transversely applied to wall paintings, sculpture, paintings on wood metallic objects, and widely practiced over the centuries. In general, the application of a metallic leaf (gold or silver) on a support by means of an adhesive compound is the practice defined as gilding technique. In some cases, the noble leaf is deposited on other metallic foil less noble, as a tin lamina, usually used to smooth the surface of the artwork to decorate. This last case, called composite lamina, was investigated through an experimental design in which ad hoc samples were performed and artificially aged with the aim to simulate the real state of conservation conditions. The study of the interactions between metallic foil and organic compounds has reveled the production of tin metal soaps due to the interaction among tin leaf with fatty acid presence in adhesive mixture. At the same time, mass fraction and chemometric studies of the major terpenic molecules that characterize the resinous part of the adhesive mixture in Art and Archaeology, were also performed. Successively, the SIMS analytical technique was adopted for real case studies in order to validate the previous results. Wall paintings cross sections of Giotto and a Giotto’s apprentice, characterized by the presence of composite lamina technique, were investigated. Moreover, the investigation was extended to the study of a silvered and painted leather artifact and an Etruscan cinerary urn decorated with gold lamina. Their investigation has made possible the knowledge of the various metal foils decoration techniques on different types of artefacts through the centuries. During this work of thesis, the features of low energy ion beam analysis technique were exploited to perform the chemical characterization of samples belonging to the cultural heritage field. In addition, cross sections belonging to a wooden triptych painted by the Flemish painter Jheronymous Bosch were also analyzed in order to identify and localize the chemical interaction and degradation products occurred among different materials, paying particular attention on the reactivity of metal-based pigments in the layers of paints. The use of secondary ion mass spectrometry and imaging were used to detected and map characteristics fragments of oil-resinous medium, metal leave, metal-based pigments, varnishes and their interactions products, with high spatial resolution. Furthermore, all the other materials composing the samples were also characterized, providing a complete overview of artworks. Finally, the ToF-SIMS dynamic operation mode was exploited to the investigation of metallic gilded artifact, obtaining a tridimensional distribution and localizing diffusion effects from the surface to the bulk of the sample.The ToF-SIMS results were supported by and in agreement with the results obtained with, FTIR in the experimental section and with optical microscopy, SEM-EDS, FTIR and XPS in all case studies investigated. ToF-SIMS ability coupled to the analytical data obtained with the traditional complementary techniques may constitute a new means for the diagnostics of conservation science, and to extend the knowledge of materials and the state of conservation of the artworks