Résultat de la recherche
Analysis and assessment of degradation of polychrome metal artworks
This work focuses on non-destructive imaging techniques to investigate degradation due to the formation of metal soaps, especially in the early stages of the reaction. Studying the initial stage of the degradation can help design preventive conservation strategies, for which the main variables involved in the process can be adjusted to slow the process down. Imaging techniques also allow for the monitoring of the process since they provide spatial information together with spectral data that can be used to locate the same area of interest repeatedly over time and, therefore, to monitor the evolution of the species investigated and follow the growth of the searched compounds on the same region of the artwork. In particular, the analytical protocol established in this thesis was aimed at investigating a specific class of cultural heritage objects, i.e., painted metals. This category refers to artworks made of a metal support that was then painted by artists using a range of industrial and homemade formulations. Specifically, in this work, oil-based formulations were considered, either utilised solely as a coating or in a mixture with a pigment to create a paint. Objects made in this manner can be found in the 15th – 16th century (e.g., miniature portraits), 18th and 19th centuries (e.g., technical objects), and 20th centuries (e.g., sculptures and paintings on copper and aluminium). The chosen approach for developing the analytical protocol relied on model samples, i.e., the simplest representation of a class of objects consisting of a metal coupon and an oil coating. Copper and zinc substrates were chosen based on their reactivity toward fatty acids. The model samples were designed following historical sources and scientific literature, and their surfaces were documented before and after coating application in terms of roughness, thickness and homogeneity of the coating. Roughness parameters and thickness of the coating were measured to highlight possible inhomogeneities and correlate them with differences in the behaviour of the two substrates. The coated samples were then artificially aged to induce the formation of metal soaps. The methodological aspects that defined the ageing parameters are presented, as well as the result of a multivariate approach based on design of experiment methods (DoE), i.e., full factorial design. All samples were monitored during artificial ageing and characterised using several non-destructive analytical techniques, such as μ-FTIR, OCT, LIBS, μ-Raman, XRD, HSI xii A. Abstract and IR thermography. FTIR 2D chemical imaging allowed us to follow the evolution of the reaction at an early stage and to infer whether the detection of the species of interest was influenced by the morphological characteristics of the samples and by the thickness of the coating. Complementary Raman analyses helped identify the crystalline species forming. Moreover, to investigate the interface between the metal and the oil coating without sampling, OCT and LIBS were employed, which provided information on the stratigraphy of the object and allowed detecting metal-organic compounds forming in contact with the metal within the first few micrometres of the oil coating. The results obtained through preliminary tests using XRD, HSI and IR thermography did not allow the detection of metal soaps and would require further investigation and optimization of the analytical parameters. However, these techniques were able to analyse other system parameters. This work showed that, granted an appropriate choice of the analytical parameters, a combination of non-invasive imaging techniques could help elucidate and monitor the evolution of the reaction of formation of metal soaps on coated and painted metals, allowing designing appropriate conservation measures to prevent future damage.