Diagnostic, Restoration and Maintenance of Outdoor Bronze Artworks: Investigating Patinas in Marine Environments and Their Stabilisation with low Environmental Impact Treatments

The conservation of outdoor bronze artworks is challenging because of the need to consider the highly complex system of alloy+patina/coating/environment. Despite the numerous reports on bronze conservation published in the scientific literature, in most cases the studies have been limited to model surfaces and laboratory-based investigations. Among the issues that remain to be addressed in detail are: (i) the corrosion of outdoor bronze surfaces in a changing environment, especially the marine environment, with respect to bronze disease; (ii) the value of the different surface products, in order to better evaluate the effectiveness of different cleaning methods; and (iii) the morphological properties of corrosion products with respect to the behaviour of protective treatments (inhibitors, coatings). Insights into the complex interactions underlining these issues can only be obtained through carefully designed experiments. In heritage science, the multi-analytical in situ approach offered by non-destructive testing (NDT) allows the properties of heritage objects, such as visual appearance, chemical composition, corrosion rate and surface texture, to be investigated in detail. Thus, with the goal of more successful conservation treatments of outdoor bronze monuments, we conducted multi-analytical NDT measurements of marine-weathered coupons and then compared the results to those obtained with several bronze artworks, mainly from the 19th and 20th centuries, in the Staglieno Monumental Cemetery (Genoa, Italy) and Certosa Monumental Cemetery (Bologna, Italy). Our primary objective was to better define the variability ranges of relevant parameters and the similarities of the surface morphologies of the coupons and those of the outdoor metal artworks exposed to different environments. Moreover, we consider the applications of the available NDT techniques to better define good practices for outdoor bronze conservation. Particular attention was paid to the identification of nantokite, a promoter of bronze disease, and its uneven distribution within patinas. We also discuss the results of preliminary tests directed at the in situ characterisation of traditional and innovative treatments of patinated bronze surfaces aimed at inhibiting the cyclic corrosion linked to the presence of CuCl.