Reproduction of archaeological patinas on bronze coupons through 15 years of intentional burial in the soil of Tharros (Sardinia, Italy)

Bronze archaeological artifacts are usually covered by alteration products (i.e., the patina) that result from the long-term interaction between the alloying elements and the soil in which they are buried (Tylecote, 1979; Scott, 2002; Ingo et al., 2019). Bronze corrosion patinas are composed of mineralogical compounds including, as the most common, copper(II) salts (carbonates, hydroxychlorides), copper(I) oxides, tin oxides or hydroxides and terrigenous species. The intentional reproduction of corrosion patinas naturally grown during a long-term burial is a challenge for the scientific community. The major problem is related to the research time frame that is usually too short compared with that required for the development of archaeological corrosion layers. However, sacrificial substrates with composition and surface features like those of the archaeological finds are essential to test and validate new conservation materials. In this study, bronze coupons were intentionally buried for 15 years in the soil of the archaeological site of Tharros (Western Sardinian coast, Italy), both in the laboratory and in situ, with the aim of reproducing corrosion patinas typical of a burial environment. The morphological, chemical and mineralogical features of the patinas were analysed and compared with those of an archaeological bronze recovered from the same site. The effect of the short-term intentional treatment in soil (for about 15 years) was evaluated with respect to that of the long-term burial (for about two thousand years) of the archaeological object. Results revealed that mechanisms controlling the corrosion of bronzes are the same for both short- and long-term interactions with soil. Time-dependent features are the thickness of the patina and the amount of cuprite (Cu2O), whereas the roughness is mainly affected by the surrounding environment. Based on these results, a corrosion patina grown during the 15 years of burial in Tharros was selected and used as a sacrificial substrate to study the effect of a cleaning treatment performed using a polyvinyl alcohol (PVA)-based gel loaded with a chelating agent (Mastrangelo et al., 2020). The gel was effective in removing the disfiguring corrosion products (i.e., contaminants from the soil) and preserving the stable and protecting patina. Moreover, the depth of cleaning can be tuned based on the conservation needs by varying the time of application. The present work was financially supported by the national PRIN 2017249YEF project and the GREENART project (Horizon Europe research and innovation program under GA no. 101060941). Ingo G.M., Riccucci C., Guida G., Albini M., Giuliani C. & Di Carlo G. (2019) - Rebuilding of the burial environment from the chemical biography of archeological copper-based artifacts. ACS Omega, 4, 11103-11111. https://doi. org/10.1021/acsomega.9b00569. Mastrangelo R., Chelazzi D., Poggi G., Fratini E., Pensabene Buemi L., Petruzzellis M.L. & Baglioni P. (2020) - Twin-chain polymer hydrogels based on poly(vinyl alcohol) as new advanced tool for the cleaning of modern and contemporary art. P. Natl. A. Sci., 117(3), 7011-7020. https://doi.org/10.1073/pnas.1911811117. Scott D.A. (2002) - Copper and bronze in art: corrosion, colorants, conservation. Getty Conservation Institute, 533 pp. ISBN 978-0-89236-638-5. Tylecote R.F. (1979) - The effect of soil conditions on the long-term corrosion of buried tin-bronzes and copper. J. Archaeol. Sci., 6(4), 345-368. https://doi.org/10.1016/0305-4403(79)90018-9.