Artificial reproduction of natural and artistic patinas on bronze mock-ups to be used as sacrificial substrates for the validation of new conservation materials

Modern, historical and artistic bronze sculptures frequently present a matt and colored surface rather than a pure metallic appearance. Color changes can be related to the occurrence of alteration phenomena or to an intentional patination designed by the artists. In the first case, bronze artworks, especially those exposed outdoor, naturally react with atmospheric pollutants, such as SOx, NOx, O3, airborne particulate matter (PM), to form green sulfate species (Chiavari et al., 2007), sometimes appreciated for the old-like appearance they provide to the object. On the contrary, artistic patinas are intentionally applied to the works of art to obtain special chromatic effects and emphasize textured surfaces with light and shade effects. Although recipes used by artists to artificially patinate modern sculptures are commonly kept secret, it is known that they are based on the use of reagents like “liver of sulfur” (K2S), copper nitrates or iron nitrates (Bongiorno et al., 2012; Crippa et al., 2019). In both cases, the surface reactivity significantly changes (Kosec et al., 2021), and tailored conservation practices are required to protect this type of materials from disfiguring degradation processes. In this work, several existing methods to artificially patinate bronze substrates were tested and critically compared with the aim of developing an easy and comprehensive standard procedure to obtain bronze patinas with different colors, structure and minero-chemical composition, representative of both natural and artistic aged surfaces. Copper and iron nitrates, copper sulfates and potassium sulfide were selected as patinating agents for both immersion and hot-brushing treatments. The morphological, chemical and mineralogical features of the patinas were studied by a multianalytical approach, including optical microscopy (OM), field emission scanning electron microscopy coupled with energy dispersion spectroscopy (FE-SEM-EDS), Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR) and x-ray diffraction (XRD). Results pointed out that by properly varying the patination procedure a set of artificial patinas, representative of different typologies of bronze artifacts surfaces, are easily obtained. Therefore, these patinas can be used as sacrificial substrates to reliably validate new conservation materials. The national PRIN 2017249YEF project and the GREENART project (Horizon Europe research and innovation program under GA no. 101060941) are gratefully acknowledged for financial support. Bongiorno V., Campodonico S., Caffara R., Piccardo P. & Carnasciali M.M. (2012) - Micro-Raman spectroscopy for thecharacterization of artistic patinas producedon copper-based alloys. J. Raman Spectrosc., 43, 1617-1622. https:// doi.org/10.1002/jrs.4167. Chiavari C., Rahmouni K., Takenouti H., Joiret S., Vermaut P. & Robbiola L. (2007) - Composition and electrochemical properties of natural patinas of outdoor bronze monuments. Electrochim. Acta, 52(27), 7760-7769. https://doi. org/10.1016/j.electacta.2006.12.053. Crippa M., Bongiorno V., Piccardo P. & Carnasciali M.M. (2019) - A characterisation study on modern bronze sculpture: the artistic patinas of Nado Canuti. Stud. Conserv., 64(1), 16-23. https://doi.org/10.1080/00393630.2018.1492253. Kosec T., Novak Ž., Fabjan E.Š., Škrlep L., Škapin A.S. & Ropret P. (2021) - Corrosion protection of brown and green patinated bronze. Prog. Org. Coat., 161, 106510. https://doi.org/10.1016/j.porgcoat.2021.106510.