Historical metals conservation: disposable substrates and high‑performing systems for cleaning and protection

The effective and reliable conservation of archaeological, historical and artistic metal objects is a challenging task. In almost all the environments, metal surfaces undergo severe corrosion attacks, irreversibly affecting their aesthetic appearance and physico-chemical stability. Conservation procedures aim to remove disfiguring and unstable compounds and to protect surfaces from further degradation. However, traditional methods involve the use of hazardous substances that represent a serious threat for both human health and the environment. These products should be replaced by novel effective and green materials to ensure a sustainable long-term protection of metal cultural heritage. In this perspective, the present research work focused on the conservation of Cu-based and Ag-based artifacts by pursuing three main objectives: I) the development of disposable mock-ups resembling patinated metal objects to be used as reference for validation studies; II) the validation of novel cleaning gels based on polyvinyl alcohol (PVA) for the controlled and selective removal of degradation compounds; III) the evaluation of the protective properties of sustainable chitosan-based coatings toward silver and bronze substrates to be preserved in indoor environments. In order to produce mock-ups with corroded surfaces that are representative of metal artifacts exposed to different degrading conditions, both in terms of composition and microstructure, patination procedures were developed and studied. The effects of a 15-years-long burial on Cu- and Ag-based alloys were extensively investigated considering the patinas appearance, composition and structure. These were compared with the effects of a hundred years-long burial experienced by archaeological finds. Immersion and hot-brushing treatments were used to reproduce the surface appearance and composition typical of bronze sculptures aged outdoor and of artifacts intentionally patinated by artists. All the patinas artificially obtained were extensively characterized by means of a multi-analytical approach involving 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), micro-Raman spectroscopy and X-ray diffraction (XRD). Results showed that the 15-years-long burial treatment successfully induced the growth of corrosion patinas similar to those of archaeological finds. The extensive characterization of the patinas allowed to rebuild the first stages of corrosion mechanisms under burial conditions and to highlight differences in terms of composition and structure related to the time spent in soil. On the other hand, the immersion and hot-brushing patination treatments provided a large and reproducible set of mock-ups with different surface features. By varying the nature and concentration of patinating solutions and the application method, it was possible to control the color, composition and morphology of the patinated surfaces. The archaeological-like and outdoor-like bronze mock-ups were used to validate the cleaning performances of novel PVA-based gels. Twin-chain polymer network hydrogels (TC-PNs), loaded with proper chelating agents, were applied on the mock-ups and their ability to gradually and selectively remove disfiguring degradation compounds was tested. Based on the compositional and structural features of the patinas, different application times were selected to deeply evaluate the efficacy and selectivity of the cleaning treatment. Results showed that PVA TC-PNs can be successfully used to achieve a gradual and selective removal of Cu(II) corrosion compounds and sediment inclusions, preserving the stable Cu(I) oxide layer. Sustainable and water-soluble protective coatings based on chitosan, both pure and functionalized with a corrosion inhibitor, namely benzotriazole (BTA), were prepared and applied on a sterling silver alloy with a polished finish and on the artistically patinated bronze mock-ups. The films applied on the reference substrates are transparent, colorless and easily removable by water. The protective properties were assessed by means of both accelerated and long-term aging tests reproducing inappropriate storage conditions. Results showed that chitosan-based coatings can be successfully used to hinder corrosion processes of both polished and patinated metal surfaces. The slight chromatic alterations of the coatings, which were observed after the aging, can be a useful starting point for future improvements of the protective formulations. Two additional studies were also performed to address the challenging identification of copper hydroxychlorides via XRD analysis and to find sustainable corrosion inhibitors for Ag-based alloys in place of the harmful BTA. Natural samples of copper hydroxychlorides were analyzed by single-crystal X-ray diffraction and the diffractograms were compared, also including the reference ones from online databases. A criterion for their identification, based on characteristic XRD reflections, was successfully found for almost all the samples. Concerning the corrosion protection of Ag surfaces, six amino acids were tested by electrochemical measurements on a sterling silver alloy and their inhibition performances were compared to that of BTA. Preliminary results showed that some amino acids containing S-bearing moieties provide an inhibition action comparable to that of BTA. Overall, the research activities enabled a thorough comprehension of metals behavior under different corrosive environments and highlighted the advantages of using sophisticated and sustainable materials in conservation practice. The procedures and methods adopted to obtain reference mock-ups can be used as a basis for developing standardized validation protocols, while the study of advanced conservation materials represents a valuable step toward tailored and more sustainable strategies for cultural heritage preservation.