Traditional and innovative treatments for the protection of patinated copper

Natural and artistic patinas are nowadays considered an integral part of the artworks, and they must be preserved during conservation treatments. In order to test and develop new protective treatments, it is fundamental to be able to produce in the laboratory artificial patinas with characteristics similar to the natural ones, since in the case of cultural heritage surfaces, they are normally applied on corroded/patinated surfaces. Preserving these patinas has become an essential, but difficult task for restorers. Artificial patinas may have quite different characteristics compared to natural ones and are often obtained by using polluting chemicals. It is therefore important to find low environmental impact procedures for their production. The traditional and commonly used method for protecting copper alloys involves a multilayer of an acrylic resin layer topped with wax. However, the acrylic layer tends to crack under temperature changes, and the wax wears off quickly. Research continues to focus on developing protective coatings to prevent corrosion, that should maintain the appearance and be non-toxic for the restorers. Recently, silane-based coatings have emerged as an eco-friendly alternative, offering high hydrophobic properties. In this study, a series of artificial patinas were produced and characterized: brochantite (CuSO4Cu(OH)2), and a mixture of basic copper chlorides (paratacamite Cu3Cl2(OH)6 and atacamite Cu2Cl(OH)3 ). In addition, a new silane-based treatment to conserve patinated copper-based alloys was proposed. Results demonstrated that the artificial patinas closely resembled natural ones and that the new silane-based coatings offered high hydrophobicity of the surface with minimal aesthetic alterations, much lower if compared with the traditional multilayered treatment. Surfaces were characterized by colour, corrosion resistance, composition and morphology. Colour was measured using spectrophotometry, while corrosion resistance was evaluated by measuring Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS). Surface morphology was examined using optical microscopy, and patina composition was analyzed by Fourier-Transform Infrared Spectroscopy (FTIR). Water contact angle (WCA) measurements were performed to evaluate surface hydrophobicity.