Protectiveness of different silane coatings on patinated ancient and modern bronzes: Part 1

The safeguard of artistic and cultural heritage is of great importance for the conservation of the historical and cultural background of a society. The works of art in bronze are an important part of this heritage and the production of effective surface protective treatments with a low environmental impact is a central and still unresolved issue. The treatment effectiveness depends on the nature of the substrate, that is on the type of bronze to be protected and on the nature of the surface patina actually present at the time of coating application. For this reason, within the M-ERA.NET project named B-IMPACT, two bronze alloys were produced by sand casting and properly patinated to obtain substrates representative of monuments exposed outdoors on which different silane-based coatings were tested. In particular, a modern silicon bronze was treated by an aqueous solution of potassium sulfide (K2S), as traditionally done in artworks of the 19 th century. Moreover, a quaternary Cu-Sn-Zn-Pb bronze was artificially aged in the laboratory by dropping synthetic acid rain (AR) to reproduce the surface of more ancient sculptures. These substrates were treated by some silane-based coatings characterized by different functional groups, such as amino, thiol, and tetrasulfide groups or long aliphatic chains. After a short curing period at 50 °C, the coated samples were exposed to a concentrated synthetic acid rain solution (ARX10) for 30 days and their corrosion behavior was monitored by electrochemical impedance spectroscopy technique (EIS). At the end of the immersion period, cathodic and anodic polarization curves were recorded and, on non-polarized samples, SEM-EDS analyses were performed. The results achieved were compared to those afforded by a benzotriazole-containing acrylic coating, normally used by restorers.