Protectiveness of different silane coatings on patinated ancient and modern bronzes: Part 2

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 19th century. Moreover, a quaternary Cu-Sn-Zn-Pb bronze was artificially aged in the laboratory by dropping synthetic acid rain 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. The most promising coatings were evaluated considering the visual aspect (colour measurement), the adherence, the UV-resistance (test in climatic chamber with an UV lamp), the resistance to run-off or stagnant exposure to synthetic acid rain (artificial ageing tests by dropping and Wet&Dry, with related characterisation of the ageing solutions and samples during and at the end of the tests). The achieved results were compared to those obtained by applying a standard benzotriazole-based acrylic coating, still the most widely used by restorers.