Deposition of silica-based films for the protection of bronzes surfaces

In the present work, two different thin films of vitreous silica were tested to protect bronze surfaces from corrosion. A thin film of glass, less than 0.3 micrometer thick, presents excellent optical transparency and chemical inertia: it is particularly resistant to environmental corrosion, protecting the underlying material surface and its thinness guarantees elasticity. We compare the behavior of films obtained from two different chemical precursors and deposited on bronze samples of parallelepiped shape whose composition is similar to material present in Venice bronzes around 800 AD. The solutions of a silazanic precursor and a tetraethylorthosilicate (TEOS) based product have been employed to form the silica films by immersion of the bronze samples. The completely covered samples after the spectroscopic and morphological analyses were exposed to a corrosion test. In particular, the use of ammonia solution allows to observe at first sight if the colorless original solution turns to blue for the formation of copper-ammonia complex indicating the presence of corrosion phenomena. The experimental results shows that the silica protective films starting from silazanic precursors provide coatings with protective, mechanical and optical properties better than the TEOS-based ones.