Evaluation of the effectiveness of coatings for the protection of outdoor terracotta artworks through artificial ageing tests

Historical-artistic heritage, when located outdoor, is heavily targeted by deterioration phenomena such as weathering and air pollution. This is especially true for terracotta artefacts, as the medium porosity which characterizes them makes their damaging easier. Nevertheless, there is limited academic research on conservation strategies with regards to coatings. Consequently, the restoration of the carbonate terracotta sculpture known as “Muro del vento” (Wind Wall) by Domenico Matteucci has become the starting point for an experimental investigation carried out in order to evaluate the effectiveness and durability - for outdoor terracotta artworks - of an array of protective coatings. In this paper, four commercial protective coatings, recommended both for natural and artificial stones, were evaluated: a vinylidene-fluoride-hexafluoropropene copolymer at 3 % in acetone, an aqueous emulsion of alkylpolysiloxane, an aqueous dispersion of functionalized silica nanoparticles, and a nanostructured and functionalized silica gel in hydroalcoholic solution. Coated and uncoated representative calcium-rich terracotta specimens were subjected to two different accelerated ageing procedures: rain runoff test and climatic chamber exposure. Concurrently, a long-term outdoor exposure was set up. The characterization of the specimens and the evaluation of the coating's performances were carried out through color, contact angles, water absorption and mass variation measurements, 3D digital microscopy, scanning electron microscopy, X-Ray diffraction, Raman micro-Spectroscopy and atomic absorption spectroscopy in order to quantify calcium release in rain. Overall, the proposed accelerated ageing procedures proved to be successful for evaluating effectiveness and durability of protective treatments on ceramic materials. Silicon-based coatings, especially nanostructured and functionalized silica gel, followed by alkylpolysiloxane emulsion, have been shown to be the most suitable coatings for outdoor terracotta artefacts, while the fluorinated coating did not provide adequate protection as it was not able to limit water absorption under runoff conditions.