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

Outdoor historical-artistic heritage is increasingly exposed to risks related to climate change and air pollution, which in the future will contribute to further accelerate the phenomena of deterioration. Unlike museum collections, it is difficult to control or limit the parameters that are crucial for the protection of an object when it is exposed outdoors. Therefore, one of the adoptable strategies consists in the direct intervention on the work of art, through the interposition of a 'barrier', such as a protective or a surface treatment, that could mitigate the effect of external agents. The aim of this research is to study new materials suitable for the conservation and restoration of outdoor terracotta artworks, a field that today has very few comparisons and bibliographic references, compared to other materials. Starting from the conservative study and the restoration of a terracotta sculpture of the Open Air Museum of Faenza, the Wind Wall by Domenico Matteucci, an experimental investigation has been carried out in order to evaluate the effectiveness, for terracotta artworks, of some protective coatings available on the market. Four coatings were selected and assessed according to their compatibility with the terracotta substrate and resistance to the main degradation agents. The selected coatings (vinylidene-fluoride-hexafluoropropene copolymer at 3% in acetone, aqueous emulsion of alkylpolysiloxane, aqueous dispersion of functionalized silica nanoparticles, nanostructured and functionalized silica gel in hydroalcoholic solution) were applied by brush on carbonate terracotta samples fired at a temperature of 980°C. Subsequently, the specimens were subjected to accelerated aging, which included a run-off test with acid rain and a climate chamber test with exposure to UVA radiation. These tests were properly set-up to investigate the interaction mechanisms between the environment and the ceramic substrate, separately highlighting the effects of heavy rain and daily variation of temperature, relative humidity and irradiation. The effectiveness of the treatments was evaluated through colorimetric analyses, water absorption capacity measurements, attenuated total reflectance infrared spectroscopy (FTIR-ATR), mercury intrusion porosimetry (PIM) and assessment of the amount of calcium ions brought into solution by rain through atomic absorption spectroscopy (AAS). On the basis of the results obtained, the performance of the coatings was evaluated and compared.