Investigating coating applications for protection of Roman frescoes

Introduction In the last decades, Climate Change has been having a significant impact on Built Heritage due to the degradation factors to which it is directly exposed. Current conservation practices consider mural paintings as an integral part of monuments and heritage sites, and advocate for their in-situ preservation whenever possible [1]. The application of protective coatings may thus play a crucial role in their conservation. In this work, a new green protective coating for outdoor ancient frescoes surfaces is presented: its development is meant to fill a void in conservation products specially designed for wall paintings, both on the market and in the literature. The formulation is obtained by optimizing a one-pot synthesis, combining the stability and the reducing characteristics of the chitosan biopolymer [2] with the hydrophobic features of silver nanoparticles (Ag NPs) [3]. To validate the protective features of the proposed coating, its characteristics have been compared with those of two commercial products, i.e. Paraloid® B72 and Proconsol®. The analyses were carried out on both the coatings and the treated surfaces of fresco mock-ups. 2. Results and Discussion For the formulation of the coating base, a gel with chitosan and Ag nanoparticles (NPs) was prepared. The main idea is to provide hydrophobic properties, by adding silver NPs, to a base made by a biopolymer that is green, has antimicrobial activity, and could help in surface protection in an outdoor environment. The chitosan has been solubilized in azelaic and lactic acid, in which Ag NPs were successfully developed by a one-pot synthesis, using calcium or sodium hydroxide as precipitating reagent. Finally, the obtained gel has been diluted with a mixture of glycerol and ethanol. For the determination of the adequate concentration of silver nitrate and of the best precipitation agent to use, the final formulation has been characterized by SEM, TEM, UV-Vis analyses. For the solvents choice, instead, rheological analyses were carried out to reach an adequate compromise between volatility and an adequate evaporation rate, also considering the nanoparticles’ stability in the different environments. Once the new coating was ready to be applied, the fresco mock-ups were treated with it and with the commercial products Proconsol® and Paraloid® B72 to compare and validate the protective features of the newly formulated coating (0.6 L/m2 applied by brush). Morphological, colorimetric (ΔE), and ATR analyses, along with hydrophobicity and water vapor permeability (δp) tests have been carried out for establishing the coatings' protective features and stability, before and after application, as well as after artificial ageing. The results show that the formulation with chitosan and AgNPs is highly promising. Paraloid® B72 treated mock-ups presented good hydrophobicity and high stability over time, but the product determined a glossy effect on the samples and a change in the surface morphology after application. Proconsol® was not effective in terms of hydrophobicity, due to the absence of hydrophobic features. The hydrophobicity of the chitosan and Ag NPs-based coating did not result superior to that of the Paraloid® B72, but the coating has a higher water vapor permeability. Moreover, it displays good morphological and colorimetric features, without glossy effects, low color variations, and stability over artificial ageing. Thus, it can be said that the new formulation with chitosan and silver nanoparticles represents a valid alternative to the existing commercial products on the market today, proving to be effective and safe. 3. Conclusions In this study, a new green coating for frescoes has been successfully developed and tested. The results indicate that the new product represents a valid alternative to Paraloid B72® and Proconsol®. It was obtained by combining the stability and the reducing characteristic of the chitosan biopolymer with the hydrophobic and antibiotic features of silver nanoparticles. The newly formulated coating is highly performing since it does not excessively alter the morphology nor the color of the materials, with no gloss, color variations lower than the human eye limit of the human eye, and chemical stability over artificial ageing. Therefore, the new green coating is promising for application on real frescoes exposed outdoors.