Authors: Souty Beskhyroun, Aline Assumpção, Bianca Gonçalves
The best way to minimize the degradation of objects is through preventive conservation methods. However, it is not always possible to create ideal environmental conditions for artifacts, so it is sometimes necessary to apply protective organic coatings to their surfaces in order to protect them from weathering agents. When the conservator selects these protective coatings, some of their properties must be taken into consideration. For instance, the coating should be transparent and should not cause any change in the original substrate. Coatings should also be reversible when possible. As the aim of conservation is to increase the longevity of historic artifacts, the long-term efficacy of coatings is another property to be considered. Finally, coatings should be compatible with the original material of the object, and they should affect neither its conservation nor its explicitness.
These considerations relate to some significant problems that can arise through the use of coatings. For example, the painted surfaces in the chapel of Holy Nail (Siena, Italy) were treated with Paraloid B-72 (an acrylic resin) to inhibit their interactions with the surrounding environment, but the coating caused a shiny yellowing effect in treated areas. In addition, Incralac (another acrylic resin) and Paraloid B72 have severely damaged underlying metals in many cases. Biodeterioration of synthetic organic coatings has also been a particular concern for conservators who use the materials for the long-term protection of outdoor monuments.
Due to these issues, further research was required in order to make improvements to these protective coatings. Studies show that polymeric nanocomposites based on nano-sized inorganic particles (clay, titanium dioxide, and zinc oxide) can enhance the coatings. In addition, like other nanomaterials used in conservation and restoration, nano-filled coatings are often a sustainable treatment option. They decrease the deposition of pollutants, soiling materials, and microbes, and they also block UV light. In these ways they reduce the need for cleaning, enabling us to save energy and materials. Clay nano-particles in particular are low in toxicity, pose few environmental risks, and have good reactivity and catalytic properties. TiO2 nanoparticles are versatile, and their production process is environmentally-friendly. It is possible to use TiO2 in aqueous dispersion, avoiding conventional toxic solvents.
However, the nanoparticles released from protective coatings may pose risks to human health and the environment at different stages of their life cycle. Furthermore, we do not know how far they can travel in the ocean, our skin, or the soil; are they compatible with our environment or are they bioaccumulative?.The negative environmental effects of TiO2 are still a matter of debate, but conservators tend to apply the material with a brush in order to reduce the risk of inhalation. Despite the extraordinary physical and chemical properties of ZnO nanoparticles, as well as their low cost, they are inherently toxic to mammals. The synthetic routes involve a large amount of chemical and stringent reactions that contaminate the environment through high energy and water consumption, heat generation, and chemical waste. Nanobiotechnology techniques for nanoparticle synthesis are able to mitigate some of these issues, and collaboration between scientists and conservators is likely to facilitate the creation of more sustainable nano products for cultural heritage conservation in the future.
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