Cultural heritage buildings and artefacts made of natural stones are subjected to deterioration, due mainly to environmental (chemical, physical, biological) agents or anthropogenic pollutants. These lead to the degradation of the stone, compromising its aesthetic appearance but also its physical integrity. Many products have been developed and used to hinder the effects of decay agents or to restore the stone material.
An innovative and very promising strategy to prevent building materials from soiling and biofouling seems to be the use of TiO2 nanocoatings, because of their ability to photo-decompose pollutants under UV irradiation, their durability and affordable costs.
TiO2-based nanocoatings have been widely applied on many stone materials such as marbles, travertines, dolostones and limestones; however, the efficiency of these coatings in such applications is still debated.
In the paper Is nano-TiO2 alone an effective strategy for the maintenance of stones in Cultural Heritage?, by E. Quagliarini, L. Graziani, D. Diso, A. Licciulli, M. D’Orazio, published on Journal of Cultural Heritage, Vol. 30, 2018, Pages 81-91, https://doi.org/10.1016/j.culher.2017.09.016, the authors investigate the performance of a nano-TiO2 dispersion in water, applied on six different types of natural stones (3 limestones, 2 sandstones and 1 tuff) usually used in Cultural Heritage, with high porosity and roughness. The self-cleaning power of the coating was evaluated by measuring its ability at discoloring an organic dye (Methylene Blue), while its anti-biofouling efficiency was assessed by an accelerated growth test under controlled climatic conditions of two algal microorganisms (Chlorella mirabilis and Chroococcidiopsis fissurarum).
Results show that even if the photocatalytic and biocide power of nano-TiO2 itself is well known in literature, its efficacy is not assured when applied on Cultural Heritage building stones, where low amounts of titania must be used not to alter their aesthetic appearance. Rough and porous stones, frequently used in Cultural Heritage, in fact, can limit the performance of TiO2 reducing its ability to keep the substrate self-cleaned or to slow down algal proliferation. In Cultural Heritage conservation the amount of TiO2 should be carefully chosen, to avoid both chromatic variations and the production of micro-cracks on the coating, which can accelerate the adhesion of microorganisms. Low amounts of TiO2 limit the formation of micro-cracks, but produce low photo-degradation of organic dyes and are not able to limit biofouling. In this way, the tested application of nano-TiO2 dispersion for the maintenance of stones in Cultural Heritage does not seem to be an effective strategy, especially with porous and rough stones.