NANO MEETS HERITAGE: NANOMATERIALS FOR THE CONSERVATION OF WORKS OF ART

Soiling, condensed water, light, atmospheric pollution, and biological attack are among the main causes for deterioration of cultural heritage. While the need to better protect our heritage assets has increased significantly in recent years, the field of conservation has remained firmly embedded with traditional practices, using conventional materials and approaches. Such methods have proven to be often incompatible with the historic substrates and lead to damage of the artefacts in the long term, due to their low effectiveness and durability. Flexibility in both tailoring properties at nanoscale and combining various functionalities in nanocomposite materials could enable cleaning, restoration, and effective prevention of the degradation processes, without damage or alteration to the historic substrate. 

The size-dependent properties of nanoparticles and large surface-to-volume ratio enhance their reactivity, and enable efficient penetration through porous substrates. The introduction of nanomaterials in the field of heritage conservation lead to significant improvements: high physico-chemical compatibility with the matrices of works of art, low toxicity, high control of their application during restoration interventions, and long-term stability. 

One of the first nanomaterials introduced in this field is nanolime (that is a water or alcoholic dispersion of Ca(OH)2 nanoparticles), which has been used for the consolidation of calcareous stones, plasters, and wall paintings. Thanks to its nanosize, nanolime shows higher reactivity, deeper penetration in the pores of the substrate, reduced carbonation time, and higher stability when compared to traditional limewater. Nowadays, the most used consolidants for both limestones and sandstones are alkoxy-silane and oligomers; however, these materials need long reaction time to form silica-gel and the latter is often characterized by micro-cracks which compromise its durability. 

To overcome these drawbacks, particle-modified consolidants, based on the introduction of different nanoparticles in pre-polymerized tetraethoxysilane, have been proposed and successfully tested for the consolidations of five cathedrals and a contemporary theatre in the framework of the European Horizon 2020 Nano-Cathedral project. 

 Nanotechnology is also used for the set-up of protective treatments for stone as they can enhance the protection effectiveness by reducing water absorption and surface wettability, and confer photocatalytic, self-cleaning, and antifouling properties. The introduction of different nanoparticles (SiO2, SnO2, Al2O3) inside polymeric media increase the stone surface roughness and impart super-hydrophobic (water contact angle > 150°) and self-cleaning properties to the stone. In addition, treatments based on titania (TiO2) nanoparticles, thanks to their depolluting and photocatalytic properties, are a preventive strategy towards stone degradation as they can promote the degradation of inorganic and organic pollutants. Titania, together with other nanoparticles such as ZnO and Ag, have also been successfully employed in treatments to inhibit and prevent biocolonization.

Nanomaterials also exhibit excellent results for the conservation of artworks based on organic materials, such as paper, textile, parchment, wood, and leather. In particular, dispersions of Mg(OH)2 and Ca(OH)2 nanoparticles have been applied to provide safe, stable, and controlled deacidification of paper. Indeed, these materials can control the acidic and oxidative degradation of manuscripts and archival documents. In recent years, nanocellulose has gained significant attention due to its high strength and stiffness, paired together with low weight, biodegradability, and renewability. 

The introduction of nanocellulose in paper, textile, and canvas conservation shows different advantages: high chemical compatibility with the fabrics, better stability, and retreatability, when compared to traditional vinyl and acrylic-based products; the small size of the crystals enables stronger interactions between nanocellulose and fibers, improving the mechanical properties of brittle fibers. Cleaning is one the most delicate phases during conservation interventions and must be selective in the treatment of dirt and unwanted coatings on works of art, without compromising the historic and artistic substrates. 

Current research efforts focus on development of innovative cleaning materials including nanostructured cleaning fluids such as oil-in-water microemulsions, solvents, and rigid gels, and biological cleaning methods such as microorganisms and hydrolytic enzymes. These innovative materials show less toxicity compared to traditional solvent blends and provide excellent cleaning effectiveness. In particular, the use of gels allows a controlled release of the fluids on the substrate, without leaving residues on the surface or compromising the properties of historical substrates and contemporary artefacts. 

Researchers have proven that nanomaterials are promising in the field of heritage conservation. However, to extend their use it is necessary to better understand the process taking place at the nano/microscale interface between treatments and cultural materials, and their durability.

Francesca Gherardi