Abstract
Considering that consolidant products are commonly used in the cultural heritage field and the titanium oxide nanoparticles (TiO2) have been used to develop photocatalyst films to induce self-cleaning property, the scientific research on consolidants doped with TiO2 is justified. However, the addition of TiO2 can affect to the physical properties of the cultural heritage object, questioning the adequacy of the procedure. In this paper, we evaluated the influence of nanoparticle TiO2 addition to two different commercial consolidant products (ethyl silicate or nano-sized silica) on the appearance and the color of a granite and the penetration through its fissure system. The stone was previously subjected to high temperature simulating the effect of a fire and the subsequent tap water jet to cool down. Therefore, different concentrations of nanocrystalline TiO2 (0.5, 1, and 3 wt %) were considered. The different compositions were also studied considering the compactness, the extent and the thickness of the superficial xerogel coating, and as well the penetration of the consolidant. The minimal TiO2 concentration tested (0.5 wt %) implied a low-medium risk of incompatibility as an intervention in cultural heritage field, because its low-medium potential as inducer of visible color changes of the granite surface. Regardless of the TiO2 content, the nano-sized silica induced surface xerogel coatings more compact and continue than those formed in the ethyl silicate coated surfaces. Higher penetration rates were identified in the granite treated with nano-sized silica colloidal solution, while ethyl silicate was only found in the first few µm. It was found that penetration could depend on the application procedure, the solvent of the consolidant and the silica particle size. The TiO2 addition reduced the penetration of the nano-sized silica consolidant.
Highlights
In the conservation of the cultural heritage field, the application of consolidant products seeks the strengthening of the deteriorated surfaces with historic and artistic value and subsequently the mitigation of their disintegration
The most notable difference was found on the surfaces coated with 0.5% TiO2, because, despite cracks being found on the coatings, a more continuous layer was detected on the surface when Nano Estel was used as consolidant (Figure 3b) while on the surface coated with Estel 1000, accumulation of product was found in fissures and voids of the surfaces (Figure 2b)
Attending to the effect on the surface, high concentration of TiO2 mixed with both consolidants induced cracked white deposits in fissures and voids detected on the surface, at the lower concentration (0.5% TiO2 ) different patterns were detected: i) in the nano-sized silica consolidant, the nanoparticles TiO2 were kept as a suspension after the polymerization; and ii) in ethyl silicate consolidant, the nanoparticles TiO2 induced cracked white deposits filling the fissures and voids as was identified for the consolidant layers with higher TiO2 contents
Summary
In the conservation of the cultural heritage field, the application of consolidant products seeks the strengthening of the deteriorated surfaces with historic and artistic value and subsequently the mitigation of their disintegration. Considering that consolidation is an irreversible treatment, scientific research is justified to improve the cohesion of the stone surface and slow down its natural deterioration. Alkoxysilanes, such as tetraethoxysilane (TEOS) products are applied due to their low viscosity and their ability to form siloxane bonds [2]. These products polymerize inside the pore through a sol–gel process obtaining a xerogel, increasing the cohesion. In the case of TEOS-based products, cracking occurred due to the TEOS/organometallic catalyst ratio [7,8]
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