Abstract
In this work, we investigated the potential of PVA-borax hydrogel for cleaning limestones and the dependence of the cleaning on the porosity of the rock and on the action time of the hydrogel treatment. Towards this goal, we used a nuclear magnetic resonance (NMR) spectrometer, developed for non-invasive and non-destructive applications on cultural heritage. T2-NMR parameters were quantified on different samples of Lecce stone and Travertine cut perpendicular (Pe) and parallel (Pa) to the bedding planes under different experimental conditions: untreated samples, treated with Paraloid B72 and cleaned with PVA-PEO-borax hydrogel applied for 4 min and 2 h. The T2 results suggest that the effectiveness of the cleaning strongly depended on the porosity of the stones. In Lecce stone, the hydrogel seemed to eliminate both the paramagnetic impurities (in equal measure with 4 min and 2 h treatment) and Paraloid B72. In Travertine Pe, characterized by a smaller pore size compared to Lecce stone, no significant effects were found regarding both the cleaning and the treatment with Paraloid B72. In Travertine Pa, characterized by a larger pore size than the other two samples, the hydrogel seemed to clean the paramagnetic agents (it worked better if applied for a longer time) but it did not appear to have any effect on Paraloid B72 removal.
Highlights
Stone represents one of the most widely employed materials for the realization of cultural heritage artefacts, including statues, fountains, and buildings.Among them, Travertine and Lecce stone are calcareous lithotypes mainly composed of calcium carbonate (CaCO3) and are commonly employed all over the Mediterranean area, in the edification of Italian historical buildings
T2-nuclear magnetic resonance (NMR) parameters were quantified on different samples of Lecce stone and Travertine cut perpendicular (Pe) and parallel (Pa) to the bedding planes under different experimental conditions: untreated samples, treated with Paraloid B72 and cleaned with PVA-PEO-borax hydrogel applied for 4 min and 2 h
The employment of PVA based hydrogels crosslinked with borax has been spreading in the field of cleaning of cultural heritage materials [38]
Summary
Stone represents one of the most widely employed materials for the realization of cultural heritage artefacts, including statues, fountains, and buildings.Among them, Travertine and Lecce stone are calcareous lithotypes mainly composed of calcium carbonate (CaCO3) and are commonly employed all over the Mediterranean area, in the edification of Italian historical buildings. Water allows the solubilization of the atmospheric sulfur dioxide (SO2) present in polluted environments, which reacts with calcium carbonate (CaCO3) to form gypsum (CaSO4·2(H2O)) [3,4], a less noble material, and one that is much more friable and absorbent than the original carbonate surface [5] These newly formed gypsum incrustations retain various aero-dispersed particles of pollutants and organic materials, giving rise to the so-called black crusts: dark areas (due to soot particles) covered by an irregular, dendrite-like hard crust [6], which represents one of the most studied and relevant problems characteristic of outdoor preserved carbonate artistic materials. Water, when retained inside the porous matrix, allows the proliferation of photosynthetic microorganisms (algae, cyanobacteria, and lichens), pioneers in the biological succession, followed by chemotropic microorganisms (fungi and bacteria), which, together, are responsible for the biodeterioration that causes aesthetic, chemical, and physical damage to the biocolonized stones (i.e., discolorations, biopitting, surface roughness, fractures, and detachments) [10,11]
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