PVA/Borax-based hydrogel for stone conservation: evaluation of cleaning performance through a multi-technical approach

Abstract

ABSTRACT: Carbonate stones used in connection with cultural heritage materials are subjected to different degradation phenomena in function of their chemical and physical properties. Two important issues that need to be addressed are: (1) the formation of corrosion products from metallic impurities or metallic artifacts in contact with the stone and (2) the photo-oxidative weathering of resins on stone which render them insoluble. In such cases, the choice of cleaning method is critical because the removal has to be confined to the contaminants and should not further damage the stone. High viscosity polymeric dispersions (HVPDs), which have good mechanical properties, high solvent and active component retention, low toxicity, and low environmental impact, seem to be a promising solution. For this thesis project, the two main HVPDs evaluated were polyvinyl alcohol cross-linked with borax (PVA/B) and its variant with agarose (PVA/B-AG) on the substrates Carrara Marble, Lecce Stone, and Travertine. For the cleaning of bronze corrosion products, ethylenediaminetetraacetic acid (EDTA) and potassium sodium tartrate (PST) were added as chelants. Based on characterization studies, all of the HVPDs pass the criteria of peelability, have minimal water desorption, retain more than half of their volatile fraction, and do not leave residues under four hours. Regarding the first issue, SEM and FORS analysis reveal that treatment using PVA/B-AG with EDTA or PST are both effective in the removal of corrosion products and the restoration of the surface close to its original state. Two applications are necessary, with the second one modulated to avoid lightening. The use of EDTA is recommended over PST due to its higher removal efficiency. FT-IR ATR results confirm the complexation of the chelants with copper, their sequestration, and their interaction with the HVPD components. Regarding the second issue, FORS analysis demonstrate the significant reduction in the redness and yellowing of the surface after the treatment with PVA/B and PVA/B-AG. Flaking off and delamination were observed in both cases, but FT-IR ATR results show the capability of the latter to adhere the acrylate polymer. Water vapor permeability test results also reinforce that the resin is being removed based on the improvement of the stone's breathability. Altogether, the results demonstrate the eco-friendliness and versatility of PVA/B-based HVPDs as a cleaning tool. This also opens avenues for its adoption not just in the removal of hydrophilic but also hydrophobic materials detrimental to cultural heritage objects.