Single-Sided Portable NMR Investigation to Assess and Monitor Cleaning Action of PVA-Borax Hydrogel in Travertine and Lecce Stone

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.

Non-Invasive Assessment of PVA-Borax Hydrogel Effectiveness in Removing Metal Corrosion Products on Stones by Portable NMR

“…This result may suggest that the PVA-based hydrogel did not completely remove the metal corrosion products from the stone. In addition, the fact that the T 1 did not return to the initial values may also suggest that the gel used has left residues in the pores, as observed in previous studies [ 12 , 29 ]. In particular, in Travertine soiled with corrosion products from the penny (Travertine 1) the T 1 and T 2 distributions have the same behavior with both T 1 and T 2 mean values that did not return to their original value after the cleaning process ( Figure 3 ).…”

“…The two T 1 and T 2 components detected for all three samples, suggest that two main different pore size compartments exist in the stones. In contrast to our previous work [ 12 ], we investigated samples at relative humidity (RH) value equal to 50 ± 3%. For this reason we did not detect the long T 2 component (around few or tens of ms) obtained in the study performed at RH = 94% [ 12 ].…”

“…In contrast to our previous work [ 12 ], we investigated samples at relative humidity (RH) value equal to 50 ± 3%. For this reason we did not detect the long T 2 component (around few or tens of ms) obtained in the study performed at RH = 94% [ 12 ]. Indeed, at RH = 50% less water molecules in vapor form wet the stone pores compared to those present at RH = 94%.…”

“…Behind the surface alterations of stone artefacts induced by the corrosion of metallic components, other factors can cause the aesthetical modification of the artwork. Among these factors, there are old restorations and protecting interventions, deposition of particulate and pollutants, graffiti, and vandalism [ 12 ]. Specifically, because of the well-known interaction among SO 2 , PM 10 and rain pH, black crusts are the most common alteration for stone artworks in cities [ 9 , 13 ].…”

“…This represents a great advantage for the conservation treatment of cultural heritage. However, the ease of gel removal, together with its capability of retention of the liquid phase and the absence of gel residues also depend on the characteristics of the surface to be cleaned [ 12 , 29 ].…”