**1. Introduction**

The surface cleaning of works of art composed of stone materials is a critical concern for conservators and restorers. The cleaning process involves the removal of dirt, dust, pollutants, metal ions, or microorganisms. In particular, all these agents continuously endanger the life of stone artworks exposed to outdoor. When a stone building, statue or monument under atmospheric conditions is in contact with metallic parts, such as clamps, pivots, or plaques [1], corrosion of the latter may be induced and the corrosion products may hinder the correct readability of the artwork [2–4]. In particular, in the case of copper and its alloys, corrosion is a chemical attack mainly promoted by the affinity of metals and pollutants (i.e., sulfur, carbon dioxide, chlorides). This process leads to a corrosive layer called patina [5,6], which can be protective (noble patina) or unprotective (vile patina) [7–9], depending on the concentration of pollutants and acid rain. As a

**Citation:** Stagno, V.; Ciccola, A.; Curini, R.; Postorino, P.; Favero, G.; Capuani, S. Non-Invasive Assessment of PVA-Borax Hydrogel Effectiveness in Removing Metal Corrosion Products on Stones by Portable NMR. *Gels* **2021**, *7*, 265. https://doi.org/ 10.3390/gels7040265

Academic Editor: Wei Ji

Received: 15 November 2021 Accepted: 13 December 2021 Published: 14 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

result, geographic location, precipitation, and pollution level all have an impact on patina composition and morphology [9,10]. So, the atmospheric exposure of copper produces the oxidation-reduction reactions leading to different corrosion products: copper(I) oxide which is red, copper(II) oxide which is black, black copper sulfide, various colored salts and nantokite, green-blue atacamite and clinoatacamite [6]. These corrosion products are responsible for the discoloration of the stone [1,11].

In the case of iron in contact with a stone artwork and in the presence of oxygen and water, it will corrode depending on the pH value of the surrounding environment [1,4,11]. Generally, corrosion is activated by acidic conditions, but it can also take place in an alkaline environment [11]. The result of iron corrosion will not be a patina adhered to the metal surface but a powdery rust layer produced by electrochemical processes [6]. So, the rust consists of the stratification of the oxides, usually green hydrated magnetite, black anhydrous magnetite and, only externally, the ferric hydroxides [6], which are responsible for cervices formation in the stone surrounding the metallic part [11]. Moreover, also when iron is covered by lead a damage to the stone should be expected because lead itself can be attacked [11]. 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 SO2, PM<sup>10</sup> and rain pH, black crusts are the most common alteration for stone artworks in cities [9,13]. The black crusts composition reflects that one of the air in which the artwork is exposed and they lead to a mechanical, aesthetical and chemical damage [9].

In this scenario, the cleaning process and the choice of the cleaning substance are of fundamental importance. In the last years, gels or gel-like systems have been widely employed for the cleaning procedure of stone artworks and cultural heritage in general [1,14–17]. Gels or gel-like systems, as well as high-viscous-polymeric-dispersion [18], have shown great potential due to their high selectivity, low toxicity, and low environmental impact [16,18,19]. Among these, there are gel-systems based on polyvinyl alcohol (PVA). PVA is a water soluble and biocompatible polymer with good resistance to mechanical stress, capable to form hydrogen bonds, thanks to its hydroxyl groups, and ion complexes [19–21]. It has emerged as a potential adsorbent because of its high swelling capacity and its resistance to dissolution, mainly due to the formation of cross-links between network chains [22]. In particular, it can capture contaminants entrapping them between the fine pores of hydrogel developed via crosslinking networks [23]. The high-water-content and porous structure networks help to diffuse the solute with contaminants [24]. Moreover, it has been used together with both natural and synthetic compounds to produce different types of hydrogels, which may also be physically or chemically cross-linked. One of these, the PVA-borax hydrogel, obtained by cross-linking of PVA with borate ions [18,20,21,25], can be described as a viscoelastic dispersion with a dynamic network. In fact, the increase in borax concentration expands the system network due to electrostatic repulsions in the polymer chain [20,25,26]. When PVA-borax aqueous gel is applied on the surface of cultural heritage for cleaning purposes, this should be able to remove degradation products by capillarity absorption through their pores [19]. To this end, Riedo et al. [20] studied the effect of polyethylene oxide (PEO) addition to the PVA-borax hydrogel. The authors showed that PEO, which is a water-soluble and biocompatible thermoplastic polymer, would increase the pore size of the system in agreement with other studies [27,28]. Moreover, PEO seemed to improve the mechanical properties of the gel and the retention of its liquid phase [20,29].

PVA-PEO-borax gels, as well as all the gel-systems, can be easily removed in one step from a surface simply by peeling. 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].

In this work the cleaning effectiveness of a PVA-PEO-borax hydrogel in removing metal corrosion products from two different porosity stones has been assessed by using a multidisciplinary and non-destructive approach, combining the single-sided portable Nuclear Magnetic Resonance (NMR) to investigate stone samples cleaning, with Scanning Electron Microscopy—Energy Dispersive Spectroscopy (SEM-EDS) to characterize the gel composition after the stone cleaning process and Raman Spectroscopy measurements to study the gel before and after the cleaning procedure.

To this end, samples of two lithotypes, Travertine and Carrara marble, have been soiled by triggering the acidic corrosion of some copper coins in contact with the stone surface. Then, a PVA-PEO-borax hydrogel was used in order to clean the stone surface. NMR relaxation times were evaluated in untreated and treated samples. Pure gel and dirty gel obtained after sample cleaning were analyzed by Raman Spectroscopy. Moreover, the composition of the dirty gel was obtained by SEM-EDS. The novelty of this study is the use of portable NMR as a non-invasive and non-destructive tool for the monitoring of the gel cleaning procedure. Indeed, the NMR protocol that we developed can be employed for in situ analyses to evaluate the cleaning efficacy and action of different gels on different materials.

## **2. Results and Discussion**
