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

Valeria Stagno,Alessandro Ciccola,Silvia Capuani,Roberta Curini,Gabriele Favero,Paolo Postorino

doi:10.3390/gels7040265

Abstract

The cleaning of buildings, statues, and artworks composed of stone materials from metal corrosion is an important topic in the cultural heritage field. In this work the cleaning effectiveness of a PVA-PEO-borax hydrogel in removing metal corrosion products from different porosity stones has been assessed by using a multidisciplinary and non-destructive approach based on relaxation times measurement by single-sided portable Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy—Energy Dispersive Spectroscopy (SEM-EDS), and Raman Spectroscopy. To this end, samples of two lithotypes, Travertine and Carrara marble, have been soiled by triggering acidic corrosion of some copper coins in contact with the stone surface. Then, a PVA-PEO-borax hydrogel was used to clean the stone surface. NMR data were collected in untreated, soiled with corrosion products, and hydrogel-cleaned samples. Raman spectroscopy was performed on PVA-PEO-borax hydrogel before and after cleaning of metal corrosion. Furthermore, the characterization of the dirty gel was obtained by SEM-EDS. The combination of NMR, SEM-EDS and Raman results suggests that the mechanism behind the hydrogel cleaning action is to trap heavy metal corrosion products, such as Cu2+ between adjacent boron ions cross-linked with PVA. Moreover, the PVA-PEO-borax hydrogel cleaning effectiveness depends on the stone porosity, being better in Carrara marble compared to Travertine.

Highlights

The surface cleaning of works of art composed of stone materials is a critical concern for conservators and restorers
The study we presented here, provides new information for the possible use of PVAborax hydrogel to clean monuments and marble statues from metal corrosion products
We used a portable Nuclear Magnetic Resonance (NMR) instrument that allows in situ measurement to study the cleaning efficacy of polyvinyl alcohol (PVA)-borax hydrogel

Summary

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 All these agents continuously endanger the life of stone artworks exposed to outdoor. 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) [7,8,9], depending on the concentration of pollutants and acid rain. 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]

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Conclusion