Detection and Identification of Possible Gel Residues on the Surface of Paintings after Cleaning Treatments

Abstract

Important features required for gels used for the cleaning of paintings are viscoelastic properties ensuring good adaptability to the morphology of the surfaces and complete gel removal after application. Poly (vinyl alcohol) (PVA)-borax gels containing different liquid phases are often used as cleaning materials, but still little is known about their ability to leave no residues. This study reports the development of an analytical method based on attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) for the identification of PVA-borax gel residues on mock-ups and on works of art after cleaning treatments. The effect of additives in the formulation (i.e., poly (ethylene oxide)) and of clearing treatments with organic solvents after the gel removal was assessed both with respect to the effectiveness of the cleaning and the presence of residues on the painted surfaces. The results obtained show that clearing the surfaces with cotton swabs and organic solvents after the application of the gel is necessary to ensure a good removal of gel residues. Moreover, Py-GC/MS analyses in single-ion-monitoring (SIM) mode are more sensitive and selective, allowing the detection of gel residues even on surfaces where no residue is detected by ATR-FTIR.

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

“…In this work, the formulation of PVA-PEO-borax in water solution developed by Riedo et al [ 20 ] has been used for the hydrogel preparation. This has shown many weaknesses, in agreement with previous studies [ 20 , 21 , 29 , 41 , 42 , 43 ]. First of all, it does not seem suitable for the surface cleaning of macro-porous materials, such as the Travertine samples, because of the difficulty in the complete removal of the hydrogel layer, which leads to solid-like residues (see Section 4.1 ).…”

“…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 ).…”

“…First of all, it does not seem suitable for the surface cleaning of macro-porous materials, such as the Travertine samples, because of the difficulty in the complete removal of the hydrogel layer, which leads to solid-like residues (see Section 4.1 ). Despite the fact that the addition of PEO increases the pore size and the liquid phase retention of the gel network, as well as improves its mechanical properties, it has been observed that the residues left on the stone surface may be due to the fact that PEO is the only polymer not chemically bonded to the network [ 20 , 21 , 29 , 42 ]. So, PEO seems the major responsible for residues together with the type and state of conservation of the sample analyzed.…”

“…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 ].…”

“…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 ].…”