Detection and identification of possible gel residues on the surface of paintings after cleaning treatments

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.