Fig. 3

FTIR 2D imaging of paintings’ mock-up samples. Top panel: visible light and 2D FTIR maps of mock-ups that were unvarnished (“paint”, top row), varnished (center row), or varnished and then cleaned with the PVA hydrogels loaded with the NSF (“cleaned”, bottom row). The tests were carried out in the wax-lined regions. For each sample, the images beside the visible map show the corresponding 2D FTIR maps, where the intensities of the following peaks were imaged: 1185–1110 cm⁻¹ (where the C-O stretching band of the C₉₋₁₁E_5,5 surfactant would be found in case of NSF residues after cleaning), 1560–1465 cm⁻¹ (CH₂ bending, wax used in the lining), 1880–1705 cm⁻¹ (C = O stretching, polyvinyl acetate-based varnish), and 3590–3380 cm⁻¹ (where the characteristic OH stretching band of PVA would be found in case of gel residues after cleaning). All maps have dimensions of 2100 × 1400 µm², each axis tick being 50 µm. The FTIR Reflectance spectra are shown in the bottom panel, each spectrum relating to a single pixel (5.5 × 5.5 µm²) of the corresponding 2D imaging map. The maps show the removal of the varnish and surface removal of wax spots, while no absorptions ascribable to PVA or surfactants from the NSF could be detected. The spectrum of a PVA hydrogel (loaded with the NSF and then let dry before acquiring the spectra) is also showed as a reference