Twin-chain polymer hydrogels based on poly(vinyl alcohol) as new advanced tool for the cleaning of modern and contemporary art

Abstract

Conservation of our cultural heritage is fundamental for conveying to future generations our culture, traditions, and ways of thinking and behaving. Cleaning art, in particular modern/contemporary paintings, with traditional tools could be risky and impractical, particularly on large collections of important works to be transferred to future generations. We report on advanced cleaning systems, based on twin-chain polymer networks made of poly(vinyl alcohol) (PVA) chains, semiinterpenetrated (semi-IPN) with PVA of lower molecular weight (L-PVA). Interpenetrating L-PVA causes a change from gels with oriented channels to sponge-like semi-IPNs with disordered interconnected pores, conferring different gel (and solvent) dynamics. These features grant residue-free, time efficient cleaning capacity and effective dirt capture, defeating risks for the artifact, making possible a safer treatment of important collections, unconceivable with conventional methods. We report as an example the conservation of Jackson Pollock's masterpieces, cleaned in a controlled way, safety and selectivity with unprecedented performance.

Keywords: contemporary art; cultural heritage conservation; modern art; poly(vinyl alcohol) hydrogel; semiinterpenetrated gels networks.

Fig. 1.

The structure of PVA cryogels at the micron scale. Confocal-microscopy images of the FT PVA gels, loaded with an aqueous solution of the green dye rhodamine 110, which preferentially interacts with the gel’s walls. A–D also include images of horizontal (α) and vertical (β and γ) sections of the imaged volumes. (A) PVA FT1 gel. The pores are cylindrical and arranged into a hexagonally packed structure (see detail of the β-plane section), with central branches from which new strands originate and arrange perpendicular to each other (see detail of the α-plane section). (B) TC-PN FT1 gel. The presence of L-PVA as semiinterpenetrated polymer leads to the formation of larger and nonoriented pores; the pore distribution resembles that of a sponge-like network (see sections along the α and β planes) (C) PVA FT3 gel. (D) TC-PN FT3 gel. In both FT3 hydrogels, the pores walls are thicker, as repeated FT cycles cause further phase separation, increasing local polymer concentration. The bottom row in A–D shows SEM images of xerogels obtained from the PVA-based gels, highlighting the presence of a wide range of pores’ diameters in TC-PNs (B and D), including pores <1 µm. Bar dimension is 20 µm.

Fig. 2.

Chord-length analysis of PVA cryogels. (A and B) Averaged chord-length distributions for the pore phase (A) and the gel phase (B) of PVA cryogels. The lines are the fitting of the data to exponential decays (SI Appendix). (C and D) The variance of chord distributions of pores with the depth of confocal stacks for PVA FT1 (C) and PVA/PVA FT1 (D).

Fig. 3.

Confocal-microscopy images of the PVA/PVA system. H-PVA is labeled in green with FITC and L-PVA in red with RBITC. Thus, the first column shows the FITC-PVA, the second column shows the RBITC-PVA, and the third and fourth columns show the sum of both components. (A) The PVA/PVA solution before the FT process. (B) The PVA/PVA gel network obtained after the FT process (1 cycle). The formation of ice crystals along preferential axes leads to the deformation of the blobs, which are distorted from spherical to elongated shapes; some of the smaller blobs coalesced together. (C) The PVA/PVA network after the first cycle of the FT process and 1 wk of curing in water. The 2D view of the top horizontal planes (α) highlights that L-PVA is preferentially localized on the gel’s walls, interacting with the green-labeled H-PVA. L-PVA is also present inside the pores, throughout the gel volume.

Fig. 4.

The diffusion of L-PVA in the TC-PN gels. (Top) FCS autocorrelation curves (marks) and fitting functions (solid lines) for a diluted solution of L-PVA in water (SI Appendix) (A), L-PVA in the pores of TC-PN FT1 gel (B), and L-PVA in the pores of TC-PN FT3 gel (C). The results of the fittings (values of the diffusion coefficient, D) are reported in SI Appendix, Table S4. (D) FRAP recovery profiles of L-PVA on the walls of TC-PN FT1 (blue markers) and FT3 (green markers) gels; the curves are flat, without recovery in the fluorescence intensity, which indicates that no diffusion of the polymer chains is detectable (SI Appendix, section S2.3).

Fig. 5.

Rheological measurements and visual aspect of cryogels. (A) Amplitude sweep curves of cryogels; the arrows indicate the cross-over of G′ and G″ in different systems. Solid and empty markers indicate G′ and G″, respectively. The oscillation strains at the cross-over follow the trend: PVA FT3 < PVA/PVA FT3 ∼ PVA FT1 < PVA/PVA FT1. Error bars are not included to facilitate the readability of the image (SI Appendix); errors do not affect cross-over trend. (B) Frequency sweeps of cryogels. Solid and empty markers indicate G′ and G″, respectively. Error bars show the SDs; when not visible, they are smaller than the markers’ size. (C) Cryogels on wooden sticks and colored logos. The increase in the number of cycles leads to opaquer and more rigid systems. CSGI, Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (Center for Colloid and Surface Science).

Fig. 6.

Cleaning tests on mockups: assessment of PVA and traditional gels. (A) Clotted painting mockup that mimics Pollock’s alkyd paintings, artificially soiled. (B) Application of a pure PVA FT1 gel (Left), a PVA/PVA FT1 TC-PN (Center), and a gellan gel sheet similar to those used in the traditional restoration practice (Right). (C) Soil removal after the application of the gels (8 min each); no additional mechanical action was carried out after the removal of the gels. D–F detail the removal efficacy of each type of gel: the soil was efficiently removed using the TC-PN (E), while partial cleaning was achieved using the pure PVA gel (D). Scarce soil removal was obtained using the gellan sheet (F), as expected considering the poor adhesion of the rigid sheet to the clotted painted surface. A and B represent areas of 9.5× 4.1 cm². C has a magnification of 1.25×. D–F have magnification of 3.4× with respect to A and B, and are magnifications of Insets (D–F) in C.

Fig. 7.

Cleaning tests on mockups: assessment of the cleaning using FTIR 2D Imaging. (A) Removal of artificial soil from an oil painting mockup (water sensitive cadmium red color) that mimics Pollock paintings (1, 2). Removal using a swab soaked with a cleaning aqueous solution. The detail in 2 shows the removal of some red pigment along with the soil (3, 4). Application of the TC-PN FT1 hydrogel sheet on the soiled surface (i.e., approach proposed in this contribution); 4 shows that only soil, and no red pigment, adheres to the gel sheet following the application (5, 6). The cleaning is completed using the TC-PN FT1 hydrogel shaped as an eraser gum; gentle mechanical action with the gel leads to the complete removal of the soil. The detail in 6 shows that no red pigment adheres to the eraser gum. (B) Assessment of the cleaning effectiveness of the TC-PNs hydrogel using FTIR 2D Imaging. The IR maps show the imaging of the bands of kaolin (present in the artificial soil mixture) in the 3,725 to 3,592 cm⁻¹ region. (Top) Pristine painted surface. (Center) Painted surface that was soiled and then cleaned using the TC-PN gel. (Bottom) Soiled painted surface. Representative spectra of the pristine, cleaned, and soiled surfaces are shown below the maps. (C) Assessment of the absence of gel residues. The IR maps (acquired on the same areas as panel b) show the imaging of the 3,440 to 3,180 cm⁻¹ region, where characteristic bands of PVA would be found in case of gel residues after cleaning. (Top) Pristine painted surface. (Bottom) Painted surface that was soiled and then cleaned using the TC-PN gel. Representative spectra of the pristine and cleaned surface are shown below the maps, along with the difference between the two spectra, showing no absorptions ascribable to PVA. abs, absorbance; max, maximum; min, minimum.

Fig. 8.

Cleaning of Pollock’s masterpieces. (Left) Two by Jackson Pollock, © Pollock-Krasner Foundation/Artists Rights Society (ARS), New York. (Right) Eyes in the Heat by Jackson Pollock, © Pollock-Krasner Foundation/Artists Rights Society (ARS), New York. (A and D) Collages showing the paintings before and after the cleaning intervention, where the soil was removed using the TC-PN FT1 hydrogels. The collages allow to better appreciate the removal of soil as the paints’ hue and brightness were brought back. The whole paintings were cleaned during the cleaning intervention. (B and C) A detail showing the gel adhering to the painting and the same area after cleaning. (E and F) A detail of the painting before and after cleaning with the TC-PN gels. B and C have a magnification of 13× with respect to A. E and F have a magnification of 20× with respect to D.