Restoration of paper artworks with microemulsions confined in hydrogels for safe and efficient removal of adhesive tapes

Significance

From Dead Sea Scrolls to Federico Fellini and Lucio Fontana drawings, pressure-sensitive tapes (PSTs) have been used as adhesive fasteners or as part of temporary conservation treatments that frequently became permanent. Their safe and efficient removal poses ethical and aesthetic questions: Adhesive tape residues damage the paper substrate and, due to discoloring, prejudice the artwork enjoyment and conservation. Selective removal without affecting the underlying support is challenging and often impossible. We tackled this issue from a physicochemical and a colloidal perspective, by designing a system where nanosized solvent droplets are confined within a hydrogel. This method has the potential to revolutionize the approaches used so far in the removal of PSTs and coatings from a plurality of materials.

Abstract

The presence of pressure-sensitive tapes (PSTs) on paper artworks, either fortuitous or specifically applied for conservation purposes, is one of the most frequent and difficult issues encountered during restoration. Aged PSTs can damage or disfigure artworks, compromising structural integrity, readability, and enjoyment. Current procedures are often inherently hazardous for artistic media and paper support. Challenged by the necessity to remove PSTs from a contemporary and an ancient drawing (20th century, by artists da Silva and Hayter, and a 16th-century drawing of one figure from the Sistine Chapel by Michelangelo), we addressed this issue from a physicochemical perspective, leveraging colloid and interface science. After a characterization of the specific PSTs present on the artifact, we selected a highly water-retentive hydrogel as the host of 23% wt/wt of “green” organic solvents uniformly dispersed within the gel in the form of nanosized droplets. The double confinement of the organic solvent in the nanodroplets and into the gel network promotes a tailored, controlled removal of PSTs of different natures, with virtually no interaction with the solvent-sensitive artwork. This noninvasive procedure allows complete retrieval of artwork readability. For instance, in the ancient drawing, the PST totally concealed the inscription, “di mano di Michelangelo” (“from Michelangelo’s hand”), a possibly false attribution hidden by a collector, which is now perfectly visible and whose origin is currently under investigation. Remarkably, the same methodology was successful for the removal of aged PST adhesive penetrated inside paper fibers of a drawing from the celebrated artist Lucio Fontana.

The invention of pressure-sensitive tape (PST) is attributed to Horace Day, a surgeon who made, in 1845, the first basic surgical sticking plaster tape by combining India rubber, pine gum, turpentine, litharge, and turpentine extract of cayenne pepper and applied that mixture to strips of fabric (1). It took almost 75 y, from Day’s first PST until the early 1920s, for the first industrial tape application to appear (1). Since then, PSTs have been used in several applications, including to mend precious mementoes, repair torn book pages, and hold together parts of lacerated documents, hence reflecting the social and cultural “fast at any cost” aspects of modern society. PSTs most frequently encountered (2) in works of art have rubber or acrylic-based adhesive pastes. Depending on their composition, PST adhesives undergo several degradation processes, which lead, at their final stages, to materials that become dark and oily and can entirely penetrate the medium; i.e., they are quite different in color, form, and function from the original. Time and experience have shown that PST(s) on paper can be disfiguring, damaging, and difficult to remove. Conservators are familiar with a variety of tape removal methods, including mechanical, immersion, poultice, rolling, and suction table (2). However, each method has some associated risks/disadvantages, which may result in undesirable changes such as skinning of the medium with mechanical removal, tidelines and media bleeding when suction techniques and poultices are used, and media bleeding and penetration of the adhesive into the cellulose fibers when using immersion treatments. For artistic, ethical, and practical purposes, it is important to find a way that ensures selective removal of the PSTs without affecting the artifact or leaving residues. Over recent years, we have designed several aqueous-based colloidal systems, where organic solvents can be confined with structural and dynamic control at the nanoscale, termed “nanostructured fluids” (NSFs), tailored to clean surfaces of artworks. These complex fluid media overcome many of the disadvantages of neat organic solvents, such as toxicity and solvent spreading (3–5). In addition, the loading of water-based NSFs within hydrogel scaffolds with high water retentiveness guarantees effective yet safe cleaning of water-sensitive artifacts (6, 7). These methods have been applied to remove soil, wax, polymeric coatings, and varnishes from a variety of important artistic surfaces (8, 9). Recently, we were confronted with the removal of PSTs from two completely different artworks: a recto-verso contemporary drawing, unusually realized by two artists at different times (the recto was realized by the Portuguese−French abstractionist Maria Helena Vieira da Silva, while Helen Phillips Hayter realized the drawing visible on the verso) and an ancient 16th-century drawing representing one of the figures of the Ascesa dei Beati, a scene of the Giudizio Universale realized by Michelangelo Buonarroti in the Sistine Chapel.

We decided to tackle this complex issue using a hydrogel, close to complete water retentiveness (6), as a scaffold to confine a nanostructured fluid composed by a surfactant that stabilizes droplets of organic solvents in water as continuous phase (NSF), tailored to remove specific PSTs. This approach allows a very sophisticated control of the cleaning system: (i) The confinement of the organic solvent—needed for PST softening—in droplets within a continuous water phase controls the penetration of the solvent in the PST, and prevents spreading in the underlying support; (ii) the confinement of the NSF within the highly retentive hydrogel network allows NSF−PST contact with no lateral migration of the liquid phase. To fine-tune the hydrogel/NSF system, we investigated the interaction with the most common PSTs, whose multilayered structure had been characterized through SEM, attenuated total reflectance (ATR)-FTIR, and thermal analysis. The NSF ability to interact with the PSTs and diffuse through the backing of the different PSTs types was studied by thermogravimetric analysis and steady-state fluorescence measurements, respectively. We then challenged the different PSTs with the hydrogel/NSF system and monitored the NSF interaction with confocal microscopy, to determine the mechanism of action and predict the outcome of PST removal in real cases. The selected system was applied to remove the PSTs from the two-mentioned artworks, leading, in one case, to the astonishing discovery of the inscription “di mano di Michelangelo” (“from Michelangelo’s hand”). Moreover, we extended the same method to the removal of strongly defacing PST residues from a 20th-century Lucio Fontana drawing, demonstrating the potentiality of the proposed methodology to fully control every step of the restoration process.

Results and Discussion

PST Models.

Fig. 1 A and B displays an ancient 16th-century drawing representing one of the figures of the Ascesa dei Beati, a scene of the Giudizio Universale realized by Michelangelo Buonarroti in the Sistine Chapel, with an aged PST, at a first inspection characterized by a backing made of cellulosic material (paper), attached on the bottom left side of the recto. Fig. 1 C and D displays a recto-verso contemporary drawing (the recto by the Portuguese−French abstractionist Maria Helena Vieira da Silva, the verso by Helen Phillips Hayter), with a PST characterized by a transparent backing on the verso side (highlighted by an arrow in Fig. 1D). These two extremely different artworks clearly demonstrate the ubiquitous nature of PSTs’ application and the complexity of their removal. In fact, several classes of PSTs can be found on both ancient and contemporary artworks, with different structural and physicochemical features. To tackle this issue with a general approach and to set up a procedure enabling the safe and efficient removal of different PSTs from works of art, we selected the most common PSTs, to investigate their physicochemical properties and address their removal. PSTs present a multilayered structure composed of a pressure-sensitive adhesive (PSA) and its carrier (backing). Minor components include a release coat, ensuring an easy unrolling of the tape, and a primer, that enhances adhesion between backing and the adhesive mass. Backing materials may include paper, fabric, cellophane, cellulose acetate, and oriented polypropylene, while PSAs include natural and synthetic rubbers, acrylic copolymers, and silicones (2, 10). Both the adhesive and the backing layer determine the fate of the PSTs, in terms of aging and degradation effects.

Fig. 1.

(A) Sixteenth-century drawing from Ascesa dei Beati, a scene of the Giudizio Universale of Michelangelo Buonarroti, Sistine Chapel, (B) detail of an aged PST on the drawing; and (C and D) 20th-century drawing (C) recto by Maria Helena Vieira da Silva and (D) verso by Helen Philips Hayter.

We selected three types of PSTs as the most common representative models of PSTs in artworks: Filmoplast P (FPP) is a PST with a cellulose-based backing, specifically designed for paper conservation; MagicTape (MT) is the popular “matte-finish tape” (which makes it invisible on paper); and ordinary tape, OT, is a very common PST for domestic usages. In Fig. 2A, SEM images of the three backings of PSTs are displayed. FPP (i.e., paper) exhibits the typical morphology of compressed cellulose fibers; MT displays a rough surface, indicating that the macroscopic matte effect is obtained through treatment of the surface; and the surface of OT appears smooth and compact at this length scale.

Fig. 2.

(A) SEM images of PSTs. (B and C) ATR-FTIR spectra of the (B) adhesives and (C) backings.

Fig. 2 shows the ATR-FTIR characterization of the adhesive (Fig. 2B) and backing (Fig. 2C) of the PST models. In all of the studied PSTs, the typical infrared pattern of acrylic adhesives, widely used in PSAs formulations, is present (Fig. 2B) (11). Conversely, meaningful differences in the composition of the PSTs are observed in the backings (Fig. 2C): cellulose for FPP, cellulose acetate for MT, and polypropylene for OT (full ATR-FTIR analysis is reported in SI Appendix, Fig. S2). As already discussed, PSTs are currently removed with procedures that can damage the artifacts, as rough mechanical processes are often extremely dangerous for the integrity of works of art. The selection of an appropriate solvent for PST removal using the classic immersion procedure is by no means trivial. The solvent should efficiently penetrate the backing and soften the adhesive. However, since the supports of artworks are generally very porous, a complete solubilization of the adhesive could lead to the penetration of the solubilized adhesive within the support, leading to uncontrolled spreading and penetration of great quantities of solvent that can be extremely dangerous for the artistic media (e.g., inks), with the risk of irreversible damage or even the loss of the artistic message.

Choice of the NSF.

With these prerequisites in mind, we chose a multifunctional system able to efficiently interact with different backings, without causing damage to the support, i.e., without affecting the painted layer or the underlying paper. Our research group designed several nanostructured aqueous-based formulations as cleaning tools for works of art, with high efficiency for the removal of hydrophobic polymer coatings from wall paintings, unwanted varnishes from easel paintings, or grime and other contaminants from modern and contemporary artifacts (12). Nanostructured fluids present several advantages for dealing with restoration issues: The confinement of organic solvent in a water-based medium limits or completely avoids the adverse effects on the support that are due to the uncontrolled spreading of the organic solvent in the porous matrix of the works of art. The use of small amounts of organic solvents, typical of NSF, improves the system in terms of environmental impact and health compatibility. Finally, the intrinsic high interfacial area of the NSFs leads to very high efficiency and selective removal of defined components from artifacts (13). One of these NSFs, termed EAPC, is composed of water, SDS, 1-pentanol, ethyl acetate (EA), and propylene carbonate (PC). This system has been fully characterized from a structural point of view using small-angle neutron scattering with contrast variation (4), determining the localization of each component in the NSF. Nanosized (major axis 12.8 nm) (4) ellipsoidal droplets mainly composed of EA are stabilized in a mixed continuous phase (water and about 20% PC) by a film of SDS and 1-pentanol. PC, mainly located in the continuous phase, is also partitioned at the micelle interface, conferring enhanced cleaning capacity due to its high dipole constant (a cartoon sketching the structure of EAPC is displayed in SI Appendix, Fig. S1). EAPC displayed high efficiency in removing acrylic coatings from artistic surfaces (5, 14). Finally, both EA and PC are classified as environmental friendly solvents. Based on chemical affinity, we can expect this NSF to efficiently penetrate the backing of FPP (thanks to the hydrophilicity and porosity of the backing), and to efficiently interact with MT backing (due to the partial solubility of cellulose acetate in ethyl acetate and propylene carbonate), while scarce interaction is expected for the OT backing. Finally, since NSFs can dewet thick acrylic coatings (15–18 μm), we can predict some swelling of the acrylic adhesives. We will show how NSF opportunely confined into hydrogels allows the removal of PSTs from artistic surfaces with unprecedented performances.

Interaction of PSTs with NSFs.

Before uploading the NSF inside the hydrogel matrix for application to the removal of PSTs from works of art, we investigated, in detail, its interactions with the most common PSTs used in the conservation practice.

The thermal degradation profiles of chemical components are robust tools for analytical characterization. We exploited differential thermogravimetric analysis (DTG) for determining the chemical modifications of PST components constituting the adhesive and backing layers, upon interaction with EAPC NSF. In SI Appendix, Fig. S4, the DTG curves of the three complete PSTs (adhesive and backing) and bare backings, before and after immersion in NSF for 48 h, are displayed. The DTG results (whose complete discussion is reported in SI Appendix) highlight (i) the absence of chemical modification of the acrylic components of PST adhesives upon interaction with the NSF, for the three PSTs; and (ii) different effects of the NSF, depending on the backings: FPP and OT show no significant variations in thermal behavior, suggesting that the NSF does not alter the chemical composition or structure of the backing. On the other hand, after interaction of MT backing with the EAPC NSF, an additive [probably a plasticizer, diethyl phthalate, commonly used in the production of cellulose acetate-based PSTs (19, 20)] is removed from the cellulose acetate film, possibly due to its high solubility in ethyl acetate, one of the components of EAPC NSF.

In summary, DTG suggests that the different backings steer the interaction pathway of the EAPC with the PSTs. To further address this point, we performed a fluorescence assay, to monitor the ability of EAPC NSF to penetrate PST backings. The three backings were layered on a quartz cuvette filled with Nile Red (NR)-labeled EAPC NSF in contact with a second cuvette filled with unlabeled EAPC NSF; NR fluorescence inside the originally unlabeled EAPC solution was then monitored for a period of 96 h (Fig. 3). The fluorescence increase due to NR diffusion across the MT backing was compared with a reference curve (SI Appendix, Fig. S6) to obtain an NR concentration profile (C_t). Fig. 3B compares, for the three backings, the concentration profiles normalized for the theoretical final concentration, C_f (C_t/C_inf). Consistently with DTG results, no penetration is observed across OT backing, while NR crosses MT and FPP backings, with a higher permeation efficiency in the first case. The dye diffusion to the second cuvette implies (i) interaction of the NSF with the backing and (ii) release of the dye from the backing to the unlabeled solution, due to concentration gradient. The data were analyzed with a model for the release of active principles from a thin film, according a power law C_t/C_inf = ktⁿ, with k dependent on the structure and geometry of the system and n related to the diffusion mode (21–24). For pure Fickian diffusion, n = 0.5 is expected, while a linear dependence over time (n = 1) is related to Case II transport, connected to swelling or relaxation of the support. The data, yielding n values of 0.47 for MT and 1.0 for FPP, can be correlated with the DTG results. EAPC closely interacts with MT backing, eventually leading to the solubilization of some of the additives (Fig. 3A), and the dye diffusion across this barrier is not hampered, resulting in a Fickian diffusion. Conversely, the possibly strong affinity for FPP cellulose backing might lead to adsorption−desorption of NR on the cellulose fibers, resulting in a non-Fickian diffusion. If we “force” the model to a pure Fickian diffusion for both backings (n = 0.5), we obtain k values of 0.05 and 0.025 for MT and FPP, respectively. The diffusion kinetics of NR across MT is considerably faster than for FPP. Despite its simplicity compared to more complex approaches (25), this model nicely captures some essential details, accounting for the different affinity of EAPC for the backings, and complement DTG results. In particular, for FPP, the interaction with cellulose probably determines a partial disruption of NSF, a nonnegligible physical adsorption of the NR on fibers, without any chemical modification of the backing (as highlighted from DTG). This leads to the overall slowing down of NR diffusion across the backing (decrease in k) and to a change of the diffusion mode (variation of n), due to a strong “matrix effect.”

Fig. 3.

(A) Fluorescence spectra of NR-labeled EAPC diffusing across MT backing over 96 h; (B) picture of cuvettes containing originally unlabeled EAPC, after 96 h contact with NR-labeled EAPC through FPP, MT, OT backings; and (C) NR diffusion kinetics through FPP, MT, and OT backings (markers); curve fit of experimental data according to C_t/C_inf = ktⁿ (continuous lines) equation; curve fit of FPP with n = 0.5 fixed (dashed line).

In summary, the NSF is able, to a different extent and with different mechanisms, to cross the backings of FPP and MT over their entire thickness, while no penetration is observed for OT. This very simple method allows ex situ selection of the NSF to achieve a slow and controlled interaction with the backings and can be applied generally to address PST removal without involving the artifact.

Interaction of PSTs with Gel-Confined NSFs.

The NSF’s confinement, key for restoration interventions, ensures control over the removal process. A partial control is guaranteed by dispersing the organic solvent as nanodroplets in the NSF, but a lateral confinement is necessary to limit spreading outside the application area. For this reason, the NSF was inserted in a semiinterpenetrating polymer network composed of poly(hydoxyethyl metacrylate) [p(HEMA)] and polyvinylpyrrolidone (PVP). The hydrogel preparation and NSF upload are described in SI Appendix, Hydrogel Preparation and Loading. Unlike the physical gels most commonly used in art conservation (e.g., agar, gellan, solvent gels) (26, 27), this system is a chemical gel, whose network is established by formation of covalent bonds. This difference ensures the absence of gel residues on support after the treatment. Depending on composition and viscoelastic properties, chemical hydrogels can be easily shaped to perfectly match the PST footprint on the artwork. In addition, the high retentiveness can further improve the confinement of the NSF, determining a controlled release of the NSF to the PST, avoiding the lateral spreading over the application area of the NSF to the very surface covered by the PST.

The interaction of p(HEMA)/PVP gels loaded with EAPC NSF with the different adhesive tapes was monitored with confocal laser scanning microscopy (CLSM). PST samples were fluorescently labeled by immersion in a 10⁻³ mM aqueous solution of rhodamine B isothiocyanate (RhBITC) and then air dried for 24 h. RhBITC physically adsorbs within the acrylic adhesive layer without altering the structural and chemical features of the PSTs. EAPC dispersions, fluorescently labeled with Rh110 10⁻² mM, were uploaded in the gel by soaking the network in the Rh110-labeled fluid overnight. For CLSM (Fig. 4), the RhBITC-labeled PSTs (red) were attached on a coverglass with the gels loaded with Rh110-labeled EAPC NSF (green) layered on top.

Fig. 4.

Confocal microscopy of PSTs with RhBITC-labeled adhesive (red) interacting with p(HEMA)/PVP gel loaded with Rh110-labeled EAPC NSF (green). (A–C) A 3D reconstruction of FPP PST after (A) 5 min, (B) 10 min, and (C) 20 min of interaction with the gel; due to the opacity of FPP backing, the green-labeled hydrogel layered on the PST is not visible in CLSM 3D reconstruction; after 20 min, the NSF penetrates the backing, which is then homogeneously fluorescently labeled (C). (D) A 3D reconstruction and (E) a 2D horizontal section of MT after 20 min of interaction, where, due to the transparency of the PST, both the initially unlabeled backing and the hydrogel are visible; after 20 min, the NSF penetrates the backing from the hydrogel (the pale green region between the red adhesive and the bright green gel). (E) A 2D section of MT PST adhesive after 20 min of interaction with the NSF: The red and the green emissions are separately displayed with the transmission (grayscale); the overlay of these images (with colocalization of the probes appearing as yellow) highlights the successful penetration of the NSF in the upper parts of the MT adhesive. (F) A 3D reconstruction and (G) a 2D vertical section of OT PST after 20 min of interaction. Due to the transparency of the PST, both the unlabeled backing and the hydrogel are visible; after 20 min, the NSF from the hydrogel is separated from the adhesive layer by the backing, which, unlike for MT and FPP, remains unlabeled. (Scale bars, 50 μm.)

In Fig. 4 A–C, some representative 3D reconstructions of FPP PST (red) interacting with p(HEMA)/PVP gel containing the Rh110-labeled EAPC (green) are displayed, after 5 min (Fig. 1A), 10 min (Fig. 1B), and 20 min and (Fig. 1C). The FPP paper backing is opaque, and, at the beginning, only the fluorescently labeled adhesive (red) is visible (Fig. 4A). With increasing incubation times (Fig. 4 B and C), the NSF progressively penetrates the backing, reaching the backing−adhesive interface after 20 min (Fig. 4C). Concerning MT (Fig. 4 D and E), PST is completely transparent and allows visualizing both the labeled PST (red) and the gel. In this case, some Rh110 is able to deeply penetrate the backing (Fig. 4D), eventually reaching the lower part of the adhesive. As a matter of fact, a horizontal section of the same sample, acquired after 20 min of incubation (Fig. 4E), highlights that the red and the green fluorescence are colocalized inside the PST. Fig. 4F displays a representative 3D reconstruction of OT PST (red) upon 20 min of interaction with the NSF (red) loaded in the p(HEMA)/PVP gel (green). In this case, the probes’ emissions are well separated, as confirmed also by the vertical section displayed in Fig. 4G.

In summary, a different penetration of the NSF loaded in p(HEMA)/PVP gels is observed in the three PST models: (i) EAPC efficiently penetrates the FPP paper structure, poorly interacting with the adhesive underlying layer; (ii) it efficiently interacts with MT, completely penetrating the backing and eventually reaching the lower part of the labeled PST (the adhesive layer); (iii) conversely, the polypropylene backing of OT strongly opposes the penetration the NSF.

From these experiments, the composition of the fluid phase diffusing inside the PSTs remains unclear. To address this issue, we designed a complementary experiment to determine the axial penetration of the components with different polarity of the complex fluid. Unlabeled PSTs were attached to coverglasses and covered with the gel loaded with a doubly labeled NSF, containing 10⁻² mM Rh 110 (green) and 10⁻² mM NR (red). The two dyes are characterized by extremely different affinities for water and organic solvents: Rh 110 is hydrophilic, while NR has a marked hydrophobic nature. The penetration degree of the two dyes can be considered a reliable representation of the penetration extents of the components of the NSF with different polarities. Fig. 5 A and B shows some representative 3D reconstructions of NR−Rh110 doubly labeled NSF/gel after 20 min of interaction with unlabeled FPP. The whole paper backing is soaked with the NSF, with homogeneous fluorescence of the dyes visible as a continuous layer overlying the adhesive (unlabeled and transparent, and thus not visible). Interestingly enough, a different distribution of the two dyes is observed: The hydrophilic green dye is more evenly distributed compared to the hydrophobic red dye. This is consistent with a higher affinity of the hydrophilic components of the NSF for the cellulosic structure of FPP. Concerning MT (Fig. 5 C and D), after 20 min of interaction, the hydrophobic red NR penetrates more deeply the PST compared to the hydrophilic green dye. Therefore, the NSF organic components have a higher affinity for the MT backing than the hydrophilic ones. Finally, Fig. 5E reports vertical sections of the NSF/gel after 20 min of interaction with unlabeled OT. The perfect colocalization of green and red, represented in yellow, highlights that the NSF components remain confined in the gel, without penetration in the backing, nor in the underlying adhesive layer.

Fig. 5.

Confocal microscopy of unlabeled PSTs interacting with p(HEMA)/PVP loaded with Rh110-labeled (green) and NR-labeled (red) EAPC NSF. (A and B) A 3D reconstruction of FPP PST after (A) 10 min and (B) 20 min of interaction; after 20 min, the unlabeled backing is green, due to the penetration of the hydrophilic components of the NSF, while the adhesive remains unlabeled. (C) A 3D reconstruction and (D) a 2D vertical section of MT PST after 20 min of interaction, with the channels displayed both separately and overlaid (with emission colocalization in yellow): The backing transparency allows visualizing the adhesive, backing, and gel, showing that the doubly labeled NSF migrates in the backing, reaching the first layer of the adhesive enriched with the NSF organic components [consistent with the higher intensity of NR fluorescence (red) compared to the hydrophilic Rh 110 fluorescence (green)]. (E) A 2D vertical section of OT PST upon 20 min of interaction, with the channels both displayed separately and overlaid (the colocalization of Rh110 and NR fluorescence is highlighted in yellow): The transparency of the backing allows visualizing adhesive, backing, and gel, showing the confinement of the NSF in the hydrogel, with no significant penetration in the backing and adhesive, which remain unlabeled. (Scale bars, 50 μm.)

Overall, these experiments provide fundamental knowledge on the interaction mechanism of the NSF with the different PSTs.

For FPP, the NSF does not chemically modify the backing and adhesive but can efficiently penetrate across the backing. When diffusing within the PST, the NSF structure is partially disrupted, and a water-enriched fluid reaches the lowest part of the backing, in contact with the adhesive. This evidence can possibly explain the non-Fickian diffusion of NR across the backing.

Concerning MT, the NSF produces chemical modifications in the backing, solubilizing some of its components (diethyl phthalate), then efficiently crosses the backing with a purely diffusive mechanism to reach the farthest region of the backing, with final adsorption inside the adhesive. In this case, the organic component of the NSF is able to penetrate at higher depths compared to the aqueous components.

In the case of OT, the NSF does not modify the backing and the adhesive and is not able to penetrate the entire thickness of the backing.

The gel-confined NSF can then remove FPP-type and MT-type PSTs from artworks, while NSFs with a higher amount of organic solvents have to be designed to tackle the OT removal.

Removal of PSTs from Artworks.

The removal of aged PSTs was performed with the same procedure for both drawings: The NSF-loaded hydrogel was applied on the PST after being shaped to match its exact profile and size. After 5 min, the softening of the PST was tested with a scalpel. Detaching was performed by a very gentle mechanical action, without risk of abrasion of the underlying paper support. Fig. 6A displays the entire procedure leading to the removal of the PST from the 16th-century drawing, while Fig. 6 B and C shows the artworks after removal. The confinement of the EAPC NSF inside the hydrogel permitted a safe application, avoiding lateral migration of the liquid. Thanks to the facile handling, which permits removal and reapplication of the loaded hydrogel if needed, the minimum time of interaction between the NSF-loaded hydrogel and the PST is readily determined.

Fig. 6.

(A) Removal of a PST from the bottom of the 16th-century drawing. The detail shown in the red square highlights the EAPC-loaded hydrogel shaped to precisely match the PST to be removed to avoid contact between the cleaning system and the artwork. (B) Detail of the drawing after removal of the PST, where the inscription “di mano di Michelangelo” appears, probably a false attribution which was concealed by the collector. (C) Contemporary drawing by Helen Phillips Hayter after removal of the PST. Inset shows the detail of PST before removal.

After the successful removal of the two PSTs from both the ancient and the contemporary drawing, an FTIR ATR analysis was performed (SI Appendix, Fig. S10). The PST removed from the Helen Phillips Hayter drawing is made of a cellulose acetate-based backing with an acrylic adhesive, which makes it consistent with the composition of the MT sample. On the other hand, the PST removed from the 16th-century drawing consists of a cellulose backing with a rubber-based adhesive. Due to the backing, we expect a mechanism of diffusion similar to FPP. Since this adhesive is different from the three selected model samples, this evidence shows that the proposed system can be applied to a large variety of PSTs.

In the discussed examples, the intervention on aged PSTs aims at softening the adhesive to facilitate the backing removal, minimizing the mechanical action needed. In some cases, the PST adhesive residues can be found even inside the paper fibers, due to complex aging processes or inappropriate solvent applications by previous restoration interventions.

Fig. 7A displays a ballpoint pen and tempera drawing by the 20th-century celebrated Italian artist Lucio Fontana, with defacing discolorations, due to aged adhesives deeply penetrated inside the paper fibers, in the absence of the backing.

Fig. 7.

Lucio Fontana, Untitled, 1956 (ballpoint pen and tempera on paper) (A) before and (B) after removal of PSTs adhesive residues upon application of the chemical hydrogel.

The ink found on this drawing is a black ballpoint pen ink from the 1950s. Ballpoint pen inks consist of organic dyes dispersed in a mixture of appropriate solvents containing various additives. These inks are very sensitive and may be altered by either organic solvents or water, depending on the specific composition (28). For these reasons, and due to the obvious difficulty of assessing the exact composition of each ink on original artworks, the treatment of drawings made using ballpoint pens as artistic media is particularly challenging to conservators, making the wet cleaning of areas containing ballpoint pen strokes almost impossible.

The use of the system proposed in this work enables the control of penetration and lateral spreading of the liquid phase, minimizing the contact with sensitive components of the artwork and limiting the possible movements of the inks. The application of the previously described hydrogel successfully allowed the softening of the penetrated adhesive and its removal (Fig. 7B).

These case studies demonstrate that a highly versatile tool is available to conservators, guaranteeing complete control of the removing fluids during all of the steps required for PST removal from paper artworks.

This confinement-based methodology ensures the achievement of unprecedented safe and efficient removal of PSTs from paper artworks, thus restoring their full readability for public enjoyment.

Materials and Methods

PST Models.

OT was purchased from Tesa (product code 56100), MT is from 3M, and FPP is from Neschen; analysis of backing was carried out after removal of the adhesive with isopropanol.

Microemulsion Preparation.

EAPC system is an oil-in-water microemulsion prepared by dissolving the SDS surfactant in water and propylene carbonate. Ethyl acetate as dispersed organic phase and the cosurfactant 1-pentanol were added drop-wise to the aqueous surfactant solution.

Hydrogel Preparation.

Semiinterpenetrating p(HEMA)/PVP hydrogel networks are obtained through free radical polymerization of HEMA using azoisobutyronitrile (AIBN) as an initiator and N,N-methylene-bis(acrylamide) (MBA) as cross-linker. Hydrogels were loaded with the nanostructured cleaning fluid (i.e., EAPC) through immersion for at least 12 h.

PST Characterization.

PST characterization was performed through SEM, ATR-FTIR, and thermogravimetry.

EAPC Nanofluid Interaction with the PSTs.

EAPC nanofluid interaction with the PSTs was evaluated through thermogravimetric analysis and a designed steady-state fluorescence kinetic experiment.

p(HEMA)/PVP EAPC Interaction with PSTs.

p(HEMA)/PVP EAPC interaction with PSTs was investigated through CLSM. Full methods and materials are available in SI Appendix.