Free-liquid trials—mock-ups
As expected, the neat cyclic silicone solvent D5 performed poorly with respect to cleaning efficacy [40, 41]. For the alkyd-based mock-up, no pigment pickup or gloss change was noted with this solvent, however slight pigment pickup was witnessed on the MSA (Magna equivalent) and oil-based Ben-Day dot mock-ups. D5 spreads on contact with paint surfaces and its slow evaporation rate delayed the visual assessment of treated surfaces. The hydrocarbon solvent Shellsol D40 also exhibited poor cleaning efficacy, and unsurprisingly caused pigment pickup on the MSA mock-ups and to some extent on the Ben-Day dot mock-ups (Additional file 1: Table S4). Deionised water exhibited an adequate cleaning efficacy and caused no detectable physical changes to both the alkyd and MSA-based mock-ups; whilst some pigment pickup was noted on the blue Ben-day dot mock-up.
The addition of tri-ammonium citrate (TAC) chelator to the DI water at a concentration of 0.5–1% w/w, resulted in increased soiling removal efficacy to a moderate level for the MSA passages, and also performed well on the alkyd priming mock-up. Aqueous solutions with added non-ionic surfactants (see Table 2) also performed well, with improved cleaning efficacy noted with increased surfactant concentrations up to 1% w/w. Whilst no visual changes were observed on the mock-up surfaces after the use of aqueous surfactant solutions, foaming was noted for these systems on all MSA paints tested (Additional file 1: Table S4). As expected, foaming was more pronounced when concentrations were higher, confirming that lower quantities (e.g. 0.5% w/w, up to 10× the critical micelle concentration [CMC])Footnote 17 were optimal. Where performed, the deionised water clearance step often resulted in additional soil removal and did not produce significant alterations to any paint surfaces.
Free-liquid selection—Whaam!
Figure 5 summarises the results for preliminary tests carried out on discreet areas of the painting,Footnote 18 with the most promising cleaning liquids, i.e. aqueous-based solutions with added TAC or non-ionic surfactant (see Table 2) at pH 5.5–6.5. None of the solutions tested caused discernible swelling or pigment pickup, however the Magna passages proved sensitive to mechanical action with prolonged swabbing action (i.e. more than 10 rolls). The 0.5% w/w TAC solution rated highly for the Magna paint passages with respect to soiling removal efficacy and paint surface integrity, however this solution proved less effective on the alkyd priming. The two non-ionic surfactant solutions did not perform as well as the TAC solution with respect to cleaning efficacy on both the alkyd priming and Magna paints, and, as was noted for the MSA mock-ups, foaming was also observed with the surfactant solutions—in this case most pronounced on the black Magna mock-up. Further tests using increased concentrations of up to 1% w/w surfactant and/or chelator (data not shown) resulted in enhanced soiling removal, nonetheless, any advantage was countered by the foaming observed for all surfactant-containing options.
Additional trials were also performed on Whaam! using pH buffered waters [19], as listed in Table 2. These were created to evaluate the effect of pH on cleaning efficacy (MES buffered waters) and to compare the effects of citric acid-buffered waters to TAC solutions. Although moderate soil removal was noted using the range of buffered waters, aqueous solutions with added TAC consistently produced optimal results across the painting surface with respect to soiling removal efficacy and the retention of paint surface character. Figure 6 shows the corresponding star diagram for the alkyd priming evaluations, where the 0.5% w/w TAC solution (at pH ~ 6.5) performed similarly to the MES water buffered to pH 6.5; however, in other areas of the painting, the TAC solution offered a more consistent soiling removal action, which was key to the success of this treatment.
Gel and emulsifier selection—hydrogel evaluations—mock-ups
During this phase, the range of confining materials, i.e. gels and emulsifiers (Table 2) were tested as prepared with deionised water only to compare their ease of use, inherent cleaning efficacy and physical properties such as transparency/opacity and relative conformation to the various paint surfaces. The results are presented for the black MSA mock-up (Figs. 7 and 8) and the oil Ben-Day dot mock-up (Fig. 9). The black MSA mock-up proved particularly useful due to the relative visibility of the cleaning tests, which informs treatment options for these relatively under-studied paints [27].
The agarose and gellan gum options offered minimal cleaning efficacy after a 1-min exposure and a second application using a 5-min exposure produced similar results. No swelling, blanching, pigment pickup or gloss change was observed on any of the mock-ups using these gels. Their application and removal was straightforward, though it was noted the gels could break apart when handled. Surface contact was also not optimal and contributed to uneven soil removal and the use of weights was not desirable for Whaam! After the removal of the gels, small water droplets were also observed on the mock-up surfaces, which required removal using a dry cotton swab. Due to these issues, the two rigid polysaccharide gels were not taken forward for further modification and evaluation.
For the Pickering silicone emulsifiers (Table 2), promising results were obtained with respect to soil removal with the KSG 350z option (see Figs. 7 and 9). However, the low evaporation rate of the clearance solvent (D5)Footnote 19 made it difficult to assess paint surfaces within a suitable period. It was also observed that the removal and clearance procedure is relatively time-consuming. Initially, clearance procedures were performed using a cotton swab dipped into D5 solvent, which was lightly dried off onto a paper towel and then rolled onto the mock-up surface. In this case, the emulsifier could be spread across the paint surface, making removal more difficult. The procedure was then modified by removing the bulk of the emulsifier with a dry swab, followed by a series of D5-dipped hand-rolled swab applications (average of 4). This extended clearance procedure resulted in paint softening, with consequent pigment pickup and surface changes noted for the MSA and Ben-Day dot mock-ups, as also noted during the free-liquid trials. As a result, the emulsifiers received a low rating for surface integrity and pigment pickup (Figs. 7 and 9), in addition to health and safety concerns around the risks associated with silicone materials.Footnote 20
Nanorestore Gel®Extra Dry (now MWR) is a rigid gel which was easy to apply and remove from paint surfaces. The gel did however partially release water onto mock-up surfaces, which was removed via blotting using a Whatman filter paper. This hydrogel resulted in a moderate cleaning efficacy (see Figs. 7 and 9), which represented an enhanced performance when compared to agarose and gellan. No residues were noted, and the paint surfaces did not appear to be affected when examined under magnification (Figs. 8 and 9).
Both Nanorestore Gel® Peggy 5 and Nanorestore Gel® Peggy 6 performed optimally regarding soil removal, ease of use, and health and safety. They also conformed more fully to the paint and canvas textures than any of the other non-spreadable gels due to their unique flexibility; where contact with the paint surface could be further improved using light finger pressure. Their application and removal required a gentle placing onto, and then peeling away of the gels from the paint surface. Microscopic examination revealed that the artificial soil layer had been substantially reduced (see Figs. 8 and 9); however, as remnants of the soil remained, longer exposure times and the addition of chelators and/or surfactants to the aqueous phase would enhance cleaning efficacy. The presence of residual soiling was naturally more evident on lighter samples, such as the alkyd priming mock-up (see Fig. 9) and the yellow MSA samples (see Additional file 1: Figure S2). No gel residues were observed for any of the Nanorestore Gels via microscopy (Figs. 8 and 9; Additional file 1: Figure S2), however it was noted that the gels were tacky enough to pick up fibres from the blotting paper (Whatman filter paper) used to dry off the gels prior to use. It was also noted that these gels can deposit a very thin layer of water onto the paint surface, which can be reduced by blotting the gels more thoroughly prior to application and/or very lightly blotting the painting surface immediately after the removal of the cleaning and/or clearance gels.
In summary, for the deionised water-prepared range of gels and emulsifiers, the Nanorestore Gel® products Peggy 5 and Peggy 6 proved to be the most suited to the mock-up samples. They offered a moderately high cleaning efficacy with the advantages of minimal mechanical action, no pigment pickup and the ability to judge the surface soon after application and clearance. Hence, these gels were taken forward to the next evaluation phase, with the Velvesil Plus silicone emulsifier also included as an initial comparison. In retrospect, the KSG-350z emulsifier would have been a more helpful choice due to its enhanced modifiability over Velvesil Plus [6, 21, 24].
Optimising gels and emulsifiers—mock-ups
The initial series of gel/emulsifier evaluations confirmed that even when using deionised water alone, the rigid gels and emulsifiers offered significant advantages over the application of the free-liquid options with respect to cleaning efficacy; minimising unwanted changes to paint surfaces and reducing mechanical action. Trials were therefore expanded to include optimised aqueous solutions (see Fig. 6). Further investigations were carried out into different tissues for blotting the gels before application which determined that non-cellulosic, non-woven Evolon tissueFootnote 21 proved helpful for drying the gels prior to use, with no fibre transfer and a comparable dry absorption capacity to Whatman paper.
Figure 10 contains an annotated diagram of the optimised aqueous gel tests performed on the black MSA mock-up, with corresponding star diagrams. Although the addition of chelators and/or surfactants improved the cleaning efficacy, it was noted that the relatively long exposures and clearance steps required for the silicone emulsifier resulted in softening of the paint and pigment pickup in some cases (e.g. for the oil Ben-Day dots); hence these materials were excluded from further trials.
As reflected in Fig. 10, among the Nanorestore Gel® products evaluated, the Peggy 5 and Peggy 6 gels proved optimal. Their unique flexibility and enhanced conformation to the substrate resulted in the most homogeneous removal of the soiling layer when compared to the Nanorestore Gel® Extra Dry (now MWR) and other gels/emulsifiers evaluated.
Similar cleaning efficacy results were obtained with both Peggy gels loaded with enhanced aqueous cleaning solutions, across the range of mock-ups. The gels loaded with TAC at 0.5% w/w consistently proved to be the most suitable, receiving high scores for all empirical criteria. For the tests where non-ionic surfactants (initial concentration 1% w/wFootnote 22) were added to the aqueous phase, it was noted that the Peggy gels tended to become slippery and difficult to handle, with a loss of surface contact. As a result, a low score was allocated for the “ease of use” parameter, and observations of changes to the paint surface (e.g. slightly patchy, matte paint after both the cleaning and clearance steps) is also reflected in Fig. 10. Additional tests were performed with reduced concentrations of the surfactants, where it was observed that for concentrations equal or higher than 0.75% w/w, the gels became difficult to handle and resulted in reduced contact with the substrate.
Figure 11 includes images of the black MSA mock-up cleaned with the Peggy 5 and Peggy 6 gels respectively. Both gels offer a similar cleaning efficacy, however as none of the options entirely removed the soiling layer, higher concentrations of surfactant and/or chelators, or longer exposure-times, or both, were considered. However, for the reasons outlined above, the use of surfactants in these gels was not taken forward. Although similar results were obtained using both Nanorestore Gel® Peggy gels; Peggy 6 was selected for preliminary testing on Whaam! as it is slightly more flexible, offers enhanced conformation to the surface, and as it is less opaque, it also offered enhanced visual access to the paint surface during application.
Cleaning system residue evaluation—mock-ups
The black MSA mock-up was also used to investigate the possible presence of cleaning system residues (gel, chelator, etc.) remaining on paint surfaces after clearance, which forms a necessary part of the evaluation of any novel cleaning materials. Alongside visual observations and microscopy (Figs. 7, 8, 9, 10, 11, and Additional file 1: Figure S2), two infrared spectroscopy techniques; ATR-FTIR and micro-reflectance 2D-imaging were employed (see “Instrumentation”).
For the range of gels and emulsifiers evaluated (Table 2), no residues were detected on the mock-ups using the ATR-FTIR system. Analysis of a select group of cleaned samples using microFTIR-2D imaging proved to be more sensitive, offering a lower detection limit of ~ 5 mg/m2. The presence of possible residues on the black mock-up surface were explored via mapping characteristic absorption bands from each cleaning material: for example, the silicon-methyl stretching band at 1260 cm−1 for the silicone emulsifier, and the C–O stretching band for polyvinyl alcohol at 1251 cm−1 for the Nanorestore® Peggy 6 gel. As noted during the ATR-FTIR analysis, the presence of absorption bands from the artificial soiling components dominated the acquired spectra. However, it was still possible to detect residues on areas cleaned with the silicone emulsifiers Shin-Etsu KSG 210 and Shin-Etsu KSG 350z, as shown in Fig. 12, though it is also noted that the proportion of emulsifier used was relatively high. The presence of silicone emulsifier residues has however been reported elsewhere, including when sponges were used (instead of swabs) for the clearance step [40]. Under the same analysis conditions, no residues were detected for any of the Nanorestore Gel® Peggy 5 and Peggy 6 gel options, shown in Figs. 13 and 14 respectively.
The Nanorestore Gel® Peggy 6 system was further investigated via simulating the cleaning procedure on a series of unsoiled MSA paints [Golden Artist Colors], see “Instrumentation”. For these samples, key evaluations were carried out using the Nanorestore Gel® Peggy 6 loaded with deionised water and with an aqueous solution containing 1% TAC w/w, with exposure times of 2 and 15 min respectively, cleared via applying a hydrogel for the same period. Peggy 6 gel residues were investigated by mapping the polyvinyl alcohol C–O stretching band at 1251 cm−1, whilst TAC residues were explored through mapping the symmetric oscillation band of the carboxylate ion at 1550 cm−1. Representative FTIR-2D imaging maps for the Azo Yellow MSA paint sample are shown in Figs. 15 and 16. In summary, there were no detectable residues on any of the unsoiled MSA mock-ups ascribable to the Peggy 6 gels and TAC solutions, confirming that any residues remaining on these paint films fall below the 5 mg/m2 instrument detection limit.