Cleaning painted surfaces: evaluation of leaching ...

Cleaning painted surfaces: evaluation of leaching phenomenon induced by solvents applied for the removal of gel residues Antonella Casoli, Zaira Di Diego & Clelia Isca

Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-014-2658-5

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Author's personal copy Environ Sci Pollut Res DOI 10.1007/s11356-014-2658-5

CHEMISTRY IN A SUSTAINABLE SOCIETY

Cleaning painted surfaces: evaluation of leaching phenomenon induced by solvents applied for the removal of gel residues Antonella Casoli & Zaira Di Diego & Clelia Isca

Received: 21 June 2013 / Accepted: 27 January 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Cleaning is one of the most important, delicate, and at the same time controversial processes in the conservation treatment of paintings. Although a strict definition of cleaning would be the removal of dirt, grime, or other accretions (surface cleaning), in the conservation field, cleaning is used in the broader meaning to include thinning/removing altered or “unwanted layers” of materials without damaging or altering the physicochemical properties of the surfaces to be preserved. The cleaning of unvarnished paintings is one of the most critical issues that are currently discussed. Several studies exist regarding different cleaning tools, such as gels, soaps, enzymes, ionic liquids, and foams, as well as various dry methods and lasers, but only a few have been performed on the risk associated with the use of water and organic solvents for the cleaning treatments in relation to the original paint binder. The aim of the study is to verify analytically the behavior of water gelling agents during cleaning treatments and the interaction of the following elements: water or organic solvents applied for the removal of gel residues with the original lipid paint binder. For this purpose, the study was conducted on a fragment of canvas painting (sixteenth to seventeenth century) of Soprintendenza per i Beni Storici, Artistici ed Etnoantropologici del Friuli Venezia Giulia (Superintendence for the Historical, Artistic and Ethnoanthropological Heritage of Friuli Venezia Giulia), Udine by means of Fourier transform infrared spectroscopy, gas chromatography/mass spectrometry, and scanning electron microscopy.

Responsible editor: Philippe Garrigues A. Casoli (*) : Z. Di Diego : C. Isca Dipartimento di Chimica, Università degli Studi, Parco Area delle Scienze 17/a, 43124 Parma, Italy e-mail: [email protected]

Keywords Cleaning surfaces . Painting . Gelling agents . FTIR spectroscopy . Gas chromatography/mass spectrometry . Scanning electron microscopy

Introduction In traditional procedures, the cleaning of the painted surface is carried out through organic solvents (to obtain solubilization) and mechanical action. The greatest drawback is the poor selectivity of organic solvents, but there is also the risk of dirt redistribution when using solvents. Moreover, they have a low environmental safety. The use of a pure solvent may cause detrimental effects: swelling or leaching of binders and varnishes. Polymers like some cellulose ethers (i.e., Klucel®) increase the viscosity of polar solvents and form gels, thus controlling diffusion onto and under the treated surface. Unfortunately, only the most polar solvents (i.e., alcohols and their mixtures) can be gelled; in addition, the adhesiveness of the gelling material can lead to permanence of residual materials on the surface, even when a proper posttreatment rinsing is carried out (Cremonesi 2004). More recently, the American conservation scientist and restorer Richard Wolbers has proposed the use of higher-viscosity solvent-surfactant gels, based on thickeners derived from polyacrylic acid (i.e., Carbopols® and Pemulens®) which possess less adhesive power and are more easily cleared off from the surface (Wolbers 1990, 2000). The permanence of residual gelling material is then minimized (Stulik et al. 2004). Aqueous methods for the cleaning of movable paintings and other polychrome works of art were originally developed in a systematic way in the 1980s also by Wolbers (Wolbers 1990, 2000, 2004). Since then, the “aqueous approach” has become well established on a worldwide scale, and in many countries, various conservators and scientists have contributed to disseminate it in the conservation communities, although

Author's personal copy Environ Sci Pollut Res

making some degree of adaptation according to local tradition, techniques, and material availability (Khandekar 2000; Hackney et al. 1990; Cremonesi 2012). The cleaning of works of art still relies on organic solvents; however, for the purpose of removing aged film-forming materials, the aqueous approach can be often performed as effectively and with a higher degree of safety for the operator’s health and for the structural integrity of the work of art (Wolbers 2000, 2004). More precisely, the general term “cleaning” refers to two distinct types of intervention, profoundly different one from the other for the concerns on ethics, expectations, result, and perception (Cremonesi and Signorini 2012): –

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“Surface cleaning,” i.e., removing soiling material from the surface of an artifact while conserving any filmforming material (a varnish, an unvarnished paint layer, etc.) as unaffected as possible The partial or complete removal of layers of altered and degraded film-forming materials (varnishes, retouchings, and overpaints)

Nowadays, technology has developed considerably and there are innovative gel systems that have several advantages over other gels used in art conservation. For example, Carretti et al. (2009) have reported on PVA-borax-based highly viscous polymeric dispersions, which can be prepared with water and several organic solvents and applied for the cleaning of artistic surfaces such as paintings. These systems are more elastic than solvent gels. As a consequence, they can be peeled off from the surface after cleaning, minimizing, or avoiding the presence of residues. They require no afterwashing to remove residues, their cleaning action is easily controlled, and they can be removed in one piece by peeling. Another example is the study of Domingues et al. (2013). They have reported on the use of chemical gels based on semiinterpenetrating p(HEMA)/PVP networks for the cleaning of water-sensitive substrates. These gels can be loaded with water or water-based systems (e.g., o/w emulsions) and have proved to be effective in removing dirt from sensitive surfaces without leaving residues and guaranteeing a controlled water release. The gel mechanical properties and the water “mobility” can be tailored to the required characteristics for specific conservation issues by changing the gel composition, making these new polymeric hydrogels the most advanced systems for the cleaning of water-sensitive artifacts. In this paper, the authors considered physical gels only as they are still widely used. Aqueous methods often represent the most appropriate means of surface cleaning an artifact, if the specific surface possesses at least a minimal degree of water tolerance; furthermore, by simply varying specific “conditions” such as pH, ionic strength, and viscosity and by adding specific “active principles”

such as surfactants, chelators, or enzymes, the same aqueous medium can be made effective in performing the “second level of cleaning,” i.e., the removal of aged, deteriorated film-forming materials. In this latter action, the aqueous medium can often prove safer, as compared to organic solvents (Wolbers 2004), with respect to the interaction with paint binding media. As an example, the aqueous medium within acidic to slightly alkaline pH does not play a significant leaching action on an oil-based paint medium; conversely, all organic solvents, regardless of their polarities, are able to leach out oil-bound paints (Hedley et al. 1990; Michalski 1990; Mecklenburg et al. 2013). Up to date, aqueous cleaning methods have received less attention, as compared to organic solvents, with respect to the analytical characterization of their action and, even more important, their interaction with the constituent materials of the painting (Burnstock and White 1990; Hedley et al. 1990; Burnstock and Learner 1992; Lang 1998; Morrison et al. 2007; Campani et al. 2007; Ledesma et al. 2008). Undoubtedly, in many instances, their intrinsic risk factor appears lower than with solvents; nonetheless, in some instances, a more accurate analytical approach is necessary to ascertain the suitability of their use on artworks, in particular addressing the issue of residues left over an artifact.

Aims of this work Water is a high-diffusing solvent, due to its low viscosity and high surface tension; furthermore, the latter property is responsible for the scarce wetting ability of water onto hydrophobic surfaces. Usually, the most important issues on the use of liquid water systems for the cleaning of paintings are related to the penetration of the solution into the paint layer and to the consequent swelling of the polymeric materials it consist of. This effect has, as a main consequence, the final reduction of both the strength and the mechanical stability of the paint layer. Gelling agents increase the viscosity of water, and this strategy is commonly used to decrease the diffusion rate and improve surface wetting when using aqueous liquid cleaning solutions. However, gelling agents are solid, nonvolatile polymers, with variable degrees of adhesiveness. Therefore, a proper rinsing procedure must be adopted to remove residual gelled material; nonetheless, even in the most successful instance, the likelihood of residual gelling material remains real (Cremonesi 2004). This study aims at addressing this critical issue, considering three gelling materials commonly used on polychrome artworks and exploiting the fragment of a canvas painting as test substrate. Aqueous solutions are often used in a gelled form when the surface to be treated has some degree of sensitivity to water. The crucial step then becomes the posttreatment clearance that inevitably has to be carried out with free water, effective on


hydrophilic gel residues but potentially harmful to the surface. To avoid this conflict, conservators often replace the aqueous clearance with a nonpolar hydrocarbon rinsing, which does not interfere with the surface. However, it remains to be verified whether such a nonpolar rinsing would still be effective in removing residual gelling material. This was one of the aims of our study. In addition, by analyzing the cotton swabs used to remove the applied gels and to carry out the following rinsing, we planned to test whether gelling could actually reduce the degree of interaction of an aqueous solution with the paint binder. The study was conducted on a fragment of a painting on canvas (sixteenth to seventeenth century) of Soprintendenza per i Beni Storici, Artistici ed Etnoantropologici del Friuli Venezia Giulia (Superintendence for the Historical, Artistic and Ethno-anthropological Heritage of Friuli Venezia Giulia), Udine by means of Fourier transform infrared spectroscopy, gas chromatography/mass spectrometry, and scanning electron microscopy coupled with energy-dispersive X-ray.

Materials and methods Samples The polychrome substrate used for this study was a 28×12-cm fragment of a painting on canvas (sixteenth to seventeenth century), of unknown provenance (Fig. 1), from which small

samples (such as sample 3A) were taken for analysis and characterization. Micro-Fourier transform infrared spectroscopy (FTIR) A preliminary characterization of organic and inorganic matter was performed by means of micro-Fourier transform infrared spectroscopy analysis. A Thermo Nicolet Nexus 5700 spectrometer coupled with a Nicolet Continuum FTIR microscope fitted with an MCT detector cooled by liquid nitrogen and a 15× Thermo-Electron Infinity Reflachromat objective with tube factor of 10× was used. The spectra were acquired in attenuated total reflection (ATR) mode in the range of 4,000–650 cm−1 at a spectral resolution of 4 cm−1. A total of 64 scans were recorded and the resulting interferogram was averaged. Gas chromatography-mass spectrometry (GC-MS) The determination of the lipid and protein material was carried out by means of GC-MS (Casoli et al. 1996). A FOCUS GC (Thermo Scientific) coupled to DSQ II (Thermo Scientific) with single quadrupole and split-splitless injector was used. The mass spectrometer was operated in the EI positive mode (70 eV). The carrier gas was helium. Separation of components was done by means of a fused-silica capillary column (RXI-5, Restek) with a 0.25-μm (30 m×0.25 mm×0.25 μm) methyl silicone (5 % phenyl) film and the injector was used in splitless mode. The analytical method used was based on a combined procedure for the characterization of lipid and proteinaceous materials on the same sample. The internal standards considered were heptadecanoic acid (50 μl of a 0.1-mg/ ml solution w/v) for the analysis of fatty acids and norleucine (50 μl of a 0.1-mg/ml solution w/v) and norvaline (50 μl of a 0.01-mg/ml solution w/v) for the amino acid analysis; the analysis was conducted on 1 mg of paint samples. Fatty acid analytical procedure The material was treated with 4 N HCl in methanol (1 ml) and n-hexane (1 ml) for 2 h at 50 °C. The n-hexane phase, which contains fatty acid methyl esters, was used for gas chromatography analysis (1 μl). Separation of the methyl ester of fatty acids was achieved following this temperature program: isothermal conditions at 80 °C for 2 min, with 20 °C/min heating up to 280 °C and isothermal conditions at 280 °C for 6 min (total run time 18 min). The mass spectra were collected in total ion current (40–500 m/z fragmentation rate).

Fig. 1 Paint fragment on canvas (sixteenth to seventeenth century)

Amino acid analytical procedure After evaporation to dryness of the methanol phase, the residues were dissolved in 6 N hydrochloric acid (2 ml) and hydrolyzed in a screw-capped container for 5 h at 100 °C in an oil bath. After evaporation to dryness, the hydrolyzed residues were esterified using 3 ml of


2 N HCl in propan-2-ol at 90 °C for 1 h. After cooling, the solvent was evaporated under vacuum and the residue of the paint was dissolved in 2 ml of dichloromethane and derivatized with 0.2 ml of trifluoroacetic anhydride at 60 °C during 1 h. After cooling, the solvent was evaporated under vacuum and the residue of the paint sample was dissolved in 500 μl of dichloromethane, then the solution was used for gas chromatography analysis (1 μl). Separation of Ntrifluoroacetyl-O-2-propyl ester amino acid derivatives was achieved following this temperature program: isothermal conditions at 60 °C for 3 min, with 25 °C/min heating up to 260 °C and isothermal conditions at 260 °C for 6 min (total run time 17.00 min). The mass spectra were recorded in selected ion monitoring (140, 126, 154, 153, 139, 168, 182, 166, 164, 184, 180, 198, 91, 190 m/z fragments). The chromatographic peak area of each analyte was integrated, corrected using response factor, and expressed in relation to the internal standard (IS), in order to obtain semiquantitative estimation. Finally, the average analyte/IS ratio was calculated and the average percentage of three runs was taken into account. The IS was subject to the same treatments as the sample. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray The interaction between the gels and the painting surface and the effects of cleaning treatments were evaluated by means of scanning electron microscopy observations (JEOL JSM6400) equipped with an energy-dispersive X-ray analyzer. The images were acquired in secondary electron mode. The elemental composition of samples was evaluated using an acceleration voltage of 20 KeV, lifetime of >10 s, and working distance equal to 15 mm. The samples (fragments of painting) were previously coated with a graphite layer (0.1 μm of thickness) by sputtering (JEOL JEE-4X).

Cleaning treatments For the cleaning tests, gels made with the following gelling agents are applied on the painting surface: 1. Nonionic cellulose ether (hydroxypropylcellulose, Klucel® G) & &

Test 1—Klucel® G in distilled water Test 2—Klucel® G in pH 5.5 acetate buffer

2. Xanthan gum, an anionic polysaccharide (Vanzan NF-C®) & &

Test 3—Vanzan NF-C® in distilled water Test 4—Vanzan NF-C® in pH 5.5 acetate buffer

3. Acrylic acid/acrylate anionic polymer (Carbopol Ultrez 21®) & &

Test 5—Carbopol Ultrez 21® neutralized with NaOH to pH 5.5–6 Test 6—Carbopol Ultrez 21® neutralized with triethanolamine (TEA) to pH 5.5–6

To prevent biodeterioration of the gels used, potassium sorbate is added in very low concentration (0.1‰) to distilled water and to the buffered solutions. For preparing the gels, 3.5 g Klucel® G or 1.5 g Vanzan NF-C® was gradually added to 50 ml of water or 50 ml of acetate buffer while mixing with a metal spatula. Then, the gels are left to rest overnight and became homogeneous. Carbopol Ultrez 21® (0.5 g) was added gradually to 50 ml distilled water; within a few minutes, it became hydrated, and the dispersion was ready for the addition of the base. Polyacrylic acid is a weak acid, and once neutralized with a strong base (i.e., sodium hydroxide), it can act as a buffer. Conversely, with the weak base TEA, this buffering ability is somehow lost. Of 1 M NaOH solution, 1.6 ml is added to reach pH 5.77, while 0.353 g TEA yielded pH 5.85. Both bases were added slowly while carefully mixing with a metal spatula. The viscosity depends mainly on the molecular weight of gelling agent. The molecular weight depends on the degree of polymerization (DP), the number of molecular substitution, and the molecular weight of the individual substituent groups. That is why the viscosity is directly proportional to the DP (the higher value of DP is equivalent of solutions more viscous and less amount of used material). The acrylic acid has the highest viscosity. The cleaning procedure is performed as follows: 1. The pH of the painting surface was measured. 2. The pH and conductivity of the gelled solutions were measured. 3. The gelled solutions were applied by brush and stirred on paint surface for 1 min. 4. The gels were left on the surface for 2 min. 5. The gels were removed with three dry cotton swabs; half of the treated area was rinsed three times with distilled water, using three distinct cotton swabs, and the other half of the treated area was rinsed three times with ligroin, using three distinct cotton swabs. 6. Fifteen minutes later, the pH of the treated area is measured. The pH measurements are carried out through a handheld pH tester with a glass electrode with flat tip. The electrode is placed on the point to be measured on the surface. The gels are applied in different surface areas of the same size in a homogeneous way. Before the applications, the cotton is washed to eliminate hydrophobic and hydrophilic residues. The treatments consist of a sequence of separated washes, with different solvents from low to high polarity:


hexane, ethanol, and distilled water. For every cotton swabs, 30 mg (±1 mg) of material is used in order to have the same quantity for all cleaning treatments and analysis of cotton. The control (washed cotton swab) is analyzed using the same procedure as the samples and not signals attributed to fatty acids are detected. After the treatments, all the cotton swabs collected (from dry gel removal and wet rinsing) are extracted in ethanol (1 ml) at 50 °C for 1 h under magnetic stirring. The solvent is evaporated under vacuum, and the residue is chemically treated for GC-MS analysis for the lipids. The use of ethanol is justified by previous studies about leaching effect on paint surface. The selected solvent shows good extraction ability of the organic compounds present in the cotton swabs after it was applied onto the artifact, for instance for the commonest saturated and unsaturated fatty acids derived from paintings (Berzioli 2010).

in the other side. In Fig. 2, the presence of organic matter was detected by bands in the range between 3,000 and 2,850 cm−1 due to the stretching of C–H groups and 3,300 cm−1 due to the stretching of N–H that is associated to the stretching of amide I (C=O 1,649 cm −1), amide II (N–H bending and C–N stretching 1,547 cm−1), and amide III (N–H bending and C– H stretching 1,319 cm−1) (Kong and Yu 2007). Furthermore, all recorded spectra showed the stretching band of C=O of carboxylic group at 1,722 cm−1 derived from lipid fraction. The sample is not incorporated in resin, so it is possible to turn the side and different points were acquired from the preparation layer. The signals at 1,411 and 869 cm−1 revealed the presence of calcite. The broad signal between 1,200 and 900 cm−1 is ascribable to the presence of matter containing Si– O bonds. The calcium sulfate bihydrate was detected by antisymmetric and symmetric water stretching at 3,500 and 3,400 cm−1 and two bands at 1,681 and 1,619 cm−1 in the O– H bending region. Stretching band at 1,106 and bending vibration mode at 668 cm−1 were derived from sulfate ion (Fig. 3).

Results and discussion Characterization of the materials of the paint fragment

Analysis of lipid and proteinaceous fractions

μ-FTIR analysis of inorganic and organic compounds

First of all, a small amount of the paint layer is scraped from an edge and it is submitted to the reaction with ammonia and hydrogen peroxide (histochemical test). The development of stable lather shows the lipid substance. The reaction in making soap (saponification) is a base hydrolysis of triglycerides to

μ-FTIR investigation is preliminary to GC-MS analysis and it has been carried out on sample 3A. It is a paint fragment that shows the painted layer in the front side and preparation layer 100 99 98 97 96 95 94

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Fig. 2 μATR FTIR spectrum of the sample 3A

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Fig. 3 Comparison between μATR FTIR spectrum of the sample 3A (blue line) and a reference spectrum of gypsum, from chalk sample (IMP00024, Kremer, 5815, GCI, tran) (black line)

make three salts (soap) and glycerol. Then, two samples (3G and 3E) are analyzed by gas chromatography/mass spectrometry. As

Fig. 4 Chromatogram of the lipid fraction of the sample 3G

FTIR spectra revealed the presence of both lipids and proteins, these classes of organic matter are investigated.

Author's personal copy Environ Sci Pollut Res Fig. 5 Chromatogram of the proteinaceous fraction of the sample 3G

The basic methodology relied on the identification of fatty acids and amino acids on the same sample. Two chromatograms are collected for each sample: the first one was from fatty acid derivatives and the second one from amino acid derivatives. The results of GC-MS lipid fraction analysis (Fig. 4) revealed the presence of a low content of azelaic acid and saturated fatty acids, principally palmitic and stearic acids. The presence of azelaic acid, whose formation is due to the oxidation of unsaturated acids, can be attributed to siccative oil. The ratios of azelaic vs. palmitic and palmitic vs. stearic are calculated (for sample 3G, A/P=0.64, P/S=1.09; for sample 3E, A/P=0.88, P/S=1.84) as useful parameters for the characterization of the lipids (Colombini et al. 2002). The considered ratios suggest that the observed lipid fraction could be derived from the presence of linseed oil, but the low values of the azelaic/palmitic ratios suggest that there may be another substance other than lipid in addition to siccative oil medium. The analysis of the amino acidic fraction by GC/MS shows the presence of proteinaceous materials in all the samples. The amino acidic profile of the sample 3G is reported in Fig. 5. The high content of glutamic and aspartic acids in the samples indicates the use of egg as a binder material. The presence of hydroxyproline, marker of collagen, allowed identifying animal glue, presumably due to the trace of the ground in the sample. In conclusion, we think that the presence of both linseed oil and egg suggests the use of tempera grassa technique. We know that the P/S ratios for a tempera grassa should be much higher,

but we believe that the evaluation of P/S ratio requires particular care: a recent study on artificially aged paint layer highlighted the possible decrease of P/S ratio with aging, due to the preferential loss of palmitic acid with respect to stearic acid, under the influence of the concentration of pigments. Moreover, fatty acids, especially palmitic and stearic acids, are abundant in the environment and may contaminate paint layer (hand contact, animal fats for lighting, residues of burning vegetable oils, etc.) (Colombini et al. 2010). Moreover, it is impossible to establish the conservation conditions and previous restorations (and the effect that they have on chemical composition) due to the unknown provenance of painting. Cleaning tests On this fragment, we selected to apply different gelled aqueous solutions, mimicking the intervention a restorer would perform, should he wish to remove soiling material from the surface. Before the cleaning treatments, pH and conductivity are measured for each gel. The surface of the paintings is a complex mixture of proteins and lipids; furthermore, the two film-forming materials, oil and egg, may not have a uniform distribution onto the surface. Therefore, we selected to measure the pH, as a practical means of characterizing the surface. The surface itself is acidic, as shown Table 1. However, values differ within about a one unit pH range. The uneven distribution of oil and

Author's personal copy Environ Sci Pollut Res Table 1 Gelled solutions used for the cleaning treatments, gel pH and conductivity, and pH of the painted surface before and after treatment

Gelled solution

Gel pH

Gel conductivity (μS/cm)

pH of painted surface before treatments

pH of painted surface after treatments

Klucel G® + distilled water Klucel G® + acetic buffer Vanzan NF-C® + distilled water Vanzan NF-C® + acetic buffer Carbopol Ultrez 21® + NaOH Carbopol Ultrez 21® + TEA

6.56 5.59 5.30 5.50 5.76 5.72

90 1,860 950 1,290 1,850 1,390

4.97 6.00 5.75 6.10 5.10 5.17

5.57 5.70 6.00 6.10 5.95 5.10

protein, together with the different pigment composition, may be responsible for these pH variations. Alkaline and even neutral aqueous solutions could ionize acidic components within the film-forming materials, fatty acids in the oil fraction as well as polypeptides in the egg fraction, and render them more hydrophilic; as a result, the surface could swell and become more susceptible to damage. Isoelectric point (pI) values for collagen and egg proteins (ovalbumins, phosvitins, and lipovitellins) lie all in the acidic range, 4.3–6.5 (Ternes 1989). Considering that pI is the point of no net electrical charge, an identical pH value then represents the least solubility in water for a protein. Based on this, our choice of an acidic pH for all gels is regarded as a “safer” condition for the cleaning intervention. As for the conductivity measurements, only the aqueous solutions are analyzed, due to the practical impossibility of measuring in a reliable way the conductivity of the surface itself, at the time this study was carried out. It must be emphasized that, without this comparison, it becomes impossible to predict whether any applied solution would be hypotonic or hypertonic with respect to the surface and whether, as

Fig. 6 Histogram: comparison of the tests (sum of corrected areas of all fatty acids detected)

a consequence of this, it could have a damaging swelling effect on the layers. Thus, the conductivity values can only be regarded as a means of characterizing the ionic concentration of the different tested solutions. After the treatments, the gels are carefully removed from the surface with dry cotton swabs; then, three wet rinses were done to remove any residual gelling material. Half of each treated area was rinsed with water and half with ligroin (dearomatized petroleum ether, 100–140 °C): this allowed to compare the effectiveness of aqueous and hydrocarbon rinsing. The cotton swabs are analyzed by means of GC-MS in order to verify any interaction of water or the solvent with the lipid fraction in the original paint binder. In all cases, the fatty acids detected were palmitic acid (C16:0) and stearic acid (C18:0). In some dried cotton swabs, the presence of azelaic acid (CC9) is also detected; in some cotton swabs from the ligroin rinsing, lauric acid (C12:0) is detected as well. Where dicarboxylic acids are not detected, they fell under the detection limit of the analytical procedure. The action of swabs is chemical and mechanical at the same

S dry removal 1A first rinsing with water 2A second rinsing with water 3A third rinsing with water 1L first rinsing with ligroin 2L second rinsing with ligroin 3L third rinsing with ligroin T1 - Klucel G® i(distilled water;) T2 - Klucel G® pH 5.5 acetate buffer ; T3 - Vanzan NF-C® distilled water; T4 - Vanzan NF-C® pH 5.5 acetate buffer; T5 - Carbopol Ultrez 21® neutralized with NaOH pH 5.5 – 6; T6 - Carbopol Ultrez 21® neutralized with Triethanolamine (TEA) pH 5.5 – 6.


time: the gels and the solvent may have chemical interactions with the painting, while the cotton swabs, rubbed on the surface, may cause mechanical extraction of some fatty acids. In fact, even dried cotton swabs, applied onto the surface without solvent, could contribute to the removal of small quantity of the original paint layer due to their mechanical action. The fatty acids extracted are expressed as average percentage ratio analyte/internal standard (C17), and for each application (dried cotton swabs, cotton swabs with water and with ligroin, separating first, second, and third washing with both solvents), the sum of all the detected fatty acids is considered. All treatments, shown in Fig. 6, revealed that the application of ligroin on the painting surface extracted larger amounts of the lipid fraction, as compared to water application. The nonpolar character of the ligroin may explain this greater action on fatty acids, compounds of prevailing hydrophobic character. This is a demonstration that organic nonpolar solvents are more effective than aqueous media in solubilizing lipids. Even when comparing all various applications, it is difficult to evaluate the effect of the different ionic concentration of the gelled solutions used: it was impossible to measure the surface conductivity; therefore, we could not establish whether a specific gelled solution would be hypotonic or hypertonic, relatively to the surface, and understand whether it would promote water migration into or out of the layers. It can be assumed that the acidity of the aqueous solution causes less ionization—compared to simple water—of the fatty acids; therefore, it gives the aqueous acidic solutions lower extracting ability towards these compounds, relatively to the action of ligroin. This seems to have occurred in test 4, but not in other cases. For test 2 (Klucel® G in acetate buffer), the ratio of the overall lipid fraction extracted (Fig. 7) through the ligroin rinsing compared to that extracted through the water rinsing shows a leaching action 14.5 times higher for ligroin with respect to water. Moreover, if we consider the ratio between the first ligroin rinsing of test 2 and the same of test 1, the leaching action is almost ten times higher for Klucel® G in acetate buffer.

Fig. 7 Histograms of the lipid fraction extracted in test 1 (left) and in test 2 (right), expressed as average percentage ratio analyte/ internal standard (IS)

Furthermore, water appeared to be more effective for the removal of gel residues. All SEM images are difficult to interpret. From them, it seems that the rinsing with water, already after the first treatment, gives a cleaning effect like layer of resin (Fig. 8). In the third rinsing, water images seem to be comparable to that of the untreated surface (Fig. 9). The image of the third washing with water of Carbopol Ultrez 21® neutralized with TEA would appear very similar to the image of an accumulation of Carbopol® found near an edge (due to the presence of a small amount of gel below the mask separation of the treatment zones) (Fig. 10).

Conclusions This research relies on the comparison of different cleaning treatments of a tempera grassa painting, based on gelled aqueous solutions. The aim of this study was twofold: –

–

First was to understand how different gelling agents, capable of producing different viscosities, would affect the diffusion of water and its leaching action—if any— toward the lipid components of the original binding medium Second was to verify whether both water and hydrocarbons could be effective rinsing solvents for removing residual gelling material and whether either or both would exert a leaching action.

In practical terms, the analytical study is conducted in this fashion: all the cotton swabs from the performed treatments, i.e., dry removals of the gels and water and hydrocarbon rinsing, are analyzed by means of GC-MS. Each treatment is repeated three times. Results were difficult to interpret; this is not surprising, and it is a rather common occurrence when conducting tests on real, rather than on simulated, paint samples; however, we believe that this approach can still provide useful information for everyday conservation practice.

Author's personal copy Environ Sci Pollut Res Fig. 8 SEM images (detection of secondary electrons) of the paint surface after treatment with Vanzan NF-C® in distilled water (test 3)

Author's personal copy Environ Sci Pollut Res Fig. 9 Photos at ×2,200 of the untreated surface in secondary electrons (a) and in backscattering (b). Photo at ×800 of the third washing with water in test 3 (Vanzan NF-C® in distilled water) in secondary electrons (c) and in backscattering (d)

No clear answer was obtained, addressing the first aim: it is not easy to point out clear differences among the abilities of the three gelling agents in controlling the diffusion of water and its interaction with the paint medium. In the choice of a specific gelling agent, another factor becomes more important: the ease of clearance of its residues from the treated surface, and this leads to the second aim of this study. Often, in cleaning treatments, water is used in a gelled form just because the surface is rather sensitive to water: once the gel is removed with dry cotton swabs, rinsing is then done with a hydrocarbon solvent (petroleum ether, Fig. 10 a Image in ×800 in secondary electrons of third washing with water in test 6 (Carbopol Ultrez 21® neutralized with TEA) and b image in ×1,100 in secondary electrons of an accumulation Carbopol® on an edge of one zone of treatment

white spirit, gasoline, etc.) to avoid the use of free water. This practice is becoming fairly common in conservation, but it still must be demonstrated that nonpolar solvents can be as effective as water, the most polar solvent, in removing residual water-soluble gelling materials. It is true that all these polymers possess both characters, hydrophilic and hydrophobic, but the latter is not sufficient to give the polymer solubility in the least polar solvents; even more so in the case of ionic polymers, like two out the three tested gelling materials: Vanzan NF-C® and Carbopol Ultrez 21®.


Relating to this topic, this study has succeeded in demonstrating that water is more effective, as compared to ligroin, in removing residual gelling materials; scanning electron microscopy analysis clearly indicates its greater action. Therefore, water should be regarded as the proper solvent for this procedure. Addressing the leaching issue, the results revealed that all treatments caused a partial extraction of the lipid fraction: in fact, palmitic and stearic acids are detected in all samples. This effect was more evident with ligroin, likely due to its nonpolar nature, which is more closely related to that of the fatty acids. Conversely, water seems to have a less invasive action towards the lipid fraction. The role of ion concentration and the differences between simple water and a buffer, given the same pH value, remain to be understood and will be the object of further research. Acknowledgments The authors wish to thank Dr. Paolo Cremonesi who suggested this research, contributed directly to the work, and provided intellectual support and technical help (including writing and result analyses) and Soprintendenza per i Beni Storici, Artistici ed Etnoantropologici del Friuli Venezia Giulia (Italy) for providing a real ancient paint. Special thanks to Paolo Casadio, official of Soprintendenza, to Nicoletta Buttazzoni, restorer, and also to Dr. Stefano Volpin, scientific consultant at Soprintendenza Beni Storico-artistici in Trento (Italy), for his advice about this paper.

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