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Analytical Sciences

Literature Study

The Analysis of Organic Painting Materials

and the Application of Nanomaterials for the

Conservation and Restoration of

Traditional Paintings

by

Paula Helena Sieber

12769266

March 2021

12 ECTS

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AgNP Silver Nanoparticles

CCNF Carboxymethylated Cellulose Nanofibrils CNC Cellulose Nanocrystals

CNF Cellulose Nanofibrils DAD Diode Array Detector DKPs 2,5-Diketopiperizines

EES Excitation Emission Spectra ESI Electrospray Ionization

FLIM Fluorescence Lifetime Imaging

FORS Fibre Optics Reflectance Spectroscopy FTIR Fourier-Transform Infrared

FT-NIR Fourier-Transform Near-Infrared GSA Gecko-like Synthetic Adhesives GC Gas Chromatography

GC-MS Gas Chromatography-Mass Spectrometry HEMA 2-Hydroxymethyl Methacrylate

HPLC High-Performance Liquid-Chromatography IR Infrared

LC Liquid Chromatography

LC-MS Liguid Chromatography coupled to Mass Spectrometry MALDI Matrix Assisted Laser Desorption Ionisation

MBA N,N’-Methylenebis(acrylamide) MIR Mid-Infrared

MMMC Methoxy Magnesium Methyl Carbonate MRS Micro-Raman Spectroscopy

MS Mass Spectrometry

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MWCNT Multi-Walled Carbon Nanotubes NIR Near-Infrared

OCT Optical Coherence Tomography o/w Oil in Water

PCA Principal Components Analysis

PDA Photo Diode Array Ultraviolet absorption detector PEOX Poly(2-ethyl-2-oxazoline)

PL Phospholipids

PCP Poly-(vinylpyrrolidine)

py-GC pyrolysis Gas-Chromatography Q-ToF Quadrupole-Time-of-Flight RF Relative Humidity

RP Reversed Phase

SAXS Small-Angle X-ray Scattering

SERS Surface-Enhanced Raman Scattering SIPNs Semi-Interpenetrated Polymer Networks TAGs Triacylglycerols

ToF Time-of-Flight TC Tropocollagen

UPLC Ultra Performance Liquid Chromatography UV Ultraviolet

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This literature study aims to give an overview of current analytical methodologies for the analysis of traditional easel and wall paintings and discusses their potential limitations and benefits. These methods comprise spectroscopic methods for the determination of organic material classes of pigments and binders, their localisation and the distinction between de-graded and intact areas. Further, Mass Spectrometry (MS)-based techniques that are im-plemented for the identification of specific organic compounds are explained.

The thesis also gives insight into the current state of the application of nanomaterials for conservational treatments on ancient paintings and discusses where interventions have al-ready been improved over traditional conservation actions. It further elucidates some ideas drawn from bionics and the application of biomolecules as paint consolidation agents.

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Abbreviations . . . I Abstract . . . III

1 Introduction 1

2 Organic Analysis of Traditional Paintings 4

2.1 Organic Materials used in Traditional Paintings . . . 5

2.1.1 Easel Paintings . . . 5

2.1.2 Mural Paintings . . . 6

2.2 Analysis of Organic pigments . . . 7

2.3 Analysis of Organic Binders . . . 19

3 Applied Nanomaterials 27 3.1 Preventive Conservation . . . 28 3.1.1 Protective Coatings . . . 28 3.1.2 De-acidification . . . 29 3.2 Remedial Conservation . . . 30 3.2.1 Cleaning of Paintings . . . 30 3.2.2 Removing adhesives . . . 34 3.3 Restoration . . . 36

4 Recommendations for Future Research 40 4.1 Background . . . 40

4.2 Binding properties . . . 41

4.3 Removability . . . 46

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Art works like paintings and murals are part of tangible cultural heritage, which is known as a legacy of humankind and allows for the relation to historical achievements of ancient populations. Benefits like enhanced social inclusion, encouraged dialogue between countries and improved life quality can be enjoyed by increasing the access to these art objects. Besides its immense social and economic value, it has also informational value. For example, for many past human activities and achievements, the only evidence comprises physical objects like written documents and records in historical and archaeological sites [1, 2].

However, paintings and murals are fragile and subjected to natural events, intentional de-struction, and neglect, which leads to a variety of irreversible degradation processes. These include environmental impacts, physical erosion, chemical processes, and disintegration by microorganisms, which all alter the physico-chemical properties of artefacts. This damage on art works can mean a loss of sometimes irreplaceable information. To avoid this, conservation interventions may be performed to protect paintings and to ensure accessibility to present and future generations [3, 4]. In general, these measures and actions comprise preventive conservation, which aim to avoid and minimize further deterioration, and complementary remedial conservation, which aim to arrest the current damaging process or reinforce the structure of the item. In cases where the art work is already highly damaged, restoration is carried out, which aims to facilitate the understanding of an item and to restore the artist’s intention [1].

Preconditions for carrying out appropriate conservation interventions is the understanding of an object by means of its material nature. To make a painting, the paint is firstly prepared by dispersing pigments in binders, and this mixture of substances is then applied in some-times multiple layers onto the support. In the case of murals, the support is a wall, and for easel paintings, it is a panel or a canvas. Finally, the painting might be finished by applying a thin varnish layer. Thus, every conservation treatment starts with the characterization of the painting’s materials and their alterations, and can be facilitated by analytical methods [5]. However, paintings can represent a challenge for chemical analysis, because a potentially wide variety of substances are often heterogeneously distributed throughout the painting. At

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this point, especially natural organic materials of traditional paintings must be mentioned as being difficult to be identified by several reasons. Because the masterpiece often is of sig-nificant age, sampling can lead to impairments of the integrity and appearance of a painting, and removing any substance is often unacceptable at all. On the other hand, if sampling is possible, only minute substances can be removed from the painting to reduce any alterations of the artwork’s integrity. Next to the extreme limitation of available sample, the amount of natural organic pigments is very low as they exhibit strong tinting power, and other organic substances like binders are often highly degraded in such old items. Further, X-ray based techniques that make the identification of inorganic pigments very easy are based on ele-mental signatures and the crystallinity of the sample, and as most natural organic pigments are only poorly crystalline and mainly consist of the same elements, these techniques are hardly useful for the analysis of organic substances in traditional paintings. However, the analysis of organic substances is an important task not only to implement an appropriate conservation strategy, but also for studying art history, to determine degradation pathways and to date and authenticate traditional paintings [6, 7, 8]. Analytical methods for organic molecules have proven to be fundamental in conservation sciences. A variety of different techniques are available and frequently being used for the analysis of traditional paintings. Spectroscopic methods can give details about artist’s painting technique and the presence of certain organic materials classes, and the location of these organic substances and degraded areas can be determined. This knowledge aids in deciding on appropriate sampling points for more sophisticated methods, which yield advanced information that is necessary for the identification of specific substances. In this sense, organic binders and pigments are success-fully identified by methods based on MS. With this obtained knowledge, safer conservation treatments can be performed that are adjusted to the unique challenges of each painting [6, 9].

When chemical knowledge about the materials is gained and degraded areas are identified, conservative interventions can be performed. Actions that are undertaken on paintings must follow the principle of minimal intervention and the principle of stability and reversibility. Thus, only interventions that are necessary to restore the stability of paintings are under-taken in the minimal possible extent. Further, any applied material must be stable in the long-term and entail easy removal [5]. However, it has become evident that traditional con-servation treatments did not underly these principles [10], which makes the development

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and chemical properties of artefacts intact whilst having a low environmental impact, which makes them a promising conservation technique in comparison to conventional materials and techniques [3].

This literature study aims to give an overview of current analytical methodologies for the analysis of organic materials that constitute traditional paintings, including easel and wall paintings. Further, the current state of the art concerning the application of nanomaterials to conserve and restore paintings is given. Since the existing literature lacks studies on the application of nanomaterials for the re-adhesion of lifted paint layers, hypotheses for possible techniques are explored and discussed before the topic is summarized.

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Paintings

Paintings can be analysed to address several questions such as identifying the present com-pounds or uncovering the artist’s painting technique. Ideally, these analyses are performed in a non-destructive way. However, traditional paintings comprise various organic materials that might not be identified by in situ techniques and thus, not all analyses can be per-formed non-destructively. Further, when chemical information about underlying layers has to be obtained, sampling is certainly needed [7].

In cases where full analysis is required to obtain extensive knowledge about a painting, the optimal analytical approach for the analysis of traditional paintings consists of the following stages: firstly, a non-invasive technique is used to scan the surface of an art object to locate sampling sites, followed by sampling on specific points of interest, and finally a micro-destructive, mostly chromatographic technique is performed to answer the posed questions. During this approach, the layering structure can be determined and heterogeneous mixtures that are not visible by the naked eye can be analysed. The order of these complementary techniques is designed to deal with the major challenges when analysing fragile paintings: only minute sample amounts can be taken from a masterpiece in order to safe its integrity and appearance in the most possible way; degradation products might be present; the com-position of the original recipes including various organic binder materials and pigments is often unknown; and chromphores in a given samples are often only present in very low con-centrations due to their high tinting power [7, 9]. Consequently, the non-invasive analysis of these substances is a difficult, and sometimes an impossible responsibility of conservation scientists [11].

In the following section, analytical methods that are useful for the characterization of organic pigments and binding materials are discussed.

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2.1 Organic Materials used in Traditional Paintings

2.1.1 Easel Paintings

Easel paintings are movable artworks with different supports, such as wood or canvas. The layering structure (see Fig. 2.1) starts with a ground layer on the support, followed by the paint layers and might be finished with a thin varnish layer, where each layer serves different functions [12]. The ground layer is applied to generate a uniform and homogeneous layer on which the paint layers exhibit good adhesion. It consists of a filler that smooths the porous support and determines the support’s colour, on which the paint layers are spread. These filler compounds are dispersed in a medium, which holds the filler grains together and allow for adhesion to the support. Before the pigments can be applied onto the support, they are dispersed in an appropriate binder, which consists of organic materials like egg white, egg yolk, casein, or synthetic polymers. Binders were used to improve the texture and to modify the properties of the painting mixtures. This binding medium can be of hydrophilic or hydrophobic nature and hence, tempera, i.e. emulsions of oil and egg, or oil and casein, can be classified next to oil paints, i.e. pure oil medium. Furthermore, lake pigments, which are produced by precipitating organic dyes onto an inert, insoluble and inorganic substrate, have also been used for easel paintings. Organic dyes can be classified regarding their chromophores: anthraquinoid dyes are mostly red, flavonoid dyes were used for yellow (flavones), red and darker (omoisoflavones) lakes, and indigoid dyes were used for blue (indigotin) and purple (brominated indigoid and indirubin) lakes [9]. Finally, a varnish may be applied, which supports depth and colours and provides protection of the paint layers. This external surface layer consists of drying oils, natural resins like dammar or mastic, organic solvents, or polymers [13, 14].

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2.1.2 Mural Paintings

Mural paintings are pictorial surfaces applied onto a wall support. The pictorial layer must adhere sufficiently onto the wall support to result in a long-lasting artwork, and this re-quirement represents an important physico-chemical aspect concerning mural paintings. To enable this, the layering structure of wall paintings begins with the Arriccio, a sand-lime mix, which was applied roughly onto the wall to increase the specific area of this 1st

stra-tum and to enhance the adhesion of the following layer. After perfect dehydration of this layer, the Intonaco was spread on top of the Arriccio. This 2nd stratum again consists of a

sand-lime mix, but this time the sand is very fine (a few µm) to generate a smooth surface and to enable easy application of the pigments. The 1st and 2nd stratum together comprise

the aerial plaster on which the painted layers were applied.

Two painting techniques, namely ‘a fresco´ and ‘a secco´, exist, and which technique the artist used depends on the stability of the pigments in Ca(OH)2 rich environments. During

the ‘a fresco´ technique, the pigments were applied when the Intonaco was still wet. Thus, during drying of the 2nd stratum, a carbonation reaction occurs which leads to the formation

of a complex carbonate network, in which stable pigments were entrapped [15]. This drying is promoted by a chemical reaction with atmospheric CO22. Ca(OH)2 (lime) is transformed

to CaCO3, which functions as the binder of the pigments to the wall, resulting in perfect

adhesion between pigment particles and between the pictorial surface and the aerial plaster. However, some inorganic pigments are unstable in the Ca(OH)2 rich environment of the

wet Intonaco. Therefore, during the ‘a secco´ technique, the pigments are applied with a binder (organic substances like egg, casein, or animal glue and sometimes lipid substances like linseed oil) after drying of the 2nd stratum to result in the final wall painting [13].

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2.2 Analysis of Organic pigments

One of the most challenging tasks is the accurate identification of organic pigments. Simple, fast and clear characterization is hampered, because they have very similar chemical struc-tures. However, this information is required for dating and authentication, and is important for the design and implementation of a safe conservation strategy and for studying art his-tory. Furthermore, mapping degraded pigments makes re-colouration possible, and it helps finding the origin of degradation and the degradation state of the artwork [7, 16].

Spectrophotometric and fluorometric techniques are both analytical methods that promise fast results without expensive sample preparation. In this context, Fibre Optics Reflectance Spectroscopy (FORS) can be used to discriminate between organic substances in paintings. For example, organic red pigments originating from animals (cochineal) or plants (mad-der) can be distinguished. Since madder based lake pigments were used since antiquity, and cochineal based lake pigments are used since Medieval times, this information helps in dating of a painting. The discrimination between these two sources is possible by FORS, be-cause their absorption bands are shifted relative to each other as exemplified by synthesized pigments applied to primed canvasses (see Fig. 2.2, a and c). However, the interpreta-tion of features in reflectance spectra is often difficult, because they show large variability and also depend on the thickness of the applied paint. On the other hand, first derivative transformation of FORS spectra showed less variability and the interpretation is also more straightforward, because spectral features are visible as inflection points of the absorption maxima seen in the original spectra (see Fig. 2.2, b and d) [17, 18].

Distinguishing between organic materials in real art samples by reflectance spectroscopy might be complicated by interferences with impurities, differences in the synthesis pathway and resulting average particle size. All these impacts can result in spectral shifts, which makes FORS often unsuitable for the identification of unknown organic substances in paint-ings [9].

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Figure 2.2: Reflectance spectra with minimum absorption values as mentioned in the litera-ture (left) and first derivative transformation with characteristic inflection points (right) of cochineal (a and b) and madder (c and d) based pigments [18]

A more sophisticated technique is fluorescence spectroscopy, which results in more mean-ingful information, even when applied to real paintings [9]. As organic pigments are quite abundant in a painting and they exhibit intense luminescence, sensitivity and specificity of fluorometric techniques is increased when compared to the analysis of absorption bands. Flu-orometric techniques provide sufficient sensitivity to distinguish different pure fluorophores and detect them even when only present in trace amounts. Nevertheless, fluorescence spec-tra and subtle differences in these specspec-tra may be undetectable in paintings due to masking effects like auto-absorption or scattering, and superposition of signals from more than one

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crimination between different fluorophores with the same excitation maximum, because some fluorescence lifetimes can be characteristic for a certain fluorophore [21]. For example, the analysis of a Renaissance wall painting from the artist Masolino da Panicale by Fluorescence Lifetime Imaging (FLIM) showed the presence of an organic lake pigment in Salome’s head-dress, which were not visible by the naked eye (see Fig. 2.3) [22]. However, central dots in both ovals, which present shorter lifetimes than the surrounding areas (3.1 ns ± 0.1 ns and 3.5 ns ± 0.1 ns, respectively), could be detected by FLIM (see Fig. 2.4). By comparison with previously recorded fluorescence spectra, it was concluded that the painter added the central pigments in a higher content – visible as a more intense red fluorescence emission, which is related to a shorter effective lifetime [22].

Thus, the identification of discoloured areas and the different amount of applied paint enables conservators to obtain profound understanding about how the original painting would have looked like, which in turn aids on the decision where retouches might be necessary. Further, knowledge about the applied and degraded pigments can be conferred to other paintings of the same artist or from the same era to help restoring their original appearance.

Figure 2.3: Colour image of Salome’s head, by Masolino da Panicale [22]

Figure 2.4: Fluorescence lifetime map of Salome’s head, by Masolino da Panicale [22]

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Analysing fluorescence lifetimes not only aids in the localisation of certain fluorophores, but the emission lifetime is also indicative for any interactions between the fluorophores and its environment, which might give insight into inappropriate storage conditions of traditional paintings. Chemical changes like ageing and oxidation of organic substances on painted surfaces, indicating a loss of original material and emphasizing the need for remedial conser-vation in order to inhibit further deterioration, can lead to alterations in fluorescence spectra due to emission quenching. Overall, measuring the fluorescence lifetime is advantageous over detecting common fluorescence spectra, because it is independent from the intensities both from fluorescence and ambient light [19]. Further, important information about appropriate sampling points for micro-destructive analysis can be gained, but the interpretation of such results might still be inadequate.

Not all organic substances exhibit fluorescence. In such cases, analytical methods based on the interaction between matter and light can be useful. One method that is based on molecular vibrations is Infrared (IR) spectroscopy. In the field of conservation of paintings, IR can be used to tackle different questions, including the identification of organic pigments [23]. For example, lac dye can be identified by Fourier-Transform Infrared (FTIR) spec-troscopy in the spectral range of 650 – 4000 cm−1 and a spectral resolution of 2 cm−1. By

using model samples of this dye, prepared by mixing the main colouring component laccaic acid with polymerized linseed oil, and analysing an area of 25 x 25 µm, characteristic IR bands could be detected and compared with reference spectra, which resulted in a positive identification of the known sample dye. Even after artificial ageing and strong photochemical degradation, the spectra were useful for identification purposes [24].

These samples were analysed as homogenous layers on glass supports. In terms of examining real paintings, these conditions might not be sufficient, because real paintings are commonly constituted of mixtures of various dyes that are spread in different ratios on canvasses or walls. In this context, Raman spectroscopy, especially Micro-Raman Spectroscopy (MRS) can be a good alternative. For this method, the laser beam is reduced from 1 mm2 to a

diameter of approximately 1 µm by high-magnification microscope objectives, which leads to a typical lateral resolution of about 1 µm, and 2 µm for in-depth resolution [25]. The ability of analysing such small sample areas is especially advantageous, when the sample is very heterogeneous as it is the case for paintings, because the analysis of a macroscopic area

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sophisticated analyses.

MRS was successfully used to identify a non-fluorescent unknown organic substance from purple areas in ancient Greek wall paintings from Santorini Island (Akrotiri, 1650 B.C.) [26]. Small lumps of these archaeological lake pigments were excavated from the wall painting and a spot of around 3 µm was excited by a 532 nm laser (Spectral resolution: 2 cm−1). The

well-preserved purple pigment was determined as a derivative of indigo, because C=C and C=O stretching motions, typically for indigoid molecules, were observed between 1580 and 1700 cm−1. A band at 307 cm−1 disclosed the substance as a bromine containing molecule and by

comparison with a reference spectrum, the purple pigment was identified as dibromoindigo (see Fig. 2.5), which is the main component of Tyrian purple. This pigment was used in Eastern and Western countries since Antiquity, and the identification helps to understand the trade and manufacturing skills of ancient populations. The analysis performed to identify Tyrian purple was conducted without any sample preparation steps, and demonstrate the successful identification of an ancient organic pigment in heterogenous matrices by MRS [26].

Figure 2.5: Raman spectra of a purple archaeological sample from the Akrotiri site (A) and Raman spectra of 6,6-dibromoindigo (B) after excitation by 532 nm (Spectra are normalised, shifted and baseline corrected) [26]

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Because organic pigments and other organic materials found in artworks might exhibit strong fluorescence, the Raman signals might be covered in a strong background. Though, spectra of pure pigments, for example laccaic acid [24], may be successfully recorded in the spectral range of 300 – 2000 cm−1 by using a laser source of higher wavelength. In this case, strong

fluorescence was observed at 532 nm, but characteristic vibrational bands were successfully recorded at 785 nm [24]. However, actual dye mixtures cannot be analysed by means of con-ventional Raman spectroscopy. This can be explained due to the various compounds found in paint samples, including proteins, saccharides, synthetic binders, and inorganic substances. All these materials contribute to the masking effect and hinder the determination of the vibrational bands of the main colouring component due to strong fluorescent background [24].

So far, the complementary techniques Raman spectroscopy and IR spectroscopy, both based on the measurement of molecular vibrations, were useful in order to identify the organic dye molecule laccaic acid, whereas FTIR spectroscopy succeeds Raman spectroscopy, as it could identify the actual dye mixture in model samples. However, these non- and minimally-invasive techniques have their limitations, as they lack sensitivity and are unsuitable to provide fingerprints of unknown substances in samples. Generally spoken, the analysis of heterogeneous samples from paintings with the aim of directly identifying the present dye molecule, but with simple sample preparation as included for the aforementioned methods, is a great analytical challenge [27].

In this sense, sample preparation is certainly required to gain more detailed information. One approach is to enhance Raman scattering whilst simultaneously quenching background fluorescence, which is advantageous as many organic dyes exhibit strong fluorescence. Hence, the advantages of Raman spectroscopy over other optical techniques, including high spatial (< 1 µm in MRS) and spectral resolution (< 0.1 cm−1) and high chemical sensitivity, can

be fully exploited. During Surface-Enhanced Raman Scattering (SERS), the generally weak Raman signals can be increased by up to 7 folds of magnitude (see Fig. 2.6). For doing so, metal particles are evaporated to coat the sample, which then intensify the Raman elec-tromagnetic field by resonance. Instead of emitting fluorescence for relaxation, the excited molecule transfers electrons to the metal colloids, which decreases the fluorescence back-ground.

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Figure 2.6: Comparison of conventional Raman scattering (left) and SERS (right) [7] Example spectra comparing conventional Raman spectroscopy of the fluorescent organic dye Alizarin and its corresponding SERS spectra showed that conventional Raman spectroscopy yielded an uninterpretable spectrum, whereas certain peaks can be clearly differentiated in the SERS spectrum (see Fig. 2.7) [28, 7, 29].

Figure 2.7: Conventional Raman (B) and SERS (A) spectra of an experimental Alizarin lake, excited by 532 nm [26]

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This technique was used for the direct identification of lac dye from a microscopic sam-ple removed from a painting, which was coated with Silver Nanoparticles (AgNP) and by analysing an area of 100 µm directly after the preparative step (excitation with 632.7 nm, see Fig. 2.8) [27]. By comparing this spectrum with the spectrum obtained by analysing pure lac dye (excitation with 632.8 nm, see Fig. 2.9) [30], some similarities are visible, but the spectrum lacks sharpness of peaks. The underlying reason for these broad peaks might be inhomogeneous peak broadening. In laser spectroscopy, signals are gained due to homogeneous peak broadening (internal), resulting from the intramolecular interactions in molecules, whereas inhomogeneous peak broadening (external) originates from external perturbations. Each molecule interacts with its own environment and experiences therefore only similar, but not identical conditions. This leads to slight changes in the frequency and width of the peak of each individual molecule leading to the measurement of the overall peak of the molecular population. This effect is especially observable in solid samples with chemical inhomogeneity as it is the case for paint samples, where the local environment of each molecule might even differ significantly [31].

Figure 2.8: SERS spectrum from lac dye obtained from a sample frag-ment [27]

Figure 2.9: SERS spectrum of pure lac dye [30]

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To conclude, the analysis of heterogeneous samples from paintings, which contain a mixture of organic materials including different pigments and binder substances needs more sophis-ticated methods with higher specificity than optical or vibrational techniques. The obtained spectral peaks are complicated to compare to references, and in the case of FORS, no further information about the analysed painting is given. Thus, assessing interactions with other organic materials and predicting possible interferences is difficult. However, it is desired to gain the maximum amount of information whilst consuming the least amount of sample, so highly sensitive analytical methods are required. In this sense, separation techniques cou-pled to MS are frequently used, because it discriminates organic materials in samples from paintings successfully and even reveals degradation by-products. Most commonly, Liquid Chromatography (LC) is performed in Reversed Phase (RP), because most analysed chro-mophores are polar and water-soluble, but the sample components are often unspecified and may have various solubilities [32].

For chromatographic techniques, complex and time-consuming sample pre-treatment procedures might be required to extract the pigments from the matrix and solubilize the analytes in an appropriate solvent, which also reduces interferences. Nevertheless, these sophisticated methods are also challenged by small sample amounts and low concentration of specific molecular markers, which are identified in order to determine the source of pigments. However, chemical modification and analyte-loss can occur during these steps and make the simultaneous and similar preparation of reference substances necessary to identify sample components. In turn, the preparation of these reference pigments needs previous knowledge about the presence of a certain pigment, which is often not the case. Therefore, colorant recovery methods were optimized in order to inhibit complete disappearance of analytes and to reduce chemical modification of the dye composition, with the aim of identifying pigments without the requirement of reference pigments [6, 33].

In this sense, genuine painting samples were taken and pre-treated, and the present organic pigments were identified without simultaneous pre-treatment of reference substances. Liguid Chromatography coupled to Mass Spectrometry (LC-MS) analysis was performed to inves-tigate samples containing organic red lake pigments from a valuable mural painting in the Convent of Tomar (Portugal) [34]. These paintings present the only pictorial cycle made exclusively with organic binders in Portugal, and the Convent of Christ in the city Tomar is classified as World Heritage [34]. Thus, sampling and sample preparation represents a deli-cate action. Three sampling points were determined based on pink/orange Ultraviolet (UV) fluorescence, which is characteristic for organic red lakes, and less than 200 µg were

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re-moved from each of these points. Sample preparation was performed by hydrolysis and for this, the samples were extracted for 4 hours in a hydrofluoric acid solution, lyophilized, re-dissolved and centrifuged before the supernatant was injected into a High-Performance Liquid-Chromatography (HPLC) instrument with Diode Array Detector (DAD), coupled to MS. Obviously, sample pre-treatment methods were complicated, time-consuming and lead to total deconstruction of the sample, however, comprehensive information could be gained. For example, two chromophores could be detected in all three samples, separated and identified, even when both were present in the same sample. Based on previous exami-nation performed with UV fluorescence, sampling locations were selected where the presence of organic red lake pigments was determined. However, the presence of different red lake pigments was not obvious at this stage, but the separation in RP showed the presence of two substances. The absorption below 500 nm, determined using a DAD which scans the absorption in UV to the visible region, indicated the presence of anthraquinone structures. By analysing the complementary information gained by elution pattern and mass spectra, the main chromophores in madder dye, i.e. alizarin and purpurin, were identified as the two organic red lake pigments used for the mural paintings. Both substances were separated in LC and eluted after 21.73 min and 23.15 min, whereas the deprotonated molecular ions [M-H]− at m/z 239 and m/z 255 and their corresponding fragmentation pattern were used

to identify alizarin as the substance eluting at 21.73 min, and purpurin as the later eluting molecule [34].

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Both substances are anthraquinone-derivatives with a different number and position of OH-substitutes as the only structural difference (see Fig. 2.10 and Fig. 2.11) [36, 35]. This high similarity makes differentiation with optical or vibrational methods difficult as shown by the previous examination by UV fluorescence. However, with the sample pre-treatment step, the pigment molecules were released from the binder and then successfully separated by LC, which made the detection of LC-DAD (nm) and LC-MS (m/z) data of each of the chromophores possible. This example shows, how sophisticated analytical methods can differentiate even between derivatives and it also illustrates the distinct advantage of MS based techniques over spectroscopic techniques for the identification of organic pigments. However, to get these results, the samples from valuable mural paintings were destroyed during laborious sample pre-treatment.

LC-MS can also aid in identifying specific dye markers, which, in return, is useful in designing new non-destructive analytical methods for future analyses. By analysis by Ultra Performance Liquid Chromatography (UPLC), detection with a Photo Diode Array Ultravi-olet absorption detector (PDA) and Quadrupole-Time-of-Flight (Q-ToF) MS, an unknown marker for the use of colourants obtained from brazilwood species was identified [37]. This marker is present, even when the main dye molecule brazilein is degraded, but so far, the structure was unknown. However, it is used to confirm the application of brazilwood-derived colouring material. Knowledge about this marker can give insight into the degradation of brazilwood and hence, confirm this marker as being a true characteristic for the use of this dye. A reference lake pigment, which was prepared following a historical recipe by the Na-tional Gallery, London, in 1989 showed degradation to a brown colour within 2 years, and a large amount of the unknown marker component was confirmed by HPLC analysis. Because the origin of this marker was not clear, three substances based on the photo-degradation pathway of brazilein and previous studies were selected as potential candidates. By compar-ing retention times and UV spectra of one of these structural classes, namely benzochrome-ones, urolithin C was identified as the unknown marker. Spectral features with absorption bands at 250 – 280 nm and 330 – 380 nm matched between the reference substance and the pigment sample, which is a strong indication for the same substitution pattern. Further, a variety of analytical methods, namely HPLC, UPLC and Gas Chromatography-Mass Spec-trometry (GC-MS), all reflecting common procedures in the field of conservation science, confirmed the brazilwood-derived marker by matching retention times, pseudo molecular ion and Two-Dimensional Mass Spectrometry (MS-MS) spectra with urolithin C reference standard [37].

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In this sense, by characterizing and identifying typically analysed substances, or even inves-tigating new specific markers, chromatographic techniques coupled to MS can potentially aid in developing new non-destructive analytical methods, that exceed common methods by means of accuracy, sensitivity, and duration of the analysis. Further, by the development of such techniques, the need for specialized personnel is decreased, and analyses of paintings is fastened, which will also have a positive impact on art sciences.

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2.3 Analysis of Organic Binders

Next to pigments, binding media are important to study, but also difficult due to their complex nature and elaborate analysis required for successful identification. However, insight into the organic binding medium used points towards certain painting techniques, aiding restoration actions, and is also important to conserve pictorial surfaces, because binding media ensure colour durability and colour perception. Knowing which organic binder has been used helps prognosing potential degradation areas and speed, and will then guide preventive conservation actions. Further, it gives insight into the degradation state, which is useful for the determination of an appropriate conservation strategy and in deciding where restorative actions might be necessary [38, 39].

Due to the translucent nature of pure binding media samples, optical spectra only show a low number of characteristic features, which will be difficult to detect when analysing real samples of a mixture of binding media and pigments. One of these reflectance features obtained by analysing pure samples are NH combination bands between 2050 – 2060 nm, obtained by FORS. Because these combination bands are only visible in egg samples, they discriminate between proteinaceous and lipid-based binding media (see Fig. 2.12, rectan-gle) [38]. However, due to the complex mixture of samples from painted art works, these weak features are difficult to observe. Next to the differentiation between biding media from different classes, it was also investigated, if different egg-based binding substances can be dis-tinguished by differences in intrinsic fluorescent properties between egg white and egg yolk. Fluorescence Excitation Emission Spectra (EES) of proteinaceous binding media show char-acteristic fingerprints, which can discriminate between egg white and egg yolk despite their similar components. Again, fingerprints are obtained when the binding media is analysed in its pure form, but it was shown that the analysis of mixtures of pigments and binding sub-stances resulted in such significant changes of the spectra, that the discrimination between egg yolk and egg white was highly complicated [40].

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Figure 2.12: FORS spectra of pure binding media on plain glass (Egg White (EW; a), Egg Yolk (EY; b), Whole Egg (WE; c), Gum Arabic (GA; d), Linseed Oil (LO; e), Poppy oil (PO; f), Walnut Oil (WO; g)) [38] with marked NH combination bands in egg samples

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It is very improbable that these fingerprints are obtained when a real sample containing other substances including fluorescent ones is under investigation. Hence, vibrational spectra might once again be more useful to discriminate between organic binders found in painted art works, because Raman spectra represent vibrations from the chemical skeleton and IR gives insight into the presence or absence of chemical groups. Thus, these features are more discriminative than data based on the absorption or emission of wavelengths. Studies have proven to successfully discriminate between binders of different and within the same organic class by combining Raman and Fourier-Transform Near-Infrared (FT-NIR) spectroscopy and applying multivariate analysis. For this, organic binders from different organic classes, including protein-, lipid- and saccharide-based ones, were prepared and applied in thin layers onto glass slides [41].

It is questionable, if these results are observable, if complex samples from real paintings are under analysis. Fluorescence background might drown weak Raman peaks, and other organic compounds would lead to a complex spectrum showing disturbing peaks that complicate feature picking.

Despite these limitations, the analysis of the Renaissance fresco il Perugino by Pietro Vannucci in an in situ approach in the Mid-Infrared (MIR) (4000 - 900 cm−1) resulted in the

finding of two different organic binders [42]. More than 100 sample points with an analysed area of about 20 mm2 were analysed and the absence of a carbonyl stretching at 1740 cm−1,

observed in lipidic binders, and the presence of amide I and II bands at 1684 cm−1 and 1585

cm−1 point towards the use of a protein-based binder in all samples. By applying a Principal

Components Analysis (PCA) algorithm, varying strength of CH features at about 2900 cm−1

were observed and it was concluded from this result that two different proteinaceous binding media were used for this fresco. By comparing these spectra with the spectra of mock mural paintings, casein was identified as the binder with weak CH features, and the other binder is most likely egg tempera or animal glue. However, due to the absence of the lipid stretching, the obtained spectrum more likely represents animal glue [42].

In this case, the natural organic binder could be identified in real art samples. However, because the sample was obtained from a mural painting with a carbonate matrix, limitations regarding the presence of other, organic substances were not present, and the informative amide I and II bands were well resolved from the carbonate bands.

In general, MIR covers the fingerprint region between 1200 – 900 cm−1, where fundamental

molecular vibrations are recorded, which result from vibrational transitions between the ground and first excitation vibrational level. These transitions are specific for each organic

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functional group and can be used to identify molecules. However, interferences with pigments are possible, which complicates identification. To overcome this limitation, analyses in the Near-Infrared (NIR) from 12000 – 4000 cm−1 are performed. Nevertheless, the NIR records

overtones of the fundamental bands, which results in weaker bands and combinations of vibrations, which broadens the bands. Thus, spectra of binders are not specific enough to discriminate between substances belonging to the same organic class [43, 17].

Overall, optical and vibrational methods for the analysis of organic binders in samples containing various organic substances would result in overwhelming spectral features, which complicate manual interpretation of spectra. These non-destructive methods are more useful for first screenings but lack specificity for detailed information that lead to the identification of substances in complex sample mixtures from paintings. Hence, Gas Chromatography (GC) and LC or pyrolysis Gas-Chromatography (py-GC) are typically performed to characterize organic materials in paintings. These materials mainly consist of lipids and proteins, and these components can be separated during an initial sample preparation step to analyse each fraction separately, or they can be analysed during the same method.

In this sense, proteinaceous binders can be analysed and differentiated by py-GC. GC-MS based analysis is especially advantageous in the case of traditional paintings, where proteins are mostly degraded, and their solubility is decreased. Hence, wet chemistry methods like LC are hindered, because the samples are difficult to bring in solution. Thus, three proteinaceous binder namely egg white, casein and animal glue, show different pyrolytic profiles at the molecular level resulting from different amino acid sequences, although all binders experience the same main thermal degradation steps during pyrolysis. An easy distinction between these binders is based on pyrolytic profiles of 2,5-Diketopiperizines (DKPs), which are formed during the first thermal degradation step under anoxic heating. Contrary to these promising results, the analysis of study samples from different periods between the 2nd century BC

to the 20th century AD showed that older and more degraded samples lead to less specific

pyrolytic profiles. Overall, a higher relative amount of non-specific polycyclic aromatic hydrocarbons was detected, and combined with the loss of DKPs and aromatic markers, the pyrolytic profiles are less distinctive for each proteinaceous binder [8]. These results show that py-GC-MS is rather disadvantageous for the determination of proteinaceous binders. Next to characterizing the use of proteinaceous binders, lipidic binders were also analysed

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oxidative transformation during drying. Thus, polyunsaturated acyclic chains are broken and low molecular weight dicarboxylic acids, namely azelaic acid, oxalic acid, sebacic acid, and suberic acid, are formed. In the case of egg yolk, 60% are made of lipids and only a low amount of polyunsaturated fatty acids is found in egg yolk-based binders. Thus, ageing leads to lesser extent to the formation of dicarboxylic fatty acids with respect to drying oils [44].

One approach of discriminating between different lipidic binders is based on the amount of dicarboxylic acids in the sample, and the ratio between these discriminative markers is determined. This concept was used to characterize the use of organic binders for the mural paintings in the Charola of the Convent of Christ, dated from the 12th century AD. For

this, < 100 µg were removed from 12 locations of the painting, which were selected during analysis by micro-FTIR spectroscopy, before they were subjected to analysis by pyrolysis GC-MS. However, because polar organic molecules like carboxylic acids have poor volatility, a derivatization step is necessary before analysis. In this sense, the samples taken from the mural painting were on-line derivatized with the methylation agent tetramethylammonium hydroxide. Hence, the volatility increased, and the derivatisation also improves the sepa-ration of polar analytes on the non-polar capillary column. Thermal degradation products of the sample were formed in the pyrolysis chamber under a temperature of 500°C within 12s, and were then analysed by GC-MS. For discrimination purposes, several properties and ratios were determined. Firstly, the total amount of fatty acid contentP

D found in the

sam-ples was determined as a further indication for the presence of the type of a lipidic binder, because more dicarboxylic acids are present when drying oils were used. Hence, a value of > 30% for P

D is determined when drying oils were used as the only binder. Secondly, the

ratio between the dicarboxylic acid azelaic acid and the monocarboxylic acid palmitic acid (A/P) was identified. Finally, to distinguish between different drying oils, the ratio between palmitic acid to stearic acid (P/S) was used, as a ratio of <2 indicates the use of linseed oil, 2.1 – 3 points towards walnut oil, and >3 is the ratio for poppy seed oil. After determining these ratios for several samples, the use of different binders could be stated. In those samples were no protein or oil marker was detected, an inorganic binder was probably used. For ten samples, all determined values were in accordance with the use of linseed oil. However, the results for some samples were inconclusive, as P/S point towards the use of linseed oil, but

P

D was not sufficient enough to account for drying oils [34, 45].

Certainly, the identification of the used lipidic paint binder is not always successful when the approach of determining certain ratios is followed, and the same applies for the analysis of proteinaceous binders. However, due to the need for derivatization in certain circumstances

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and complete destruction of the sample, there is no possibility to re-analyse the same sample by more promising techniques, and removing more samples from paintings brings ethical complications. Further, as both approaches are limited to the analysis of one organic class only, certain information is lost. However, the combined knowledge about the presence of lipidic and proteinaceous marker would be most informative about the use of a specific organic binder.

In this sense, soft ionization methods like Matrix Assisted Laser Desorption Ionisation (MALDI) and Electrospray Ionization (ESI) must be mentioned as promising alternatives in MS based methods. MALDI and ESI lead to increased application of MS in omics stud-ies and is also appreciated in Cultural Heritage studstud-ies. Since the sample amount needed for inherently sensitive methods like MALDI or nano-LC-ESI-MS is in the range of micro-grams, these analytical methods are very attractive for analysing the organic binder content in paintings [46]. Organic paint binder can be fast and completely identified in complex mixtures by means of analysis with MALDI-Time-of-Flight (ToF)-MS. The soft ionization method MALDI comes with the distinct advantage of being a fast method, but still offering high sensitivity, which is especially crucial for the analysis of limited sample amounts from traditional paintings. However, in order to gain most comprehensive information to char-acterize the present organic binders, the samples have to be prepared in such a way that both lipidic and proteinaceous fractions can be analysed. For this, the samples (50 – 100 µg) are pulverized and proteins and lipids are extracted following the Bligh-Dyer method, needing about 1 hour in total. Chloroform and methanol are added, and after vortex-mixing, ultra-sonification and centrifugation, the two separated layers can be analysed individually. In this sense, Triacylglycerols (TAGs) from drying oils, and Phospholipids (PL) from egg are extracted and solved in the organic phase, and this approach was tested by using linseed oil, egg and a mixture of both for analysis. It can be seen that signals from polar lipids, i.e. PL, are in the range between m/z 700 – 820, whereas neutral lipids, i. e. TAGs, are found between m/z 850 – 920. Hence, the extraction procedure can recover both lipids, and can be useful in order to discriminate between organic binders from both groups. However, ageing and interaction with pigments oxidizes PL and TAGs, leading to mass shifts of +16 and +32 Da, and β-cleavage products are visible in the lower m/z range. After 2 years of ageing, only degradation products and no native or oxidized PL and TAGs are detectable anymore and thus, the analysis of marker ions is focused on degradation products from these

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Protein and lipid fractions of samples taken from The Crucifix (Isole Tremiti, Italy), a 12th to 13th century panel painting, were characterized [48]. For this, lipids and proteins

were extracted from 50 - 100 µg sample following the Bligh-Dyer method, and the organic phase was faced to lipid MALDI analysis. The upper face was incubated over night for tryptic digestion before it was subjected for peptide analysis. The organic fraction did not reveal characteristic marker ions of drying oils, i.e. TAGs, however, degradation products of PL were present, as found in lecithin of egg yolk. Hence, both findings indicate the use of an egg-based binder. Further, the tryptic digest revealed numerous m/z values, which were compared to external and internal databases, and with various protein-based materials found in paintings. However, chemical modification of proteinaceous binders due to ageing prevents straightforward identification. Oxidation, deamination, or loss of water can result in mass shifts, which hinders direct comparison of aged samples with m/z values of unaltered peptides. Therefore, possible modifications of several amino acids have to be considered, before databases can be searched for matches. The obtained m/z ions could be identified as proteins and peptides from egg yolk and egg white, but mostly only oxidized and/or deaminated peptides were attributed. It was concluded that most likely whole egg was used as a binder, as features in accordance with both egg white and egg yolk were attributed [48]. Even though degradation of the samples must be taken into account in order to draw conclusions regarding the presence of certain organic substances used as binder, this study shows that MALDI-ToF-MS is a very powerful technique, capable of obtaining detailed information in old and severely degraded samples from traditional paintings. However, in order to get these results, the sample pre-treatment is quite laborious. The organic fraction must be separated, and the proteinaceous content must be enzymatically cleaved before both parts are analysed individually. In general, such pre-analytical steps are commonly needed to characterize organic binders by GC and LC or py-GC. However, lipidic and proteinaceous binder can be analysed in one run by means of MALDI-MS without the need of laborious solvent extraction. For this, 100 µg of sample were applied onto a MALDI plate and wet chemistry steps required for protein digestion were directly performed on the paint fragment. The samples were added as such or after finely grinding onto the plate, on which graphite paint was previously applied. Graphite enhances thermal and electoral conduction and hence, it favours laser desorption ionization increasing MALDI signal intensity. Then, a trypsin solution was deposited on the samples for on-spot protein digestion, which took 10 minutes. This procedure was performed for replica samples, including artificially aged ones to discover the ability to detect degradation products, and also to paint fragment from the

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polyptych "Christ in Pity, St. Louis of Toulouse, St. Francis of Assisi, St. John Baptist, St. Anthony of Padua" (1467) by Antonio Vivarini (Pinacoteca Provinciale "Corrado Giaquinto", Bari, Italy) [46]. Although the organic fraction was not separated from the proteinaceous content, but was simultaneously analysed, lipids and peptides were successfully identified in aged samples. Hence, hydrolysed native PL and TAGs are visible in the m/z range from 470 – 610, β-scission products from PL can be seen from m/z 620 – 700, intact and mono-oxidized PL are detected in the m/z range of 750-820 and TAGs are observed from m/z 850 – 900. These results are in accordance with the results obtained after sample pre-treatment by solvent extraction, but the entire method only required 15 minutes. Validation of the method also showed that all peptides found for different proteinaceous binders mentioned in the literature are found in a range between m/z 900 – 4,000. These findings strongly suggest the ability of this simplified pre-analytical procedure for the discrimination of natural organic binders. Thus, the analysis of the genuine paint fragments showed the presence of peptides attributed to the use of an egg-based binder, and also the presence of PL and their degradation products [46].

Overall, the conclusive identification of organic paint binders is challenged by several factors. Non-destructive methods are limited by the presence of organic pigments that lead to a noisy background, and separate determination of lipids and proteins by pyrolysis GC-MS shows limitation in highly degraded samples. Nevertheless, even highly degraded samples show promising results when analysed by MALDI-TOF-MS and the laborious sample pre-treatment procedure can be replaced by a fast on-plate tryptic digest, without compromising significance of the results. Hence, detailed knowledge about the used organic binder can be obtained by highly sensitive methods.

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When the painting is well understood, meaning the organic materials are characterized and the degradation state is determined, an appropriate conservation plan can be made. These conservation interventions must respect the integrity of the painting and have to conform the principle of being minimally invasive, highly re-treatable and stable. However, conventional treatments showed to lack stability, altered the appearance of paintings and are sometimes hardly removable. Hence, new technologies based on nanotechnology are being developed and applied for the conservation and restoration of painted artworks. The focus is on physico-chemical stability and compatibility of the new applied material with the painting, which leads to durability of the treatment [13].

Nanotechnology refers to the design, production and use of materials, whose size is in the nanometer range in at least one dimension [49]. This leads to structures having a large surface to volume ratio and because materials behave very differently at the nanoscale, properties and effects are very different as the same component in the bulk. These include unique electronic, optical, magnetic and chemical properties, and very complicated chemical reactions taking place at the surface of the nanoparticle mostly remain unknown [49]. Thus, the small dimensions lead to alterations of the mechanical properties, like super-plasticity or enhanced diffusivity and the large surface area improves the chemical reactivity of the material [15]. The latter can be explained, because in contrast to bulk materials, most of the atoms are located at the surface of the nanoparticle and are therefore in a different environment. All these characteristics can be beneficial for various applications, including the restoration of paintings [50]. Nanomaterials, in contrast to conventional materials like polymers, promise to keep the physical and chemical properties of artefacts whilst having a low environmental impact, which makes them a promising conservation technique [3]. The very first method, which was the application of colloidal calcium hydroxide particles to the restoration of Santi di Tito’s wall paintings, already showed an immense improvement over pre-existing methods [51]. In the following sections, examples of the use of nanotechnology for the conservation and restoration of cellulose based art works are presented.

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3.1 Preventive Conservation

Preventive conservation interventions aim to minimize or even avoid deterioration and loss of painting materials that might appear in future. These include indirect measures like envi-ronmental control and elucidating the correct storage conditions, where further degradation processes are prevented, and it also includes direct actions. Coatings might be applied to generate a protective barrier against pollution and dirt, and thus prevents more harsh in-terventions like cleaning of sensitive paintings, and de-acidification might be performed on items that are prone to loosen their strength due to acidic conditions [1].

3.1.1 Protective Coatings

In the late 19th century, the use of varnishes to protect paintings and to increase gloss and colour saturation started to decline [44]. The resulting matte surfaces are more difficult to consolidate and restore, because any added protective varnishes would change the visible ap-pearance. But recently, with the help of nanotechnology, a promising alternative to common polymeric varnishes was developed. This innovative coating consists of TiO2 nanoparticles that are embedded into Poly(2-ethyl-2-oxazoline) (PEOX). The application of compositions with different concentrations (16, 28 and 44 wt%) of TiO2 onto matte black painting models

showed that the surface diffusive properties were mimicked. In this sense, pure polymers applied on such models decrease the light scattering, which results in a smoothened and darkened surface. However, the nanocomposite films containing TiO2 nanoparticles

matti-fied the painting’s surface, because these nanoparticles have a high refractive index leading to induced light scattering. Next to this observation, some transmittance was lost due to Rayleigh scattering of built nanoparticle clusters in the polymeric matrix, even though the nanocomposite films were optically transparent. By using different concentrations of nanoparticles, the refractive index of the film could even be tailored. Furthermore, an inves-tigation into the behaviour of the nanocomposite films and polymeric coatings after natural ageing for 2 years and artificial ageing resulted in contradictory results: After natural ageing, oxidation products of the polymeric matrix was observable in the purely polymeric coating only, whereas the polymers applied with nanoparticles were found to be stable. However, after artificial ageing using a Xenon lamp, the pure polymer was found to be stable, whereas

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and TiO2 in higher concentrations (28 and 44 wt% of TiO2 in the polymeric matrix)

can-not completely be removed after a single cleaning treatment. Thus, this study represents rather a proof-of-concept of protective coating, and not a ready-to-use-material. However, the application on a real contemporary painting showed that the nanocomposite offers the best results from the optical point of view. However, due to the de-polymerization of the polymers catalysed by TiO2, further investigations using less photo-active nanoparticles is

needed in order to use this concept for mattifying coatings [44].

3.1.2 De-acidification

In the case of canvas paintings, the ageing of varnishes and drying oils leads to the production of acids which catalyse a depolymerization reaction of the canvas and hence, favour the degradation of the cellulose fibres [12]. Cellulose-based materials, including wood panels, canvases and paper, degrade due to acidification processes and result in chemical disruptions of the cellulose polymer by hydrolysis. This leads to the loss of strength, and because the entire process is catalysed by acids, de-acidification is an action that slows down or even stops this chemical route. This preventive treatment is unfortunately not able to restore those mechanical properties that have already been lost, therefore an early action is most useful in order to restore canvases [15].

Using dry powdered chalk is the simplest method to de-acidify canvases and paper, but the limited penetration of the powder into the material is disadvantageous. Traditionally, aqueous solutions of alkaline hydroxides have been used to de-acidify canvases and paper, but the additional moisture can lead to disadvantageous swelling of the hygroscopic cellulosic canvas, the glue and the paint. Further, hydrophilic cellulose materials de-polymerize further when exposed to strongly alkaline conditions. Hence, non-aqueous solvents are a promising alternative. However, the only chemical that is reported to be used for de-acidification action in a non-aqueous solution is Methoxy Magnesium Methyl Carbonate (MMMC), which is commonly known under the Wei t’O method [15].

Nanoparticle dispersion in non-aqueous solvents are an alternative with increased effec-tiveness over MMMC: in contrast to bulk materials, there are more atoms on the surface of the nanoparticle, which increases the reactivity at the surface, where acids can be neutralized [15]. Thus, inorganic nanoparticles can be solved in non-aqueous solvents like iso-propanol. In comparison to aqueous solvents, iso-propanol showed to prevent agglomeration of the nanoparticles, and it disrupts pigments only minimally when applied to paintings. Further, the application on paintings is easy, because it readily evaporates from the artwork,

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bypass-ing removal of the solvent from the paintbypass-ings’ surface, whereby nanoparticles can integrate towards the artefacts for de-acidification reactions. The latter point is a big advantage of using nanotechnology instead of conventional treatments, because those are only able to de-acidify up to 0.5 cm below the surface, whereas nanosized components reach deeper structures through penetration via capillary tubes present in cellulose materials [10].

In this sense, alkaline nanoparticles [Ca(OH)2] were applied on 19th century linings, that

were subjected to natural ageing of about two years, to inhibit acidification. Although de-acidification cannot restore lost mechanical properties, treated samples showed elasticity similar to that of unaged canvases. This property was tested by gradually increasing Relative Humidity (RF)) and stretching the lining in order to investigate the response towards this environmental change. The mechanical properties were altered after the treatment, because the treatment lead to the production of a coating that hindered the penetration of moisture into the structures. Further, the de-acidification treatment using alkaline nanoparticles build an alkaline reservoir within the canvas, which acts as a long-term protection against further acid attack [52]. Overall, these findings show several advantages of nanotechnology-based de-acidification treatments. Not only can lost mechanical properties be restored, but the formation of the protective coating against humidity is also beneficial in terms of swelling of the canvas, may have further tremendous effects on paint layers. Hence, this coating and the build alkaline reservoir act as a long-term protection against different negative impacts, that paintings are subjected to.

3.2 Remedial Conservation

Remedial conservation interventions refer to direct actions and measures that are focused to inhibit ongoing degradation processes and to reinforce the structure. Thus, during cleaning interventions, dirt or previously applied varnishes might be removed, because they might negatively interfere with the painting’s surface and alter its appearance. Previously applied additional new canvasses might also sometimes be removed again, because degradation of the adhesive can have tremendous effects for the painting [1].

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light leads to molecular changes in external layers of paintings. Thus, the protective varnish layer is affected by the combined action of light and air, leading to natural ageing, which alters the chemical and mechanical properties and with it the solubility and removability of these resins [10]. These naturally aged varnishes, and depositions of dirt have to be re-moved in order to restore the original appearance of an artwork. However, this cleaning intervention constitutes an irreversible intervention, aiming to selectively ultimately remove undesired materials [3]. Traditionally, this is done using solvents, but this entails certain disadvantages. Organic solvents are toxic and bear a risk for the conservator and the environ-ment. Next to this, the application of organic solvents is only poorly controllable, and hence, parts of the object may swell due to excessive solvents and pigments and binders might leach into neighbouring areas or different layers, leading to the even deeper penetration of solubi-lized coatings within the pores of masterpieces [53]. Thus, the use of pure organic solvents was replaced by gel technology, which enables better control over the cleaning liquid [10]. Several systems have been developed, also including solvent-free methods including micellar system and microemulsions, which are so far the most advanced cleaning fluids to remove hydrophobic substances from sensitive surfaces [53].

These systems include gel technology, in which the cleaning fluid is confined within a network of polymer chains, allowing for a gradual release of the fluid without penetration into the paint layers. Thickeners and polysaccharide gels have been used, but they have several limitations including inadequate fluid retention making the application to sensitive works impossible, and difficult removal of the applied system leaving residues on the surface. Next to these physical gels, whose networks are comprised of non-covalent bonds, chemical gels were developed which possess the advantages of being more structured and cohesive, and their removal is simple and residue-free. Unfortunately, gel cleaning techniques might still be inappropriate, because they are difficult to be completely removed from porous objects like easel and wall paintings. This is, where nanotechnology plays a crucial role, as it has shown several improvements. In this sense, responsive gels, i.e. gels that react to an external stimulus, have been developed using nanotechnology [53, 10, 54, 12].

By crosslinking ferric magnetic nanoparticles into a copolymer-based network (see Fig. 3.1), a gel was developed that can be easily removed using a permanent magnet. This makes the application to porous objects possible, but it is also advantageous, because the artefact itself is not touched, as the sponge is shaped and removed at a distance (see Fig. 3.2). Due to the magnetic nanoparticles with the ability of self-organization into a highly ordered superstructure, the resulting sponge can be cut and shaped into appropriate sizes. Further, the sponge can be easily loaded with microemulsions and various additives such as different

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solvents, which makes fine-tuning for the application to different paintings possible. During the cleaning process, the loaded microemulsion moves through the heterogeneous pores of the gelled network towards the surface of the painting, where it solubilises and compartmentalizes undesired materials and drags them into the gel. After cleaning and drying the sponge, it can be reused.

Figure 3.1: Schematic illustration of a nano-magnetic gel [55]

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Using the nanomagnetic sponge, an aged layer of paraloid B72 was completely removed from a marble sample and the application to the surface of a fresco showed the possibility of nanomagnetic sponges to clean genuine artworks. Though, cleaning using the nanomagnetic sponge is limited by the unknown amount of residue that can be absorbed by the sponge, and the application to artworks containing iron-containing pigments could lead to the dragging of pigments towards the magnetic nanosystem of the magnet itself [55, 10].

Nanotechnology also lead to the development of more promising cleaning systems. For the delivery of nanostructured cleaning fluids, a hydrogel based on Semi-Interpenetrated Polymer Networks (SIPNs), that can be loaded with complex cleaning fluids for the re-moval of aged varnishes, was developed. For these networks, 2-Hydroxymethyl Methacrylate (HEMA), which adds mechanical strength to the system, and N,N’-Methylenebis(acrylamide) (MBA), which is responsible for the hydrophilicity of the system, were cross-linked in the presence of linear chains of Poly-(vinylpyrrolidine) (PCP). However, chemical reaction only occurred between HEMA and MBA, whereas PCP is interpenetrated in the polymeric net-work and increases the loadability of the netnet-work. Thus, SIPNs hydrogels can be tuned in order to adjust hydrophilicity and porosity, which enables the optimization of the liquid release kinetics and swelling properties. Hence, in contrast to commonly implemented sol-vent cleaning, the release of solsol-vent is smaller and can be controlled. Further, SIPNs show high versatility and can be used for the removal of hydrophilic substances like grime, and hydrophobic layers like polymeric varnishes, as they can be loaded with be loaded with Oil in Water (o/w) microemulsion of varying ratios [53, 56].

Nanotechnology not only shows advantages over common cleaning systems in the case of the delivery system, but also the cleaning fluid itself. As mentioned, organic solvents are commonly used for cleaning procedures, and for the innovative cleaning system based on SIPNs, the organic solvent is dispersed as nanosized droplets in an aqueous phase with the help of surfactants. Thus, the surrounding phase is water, reducing the risk for conservators. Further, because the fluid can be loaded onto the hydrogel without losing the cleaning effectiveness of the organic solvent and by the confinement of the cleaning fluid into the highly water retentive SIPNs hydrogel, the fluid can be released gradually allowing for the control of the cleaning action. Two cleaning fluids were selected based on their ability to swell and solubilize polymeric coatings and loaded onto SIPNs. The first one, EAPC, is already commonly used to clean wall paintings, whereas the second one, MEB, was specifically developed and included a less polar molecule, butyl acetate, which was added to increase the range of materials that can be swollen and removed. After loading, an investigation by

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