Advantages of the Use of SR-FT-IR Microspectroscopy: Applications

Anal. Chem. 2005, 77, 3444-3451

Advantages of the Use of SR-FT-IR Microspectroscopy: Applications to Cultural Heritage Nati Salvado´,*,† Salvador Butı´,† Mark J. Tobin,‡ Emmanuel Pantos,‡ A. John N. W. Prag,§ and Trinitat Pradell⊥

Departament d’Enginyeria Quı´mica, EPSEVG, Universitat Polite` cnica de Catalunya, Avinguda Vıćtor Balaguer s/n, 08800 Vilanova i la Geltru´ , Barcelona, Daresbury Laboratory, Kecwick Lane, Warrington WA4 4AD, UK, The Manchester Museum, University of Manchester, Manchester M13 9PL,UK, and Departament Fı´sica i Enginyeria Nuclear, ESAB, Edifici ESAB, Campus del Baix Llobregat, Universitat Polite` cnica de Catalunya, Avinguda del Canal Olı´mpic s/n, 08860 Castelldefels, Barcelona

Synchrotron radiation Fourier transform infrared (SR-FTIR) microspectroscopy represents an advance over conventional FT-IR spectroscopy because it gives a higher signal/noise ratio at the highest spatial resolution due to the high brightness and collimation of synchrotron radiation. It has been successfully applied to the study of ancient paintings, alteration and corrosion layers which are heterogeneous microlayered materials made by complex mixtures of organic and inorganic compounds. Moreover, the high brightness attribute allows FT-IR spectra to be routinely obtained directly on the surfaces of the objects and opens the possibility for nondestructive testing of museum objects. We present in this paper a selection of applications of SR-FT-IR to the analysis of ancient paintings, alteration and corrosion layers where the technique has proven to be especially useful: first, the separation and identification of pigment microparticles from ancient Roman wall paintings; second, the determination of the binding media and the byproducts resulting from the interaction between binders and pigments from medieval altarpieces; and third, the study of the surface corrosion layers of a bronze helmet by means of direct analysis of the surface.

INTRODUCTION Synchrotron radiation sources produce high-brightness light from the infrared to the hard X-ray.1,2 The high collimation of the beam allows the examination of very small samples with lateral dimensions of a few micrometers. SR-FT-IR has already been * Corresponding author. Phone: +34938967717. Fax: +34938967700. E-mail address: [email protected]. † Departament d’Enginyeria Quı´mica, EPSEVG, Universitat Polite ` cnica de Catalunya. ‡ Daresbury Laboratory. § University of Manchester. ⊥ Departament Fı´sica i Enginyeria Nuclear, ESAB, Universitat Polite`cnica de Catalunya. (1) Munro, I. H. J. Synchrotron Radiat. 1997, 4, 344-358. (2) Margaritondo, G. J. Synchrotron Radiat. 1995, 2, 148-154.

3444 Analytical Chemistry, Vol. 77, No. 11, June 1, 2005

applied to different field,s3-5 such as food science and analysis,6 biology and biomedicine,7,8 and geochemistry,9 among others. These advantages open up numerous applications in the field of cultural heritage. The study and analysis of ancient objects presents many difficulties, but it is of the highest interest because it can provide historical information on early technologies (raw materials processing, synthetics) and ancient trade and commercial routes (i.e., origin of raw materials). The artworks studied are made up of a large variety of materials (metal, ceramic, glass, mineral, binders, glues, varnish, dyes, lake pigments, leather, cloth, bone, ...); they may be natural (of mineral or biological origin) or synthetic and may appear in large quantities or at trace levels. Normally, the objects are made not of a single material but of complex mixtures. For instance, microscopic particles of pigments are mixed with a binder and applied over a preparation layer, forming a composite layer of submillimeter thickness. Although an art object may be big, the materials under investigation are normally applied by forming successive layers of mixtures of materials (paintings, glazes, surface decorations, etc.). Moreover, this mixing of materials may result in the formation of small amounts of reaction compounds that may affect the stability of the object. Another issue is the age of the objects and their state of conservation: the objects may have been kept under inadequate conservation conditions and may have been affected by weathering while in storage, during burial, or after floods. Consequently, the study of corrosion and alteration surface layers is of great interest with regard to the conservation and possible restoration of the artwork. The alteration layer is defined as the outermost layer that is in contact with the environment which, with the passing (3) Reffner, J. A.; Martiglio, P. A.; Williams, G. P. Rev. Sci. Instrum. 1995, 66 (2), 1298-1302. (4) Kevin Gillie, J.; Hochlowski, J.; Arbuckle-Keil, G. A. Anal. Chem. 2000, 72, 71R-79R. (5) Dumas, P.; Tobin, M. J. Spectrosc. Eur. 2003, 15 (6), 17-23. (6) Yu, P. Br. J. Nutr. 2004, 92, 869-885. (7) Kneipp, J.; Miller, L. M.; Joncic, M.; Kittel, M.; Lasch, P.; Beekes, M.; Naumann, D. Biochim. Biophys. Acta 2003, 1639, 152-158. (8) Wetzel, D. L.; Williams, G. P. Vib. Spectrosc. 2002, 30, 101-109. (9) Benning, L. G.; Phoenix, V. R.; Yee, N.; Tobin, M. J. Geochim. Cosmochim. Acta 2004, 68, 729-741. 10.1021/ac050126k CCC: $30.25

© 2005 American Chemical Society Published on Web 04/19/2005

of time, can react with environmental components and adsorb foreign particles from the environment. Finally, the limited availability of such objects, especially for museum artworks, requires either the use of nondestructive in situ tests or very limited sampling. In situ tests demand analytical techniques with microscopic capabilities able to handle large objects at the same time. Furthermore, the small sample size or low sample weight requires the use of microanalytical techniques, such as optical microscopy (OM); scanning electronic microscopy (SEM) with energy dispersive X-ray analysis (EDX); infrared spectroscopy (FT-IR); Raman spectroscopy; X-ray diffraction (XRD); and, sometimes, separation techniques, such as gas chromatography (GC), high performance liquid chromatography (HPLC), or capillary electrophoresis (CE). Their use, together with specific sample preparation methodologies, enables greater understanding of ancient materials. SR-XRD and FT-IR have recently been employed10,11 for the study of ancient paintings, pigments, and alteration and corrosion layers. The analysis of small and/or heterogeneous samples, such as paintings and corrosion products, requires the smallest probe and the highest spectral resolution possible. SR-FT-IR has 100 to 1000 greater brightness than conventional IR sources and gives a high spectral signal to noise at diffraction-limited spatial resolution.5,12-15 Moreover, SR-FT-IR microspectroscopy allows the examination of areas as small as 5 µm2 with negligible heating effects on the sample.16 This can be achieved either in transmission or in reflection mode,17 with reflection measurements possible from small sample fragments or directly from the surface of the object. SR-FT-IR is, thus, a powerful molecular spectroscopy technique for the characterization of painting materials. The layered structure of paint samples formed by a complex mixture of compounds and the presence of minor compounds (e.g., impurities, products of aging) makes it necessary to perform analyses at the microscopic level. SR-FT-IR provides high-quality IR spectra that facilitate pinpointing and separation of the compounds present and their interpretation. A key advantage is the ability to characterize corrosion and surface alteration layers in reflection mode without the need to extract samples, working directly on the surface of large objects. Thus, it is a nondestructive technique. As mentioned above, all these advantages of SR-FT-IR are wellknown to practitioners in other research areas.3-9 Of particular importance is the information content that can be visualized in the form of two-dimensional chemical maps,5 a very powerful means of providing a concise overview. In this work, we have used SR-FT-IR to help identify components present in clearly observable heterogeneities within the samples. As such, we have not needed to make use of the mapping capabilities of the instrumentation. (10) Salvado´, N.; Pradell, T.; Pantos, E.; Papiz, M. Z.; Molera, J.; Seco, M.; Vendrell-Saz. M. J. Synchrotron Radiat. 2002, 9, 215-222. (11) Salvado´, N.; Butı´, S.; Tobin, M.; Pantos, E.; Pradell, T. IRUG6 proceedings: Firenze 2004, 296-301. (12) Carr, G. L. Vib. Spectrosc. 1999, 19, 53-60. (13) Lobo, R. P. S. M.; LaVeigne, J. D.; Reitze, D. H.; Tanner, D. B.; Carr, G. L. Rev. Sci. Instrum. 1999, 70 (7), 2899-2904. (14) Carr, G. L. Rev. Sci. Instrum. 2001, 72 (3), 1613-1619. (15) Smith, T. I. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 483, 565-570. (16) Martin, M. C.; Tsvetkova, N. M.; Crowe, J. H.; R.; McKinney, W. R. Appl. Spectrosc. 2001, 55/2. (17) Gensch, M.; Hinrichs, K.; Ro¨seler, A.; Korte, E. H.; Shade, U. Anal. Bional. Chem. 2003, 376, 626-630.

Figure 1. Schematic diagram of the SR-FT-IR beamline 11.1 at the Synchrotron Radiation Source, Daresbury Laboratory, Warrington, UK.

In this paper, we outline three different applications of SR-FT-IR microspectroscopy to demonstrate its capabilities and relevance to the cultural heritage field. The first example describes the characterization of the pigments present in the wall paintings from a room of a rural Roman villa in l’Espelt (OÅ dena, Catalunya) and dated, according to the archaeological data 18 to the 2nd century AD. The villa was set on cultivated land, a fundus, which formed an agricultural unit. The pigments studied here are from a room decorated with polychromatic stucco. The second example concerns the study of the innovation of the binding media in mediaeval paintings, the altarpiece of the “Conestable” (15th century AD) of Jaume Huguet in the chapel of Sainte AÅ gata in Barcelona. Jaume Huguet is one of the most representative painters of Catalan Gothic painting, and his work is associated with the innovations of this period between the Gothic and the Renaissance.19 Finally, the determination of the corrosion products on a 7th century BC Corinthian-type bronze helmet (The Manchester Museum, UK) by direct measurement on the surface. This helmet is a fine example of ancient Greek technology, made of a single piece of bronze. The art of making such helmets survived until around 1500 AD, but its secrets have now to be relearned. SR-XRD and neutron diffraction have also been used to study the bronze composition and aspects of its method of manufacture.20 EXPERIMENTAL SECTION Instrumentation. SR-FT-IR data were obtained on beamline 11.1 on the synchrotron radiation source at Daresbury Laboratory, Warrington, UK (Figure 1). The NEXUS FT-IR spectrophotometer is equipped with a Nicolet Continuµm microscope and MCT detector cooled with liquid N2, measuring range 650-4000 cm-1. The collimated synchrotron beam is introduced directly into the spectrophotometer and toward the IR microscope. The beam is very stable, with a high brightness, and allows a 10 µm2 spot to be focused on with most of the synchrotron beam. The spectra are displayed as absorbance spectra using the Nicolet OMNIC software. (18) Enrich. J. Director of the excavation of the Roman Villa in l’Espelt; Personal communication, 2004. (19) Salvado´, N. PhD Thesis, Universitat de Barcelona, 2001. (20) Pantos, E.; Kockelmann, W.; Chapon, L.; Lutterotti, L.; Bennet, S. L.; Tobin, M. J.; Mosselmans, J. F. W.; Pradell, T.; Salvado, N.; Butı´, S.; Garner, R.; Prag, A. J. N. W. Nucl. Instrum. Methods Phys. Res., Sect. B, in press.

Analytical Chemistry, Vol. 77, No. 11, June 1, 2005

3445

Figure 2. Photo of the instrument (spectrophotometer and microscope) and accessories used in order to collect spectra of large objects in reflection mode.

The synchrotron beam is focused onto the sample stage of the microscope, giving a near-diffraction-limited spot size of 8 × 8 µm on the sample (fwhm). Measurement of the IR absorbance of a sample can be made in transmission mode or in reflectance mode. For transmission measurements, the sample is usually mounted on an IR transmitting substrate, such as KBr, or BaF2. Alternatively, a diamond compression cell may be used to aid in the flattening of multilayer samples. Reflectance measurements require a highly polished surface for optimal collection of the reflected light, and a dispersion correction of the reflectance spectra is required to recover the absorbance spectrum. To enable reflectance spectra to be collected from large samples, a sideport accessory is employed to direct the infrared beam sideways from the microscope onto the sample (Figure 2). Materials and Methods. We used three general methods to obtain the infrared spectra for three different situations: transmission through a diamond cell to analyze heterogeneous micrometric samples, reflection to measure submillimetric cross sections of painting samples, and reflection using an extension attachment to study the surface of a large object that the standard geometry of the sample holder could not accommodate. Method 1. Transmission spectra were collected from the samples placed on a diamond cell viewed through the microscope system. Micrometric samples were first placed and pressed in the diamond cell, and then only one window of the cell (the one on which the sample was well distributed) was used for the measurement. The microscope allowed a single particle/compound to be pinpointed. This was crucial in separating overlapping spectra obtained from the whole layer. One of the main problems is the selection of particles/compounds from the layer, because microscopic contaminations may represent a significant fraction of the material, resulting in misleading identification of the painting layer. For this reason, microparticles/compounds carefully selected from each layer were used. The IR spectra were recorded as the average of 128 scans, at 4 cm-1 resolution with a 10-µm2 spot size. Method 2. Reflection spectra of submillimetric samples were collected with the microscope operating in reflectance mode, at 4 cm-1 resolution, 128 scans, and 10-µm2 spot size. All spectra were corrected against a background spectrum collected from a 100% reflecting gold mirror. The high signal-to-noise ratio achieved 3446


at this spatial resolution improves the quality of the spectra and, thus, opens the possibility for making measurements in reflection mode. The surface of the sample is analyzed directly without breaking up the sample into smaller fragments. This is especially important when the sample consists of a microsequence of different layers to obtain separate information from each of them. A polished cross section of the painting sample is obtained by first embedding it in polyester resin; cutting the section with a diamond saw; and finally, polishing the section with diamond paste. The Kramers-Kronig transformation to correct for dispersive effects was applied when converting reflectance spectra into absorbance spectra. Method 3. To extract information directly on the surface of a large object, a SpectraTech side-port reflectance accessory was employed. The accessory allowed beam focusing of approximately 10 × 10 µm. In addition, an x-y-z translation stage was assembled to allow the large object to be focused in front of the objective and translated in the beam. This system allows the analysis of pieces of different geometric shapes and large objects that cannot be placed directly under the microscope. It is worth noting that this mechanism allows working directly on the surface of a large object and that in this case, sampling is not required. This method introduces a new nondestructive technique for the analysis of artworks. Samples and Reference Materials. The painting samples from both the Roman wall painting and the gothic altarpieces were obtained by cutting small (of some hundreds of micrometers full size) fragments of the painting containing both chromatic and preparation layers. They consisted of a multilayered mixture of pigments, binding media, and alteration products. Cross sections of the samples were prepared by embedding the fragments in a polyester resin polymerized by a peroxorganic catalyzer (Fontanals Composites, Barcelona); then they were cut with a diamond saw of 0.15-mm thickness and, finally, polished with diamond paste (1 µm). The reference materials were either reagent grade chemicals or natural products. The carminic acid, the lead carbonate, and the lead basic carbonate were analytical grade supplied by Sigma Aldrich Chemicals. Copper acetate monohydrate and copper hydroxide carbonate were supplied by Probus. The verdigris (copper acetate) was supplied by Zecchi (Firenze). The binding media such as egg yolk, egg white, animal glue, and drying oil were prepared in our laboratory from natural sources. Some other drying oils and animal glues were supplied by Zecchi (Firenze). Because aging is very important for the binders and, in general, organic materials, preparations of all of these compounds which have been prepared in our laboratory and aged for at least 5 years were also analyzed. The aged materials show some alterations in some absorption bands, which are also found in the ancient materials. RESULTS AND DISCUSSION Roman Wall Paintings. The pigments decorating a room in the archaeological site of Espelt show a range of colors: white, red, blue, green, and black. The pigments identified were, respectively, dolomite, haematite, Egyptian blue, celadonite, and carbon black. They were studied by both SR-XRD and SR-FT-IR techniques. The painting technique corresponds to “fresco”, using

the overlay of color layers to produce the decorative motifs. The “fresco” technique consists of the application of a calcium hydroxide layer over which, while wet, a water suspension of the color pigment is applied. After drying and being in contact with the atmosphere, the calcium hydroxide transforms to calcium carbonate, fixing the pigment. SR-FT-IR allows pinpointing and separating the minute crystals of the pigment from the carbonate matrix. Figure 3 shows the infrared spectra obtained from the blue particles (Figure 3a), the green pigment (Figure 3b), and the white pigment (Figure 3c). The Blue Pigment. The infrared bands observed at 668, 756 cm-1 symmetric Si-O-Si stretching, and 1012, 1064, 1162 cm-1, and shoulder at 1255 cm-1 asymmetric Si-O-Si stretching must be attributed to a double silicate of copper and calcium, CaCuSi4O10 cuprorivaite, a compound corresponding to the main constituent of Egyptian blue pigment21-23 (Figure 3a). SR-XRD showed that apart from cuprorivaite, cristobalite (SiO2) and quartz (SiO2) are present, but in smaller quantities. The presence of cristobalite is characteristic of the synthesis of this pigment and indicates that the temperature reached was 1000 °C.24 Egyptian blue was the first synthetic pigment prepared by man of which we have evidence. The first samples were found in Egypt and correspond to the third millennium BC. Egyptian blue was used extensively in the Roman period but suddenly, with the collapse of the western Roman empire, it went out of use. The Egyptian blue pigment was synthesized to substitute for lapislazuli, a semiprecious stone which was very expensive. The synthesis of Egyptian blue is linked to glass technology and demonstrates the high technical level achieved in ancient times.21,23,25 The Green Pigment. The green earths have been used by different cultures all over the world, from ancient India to the Romans, as well as by the American Indians. They are by far the most common green pigments used in Roman wall paintings. During the Middle Ages, green earths continued to be extensively used until the Renaissance, when, coinciding with the appearance of oil painting techniques, their use declined; however, they continue to be used today.26,27 The group known as green earths includes celadonites, glauconites, and chlorites. Although chlorites can be easily differentiated by XRD, celadonites and glauconites are difficult to identify by XRD because of their equivalent structure and similarities in chemical composition. Although they can be differentiated by morphological and structural criteria,27,28 their infrared spectra show differences in the absorption bands corresponding to structural OH and Si-O groups.26,29 Celadonite and glauconite are both layer silicates of the dioctahedral mica group (21) Mirti, P.; Appolonia, L.; Casoli,; Ferrrari R. P.; Laurenti, E.; Amisano Canesi A.; Chiari G. Spectrochim. Acta 1995, 51A, N3, 437-446. (22) Bruni, S.; Cariati, F.; Casadio, F.; Toniolo, L. Vib. Spectrosc. 1999, 20, 1525. (23) Baraldi, P.; Bondoli, F.; Fagnano, C.; Ferrari, A.; Tinti, A.; Vinella, M. Ann. Chim. 2001, 91, 679-692. (24) Pradell, T.; Hatton, G.; Salvado´, N. Tite M. S. In preparation. (25) Riederer, J. Artists’ Pigments, A Handbook of Their History and Characteristics, 3. Oxford University Press: Oxford, 1997; pp 23-45. (26) Grissom, C. A. Artists’ Pigments, A Handbook of Their History and Characteristics, 1. Oxford University Press: Oxford, 1986; pp 141-167. (27) Hradil, D.; Grygar, T.; Hradilova, J.; Bezdicka, P. Appl. Clay Sci. 2003, 22, 223-236. (28) Bearat, H.; Pradell, T. Proccedings of the International workshop on Roman wall paintings: materials, techniques, analysis and conservation. Fribourg, 1997.

Figure 3. SR-FT-IR microspectroscopy spectra (128 scans, 4 cm-1 resolution, spot size 10 µm × 10 µm) taken from the Roman wall paintings from Espelt corresponding to (a) blue paint; particles of the pigment have been isolated, and the spectrum shown corresponds to the pure compound cuprorivaite (CuCaSi4O10), a synthetic copper containing a silicate known as Egyptian blue; (b) green paint; the absorption bands corresponding to celadonite, 3600, 3556, 3533, 1074, 1018 and 979, 800, and 681 cm-1, and some extra bands, 2522, 1790, 1478, 856, and 713 cm-1, which are associated with aragonite are shown; (c) white paint; the spectrum corresponds to dolomite, bands at 2528, 1821, 1451, 1081, 882, and 730 cm-1, but aragonite is also detected.

with an elemental composition of Al, Si, K, Mg, FeII, FeIII, O, and H. Si is the central atom of the tetrahedral sites; Al, FeIII, Mg, and FeII are the central atoms in the octahedral sites; and K is in Analytical Chemistry, Vol. 77, No. 11, June 1, 2005

3447

Figure 4. SR-FT-IR microspectroscopy spectra (128 scans, 4 cm-1 resolution, spot size 10 µm × 10 µm) corresponding to (a) linseed drying oil aged for 7 years; (b) green paint taken from the altarpiece “El Conestable” from Jaume Huguet (15th century AD) where a mixture of bands related to an aged linseed drying oil and a metal carboxylate are identified; (c) copper carboxylate isolated from the green paint and; (d) enlargement of the 2000-1300 cm-1 region corresponding to the three spectra: (a) thin solid line, (b) dashed line, and (c) thick solid line.

the interlayer. The main difference between glauconite and celadonite is the total number of the trivalent cations (Al, FeIII) in the octahedral site that in celadonite is similar to the total number of divalent cations; this results in a dominant R3+-R2+ OH-stretching band in the IR spectra. The green color depends on the ratio between the divalent and trivalent iron ions. The infrared spectrum of the green pigment exhibits absorption bands characteristic of celadonite. That is, the absorption bands at 3600, 3556, and 3533 cm-1 are assigned to the hydroxyl stretching, a shoulder at 1074; bands at 1018 and 979 cm-1 are related to the Si-O stretching; and finally, the two bands at 800 and 681 cm-1 are related to the R-O-H bending vibration where R is the octahedral ion26,29 (Figure 3b). The additional bands at (2546), 2522, (2499), 1790, 1478, 856, 713, (700) cm-1 found in the pigment are related to the presence of aragonite CaCO3.30 Aragonite is

found in the green earth as well as in the white pigment, dolomite, CaMg(CO3)2 (Figure 3c), but not in any of the other colors. The presence of aragonite must be related to the presence of magnesium in the green and white pigments. Aragonite precipitates instead of calcite in carbonate solutions for Mg2+/ Ca2+ > 1,31 indicating that the carbonate precipitation was produced in the presence of the pigments. Therefore, its presence may be used as an indicator of the fresco technique. Gothic Altarpieces. Previous studies of the pigments were performed by using scanning electron microscopy (SEM) equipped with an EDS microanalysis detector, SR-XRD, and FT-IR.10 From these, the identification of the pigments and binding media was accomplished. Jaume Huguet applied the pigments using the egg tempera technique.19,32 However, SR-FT-IR has given a deeper insight, in particular, for the green painting layers and the red

(29) Drits, V. A.; Dainyak, L. G.; Muller, F.; Besson G.; Manceau A. Clay Min. 1997, 32, 153-179. (30) Jones, G. C.; Jackson, B. Infrared Transmission Spectra of Carbonate Minerals; Chapman & Hall, Ed.: London, 1993.

(31) Drever, J. I. The Geochemistry of Natural Waters; Prentice Hall: New Jersey, 1997; p 223. (32) Salvado´, N.; Seco, M.; Vendrell-Saz. M. Butlletı´ MNAC, Barcelona 2001, 5, 47-58.


Figure 5. SR-FT-IR microspectroscopy spectra (128 scans, 4 cm-1 resolution, spot size 10 µm × 10 µm) corresponding to (a) polished cross section of a dark red layer from the altarpiece “El Conestable” from Jaume Huguet (15th century AD), the spectrum was obtained in reflection mode; (b) the same dark red layer, but the spectrum was obtained in transmission mode from a small red particle extracted from the sample, the bands 1078 and 1044 cm-1 correspond to a carminic acid, and the rest of the bands to proteinaceous matter; and (c) reference spectrum from a carminic acid; the spectrum was obtained in transmision mode.

lakes. The complexity of the materials and the chemical reactions between pigments and binders make it an especially interesting case study. The Green Painting Layers. The green pigments used by Jaume Huguet were determined to be a synthetic pigment containing a complex mixture of green copper compounds, such as copper basic carbonate (structure of malachite CuCO3‚Cu(OH)2), copper hydroxychlorides (structure of atacamite Cu2(OH)Cl3, paratacamite Cu2(OH)Cl3, and calumetite Cu2(OH,Cl)‚2H2O) and hydrated copper acetates (monohydrate copper acetate, basic copper acetates).10,11 The pigment was sintered, following a recipe similar to that described by Theophilus in De Diversis Artibus.33 Huguet mixed the green pigment with yellow, a synthetic lead stannate (2PbO‚SnO2), or white, a synthetic lead carbonate (hydrocerussite, Pb3(CO3)2(OH)2, and cerussite, PbCO3). The binding medium identified in the green paintings was a mixture of protein (egg) and drying oil. In fact, the green pigment was mixed with drying oil, and the white and the yellow pigments with egg. It must be noted that the use of a drying oil is specific to the green pigment, as it has never been found in any other colors of the altarpieces studied. The difficulty of studying these layers is the heavy overlap of the absorption bands with contributions of all the compounds of the green pigment, the yellow and white pigments, and protein and drying oils from the binding medium, as well as the chemical reactions between the pigments and binding media. The drying or aging process of oil is an oxidative polymerization process from which fatty acids with a lower molecular weight, dicarboxylic acids, and radicals (peroxo radicals) may be produced.34 The 3000-2800 cm-1 region bands are due to the CH stretching vibrations of the methylene and terminal methyl groups of fatty acid chains of the triglycerides. Figure 4a shows the (33) Hawthorne, J. G.; Smith, C. S. Theophilus on Divers Arts: The Foremost Medieval Treatise on Painting Glassmaking and Metalwork; Dover: New York, 1979. (34) Mills, J. S.; White, R. The Organic Chemistry of Museum Objects; Butterworth: London, 1987.

spectrum taken from aged linseed oil. The characteristic CH stretching vibration bands appear at 2926 cm-1 (asymmetric stretching CH2) and 2854 cm-1 (symmetric stretching CH2). The CdO stretching absorption of the triglyceride ester linkage appears at ∼1740 cm-1; our aged linseed oil, Figure 4a, has a band at 1737 cm-1. The free fatty acids exhibit a vibration band corresponding to the carboxylic acid group (-COOH) that appears at ∼1710 cm-1. We can see that in our aged linseed oil, Figure 4a, a band appears at 1713 cm-1. Finally, the interval between 1500 and 900 cm-1 is a fingerprint region, and the band at 721 cm-1 corresponds to CH2 rocking, both related to the fatty acids. The IR spectrum shown in Figure 4b indicates the presence of a mixture of the green pigment and the organic compounds associated with the binding media. In this case, the free fatty or dicarboxylic acids resulting from the aging of the drying oil can react with the pigments and produce new chemical compounds, such as fatty acid salts of metal cations (copper, lead, and probably, tin).35,36 SR-FT-IR using method 1 allowed us to isolate and clearly identify a copper(II) fatty acid salt in Figure 4c. For instance, the salt determined shows bands at 1585 cm-1 corresponding to the carboxylate asymmetric stretching, at 1470 cm-1 corresponding to δ CH2 and at 1411 cm-1 corresponding to the carboxylate symmetric stretching37 (Figure 4c). In particular, an absorption band at 1585 cm-1 of carboxylate stretching (IR νas COO) has been observed in the literature corresponding to a copper stearate (octadecanoic acid copper salt).38,39 Therefore, the presence of this band reveals the conversion of the carboxylic acids to carboxylate (35) Burmester, A.; Koller, J. Recent Advances in the Conservation and Analysis of Artifacs; Summer Schools Press: London 1987; pp 97-103. (36) Salvado´, N.; Molera, J.; Vendrell-Saz, M. Anal. Chim. Acta 2003, 479, 255263. (37) Redondo, M. I.; Garcıá, M. V.; Gonza´lez-Tejera, M. J.; Cheda, J. A. R. Spectrochim. Acta 1995, 51A (N3), 341-347. (38) Gunn, M.; Chottard, G.; Rivie`re, E.; Girerd, J.; Chottard J. C. Stud. Conserv. 2002, 47, 12-23. (39) Robinet, L.; Corbeil, M. C. Stud. Conserv. 2003, 48, 23-40.


3449

ions. Sometimes, a band at 3571 cm-1 or ∼3450 cm-1 related to OH stretching of hydroperoxides (ROOH) or alcohols (ROH)s products related to the aging processsalso appears. The small spot size allowed the separation and full identification of these reaction compounds. The identification of all the compounds present in the painting layers, either original or produced as a reaction of the pigment with the organic media, was extremely important in the study of the alteration mechanisms found in these layers. The Red Lake. The red pigment was identified as an organic lake (carminic acid). The presence of alum, determined by SR-XRD and SEM-EDX, is an indication of the process used to manufacture the pigment. Alum was used to precipitate the lake pigment, following ancient textile production techniques. In this case, the use of high-brightness SR infrared light improves the signal-to-noise quality of the spectra and, thus, opens the possibility for making measurements in reflection mode on these materials (method 2). Cross sections of the painting samples prepared as explained in the Materials and Methods section were studied. The sections were polished with diamond paste with the necessary precautions to avoid contamination. Figure 5a shows the SR-FT-IR spectra obtained from a dark red paint in reflection mode after application of the Kramers-Kronig transformation. A reference spectrum of a carminic acid (1078 and 1044 cm-1 absorption bands)40,41 is shown in Figure 5c. The bands, characteristic of the carminic acid, are clearly identified in the sample. This dark red layer shows the presence of a proteinaceous material (bands at 1650, 1542, 1450, 1400, and 1240 cm-1) which belongs in part to the binding medium (egg white) but which must also be related to the animal origin of the red pigment itself. As a comparison, in Figure 5b, we show the SR-FT-IR spectrum corresponding to the red paint in transmission geometry using method 1. Although the quality of the spectrum is not as high as in transmission geometry, mainly due to the higher roughness of the surface, the reflection mode opens the door to the possibility of identifying painting layers when the extraction of the sample is not possible. Corinthian-Type Bronze Helmet. In the field of analysis of objects with historical or artistic interest, the extraction of samples from the objects is limited and, in some cases, may not be possible. In these cases, the analysis has to be able to work directly on the objects, and obviously, the technique used must be nondestructive. Both XRD and FT-IR are nondestructive, but the geometry and size of the objects and the resolution of the techniques working in reflection mode heavily limit their application. Although FT-IR working in reflection on unpolished surface mode gives poorer results than in transmission mode, the use of SR helps to overcome this problem. Moreover, the detection system should be adapted to focus on the surface of a large 3D object using method 3. Although only the compounds present on the surface can be determined, the technique does allow selection of different areas of the surface of the object; thus, a choice of representative sampling points may be measured without extracting samples from the object. One such important museum object we studied was a Corinthian-type bronze helmet.20 The corrosion products were analyzed, (40) Flieder, F. Stud. Conserv. 1968, 13, 87-97. (41) Schweppe, H.; Rossen-Runge, H. Artist’s pigments. A Handbook of Their History and Characteristics, 1. Oxford University Press: Oxford, 1986; pp 255-283.


Figure 6. SR-FT-IR microspectroscopy spectra (128 scans, 4 cm-1 resolution, spot size 10 µm × 10 µm) obtained in reflection mode directly over the surface of a Corinthian-type bronze helmet from the Manchester Museum. The spectra were obtained from (a) the noseguard where hydrocerussite, 2PbCO3‚Pb(OH)2, bands are marked with a cross and from (b) the helmet itself, where malachite, CuCO3‚ Cu(OH)2, bands are marked with a dot. Both compounds result from the alteration of the alloy.

and from them, it was possible to determine the authenticity of the noseguard piece soldered onto the rest of the helmet. A protein compound associated with animal glue was determined, distributed all over the helmet; this glue was very likely applied to prevent further corrosion before the helmet was acquired by the Museum. Differences were observed between the corrosion products identified on the noseguard piece and the rest of the helmet. In particular, hydrocerussite 2PbCO3‚Pb(OH)2 was found only on the noseguard piece (Figure 6a), whereas malachite CuCO3‚Cu(OH)2 was found all over the rest of the helmet (Figure 6b). The presence of hydrocerussite indicates that the noseguard piece was made of a lead-containing copper alloy, but the rest of the helmet was made of a lead-free copper alloy. Consequently, it can be concluded that the noseguard must be a replacement of the original piece. This is consistent with SR-XRF, SR-XRD, and neutron diffraction measurements of the helmet, which have shown that a brass (Cu-Zn) alloy containing some lead was used to produce the noseguard piece, but the rest of the object is made of a bronze (Cu-Sn) alloy.20 Brass has a high welding capability, ensuring a good adherence to the bronze helmet. This study shows how SR-FT-IR spectra taken on the surface of museum objects may give a lot of information about the object itself and about the alteration/corrosion products without any destructive

sampling. The method described allows the study of 2D and 3D objects with a spatial resolution of 10 micrometers or bellow. This could be extremely interesting, for instance, for the study of manuscripts (inks, paintings, alterations) and the surface degradation and corrosion of large museum objects. CONCLUSIONS SR-FT-IR microspectroscopy has been demonstrated to be an ideal tool for the characterization of heterogeneous and complex micrometric mixtures of art materials. It has been used in this work for the study and analysis of ancient paintings, revealing the nature of the pigments, binding media, and reaction compounds, as well as aging and alteration products. Microspectroscopic studies were carried out on single microparticles extracted from submillimetric samples. We have also demonstrated the capabilities of the technique in reflection mode either on small samples or directly on large objects. SR-FT-IR in this mode fulfils the requirement necessary to be considered as a truly nondestructive technique for cultural heritage applications.

ACKNOWLEDGMENT This work has been developed and funded in part by the Memorandum of Understanding Agreement between the Patrimoni UB-UPC (Universitat de Barcelona and Universitat Polite`cnica de Catalunya) and CCLRC-Daresbury Laboratory and in part by CCLRC Grant 41127. Dr. T. Pradell has been funded by Ministerio de Educacion y Ciencia Grant no. MAT2004-01214. We acknowledge Mr. J. Pradell for his advice and help in sampling and Mr. J. M. Xarrie´, Head of the Serveis de Restauracio´ de Beńs Mobles (Generalitat de Catalunya) for the access given to the gothic altarpieces studied. We also thank the Museu de la pell d’Igualada i Comarcal de l’Anoìa and ArqueoCat for their assistance and permission to access the wall paintings from the Roman villa from Espelt.

Received for review January 22, 2005. Accepted March 30, 2005. AC050126K


3451

Advantages of the Use of SR-FT-IR Microspectroscopy: Applications

Recommend Documents