- Email: [email protected]
Microanalysis of artworks: IR microspectroscopy of paint cross-sections
Vibrational Spectroscopy 53 (2010) 77–82
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Microanalysis of artworks: IR microspectroscopy of paint cross-sections Robyn Sloggett a, Caroline Kyi a, Nicole Tse a, Mark J. Tobin b, Ljiljana Puskar b, Stephen P. Best c,* a
Centre for Conservation of Cultural Materials, University of Melbourne, Victoria 3010, Australia Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia c School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 September 2009 Received in revised form 21 January 2010 Accepted 27 January 2010 Available online 4 February 2010
FTIR microspectroscopy of a highly stratified paint cross-section from a late nineteenth century commercial building is described. The use of a Ge attenuated total reflectance (ATR) microscope objective and synchrotron IR source allow collection of high-quality spectra from a 5 mm 5 mm area of sample in contact with the Ge-ATR crystal. These measurements reflect the clear distinction between the major components that comprise the different paint layers and, more importantly, reveal differences in the local composition within visually similar regions, assisting in the identification of the minor components of the layer. While responsible for the appearance, intensely colored pigments are often present in low-concentration and give vanishingly weak spectral signatures. When combined with high spatial resolution, the presence of highly pigmented imperfections in the paint layer is shown to provide a means of identification of highly diluted pigments. ß 2010 Elsevier B.V. All rights reserved.
Keywords: ATR microscopy IR spectroscopy Paint artwork Synchrotron IR microscopy
1. Introduction Cultural material objects and built heritage offer a tangible link to, and enable a reading of, the past. The detail able to be extracted from these objects depends both on the identification of the major and minor components and knowledge of their spatial distribution. These details are important both in terms of our ability to properly ‘read’ these physical documents of our past and to establish effective protocols for their conservation. The analysis of the pigment layers, binders and substratum in a sample of paint can reveal common practice, technological developments and the availability of materials which, in turn, can provide the evidence needed to establish provenance and authenticity. Scientific analysis has an important part to play in understanding the development and shifts in cultural and technological knowledge in societies. The focus of this manuscript is the application of infrared microspectroscopy to the analysis of the components of highly stratified paint cross-sections with the very high spatial resolution (5 mm) made possible by the use of a synchrotron IR source. Vibrational spectroscopy offers important advantages for the study of pigments, these centre on the specificity of the results to the molecular or crystal form and the availability of sampling approaches that permit non-destructive analysis with high spatial resolution [1–4]. Owing to the relative ease of conducting experiments with resolution close to 1 mm, the initial vibrational
Abbreviations: ATR, attenuated total reflection; FTIR, Fourier transform infrared. * Corresponding author. Tel.: +61 3 83446505; fax: +61 3 93475180. E-mail address: [email protected] (S.P. Best). 0924-2031/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.01.021
studies of pigments mostly utilized Raman microscopy, and these showed that highly specific spectra could be obtained from single pigment grains contained within the paint medium [1–5]. Despite the considerable strengths of Raman spectroscopy for applications of this sort, there are a number of limitations inherent to the technique and these mostly are associated with its low sensitivity, in the absence of resonance enhancement, and potential interference from fluorescence processes. While the latter issue may be resolved in most cases by use of long wavelength excitation the former is problematic when analysis is concerned with species present in low-concentration. IR spectroscopy is complementary to Raman in terms of the character of the molecular vibrations giving intense spectral features, the wavelengths of light used to probe the molecular vibrations and the inherent sensitivity of the technique. Consequently, IR spectroscopy presents particular advantages for the study of organic pigments, binders, varnishes and solvents. The spatial resolution of the technique is limited by the characteristics of the IR source and the diffraction limiting focus, this being wavelength-dependent. Laboratory-based IR transmission or reflectance microscopy has a useful spatial resolution of ca. 50 mm and this limits the applicability of the method to analysis of paint cross-sections. Improved spatial resolution may be achieved by the use of objectives that incorporate attenuated total reflectance (ATR), particularly in cases where the crystal has a high refractive index [6]. The application of ATR-FTIR microscopy to multi-layered crosssections with single element and focal plane array detectors have recently been published [7–10]. The high brightness of the synchrotron IR source presents enormous advantages for highresolution imaging applications where a spatial resolution of 5 mm
78
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
Fig. 1. Exterior view of the Provincial Hotel, Melbourne, Victoria.
in the mid-IR region is routinely available on many such instruments, including that at the Australian Synchrotron [11]. Synchrotron-based FTIR microscopy has been applied to the study of extracts from microscopic paint samples pressed between the windows of a diamond clamp cell [12]. The present investigation involves the study of paint cross-sections by synchrotron-based ATR-FTIR microscopy.
are likely to have been used in the paint application. A key focus of our research interest is the materials and methods used in 20th Century painting. In this context the cross-sections from the Provincial Hotel provide an ideal test case for the use of IR microscopy for the elucidation of this information.
1.1. Overview
A range of integrated tools and techniques are needed for the investigation of cultural material. Where possible conservators avoid taking samples from objects and through-air in situ examination is preferred. However, when interest is focused on the substratum or on individual layers within a paint matrix, it is often necessary to remove a sampled cross-section. The samples examined in this study were acquired from previously detached areas associated with the late 19th century fac¸ade of the Provincial Hotel. Because such a sample will usually contain a number of variable materials, such as resins, oils, inorganic and organic pigments and the like, and because there is an ethical need to remove as little of the original material as possible from the object, a battery of analytical techniques is generally required. This means that analysis generally starts with the least interventive procedure; that is, one that will provide information while leaving the sample intact [13]. IR microspectroscopy is a key technique in this category [14].
The Provincial Hotel (Fig. 1) is a landmark in Melbourne’s inner city suburb of Fitzroy. It is a building that represents the changing socio-economic role of Melbourne inner city pubs. Originally a suburb with a strong working class demographic, Fitzroy is now part of the expensive and highly desirable inner city and is home to a completely different demographic. The facade of the building supports a palimpsest of decorative schemes and painted signage popular from the 1880s through to the 1950s. The complexity of paint layers is reflected in the dense stratigraphy of cross-section samples (Fig. 2). The binders used in each of the paint layers remains unknown and would reflect the history of paint technology over the last century. Lime-washes, oils and acrylics
1.2. Sampling
2. Material and methods 2.1. Preparation of paint cross-sections
Fig. 2. Visible image obtained using the IR microscope of the polyester-embedded paint cross-section from the exterior wall of the Provincial Hotel. The most recently painted layer corresponds to L8.
Samples were embedded in a polyester resin using a methylethyl ketone peroxide catalyst [15]. After curing for several days (at least two days is required before the resin reaches required hardness) the polyester block was cut to reveal the sample giving a block approximately 17 mm 10 mm 3 mm in dimension. The sample exposed face was polished using various grades of abrasives and ultimately a 3 mm grade diamond polish. In addition, fresh sample surfaces were obtained by microtoming selected samples for comparison with those which had been polished. In this way it was possible to ascertain the impact of contamination of the sample induced by the polishing process [15].
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
79
2.2. Infrared microscopy Spectra were recorded using a Bruker Hyperion 2000 microscope in conjunction with a Vertex V80 FTIR spectrometer fitted with a liquid-nitrogen cooled narrow-band MCT detector and using the IR beamline of the Australian Synchrotron as the radiation source. Measurements were made using a Ge-ATR objective (refractive index, Ge = 4.00 at l = 10 mm) and all measurements were conducted using an optical aperture of 20 mm 20 mm where, owing to refraction, this corresponds to a 5 mm 5 mm sampling region on the surface of the Ge crystal. The spatial resolution of the Australian Synchrotron IR microscope has previously been verified by measurement of photolithographically generated polymer targets on CaF2 support [11]. Contact between the 100 mm diameter tip and the sample was maintained by application of a force of ca. 0.6 N, the lowest available setting for the ATR objective. Examination of the sample at the conclusion of measurements revealed minor indentation but no indication of redistribution of the components of the sample. The absorbance spectra presented were calculated using a spectrum obtained from a single crystal of CaF2 as reference. The advantage of this approach, over the more common practice of using an air gap, is that reference spectra could be measured using the Hyperion 2000 microscope without disturbing the air purge about the sample. While CaF2 absorbs at wavenumbers below 1000 cm 1 this did not result in a noticeable distortion of the spectra between 1000 and 750 cm 1. A spectral resolution of 4 cm 1 was used and high-quality spectra could be obtained by coaddition of 32 double-sided inteferograms (64 scans) with an overall measurement time per point of ca. 28 s. The Bruker Opus version 6.5 software was used to control data collection. The automated XY stage permitted mapping experiments and in these cases spectra were recorded from a 5 mm spaced grid and a fresh reference spectrum was generally recorded at intervals of 20 sample spectra. No correction was applied to take into account the wavelength-dependent penetration depth, consequently, relative to transmission spectra the intensities of the higher wavenumber bands are attenuated and there are shifts of the intense absorption bands to lower wavenumber. 3. Results A typical paint cross-section removed from the Provincial Hotel is shown in Fig. 2. The upper layers of the image of the crosssection correspond to the original base coats, including the priming layers and earlier color schemes. IR spectra were recorded as line scans across the width of the sample and also from rectangular areas at specific locations from the sample. The latter approach affords the advantage of allowing an assessment of the variation of the spectra from a region that appears to be homogeneous on visual inspection. Further, irregularities in the surface topology, mostly introduced in the course of microtoming, were easily avoided by careful selection of the sampling region. The sampling regions discussed in this investigation are indicated by the labels shown in Fig. 2. Representative IR spectra obtained from the polymer resin and eight of the distinct horizons of the sample (L1–L8) are shown in Fig. 3. Owing to the long-wavelength cutoff of the narrow-band MCT detector the spectral range is limited to 3800–750 cm 1. While there is considerable variation in the spectra obtained within the sample regions the initially painted layers (L1–L3) feature strong bands near 1000 and 1400 cm 1 that are attributed, respectively, to ochre/kaolinite and calcium carbonate. Layer L1 is marked by a dark discoloration with a series of weak bands with a profile similar to that of the polyester polymer with strong bands near 1000 cm 1 (Fig. 3) that match those from IR spectra of various
Fig. 3. Representative Ge-ATR infrared spectra obtained from the polyester support and the regions L1–L8 indicated in Fig. 2. In each case the spectrum was obtained from a 5 mm 5 mm sample region.
sources of ochre/kaolinite listed on the IRUG spectral database (IMP0023, IMP00032 and IMP00082) [16]. The weaker, higher wavenumber, bands are most likely due to an organic resin binder, however some contamination due to ingress of the polyester polymer into the outermost layer of the sample cannot at this stage be ruled out. At least four layers of paint are associated with L2 and L3 where the base color ranges from cream to a pale orange. The paler layers of L2 contain predominantly calcium carbonate with the bands at 1400 and 873 cm 1 being in close agreement with those expected for calcite [17]. Additional features, consistent with the presence of an organic binder and ochre/kaolinite are distributed within this region, albeit with lower relative intensity. Within L3 the bands due to ochre/kaolinite are much more intense and the organic component of the spectrum differs from that of the polyester support (Fig. 3). Additionally, higher-wavenumber bands above 3500 cm 1 track in intensity with the intense band near 1000 cm 1. These features are most likely due to hydroxyl groups associated with the clay. A distinct, pale blue layer (L4) of ca. 28 mm thickness separates the predominantly orange and lilac regions of the sample. The IR
80
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
spectrum from L4 is similar to that of the polyester support although there are differences in the relative intensities of the CH stretches (near 2900 cm 1) and the 1722-cm 1 band. The spectral features in this layer due to the organic component differ in relative intensity but not in form and most closely match that of an alkyd-based paint (e.g. IMX00002) [18]. The lilac-colored paint layers (from L5 to L6) encompass a region ca. 135 mm thick containing significant organic (alkyd) components together with bands attributable to sulfate, and silicates (L5) which are most likely attributed to gypsum [17] and ochre/kaolinite [18]. Within L6 the main inorganic component is due to calcite. The following, darker, layer L7 gives very intense bands indicating the presence of sulfate (gypsum) together with alkyd resin and hydroxide. An additional, as yet unidentified, component is marked by the presence of a prominent band at 1620 cm 1. The final layer, L8, has a somewhat irregular interface with L7 and is painted with an organic (polyester or alkyd) based paint. 4. Discussion 4.1. Impact of spatial resolution on spectral analysis While the foregoing discussions reflect the well-recognised suitability of IR spectroscopy to identification of the main components of the paint sample, the main object of this investigation is the evaluation of ATR-based IR microscopy for analysis of paint cross-sections. Of central importance is the level of spatial resolution needed for effective analysis. This point is addressed through closer consideration of layers L2 and L4. As already noted, L2 gives IR spectra dominated by bands readily attributed to calcium carbonate (calcite) together with weaker bands below 1750 cm 1 that are most likely due to an organic binder and a band near 1000 cm 1 attributed to ochre/ kaolinite. Spectra obtained from a 5 mm spaced 5 6 grid from a region of sample of homogeneous visual appearance are shown in Fig. 4. For these measurements the IR beam addresses a 5 mm 5 mm area of the sample, that is less than 0.5% of the 100 mm diameter of the ATR crystal. Variation in contact between the heterogeneous sample and the area interrogated by the IR beam will impact on the spectra and this may be manifested by changes in bandshape (e.g. the 873-cm 1 band in Fig. 4), baseline shifts or a loss in intensity over the spectrum. Notwithstanding the overall similarity of the spectra there are clear variations that are apparent for the spectra shown in Fig. 4. The presence of ochre/kaolinite is largely concentrated in the lower two rows and an additional organic component, marked by the appearance of the band at 1537 cm 1, apparent in the upper layers of the sample grid. There appears to be significant distortion of the band profile of the antisymmetric carbonate stretching mode (ca. 1400 cm 1). This may be due to the form of the carbonate (amorphous/calcite) or interaction between the calcium carbonate and other components of the medium, in this case kaolinite and/or water. It is noted that the sharp carbonate bending mode (ca. 873 cm 1) occurs with a constant relative intensity and wavenumber. While adding to the complexity of the analysis, the availability of spectra with high spatial resolution provides significant advantages for the identification of the components present in the paint sample. Significant problems are associated with the identification of pigments that are highly dilute within the matrix. In very favourable cases it is possible to overcome this problem by the use of resonance-enhanced Raman measurements, this being clearly illustrated by objects decorated with lapus lazuli, or its synthetic form ultramarine [5]. Layer L4 of the sample from the Provincial Hotel (Fig. 2) has a pale blue hue, where spectra obtained from this region have a profile matching closely that of
Fig. 4. Ge-ATR infrared spectra obtained from a 6 5 grid covering a 25 mm 20 mm area at sample point L2. Spectra are overlaid according to their row (the number scheme for the lower left and upper right sample points are indicated in the optical image of the sample).
polyester or alkyd resins. In addition there are darker regions that appear to be due to impurities within the paint horizon. The spectra obtained from a region including an impurity of 7 mm 5 mm dimension are shown in Fig. 5. Spectra obtained from the impurity give a distinct band at 2088 cm 1. It is noted also that there is a difference in registration between the optical and IR images by 15 mm in the vertical. A similar offset can be discerned from the spectral images that include visually distinct markers. The strong band at 2088 cm 1, associated with the impurity particle, is characteristic of a material containing strongly bound species such as cyanide. Prussian blue, one of the first synthetic pigments, is a mixed-valence iron cyanide compound of idealised formula Fe7(CN)18 with variable amounts of water and other cations. The transmission spectrum of a Winsor and Newton alkyd Prussian blue paint sample IMX00003 [18] features a CN stretching band at 2095 cm 1. IR microscopy of a sample of Prussian blue paint using the Ge-ATR objective give bands near 2083 cm 1, where the exact wavenumber of the band maximum depends on the absorbance of the sample. The apparent shift of the band position, together with the distortion of the leading edge of the band profile, is commonly observed for spectra obtained using ATR methods. The penetration depth of the evanescent wave depends on the refractive indices of the ATR crystal and the sample. While the refractive index of a material generally is not highly wavelength-dependent, this situation breaks down near a strong absorption band and it is this that leads to the spectral distortions. When these effects are taken into account there is an excellent match between the spectra obtained from the dark regions of L4 and spectra of
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
81
4.3. Spectral analysis While offering significant advantages in terms of sampling, issues related to the wavelength-dependent variation in the penetration depth of the evanescent wave can create problems with the interpretation of the spectral results. The main difficulty in this regard is the large shift of the band maximum for strongly absorbing samples. These effects are likely to complicate multicomponent analysis for IR spectra recorded using this approach. Notwithstanding these issues, the present study suggests that the spectral data are extremely rich and welldefined and this provides support for the proposition that information regarding both major and minor components of the paint layer may be obtained using IR spectra obtained using ATR microscopy. 5. Conclusions
Fig. 5. Ge-ATR infrared spectra obtained from columns of sample points within a 30 mm 20 mm area within layer L4.
authenticated samples of Prussian blue. Consequently the pale blue effect apparent in layer L4 is due to a very dilute mixture of Prussian blue in an alkyd-based paint. Over the majority of the paint layer the concentration of pigment is insufficient to give a clearly defined band near 2090 cm 1, but in regions where there is aggregation the identity of the pigment is readily determined. 4.2. Sampling issues Even within a thin paint cross-section there can be considerable variance in the physical characteristics of the sample [16]. This is dependent on both the pigment and the medium in each layer. For example some pigments absorb, or are coated with, more medium during production often resulting in a softer sample or others may contain a resin that develops substantial cross-linking to give a hard material. For ATR microscopy the hardness of the sample becomes a significant issue because even at minimum pressure the 100 mm diameter of the ATR crystal exerts significant force on the sample leading to indentation of reasonably hard samples through to reconstitution of ‘‘soft’’ materials. For the paint cross-section of the Provincial hotel examined in this work, the collection of spectra is accompanied by relatively minor indentation and this may be discerned by close inspection of Fig. 2. A similar level of indentation was obtained when sampling the polyester support. In no case in the present investigation was there any indication of problems due to redistribution of the sample during the course of the spectral measurements.
The present study of the highly stratified paint cross-section from the Provincial Hotel demonstrate clearly that excellent quality IR spectra can be routinely obtained from the surfaces of non-transparent samples and with excellent spatial resolution using ATR microscopy. While there is potential for contamination of the sample by the embedding polymer, the extent of this problem was not found to be significant with similar spectra obtained from polished and microtomed samples. It is less clear whether there is inundation into the outer layers of the paint crosssection during the curing process. We note that approaches such as encapsulating the paint fragment in KBr prior to embedding in polyester [10] may help resolve questions of this sort. Notwithstanding issues related to sample hardness, the approach is well matched to the requirements of pigment analysis from paint cross sections and the spectra thereby obtained are extremely rich and provide information both from major organic and inorganic pigments as well as from the components of the medium. The very high spatial resolution able to be obtained when using a synchrotron IR source is critical to the usefulness of the approach. The spatial distribution of the components of the medium has a length scale in the micron range and spectra obtained with spatial resolution poorer than about 10 mm will generate averaged spectra from which it is much more difficult to identify the component parts. This proposition is reflected by the variation of the spectra obtained from an apparently homogeneous region of a sample (Fig. 4) and identification of Prussian blue as the pigment responsible for the pale blue color of layer L4 (Fig. 5). While much more work needs to be done to refine the strategies that will yield a more complete identification of the components of the sample, these results suggest that efforts directed toward that end will yield results that will be of practical importance to workers in as diverse fields as art history and forensic science. Acknowledgements This research was undertaken on the infrared microspectroscopy beamline at the Australian Synchrotron, Victoria, Australia. The University of Melbourne is thanked for the award of foundation investor beamtime during which initial investigations were undertaken. The award of a Linkage grant from the Australian Research Council is gratefully acknowledged. References [1] R.J.H. Clark, C. R. Chim. 5 (2002) 7–20. [2] P. Vandenabeele, H.G.M. Edwards, L. Moens, Chem. Rev. 107 (2007) 675–686. [3] C.L. Aibeo, S. Goffin, O. Schalm, G. van der Snickt, N. Laquiere, P. Eyskens, K. Janssens, J. Raman Spectrosc. 39 (2008) 1091–1098. [4] H.G.M. Edwards, Pract. Spectrosc. 28 (2001) 1011–1044.
82 [5] [6] [7] [8]
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
S.P. Best, R.J.H. Clark, R. Withnall, Endeavour (New Series) 16 (1992) 66–73. L. Lewis, A.J. Sommer, Appl. Spectrosc. 53 (1999) 375–380. A. Rizzo, Anal. Bioanal. Chem. 392 (2008) 47–55. M. Spring, C. Ricci, D.A. Peggie, S.G. Kazarian, Anal. Bioanal. Chem. 392 (2008) 37– 45. [9] E. Joseph, S. Prati, G. Sciutto, M. Ioele, P. Santopadre, R. Mazzeo, Anal. Bioanal. Chem. 396 (2010) 899–910. [10] R. Mazzeo, E. Joseph, S. Prati, A. Millemaggi, Anal. Chim. Acta 599 (2007) 107–117. [11] http://www.synchrotron.org.au/index.php/aussyncbeamlines/infrared-micro/ technical-information2, accessed 17 January 2010.
[12] N. Salvado, S. Buti, J. Nicholson, H. Emerich, A. Labrador, T. Pradell, Talanta 79 (2009) 419–428. [13] J. Plesters, Stud. Conserv. 2 (1965) 11–157. [14] N. Khandekar, Rev. Conserv. IIC (2003) 52–64. [15] E. Veckert, Stain Technol. 35 (1963) 261–265. [16] M. Derrick, L. Souza, T. Kielich, H. Florisheim, D. Stulik, J. Am. Inst. Conserv. 33 (1994) 227–245. [17] RRUF Project, http://rruff.info/, accessed 14 January 2010. [18] Infrared and Raman Users Group, Spectral Database, http://www.irug.org/, accessed 23 October 2009.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Microanalysis of artworks: IR microspectroscopy of paint cross-sections Robyn Sloggett a, Caroline Kyi a, Nicole Tse a, Mark J. Tobin b, Ljiljana Puskar b, Stephen P. Best c,* a
Centre for Conservation of Cultural Materials, University of Melbourne, Victoria 3010, Australia Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia c School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 September 2009 Received in revised form 21 January 2010 Accepted 27 January 2010 Available online 4 February 2010
FTIR microspectroscopy of a highly stratified paint cross-section from a late nineteenth century commercial building is described. The use of a Ge attenuated total reflectance (ATR) microscope objective and synchrotron IR source allow collection of high-quality spectra from a 5 mm 5 mm area of sample in contact with the Ge-ATR crystal. These measurements reflect the clear distinction between the major components that comprise the different paint layers and, more importantly, reveal differences in the local composition within visually similar regions, assisting in the identification of the minor components of the layer. While responsible for the appearance, intensely colored pigments are often present in low-concentration and give vanishingly weak spectral signatures. When combined with high spatial resolution, the presence of highly pigmented imperfections in the paint layer is shown to provide a means of identification of highly diluted pigments. ß 2010 Elsevier B.V. All rights reserved.
Keywords: ATR microscopy IR spectroscopy Paint artwork Synchrotron IR microscopy
1. Introduction Cultural material objects and built heritage offer a tangible link to, and enable a reading of, the past. The detail able to be extracted from these objects depends both on the identification of the major and minor components and knowledge of their spatial distribution. These details are important both in terms of our ability to properly ‘read’ these physical documents of our past and to establish effective protocols for their conservation. The analysis of the pigment layers, binders and substratum in a sample of paint can reveal common practice, technological developments and the availability of materials which, in turn, can provide the evidence needed to establish provenance and authenticity. Scientific analysis has an important part to play in understanding the development and shifts in cultural and technological knowledge in societies. The focus of this manuscript is the application of infrared microspectroscopy to the analysis of the components of highly stratified paint cross-sections with the very high spatial resolution (5 mm) made possible by the use of a synchrotron IR source. Vibrational spectroscopy offers important advantages for the study of pigments, these centre on the specificity of the results to the molecular or crystal form and the availability of sampling approaches that permit non-destructive analysis with high spatial resolution [1–4]. Owing to the relative ease of conducting experiments with resolution close to 1 mm, the initial vibrational
Abbreviations: ATR, attenuated total reflection; FTIR, Fourier transform infrared. * Corresponding author. Tel.: +61 3 83446505; fax: +61 3 93475180. E-mail address: [email protected] (S.P. Best). 0924-2031/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.01.021
studies of pigments mostly utilized Raman microscopy, and these showed that highly specific spectra could be obtained from single pigment grains contained within the paint medium [1–5]. Despite the considerable strengths of Raman spectroscopy for applications of this sort, there are a number of limitations inherent to the technique and these mostly are associated with its low sensitivity, in the absence of resonance enhancement, and potential interference from fluorescence processes. While the latter issue may be resolved in most cases by use of long wavelength excitation the former is problematic when analysis is concerned with species present in low-concentration. IR spectroscopy is complementary to Raman in terms of the character of the molecular vibrations giving intense spectral features, the wavelengths of light used to probe the molecular vibrations and the inherent sensitivity of the technique. Consequently, IR spectroscopy presents particular advantages for the study of organic pigments, binders, varnishes and solvents. The spatial resolution of the technique is limited by the characteristics of the IR source and the diffraction limiting focus, this being wavelength-dependent. Laboratory-based IR transmission or reflectance microscopy has a useful spatial resolution of ca. 50 mm and this limits the applicability of the method to analysis of paint cross-sections. Improved spatial resolution may be achieved by the use of objectives that incorporate attenuated total reflectance (ATR), particularly in cases where the crystal has a high refractive index [6]. The application of ATR-FTIR microscopy to multi-layered crosssections with single element and focal plane array detectors have recently been published [7–10]. The high brightness of the synchrotron IR source presents enormous advantages for highresolution imaging applications where a spatial resolution of 5 mm
78
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
Fig. 1. Exterior view of the Provincial Hotel, Melbourne, Victoria.
in the mid-IR region is routinely available on many such instruments, including that at the Australian Synchrotron [11]. Synchrotron-based FTIR microscopy has been applied to the study of extracts from microscopic paint samples pressed between the windows of a diamond clamp cell [12]. The present investigation involves the study of paint cross-sections by synchrotron-based ATR-FTIR microscopy.
are likely to have been used in the paint application. A key focus of our research interest is the materials and methods used in 20th Century painting. In this context the cross-sections from the Provincial Hotel provide an ideal test case for the use of IR microscopy for the elucidation of this information.
1.1. Overview
A range of integrated tools and techniques are needed for the investigation of cultural material. Where possible conservators avoid taking samples from objects and through-air in situ examination is preferred. However, when interest is focused on the substratum or on individual layers within a paint matrix, it is often necessary to remove a sampled cross-section. The samples examined in this study were acquired from previously detached areas associated with the late 19th century fac¸ade of the Provincial Hotel. Because such a sample will usually contain a number of variable materials, such as resins, oils, inorganic and organic pigments and the like, and because there is an ethical need to remove as little of the original material as possible from the object, a battery of analytical techniques is generally required. This means that analysis generally starts with the least interventive procedure; that is, one that will provide information while leaving the sample intact [13]. IR microspectroscopy is a key technique in this category [14].
The Provincial Hotel (Fig. 1) is a landmark in Melbourne’s inner city suburb of Fitzroy. It is a building that represents the changing socio-economic role of Melbourne inner city pubs. Originally a suburb with a strong working class demographic, Fitzroy is now part of the expensive and highly desirable inner city and is home to a completely different demographic. The facade of the building supports a palimpsest of decorative schemes and painted signage popular from the 1880s through to the 1950s. The complexity of paint layers is reflected in the dense stratigraphy of cross-section samples (Fig. 2). The binders used in each of the paint layers remains unknown and would reflect the history of paint technology over the last century. Lime-washes, oils and acrylics
1.2. Sampling
2. Material and methods 2.1. Preparation of paint cross-sections
Fig. 2. Visible image obtained using the IR microscope of the polyester-embedded paint cross-section from the exterior wall of the Provincial Hotel. The most recently painted layer corresponds to L8.
Samples were embedded in a polyester resin using a methylethyl ketone peroxide catalyst [15]. After curing for several days (at least two days is required before the resin reaches required hardness) the polyester block was cut to reveal the sample giving a block approximately 17 mm 10 mm 3 mm in dimension. The sample exposed face was polished using various grades of abrasives and ultimately a 3 mm grade diamond polish. In addition, fresh sample surfaces were obtained by microtoming selected samples for comparison with those which had been polished. In this way it was possible to ascertain the impact of contamination of the sample induced by the polishing process [15].
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
79
2.2. Infrared microscopy Spectra were recorded using a Bruker Hyperion 2000 microscope in conjunction with a Vertex V80 FTIR spectrometer fitted with a liquid-nitrogen cooled narrow-band MCT detector and using the IR beamline of the Australian Synchrotron as the radiation source. Measurements were made using a Ge-ATR objective (refractive index, Ge = 4.00 at l = 10 mm) and all measurements were conducted using an optical aperture of 20 mm 20 mm where, owing to refraction, this corresponds to a 5 mm 5 mm sampling region on the surface of the Ge crystal. The spatial resolution of the Australian Synchrotron IR microscope has previously been verified by measurement of photolithographically generated polymer targets on CaF2 support [11]. Contact between the 100 mm diameter tip and the sample was maintained by application of a force of ca. 0.6 N, the lowest available setting for the ATR objective. Examination of the sample at the conclusion of measurements revealed minor indentation but no indication of redistribution of the components of the sample. The absorbance spectra presented were calculated using a spectrum obtained from a single crystal of CaF2 as reference. The advantage of this approach, over the more common practice of using an air gap, is that reference spectra could be measured using the Hyperion 2000 microscope without disturbing the air purge about the sample. While CaF2 absorbs at wavenumbers below 1000 cm 1 this did not result in a noticeable distortion of the spectra between 1000 and 750 cm 1. A spectral resolution of 4 cm 1 was used and high-quality spectra could be obtained by coaddition of 32 double-sided inteferograms (64 scans) with an overall measurement time per point of ca. 28 s. The Bruker Opus version 6.5 software was used to control data collection. The automated XY stage permitted mapping experiments and in these cases spectra were recorded from a 5 mm spaced grid and a fresh reference spectrum was generally recorded at intervals of 20 sample spectra. No correction was applied to take into account the wavelength-dependent penetration depth, consequently, relative to transmission spectra the intensities of the higher wavenumber bands are attenuated and there are shifts of the intense absorption bands to lower wavenumber. 3. Results A typical paint cross-section removed from the Provincial Hotel is shown in Fig. 2. The upper layers of the image of the crosssection correspond to the original base coats, including the priming layers and earlier color schemes. IR spectra were recorded as line scans across the width of the sample and also from rectangular areas at specific locations from the sample. The latter approach affords the advantage of allowing an assessment of the variation of the spectra from a region that appears to be homogeneous on visual inspection. Further, irregularities in the surface topology, mostly introduced in the course of microtoming, were easily avoided by careful selection of the sampling region. The sampling regions discussed in this investigation are indicated by the labels shown in Fig. 2. Representative IR spectra obtained from the polymer resin and eight of the distinct horizons of the sample (L1–L8) are shown in Fig. 3. Owing to the long-wavelength cutoff of the narrow-band MCT detector the spectral range is limited to 3800–750 cm 1. While there is considerable variation in the spectra obtained within the sample regions the initially painted layers (L1–L3) feature strong bands near 1000 and 1400 cm 1 that are attributed, respectively, to ochre/kaolinite and calcium carbonate. Layer L1 is marked by a dark discoloration with a series of weak bands with a profile similar to that of the polyester polymer with strong bands near 1000 cm 1 (Fig. 3) that match those from IR spectra of various
Fig. 3. Representative Ge-ATR infrared spectra obtained from the polyester support and the regions L1–L8 indicated in Fig. 2. In each case the spectrum was obtained from a 5 mm 5 mm sample region.
sources of ochre/kaolinite listed on the IRUG spectral database (IMP0023, IMP00032 and IMP00082) [16]. The weaker, higher wavenumber, bands are most likely due to an organic resin binder, however some contamination due to ingress of the polyester polymer into the outermost layer of the sample cannot at this stage be ruled out. At least four layers of paint are associated with L2 and L3 where the base color ranges from cream to a pale orange. The paler layers of L2 contain predominantly calcium carbonate with the bands at 1400 and 873 cm 1 being in close agreement with those expected for calcite [17]. Additional features, consistent with the presence of an organic binder and ochre/kaolinite are distributed within this region, albeit with lower relative intensity. Within L3 the bands due to ochre/kaolinite are much more intense and the organic component of the spectrum differs from that of the polyester support (Fig. 3). Additionally, higher-wavenumber bands above 3500 cm 1 track in intensity with the intense band near 1000 cm 1. These features are most likely due to hydroxyl groups associated with the clay. A distinct, pale blue layer (L4) of ca. 28 mm thickness separates the predominantly orange and lilac regions of the sample. The IR
80
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
spectrum from L4 is similar to that of the polyester support although there are differences in the relative intensities of the CH stretches (near 2900 cm 1) and the 1722-cm 1 band. The spectral features in this layer due to the organic component differ in relative intensity but not in form and most closely match that of an alkyd-based paint (e.g. IMX00002) [18]. The lilac-colored paint layers (from L5 to L6) encompass a region ca. 135 mm thick containing significant organic (alkyd) components together with bands attributable to sulfate, and silicates (L5) which are most likely attributed to gypsum [17] and ochre/kaolinite [18]. Within L6 the main inorganic component is due to calcite. The following, darker, layer L7 gives very intense bands indicating the presence of sulfate (gypsum) together with alkyd resin and hydroxide. An additional, as yet unidentified, component is marked by the presence of a prominent band at 1620 cm 1. The final layer, L8, has a somewhat irregular interface with L7 and is painted with an organic (polyester or alkyd) based paint. 4. Discussion 4.1. Impact of spatial resolution on spectral analysis While the foregoing discussions reflect the well-recognised suitability of IR spectroscopy to identification of the main components of the paint sample, the main object of this investigation is the evaluation of ATR-based IR microscopy for analysis of paint cross-sections. Of central importance is the level of spatial resolution needed for effective analysis. This point is addressed through closer consideration of layers L2 and L4. As already noted, L2 gives IR spectra dominated by bands readily attributed to calcium carbonate (calcite) together with weaker bands below 1750 cm 1 that are most likely due to an organic binder and a band near 1000 cm 1 attributed to ochre/ kaolinite. Spectra obtained from a 5 mm spaced 5 6 grid from a region of sample of homogeneous visual appearance are shown in Fig. 4. For these measurements the IR beam addresses a 5 mm 5 mm area of the sample, that is less than 0.5% of the 100 mm diameter of the ATR crystal. Variation in contact between the heterogeneous sample and the area interrogated by the IR beam will impact on the spectra and this may be manifested by changes in bandshape (e.g. the 873-cm 1 band in Fig. 4), baseline shifts or a loss in intensity over the spectrum. Notwithstanding the overall similarity of the spectra there are clear variations that are apparent for the spectra shown in Fig. 4. The presence of ochre/kaolinite is largely concentrated in the lower two rows and an additional organic component, marked by the appearance of the band at 1537 cm 1, apparent in the upper layers of the sample grid. There appears to be significant distortion of the band profile of the antisymmetric carbonate stretching mode (ca. 1400 cm 1). This may be due to the form of the carbonate (amorphous/calcite) or interaction between the calcium carbonate and other components of the medium, in this case kaolinite and/or water. It is noted that the sharp carbonate bending mode (ca. 873 cm 1) occurs with a constant relative intensity and wavenumber. While adding to the complexity of the analysis, the availability of spectra with high spatial resolution provides significant advantages for the identification of the components present in the paint sample. Significant problems are associated with the identification of pigments that are highly dilute within the matrix. In very favourable cases it is possible to overcome this problem by the use of resonance-enhanced Raman measurements, this being clearly illustrated by objects decorated with lapus lazuli, or its synthetic form ultramarine [5]. Layer L4 of the sample from the Provincial Hotel (Fig. 2) has a pale blue hue, where spectra obtained from this region have a profile matching closely that of
Fig. 4. Ge-ATR infrared spectra obtained from a 6 5 grid covering a 25 mm 20 mm area at sample point L2. Spectra are overlaid according to their row (the number scheme for the lower left and upper right sample points are indicated in the optical image of the sample).
polyester or alkyd resins. In addition there are darker regions that appear to be due to impurities within the paint horizon. The spectra obtained from a region including an impurity of 7 mm 5 mm dimension are shown in Fig. 5. Spectra obtained from the impurity give a distinct band at 2088 cm 1. It is noted also that there is a difference in registration between the optical and IR images by 15 mm in the vertical. A similar offset can be discerned from the spectral images that include visually distinct markers. The strong band at 2088 cm 1, associated with the impurity particle, is characteristic of a material containing strongly bound species such as cyanide. Prussian blue, one of the first synthetic pigments, is a mixed-valence iron cyanide compound of idealised formula Fe7(CN)18 with variable amounts of water and other cations. The transmission spectrum of a Winsor and Newton alkyd Prussian blue paint sample IMX00003 [18] features a CN stretching band at 2095 cm 1. IR microscopy of a sample of Prussian blue paint using the Ge-ATR objective give bands near 2083 cm 1, where the exact wavenumber of the band maximum depends on the absorbance of the sample. The apparent shift of the band position, together with the distortion of the leading edge of the band profile, is commonly observed for spectra obtained using ATR methods. The penetration depth of the evanescent wave depends on the refractive indices of the ATR crystal and the sample. While the refractive index of a material generally is not highly wavelength-dependent, this situation breaks down near a strong absorption band and it is this that leads to the spectral distortions. When these effects are taken into account there is an excellent match between the spectra obtained from the dark regions of L4 and spectra of
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
81
4.3. Spectral analysis While offering significant advantages in terms of sampling, issues related to the wavelength-dependent variation in the penetration depth of the evanescent wave can create problems with the interpretation of the spectral results. The main difficulty in this regard is the large shift of the band maximum for strongly absorbing samples. These effects are likely to complicate multicomponent analysis for IR spectra recorded using this approach. Notwithstanding these issues, the present study suggests that the spectral data are extremely rich and welldefined and this provides support for the proposition that information regarding both major and minor components of the paint layer may be obtained using IR spectra obtained using ATR microscopy. 5. Conclusions
Fig. 5. Ge-ATR infrared spectra obtained from columns of sample points within a 30 mm 20 mm area within layer L4.
authenticated samples of Prussian blue. Consequently the pale blue effect apparent in layer L4 is due to a very dilute mixture of Prussian blue in an alkyd-based paint. Over the majority of the paint layer the concentration of pigment is insufficient to give a clearly defined band near 2090 cm 1, but in regions where there is aggregation the identity of the pigment is readily determined. 4.2. Sampling issues Even within a thin paint cross-section there can be considerable variance in the physical characteristics of the sample [16]. This is dependent on both the pigment and the medium in each layer. For example some pigments absorb, or are coated with, more medium during production often resulting in a softer sample or others may contain a resin that develops substantial cross-linking to give a hard material. For ATR microscopy the hardness of the sample becomes a significant issue because even at minimum pressure the 100 mm diameter of the ATR crystal exerts significant force on the sample leading to indentation of reasonably hard samples through to reconstitution of ‘‘soft’’ materials. For the paint cross-section of the Provincial hotel examined in this work, the collection of spectra is accompanied by relatively minor indentation and this may be discerned by close inspection of Fig. 2. A similar level of indentation was obtained when sampling the polyester support. In no case in the present investigation was there any indication of problems due to redistribution of the sample during the course of the spectral measurements.
The present study of the highly stratified paint cross-section from the Provincial Hotel demonstrate clearly that excellent quality IR spectra can be routinely obtained from the surfaces of non-transparent samples and with excellent spatial resolution using ATR microscopy. While there is potential for contamination of the sample by the embedding polymer, the extent of this problem was not found to be significant with similar spectra obtained from polished and microtomed samples. It is less clear whether there is inundation into the outer layers of the paint crosssection during the curing process. We note that approaches such as encapsulating the paint fragment in KBr prior to embedding in polyester [10] may help resolve questions of this sort. Notwithstanding issues related to sample hardness, the approach is well matched to the requirements of pigment analysis from paint cross sections and the spectra thereby obtained are extremely rich and provide information both from major organic and inorganic pigments as well as from the components of the medium. The very high spatial resolution able to be obtained when using a synchrotron IR source is critical to the usefulness of the approach. The spatial distribution of the components of the medium has a length scale in the micron range and spectra obtained with spatial resolution poorer than about 10 mm will generate averaged spectra from which it is much more difficult to identify the component parts. This proposition is reflected by the variation of the spectra obtained from an apparently homogeneous region of a sample (Fig. 4) and identification of Prussian blue as the pigment responsible for the pale blue color of layer L4 (Fig. 5). While much more work needs to be done to refine the strategies that will yield a more complete identification of the components of the sample, these results suggest that efforts directed toward that end will yield results that will be of practical importance to workers in as diverse fields as art history and forensic science. Acknowledgements This research was undertaken on the infrared microspectroscopy beamline at the Australian Synchrotron, Victoria, Australia. The University of Melbourne is thanked for the award of foundation investor beamtime during which initial investigations were undertaken. The award of a Linkage grant from the Australian Research Council is gratefully acknowledged. References [1] R.J.H. Clark, C. R. Chim. 5 (2002) 7–20. [2] P. Vandenabeele, H.G.M. Edwards, L. Moens, Chem. Rev. 107 (2007) 675–686. [3] C.L. Aibeo, S. Goffin, O. Schalm, G. van der Snickt, N. Laquiere, P. Eyskens, K. Janssens, J. Raman Spectrosc. 39 (2008) 1091–1098. [4] H.G.M. Edwards, Pract. Spectrosc. 28 (2001) 1011–1044.
82 [5] [6] [7] [8]
R. Sloggett et al. / Vibrational Spectroscopy 53 (2010) 77–82
S.P. Best, R.J.H. Clark, R. Withnall, Endeavour (New Series) 16 (1992) 66–73. L. Lewis, A.J. Sommer, Appl. Spectrosc. 53 (1999) 375–380. A. Rizzo, Anal. Bioanal. Chem. 392 (2008) 47–55. M. Spring, C. Ricci, D.A. Peggie, S.G. Kazarian, Anal. Bioanal. Chem. 392 (2008) 37– 45. [9] E. Joseph, S. Prati, G. Sciutto, M. Ioele, P. Santopadre, R. Mazzeo, Anal. Bioanal. Chem. 396 (2010) 899–910. [10] R. Mazzeo, E. Joseph, S. Prati, A. Millemaggi, Anal. Chim. Acta 599 (2007) 107–117. [11] http://www.synchrotron.org.au/index.php/aussyncbeamlines/infrared-micro/ technical-information2, accessed 17 January 2010.
[12] N. Salvado, S. Buti, J. Nicholson, H. Emerich, A. Labrador, T. Pradell, Talanta 79 (2009) 419–428. [13] J. Plesters, Stud. Conserv. 2 (1965) 11–157. [14] N. Khandekar, Rev. Conserv. IIC (2003) 52–64. [15] E. Veckert, Stain Technol. 35 (1963) 261–265. [16] M. Derrick, L. Souza, T. Kielich, H. Florisheim, D. Stulik, J. Am. Inst. Conserv. 33 (1994) 227–245. [17] RRUF Project, http://rruff.info/, accessed 14 January 2010. [18] Infrared and Raman Users Group, Spectral Database, http://www.irug.org/, accessed 23 October 2009.