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The application of Raman Spectroscopy for the Non-destructive Analysis of Art Objects Peter Vandenabeele and Luc Moens Ghent University ­ Laboratory of Analytical Chemistry Proeftuinstraat 86 ­ B­9000 Ghent Tel. +32 9 264 66 23 ­ Fax. +32 9 264 66 99 Contact E-mail: peter.vandenabeele@rug.ac.be

Introduction

The spectroscopic examination of artworks is of high importance for conservators, art-historians and keepers of private or public museum collections. These investigations reveal information, which is of general (art-) historical interest: the knowledge of the artistic materials that were used and were available at a certain period in a particular region and the dissemination of new methods provide information about the interactions between distinct cultures and about trade routes. The knowledge on the artists' materials that were available in particular regions and periods can help in dating artefacts. The retrieval of pigments with a well-known date of invention enables to date the artefact post quem. Other materials are known to have disappeared from the artists' palette, because they were substituted by others and retrieving enables to date artefacts ante quem. Finding anachronisms in the materials that were used, is a straightforward way for the exposure of counterfeit masterpieces. Another method consists of the comparison with the materials that were used in known works of the same artist. If it is well-established that a certain artist in a large group of works from a particular period never used certain pigments, finding these materials in a suspect painting deepens the suspicion and invites for further examinations. Besides these reasons for the spectroscopic examination of objects of art, an important purpose of this work is to help conservators in finding the reasons of the deterioration of a certain artwork and helping them in optimising the conditions of conservation. The main purpose of any analytical examination of artefacts should be to gain as much information as possible in a non-destructive way.
Molecular Raman spectroscopy is well-suited for this purpose: it enables the identification of inorganic [1] and organic pigments [2], as well as binding media and varnishes [3]. By focusing a laser on a sample the intensity of the inelastically scattered light is plotted against the Raman wavenumber, which is proportional to the difference in energy between the laser and the scattered light. By using a microscope to focus the laser beam on the sample, i.e. micro-Raman spectroscopy, one can get spectra from particles with a diameter down to 1 µm, which is about the usual dimension of artists' pigments. This high lateral resolution can be used for the examination of
1. embedded stratigraphic samples, of
2. micro-samples, or even for the
3. direct investigation of artefacts that can be positioned under the microscope. For large artefacts
4. fibre optics can be used for remote investigation. In this case there is loss of spatial resolution to some mm².
In this work examples of these four different ways of application of Raman spectroscopy are given.

Experimental

Micro-Raman investigations are performed using a dispersive Renishaw System­1000 Ramanspectrometer. The device is fitted with a diode laser operating at 50 mW and 780 nm. An Olympus BH-2 microscope is used to focus the laser on the sample, using objective lenses of 5x, 20x, 50x and 80x magnification. The microscope is equipped with binoculars and a colour video camera, allowing to position the sample and to select a specific region for investigation. The backscattered light passes through a holographic notch filter (HNF), which rejects the abundant elastic Rayleigh scattering, to avoid outshining of the weaker Raman signal. The backscattered light is dispersed by using a 1200 lines / mm grating and is detected on a Peltier-cooled CCD-detector. This configuration allows to record spectra with a spectral resolution of ca. 1 datapoint / cm^-1 in the spectral region between 150 and 2500 cm^-1. The fibre-optics Ramanspectra are recorded on a Bruker FT-Raman spectrometer, allowing to record spectra from 250 cm^-1 onwards.

Direct Analysis of 19^th Century Porcelain Cards

Micro-Raman spectroscopy can be used for the non-destructive direct analysis of small objects of art that can be positioned under the microscope optics of the spectrometer. Examples of such small objects of art are 19^th Century porcelain cards. These luxurious products served as status symbols of the bourgeoisie and were used as business cards or invitations for parties. The cards were manufactured in a combined process of printing the text and illustration and colouring by hand. In Flanders only a confined number of printers fabricated these cards and the number of colours that were used was limited. It was our purpose to reveal which colorants were used in these cards. Therefore several porcelain cards were analysed directly under the Raman microscope.
In Fig. 1 the geometry for direct micro-Raman analysis is shown together with some Raman spectra from the investigation of a porcelain card of the Ghent University. This porcelain card, which is dated 1843, was released on the occasion of two fancy dress balls that were organised by the students of the university. As is shown in Fig. 1, the recorded spectra are of sufficient quality to allow identification of the pigment material. The bright blue areas are painted with a Prussian blue (Fe₄[Fe(CN)₆]₃) containing paint. This blue iron-containing pigment was for the first time synthesised by Diesbach in Berlin in 1704. It can easily be identified via the band at ca. 2150 cm^{- 1}, which is caused by the v(CN) stretch. The darker blue areas are painted with ultramarine (Na_8..10Al₆Si₆O₂₄S_2..4), while the spectrum of the red paint in Fig. 1.3.c is that of vermilion (HgS). The yellow areas are painted with a chrome yellow (PbCrO₄) containing paint. The card is whitened with white lead (2 PbCO₃ · Pb(OH)₂).

1.1 Geometry for direct analysis. 1.2 Porcelain card of the Ghent University: invitation for a masquerade by the students (dated 1843). 1.3 Raman spectra of coloured regions of this porcelain card: a. Bright blue: Prussian blue, b. Dark blue: Ultramarine, c. Red: Vermilion, d. Yellow: Chrome yellow, e. White: White lead.
Fig 1: Direct analysis of a 19th Century porcelain card.

Raman Analysis using Fibre Optics

Direct analysis of art objects with larger dimensions necessitates the use of fibre optics. The panel painting Baby Elephant was painted by Lucebert, a member of the Cobra movement. For the non-destructive investigation of that 20^th Century panel painting fibre optics FT-Raman was used in order to identify the pigments in the artwork. The painting is part of the collection of the S.M.A.K., the municipal museum of contemporary art in Ghent (Belgium). The spectra that are presented in Fig. 2 are of sufficient quality for identification of the pigments. The Raman spectrum of titanium white is presented, together with the organic pigments PY 3 and PR 4. PY 3 is a Hansa yellow mono-azo pigment, PR 4 is of the b-Naphthol type.

Fig 2: Fibre optics Raman analysis of Baby Elephant. a. Titanium white; b1. Green area; b2. Reference of PY 3; c1. Red area; c2. Reference of PR 4.

Micro-Raman Examination of Embedded Stratified Samples

Another possibility for the analysis of painted artefacts is the investigation of the superposed layers in small samples of the paint, embedded in a resin. These stratified samples are frequently used by conservators for the examination of the structure of paintings, polychrome sculptures or furniture, and allow to identify the frequently observed overpaintings. Thanks to the good lateral resolution it is possible to record with Raman spectroscopy spectra of the different layers of an embedded stratigraphic sample. In Fig. 3 a Ramanspectrum of a stratified sample from a historical table (dated ca. 1500) is presented. The spectrum of the greenisch layer reveals the presence of Prussian blue (Fe₄[Fe(CN)₆]₃), but the spectrum also contains the features of starch, which may have been used as binding medium. Barite can be added during the production process of the pigment or may be used as a filler in the paint.

Fig 3: Ramanspectrum out of the greenisch layer of a stratigraphic sample (bottom) in combination with reference spectra from Prussian blue, starch and barite.

Gentle Micro-sampling Method and a Combined Method Approach

Transport of precious objects of art to the laboratory is not always possible. and sampling is required. The largest disadvantage of stratigraphic samples is that this method causes visible damage to the artefact. To overcome this severe disadvantage a microsampling method was developed, consisting of gently rubbing a dry and clean cotton bud (Q-tip) over the paint surface [4]. Without leaving a trace on the artefact, a small amount (ca. < 1 µg) of pigment and binding medium is removed from the top layer, which largely meets the needs for multiple investigations using micro-Raman spectroscopy and other micro-analytical methods. An example of this approach is given by the examination of two Egyptian burial masks [5]. Due to the fragility of these antique artefacts, it was impossible to transport these to the laboratory. Therefore, micro-samples were taken, using the Q-tip method. The samples were examined by a combined approach of molecular micro-Raman spectroscopy and (atomic) total-reflection X-ray fluorescence analysis (TXRF). Some Ramanspectra are shown in Fig. 4. TXRF analysis showed that the beige sample contained considerable amounts of As, being an indication for the presence of the orange realgar (AsS) or yellow orpiment (As₂S₃). Raman analysis revealed that the yellow particles were none of these pigments, but were identified as para-realgar, a yellow photo-degradation product of realgar (Fig. 4). This brings up some interesting questions for art-history and conservation science. One could wonder whether the ancient Egyptians applied the orange realgar (which degenerated on the mask to para-realgar) or a yellow pigment. Were they aware of the photo-degradation of realgar and did they know the difference between orpiment and para-realgar?

Fig 4: Raman investigation of micro-samples from Egyptian burial masks. a­b. A combined spectrum of vermilion (Hgs, a.) and haematite (Fe2O3, b.). c. Ramanspectrum of para-realgar (AsS) and (d.) realgar (AsS)

Conclusion

In this work the application of Raman spectroscopy in the field of non-destructive art analysis is demonstrated. The method can be used for direct analysis of small stratified samples embedded in a resin, for micro-samples, taken by the gentle Q-tip method and for direct analysis. Small artefacts can be analysed as such under the Raman microscope, for large objects of art fibre optics can be applied. Molecular Raman spectroscopy is applicable to the identification of inorganic pigments, of 20^th Century synthetic organic colorants and for the identification of the binding media, which makes the method suitable for art-analysis of modern as well as ancient artworks.

Acknowledgements

The authors thank Alex von Bohlen, Reinhold Klockenkämper, Howell Edwards, Paul De Paepe, Frederika Huys, Freya Joukes, Georges Dewispelaere and Francis Verpoort for their help and support.

References

1. B. Wehling, P. Vandenabeele, L. Moens, R. Klockenkämper, A. von Bohlen, G. Van Hooydonk and M. De Reu, Mikrochim. Acta 130, 253-260 (1999).
2. P. Vandenabeele, L. Moens, H.G.M. Edwards and R. Dams, A Raman Spectroscopic Database of Azo-pigments and application to Modern Art Studies, J. Raman Spectr. 2000, In Press.
3. P. Vandenabeele, B. Wehling, L. Moens, H. Edwards, M. De Reu and G. Van Hooydonk, Anal. Chim. Acta 407, 261-274 (2000).
4. L. Moens, W. Devos, R. Klockenkämper and A. von Bohlen, J. Trace and Microprobe Techn. 13, 119-139 (1995).
5. P. Vandenabeele, A. von Bohlen, L. Moens, R. Klockenkämper, F. Joukes and G. Dewispelaere, Spectroscopic Examination of Two Egyptian Masks: a Combined Method Approach, Anal. Letters 2000, Submitted.

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