Raman microscopy in art history and conservation science



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Conservation and ancient technology studies
The characteristics of Raman microscopy that make it so well suited for the applied research mentioned above also allow it to be used to great effect for basic research in conservation and archaeology. This type of work involves understanding the chemistry and degradation mechanisms of historical materials and pigments, assessing the effectiveness of conservation procedures and rediscovering the ancient technologies required for the production of certain pigments. The structural sensitivity of Raman microscopy has been used to specify S3-., S2-. and another unidentified species, as the principal chromophores in a series of differently-coloured ultramarine pigments [5]; to investigate the transformation process for the light-induced conversion of realgar to pararealgar through an intermediate phase [23]; and to optimize the experimental conditions for the non-destructive analysis of various lead pigments by Raman microscopy [88, 89]. Further investigation into the thermal decomposition of lead compounds by several techniques including Raman spectroscopy has elucidated better the ancient process by which red lead was generated from the roasting of lead white [117]. Notably, a previously unknown intermediate phase of lead white was discovered. The formation of Pb3O4 was found to depend on the amount of oxygen, the temperature and the duration of heating, and these conditions were shown to effect the production of lead(II) oxides that either superseded the formation of red lead or greatly influenced the final shade of that pigment.

The investigation of degraded organic materials has also been pursued. The ability of FT-Raman spectroscopy to monitor structural differences between ancient vellums and parchments has been shown, although the results suggest that the ability of this technique to indicate their state of degradation is questionable [118]. A similar Raman analysis combined with IR and Brillouin spectroscopies examined the molecular changes imposed on parchment that has undergone burning and restoration [119]. Although the Raman technique is particularly well suited to monitoring changes in the inorganic components of the manuscript substrates, the analysis of the organic components relied heavily on the IR data. A study of the structural effects of various bleaching agents on ancient papers also utilized FTIR and Raman spectroscopy [120]. In this study, the IR data were significantly complicated by the high water content of the papers both before and after treatment with aqueous bleaching solutions. Moreover, the IR analysis required that portions of the paper be destroyed in making IR-transmissive KBr pellets. The Raman data showed more clearly in this instance the apparent oxidation of cellulose to form carbonyl moieties. These spectra also revealed residual carbonate ions from treatment of the papers with 2% sodium percarbonate (2Na2CO3.3H2O2) solution, even after washing.

The examination of an ink print suffering from a powdery white bloom revealed that the efflorescence was p-toluenesulfonamide, the active ingredient in the bleach Chloramine-T, most likely a residue of a recorded, but unspecified, previous restoration process [121]. The same analysis showed the presence of CaCO3, which could have been formed on the surface of the print from atmospheric action on Ca(OH)2 remaining from a de-acidification treatment.

In a clever experiment, spectroscopists working with conservators have used Raman microscopy to examine the surface of faded and tarnished daguerreotypes [122]. These artefacts, the earliest form of photography, are composed of copper plates with a nano-particulate coating of silver/mercury amalgam that was used to record the image. The density of the metal particles provides the shading of the photograph. Disappointingly, it was found that conventional Raman spectra collected from tarnished areas of the daguerreotypes provided only weak spectral features consistent with the presence of silver oxidation products. However, deposition of dye molecules on the sub-micrometre structured layer of the image generated surface-enhanced Raman spectra (SERS, [2]) of the dye in

contact with the nanoparticles. SERS spectra of brilliant cresyl blue and fast cresyl violet dyes were collected and suggest that the technique might be used for enhancing or retrieving faded images on degraded daguerreotypes.

Much attention has been raised by the possibility of using lasers in the cleaning of paintings and stone artefacts. Raman microscopy has been employed in a study to test the sensitivity of various pigments to UV laser light [123] and to monitor the integrity of underlying stone when being cleaned with pulsed NIR radiation [124]. Others have determined the limits of detection for Raman spectroscopy when analysing mixtures of calcite, gypsum and aragonite (orthorhombic CaCO3) [125]. The technique is shown to be superior to XRD as a means of monitoring marble degradation by sulfurous pollution, as demonstrated on a marble sample from the Athens National Garden.

Raman spectroscopy was used in an investigation of the mechanisms responsible for the curing and degradation of oil paint films [126]. The temporal evolution of Raman spectra taken from white paint films (ZnO or TiO2 in safflower oil) exposed to UV radiation did not reveal evidence of either radical termination reactions or Diels-Alder type crosslinking, the two mechanisms which are believed to bring about the 'drying' of oil paints. Rather, the spectral responses - a decrease in the intensity of the C=C stretch (1660 cm"1) and a temporary increase in that of the RO-OH stretch (870 cm"') - have been interpreted as evidence for a hydroperoxide addition reaction to carbon-carbon double bonds as the principal curing mechanism for these oil paints. However, ZnO and TiO2 are known to generate both singlet oxygen and OH radicals upon irradiation [127], and therefore these pigments, chosen for their resistance to heating from the UV lamps, in fact add further variables that should have been considered. Although the Raman technique is thus shown to be applicable to the investigation of complex reactions in the ageing of oil paints, the possible influence of these powerful oxidants on the paint samples renders the conclusions of this study suspect.

Conclusion

A strong argument can be made for the inclusion of modern Raman microscopes among the analytical tools commonly found in the museum laboratory. In order to realize this role, however, many of the historical perceptions of Raman microscopy as a spectroscopic novelty with limited applications and numerous instrumental limitations must be dismissed. Furthermore, future work using Raman microscopy must move beyond the proof-of-concept stage that currently dominates the literature and begin to tackle in-depth art historical projects and pressing scientific problems where its many advantages can be realized. This work must also begin to integrate fully the expertise of the spectroscopist, curator and conservator and result in publications that ultimately reach the intended audience, either by appearing in conservation journals or through a new awareness among the conservation community of the developing location for these articles in applied spectroscopy journals. As the review portion of this article shows, Raman microscopy already has a strong record of applications in conservation research, and recent increasing interest among conservation scientists is likely to lead to greater numbers of museums investing in this powerful technique.
principal chromophores in a series of differently-coloured ultramarine pigments [5]; to investigate the transformation process for the light-induced conversion of realgar to pararealgar through an intermediate phase [23]; and to optimize the experimental conditions for the non-destructive analysis of various lead pigments by Raman microscopy [88, 89]. Further investigation into the thermal decomposition of lead compounds by several techniques including Raman spectroscopy has elucidated better the ancient process by which red lead was generated from the roasting of lead white [117]. Notably, a previously unknown intermediate phase of lead white was discovered. The formation of Pb3O4 was found to depend on the amount of oxygen, the temperature and the duration of heating, and these conditions were shown to effect the production of lead(II) oxides that either superseded the formation of red lead or greatly influenced the final shade of that pigment.

The investigation of degraded organic materials has also been pursued. The ability of FT-Raman spectroscopy to monitor structural differences between ancient vellums and parchments has been shown, although the results suggest that the ability of this technique to indicate their state of degradation is questionable [118]. A similar Raman analysis combined with IR and Brillouin spectroscopies examined the molecular changes imposed on parchment that has undergone burning and restoration [119]. Although the Raman technique is particularly well suited to monitoring changes in the inorganic components of the manuscript substrates, the analysis of the organic components relied heavily on the IR data. A study of the structural effects of various bleaching agents on ancient papers also utilized FTIR and Raman spectroscopy [120]. In this study, the IR data were significantly complicated by the high water content of the papers both before and after treatment with aqueous bleaching solutions. Moreover, the IR analysis required that portions of the paper be destroyed in making IR-transmissive KBr pellets. The Raman data showed more clearly in this instance the apparent oxidation of cellulose to form carbonyl moieties. These spectra also revealed residual carbonate ions from treatment of the papers with 2% sodium percarbonate (2Na2CO3.3H2O2) solution, even after washing.

The examination of an ink print suffering from a powdery white bloom revealed that the efflorescence was p-toluenesulfonamide, the active ingredient in the bleach Chloramine-T, most likely a residue of a recorded, but unspecified, previous restoration process [121]. The same analysis showed the presence of CaCO3, which could have been formed on the surface of the print from atmospheric action on Ca(OH)2 remaining from a de-acidification treatment.

In a clever experiment, spectroscopists working with conservators have used Raman microscopy to examine the surface of faded and tarnished daguerreotypes [122]. These artefacts, the earliest form of photography, are composed of copper plates with a nano-particulate coating of silver/mercury amalgam that was used to record the image. The density of the metal particles provides the shading of the photograph. Disappointingly, it was found that conventional Raman spectra collected from tarnished areas of the daguerreotypes provided only weak spectral features consistent with the presence of silver oxidation products. However, deposition of dye molecules on the sub-micrometre structured layer of the image generated surface-enhanced Raman spectra (SERS, [2]) of the dye in

contact with the nanoparticles. SERS spectra of brilliant cresyl blue and fast cresyl violet dyes were collected and suggest that the technique might be used for enhancing or retrieving faded images on degraded daguerreotypes.

Much attention has been raised by the possibility of using lasers in the cleaning of paintings and stone artefacts. Raman microscopy has been employed in a study to test the sensitivity of various pigments to UV laser light [123] and to monitor the integrity of underlying stone when being cleaned with pulsed NIR radiation [124]. Others have determined the limits of detection for Raman spectroscopy when analysing mixtures of calcite, gypsum and aragonite (orthorhombic CaCO3) [125]. The technique is shown to be superior to XRD as a means of monitoring marble degradation by sulfurous pollution, as demonstrated on a marble sample from the Athens National Garden.

Raman spectroscopy was used in an investigation of the mechanisms responsible for the curing and degradation of oil paint films [126]. The temporal evolution of Raman spectra taken from white paint films (ZnO or TiO2 in safflower oil) exposed to UV radiation did not reveal evidence of either radical termination reactions or Diels-Alder type crosslinking, the two mechanisms which are believed to bring about the 'drying' of oil paints. Rather, the spectral responses - a decrease in the intensity of the C=C stretch (1660 cm"1) and a temporary increase in that of the RO-OH stretch (870 cm"') - have been interpreted as evidence for a hydroperoxide addition reaction to carbon-carbon double bonds as the principal curing mechanism for these oil paints. However, ZnO and TiO2 are known to generate both singlet oxygen and OH radicals upon irradiation [127], and therefore these pigments, chosen for their resistance to heating from the UV lamps, in fact add further variables that should have been considered. Although the Raman technique is thus shown to be applicable to the investigation of complex reactions in the ageing of oil paints, the possible influence of these powerful oxidants on the paint samples renders the conclusions of this study suspect.

Conclusion


A strong argument can be made for the inclusion of modern Raman microscopes among the analytical tools commonly found in the museum laboratory. In order to realize this role, however, many of the historical perceptions of Raman microscopy as a spectroscopic novelty with limited applications and numerous instrumental limitations must be dismissed. Furthermore, future work using Raman microscopy must move beyond the proof-of-concept stage that currently dominates the literature and begin to tackle in-depth art historical projects and pressing scientific problems where its many advantages can be realized. This work must also begin to integrate fully the expertise of the spectroscopist, curator and conservator and result in publications that ultimately reach the intended audience, either by appearing in conservation journals or through a new awareness among the conservation community of the developing location for these articles in applied spectroscopy journals. As the review portion of this article shows, Raman microscopy already has a strong record of applications in conservation research, and recent increasing interest among conservation scientists is likely to lead to greater numbers of museums investing in this powerful technique.

38 Edwards, H.G.M., Farwell, D.W., Newton, E.M., Perez, F.R. and Villar, S.J., 'Raman Spectroscopic Studies of a 13th Century Polychrome Statue: Identification of a 'Forgotten' Pigment', Journal of Raman Spectroscopy 31, 2000, pp. 407-13.

39 Mills, J.S. and White, R., Organic Chemistry of Museum Objects, Butterworth-Heinemann, Oxford, 1994.

40 Clark, R.J.H. and Gibbs, P.J., 'Identification of Lead(II) Sulfide and Pararealgar on a 13th Century Manuscript by Raman Microscopy', Chemical Communications 1997, pp. 1003-4.

41 Clark, R.J.H., 'Pigment Identification by Spectroscopic Means: An Arts/Science Interface', Comptes Rendus Chimie 5, 2002, pp. 7-20; Brown, K.L. and Clark, R.J.H., to be published.

42 Cariati, F. and Bruni, S., 'Raman Spectroscopy', in Ciliberto, E. and Spoto, G., eds, Modern Analytical Methods in Art and Archaeology, Wiley, New York, 2000, pp. 255-78.

43 Chalmers, J. and Griffiths, P.R., eds, The Handbook of Vibrational Spectroscopy, Wiley, New York, 2001.

44 Clark, R.J.H. and Gibbs, P.J., 'Analysis of 16th Century Qazwini Manuscripts by Raman Microscopy and Remote Laser Raman Microscopy', Journal of Archaeological Sciences 25, 1998, pp. 621-9.

45 Vandenabeele, P., Verpoort, F. and Moens, L., 'Non-destructive Analysis of Paintings Using Fourier Transform Raman Spectroscopy with Fibre Optics', Journal of Raman Spectroscopy 32, 2001, pp. 263-9.

46 Clark, R.J.H. and Huxley, K., 'Raman Spectroscopic Study of the Pigments on a Large Illuminated Qur'an Circa Thirteenth Century', Science and Technology for Cultural Heritage 5,1996, pp. 95-101.

47 Burgio, L. and Clark, R.J.H., 'Comparative Pigment Analysis of Six Modern Egyptian Papyri and an Authentic One of the 13th Century BC by Raman Microscopy and Other Techniques', Journal of Raman Spectroscopy 31, 2000, pp. 395-401.

48 Burgio, L., Clark, R.J.H. and Williams, K.P.J., 'The Use of Raman Spectroscopy in the Art World', in McCrone, W.C. and Weiss, R.J., eds, Fakebusters II, Scientific Detection of Fakery in Art, SPIE, Chicago, 2001, pp. 138-49.

49 Edwards, H.G.M., Farwell, D.W., Seddon, T. and Tait, J.K.F., 'Scrimshaw: Real or Fake? A Fourier-Transform Raman Diagnostic Study', Journal of Raman Spectroscopy 26,1995, pp. 623-8.

50 Edwards, H.G.M. and Farwell, D.W., 'Ivory and Simulated Ivory Artefacts: Fourier Transform Raman Diagnostic Study', Spectrochimica Ada A 51, 1995, pp. 2073-81.

51 Edwards, H.G.M. and Farwell, D.W., 'Fourier Transform-Raman Spectroscopy of Amber', Spectrochimica Ada A 52, 1996, pp. 1119-25.

52 Schrader, B., Schulz, H., Andreev, G.N., Klump, H.H. and Sawatzki, J., 'Non-destructive NIR-FT-Raman Spectroscopy of Plant and Animal Tissues, of Food and Works of Art', Talanta 53, 2000, pp. 35-45.

53 Derbyshire, A. and Withnall, R., 'Pigment Analysis of Portrait Miniatures Using Raman Microscopy', Journal of Raman Spectroscopy 30, 1999, pp. 185-8.

54 Corset, J., Dhamelincourt, P. and Barbillat, J., 'Raman Microscopy', Chemistry in Britain 1989, pp. 612-6.

55 Guineau, B., 'Analyse Non Destructive des Pigments par Microsonde Raman Laser: Exemples de l'Azurite et de la Malachite', Studies in Conservation 29, 1983, pp. 35-41.

56 Vezin, J., 'La Microsonde Raman Laser: Un Nouvel Instrument d'Analyse des Pigments dans les Enluminures', Scriptorium 38, 1984, pp. 325-6.

57 Hughes, S., 'Blues for the Chemist', New Scientist 22/29, 1990, pp. 21-4.

58 Guineau, B., 'Microanalysis of Painted Manuscripts and of Colored Archaeological Materials by Raman Laser Microprobe', Journal of Forensic Science 29, 1984, pp. 471-85.

59 Guineau, B., Coupry, C, Gousset, M.T., Forgerit, J.P. and Vezin, J., 'Identification de Bleu de Lapis-Lazuli dans Six Manuscrits a Peintures du XHth Siecle Provenant de PAbbaye de Corbie', Scriptorium 40, 1986, pp. 157-71.

60 Withnall, R., Clark, R.J.H., Cooksey, C.J. and Daniels, M.A.M., 'Non-destructive, In Situ Identification of Indigo/Woad and Shellfish Purple by Raman Microscopy and Visible Reflectance Spectroscopy', Dyes in History and Archaeology 11, 1993, pp. 19-24.

61 Best, S., Clark, R., Daniels, M. and Withnall, R., 'A Bible Laid Open', Chemistry in Britain 29, 1993, pp. 118-22.

62 Best, S.P., Clark, R.J.H., Daniels, M.A.M., Porter, C.A. and Withnall, R., 'Identification by Raman Microscopy and Visible Reflectance Spectroscopy of Pigments on an Icelandic Manuscript', Studies in Conservation 40, 1995, pp. 31-40.

63 Clark, R.J.H., 'Raman Microscopy: Application to the Identification of Pigments on Medieval Manuscripts', Chemical Society Reviews 24, 1995, pp. 187-96.

64 Clark, R.J.H., Gibbs, P.J., Seddon, K.R., Brovenko, N.M. and Petrosyan, Y.A., 'Non-Destructive in situ Identification of Cinnabar on Ancient Chinese Manuscripts', Journal of Raman Spectroscopy 28, 1997, pp. 91-4.

65 Burgio, L., Ciomartan, D.A. and Clark, R.J.H., 'Pigment Identification on Medieval Manuscripts, Paintings and Other Artefacts by Raman Microscopy: Applications to the Study of Three German Manuscripts', Journal of Molecular Structure 405, 1997, pp. 1-11.

66 Burgio, L., Ciomartan, D.A. and Clark, R.J.H., 'Raman Microscopy Study of the Pigments on Three Illuminated Mediaeval Latin Manuscripts', Journal of Raman Spectroscopy 28, 1997, pp. 79-83.

67 Clark, R.J.H. and Gibbs, P.J., 'Raman Microscopy of a 13th-century Illuminated Text', Analytical Chemistry 70, 1998, pp. 99A-104A.

68 Burgio, L., Clark, R.J.H. and Gibbs, P.J., 'Pigment Identification Studies in situ of Javanese, Thai, Korean, Chinese, and Uighur Manuscripts by Raman Microscopy', Journal of Raman Spectroscopy 30, 1999, pp. 181-4.

69 Burgio, L., Clark, R.J.H. and Toftlund, H., 'The Identification of Pigments Used on Illuminated Plates from 'Flora Danica' by Raman Microscopy', Ada Chemica Scandinavica 53, 1999, pp. 181-7.

70 Clark, R.J.H., 'Raman Microscopy: Sensitive Probe of Pigments on Manuscripts, Paintings and Other Artefacts', Journal of Molecular Structure 480-481, 1999, pp. 15-20.

71 Wehling, B., Vandenabeele, P., Moens, L., Klockenkamper, R., von Bohlen, A., Van Hooydonk, G. and de Reu, M., 'Investigation of Pigments in Medieval Manuscripts by Micro Raman Spectroscopy and Total Reflection X-Ray Fluorescence Spectrometry', Mikrochimica Ada 130, 1999, pp. 253-60.

72 Bicchieri, M., Nardone, M. and Sodo, A., 'Application of Micro-Raman Spectroscopy to the Study of an Illuminated Medieval Manuscript', Journal of Cultural Heritage 1, Suppl. 1, 2000, pp. S277-9.

73 Andalo, C, Bicchieri, M., Bocchini, P., Casu, G., Galletti, G.C., Mando, P.A., Nardone, M., Soda, A. and Plossi Zappala, M., 'The Beautiful "Trionfo d'Amore" Attributed to Botticelli: A Chemical Characterisation by Proton-Induced X-ray Emission and Micro-Raman Spectroscopy', Analytica Chimica Ada 429, 2001, pp. 279-86.

74 Edwards, H.G.M., Farwell, D.W., Perez, F.R. and Garcia, J.M., 'Mediaeval Cantorals in the Valladolid Biblioteca: FT-Raman Spectroscopic Study', The Analyst 126, 2001, pp. 383-8.

75 Davey, R., Gardiner, D.J., Singer, B.W. and Spokes, M., 'Examples of Analysis of Pigments from Fine Art Objects by Raman Microscopy', Journal of Raman Spectroscopy 25, 1993, pp. 53-7.

76 Singer, B.W., Gardiner, D.J. and Derow, J.P., 'Analysis of White and Blue Pigments from Watercolours by Raman Microscopy', The Paper Conservator 17, 1993, pp. 13-19.

77 Coupry, C, Lautie, A., Revault, M. and Dufilho, J., 'Contribution of Raman Spectroscopy to Art and History', Journal of Raman Spectroscopy 25, 1994, pp. 89-94.

78 Clark, R.J.H., Cridland, L, Kariuki, B.M., Harris, K.D.M. and Withnall, R., 'Synthesis, Structural Characterisation and Raman Spectroscopy of the Inorganic Pigments Lead Tin Yellow Types I and II and Lead Antimonate Yellow: Their Identification on Medieval Paintings and Manuscripts', Journal of the Chemical Society, Dalton Transactions 1995, pp. 2577-82.

79 de Oliveira, L.F.C., Boscan, J.D.R.P., Santos, P.S. and Temperini, M.L.A., 'Identification of Pigments from Candido Portinari's Oil Painting "Portrait of Murilo Mendes" by Raman Microscopy',




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