Liang, H

Martin-Simpson, M

Peric, B

Spring, M

English

Nottingham Trent Institutional Repository (IRep)

Spectral transparency of historic artists' pigments Borislava Peric, Sophie Martin-Simpson, Marika Spring and Haida Liang Keywords: infrared imaging, infrared refiectography, optical coherence tomography, spectral reflectance, paint, pigments Introduction Following the invention of infrared reflectography (IRR) for the imaging of underdrawings, studies have been con-ducted to determine directly or indirectly the optimum spec-tral window for IRR (van Asperen de Boer 1969; Delaney et al. 1993,2005). However, there has not been a comprehen-sive survey of the transparency of historic artists' pigments over the full near infrared (NIR) range. The recent develop-ments of another non-invasive imaging technique, optical coherence tomography (OCT), which enables non-contact imaging of the stratigraphy of paint layers, has prompted a renewed interest in studying the transparency of pigments in the NIR range (Liang et al. 2004, 2005a; Targowski et al. 2004; Szkulmowska et al. 2007). A set of paint-outs consisting of a wide variety of historic artist's pigments (~50) in both egg tempera and linseed oil has been prepared. The pigments were chosen to be repre-sentative of what is found on paintings, and the composi-tions of the pigments were verified with energy-dispersive X-ray analysis (EDX), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). The samples were prepared with known pigment volume concentration and thickness. In order to measure the transparency of the paint layers, one set of paint-outs was prepared over thin glass microscope slides. Spectral measurements In order to determine the best spectral windows for NIR imaging, either to reveal underdrawings in the case of IRR or to penetrate deeper into the paint layers in the case of non-contact cross-section imaging using OCT, the relative spectral transparency of the paint samples needs to be meas-ured. The transparency of a paint layer depends on both the scattering and absorption properties, since light is both scattered and absorbed when it travels through the layer. For paint layers, the depth range of OCT is limited by multiple scattering rather than absorption, since OCT has high sen-sitivity and dynamic range. For a strongly scattering paint layer (painted on glass), we expect to find the backscattered light and hence the reflectance to be high, and independ-ent of whether the sample was placed on a white or black background. A highly absorbing paint layer would have low reflectance on a white or black background. In contrast, a highly transparent layer will have high reflectance when it is placed on a white background but low reflectance when placed over a black background. A n Ocean Optics HR2000 fibre optic spectrometer (200-1100 nm), a Polychromix DTS 1700 (900-1700 nm) and DTS 2500 (1700-2500 nm) fibre optic spectrometer were used to measure the spectra between 400 and 2500 nm. The spectral resolutions of the three spectrometers are 0.9, 12 and 22 nm respectively. Results and conclusions By comparing the spectral reflectance over white and over black, we find that almost without exception all paint samples have best transparency (or least extinction) at -2.2 um. There are six pigments that have slightly better (but comparable) transparency in other regions of the spectra. The general trend is that the pigments in linseed oil or egg tempera are more transparent with increasing wavelength over the VIS-NIR spectral range. This comprehensive survey of VIS-NIR reflectance spectra also provides a reference library for the identifica-tion of historic artists' pigments. By extending the wave-length range into the NIR, additional spectral features that are unique to each pigment are revealed, allowing more conclusive identification (Bayerer 1996; Bacci 2000; Liang et al. 2005b). The positions of the peaks in the spectra are found to shift whenever the concentration or the particle sizes changes, which needs to be considered when identi-fying pigments based on their spectra. The successes and limitations of non-invasive identification of pigments on 185 C O N S E R V A T I O N S C I E N C E 2007 actual paintings using this VIS-NIR reference spectral library along with newly developed algorithms for spectral identification will be reported in a future publication. Acknowledgements This work is supported by an EPSRC Case award, the National Gallery, London, UK and the Leverhulme Trust, UK. We would like to thank Dr David Saunders for encour-agement and for helping to secure the Case funding while he was still at the National Gallery. References Bacci, M . 2000. 'UV-VIS-NIR, FT-IR, and FORS spectroscopies', inE . Ciliberto andG. Spoto (eds),Modern Analytical Methods in Art and Archaeology. New York: John Wiley & Sons, 321-61. Bayerer, F. 1996. Die Untersuchung von Kunstobjekten mitHilfe der bildgebenden Spektroskopie. Munich: Herbert Utz. Delaney, J., Metzger, C.,Walmsley, E . and Fletcher, C. 1993. 'Examination of the visibility of underdrawing lines as a function of wavelength', in J. Bridgland (ed.), ICOM-CC 10th Triennial Meeting Postprints. LosAngeles: Getty Conservation Institute, 15-19. Delaney, J.K., Walmsley, E . , Berrie, B . H . and Fletcher, C.F. 2005. 'Multispectral imaging of paintings in the infrared to detect and map blue pigments', Sackler NAS Colloquium: Scientific Examination of Art: Modern Techniques in Conservation and Analysis. Washington, DC: National Academies Press, 120-36. Liang, H . , Gomez Cid, M . , Cucu, R., Dobre, G., Jackson, D. , Pannell, C , Pedro, J., Saunders, D. and Podoleanu, A . 2004. 'Application of O C T to examination of easel paintings', in Second European Workshop on Optical Fibre Sensors. Proceedings o/SPIE 5502: 378-81. Liang, H . , Gomez Cid, M . , Cucu, R., Dobre, G., Podoleanu, A . , Pedro, J. and Saunders, D. 2005a. 'En-face optical coherence tomography: a novel application of non-invasive imaging to art conservation,' Optics Express 13(16): 6136. Liang, H . , Saunders, D. and Cupitt, J. 2005b. 'Anewmultispectral imaging system for examining paintings', Journal of Imaging Science and Technology 49: 551-62. Szkulmowska,A., Gora, M . , Targowska, M . , Rouba, B. , Stifter, D. , Breuer, E . and Targowski, P. 2007. 'The applicability of optical coherence tomography at 1.55 mm to the examination of oil paintings', in J. Nimmrichter, W. Kautek, and M . Schreiner (eds), Lasers in the Conservation of Artworks, LACONA VI Proceedings. Berlin: Springer Verlag. Targowski, P., Rouba, B . , Wojtkowski, M . and Kowalczyk, A . 2004. 'The application of optical coherence tomography to non-destructive examination of museum objects', Studies in Conservation 49: 107-14. van Asperen de Boer, J.R.J. 1969. 'Reflectography of paintings using an infra-red vidicon television system', Studies in Conservation 14: 96-118. Authors' addresses • Borislava Peric, School of Science and Technology, Notting-ham Trent University, Clifton Lane, Nottingham NG11 8NS, U K . • Sophie Martin-Simpson, School of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U K . • Marika Spring, Scientific Department, The National Gallery, London W C 2 N 5DN, U K . • Haida Liang, School of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U K . (haida. hang @ntu. ac.uk) 186 

Anewmultispectral imaging system for examining paintings',

Application of O C T to examination of easel paintings',

En-face optical coherence tomography: a novel application of non-invasive imaging to art conservation,'

Reflectography of paintings using an infra-red vidicon television system',

The applicability of optical coherence tomography at 1.55 mm to the examination of oil paintings',

The application of optical coherence tomography to non-destructive examination of museum objects',

Spectral transparency of historic artists' pigments

Spectral transparency of historic artists' pigments

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