Durham Research Online
Total Page:16
File Type:pdf, Size:1020Kb
Durham Research Online Deposited in DRO: 02 November 2017 Version of attached le: Published Version Peer-review status of attached le: Peer-reviewed Citation for published item: Mulholland, R. and Howell, D. and Beeby, A. and Nicholson, C. E. and Domoney, K. (2017) 'Identifying eighteenth century pigments at the Bodleian library using in situ Raman spectroscopy, XRF and hyperspectral imaging.', Heritage science., 5 (1). p. 43. Further information on publisher's website: https://doi.org/10.1186/s40494-017-0157-y Publisher's copyright statement: c The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Additional information: Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details. Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk Mulholland et al. Herit Sci (2017) 5:43 DOI 10.1186/s40494-017-0157-y RESEARCH ARTICLE Open Access Identifying eighteenth century pigments at the Bodleian library using in situ Raman spectroscopy, XRF and hyperspectral imaging R. Mulholland1*, D. Howell1, A. Beeby2, C. E. Nicholson3 and K. Domoney4 Abstract There are multiple challenges in analysing pigments in historic watercolour paintings on paper, and typically non- invasive, in situ methods are required. Recent developments in portable analytical instrumentation have made this more accessible to heritage institutions, but many commercial systems are not optimised for the specifc require- ments of manuscripts and works on paper. This paper describes the successful use of Raman spectroscopy, X-ray fuorescence spectroscopy (XRF) and hyperspectral imaging to identify and map watercolour pigments used by the eighteenth century botanical illustrator, Ferdinand Bauer, and demystify the unusual colour code system found in his sketches. The value, delicate nature and large size of these paintings necessitated the use of using in situ, non- contact methods of analysis. A portable, bespoke Raman spectrometer specifcally designed for analysing pigments from works on paper was used together with a bespoke portable Fibre optic refectance spectrometer, portable X-Ray Fluorescence spectrometer and a hyperspectral imaging sensor. The results demonstrate that although there is a signifcant compromise between achieving good Raman spectroscopic results from artists’ pigments and using suf- fciently low laser power densities so as not to cause damage to the pigments, good results could be obtained with this portable system, particularly when combined with XRF, fbre optic refectance spectroscopy (FORS) and hyper- spectral imaging. Eight pigments were identifed unequivocally from 125 watercolour paintings analysed, suggesting that Bauer used a more traditional and more limited palette than previously considered, and that his palette changed signifcantly in his later paintings. Similar pigments identifed by the authors on colour chart that was discovered in 1999 in Madrid and attributed to Bauer, add weight to the attribution of this chart to Bauer. The data provides a much deeper insight into Bauer’s colour annotations, and how he was able to achieve such an impressive degree of colour fdelity in his work. Keywords: Portable Raman spectroscopy, FORS, Portable XRF, Ferdinand Bauer, Watercolour painting, Pigment Analysis, Botanical illustration Background plants and 293 paintings of animals he produced for the Te remarkable story of the Flora Graeca, one of the endeavour during the period 1788–1794 has not been most magnifcent printed books of the eighteenth cen- scrutinised. In particular, Bauer’s palette and his use of tury and the work of its artist, Ferdinand Bauer, has a complex numerical colour code system used for both been well documented [1–4]. However, the methods the Flora Graeca and the lesser known and unpublished and materials Bauer utilised in order to reproduce col- Fauna Graeca have not been extensively discussed. our with astonishing accuracy in the 966 paintings of Tis study was undertaken to determine the efective- ness of non-destructive, in situ methods to identify pig- ments used in Bauer’s watercolour paintings and gain *Correspondence: [email protected] insight into how the artist worked in the feld. Part of a 1 Heritage Science, Weston Library, Bodleian Libraries, University of Oxford, Broad Street, Oxford OX13BG, UK larger research project at the Bodleian Library, the study Full list of author information is available at the end of the article was instrumental in unravelling the artist’s colour code © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Mulholland et al. Herit Sci (2017) 5:43 Page 2 of 19 and understanding how his palette evolved over time by the Raman system are manifold: it is sensitive enough to comparing pigments from 125 paintings from the Flora identify many artists’ pigments at laser power densities and Fauna Graeca with those identifed in both an early low enough to prevent photodegradation of light sensi- painted colour chart [1, 2, 5] and in a number of later tive materials (20 W cm−2), it allows for the sampling of paintings [6]. a very small area (ca. 50 μm) compared to that of com- Previous studies have used Raman spectroscopy, XRF, mercial portable Raman spectrometers, and it has a FORS and imaging spectroscopy for the non-invasive modular design that allows for a wide variety of sampling study of watercolour pigments in both manuscripts and confgurations, including the use of several diferent laser paintings on paper [7–13]. Although more interventive wavelengths. technologies such as Surface Enhanced Raman Spectros- In recent years, hyperspectral imaging and imag- copy (SERS) have yielded better results with watercolour ing spectroscopy have found numerous applications in pigments, [14] these require either sampling, or direct the study and analysis of works on paper. It has been contact with the surface of the object, something that used for, amongst other applications, identifying stains is often not permitted in heritage collections. Further- on paper [18] and monitoring the condition of historic more, portable analytical studies are generally reliant on documents over time [19]. For artists’ pigments, the commercial instruments designed for general analytical value of hyperspectral scanning is to provide a digi- work, rarely optimised for works on paper. Te follow- tal image of the entire surface of an object, from which ing study involves the use of bespoke Raman spectrom- multiple analytical approaches can be taken (pseudo- eter and FORS systems, which have built entirely for the colour mapping, principle component analysis and spec- purposes of analysing pigments in works on paper and tral angle mapping etc.). Delaney and others [11–13] manuscripts. have demonstrated the value of using both fbre optic Te reduction in cost, and wider availability of non- refectance spectroscopy (FORS) together with imag- invasive, portable instrumentation have facilitated identi- ing spectroscopy and hyper/multispectral imaging to fcation of artists’ pigments in heritage objects for which visualise and map the location of pigments in objects as it would have been otherwise impossible [7–9]. In addi- diverse as paintings by Picasso and medieval illuminated tion to portable XRF, the availability of portable Raman manuscripts. Hayem–Ghez et al. [13] have also suc- systems has also increased in recent years. Te relatively cessfully demonstrated the validity of using hyperspec- low cost of these systems have made them an attractive tral pseudo-colour composites to diferentiate between prospect for heritage institutions. However, the laser visually similar pigments. Terefore, the combination of powers employed by many portable Raman systems portable Raman spectroscopy, FORS and XRF analysis are typically set at levels to provide the optimal signal- together with Hyperspectral imaging can be considered to-noise ratio for robust samples but often exceed the a valuable