Identifying eighteenth century pigments at the Bodleian library using in situ Raman spectroscopy, XRF and hyperspectral imaging

R Mulholland,A Beeby,D Howell,K Domoney,C E Nicholson

doi:10.1186/s40494-017-0157-y

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 specific requirements of manuscripts and works on paper. This paper describes the successful use of Raman spectroscopy, X-ray fluorescence 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 specifically designed for analysing pigments from works on paper was used together with a bespoke portable Fibre optic reflectance spectrometer, portable X-Ray Fluorescence spectrometer and a hyperspectral imaging sensor. The results demonstrate that although there is a significant compromise between achieving good Raman spectroscopic results from artists’ pigments and using sufficiently 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, fibre optic reflectance spectroscopy (FORS) and hyperspectral imaging. Eight pigments were identified 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 significantly in his later paintings. Similar pigments identified 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 fidelity in his work.

Highlights

The remarkable story of the Flora Graeca, one of the most magnificent printed books of the eighteenth century and the work of its artist, Ferdinand Bauer, has been well documented [1,2,3,4]
The results of the survey were useful in determining that many common eighteenth century watercolour pigments that Bauer may have used (e.g. Brazilwood, madder lakes, sap green, and copper blues and greens) did not produce Raman spectra using any of the excitation wavelengths tested at power levels acceptable for the study of the paintings
This study has demonstrated through the analysis of approximately ten percent of Bauer’s paintings for the Flora Graeca using non-invasive, in situ methods, that Bauer used a fairly simple and traditional palette for the period, and that together with his colour code system, he used a relatively small number of pigments to achieve almost perfect colour fidelity in his work

Summary

Introduction

The remarkable story of the Flora Graeca, one of the most magnificent printed books of the eighteenth century and the work of its artist, Ferdinand Bauer, has been well documented [1,2,3,4]. The methods and materials Bauer utilised in order to reproduce colour with astonishing accuracy in the 966 paintings of Mulholland et al Herit Sci (2017) 5:43 and understanding how his palette evolved over time by comparing pigments from 125 paintings from the Flora and Fauna Graeca with those identified in both an early painted colour chart [1, 2, 5] and in a number of later paintings [6]. Previous studies have used Raman spectroscopy, XRF, FORS and imaging spectroscopy for the non-invasive study of watercolour pigments in both manuscripts and paintings on paper [7,8,9,10,11,12,13]. The following study involves the use of bespoke Raman spectrometer and FORS systems, which have built entirely for the purposes of analysing pigments in works on paper and manuscripts

Methods

Results

Conclusion