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

Abstract

There are multiple challenges in analysing pigments in historic watercolour paintings on paper, and typically noninvasive, 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, noncontact 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.

A blue can conceal another! Noninvasive multispectroscopic analyses of mixtures of indigo and Prussian blue

“…HSI allows us to obtain RGB (R = 650 nm, G = 540 nm, B = 450 nm) and InfraRed False Color images (IRFC, R = 900 nm, G = 650 nm, B = 540 nm). In accordance with previous articles 24,26 indigo depositions appear as bright red in the IRFC image, whereas PB layers appear as black (for high concentration of the pigment) to violet (for low concentrations) ( Figure 2).…”

“…Infrared radiation usually probes the samples through a thickness up to several tens of micrometers, sometimes preventing the identification of pigments because of the overlapping of the spectral signatures of the pigment with the signals originating from paper and/or the adhesives . As demonstrated by Cavaleri et al, visible reflectance spectra can be significantly influenced by the binder or by the dilution of the pigment and thus result in an incomplete identification of the coloring materials.…”

Comparing the effectiveness of hyperspectral imaging and Raman spectroscopy: a case study on Armenian manuscripts

“…Previous studies [50,85,86] have shown that different lasers may increase the identifiable range of pigments, with 532 and 785 nm being other commonly used wavelengths. 785 nm has proven to be the most effective at pigment identification but requires an increase in the applied power [50,85,86] (to achieve good S/N ratios) which can cause damage to the object of analysis [50]. 532 nm has been shown to be better than 635 nm only for the identification of blue pigments [50,85] but in general suffers from increased fluorescence [86].…”

“…785 nm has proven to be the most effective at pigment identification but requires an increase in the applied power [50,85,86] (to achieve good S/N ratios) which can cause damage to the object of analysis [50]. 532 nm has been shown to be better than 635 nm only for the identification of blue pigments [50,85] but in general suffers from increased fluorescence [86]. It is therefore possible that the use of an excitation laser with a wavelength of 785 nm would identify more pigments, but it may also require the application of more power than we are comfortable with for valuable historic documents.…”

“…Such false colour composite images have recently been successfully used to aid the identification of watercolour pigments in eighteenth century botanical illustrations by Ferdinand Bauer [50], this study however focusses on the former identification method. 2 There is no such database suitable for the identification of pigments and so one must be manufactured by the analyst for each new study.…”