A holistic multimodal approach to the non-invasive analysis of watercolour paintings

Abstract

A holistic approach using non-invasive multimodal imaging and spectroscopic techniques to study the materials (pigments, drawing materials and paper) and painting techniques of watercolour paintings is presented. The non-invasive imaging and spectroscopic techniques include VIS-NIR reflectance spectroscopy and multispectral imaging, micro-Raman spectroscopy, X-ray fluorescence spectroscopy (XRF) and optical coherence tomography (OCT). The three spectroscopic techniques complement each other in pigment identification. Multispectral imaging (near-infrared bands), OCT and microRaman complement each other in the visualisation and identification of the drawing material. OCT probes the micro-structure and light scattering properties of the substrate, while XRF detects the elemental composition that indicates the sizing methods and the filler content. The multiple techniques were applied in a study of forty-six nineteenth-century Chinese export watercolours from the Victoria and Albert Museum (V&A) and the Royal Horticultural Society (RHS) to examine to what extent the noninvasive analysis techniques employed complement each other and how much useful information about the paintings can be extracted to address art conservation and history questions. A micro-destructive technique of micro-fade spectrometry was used to assess the vulnerability of the paintings to light exposure. Most of the paint and paper substrates were found to be more stable than ISO Blue Wool 3. The palette was found to be composed of mostly traditional Chinese pigments. While the synthetic pigment, Prussian blue, made in Europe, was found on some of the paintings, none was found on the RHS paintings accurately recorded as being between 1817 and 1831 even though it is known that Prussian blue was imported to China during this period. The scale insect dyes, lac and cochineal, were detected on nearly every painting including those that fall within the identified date range. Cochineal is known to have been imported to China in the same period. While carbonbased ink was used for the drawings in the V&A paintings, graphite pencil was used in most of the RHS paintings. The majority of RHS paintings were on Western papers, but nearly all of the V&A paintings were on Chinese papers. Nearly all of the V&A painting substrates and hardly any of the RHS paintings were sized with alum. Elements such as Cu, Zn, Ti and Pb were detected on nearly all of the RHS papers regardless of whether they were Chinese or Western papers. These elements were largely absent from the V&A papers. The differences between the two collections reflect their discrete origins and intended purposes.

The origins of the Selden map of China: scientific analysis of the painting materials and techniques using a holistic approach

“…These techniques have not been applied to the map yet due to the logistics of access and the unavailability in 2009 of some of the instruments or at least ones in a portable form that could be applied directly to the large map. A recent paper published by some of us has shown in detail the complementary use of a suite of non-invasive techniques ranging from spectral imaging to Raman, XRF and OCT that was applied directly to Chinese export water colour paintings of the nineteenth century demonstrating the advantages of a holistic multi-technique approach [14].…”

“…Spectral imaging at a spectral resolution of 40-50 nm is sufficient for identification of most pigments except for those based on anthraquinone dyes such as madder and scale insect dyes (e.g. lac and cochineal) and cobalt blue pigments such as smalt [14]. The hyperspectral version of PRISMS has sufficient spectral resolution to detect these, but it was not available in 2009.…”

“…As demonstrated in previous studies [14,17], spectral reflectance in the visible and near infrared can be used for the identification of single pigments or multiple pigments using a Kubelka-Munk (KM) [27] fit with a comprehensive spectral library of historic artists' paints. KM models calculate the resulting spectral reflectance from a mixture of single pigment paints using a reference paint spectral library.…”

“…The concentration of each pigment component is set as a free parameter such that the KM model using the reference spectral library gives a best fit to the unknown spectrum with an indicator for goodness of fit for each combination of pigments. It can also be used simply to test if a certain combination of pigments can produce a good fit to the unknown spectrum [14]. The reference spectral library used was one that was suited to the identification of historic Chinese artists' paints with animal glue as a binding medium and Chinese Xuan paper as a substrate [14].…”

Spectral imaging of Dutch gilt leather for improved conservation strategies

“…This technique has also proved applicable for monitoring the partial cleaning of (yellowed aged) varnishes [9]. Another interesting application is pigment charac-terisation, which unfortunately remains complicated, as some pigments do not have very distinct spectral features, for instance yellow pigments [10]. The characterisation of pigment mixtures present in decorative paints is an even more demanding challenge; however, success is possible [6].…”