Fibre optic reflectance spectroscopy and multispectral imaging for the non-invasive investigation of Asian colourants in Chinese textiles from Dunhuang (7th-10th century AD)

Abstract

• The archaeological complex of Dunhuang (northwestern Gansu, China) is considered a pearl on the Silk Road and the content of its caves revolutionised oriental studies. The British Museum hosts a significant number of textiles and textile fragments from the site. Although mostly catalogued and studied from the point of view of textile production, weaving techniques and iconography, they have never undergone dye analysis, which is here presented for the first time. Due to the value, preciousness and fragility of these textiles, sampling is not always an option. Therefore, a non-invasive approach combining multispectral imaging (MSI) and fibre optic reflectance spectroscopy (FORS) was tested. The final aims of this research were to explore the advantages and limitations of such an approach with application to Asian dyes and ultimately to provide new insights into the technological skills related to dyestuff production and use in ancient China. To achieve these aims, twenty-nine reference specimens of Asian dyes were sourced, including sappanwood (Biancaea sappan), sandalwood (Pterocarpus santalinus), different species of madder (Rubia spp.), rosewood (Dalbergia sp.), rhubarb (Rheum emodi), dragon's blood (Daemonorops sp.), Indian lac (Kerria lacca), gromwell (Lithospermum erythrorhyzon), safflower (Carthamus tinctorius), amur-cork tree (Phellodendron amurense), gamboge (Garcinia hanburyi), violet (Viola yedoensis), pagoda tree (Sophora japonica). Silk was dyed using these raw materials and the reference samples were analysed by colorimetry before and after ageing. These measurements highlighted the most light-sensitive dyes to be turmeric, safflower, sappanwood and sandalwood. The reference samples were also used to create a database of reflectance and apparent absorption spectra obtained by FORS, as well as to record the reflectance, luminescence and absorption properties of specific dyes by MSI. The combined information obtained from MSI and FORS brought to the fore the signature behaviour of certain dyes (protoberberine-based dyes, safflower, gromwell, tannins, madder, sappanwood, Indian lac and indigo), which enabled their unequivocal identification and the mapping of their distribution in thirty-one Dunhuang textiles from the British Museum's collection. The data presented in this work will be beneficial to other researchers and conservators investigating textiles from the Silk Road using a non-invasive approach and for the assessment of textiles and their suitability for display.