Studying colourants: in the lab

Detail from an Andean textile

Solvent extraction of the colourant from reference textile samples before HPLC analysis

To preserve sensitive dyes and components indicative of source and informative of dyeing technology the analytical approach is based on 'soft' extraction methods, using chromatographic techniques (HPLC-PDA) to give detailed molecular characterisation of colourants and sources. A multianalytical approach (including SEM, XRF and FTIR spectroscopy) is also necessary to investigate mordants and production technologies.

Study of Andean colourants as part of this project began with research into existing sources. Original Spanish chronicles are a major source of information about plants used to dye textiles in the Andean region, while archaeological and scientific literature reporting analyses of ethnographic and archaeological textiles have also been studied. The information obtained about colourant sources is being collated in a database which will be updated throughout the project, and records a range of information including common and Latin plant names, plant parts used for dyeing, colour obtained, analytical data (if available) and, for archaeological material, culture that used it, when and where. This will therefore help identify specific uses of dyes for different cultures or different geographic regions and evolutions in dye use.

High Performance Liquid Chromatography (HPLC) is one of the most efficient techniques to characterise colourants at a molecular level and the analytical approach used in this study has been adapted from that previously used for the study of European dyestuffs and requiring tiny fibre/colourant samples. Prior to analysis the colourant components are extracted from their support (fibre). Particular emphasis has been placed on developing extraction methods that, while not always fully quantitative: maximise the information that can be obtained from a sample; are suitable for the widest range of colourant classes and preserve highly sensitive dyes which may be present in mixtures; preserve the glycoside content and other chemically labile components which can be highly informative of the biological source and/or dyeing or pigment-making.

In parallel, a multianalytical approach has been developed to investigate mordants and production technologies. A range of analytical and imaging techniques including scanning electron microscopy (SEM), X-ray fluorescence analysis (XRF), Raman and Fourier transform infrared (FTIR) spectroscopy and multispectral imaging are being explored. This will be complemented by artificial ageing studies to address colourant stability, identify degradation markers and investigate deterioration mechanisms allowing both comparison of differential environmental preservation and surveying of British Museum object conditions.