Identification of dyestuffs in historical textiles: strong and weak points of a non-invasive approach

The dyers of the past exploited a number of natural dyestuffs and dyeing procedures to obtain countless beautiful colours. It is hard to disclose which dyestuff has been used to dye a historical or an archaeological textile through non-invasive analytical techniques; therefore invasive analytical strategies, which require sampling, are generally employed to face this task. Some useful indication may nevertheless derive from the electronic transitions of dyes supported on the fibres and the signals can be easily recorded by reflectance spectroscopy in the visible range of the electromagnetic spectrum. The use of portable instruments equipped with fibre optics allows operation in-situ directly on the surface of the samples, through a non-invasive approach which is particularly suitable for the inspection of historical or archaeological samples. In this paper, the strong and weak points of visible fibre optics reflectance spectroscopy are discussed, in order to highlight its possible application as a first non-invasive step of a multi-technique analytical strategy devoted to the recognition of natural dyestuffs in ancient textile artefacts. Visible fibre optics reflectance spectra were collected from a large set of reference samples (wool and silk) coloured through various dyestuffs and dyeing procedures, in order to identify the signals of the coloured species that impregnate the fibres. The position of minimum, maximum and inflection points in the reflectance spectrawas evaluated and critically discussed, in order to identify the signals which are not contingent to dyeing procedures or to the fibre typology, and that can be therefore confidently used for analytical purposes. In particular, wavelength intervals were defined, in which the characteristic signals of the investigated dyestuffs (indigo, woad, Saxon blue, logwood, cochineal, madder, brazilwood, old fustic, weld, turmeric and saffron) are expected. It emerged that visible fibre optics reflectance spectroscopy is poorly selective for yellow dyes based on curcumins and flavonoids, while it generally succeeds in detecting the other dyestuffs considered herein. The information obtained from the reference sample set was then exploited to analyze an embroidered cloth dated from the seventeenth century. Preliminary information obtained via visible fibre optics reflectance spectroscopy was confirmed, or integrated, by the results obtained through liquid chromatography coupled with diode-array detection and mass spectrometry on extracts obtained from some coloured threads sampled from the historical cloth.