Non-invasive identification of dyed textiles by using VIS-NIR FORS and hyperspectral imaging techniques

Natural dyes have been utilized since ancient times to produce colorful garments and, at the same time, each culture has developped its own traditional designs and techniques during the centuries. Hence, in order to accomplish a proper conservation program of these artefacts, it is fundamental the importance of studying the used dyes on historic textiles. Traditionally, the identification is carried out through invasive or micro-invasive techniques such as HPLC or Raman-SERS. However, a first screening using non-invasive approaches, that could tentatively identify some of the dyes in a contactless mode, is strongly reccommended. The present study focuses on the application of non-invasive spectroscopic techniques, namely FORS and hyperspectral imaging in the visible and near infrared range, for the identification of dyes on textiles. Twelve natural dyestuffs (twelve from plant, two insect dyes and a lichen) and three mordants (KAl(SO4)2, CuSO4 ∙ 5H2O, FeSO4 ∙ 7H2O) were selected and combined to create a set of wool samples whose reflectance spectra, acquired using FORS technique, were the basis for a spectral database. The same set was used to test the Specim IQ hyperspectral camera, with positive results for a non-invasive investigation on wider areas. The application of Spectral Angle Mapping (SAM) gave the possibility to map directly on the image pixel with similar spectral features. Finally, colorimetric data were acquired with a spectrocolorimeter to obtain chromaticity coordinates.