Noninvasive identification of turmeric and saffron dyes in proteinaceous textile fibres using Raman spectroscopy and multivariate analysis

We used Raman spectroscopy combined with multivariate analysis to study noninvasively naturally and artificially aged samples of silk and wool fibres dyed with turmeric and saffron dyes, following historical recipes. The Raman spectra obtained from all the samples using the two excitation lasers (785 and 1064 nm) showed the characteristic peaks of each dye and fibre. The 1064-nm macro FT-Raman offered more information regarding the degradation of the fibre substrate, whereas the mu-Raman at 785-nm spectra proved more suitable in the identification of dye characteristic peaks. We also performed colorimetric measurements to evaluate the colour change upon artificial ageing and compared them with the information obtained with Raman spectroscopy. The results suggest saffron has a higher lightfastness than turmeric and that silk fibres suffer higher fading and more evident degradation than wool. Raman spectroscopy informs about the chemical composition of the colourants and fibres and their changes upon ageing without sample pretreatment. Finally, we analysed a historical wool sample coming from a 19th-century Persian carpet, and we identified turmeric.