Historical organic dyes : characterisation of reference materials and identification on wool by non-destructive spectroscopic techniques

In the present work, a wide database of natural organic dyes used in antiquity especially in textile dyeing was created, containing for about 30 colorants the analytical data obtained by several chromatographic and spectroscopic tools. A special attention was paid to surface-enhanced Raman spectroscopy (SERS): the good performances of NaClO4 1.8 M as aggregating agent were highlighted in comparison with those of other electrolytes employed in previous studies, especially when added to the silver colloid after the analyte. In particular, the SERS spectra of 11 dyes (elderberry, dragon’s blood, sandalwood, annatto, safflower, old fustic, gamboge, catechu, kamala, aloe and sap green) were collected for the first time, integrating the data already published about this subject. Moreover, a systematic study was carried out about the applicability of different spectroscopic tools, i.e. visible reflectance spectroscopy, Fourier-transform infrared spectroscopy (FTIR), attenuated total reflection-FTIR and FT-Raman, to the identification of such dyes on wool samples dyed in our laboratory according to the ancient recipes, in a totally non-destructive way. Several methods of data analysis, such as Principal Components Analysis and library search, were finally applied for identification purposes to the spectra obtained from dyed wool.