Representative Raman spectra of the four chemical families of
the corpus are presented on Fig. 1. All these organic compounds
present comparable functional groups, leading to similarities in
their vibrational signatures. Specific features can however help
distinguishing among them [10,22,40]. On Fig. 1, abroad Raman
band at 3250–3270 cm−1 assigned to the stretching vibration of
the OH groups [6] is present for glues and gums (Fig. 1a and b), but
absent from oils and resins (Fig. 1c–f). Then, a weak band at