Potentialities of non-invasive spectroscopic methods for the identification of natural organic dyes of interest in art and archaeology

In this study several non-invasive spectroscopic methods were applied to the characterisation of natural organic dyes widely used in antiquity for dyeing textiles and in painting. A large number of colorants belonging to different molecular classes (e. g. indigoids, anthraquinones, flavonoids, carotenoids and others) was thus commercially provided as a reference for identification purposes, and an extensive database was built, containing for each dye analytical information obtained by using different techniques: reflectance visible spectroscopy, fluorescence spectroscopy, normal and resonance Raman and FT-Raman spectroscopies, Fourier-transform infrared spectroscopy (FTIR). A micro-analysis mode was assessed for all the techniques, in order to evaluate their potential applicability to real samples, which are often available in limited amounts. In-situ investigations by visible, fluorescence and Raman spectroscopies would be also achievable thanks to the use of our portable instrumentation. The real effectiveness of non-invasive spectroscopic techniques was then proven when applied to samples of archaeological and artistic interest: a purplish bone fragment found in the tomb of the martyrs Gervase and Protase in the Basilica of Sant’Ambrogio, Milano (IV century A.D.), and illuminations on Flemish manuscripts (XV century) were analysed, allowing in both cases the identification of the dyes as Tyrian purple and anthraquinonic lakes respectively.