Non-invasive identification of coloring materials based on terahertz continuous-waves (THz-CW) spectroscopy

Techniques based on terahertz (THz) radiation allow non-destructive and non-invasive analysis, making them highly interesting technologies in the Cultural Heritage field. Moreover, THz radiation has a low photon energy (~4.2 meV at 1 THz) which assures no molecular ionization and therefore, it cannot cause any damage to the materials under investigation nor issue to the operators. For these reasons, this study aimed to obtain the spectral response for pigments and dyes with terahertz continuous wave spectroscopy (THz-CW) in the spectral range 0.1-3 THz with a compact and portable experimental set-up with high-frequency resolution (in the order of 100 MHz) in transmission mode. The results demonstrated that THz-CW spectroscopy can be considered a complementary methodology for Cultural Heritage related materials’ characterization since it allowed the extraction of the optical parameters (absorption coefficient and refractive index) in the spectral range of interest for the compounds. Furthermore, a preliminary characterization of compounds of interest was performed following a multi-analytical approach, based on ED-XRF and FORS spectroscopies. Moreover, since mixtures were often used to achieve desired hues, add shading, darken the mineral pigment, or correct the faded dye, some mixtures were investigated. The resulting experimental spectra obtained with THz-CW spectroscopy were then theoretically calculated as the weighted linear combination of the absorbance of each pure component. The results showed that it is possible to differentiate single components in mixtures, proving that THz-CW is a reliable methodological high-resolution approach in the Cultural Heritage field.