Terahertz spectroscopy applied to the analysis of artists' materials

Terahertz (THz) spectroscopy and imaging have been actively studied in these decades. THz waves (0.1-10 THz) are non-invasive, can penetrate opaque materials, and can be used to obtain fingerprint spectra whose characteristics depend upon molecular and intermolecular behavior. Unlike the mid-infrared region, no commercial spectral library is available for the THz region. Consequently, a spectral database of artists' materials was developed in order to include THz spectroscopy among conservation science techniques. Most pigments and some synthetic polymers have characteristic fingerprint spectra in the THz region. With this technique most of these materials used in paintings can be identified alone or as paint, a combination of pigments and binders. Although the meaning of the spectral features has so far not been fully explained, previous studies on optical materials suggest that phonon absorption and the behavior of hydrogen bonds contribute to their spectral features. Examples of THz spectra of various art materials are discussed on the basis of measurements obtained using a conventional Fourier transform system.