Discrimination of natural and synthetic forms of azurite. An innovative approach based on high-resolution terahertz continuous wave (THz-CW) spectroscopy for cultural heritage

It is not always possible to discriminate natural and synthetic forms of pigments in a non-invasive way. In this work, we demonstrate that THz spectroscopy allows for this selective discrimination of chemically related compounds. In the presented results, we show the spectral characterization of the following copper carbonates: natural azurite and two of its synthetic forms, bice blue and blue verditer. In particular, the physical properties of the two synthetic forms were characterized for the first time in the terahertz spectral range by exploiting a high-resolution coherent terahertz Continuous Wave (THz-CW) spectrometer. The findings reveal distinct absorption spectra for the three chemically related pigments, showing specific fingerprints that can be used to discriminate the natural compound from the synthetic forms. In fact, we show that by THz-CW it was possible to discriminate those pure components in binary mixtures and quantify the single components contribution. This identification is of great interest to assist in dating artworks or evidence restoration and retouching treatments. The results demonstrate that THz-CW spectroscopy can pave the way to an innovative approach to the Cultural Heritage field.