Pigments, minerals, and copper-corrosion products: Terahertz continuous wave (THz-CW) spectroscopic characterization of antlerite and atacamite

In this work, the optical properties of atacamite, antlerite and azurite are characterized for the first time by exploiting a high-resolution coherent THz-CW spectrometer in the spectral range 0.1-2.8 THz adopting transmission configuration; the spectral response of atacamite and antlerite is reported for the first time in literature in this frequency range. The absorption spectra of the two copper-based pigments showed the presence of distinct absorption peaks defining their fingerprints. In particular, atacamite presents a peak centered at 2.45 THz and antlerite presents two spectral fingerprints centered at 1.77 and 2.52 THz; the retrieved average refractive index is 1.19 and 1.29, respectively. The absorption spectrum of azurite is in good agreement with previous works in the far infrared region and presents two sharp peaks centered at 1.83 and 2.23 THz with an average value of the refractive index of 1.65. Furthermore, by means of the same technique, it is possible to discriminate and quantify pure components in binary mixtures. This result is of great interest for the identification of pigments and metallic corrosion products, and it demonstrates that THz-CW spectroscopy can open the way to an innovative approach for the Cultural Heritage field.