Terahertz Spectroscopy in Reflection Configuration for the Identification of Mineral Pigments

This work proposes an experimental procedure to perform far-infrared vibrational study of inorganic and mineral pigments using terahertz time-domain spectroscopy in reflection configuration and a method to overcome the limitations due to scattering and absorption effects, which make the signals too weak to produce observable and detectable structures. Ten pigments used in ancient frescoes were chosen as samples for determining the pro and cons of the application of this technique on this class of materials. Specifically, Cu-based (Azurite, Malachite and Egyptian blue), Pb-based (Minium and Massicot), Fe-based (iron oxide yellow, dark ochre, hematite and Pompeii red) pigments and mercury sulphide (Cinnabar) were investigated. The spectroscopic characteristics of these samples set were investigated in the far- and mid-infrared frequencies, as well as the Raman active modes. Terahertz spectroscopy showed to be able to discriminate molecular phases of metal, detecting the absorption frequencies below 200 cm-1 of Azurite (61.0, 74.7, 130.9, 137.71, and 144.3 cm-1) but being blind to Malachite. In reflection geometry, Egyptian blue (69.0 cm-1), Minium (54.7, 62.8, 71.3, and 84.0 cm-1) and Cinnabar (39.0, 41.4, and 88.0 cm-1) have absorption peaks in the far-infrared region, confirming the results performed in transmission configuration in other experimental works and the ones determined by theoretical ss-DFT calculations [1, 2]. Terahertz spectroscopy however failed in the detection of Fe-based pigments, because they absorb infrared radiation above 220 cm-1, out of the investigated spectral range. This work demonstrates the effectiveness of THz spectroscopy in reflection configuration for the characterization of these inorganic pigments and encourages the successful analysis of real samples, contributing to the creation of a database to guide the study of archaeological frescoes.