Raman spectroscopy

Raman spectroscopy (i-Raman Plus Spectrometer, inVia Raman Spectrometer, Jobin-Yvon T6400 Triple-Monochhromator), named for the Indian physicist Sir Chandrasekhara Venkata Raman, is a technique that relies on the inelastic scattering of monochromatic light by atomic vibrations in the visible, near-infrared, and near-ultraviolet range. In particular, Raman spectroscopy is used to provide vibrational information specific to different chemical bonds and molecules of any non-metallic solid, i.e. minerals, synthetic crystals, and glasses, as well as of inorganic and organic compounds in gaseous, liquid, or solid state. Given that Raman spectroscopy is a truly non-invasive and non-destructive analytical method, it became  very attractive to the field of Cultural Heritage over the last few decades, where sampling is strongly undesirable or completely prohibitive. Thus, it has proved to be ideal for the identification and material characterization of mineral-phase compounds such as pigments, seals, clay tablets, gems, ceramics, and degradation products on the surface of historic monuments. Like IR, Raman spectroscopy can probe the vibrational modes of amorphous phases and C-containing compounds, being therefore a useful tool for the analysis of carbonaceous materials on various writing supports and of organic phases inhabiting cracks in inscribed marble.

Recently, a novel methodological approach based on Raman spectroscopy was established for the non-destructive quantitative determination of the chemical composition and crystal structure of minerals that can be found in rock-based writing supports (see Case Study Aspiotis et al., 2022, 2023). Detectable material differences between inscribed and non-inscribed areas of marble-based written artefacts can be identified by Raman spectroscopy. Particularly, the introduction of cracks and generally extended structural defects during the inscribing process facilitate the introduction of foreign material into those voids. Thus, the content of molecular inclusions or weathering-related products should be higher between engraved areas than away from them. The ability of Raman spectroscopy to quantitatively mark a guest secondary phase in a host matrix via the guest-to-host peak intensity ratio can be used to map the lateral distribution of cracks induced during the inscribing process (see Case Study, Aspiotis et al., 2021). Hence, it can potentially be used to trace lost text or to indicate an external interference such as human activity.