A multi-scale spectroscopic approach unveiling hidden manipulation and restoration treatments in mineralogical specimens

Conservation conditions assessment of cultural heritage objects is a complex task, requiring the distinction between original and manipulated areas and the identification of materials employed during previous interventions. At the same time, the analysis and detection of exogenous materials should minimise or avoid sampling and prevent sample degradation. In this study, a multi-analytical and multi-scale method is developed to achieve these goals, combining non-invasive hyperspectral imaging in the short-wave infrared region (HSI-SWIR) with micro-invasive FTIR and micro-Raman spectroscopies. The top-down workflow lies on a first macroscopic imaging by HSI-SWIR, allowing to map spatial distributions of both inorganic and organic compounds. However, a deep diagnostic capability with this sole technique is limited. To enhance the spatial distribution estimation of restored portions of the object, a novel ORganic Index (ORI) based on characteristic CH absorptions in the SWIR region is introduced. ORI maps improve the detection of organic materials compared to UV-induced fluorescence imaging and Spectral Angle Mapper classification, enhancing the identification of regions of interest for further analyses. Micro-invasive FTIR and Raman spectroscopies are thus also implemented, enabling molecular-level unambiguous identification of different restoration materials, including polycyanoacrylates, epoxy resins, terpenoid resins, lime-based mortars. Mineralogical specimens embedded in geological matrices are selected as test materials due to their complex three-dimensional geometry and the lack of previous systematic analytical investigations. The proposed methodology represents a reliable non-destructive approach for the study of restoration practices, authenticity assessment, and conservation of complex cultural heritage objects.