In-situ molecular investigation of ancient and restoration mortars from the theatre of Tindari (Sicily, Italy) by DRIFTS and µ-Raman spectroscopy

Abstract

We report an in-situ multi-technique study combining Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and µ-Raman spectroscopy for the molecular characterization of five mortar areas from the theatre of Tindari (Sicily, Italy). The selected areas, chosen as representative case studies within a broader diagnostic campaign, include three ancient mortars attributed to the Roman transformation phase of the monument, and two modern conglomerate- and cement-based materials employed in restoration works. The ancient areas are located on the podium and on the intrados of the vault of the annular corridor encircling the orchestra, while the modern restoration mortars are located on the scaena. DRIFTS spectra provided information averaged over the superficial portion investigated, whereas µ-Raman spectroscopy allowed the analysis of individual grains and localized phases. The analyses revealed clear compositional differences among the investigated mortars, mainly involving quartz-, calcite-, clay-rich, and cement-related phases, together with Raman markers deriving from carotenoid (beta-carotene)-containing biological colonization. The comparison between DRIFTS and µ-Raman data demonstrates the complementarity of the two techniques for rapid in situ screening of heterogeneous historical mortars. In particular, DRIFTS rapidly characterizes the overall molecular composition, whereas µ-Raman resolves individual mineral constituents and localized alteration products. Noteworthy, the identification of mineral phases, restoration materials, and biodeterioration markers provides a valuable basis for targeted analytical investigations and informed conservation planning.