Material Composition, Microstructure and Durability of Bishnupur Terracotta (India): Insights from Multi-Analytical Investigation and Implications for Heritage Conservation

This study investigates the material composition, microstructure, and durability of historical terracotta from the Bishnupur temples (West Bengal, India) using an integrated analytical approach combining X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). SEM image analysis reveals a polymodal particle size distribution (18.59–25.79 μm) and surface porosity of ~12–25%, indicating a packing-controlled microstructure. XRD results show a quartz-dominated mineralogy with clay minerals acting as interstitial binding phases. The characteristic basal kaolinite reflection (2θ ≈ 12.4°) identified in samples VT-1 to VT-4 confirms the persistence of kaolinite within the ceramic matrix. FTIR spectra indicate a silicate-rich system with hydroxyl and minor organic components facilitating hydration-mediated interactions. Thermal analysis demonstrates high stability (mass loss ≈2.8 ± 0.3%; Thermal Stability Index ≈0.97), with major transformations between 400 and 500°C attributed to clay dehydroxylation. Capillary forces (~10−6 N) dominate over Van der Waals forces (~10−10 N), producing a microstructural cohesion index of ~0.20–0.25 and supporting moisture-driven cohesion. The dense surface layer and relatively porous interior reflect controlled raw‑material selection and firing practices, contributing to long-term durability under humid environmental conditions and providing a scientific basis for the conservation of historic terracotta structures.