Multi-Analytical Characterization of the Earthen Plaster and Ground Layer from the Nirvana Platform in Cave No. 47 of the Kizil Grottoes, Xinjiang, China

The Nirvana platform in Cave No. 47 of the Kizil Grottoes preserves an important example of early Buddhist earthen plaster and ground layer manufacture along the Silk Roads. This study investigated the material composition and stratigraphic sequence of its earthen plaster and ground layer using optical microscopy (OM), micro-Raman spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDX), X-ray diffraction (XRD), micro-Fourier transform infrared spectroscopy (µ-FTIR), laser-diffraction particle-size analysis (LD-PSA), indirect enzyme-linked immunosorbent assay (ELISA), and archaeobotanical observation. The results reveal a differentiated multilayer system consisting of a coarse clay layer, a fine clay layer, an anhydrite-dominated white ground layer, and a hematite-bearing, anhydrite-rich red ground layer. The coarse clay layer exhibited a broader particle-size distribution and contained abundant chopped straw and barley husks. The fine clay layer was more homogeneous and contained numerous keratinous animal hairs, compatible with a provisional attribution to cattle. A weakly positive ELISA response suggests the possible presence of an ovalbumin-containing material within the earthen plaster. The results demonstrate systematic differentiation of mineral fractions and organic inclusions across successive layers. The identified sequence reflects local technological choices within broader Asian traditions and provides a material basis for understanding and conserving the Nirvana platform.