Multi-analytical Investigation of the Composition and Binders Used in the Earthen Support Layer of Fifth–Fourteenth Century CE Painted Fragments From Bezeklik, China

Composition, microstructure, and binding media of the decorative earthen plaster fragments from the Buddhist cave temple of Bezeklik (fifth–fourteenth century CE), China, were analyzed by X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, Fourier-transform infrared spectroscopy, sieve analysis, and other methods. In addition, gas chromatography–mass spectrometry was used for identification of the organic adhesive found mixed in the earthen plaster. Studies indicated the presence of proteinaceous material, mainly beeswax, with animal glue, pine resin, and plant sterol mixed in the earthen plaster as the biopolymer. The organic additives improved the basic qualities of the mud by acting as stabilizers, hardeners, and water proofers. The loamy sand soil sourced for Bezeklik plasters shows low cohesion and disaggregation of grains occurred for want of clay-sized particles in the plaster. Due to frequent handling and transportation, the fragments are now showing distress in the form of loss of earthen support and paint layers. Based on the analytical findings, traditional binders such as sepiolite clay, slaked lime, and fish glue were introduced and fragments consolidated for display.