The composition and technology of the 3-4th century CE decorative earthen plaster of Pithalkhora caves, India

This research focuses on the characterization of ancient decorative earthen plaster of India's Pithalkhora group of caves. The restoration strategies being planned for the earthen plaster demand detailed scientific investigation to understand technology and composition so as to prepare matching plaster. The mineralogical, micro-structural and chemical characterization of the earthen plasters have been investigated by analytical tools like petrological microscope, XRF, CHN analysis, laser scattering particle size analyzer, FTIR, XRD, Scanning Electron Microscopy, thermal analyzer (DGA/DTA) as well as on-site observations. A comparative study with nearby sites portrays the earthen plaster of Pithalkhora coarser (silt: 62-100%, sand: 2-2.2%) with traces of clay (0-2%); hence it may be prone to damage by grain breakages. The chemical composition confirms that the aggregates mixed with the earthen plaster belong to that of locally available basaltic origin. SEM studies show addition of quartz and traces of clay (illite and montmorillonite) to augment the flexibility and performance of the plaster. The restoration plaster should be synthesized with aggregates of locally available basaltic rock mixed with slaked lime in addition with rice husk and other vegetal fibres as vegetal additive to overcome the shrinkage of the plaster.