Calcite polymorphs in historic plasters of India’s arid region – execution technique, composition and characterization

Abstract

: The lime works in India’s desert region were executed by a technique that is characteristically different from other parts of the country. The area is very hot and the high evaporation of water due to heat and less rain has caused salinity to the groundwater. Due to local climatic conditions, the decorative works of Rajasthan were executed on a shiny white plaster by a technique locally named Ala-Gila which is sometimes compared with the true fresco of Italy. The Persian colonial arts also flourished in this part and the color palette suddenly changed from vibrant to more shiny colors in the paintings. In this work, the microscopic observation of thin section was utilized to investigate geological and fine morphological features of the 16th century Amber fort lime plaster-a WHS site and a very popular tourist destination. The FTIR, XRD, and SEM photomicrographs of the plasters showed the stabilization of different crystalline phases of polymorphs of calcium carbonate. The chemical analysis and SEM-EDX data denote dolomitic limestone probably sourced for plaster works. The high amount of magnesium present in the raw material, the environmental conditions at the time of the application of the plaster, pH, etc. have stabilized calcite polymorphs in the plaster. The calcite meta-stable phases have undergone dissolution with time making the plaster weak. This has increased porosity, permeability, and resultant lowering of plaster's mechanical strength. The thermal analysis and cementation index of the plaster showed the use of non-hydraulic binder-rich air-lime for plaster works. The granulometric study showed the mixing of reddish-brown sand-size grains, and the aggregates were probably from the same location for major construction activity. The low proportion of aggregates led to reduced mechanical strength, and the is vulnerable to damage. It is to a major restoration of the ancient lime works with compatible materials as per analytical data