An Investigation of re-Carbonation Process and Formation of Secondary Calcite in the Texture of Ancient Potteries by Petrographical Method

Re-carbonation process of calcareous phases in the texture of ancient potteries is important regarding the burial environment, pathology and prevention of future damages. If the decay process of carbonatic phases (e.g. calcite CaCO3 and dolomite Ca,Mg(CO3)2) during firing is not completed or calcium oxide (CaO) is not transformed to stable phases at high temperature (as a result of stop the firing), this can be combined with moisture (water or water vapor) and calcium hydroxide (portlandite: Ca(OH)2) can be formed. Crystallization pressure followed by increasing the volume from new growth phase can cause crevices (the narrow cracks), cracks and separations of the structure of pottery. Furthermore, portlandite can absorb carbon dioxide to form secondary calcite crystals. Formation of secondary calcite occurs after decomposition of gehlenite and anorthite in moist and acidic environment. Carbonate phase is very sensitive to the acidic environment and, by absorbing H+, it is transformed to bicarbonate phase which is soluble in water. In addition, solubility of portlandite in water is well known and therefore the presence of carbonate grains in the matrix of pottery during conservation and restoration process is remarkable.The purpose of this paper is to review re-carbonation process through investigating the thin sections of ancient potteries by means of polarize microscope. Pores generated from decomposition of carbonate grain, re-carbonation and formation of new crystalline phases (such as portlandite and secondary calcite) can be detected by this method.Petrographical investigation of thin section of ancient potteries implies that the type and origin of carbonatic phase, burial environment and its effect on decomposition of carbonates, and formation of secondary products (portlandite & secondary calcite), are important parameters for investigating the re-carbonation process.