Thermal behaviour of limestone and monocrystalline calcite tempers during firing and their use in ancient vessels

Limestone and monocrystalline calcite tempers (grains) are abundant in ancient pottery. In pottery from the Canaan area the former is common in Iron Age storage and table-ware vessels and the latter is present in cooking pots. Limestone is much more widespread than monocrystalline calcite and the potters used it often as tempers when manufacturing pottery vessels, but usually not for cooking pots. While defects appear frequently around limestone tempers, they do not appear around monocrystalline calcite ones. This study examines the reason for using the latter tempers rather than the former ones. Raw materials of carbonate tempers in a clay matrix were fired and the decarbonation process was followed by quantitative IR thermospectrometry. The results indicate that the monocrystalline calcite tempers prevent formation of defects in the cooking pots during firing or during use. The reasons for this are higher thermostability at elevated temperatures, lower intensity of decarbonation, and retention of grain shape, as compared to limestone tempers.