The ratios of the stable isotopes of carbon (13C/12C) and oxygen (18O/16O) in calcite in clays that contain carbonate and that were fired at temperatures between 500 and 700 °C displayed a good correlation with the firing temperature for both isotopes. The isotopic composition indicated that the original carbonate was either completely decomposed or that it exchanged with environmental CO2 to obliterate the original isotopic signature. The isotopic effects seem to be kinetically controlled. The isotope thermometer is used to compare refiring temperatures of pottery which was fired under controlled conditions. The differences between calculated and actual temperatures did not exceed 132 °C and in most cases was about 50 °C or less. Isotopic analysis of pottery from Tel Miqne‐Ekron and Deir el Balah, dated to the 13th–12th centuries BC, gave an average ancient firing temperature of about 600 °C. If shown to be of general validity, then perhaps this technique can be used also for estimating preparation temperatures of ancient mortar, especially when preserved in dry climates where later changes would be minimized.
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