Mortar dating: A new procedure in the Zagreb Radiocarbon Laboratory

Mortar can be dated by radiocarbon dating of binder carbonates created during mortar hardening. However, the most prominent problem in mortar dating is selecting the carbonates from binder without impurities used in mortar production that lead to overestimated ages. To this day there is no consensus on the unique way of sample preparation that would always provide the true date. At the Zagreb Radiocarbon Laboratory, Croatia, we established a procedure for graphite preparation form AMS radiocarbon dating of mortar that we find simple and cost-effective. The 14C activity of graphite samples prepared in the Zagreb Radiocarbon Laboratory is measured at the Center for Applied Isotope Studies (CAIS), University of Georgia, Athens, USA. The basis of the procedure is collection of mortar particle fraction 32 - 63 μm which is hydrolyzed by H3PO4 and CO2 fractions are collected after 3, 10, 30 s and until the end of reaction. The rule was established that the first fraction is the reliable for date of mortar, and it should not exceed 20 % of total CO2 yield. Applying this rule, for some samples the collection of CO2 fractions has to be repeated, either by prolonging the periods, or slowing the reaction by lowering temperature at fixed (the same) time periods. In order to validate the procedure two laboratory mortars containing 26 % and 48 % of binder carbonate (Šustić et al. 2012) were prepared and yielded 97.7 ± 0.3 pMC and 100.6 ± 0.3 pMC, respectively. In addition, seven original archeological mortars were used to test the method. For five archeological samples the expected age of samples was estimated by the sample provider, one sample true date was estimated from two charcoal samples associated with the sample formation and one sample true date was obtained from a tombstone with engraved date of death. Two laboratory and five archaeological samples showed acceptable results, justifying the applicability of the proposed procedure. However, two archaeological samples sampled from an altar in a cave showed a14C higher than 100 pMC, suggesting that these mortars were still active, leading to conclusion that mortars from caves, due to specific conditions during mortar hardening, are not representative for radiocarbon dating. Šustić, I., Barešić, J. & Šipušić, J. (2012) Determination of hardened binder initial composition. Zement - Kalk - Gips international, 65 (10), 70-78