Radiocarbon dating of historical mortars

The method of radiocarbon dating of mortar has been a Holy Grail since the revolution in archaeology when Libby introduced the radiocarbon dating method in 1946. However, there is still no established universal method for mortar sample preparation that would always provide a true date. The method relies on the principle that during mortar hardening, atmospheric CO2 is introduced in the mortar structure in the form of calcium carbonate, preserving the information related to the 14C activity of the atmosphere at the time of its formation. This lime-carbonate is, however, difficult to extract since it is chemically and crystallographically identical to the geogenic limestone carbonate used in mortar production, which is of different origin and does not contain 14C. Limestone remains can be present due to unburned particles or it could also have been used as aggregate. If the lime-carbonate is not completely separated from the geogenic limestone, the obtained dates will appear inaccurately old. Erroneous dates could also be obtained due to recrystallised carbonates that were precipitated from environmental waters, if the construction survived fire and carries information of the fire event, or the hardening process stopped and restarted and, consequently, carries information of the later dates. Due to all the difficulties, archaeologists closely collaborate with analysts and special attention is paid to sampling principles (Daugbjerg et al. 2020). In recent years, with the introduction of accelerator mass spectrometry, mortar dating has seen a significant improvement. International mortar intercomparison studies named MODIS have been conducted (Hajdas et al. 2017), further exchanging and promoting different approaches, gathering ideas and improving the method for the radiocarbon dating of mortars. Here, the different approaches to lime-carbonate separation and the currently most promising results for mortar dating will be presented