Assessment of residual geogenic carbon in mortars concerning radiocarbon dating

Quicklime samples were collected from six vertical layers (L1-L6) of a feedstock calcined in a traditional single-batch wood-fired kiln and assessed. Three samples were well-burned and three under-burned. The quicklime was slaked in an excess of water and the presence of unburned particles was investigated after settling it into putty. The putty was assessed as bulk and also at three depth levels. Thermal analysis determined the CO2 residua in the quicklime samples. Cathodoluminescence detected individual unburned particles and image analysis was used for their quantification. Settling of the putties led to a considerable reduction of geogenic particles in the layers above the bottom. This was also confirmed by the stable isotope analysis. In the case of the putties made from well-burned quicklime, the delta 13C values of samples L4 and L5 ranged from -25.5 parts per thousand to -20.5 parts per thousand VPDB, and the delta 18O values ranged from -17.5 parts per thousand to -16.5 parts per thousand VPDB. The fractionation was likely affected by the division according to the particle size during the sedimentation. The results of the 14C analysis correlate with the quantified percentage of cathodoluminescent particles.