Radiocarbon dating of mortars: Contamination effects and sample characterisation. The case-study of Andalusian medieval castles (Jaén, Spain)

Since 1960s, mortars have been exploited as a potential material for radiocarbon dating and, despite the fact that this methodology appears very simple in its principles, some measured radiocarbon ages were definitely different from the expected historic ages. The paper concerns to the characterisation of mortars from three Andalusian castles (Spain) by means of different mineralogical techniques in order to control “contamination effect” that could affect age estimation. Several mixtures of binders and aggregates composed the archaeological mortars; lime-based or gypsum-based binders were distinguished whereas the aggregates varied from carbonates to intrusive igneous rocks. The radiocarbon dating provided good results for lime-based mortars with silicate aggregate; conversely, for other specimens the method for sample preparation (Cryo2SoniC) was improved increasing the ultrasonic time and decreasing the mesh size. The research points out the importance of a multi-analytical approach aimed at improving a widely accepted protocol for 14C dating of archaeological mortars.