Sampling problems in the radiocarbon dating of old mortars and plasters with the "pure lime lumps" technique.

Several scientific papers describe the radiocarbon dating of lime mortars and plasters [Folk and Valastro, 1976; Van Strydonck et al., 1992; Hale et al., 2003; Nawrocka et al., 2005] carried out using the 14C dating contained in the calcium carbonate precipitated during the hardening process of lime. As the content of 14C contained in the newly formed calcium carbonate reflects the 14C concentration in the atmosphere at the time of hardening, this material can be used for the radiocarbon dating of old mortars and plasters. Although the method is quite simple in its basis principle, relevant issues come from the contamination problems with other carbon sources. Grains of carbonate sand or underburned pieces of the same limestone used to produce the lime, that are originally mixed with the binder cannot be, in fact, completely removed from the mixtures. These materials do not contain radioactive carbon but behave as the carbonated lime during the analyses. For overcoming this problem, over the past years, new techniques for the sample preparation have been developed and among these techniques, the so called “pure lime lumps” represents a fast and reliable method. The technique is based on the use of lumps of pure lime, very often embedded in old lime based mixtures as material for the radiocarbon dating. Because these lumps are made of pure lime any contamination problem is avoided. However, despite the remarkable results already achieved with this technique [Pesce et al., 2009; Pesce et al., 2012; Pesce et al., 2013], the sample collection is still a very important issue for a successful application of the radiocarbon dating. This contribute describes the main sampling problems faced during a research on the accuracy and precision of the "pure lime lumps" technique, founded by the University of Genoa2 and carried out together by the University of Genoa and the and CEDAD in Italy and the University of Bath in United Kingdom.