Study on the accuracy and precision of the 'pure lime lumps' technique for the radiocarbon dating of old lime based materials

The recently developed “pure lime lumps” technique for the radiocarbon dating of old limebased mixtures has been applied with interesting results over several years. Contrary to alternative techniques, which involve physical and/or chemical pre-treatment of specimens, this technique is based on a careful selection of a specific type of lump usually embedded in old lime-based mixtures. The founding principle underlying this technique is, in fact, the use of the “pure lime lumps”. These are lumps of pure lime which are not mixed with the aggregate and thought to originate from imperfect mixing of the wet mix or from an incomplete slaking process of the lime. Pure lime lumps contain the same lime as that used in other parts of the mixtures but, importantly, as they are not mixed with aggregate they are free of contaminants such as sand grains or under burned pieces of limestone. For this reason this technique can dramatically reduce the errors in the radiocarbon dating. Although this technique has been used in several cases, a comprehensive study of its accuracy and precision is still missing. This paper will address this issue by showing radiocarbon results of 5 specimens of pure lime lumps coming from the same part (i.e. same stratigraphic unit in archaeological terms) and from the same depth of the external surface of a specifically chosen wall. The wall, in fact, is the façade of a medieval building located in the old city of Genoa (Italy) where it is based also one of the most experienced Italian research centres for the building archaeology. This wall has been dated independently from radiocarbon through the others archaeological methods commonly used in building archaeology such as mensiochronology of bricks. The lumps collected in this wall have been characterised with several techniques such as Raman spectroscopy, scanning electron microscopy and thermal analysis and then sent to the AMS laboratory.The precision of the radiocarbon dating results has been determined from the scattering of the data obtained from the analysed terms while accuracy has been estimated by comparing the 14C results with those obtained by the archaeological analysis of the wall.