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Historic Lime-Mortar 14C Dating of Santa María La Real (Zarautz, Northern Spain): Extraction of Suitable Grain Size for Reliable 14C Dating

This paper describes a method for effective separation of the pure binder fraction of lime mortars for reliable radiocarbon dating. The methodology allows removal of the detrital carbonate fraction and the unburnt limestone particles, obtaining particles of under 1 μm. The extracted fraction ensured that all carbonate has been generated by slaked lime carbonation. Consequently, the measured carbon corresponds to atmospheric carbon. The proposed method allows to obtain pure datable binder, simplifying considerably the performance of radiometric measurements because dating other grain-size fraction is unnecessary. In order to prove the effectiveness of binder refining, the extraction method has been applied to 5 lime mortars of different archaeological periods from the perimeter walls of Santa María la Real parish church (Zarautz, northern Spain).