An innovative method to select a suitable fraction for mortar 14C dating: the Cryo2SoniC protocol.

To date, mortar radiocarbon (14C) dating represents one of the main “open issues” involving the whole radiocarbon community because, after about 50 years of experimentation, the possibility to evaluate absolute chronologies for these artifact by radiocarbon remains still uncertain. This work describes i) the development of a new methodology, called Cryo2SoniC, aimed to select a fraction of mortars recording the time of setting, ii) the evaluation of Cryo2SoniC main drawbacks and successes, when it is applied on archaeological mortar. The proposed method is based upon a physical procedure (ultrasonication) isolating only the binder calcite formed by means of the absorption of atmospheric CO2 during the mortar setting. Proposed procedure was tested, at first, on a series of synthetic mortars produced in the laboratory and lime lumps, using a first version of the separation method (i.e. CryoSoniC) and then applying the ungraded version, Cryo2SoniC, on a series of archeological samples from different important historic monuments and excavations. All measured radiocarbon ages were compared with their chronological reference allowing accuracy evaluation. Radiocarbon dating was performed on the selected fractions using high-precision (i.e. 0.3% prescision) Accelerator Mass Spectroscometry (AMS). Mineralogical investigations for mortar charcterization were performed on original mortars by X-Ray Diffraction (XRD) of powders, observations of thin section with Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). An inter-comparison exercise has been performed between CIRCE and the Poznan Radiocarbon Laboratory to verify the efficiency of different separation methods (Cryo2SoniC vs stepped digestion) performed on the same samples. InfraRed Fourier Transformed Spectroscopy (FTIR), SEM and ChatodoLuminescence (CL) analyses were performed to verify eventual dead carbon contamination affecting Cyo2SoniC produced fractions and consequentially develop a diagnostic tool i) to gather information about observed pitfalls of the methodology ii) to preliminarly predict the quality of CryoSoniC isolated fractions. Final balance of this research is the reliability and efficiency of the Cryo2SoniC method on a widespread spectra of mortar typologies, highlighting its weak points and suggesting alternative solutions such as the purification of lime lumps incased in mortars.