Radiocarbon dating mortar: The identification of a Medieval Irish round tower using a multi-method inter-comparative approach

Investigations were carried out to establish the chronology of a building from the city of Derry, Northern Ireland. The date of this structure, previously assumed to be a 17th century windmill, was examined by application of radiocarbon dating to the lime mortar. Multiple sample preparation methods (cryo-breaking, mechanical, suspension) were used to isolate a series of lime binder fractions of different grain sizes. Combined with sequential dissolution and subsequent radiocarbon dating this permitted a) the presence of multiple carbonate components to be identified and, b) aided by inter-comparison of the dissolution profiles and the presence of well-defined plateaus in the age profiles, those components and dates associated with the building's construction to be resolved. Interpretation of the results was further enhanced by assessing the efficacy of preparations methods through application of particle size analysis, x-ray diffraction, thermogravimetric analysis and Fourier transform infrared spectroscopy. This work revealed that the building can now be recognized as the remains of a mid to late 13th century Irish round tower and, as such, the only building surviving from the Derry's Medieval monastic era, therefore making it of great cultural and historical significance.