Complications hindering radiocarbon dating of aerial lime mortars

Abstract Lime mortars are used to establish the chronology of historic buildings. The process seems straightforward since atmospheric 14 C is fixed into the calcite of the binder during the setting of the mortar. Nevertheless, the radiocarbon dating of mortars can present certain complications, leading to inconclusive dating results. Mineralogical studies are essential to select suitable samples for dating and avoid potential complications. Since the nature of the aggregates is a potential source of contamination the microtexture and mineralogy of the mortar components were determined using an optical polarising microscope (OM) on polished thin sections. The binder was analysed using Scanning Electron Microscopy (SEM) and the mineral composition was determined by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), micro-Raman analysis and Fourier Transform Infrared spectroscopy (ATR FT-IR). The most frequent complications faced by the authors during the radiocarbon dating of mortars were dolomitic, magnesian or impure limestone aggregates yielding layered double hydroxide (LDH) phases. The LDHs constituted a potential contaminant mineral phases for lime mortar radiocarbon dating preventing obtaining reliable ages. The results obtained in Castillo de Portilla (Álava) predate the historical period and the age discrepancy is related to the LDH content. The state of ruin of the building or the protection of built heritage could complicate the accurate sampling of lime mortar for radiocarbon dating. Rejuvenated ages at Irulegi Castle are attributed to the incorporation of inorganic carbon resulting from the recrystallization of the binder carbonates. The use of buried samples, as at San Juan de Barbadillo, can lead to rejuvenated ages due to the incorporation of recent organic carbon into the system.