Mortar dating using AMS ¹⁴C and sequential dissolution: Examples from medieval, non-hydraulic lime mortars from the Åland Islands, SW Finland

Non-hydraulic mortars contain datable binder carbonate with a direct relation to the time when it was used in a building, but they also contain contaminants that disturb radiocarbon dating attempts. The most relevant contaminants either have a geological provenance and age or they can be related to delayed carbonate formation or devitrification and recrystallization of the mortar. We studied the mortars using cathodoluminescence (CL), mass spectrometry (MS), and accelerator mass spectrometry (AMS) in order to identify, characterize, and date different generations of carbonates. The parameters- dissolution rate, ¹³C/¹²C and ¹⁸O/¹⁶O ratios, and ¹⁴C age-were measured or calculated from experiments where the mortars were dissolved in phosphoric acid and each successive CO₂ increment was collected, analyzed, and dated. Consequently, mortar dating comprises a CL characterization of the sample and a CO₂ evolution pressure curve, a ¹⁴C age, and stable isotope profiles from at least 5 successive dissolution increments representing nearly total dissolution. The data is used for modeling the interfering effects of the different carbonates on the binder carbonate age. The models help us to interpret the ¹⁴C age profiles and identify CO₂ increments that are as uncontaminated as possible. The dating method was implemented on medieval and younger mortars from churches in the Åland Archipelago between Finland and Sweden. The results are used to develop the method for a more general and international use.