Successful AMS ¹⁴C Dating of Non-Hydraulic Lime Mortars from the Medieval Churches of the Åland Islands, Finland

TLDR

Well-preserved mortar is abundantly available from every building stage, making it a valuable dating material. The study used AMS dating on 150 Åland mortar samples, integrating dendrochronology, petrology, chemical testing, and dissolution modeling to establish reliability criteria for interpreting 14C results. The research established a chronology for 12 of 14 churches, with 80 % of the samples yielding conclusive 14C dates that agreed 95 % with wood or dendrochronology age controls.

Abstract

Fifteen years of research on accelerator mass spectrometry (AMS) radiocarbon dating of non-hydraulic mortar has now led to the establishment of a chronology for the medieval stone churches of the Åland Islands (Finland), where no contemporary written records could shed light on the first building phases. In contrast to other material for dating, well-preserved mortar is abundantly available from every building stage. We have gathered experience from AMS dating of 150 Åland mortar samples. Approximately half of them have age control from dendrochronology or from 14 C analysis of wooden fragments in direct contact with the mortar. Of the samples with age control, 95% of the results agree with the age of the wood. The age control from dendrochronology, petrologic microscopy, chemical testing of the mortars, and mathematical modeling of their behavior during dissolution in acid have helped us to define criteria of reliability to interpret the 14 C results when mortar dating is the only possibility to constrain the buildings in time. With these criteria, 80% of all samples reached conclusive results, and we have thus far been able to establish the chronology of 12 out of the 14 churches and chapels, while 2 still require complementary analyses.