Projektet Ålands kyrkor och murbruksdatering – rapport från en metodutveckling

Resumé

The Åland Churches and Mortar Dating – State of Research from the Development of a Method.
By Åsa Ringbom, Jan Heinemeier, Alf Lindroos & Àrny Sveinbjörnsdottir.

Why mortar dating? The project the Åland Churches was initiated with the aim to finally reach a reliable chronology for a group of stone churches upon which there had been deep disagreement. Since there are no contemporary sources to shed light on the matter, and since coins and artefacts cannot date buildings, there was an obvious need for objective scientific methods on a larger scale. The method of dendrochronology was widely applied on wooden structures in the churches in 1991-1992. Even if dendrochronology could not date the first building stages of the churches, due to repairs and fires, this method was very important in providing firm dates for secondary structures, and for comparative research. Mortar is not an organic material. Yet, absorbing CO2 from the atmosphere during the hardening process, makes mortar an ideal material for 14C dating. Compared to all other datable materials, mortar has the advantage of being found in the original in large quantities, from all the different building stages. Thus, the project focused on developing the method of mortar dating. Since 1994, reliable results have been reached, thanks to the introduction 14C AMS (Accelerator Mass Spectrometry) analysis. To begin with the analysis took place in the 14C AMS Dating Centre at Aarhus University, Denmark. The project turned international in 1997 when the method was tested in classical archaeology, upon mortars from the margins of the Roman Empire, and on hydraulic pozzolana mortars from Rome itself. Since 2005 the collaboration for the analysis includes the Oxford Radiocarbon Accelerator Unit, and the NSF-Arizona AMS Laboratory, in Tucson. Our conclusion today is that non hydraulic lime mortars, whether medieval or classical, are well suited for mortar dating. From Åland, 96% of all the mortars with age control, have yielded convincing results (for further information see www.kyrkor.ax). It is hardly surprising that hydraulic pozzolana concrete, with an entirely different chemistry, is more difficult. But even so, several important structures in Rome have yielded an age well known from historical sources and brick stamps. From our vast experience we have been able to identify different criteria of reliability for interpreting results where mortar dating is the only method available.