19 years of mortar dating: Learning from experience

ABSTRACT. Since 1994, our team has gained extensive experience applying accelerator mass spectrometry (AMS) radiocarbonanalysis for mortar dating, totaling over 465 samples and 1800+ measured CO2 fractions. Several samples have beenanalyzed repeatedly. The research covers both Medieval and Classical archaeology. We therefore believe our experience canbe helpful when developing preparation procedures for different kinds of mortars in different areas and in varying chronologies.So far, the main areas of interest have been (a) the churches of the Åland Islands (in the archipelago between Finlandand Sweden); (b) the churches in the Åboland Archipelago (SW Finland); (c) sites in the Iberian Peninsula including Torrede Palma (a Roman village in Portugal); and (d) Rome, Pompeii, and Herculaneum (Italy). Most of the analyses before 2000were hydrolized in only two CO2 fractions per sample, and reliability criteria were defined on the basis of how well the agesof the two fractions agree with each other. These criteria have proved most helpful in determining the reliability of 14C mortaranalyses. Different types of mortar have been investigated, including lime mortars made both from limestone and marble,pozzolana mortars, fire-damaged mortars, and mortars based on burnt shells. Most importantly, separate lime lumps sampledfrom these mortars have been analyzed sporadically and recently more systematically. The research also includes differenttypes of hydrolysis applied in the pretreatment. In addition to using 85% phosphoric acid (H3PO4), the experimental researchincludes tests with smaller concentrations of phosphoric acid, and tests based on 2–3% hydrochloric acid (HCl) dissolutions.To characterize the dissolution process, results are presented as age profiles of 2–5 CO2 fractions. In our experience, pozzolanamortars have been difficult to date, and HCl dissolution should be used only in special cases and in complementary tests.