Characterization of Historic Mortars Related to the Possibility of Their Radiocarbon Dating, Mikulčice and Pohansko Archaeological Sites

The archaeological sites of Mikulčice and Pohansko (South Moravia – the Czech Republic) belong to the oldest and the most important localities of Slavic settlement in Central Europe. A number of historic mortars sampled here were collected in order to study their composition, mortar structural characteristics and raw materials provenance. The aim of this study was not only the comparison and characterisation of the historical mortars from these archaeological sites, but we also evaluated the suitability of these mortars to be dated by14C analysis. The samples were characterised by several analytical techniques as polarised light and scanning electron microscopy, thermal analyses or quantitative X-ray diffraction. Stable isotope analyses and cathodoluminescence were also performed. The collected mortars contained a considerable amount of lime particles that can adversely affect the possibility of14C radiocarbon dating. According to the results of the analyses, the samples from both localities had a similar character. Mortars were very rich in the binder and contained unburnt limestone fragments that occurred frequently. Unburnt fragments were classified mostly as a micritic limestone contained bioclastic fragments. This material was determined as Ernstbrunn limestones according to the composition and structure. Stable isotope analysis also suggested that all studied lime samples came from a single source. The presence of geogenic carbonates (not fully burnt lime) affects the resulting14C age of the analysed samples fundamentally. The character of mortars leads to a discussion on how to adapt the separating procedures of the individual fractions and avoid geogenic carbon contamination.