A03_P10

Extensive survey on radiocarbon dating of organic inclusions in historical mortars

Pachnerova Brabcova K¹,  Kundrat P¹, Krofta T¹, Suchy V¹, Petrova M¹, John D^1,2, Kozlovcev P³, Kotkova K³, Fialova A³, Valek J³, Svetlik I¹

¹Nuclear Physics Institute of the CAS, Praha, Czech Republic, ²Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Praha, Czech Republic, ³Institute of Theoretical and Applied Mechanics of the CAS, Praha, Czech Republic

Direct radiocarbon dating of historical mortars remains challenging due to complex processes during mortar maturation that can mislead the interpretation. Organic inclusions in the mortars, such as charcoals, seeds, microbiotas, wood, or bones, represent important alternative or complementary dating material providing the date as a terminus post quem.

This work adds to the determination of how reliable such organic inclusions can be for radiocarbon dating. For the analysis, we have collected 129 charcoals from five different early to late medieval castles and churches located throughout the Czech Republic. The architectural objects were chosen to meet the following criteria:  i) known age documented in written sources and optimally also obtained through other dating method or complemented with radiocarbon dating of samples other than mortar-derived ones; ii) known history of repairs and maintenance; iii) age falling before 1650 AD to avoid issues related to inconclusive radiocarbon dating in the early modern period.

Most of the samples were measured on our new AMS MILEA. The results demonstrate that a notable fraction of inclusions corresponds to charcoals from old wood, often a few hundred years old when raising the object, presumably originating from timber cut-offs. For some sites we found several samples related to later repairs, despite trying to limit the sampling areas to original masonry only. Based on this comprehensive study, we recommend a rather high number of samples be collected per dated object, at least 5 - 10 for sites with uncomplicated building history, so that radiocarbon dating provides reliable results.