Radiocarbon dating of charcoals in mother mortars

Full text: Radiocarbon dating of historical mortars remains challenging due to the complex processes that take place during mortar maturation, which, if not reflected, lead to misleading rejuvenation or ageing of the results. Organic remains embedded in the mortars, such as wood charcoals, seeds, microbiotas, wood, or bones, are also available for dating; however, such samples are often perceived as unsuitable for the possibility of yielding older age which differs from the age of the mother mortar. A typical example frequently mentioned in the literature is the problem of so-called "old wood" based on the fact that even very old trees can be used as fuel for burning limestone, resulting in the embedded charcoals being older than the mortars. This work adds to the determination of how reliable such organic inclusions can be for radiocarbon dating, with special attention paid to wood charcoals. For the analysis, we have collected 192 charcoal samples from seven different historical castles and churches located throughout the Czech Republic. The chosen architectural objects 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 samples; ii) known history of repairs and maintenance; iii) the age falling into the period before the erratic wiggles of the 14C calibration curve in the early modern period — before 1650 AD. In the context of the Czech Republic history, it resulted in the selection of early to late medieval objects. The occurrence of charcoals varied significantly from object to object, ranging from 0.003 to 0.033 charcoals per m². The samples were carefully examined, cleaned under an optical microscope and treated with an acid-base-acid procedure. About 10% of the samples dissolved completely, thus reducing the final number of the charcoal samples. The surviving samples were then combusted and finally graphitized using metallic zinc as a sole reduction agent. Accelerator mass spectrometry (AMS) measurements were performed using the MICADAS instrument by the Hertelendi Laboratory of Environmental Studies in Debrecen, ATOMKI, Hungary or the MILEA instrument by the Nuclear Physics Institute, Czech Republic. The AMS results were compared with the known age of the castles. The first results did not confirm the extended use of very old wood for burning limestone, as most of the analysed charcoals were not older than the mother mortars. On the contrary, several samples were found to be younger. The sources of charcoal contamination during the long-term exposure when embedded in the mortars, as well as the possibility of its removal, are discussed in this contribution.