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Project

Pre-screening of anthropogenic lime carbonates for 14C dating (PALc)

Mortars, or more generally anthropogenic lime carbonates, have been used for many centuries to build elements of architecture. The oldest known use of mortar dates from the Neolithic period. The production process of lime mortars consists of heating geological limestone (CaCO3) to produce quicklime (CaO) which is then blended with water to form slaked quicklime (Ca(OH)2) and mixed with aggregates (sand) to create the mortar. The hardening is due to a carbonation reaction: atmospheric CO2 is incorporated to the mortar and gives rise to the formation of calcium carbonate (CaCO3), then called anthropogenic lime carbonates. This carbonation process theoretically allows radiocarbon dating of the mortars since the quantity of 14C absorbed is relative the 14C quantity in atmosphere at a certain time. Radiocarbon dating of homogeneous mortars, which have fully carbonated and don’t contain any geogenic carbonates (such as unburnt fragments of the geological limestone), calcareous impurities coming from the aggregates (carbonate grains such as shells) or secondary carbonate formation, can be reliable and gives often good agreements with historical dates. However, more often mortars are very heterogeneous and the radiocarbon dates obtained don’t correspond to the expectations. The current procedure used for CO2 extraction before dating is generally based on stepwise acid hydrolysis. It is assumed that the different carbonate sources dissolve at different rates although empirically this assumption seems not always valid. This project aims to develop a pre-screening method, using different analytical techniques such as polarized light microscopy and cathodoluminescence, to verify the “mortar quality” in order to decide if the radiocarbon date will be reliable. The development of a pre-screening technique to identify mortars suitable for radiocarbon dating will be beneficial for both heritage scientists and archaeologists or art historians since 14C analyses will only be performed on potentially datable mortars providing therefore more confidence in the obtained results. On another level, improving accuracy of 14C dating of lime-based mortars and renders found in monuments and archaeological remains will contribute to the cultural and touristic valorisation of these historic sites.

Date:15 Dec 2020 →  Today
Keywords:14C dating, historic site, mortar quality
Disciplines:Geology not elsewhere classified, Tourism not elsewhere classified