A03_P22

Pre-screening of lime mortars for ¹⁴C dating – Preliminary results

Wojcieszak M^1,2, Fontaine L¹, Hayen R¹, Elsen J³, Van den Brande T¹,  Oostvogels A¹, Ligovich G¹, Rich M¹, Boudin M¹

¹Royal Institute for Cultural Heritage (KIK-IRPA), Brussels, Belgium, ²Evolutionary Studies Institute (ESI), University of the Witwatersrand, Johannesburg, South Africa, ³Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium

The oldest known use of lime mortars dates from the Neolithic period. The production process incorporates atmospheric CO₂ and gives rise to the formation of calcium carbonate (CaCO₃). Radiocarbon dating of anthropogenic lime carbonates was first developed in the 60’s. These first results were very promising and the French researchers Jacques Labeyrie et Georgette Délibrias already pointed out that microscopic observation should be performed to check out the eventual presence of foraminifera (microfossils) susceptible to false the date result, since they are made of very ancient carbonates. Other types of carbonates can be present in mortars because of the mortar formation process and possible subsequent weathering. These carbonate contaminations are unburnt or underburnt limestone used for lime production, fossil carbonates such as shell fragments in sands, reused limestone and mortars crushed and used as aggregate, or specific minerals containing carbonates. Their presence results in a lower ¹⁴C/¹²C ratio which generates an older date, as they are radiocarbon free. Another category of carbonates are secondary carbonate deposits, which can cause an increase of the ¹⁴C/¹²C ratio but can also incorporate older carbon. Using the stepwise acid hydrolysis method on the binder (and when possible pure lime inclusions dating), this project aims to develop a pre-screening method, by means of different analytical techniques such as polarized light microscopy, Fourier transformed Infrared spectroscopy, cathodoluminescence and thermal analyses (TGA-DSC) to verify the “mortar quality” in order to decide whether the radiocarbon date will be reliable or not.