Method for dating archaeological lime mortar : characterization, extraction, datation, validation. – MEMOAr

MEMOAr focuses on a key element of research in building archaeology: the dating of construction phases. Currently, the study of lime mortars allows us to distinguish these phases but not to date them. The 14C dating method for lime mortars gives unreliable results because these are very heterogeneous materials: they can contain several sources of carbon which lead to dating errors. It is therefore necessary to identify and select the carbonates that really crystallized during the setting of the mortar in order to extract the right carbon to date. The identification and characterization of mortars is therefore an essential preliminary step before any dating in order to assess the risks of errors and the validity of the date obtained.
We propose an innovative method combining the precise identification of areas containing neoformed carbonates by LIBS (Laser-Induced Breakdown Spectroscopy) imaging, and the targeted extraction of carbon by laser ablation at high spatial resolution. Artificial intelligence will be used to create masks and drive targeted extraction on selected areas. The amount of residual 14C will then be measured by accelerator mass spectrometry (AMS). The first experiments carried out by the team members have enabled the design of the prototype and the validation of the concept.
The dating tests will be carried out both on experimental samples (developed in the laboratory and of known composition) and on samples from sites already dated by cross-dating methods (ancient and medieval sites from different French regions). All dating elements will be discussed and integrated into a Bayesian analysis that will allow us to rigorously validate the results obtained.
MEMOAr relies on a consortium of archaeologists, archaeometers, physicists, chemists and materials specialists (Lyon1, Lyon2, Bordeaux-Montaigne and CNRS). A researcher from the Royal Institute for Artistic Heritage in Brussels will also be consulted as an expert advisor. We plan to recruit a doctoral student (3 years) and a post-doctoral student (1 year) with interdisciplinary profiles.
The project will provide methodological tools dedicated to lime dating, accessible within an internationally open platform (ILMTech technological and scientific platform). All the data produced will be available in a dedicated space currently being developed within the Equipex+ project, ESPADON-PATRIMEX+. This database will be open and accessible to all. The entire archaeological community will thus have access to high-performance tools for dating lime mortars.