Optimization of the sample selection procedure for radiocarbon dating of historical mortars

Radiocarbon dating, a widely used technique in archaeological research and cultural heritage preservation, usually involves organic materials for age determination. However, its potential applications extend to inorganic carbon-based materials such as mortar, provided that the radiocarbon datable fraction, calcite (so-called anthropogenic calcite), can be effectively isolated. Mortar, a complex mixture of binder, aggregates, and additives, offers a promising way for absolute chronology, but also presents a particular challenge because of its heterogeneous nature. Contaminations originating from geogenic sources, and the formation of secondary calcite require rigorous sample selection procedures. This PhD thesis aims to optimize the procedures for effective use of radiocarbon dating for mortars, from sampling to 14C concentration dating. During these three years, I have worked at DST-UNIFI, ISPC-CNR and INFN-LABEC in Florence. At DST-UNIFI and ISPC-CNR, mortar characterization and new strategies for powder characterization were performed. At INFN-LABEC, sample preparation process and radiocarbon measurements were carried out. A comprehensive approach was formulated that aimed to use various mineralogical, petrographic, and chemical analyses to identify and select mortar samples suitable for radiocarbon dating. Techniques such as carbonation tests, optical and electronic microscopy, X-ray diffraction, thermogravimetric analysis and infrared spectroscopy were carried out to determine the composition of all the constituents of the mixture: the nature of the binder and aggregates, the presence of additives and the hydraulic behaviour. In particular, the integration of high-resolution techniques, including FPA-FTIR and µXRPD, facilitated the study of the distribution CaCO3 polymorphs in mortar samples. The goal was to understand how these phases are formed and to assess the feasibility of dating them when they are in the binder. After characterization, we proceeded to mechanical selection of the binder-enriched bulks or lumps. In order to distinguish between anthropogenic and geogenic calcite, non-destructive techniques were used to identify the most suitable samples. Experimental studies using standard samples of geogenic and anthropogenic calcite examined the effectiveness of ATR-FTIR and micro-Raman spectroscopy in distinguishing calcite formed by different mechanisms. The results are promising and expand the repertoire of tools for characterizing non-standard samples. The introduction of new techniques in conjunction with established methods was aimed at selecting anthropogenic sample powders for AMS measurement and greatly simplifying the procedure in terms of time and cost. To prepare the samples for dating, the experimental set-up used for our study includes a new acidification line coupled with the microsamples set-up. The acidification line allowed us to choose the time range to collect the CO2. The microsamples set-up provided us the ability to reduce the mass required for dating so that the pre-treatment remained highly selective, and we were even able to analyse individual lumps. The optimized procedure was applied to mortar samples from historic Florentine buildings and public buildings in Pompeii, both known for their hydraulics. In the case of the Florentine buildings, the procedure facilitated the reconstruction of the construction phases. In Pompeii, however, complications arose due to mortar composition and the presence of geologic carbonates. Nevertheless, the radiocarbon results were consistent with the characterization results, highlighting the need for careful analysis. The final section of this study presents a case in the Church of St. Philip, where binder mortars were not suitable for radiocarbon dating. However, a detailed characterization identified a datable component within the mixture, namely the straw fragments used as an additive for the painted plasters. The optimized microsample line enabled the successful dating of straw fragments that revealed the two construction phases of the church. In summary, this PhD thesis provides a guide for the selection and characterization of mortar specimens. It presents innovative techniques and emphasizes the importance of using non-destructive methods for powder characterization to allow the reuse of unique samples for dating purposes. The application of these techniques in different historical contexts and to various types of binder mortars allows us to assess the potential of radiocarbon dating.