Mortar radiocarbon dating: Cryo2sonic sample preparation procedure. method and applications

The absolute chronology of archaeological contexts is mainly based on the radiocarbon dating of organic materials. This analytical approach often provides misleading results when the goal is to find a surprising relationship between organic matter and archaeological structures. Thus, discovering new radiocarbon dating methodologies on mortar samples, represents one of the main “open problems” involving the entire radiocarbon community. Mortars are heterogeneous building materials composed of a mixture of inorganic and/or organic binders, and inert with sandy dimensions. Indeed, the lime binder in mortar can be used to date archaeological structures, since the carbon dioxide absorbed during the setting of the mortar probably reflects the14C content existing in the atmosphere at that time. The main sources of carbon dioxide, that potentially contribute to a biasing in the final measure, are: the residues of primary carbonates (calcination relics) originating from the incomplete limestone combustion during the process of producing lime; aggregates containing carbonates used as inert materials during the production phases of the mortar; and newly formed carbonates precipitated after the interaction between running water or rain. Therefore, the preparation of the samples must eliminate the contamination (aggregate, calcination relics or crystallization of new calcite) which must be separated from the carbon belonging to the original binder. The methods, now more commonly used for the preparation of mortar samples, consist of a mechanical pre-treatment or a chemical treatment. This paper illustrates the so-called Cryo2Sonic procedure, and how it has been improved over time (previously it was “CryoSonic”), and developed with the use of other complementary techniques. Moreover, various case studies will be presented in which this radiocarbon dating protocol has been applied to mortars of different structures belonging to different phases and historical contexts. The results have not always been good, as there are many sources of contamination from the different origins of CO2. Our goal is to eliminate contamination as much as possible, or at least try to limit the effect of CO2 contamination