Radiocarbon dating of lime mortars, towards a comprehensive approach to separate natural from anthropogenic lime carbonates

Abstract eng:
Dating of mortars by means of radiocarbon dating is thought of being a promising technique for absolute dating of archaeological sites, monumental constructions and decorative finishes. Whenever samples can be collected near to the original external surface, the determination of the 14C age of the atmospheric carbon dioxide bound within the mortar the moment it hardened will reflect its age. Radiocarbon dating of mortars has known some successes over the past decades, certainly where it concerned pure lime putty mortars. Hydraulic lime mortars or lime mortars containing contaminants from a geological provenance, such as (roughly or finely) ground limestone or underburned limestone fragments, or contaminants related to delayed carbonate formation often still prove difficult to date. Radiocarbon dating of mortars generally comprises the acid dissolution of a powdered sample to obtain the 14C ages and stable isotope profiles of successive CO2 increments, which are used to model the interfering effects of the different carbonate sources. As a clear separation of the various carbonates is not necessarily possible, interpretation and stability of the results is not always straightforward. The standard separation technique by chemical reaction is based on tiny differences in bonding energy due to different crystallographic properties of the carbonates. Besides chemical attack, calcination of the lime carbonates is an alternative to release the carbon dioxide. Thermogravimetric analysis is applied to study the release of CO2 of putty lime and different types of limestone by means of step heating as an alternative preparation method for mortar dating.

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