Historic Lime Mortars Composition and Terminology for Radiocarbon Dating—case Studies Based on Thin-Section Petrography and Cathodoluminescence

Abstract

Since the first developments of anthropogenic lime materials radiocarbon (14C) dating in the 1960s, numerous studies have been undertaken and developed to investigate the topic further. Historic mortars are complex composite and open system materials that can incorporate a large range of components. Due to the complexity of the historic lime mortars composition, they are not part of a routine protocol in most radiocarbon laboratories and reliable dating is not always achieved. A thorough characterization needs to be performed and different preparation methods can be considered as a function of their compositions. A vast range of terms are employed to qualify the lime mortars components and alterations that can possibly have an influence on the dating result. Here, a detailed description of these components and the various terms used is listed. To illustrate this, images obtained by thin-section petrography and cathodoluminescence are presented in addition to radiocarbon results using stepwise acid hydrolysis on Belgian mortars having different provenance, state, age and composition. Depending on the type of aggregate used, the type of binder and its conservation state, the eventual presence of weathering carbonates and the assumed speed of the carbonation process, the reliability of radiocarbon measurements using the stepwise acid hydrolysis technique is discussed and confronted with presumed historical constraints.