Integrated multi-analytical screening approach for reliable radiocarbon dating of ancient mortars

Abstract

Radiocarbon dating of the carbonate binder of historical mortars is a strategic research topic not lacking in complexities. The critical step is the separation of anthropogenic CaCO3-binder from other carbonate sources that could severely affect the resulting dates. Here we present a complete procedure for the processing and characterization of difficult mortars and of the separated binder fractions in order to assess a priori the chances of positively dating the mortar, and produce a binder fraction yielding the most reliable radiocarbon dates possible. Two complex architectural case studies from Northern Italy are presented and discussed in detail: the churches of Santa Maria Maggiore (Lomello, Pavia) and Santa Maria (Torba, Varese). The results support that both the reliability assessment and the successful radiocarbon dating are possible through a multi-analytical approach encompassing mineralogical and petrographic characterization, X-ray powder diffraction, scanning electron microscopy, measurement of carbon and oxygen stable isotopes, and optical cathodoluminescence.

Characterization and Selection of Mortar Samples for Radiocarbon Dating in the Framework of the Modis2 Intercomparison: Two Compared Procedures

“…Stages of alteration/degradation of the silicate and dolomite aggregates are observed, probably due to the establishment of hyperalkaline condition (Weber et al 2015). The presence of Mg in the system promotes the precipitation of aragonite, as detected by XRPD, and the formation of magnesium silicate hydrate (M-S-H) phases Ricci et al 2022) (Supplementary material, Fig. S3).…”

“…In order to separate the binder fraction from other contaminant sources, as carbonate aggregates, a multistep purification protocol based on size selection by wet sieving has been developed. (Marzaioli et al 2008;Terrasi et al 2008;Nonni et al 2013;Addis et al 2019;Ricci et al 2019Ricci et al , 2020Ricci et al , 2022. MI-FI approach aimed at testing an ATR-FTIR characterization of the bulk mortar, based on the work of Regev et al (2010), in order to provide a first discriminant technique to select the most suitable material to be dated.…”

Characterization of Historic Lime Mortars from the Arch of San Martin to Identify the Construction Phases of the City Wall of Burgos (Spain)

“…Geological carbonates and the newly formed carbonated mineral phases during the manufacture of mortars are the most common contaminants, leading to overestimating radiocarbon dates. Secondary carbonates formed during alteration processes also lead to underestimation of the radiocarbon ages [31,36,37]. To avoid contaminants, several techniques are proposed to separate the pure binder fraction, including mechanical [30,31,34,38] and chemical separation methods [29,[39][40][41].…”