Selecting the Most Reliable ¹⁴C Dating Material Inside Mortars: the Origin of the Padua Cathedral

Abstract

In order to radiocarbon (14C) date a building, several components of the mortar could be used, such as the mortar binder, the lime lumps, the charcoal particles and shell fragments eventually present among the aggregates. In particular, the mortar binder requires a purification treatment in order to separate it from other sources of carbon, which could change the 14C signature of the binder invalidating the dating process. Here, we present the application of the Cryo2Sonic method to 14C dating of the ancient building structures unearthed during excavation at the Padua Cathedral complex. The dated samples were pretreated by using Cryo2Sonic method and the improved Cryo2Sonic version 2.0, recently developed by introducing additional steps such as centrifugation of the mortar suspension and gravimetric sedimentation of the binder fractions. The Cryo2Sonic version 2.0 relies heavily on the characterization of the mortar and of the purified binder fractions, allowing the isolation of a reliable 14C datable mortar fraction. Through this new method, the 14C dating of different ancient structures excavated next to the Padua Cathedral allow to identify the first religious complex of the city of Padua (3rd–4th centuries AD).

The Cannero Castle (Italy): Development of Radiocarbon Dating Methodologies in the Framework of the Layered Double Hydroxide Mortars

“…30 g of manually disaggregated material for each sample were subjected to the separation protocol in order to extract the SGs from the other mortar components based on their grain sizes. The procedure consists in a sonication and wet gravimetric sedimentation in ultrapure decarbonated water for 24 hr, centrifugation and filtration of the fine fraction (Nonni et al 2017;Addis et al 2019). The obtained SGs were analyzed by XRPD in order to evaluate the presence of contaminants for the radiocarbon dating.…”

“…The complexity of ancient mortar systems requires the development of effective methods both for the characterization and the separation of the binder fractions for mortar dating. In this framework, sample preparation procedures have been implemented over time in order to isolate the pure carbonate binder fraction removing any other contaminant, but the issue is still not completely resolved (Sonninen and Jungner 2001;Nawrocka et al 2005;Lindroos et al 2007;Marzaioli et al 2014;Addis et al 2019).…”

“…The characterization is crucial: it provides useful data that helps to determine whether the sample is suitable for dating and which criteria are needed for a more efficient separation of the carbonate fraction of interest. Moreover, in order to separate the binder from other contaminant sources, a multistep purification protocol based on size selection by wet sieving has been developed (Addis et al 2019). Nevertheless, if the mortar has been affected by hydraulic reaction processes, LDH phases may be present in the isolated binder fraction prepared for the dating process.…”

Luminescence reveals variations in local structural order of calcium carbonate polymorphs formed by different mechanisms

“…Recent advances in this field have shown that the integrity of pyrogenic CaCO 3 crystals formed during plaster/mortar carbonation, i.e. the time of the reaction between Ca(OH) 2 and atmospheric CO 2 , is crucial for the preservation of the original 14 C signature necessary to obtain accurate age determinations 52,57,58 . Thus, SEM-CL and LIF can help discard unsuitable samples.…”

The State-of-the-Art of Dating Techniques Applied to Ancient Mortars and Binders: A Review

“…The joint Padova-Caserta research group also advocates a further checking of the purity of the separated binder fraction between steps 3 and 4 in order to control the material that is actually bound to be dated (see Figure 2 of Addis et al 2019). The dateable fraction of the carbonate binder may be tentatively defined as "the separated fine fraction of anthropogenic carbonate that has been experimentally screened for the absence of secondary carbonate, geogenic carbonate, and spurious CO 2 -containing phases".…”