Multi-analytical procedure for radiocarbon dating of Pozzolana mortars from public buildings in Pompeii

Abstract

Roman mortars were typically composed of an air-hardening lime mixed with materials exhibiting pozzolanic behaviour (e.g., pozzolana or cocciopesto), which conferred hydraulic properties to the material. So far, many characterizations of pozzolanic mortars have been carried out, revealing their complexity. The feasibility of radiocarbon dating for such mortars remains debated, with published data even suggesting that it may not be a viable method. There is currently no comprehensive study establishing a correlation between analytical characterization results and the causes of unreliable radiocarbon dating.

We have recently proposed a characterization procedure which is directly applied to the sample to be dated to allow identifying the origin of calcite. In this study, the same procedure is applied to pozzolana mortars. Samples were selected from public monuments in the Forum of Pompeii, including the Eumachia Building, the Temple of Genius Augusti, the Tabularium and the Temple of Apollo. Following the procedure, powders from binders or lumps were analyzed using XRPD, OM-CL, ATR-FTIR, micro-Raman, SEM-EDS techniques to identify the origin of calcite.

The CO ₂ from selected inorganic fractions of the mortar samples was extracted by dissolution with H ₃ PO ₄ and then converted into graphite for subsequent measurements by accelerator mass spectrometry (AMS) using the so-called Lilliput graphitization line at INFN-LABEC in Florence.

For five out of characterized fifty-three samples, the mortar shows mineralogical, petrographic, and geochemical characteristics which are suitable for dating. Two samples indeed yield ages consistent with the archaeological phases. The remaining samples appeared older than expected, highlighting the critical role of the nature of the material and its influence on dating results.