This commentary aims at raising awareness and fostering a discussion on the need of a new approach to the radiocarbon (14C) dating of historic mortars. Over the last decades, important advancements have been made in the application of the 14C dating methods to lime mortar samples, including the use of lime lumps instead of generic pieces of mortar. However, a relevant number of results in disagreement with the chronological framework of the related archaeological cases are published every year without a clear understanding of the reasons for such results. This suggests that further developments to the methodology are needed. The commentary argues that to further develop this particular application of the 14C dating method, a new, more holistic approach is needed that moves away from the very “applied” approach that dominated the last decades and focuses more on the causes of contamination and the mechanism of the reactions involved. Two actions are suggested that can immediately improve our ability to critically assess the results obtained: the publication of a chemical and mineralogical characterization of the binding fraction for the dated mortars, and the publication of sampling depth for each dated sample.

Dating organic inclusions in mortars such as charcoals is a useful alternative or complementary method to dating mortars themselves, helping to estimate the building age. To assess the limitations of this dating approach, organic inclusions were searched for in surface mortar layers of six early to late medieval buildings in the Czech Republic with relatively well-known age. Altogether, 123 samples were found. About 80% were successfully radiocarbon (14C) dated. However, only 66% originated from wood relatively young when used in lime burning. To judge which samples are relevant to the actual building date, sufficient statistics is crucial. We recommend dating at least 5–10 samples, i.e., collecting 6–12 samples, for a site with uncomplicated building history, or per building phase. Otherwise, unrealistically old or young dates might be obtained. With the recommended statistics, inclusion-based dating provides building ages with uncertainty of 50–100 years.

Rýzmburk Castle is one of the largest and most important medieval castles in Bohemia, documented since 1250 AD. Its North tower is assumed to be built in 1260–1300 AD. To test this assumption, the surface layers of mortar were inspected for the presence of charcoals suitable for radiocarbon dating, and 10 charcoals were found. The charcoals probably originated from wood used for lime burning. The results of radiocarbon dating using accelerator mass spectrometry agree with the historical estimation. Single post-1287 sample indicates that the building date might be refined to 1287–1300 AD.

This work shows the results of optically stimulated luminescence (OSL) and radiocarbon (14C) dating applied to mortars of historical structures in northern Italy. All the results are compared with archaeological evidence and thermoluminescence (TL) dating of bricks. The main issue for OSL mortar dating is that the quartz grains contained in the mortar may be only partially bleached, leading to an overestimation of the sample age. In order to identify the best protocol to apply, both multi-grain (MG) and single grain (SG) methods were used. The minimum age model (MAM) statistical approach was applied to refine their accuracy. However, the identification of the bleached grains is not always successful, indicating that further investigations are needed to develop suitable dating protocol. For the 14C technique, a crucial aspect is the selection of anthropogenic calcite. In this work the mortars were treated using a Cryosonic method to select anthropogenic calcite from raw material, and the obtained powder was sieved to select the finer fraction. Unfortunately, only in two cases an acceptable amount of sample could be obtained. All the fractions were dated via accelerator mass spectrometry (AMS), and the results compared with independently obtained dates. The results show that the execution of the dating analysis requires previous characterizations to assess the nature of the mortar components and avoid unusable fractions.

During the study of some decay processes in marble covering the façade of the Florence Cathedral (1870-1887), vaterite was found as the principal component of some bonding mortars of inlay decorations.

Vaterite is one of the three polymorphous phases of CaCO3 and crystallizes in the hexagonal system, dihexagonal bipyramidal class (Kamhi, 1963; Sato and Matsuda, 1969): it is unstable under normal environmental conditions (Deer et al., 1964).