The most recent workshop on mortar dating (25–27 Oct. 2018, Bordeaux, Montaigne University, France), which closely followed the publication of an extensive round robin-exercise involving several laboratories, was an opportunity to review the history and challenges of mortar dating methods and procedures currently in use. This review stems from the keynote lectures presented at the meeting, and wishes to summarize recent results, present trends, and future challenges. Three major areas are brought into focus (1) radiocarbon (14C) dating of complex mortars: can we assess the chances of successful dating?, (2) 14C dating of archaeological carbonate materials: difficulties, new directions and applications, and (3) single grain optically stimulated luminescence (OSL) dating of mortars in architectural archaeology: the current state of the art. This paper reflects the material presented by the authors and discussed at the workshop.

Hydraulic lime binders are considered a technological marvel which revolutionized construction techniques in antiquity. The core material is made of a binder that is a mixture of calcite and hydraulic phases, which are amorphous silicate compounds that nanostructurally polymerize into insoluble phases that harden even underwater, formed during the reaction between lime and reactive silicates such as volcanic ash. This is also what makes hydraulic lime so hard to radiocarbon (14C) date. These insoluble phases contain carbonates that may set centuries following their application, resulting in younger ages, which may contaminate the calcite fraction that is favorable for 14C dating. This calcite fraction forms upon the incorporation of atmospheric carbon dioxide during the setting of the hydrated lime. Therefore, different characterization methods are being constantly developed for identifying and characterizing the components of hydraulic lime-binders. In this work, we present a rapid characterization technique based on Fourier transform infrared spectroscopy (FTIR) that characterizes the atomic disorder and chemical environment of the carbonates and silicates fractions in the binder. The atomic disorder of the calcite crystallites was determined by the ν2 and ν4 vibrational modes, and the silicates were characterized by the main peak asymmetry and full width at half maximum (FWHM). Different hydraulic binders from Caesarea Maritima were examined, including Herodian mortars from the underwater breakwater and on-land plasters and mortars from the portʼs warehouse and vaults. Hydraulic binders, in which the calcite fraction in the binder shows atomic disorder that is comparable to modern plaster binders, was associated with silicates that have asymmetry and FWHM of clays and quartz. These materials are considered to be in good preservation state for 14C dating since their carbonates crystallites are disordered and did not interact with the environment chemically to form stable and ordered crystals. Interestingly, the atomic disorder of binders that underwent chemical alterations and recrystallization processes, are associated with reactive silicates aggregates such as volcanic ash (pozzolana). These results suggest a new way to prescreen materials for radiocarbon dating based on the composition of lime-binders and preservation state of the carbonate fraction and hydraulic products.

Accurate radiocarbon (14C) dating of lime mortars requires a thorough mineralogical characterization of binders in order to verify the presence of carbon-bearing contaminants. In the last 20 years, cathodoluminescence (CL) has been widely used for the identification of geologic calcium carbonate (CaCO3) aggregates and unreacted lime lumps within the particle size fraction selected for carbon recovery. These components are major sources of older and younger carbon, respectively, and should be removed to obtain accurate age determinations. More recently, laser-induced fluorescence (LIF) has provided another means of investigating the preservation state and composition of CaCO3 binders. Considered the growing interest of the mortar dating community in the latest advancements of these analytical methods, here we review the principles of CL and LIF of CaCO3, their instrument setup, and their application to the characterization of ancient lime mortars used for 14C dating. In addition, we provide examples of SEM-CL and LIF analyses using high-resolution instrumentation, we discuss current issues and propose future lines of research.

When sampling mortars for radiocarbon (14C) dating it is crucial to ensure that the sample has hardened rapidly relative the resolution of the dating method. Soft and porous lime mortars usually fulfill this criterion if the samples are taken from an uncovered surface from less than a few centimeters deep. However, hard, concrete-like mortars may be impermeable for carbon dioxide and even the outermost centimeters may still contain uncarbonated calcium hydroxide. These mortars may harden very slowly and contain carbonate that formed centuries or even millennia after the original building phase, and they can still be alkaline and capture modern 14C, causing younger 14C ages than the actual construction age. Another problem is reactivation of the binder carbonate if it has been partly decarbonated during a fire later on in its history. It will be shown that these young carbonates dissolve rapidly in phosphoric acid and in many cases a reasonable 14C age can be read from 14C profiles in sequential dissolution if the measurements from initially formed carbon dioxide are disregarded. However, if a mortar was made waterproof deliberately by adding crushed or ground tile, as in Roman cocciopesto mortars, it may be very difficult to get a conclusive dating.

Carbonaceous mortars from Novae (Bulgaria) contain local loess, crushed bricks and ceramic dust (pozzolanic materials). The reaction between lime and pozzolanic additives occurs easily and affects the rate and course of leaching reaction of carbonates in orthophosphoric acid during the sample pretreatment for dating. The composition of the Bulgarian mortars does not allow for unambiguous conclusions about chronology, but together with the observations of experimental mortars, gives new guidelines in terms of pozzolanic mortar application for dating. The presented research illustrates the possible reasons of difficulties with obtaining the appropriate portion of gas for radiocarbon (14C) measurement. To verify the relative chronology of legionary baths complex in Novae, the charcoals samples were also dated in addition to the mortar.

The ages of mortars and plaster can help reveal the history of monuments, their construction, or restoration times. However, these anthropogenic carbonates pose a challenge when it comes to separation of the atmospheric radiocarbon (14C) signal of the CO2 fixed in the mortar at the time of consolidation, i.e., the time of binder formation. The variety and heterogeneity of mortars require individual assessments of each sample and 14C results. Here we present our current preparation method and summarize experience based on results collected during the last 20 years of mortar dating at the ETH laboratory.

The monastery of St. John in Müstair, a UNESCO world heritage site, preserves archaeological remains and stone structures dated from the 8th century to the present. It has been extensively studied archaeologically so that numerous samples of historical materials, including mortar, are available for study. In addition to that, some of the structures have been precisely dated with dendrochronology. The monastery is located in a region characterized by dolomite rocks and the mortars are therefore of dolomitic nature, being perfectly suited to test the possibility of being dated with 14C. Furthermore, the presence of embedded carbon fragments has provided additional independent data to support or deny the results of mortar dating. A comparison of the results obtained from radiocarbon (14C) dating of bulk mortars, sieved fractions enriched in binder, lime lumps and carbon fragments, for two samples is presented, in relation to the petrographic characterization and the mineralogical phase content. This preliminary study shows that the dating of 14C can potentially be applied to the mortar of Müstair, as results in accordance with the established chronologies have been obtained for one sample. However, if the dolomitic sand contamination is very high, further studies are needed to develop a specific sample preparation technique.

The mortar samples of the Castle of Cannero (Lake Maggiore, Italy) have been characterized and radiocarbon (14C) dated. The presence of LDH phases was identified. The hydraulic reaction was evaluated by a multi-analytical approach. Careful extraction, preparation and purification of the binder fraction have been performed. Contaminations due to LDH phases have been removed allowing reliable absolute dating of the structures.

Non-hydraulic lime-based mortars represent only part of the binding materials found in archaeological and historical structures, and a new challenge is the application of 14C dating techniques on mortars that feature hydraulic reactions. This research work aims at 14C dating a series of Mg-rich hydraulic mortars from the Castle of Cannero (Lake Maggiore, Italy), from which both charcoals and mortar samples were collected. A multi-analytical approach employing X-ray powder diffraction (XRPD), optical microscopy (OM), and scanning electron microscopy/energy-dispersive microanalysis (SEM-EDS) was adopted in order to carefully characterize the samples. A wet gravimetric separation for the extraction of the fine fraction mainly composed by the mortar binder was carried out and the binder fraction was characterized by XRPD in order to investigate the presence of contaminants. The binding fractions are characterized by the widespread occurrence of hydrotalcite-type minerals, considered contaminants in 14C dating of mortars because of their capability to exchange carbonate anions even after the hardening process. A further purification treatment by thermal decomposition was performed before 14C dating by AMS. The obtained dates were consistent with archaeological expectations, confirming the potential of the developed purification methodology for hydraulic mortars dating.

Radiocarbon (14C) dating of anthropogenic carbonates (CaCO3) such as ash, lime plaster and lime mortar, has proven a difficult task due to the occurrence of a number of contaminants embedded within the CaCO3 pyrogenic binder. These include 14C-free geologic components and/or secondary phases bearing an unknown amount of 14C, and thus the alteration of the original pyrogenic isotopic signature of the material results in major age offsets when carbon recovery is performed through acid hydrolysis. Here we present a characterization/quantification approach to anthropogenic carbonates that includes Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, thin section petrography, thermogravimetric analysis and scanning electron microscopy coupled with high-resolution cathodoluminescence, with which we identified the pyrogenic CaCO3 fraction in an aerial lime plaster and two hydraulic mortars. The preserved pyrogenic component was then isolated by density separation and its purity checked again using FTIR. Carbon was recovered through thermal decomposition in vacuum. The resulting 14C age matches the expected age of the lime plaster, whereas hydraulic mortars are slightly offset due to the carbonation of calcium hydroxide lumps. This approach highlights the importance of a dedicated characterization strategy prior to dating and may be applied to aerial lime plasters to obtain accurate ages.

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.

After the last damaging earthquake in 2012, an anti-seismic reinforcement project of the cathedral of Modena was designed giving us the opportunity to investigate and date the building materials. Radiocarbon (14C), optically stimulated luminescence (OSL), and thermoluminescence (TL) dating techniques were performed on the vaults with the aim to (1) clarify the construction timing, (2) define the history of the restorations, and (3) explore the possible correlation of the main restoration works to the earthquake chronology deduced from the historic catalog. Preliminary results show that medieval older bricks were reused for most of the original construction. Only lime and non-gypsum mortar was used for the original construction in the 15th century and for later repair of damage caused by earthquakes in the 16th and 17th centuries. Gypsum mortar was used for later repair in the 18th century. The results show much stronger damage due to earthquakes than previously thought.

There are few papers that focus on optically stimulated luminescence (OSL) dating of earthen mortars. These mortars are abundant in historical buildings in northwestern Spain. The Basílica da AscensiónyForno da Santa building is an unfinished church built on a previous structure that was transformed into a crypt (Allariz, Ourense, NW Spain). Previous archaeological studies established a sequence of phases of construction, the first dating back to the Iron Age, with significant changes occurring in the Early and Late Medieval ages. The only datable material in the crypt is earthen mortar. Thus, eight mortar samples (seven joint mortars and one wall infill) were taken, seven of them dated by OSL. The dose rate was assessed, and the expected equivalent doses estimated based on the established archaeological age. Several grain sizes (from fine to coarse) were used in small multigrain aliquots to assess the equivalent doses and ages. No evidence of partial bleaching was observed in most samples and grain sizes. The resulting ages are younger than expected for most samples. This is explained by the fact that joints were repaired with new mortar from the 16th century onwards.

Dating lime mortar shows great potential for establishing the chronology of a construction. The basic premise of mortar dating by optically stimulated luminescence (OSL) is that quartz in the sand used for making mortar has been optically zeroed during the preparation process (optical bleaching). The moment to be dated is the last exposure of sand grains to light, before being embedded within the masonry and hidden from light. However, the main problem is the frequent partial and heterogeneous bleaching of grains, and this led us to use the single grain technique (SG-OSL) systematically. Some theoretical and experimental aspects of a new statistical treatment (the EED model, as exponential exposure distribution) are detailed and discussed. Our experience shows that SG-OSL dating of mortars is successful in a majority of situations. In a minority of cases (around 15%) difficulties originate when there is inappropriate OSL behavior of grains, and thus OSL dating is not possible. In the other cases, good agreement was obtained between OSL ages and the reference ones for a series of samples from a variety of ages and situations, even in the case of poorly bleached material. Anyway, the present situation of OSL dating methodology justifies the systematic use of SG-OSL in the dating of masonry today.

Electron paramagnetic resonance (EPR) spectroscopy is a well-established method of dating based on trapped charges, applied to various crystalline materials, including carbonates, bones, and teeth. It provides a detailed insight into the structure of radiation defects—paramagnetic centers generated by irradiation, without the need of a painstaking sample preparation, often challenging in other methods. Using EPR we studied the effect of γ radiation on lime mortars and plasters from ancient settlement Hippos in Israel, in order to analyze the process of defect generation. Analysis of the complex spectra revealed the presence of radiation-induced species, including CO2–, NO32– and organic radical. Using an artificial UV source, we generated relatively strong signals of paramagnetic centers, analogous to those created by γ irradiation, reaching their maximum intensity after 5–6 hr of UV exposure. Our results confirm the previous reports that radiation defects can also be generated, instead of bleached, in calcite by UV radiation, which is crucial for identifying the issues related to light exposition, affecting the accuracy of age determinations in trapped-charge dating methods.