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Dating lime mortar samples using the radiocarbon (14C) method can be difficult. This is because the contamination is similar to the primary dating material (CaCO3) and consequently difficult to remove. Mortar can also have late-in-formation pyrogenic carbonate from interactions with the environment after the initial hardening phase, such as recrystallization, fire damage or delayed hardening. When 14C dating a system of primary dating material, contamination and late-in-formation pyrogenic carbonate, one approach is multi-fraction dating with conclusiveness criteria. If a sample has sufficient contamination or late-in-formation pyrogenic carbonate, the criteria evaluate the result inconclusive. To improve inconclusive results from such samples, this study investigates sample preparation by thermal decomposition. Here samples that were inconclusively dated by the authors’ traditional method, sequential dissolution with 85% phosphoric acid, are investigated further. This study finds that CO2 thermally decomposed at low temperatures contains some late-in-formation pyrogenic carbonate. By rejecting CO2 decomposed at low temperatures, Kastelholm castle and Kimito church in Finland are conclusively and accurately dated. Furthermore, a preheating method removes some late-in-formation carbonate, but not enough for a conclusive result. Finally, thermal decomposition finds difficulty in discerning binder carbonate from limestone and marble contamination.
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.
Lime lumps and bulk mortars show different 14C contamination when analyzed in several CO2 fractions isolated from the effervescence of an ongoing hydrolysis reaction. Age profiles of both materials are therefore highly complementary and together they can provide a reliable date. Furthermore, they can also reveal the complexity of the radiocarbon (14C) distribution within the mortar and thus prevent over-interpretation of the data. The lime lump versus bulk mortar dating data presented here has been collected over 22 years, with only a small fraction of the results so far published internationally. Since there has been an increasing interest in mortar dating over recent years with a special focus on lime lumps, and since many laboratories have just begun mortar dating experiments, we wish to present some of the extensive data that already exist. Previously published data from 15 lime lumps (including 34 14C measurements from sequential dissolution) and 43 new 14C measurements from 17 lime lumps are presented here. The samples are from medieval Finland and Sweden, classical Rome and medieval Italy, and the Roman Jerash (Gerasa), Jordan.
Absolute dating of mortars is crucial when trying to pin down construction phases of archaeological sites and historic stone buildings to a certain point in time or to confirm, but possibly also challenge, existing chronologies. To evaluate various sample preparation methods for radiocarbon (14C) dating of mortars as well as to compare different dating methods, i.e. 14C and optically stimulated luminescence (OSL), a mortar dating intercomparison study (MODIS) was set up, exploring existing limits and needs for further research. Four mortar samples were selected and distributed among the participating laboratories: one of which was expected not to present any problem related to the sample preparation methodologies for anthropogenic lime extraction, whereas all others addressed specific known sample preparation issues. Data obtained from the various mortar dating approaches are evaluated relative to the historical framework of the mortar samples and any deviation observed is contextualized to the composition and specific mineralogy of the sampled material.
Seven radiocarbon laboratories: Åbo/Aarhus, CIRCE, CIRCe, ETHZ, Poznań, RICH, and Milano-Bicocca performed separation of carbonaceous fractions suitable for 14C dating of four mortar samples selected for the MOrtar Dating Inter-comparison Study (MODIS). In addition, optically stimulated luminescence (OSL) analyses were completed by Milano-Bicocca and IRAMAT-CRP2A Bordeaux. Each laboratory performed separation according to laboratory protocol. Results of this first intercomparison show that even though consistent 14C ages were obtained by different laboratories, two mortars yielded ages different than expected from the archaeological context.
Since 1994, our team has gained extensive experience applying accelerator mass spectrometry (AMS) radiocarbon analysis for mortar dating, totaling over 465 samples and 1800+ measured CO2 fractions. Several samples have been analyzed repeatedly. The research covers both Medieval and Classical archaeology. We therefore believe our experience can be helpful when developing preparation procedures for different kinds of mortars in different areas and in varying chronologies. So far, the main areas of interest have been (a) the churches of the Åland Islands (in the archipelago between Finland and Sweden); (b) the churches in the Åboland Archipelago (SW Finland); (c) sites in the Iberian Peninsula including Torre de Palma (a Roman village in Portugal); and (d) Rome, Pompeii, and Herculaneum (Italy). Most of the analyses before 2000 were hydrolized in only two CO2 fractions per sample, and reliability criteria were defined on the basis of how well the ages of the two fractions agree with each other. These criteria have proved most helpful in determining the reliability of 14C mortar analyses. Different types of mortar have been investigated, including lime mortars made both from limestone and marble, pozzolana mortars, fire-damaged mortars, and mortars based on burnt shells. Most importantly, separate lime lumps sampled from these mortars have been analyzed sporadically and recently more systematically. The research also includes different types of hydrolysis applied in the pretreatment. In addition to using 85% phosphoric acid (H3PO4), the experimental research includes tests with smaller concentrations of phosphoric acid, and tests based on 2–3% hydrochloric acid (HCl) dissolutions. To characterize the dissolution process, results are presented as age profiles of 2–5 CO2 fractions. In our experience, pozzolana mortars have been difficult to date, and HCl dissolution should be used only in special cases and in complementary tests.
AMS-bascd radiocarbon dating was applied to Medieval lime mortars made from burned shells and aggregate including both shore sediments and neovolcanic rocks. Three mortar samples from the city of Leiden near Amsterdam were prepared using the same kind of acid hydrolysis technique as has been earlier used for dating mortars made from burned marble and limestone. Five consecutive CO2 fractions were collected from each sample to form age profiles as functions of the dissolution progress index. One of the samples, from a brick wall of known age, was taken as a reference from the Pieterskerk church. Two other samples were taken from the Burcht circular stronghold on a former island on the Rhine River. The age of Burcht is less well known; thus, the presented results are a contribution to an ongoing discussion on its history.
This study focuses on radiocarbon dating of mortars that have withstood city fires and display visible fire damage effects. Some fire-damaged and undamaged original Medieval mortars from the same site have also been tested. The mortars were heated at different temperatures and then analyzed using the same preparation procedures as in 14C dating of mortars to see what kind of changes the heating would introduce to the mineralogy, chemistry, and the carbon and oxygen isotope ratios. We found that decarbonation during heating starts at ∼600 ° and recarbonation starts as soon as the temperature drops. Already after a few days, most of the lost CO2 has been replaced with atmospheric CO2. The renewed carbonates are readily soluble in the acid hydrolysis process and their carbon and oxygen isotopes have a light signature. Fire-damaged historical mortars display the same features. If a long time has elapsed between hardening of the original mortar and the fire, the new carbonates have 14C concentrations that point to the fire event rather than to the building event. In several cases, the fire-damaged mortars have an easily soluble carbonate fraction with a 14C age that could be related to a major fire event, but still most of the soluble carbonate yields a 14C age that seems like a reasonable age for the original construction.
Fifteen years of research on accelerator mass spectrometry (AMS) radiocarbon dating of non-hydraulic mortar has now led to the establishment of a chronology for the medieval stone churches of the Åland Islands (Finland), where no contemporary written records could shed light on the first building phases. In contrast to other material for dating, well-preserved mortar is abundantly available from every building stage.
We have gathered experience from AMS dating of 150 Åland mortar samples. Approximately half of them have age control from dendrochronology or from 14C analysis of wooden fragments in direct contact with the mortar. Of the samples with age control, 95% of the results agree with the age of the wood. The age control from dendrochronology, petrologic microscopy, chemical testing of the mortars, and mathematical modeling of their behavior during dissolution in acid have helped us to define criteria of reliability to interpret the 14C results when mortar dating is the only possibility to constrain the buildings in time. With these criteria, 80% of all samples reached conclusive results, and we have thus far been able to establish the chronology of 12 out of the 14 churches and chapels, while 2 still require complementary analyses.
Non-hydraulic mortars contain datable binder carbonate with a direct relation to the time when it was used in a building, but they also contain contaminants that disturb radiocarbon dating attempts. The most relevant contaminants either have a geological provenance and age or they can be related to delayed carbonate formation or devitrification and recrystallization of the mortar. We studied the mortars using cathodoluminescence (CL), mass spectrometry (MS), and accelerator mass spectrometry (AMS) in order to identify, characterize, and date different generations of carbonates. The parameters—dissolution rate, 13C/12C and 18O/16O ratios, and 14C age—were measured or calculated from experiments where the mortars were dissolved in phosphoric acid and each successive CO2 increment was collected, analyzed, and dated. Consequently, mortar dating comprises a CL characterization of the sample and a CO2 evolution pressure curve, a 14C age, and stable isotope profiles from at least 5 successive dissolution increments representing nearly total dissolution. The data is used for modeling the interfering effects of the different carbonates on the binder carbonate age. The models help us to interpret the 14C age profiles and identify CO2 increments that are as uncontaminated as possible. The dating method was implemented on medieval and younger mortars from churches in the Åland Archipelago between Finland and Sweden. The results are used to develop the method for a more general and international use.
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