INTRODUCTION
Mortar dating is of interest for many archeological and art historian studies. Radiocarbon dating of mortar binder is especially interesting, however prone to many obstacles. Though the idea of 14C dating mortar binder appeared almost at the dawn of the radiocarbon method (Labeyrie and Delibrias Reference Labeyrie and Delibrias1964; Stuiver and Smith Reference Stuiver and Smith1965; Baxter and Walton Reference Baxter and Walton1970), only since the development of the accelerator mass spectrometry this possibility was researched more extensively (Van Strydonck et al. Reference Van Strydonck, Van Der Borg, De Jong and Keppens1992; Heinemeier et al. Reference Heinemeier, Jungner, Lindroos, Ringbom, von Konow and Rud1997).
As mortar is heterogeneous and inconsistent matrix, there are different approaches to date, so the method for radiocarbon mortar dating needs systematization and verification. Therefore, the first Mortar Dating Intercomparison Study (MODIS) was conducted (Hayen et al. Reference Hayen, Van Strydonck, Boaretto, Lindroos, Heinemeier, Ringbom, Hueglin, Michalska, Hajdas and Marzaoili2016, Reference Hayen, Van Strydonck, Fontaine, Boudin, Lindroos, Heinemeier, Ringbom, Michalska, Hajdas and Hueglin2017; Hajdas et al. Reference Hajdas, Lindroos, Heinemeier, Ringbom, Marzaioli, Terrasi, Passariello, Capano, Artioli and Addis2017; Michalska et al. Reference Michalska, Czernik and Goslar2017). The intercomparison showed high agreement of results among the laboratories for non-hydraulic mortars and the general conclusion was that the dating success depended on the mortar quality.
The second mortar dating intercomparison MODIS2 was conducted in 2020. Three non-hydraulic mortar samples were distributed as a 1-cm3-piece and as <150 µm grain-size fraction. The first sample (MODIS2.1) was from AD 14th century tower of the church of Saltvik on the Åland Islands, Finland, in the central part of the Baltic Sea. The church has a dendrochronological date of AD 1381 (date also provided by the organizers), which was confirmed by mortar 14C dates (Heinemeier et al. Reference Heinemeier, Ringbom, Lindroos and Sveinbjörnsdóttir2010; Ringbom Reference Ringbom2011a). The second sample (MODIS2.2) was collected from the attic of the chancel of the church of Hamra located on the southernmost tip of the Swedish island of Gotland. The date of the mortar should be after 1300 AD, when the church was rebuilt (Roosval Reference Roosval1911) but no later than AD 1361, since the island suffered from depression after the plague during the 1350s and the invasion by the Danish king Valdemar Atterdag in 1361. The mortar from the church was also previously dated to cal AD 1310–1360 (Ranta et al. Reference Ranta, Hansson, Lindroos, Ringbom, Heinemeier, Brock and Hodgins2009). The third sample (MODIS2.3) is from the early Christian Basilica of Santa Eulalia in Mérida, western Spain. The sample is from the basement in the inner corner of the north/northwest wall of the shrine erected in honor of Santa Eulalia that died in AD 304. Since on top of it a basilica was built in her honor in AD 570 (Mateos Cruz Reference Mateos Cruz1999; Pereira Reference Pereira2015) the sample should be dated to AD 304–570. The main focus on this intercomparison was to evaluate reproducibility of the mortar radiocarbon dating laboratories.
The Zagreb Radiocarbon Laboratory approach to mortar dating was to select and analyze lime lumps (Sironić et al. Reference Sironić, Borković, Barešić, Krajcar Bronić, Cherkinsky, Kitanovska, Štrukil and Čukovska2019). Although often reliable (Lindroos et al. Reference Lindroos, Ringbom, Heinemeier, Hodgins, Sonck-Koota, Sjöberg, Lancaster, Kaisti, Brock, Ranta, Caroselli and Lugli2018), lime lumps can also yield too young dates (Nawrocka et al. Reference Nawrocka, Czernik and Goslar2009), and not all mortars contain lime lumps. Current approach involves extraction of the “middle” grain fraction (MGF, 32–63 μm), sequential dissolution with orthophosphoric acid and collection of CO2 gas portions (e.g. Lindroos and von Konow Reference Lindroos and von Konow1997; Heinemeier et al. Reference Heinemeier, Ringbom, Lindroos and Sveinbjörnsdóttir2010; Michalska Reference Michalska2019; Barret et al. Reference Barrett, Donnelly and Reimer2020). According to Sonningen and Jungner (Reference Sonninen and Jungner2001), the MGF is the best grain size span for discrimination between geogenic and anthropogenic carbonate using chemical (hydrolysis) reactivity.
For the MODIS2, the dates of the first gas portions for the samples were reported as the intercomparison result. The first gas portion was collected so that the amount of produced CO2 was maximum 20% of the amount of total CO2 (Ringbom et al. Reference Ringbom, Heinemeier, Lindroos and Brock2011b).
Since the first gas portion (no matter how small) can still contain a portion of “dead carbon,” here we also use extrapolation of the data principle (based on Folk and Valastro Reference Folk1976, similar to one described in Van Strydonck et al. Reference Van Strydonck, Dupas, Dauchot-Dehon, Pachiaudi and Marechal1986, Reference Van Strydonck, Hayen, Boudin, Van den Brande, Burguera, Ramis and De Mulder2015) and report the extrapolated results as well. We used first two gas portions for obtaining extrapolated result. They were collected so they conformed to conditions established experimentally (Sironić et al. Reference Sironić, Cherkinsky, Borković, Damiani, Barešić, Visković and Krajcar Bronić2023). The conditions were derived from the MGF hydrolysis kinetics in a way that is described in the Methods section. We also used the last (third) CO2 gas portion to evaluate geogenic carbon influence and, in comparison with the first two gas portions, to indicate presence of recrystallization, shown as significantly 14C younger fractions.
An array of analysis can be used for characterization of mortar matrix (Daugbjerg et al. Reference Daugbjerg, Lindroos, Heinemeier, Ringbom, Barrett, Michalska, Hajdas, Raja and Olsen2020 and references therein). For the prescreening/characterization of the samples we use carbonate content expressed as CaCO3, kinetics curve of mortar hydrolysis with H3PO4, and FTIR. Some authors (Toffolo et al. Reference Toffolo, Regev, Dubernet, Lefrais and Boaretto2019; Calandra et al. Reference Calandra, Cantisani, Salvadori, Barone, Liccioli, Fedi and Garzonio2022) use FTIR also for determination of the extracted binder carbonate purity of the grain fraction used for 14C dating. The shape of 14C content vs. CO2-gas amount curve (e.g. Lindroos et al. Reference Lindroos, Ringbom, Heinemeier, Hodgins, Sonck-Koota, Sjöberg, Lancaster, Kaisti, Brock, Ranta, Caroselli and Lugli2018) in case of sequential dissolution also gives insight into possible problems with recrystallization and amount of “dead carbon” content in analyzed grain fraction (Daugbjerg et al. Reference Daugbjerg, Lindroos, Heinemeier, Ringbom, Barrett, Michalska, Hajdas, Raja and Olsen2020).
Here we compare our reported and extrapolated results to the known dendrodated and historical data provided by the intercomparison organizers, and not to the results obtained from the other radiocarbon laboratories that took part in the MODIS2. We use the method applied in the Zagreb Radiocarbon Laboratory that we find to be most cost and time effective.
METHODS
Sample Preparation
The obtained samples in form of <150 µm particle size fraction were sieved and 32–63 μm particle size (MGF) was used for further processing. The MGF was analyzed by Fourier-transform infrared attenuated total reflection (FTIR-ATR) for purity, and also for carbonate content expressed as CaCO3.
For the method used for reporting the results, MGF was reacted with 85% H3PO4, at room temperature (23–27°C) on vacuum unit. Three successively formed gas portions of CO2 were collected using liquid nitrogen in 0–3 s, 3–15 s and from 15 s until the end of the reaction. Each CO2 gas portion was split for δ13C analysis and graphitized for 14C AMS analysis. The first gas portion were reported as the true age taking into account the course of reaction of sequential dissolution (Figure 2).
Extra steps were performed for the method using extrapolation principle. Hydrolysis kinetic curve of MGF with H3PO4 (85%) was created by monitoring pressure increase in time during reaction of sample conducted at 25°C or with reaction vessel immersed in ice cold water. The cumulative CO2 yield (p C/p tot, p C–cumulative pressure at time point, p tot–total pressure when the whole batch of sample is reacted) at which the curve changes slope (starts to stagnate) (p C/p tot-k) was determined. The time span for the collection of the first gas portion was selected so that the amount of produced CO2 is <25% p C/p tot-k and, for the sum of the first and second CO2 portion, <100% p C/p tot-k. If the time span for collecting first gas portion was considered to be too short at 25°C (< 2 s for the first portion), it could be extended by performing the reaction with vessel immersed in ice water. The third gas portion (until the end of the reaction) was also collected. All collected gas portions were prepared for 14C and 13C analyses. For 14C analysis graphites were prepared by using Zn reduction on Fe (Krajcar Bronić et al. Reference Krajcar Bronić, Horvatinčić, Sironić, Obelić, Barešić and Felja2010; Sironić et al. Reference Sironić, Krajcar Bronić, Horvatinčić, Barešić, Obelić and Felja2013).
FTIR Analysis
FTIR-ATR spectra were recorded using a PerkinElmer UATR Two spectrometer in the range 450 cm–1 to 4000 cm–1, with a spectral resolution of 4 cm–1 and total of 8 scans accumulated. Automatic ATR correction algorithm was applied to account for relative intensity shift in the collected FTIR spectra. The ν2/ν4 peak intensity ratio of the out-of-plane bending (ν2 = 873 cm–1) and in-plane bending (ν4 = 712 cm–1) of calcite was calculated by drawing the baseline between the closest minima on the sides of these two peaks and reading the peak intensity values (Chu et al. Reference Chu, Regev, Weiner and Boaretto2008).
14C and 13C Analysis
Carbon isotope analysis of CO2 gas (δ13C) and of graphites (14C/13C ratio) were performed at the Center for Applied Isotope Studies, Georgia (CAIS). The δ13C values, measured on Isotope Ratio Mass Spectrometer (IRMS), are expressed in per mill relative to Vienna Pee Dee Belemnite (Brand et al. Reference Brand, Coplen, Tyler, Vogl, Rosner and Prohaska2014) and have uncertainty of 0.1‰ that was calculated based on multiple measurements of the secondary standards. 14C/13C values were measured on 0.5 MeV accelerator mass spectrometer (AMS) at the CAIS (Cherkinsky et al. Reference Cherkinsky, Culp, Dvoracek and Noakes2010). 14C values are normalized to δ13C of –25‰ and expressed as fraction modern (F 14C) and as age before present (BP; Stuiver and Polach Reference Stuiver and Polach1977; Mook and van der Plicht Reference Mook and van der Plicht1999). 14C conventional ages were calibrated by OxCal 4.4 software (Bronk Ramsey Reference Bronk Ramsey2009; Bronk Ramsey and Lee Reference Bronk Ramsey and Lee2013) and IntCal20 calibration curves (Reimer et al. Reference Reimer, Austin, Bard, Bayliss, Blackwell, Bronk Ramsey, Butzin, Cheng, Edwards and Friedrich2020).
Principle of Date Extrapolation from Sequential Dissolution Gas Portions
Here we use idea of linear extrapolation, and it should also be pointed out that there could also be used exponential model (the Texas method, Folk and Valastro Reference Folk1976; Van Strydonck et al. Reference Van Strydonck, Van Der Borg, De Jong and Keppens1992; Lindroos et al. Reference Lindroos, Heinemeier, Ringbom, Brasken and Sveinbjornsdottir2007). The ages using this principle should be considered as the minimal ages.
In order to create curve expressing changes in F 14C with increase in CO2 pressure in each point, cumulative values should be calculated, because the measured vales (for F 14C and amount of CO2) correspond only to certain extracted portion of CO2 gas. Cumulative F 14CC and p C/p tot values at the end of each collected gas portions were calculated using:
$${p_{\rm{C}}}/{p_{{\rm{tot}},{\rm{n}}}} = \;\mathop \sum \limits_{i = 1}^n p/{p_{{\rm{tot}},{\rm{i}}}}{\rm{\;}}$$
$${F^{14}}{{\rm{C}}_{C,{\rm{n}}}} = \;{{{\mathop \sum \nolimits_{i = 1}^n {F^{14}}{{\rm{C}}_{\rm{i}}} \cdot p/{p_{{\rm{tot}},{\rm{i}}}}}}\over{{{p_{\rm{C}}}/{p_{{\rm{tot}},{\rm{n}}}}}}}$$
where F 14Ci is fraction modern of each collected CO2 portion and p/p tot,i are yields of each of those portions up to the nth one, while F 14CC,n is cumulative activity for each CO2 portion from zero point to the end point of that portion and p/p tot,i are corresponding cumulative yields. If it is assumed that with reaction development (with time and increase in p C/p tot) produced CO2 contains more geogenic (“dead”) carbon, it can be deduced that a theoretical initial point contains pure anthropogenic CO2 at time t = 0 s, i.e. p C/p tot = 0. To extrapolate the radiocarbon result p C/p tot vs F 14CC correlation was used. The extrapolated F 14CC value (at p C/p tot = 0) was calculated as an intercept of linear equation defined by the p C/p tot and F 14CC values only for the first two cumulative portions, points (p C/p tot,1, F 14CC,1.) and (p C/p tot,2, F 14CC,2). This principle of extrapolation is valid only for conditions: p C/p tot,1 < 25% p C/p tot-k and, p C/p tot,2 <100% p C/p tot-k (Sironić et al. Reference Sironić, Cherkinsky, Borković, Damiani, Barešić, Visković and Krajcar Bronić2023). The uncertainty for the extrapolated value was assessed from uncertainty propagation of the same two points.
RESULTS
Characterization
Characterization of the samples was performed by means of FTIR analysis (Figure 1, Table 1), carbonate yields (Table 1), hydrolysis kinetics curves (Figure 2) and cumulative radiocarbon results, i.e. curve p C/p tot vs F 14CC. (Figure 3).
FTIR-ATR analysis of mortar samples with particle size of 32–63 µm: (c) calcite, (k) kaolinite, (g) gypsum and (q) quartz. Strong absorption signals of kaolinite in MODIS2.3 confirmed low carbonate content in the sample.
Hydrolysis kinetics curves for MODIS2 and points reconstructed form amount of CO2 portions in time obtained during sequential dissolution.
Results of carbonate content (calculated as CaCO3) of 32–63 µm grain fraction (sieved from fraction <150 µm) and FTIR qualitative mineral analysis for each MODIS2 sample and δ 13C and radiocarbon dates of each CO2 sequential dissolution gas portions of the three MODIS2 samples: sample ID and laboratory numbers (Z—sample id, A—graphite ID number, Zagreb Laboratory and UGAMS—graphite ID number, CAIS), step hydrolysis time fraction, m SD—mass of sieved fraction for sequential dissolution, m(Cfr) —mass of carbon in each CO2 portion, p/p tot –percentage of carbon in each individual gas portion compared to total carbon content for sequential dissolution, F 14C—fraction modern, δ13C (with 0.1‰ uncertainty) and 14C date of each collected CO2 portion.
* Reaction conducted with reaction vessel immersed in ice cold water, nm—not measured.
Cumulative F 14CC vs p C/p tot for all MODIS2 samples (Table S1, Supplementary data), and extrapolated result (in the black rectangle) obtained from the first two CO2 portions. All results are obtained at 25°C, and “MODIS2.3–repeated, ice” was obtained with reaction vessel immersed in ice cold water.
The FTIR analysis is used since it can provide qualitative data in a simple, fast and reliable manner. For the more detailed mineralogical analysis, providing information about recrystallization (which can give both too old and too young results) or magnesite (in a case of dolomitic lime mortars, giving too young result) presence in mortar, petrography, XRD or SEM analysis should be used (Daugbjerg et al. Reference Daugbjerg, Lindroos, Heinemeier, Ringbom, Barrett, Michalska, Hajdas, Raja and Olsen2020). The carbonate yield can imply if the mortar is non-hydraulic or hydraulic. The hydrolysis kinetic curves are mainly used for determining the time periods for collecting desired portions, however, higher p C/p tot-k values imply higher portion of anthropogenic (binder) carbonate.
The final characterization of the mortars is also checked with the cumulative radiocarbon results, i.e. curve p C/p tot vs. F 14CC, which should generally be descending with later portions (e.g. Heinemeier et al. Reference Heinemeier, Ringbom, Lindroos and Sveinbjörnsdóttir2010). If changes in the trends are observed, e.g. to higher F 14CC, or there is a steep fall of F 14CC, recrystallization is suspected making the initial portion unreliable. Otherwise, the F 14CC of the first portion (amount of CO2 portion < 20% of the amount of total CO2, Ringbom et al. Reference Ringbom, Heinemeier, Lindroos and Brock2011b) or the extrapolated value (Sironić et al. Reference Sironić, Cherkinsky, Borković, Damiani, Barešić, Visković and Krajcar Bronić2023) provide the best estimated date of the mortar.
An initial screening of MODIS2 samples by FTIR showed qualitative differences among them. The most prominent features in all FTIR-ATR spectra of analyzed mortars were absorption bands of calcite as a major component, found at 712, 873, 1408, 1796, and 2513 cm–1 (Chu et al. Reference Chu, Regev, Weiner and Boaretto2008; Poduska et al. Reference Poduska, Regev, Berna, Mintz, Milevski, Kahalaily, Weiner and Boaretto2012). Mineralogical analysis also revealed that all MODIS2 samples contained a small amount of quartz based on the presence of signals at 695, 779, 798, and 1163 cm–1. Characteristic absorption signals at 3526, 3400, 1622, 1107, 1006, 667, and 599 cm–1 clearly correlated with gypsum as the third component in MODIS2.2 sample. However, in comparison with MODIS2.1 where kaolinite clay constitutes the rest of the composition, MODIS2.2 appeared to have the highest carbonate content in semi-quantitative terms. On the other hand, MODIS2.3 sample was made up of a substantial amount of kaolinite clay as indicated by typical absorption bands at 3670–3695 cm–1 and 1640 cm–1, as well as a strong signal at 1009–1023 cm–1, with expected low levels of carbonate content. These findings were later supported by carbonate yield (as CaCO3) analysis (Table 1).
An anthropogenic portion of the analyzed MODIS2 samples was evaluated through calculation of the ν 2 /ν 4 ratio (at 873 and 712 cm–1, respectively) of the corresponding CaCO3 vibration intensities in the collected FTIR-ATR spectra. The ν 2 /ν 4 ratio values were consistently found in the range from 5.0–5.9, as opposed to pure geogenic calcite for which a value of 2.3 or similar would be expected (Chu et al. Reference Chu, Regev, Weiner and Boaretto2008).
Hydrolysis kinetics curves (p C/p tot vs. time, Figure 2) were obtained for all the MODIS2 samples at room temperature and for samples MODIS2.1 and MODIS2.3 also in ice water. The points reconstructed from the amount of CO2 portions collected in time for 14C analysis during sequential dissolution for all the samples at the room temperature and for MODIS2.3 in ice water are shown in Figure 2.
It can be observed that the steepness (change in p/p tot in time) for the kinetics curves depended on the temperature of samples, largely slowing down the reaction in ice water compared to room temperature. The first two CO2 portions from sequential dissolution fitted the desired gas portion (<25% p C/p tot-k for the first and <100% p C/p tot-k for the first and the second CO2 portion) of the total yielded CO2 for all the three samples. For the sample MODIS2.3, points reconstructed from the sequential dissolution in ice water did not coincide with curve obtained from hydrolysis kinetics obtained with the same conditions. This was probably due some error in manual measurement of time during collection of gas portions from sequential dissolution.
Carbon Iotopes and Radiocarbon Age Results
Carbonate content (calculated as CaCO3) of all MODIS2 samples 32–63 µm grain fraction, as well as carbonate content in grain fraction, CO2 yield of each gas portion (p/p tot), carbon isotope data and radiocarbon date results for all sequential dissolution CO2 portions are presented in Table 1. The dates for each sample that were reported to the MODIS2 organizers as the age of the mortar were all 0–3 s gas portions (first gas portions) conducted at the room temperatures (Z-7176, Z-7185 and Z-7188 for MODIS2.1, MODIS2.2 and MODIS2.3, respectively).
The curve displaying relation of cumulative p C/p tot vs. F 14CC (Figure 3, Table S1, Supplementary data) shows steady decrease of F 14C for later portions for all the samples. This is generally expected behavior for samples without recrystallized carbon (Heinemeier et al. Reference Heinemeier, Ringbom, Lindroos and Sveinbjörnsdóttir2010; Lindroos Reference Lindroos, Ringbom, Heinemeier, Hodgins, Sonck-Koota, Sjöberg, Lancaster, Kaisti, Brock, Ranta, Caroselli and Lugli2018), implying that the first portion contains only anthropogenic carbon. Extrapolated values, because of uncertainty propagation, have almost twice the uncertainty of the reported values.
All the extrapolated results are higher than for the corresponding first portions. Results for repeated analysis of MODIS2.3 sample conducted at lower temperature (reaction vessel immersed in ice cold water) agree very well to the analysis at the room temperature (Table 2).
Reported and extrapolated radiocarbon ages and corresponding calibrated dates for 68.3% probability compared to the expected age. Ranges marked gray fit/overlap with the expected date.
*Repeated reaction with vessel immersed in ice cold water, shaded calibrated dates overlap with expected ranges).
δ13C values for all the portions increase with later CO2 portions for all the samples, which is usually observed due to kinetic effect (Nawrocka et al. Reference Nawrocka, Czernik and Goslar2009; Heinemeier et al. Reference Heinemeier, Ringbom, Lindroos and Sveinbjörnsdóttir2010), but can also be influenced by increase in amount of the dead carbon.
DISCUSSION
Radiocarbon ages and corresponding calibrated dates for reported and extrapolated values for each sample (Table 2 and Figure 4) are compared to the expected dates. Calibrated ages are given with 68.3% probability. Radiocarbon ages are not rounded according to the convention.
Calibration curves of the reported and extrapolated results for MODIS2 samples in comparison to the expected dendrodated and historical dates (blue line and rectangles, respectively).
Sample MODIS2.1 for both the reported and extrapolated calibrated dates agrees very well with the dendro-dated wood. Also, the whole span of both results fit into the expectation that the building belongs to the 14th century. The influence of the geogenic carbon is very low (Figure 3) and accounts to about 0.9% (calculated from F 14CC, Table S1, Supplementary data), so the difference between the first and the final gas portion is only 100 14C years. This is the main reason that the reported and extrapolated result fit one another.
The reported date for the MODIS2.2 sample in the 68.3% posterior probability does not overlap with the expected historical span of dates. With 95.4% (2σ) probability calibrated dates for reported value are cal AD 1281–1319 (49.5%) and cal AD 1359–1389 (45.9%) overlapping the historical dates in period AD 1300–1319 and AD 1359–1361. Higher probability range (99.7%, 3σ) (calibrated dates are cal AD 1276–1327 (51.8%) and cal AD 1351–1395 (47.9%)) overlap the historical dates in periods AD 1300–1327 and AD 1351–1361. However, by moving about 100 years later, with extrapolation, extrapolated dates fit completely with 68.3% (1σ) probability, yielding overlapping date span AD 1321–1358. The difference between the extrapolated and the F 14C of the complete CO2 portion is 0.09, which would approximate to about 10% of the geogenic component (Table S1, Supplementary data), making the difference between the reported and extrapolated result significant for dating. The stable isotope data are influenced mainly by the kinetic effect, which is particularly emphasized in very low value (–31.5‰) for the first CO2 portion in ice of MODIS2.3. However, the highest δ13C of the MODIS2.2 value (–8.8‰) for the last CO2 portion compared to the other samples, and the biggest difference between its first and the last CO2 portions (from –15‰ to –8.8‰) implies largest influence of the geogenic/dead carbon contamination in the last portion.
The historical date span for the MODIS2.3 sample is quite wide; 266 years. Both reported and extrapolated dates (in 68.3% probability) overlap in about one third of the older part of the historical date span. If both dates are considered, when dating the event of erection of the St. Eulalia shrine, the date range can be shortened to from AD 305 to cal AD 375 (reported date) or to cal AD 430 (extrapolated date), both for 94.5% posterior probability. We cannot assume normal probability distribution of the true age between the set historical range AD 304–570. It would be more probable that the shrine was erected closer to the time of death of St. Eulalia, which is here implied by the reported and extrapolated results for the sample MODIS2.3.
Similar to MODIS2.1 sample, the amount of geogenic carbon of MODIS2.3 is low, 2.9% and 2.6% at the room temperature and in ice-water, respectively (Table S1), and so the difference between the reported and extrapolated values is not crucial. The difference of this sample compared to the samples MODIS2.1 and MODIS2.2 is visible in the carbonate content (which is only 46%) and in amount of other material present as visible in FTIR spectra.
Another feature that makes the extrapolated values more reliable than the reported ones is wider uncertainty that considers both information from the first and the second gas portion.
CONCLUSION
We present the Zagreb Radiocarbon Laboratory, Zagreb, Croatia and Center for Applied Isotope Studies, University of Georgia, USA, radiocarbon results of the three mortar samples from the second international mortar dating intercomparison MODIS2. We compared our results to the expected historical age spans provided by the organizer.
The characterization of the samples was performed by FTIR, carbonate composition, hydrolysis kinetic curve of the 32–63 µm grain fraction and trend of radiocarbon results of three successive CO2 portions obtained by sequential dissolution of 32–63 µm grain fraction by phosphoric acid. We reported the date of the first CO2 portion as the date of mortar. All reported results for all three MODIS2 samples fitted the expected historical and dendrodated results. We also used the MODIS2 samples to test the principle of extrapolating results obtained from sequential dissolution. This approach is currently applied for mortar dating in the Zagreb Radiocarbon Laboratory. All the extrapolated results also fitted the expected historical ranges, and in case of sample MODIS2.2 the overlapping in calibrated extrapolated calendar dates with the historical period was better than for the reported one. This implied that the extrapolation principle can be used for eliminating dead carbon contamination in samples with higher contribution of geogenic carbonate.
The reported approach for radiocarbon mortar dating works for non-hydraulic mortars without recrystallization. Although the trend in radiocarbon activities for collected successive CO2 portions implied no presence of recrystallization, only three gas portions are not enough as definite proof, and, in the future, use of petrography and XRD analysis will be required prior to dating.
ACKNOWLEDGMENTS
FTIR-ATR was acquired through the Croatian Science Foundation (grant no. 9310).
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2024.75
