A considerable share of tangible cultural heritage consists of ruins and buildings. Some of them are monumental and of historical interest, some of them of local interest or remaining to be discovered. Often such objects lack a chronologic frame, and the method that would provide a time for buildings is not a straightforward radiometric dating. For millennia mortar was the prevailing one among other materials used in constructions until the early 1st half of the 20th century. Following the industrial revolution and expansion of new technologies, the traditional mortar has been replaced by cement. This industrial product, although developed using experience gained over millennia, is rather useless for radiocarbon dating as it contains old carbon additions, resulting in ages as high as tens of thousands of years.
Mortars, except for hydraulic, pozzolana and cocciopesto (Ringbom et al., 2011), can be more suitable for radiocarbon dating. The mechanism of binding CO2 from the atmosphere shown by the slacked carbonate oxide (carbonate hydroxide) is a perfect analog to the photosynthetic path of building carbon into organic matter. Early on, radiocarbon researchers tried to apply this method to dating archeological and historical monuments. The first results were encouraging (Labeyrie and Delibrias, 1964) but followed by less successful attempts (Stuiver et al., 1965). The main challenge in the preparation of mortars is the separation of old carbon, which might be included in the binder due to incomplete burning. Contamination can also be added together with aggregates such as sand and gravel. Although other complications of 14C signal in mortars can occur, such as delayed hardening, fire damage, or formation of new carbonates, the old carbon is the prevailing problem. Therefore, the first attempt is to achieve the most reliable 14C ages focused on the removal of geological carbonate. The observed difference between the reactivity of the binder, which dissolves faster, and the limestone has been used to separate the contaminant.
A ‘revolution’ has only been brought about by the application of the AMS method (Nelson et al., 1977).
The minimal quantities of carbon needed for the AMS 14C analyses open an opportunity for measurements of multiple dissolution fractions. Developed independently by two teams (Heinemeier et al., 1997), the method has been modified and adopted by few laboratories. Also, a little different approach has been proposed by Nawrocka et al. (2005) and Marzaioli et al. (2011). As well, this method underwent various modifications and, at some stage, has been combined with sequential dissolution (Michalska and Czernik, 2015).
The reliability of the radiocarbon ages of mortar has been debated ever since the first disappointing results of Stuiver et al. (1965) have been published. A considerable effort has been made by the mortar 14C dating community to establish the procedure and protocols as well as quality control (Ringbom et al., 2014). The results of MODIS inter-comparison exercise (Hajdas et al., 2017 and Hayen et al., 2017) have shown that some of the mortars cannot be dated by 14C method and that the understanding of mortar geochemical characteristics is a key to understating these problematic results (Michalska et al., 2017). Here, we present results of radiocarbon dating of a monument, which has been dated only by typology.
The site of Hohenrätien (GR) is located on a rock rising 250 m above the Viamala Valley (Fig. 1a) overseeing the roads of the San Bernardino and the Splügen Pass, which connect the Swiss Valley of Hinterreihn with the Italian Valle San Giacomo and Chiavenna (Fig. 1b).
The transit from Northern Italy to the Rhine Valleys appreciated since the Bronze and the Iron Age, was also used by the Romans. In the medieval ages, the strategic location was chosen for the construction of the castle, which in years 1996−1997 was a subject of archeological prospections and investigations. Moreover, in 1999, the owner of the castle discovered additional remains of older construction. The archeological excavations 2001−2004 documented an early Christian church (Gairhos and Janosa, 2011). The latter phases of constructions of the whole monument could be dated by dendrochronology, and a wiggle-matching of 14C dated tree rings to 1180−1210 AD (Gairhos et al., 2005). However, the earlier phases could only be dated by typology; therefore, the radiocarbon dating of mortar is one possibility to provide a numeric date on the monument.
The location of the three mortar samples collected from the remains of construction A (Bau A) is shown in Fig. 2.
Preparation of mortar for radiocarbon dating followed the protocol developed so far at ETH Laboratory (Hajdas et al., 2017 and Hajdas et al., 2020). The principle of the method is a separation of suitable grain size and discrimination between anthropogenic and geogenic carbonate by a different dissolution time. Two samples (Nos. 891 and 894) were prepared using the method of sequential dissolution, which targets the fast-dissolving component of the binder. In the case of sample No. 891, two different grain size fractions: 45−63 μm and 32−63 μm were analyzed The sample No.891 has been prepared using an old protocol, and as modifications has been implemented, new standard fraction 45−63 μm was re-done. Sample No. 894 was submitted later and prepared using the new standard procedure only.
The outcome of the radiocarbon dating performed on samples from the Hohenrätien old parish church is summarized in Table 1. With the exception of one sample, radiocarbon ages of the fast fractions (1st and 2nd,
Results of the 14C AMS analysis of mortar samples. All the samples but lime lump (LL) were analyzed using GIS. Combined and calibrated ages were obtained using OxCal v4.3.2 (only for 14C ages evaluated as accurate).
Lab Code | Sample/Fraction (μm) | Dissolution time (s) | 14C age ±1 sigma (BP) | Calibrated age (95.4% conf. level) (AD) | μg C |
---|---|---|---|---|---|
ETH-65530 | 891, 45−63 | 1−3 | 1534±84 | 348−656 AD | 95 |
ETH-65530 | 891, 45−63 | 4−6 | 1688±88 | NA | 109 |
ETH-65530 | 891, 45−63 | 7−9 | 2108±102 | NA | 108 |
ETH-65530 | 891, 45−63 | 10−12 | 2621±101 | NA | 104 |
ETH-65530 | 891, 32−45 | 1−3 | 1605±74 | 257−606 AD | 60 |
ETH-65530 | 891, 32−45 | 4−6 | 1737±83 | NA | 105 |
ETH-65530 | 891, 32−45 | 7−9 | 2144±103 | NA | 88 |
ETH-65530 | 891, 32−45 | 10−12 | 2461±105 | NA | 96 |
ETH-69913 | 894, 45−63 | 1−3 | 1386±64 | 543−770 AD | 81 |
ETH-69913 | 894, 45−63 | 4−6 | 1581±63 | NA | 99 |
ETH-69913 | 894, 45−63 | 7−9 | 1664±77 | NA | 91 |
ETH-69913 | 894, 45−63 | 10−12 | 2254±76 | NA | 82 |
ETH-85506 | 897, LL (lime lumps) | Total dissolution | 1569±24 | 422−547 AD | 1200 graphite |
Combined | 891 & 894 | all 1−3 s | 1495±41 | 430−646 AD | X2-Test: df=2 T=5.3 (5% 6.0) |
Combined | 891 & 894 & 897 | all 1−3 s & LL | 1551±21 | 427−559 AD | X2-Test: df=3 T=7.7 (5% 7.8) |
Figure 3 shows all the results of the radiocarbon dating of all the samples and their evaluation. Following the procedure outlined in Hajdas et al., (2020), the radiocarbon ages of the fast-dissolving fractions: 1st: 1−3 s and 2nd: 4−6 s, have the potential of providing the accurate 14C signal for the time of binding the mortar. The slight increase in the ages of the 3rd and 4th fractions indicates the presence of the old, geological component. To establish a laboratory procedure for calculating the following is applied: only the 1st,
Radiocarbon dating of the mortar provides the potential to date archeological and historic buildings. The early church at the Hohenrätien is an excellent example of the potential for the numeric dating method to be applied to mortar. The resulting radiocarbon ages of the three samples date the monument to the period between 257 and 770 AD. The wide range of calendar ages is due to the nature of the calibration curve and the age plateau between 420 and 530 AD. Nevertheless, the combined age of the fast component of the mortar and a single lime lump results in an age of 427−559 AD, confirming the typological dating. In summary, this study adds information about the reliability in using the 1st,