engleski

Radiocarbon dating of mortar: Case study of the Aqueduct in Skopje

Dating of mortar has been a challenge since the early days of the radiocarbon dating method. The idea is to date only the carbon fixed in mortar at the time of binding (i.e., the binder). However, results of ¹⁴C mortar dating are often compromised with dead carbon contamination originating from unreacted limestone during preparation of quick lime. Also, non-binder carbon can be incorporated into mortar sample (1) due to precipitation of secondary carbonates from environment, (2) in case of fire accidents, it is replaced with the carbon from the atmosphere at the time of accident, or (3) if the mortar is too alkaline, it still incorporates carbon from the atmosphere. Therefore, an effort is placed into developing of a method that can isolate strictly the binder carbon, with the help of the accelerator mass spectrometry (AMS) measurement technique enabling analyses of various phases of mortar. However, no reliable procedure has yet been established for each mortar type (Hajdas et al., 2017). The Zagreb Radiocarbon Laboratory was approached with a request to determinate time of the Aqueduct in Skopje (FYR Macedonia) construction through ¹⁴C dating of mortar samples. The Aqueduct is one of the landmarks of Skopje, a monumental building more than 385 m long. It was a part of a water-supply system with a total length of about 10 km. The age of the Aqueduct is not known – several hypotheses place it to periods between 6th and 16th century. Six mortar samples from different positions of the eastern façade and the upper parts with remains of tubes were collected in July 2017, paying attention that samples were not deteriorated and were situated at higher levels with less possibility of containing the secondary carbon. In the Laboratory, the maximum 5 cm of the mortar from the surface was subsampled, than broken by alternately changing temperatures from -198 °C to 80 °C. In order to extract only the carbon from the binder three strategies for separation of binder carbon were used: 1) mechanical separation of calcite inclusions formed during mortar hardening (not possible for all mortars), 2) selection on the basis of particle size and the ability to suspend in water induced by ultrasonic shock, and 3) collection of at least two fractions of CO₂ produced by reaction of calcite with acid. With the premise that all mortars originated from the same era, the results of ¹⁴C dating of each fraction were combined to yield a plausible date of the Aqueduct building. Some results were eliminated due to very low δ¹³C values combined with modern ¹⁴C dates and some implied dead carbon contamination. The most reliable results were obtained from two inclusions and one suspended fraction placing the time of the Aqueduct construction in 15-16 century AD. The analyses will be presented and possibility of using the method for future mortar dating will be discussed. References Hajdas et al. 2017. Preparation and Dating of Mortar Samples—Mortar Dating Inter-Comparison Study (MODIS). Radiocarbon 59(06):1845 - 185

14C ; mortar ; Aqueduct ; Skopje

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