A new technique for the radiocarbon dating of mortar




Valastro, Salvatore, 1931-

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Development of a modified mortar dating technique by the ¹⁴C method is described. This procedure may be very valuable in the detailed dating of the construction of ancient buildings and may even resolve different events or phases of construction in the same structure. Carbon-14 analysis of samples is commonly carried out on wood or charcoal derived from wood. The dates obtained from this type of samples often give ages which are too old because they may well refer to the time when the tree was still young and therefore may not reflect the age of construction. Mortar dating, however, refers to the time of building construction because contemporary ¹⁴C in the atmosphere is imbibed by the mortar during the setting or hardening process. Earlier mortar dating attempts were complicated by dead carbon (carbon containing no contemporary ¹⁴C isotope) contributed by limestone or marble contained in the aggregate The new technique is dependent on a method which separates nearly all of the aggregate from the mortar and then evolves only the first fraction of CO₂ during the acidification process. Since the ¹⁴C content of the mortar was totally absorbed as part of atmospheric CO₂, the radiocarbon assay of the sample will give an age equivalent to the setting or hardening process. Five samples of mortar from Stobi were processed and assayed as a preliminary test of the technique. Stobi is an archaeological site in Macedonian Yugoslavia that was an important provincial town during Roman and early Byzantine time. A small sample taken from the narthex floor of the Central Basilica, dated archaeologically as late 4th or 5th century A.D., gave a radiocarbon date of 260 ± 180 A.D. (sample no. Tx-1431). Another sample (Tx-1944) from the same Basilica Floor 1 produced two dates with an average of 372 ± 60 A.D., which is in excellent agreement with the archaeological age estimate. Two samples from the Theater produced six dates averaging 233 ± 32 A.D. (no. Tx-1941, Tx-1942); the archaeological age estimate places the Theater in the late 1st to early 2nd century A.D. Sample no. Tx-1943 taken from the principal construction phase of the Episcopal Basilica, whose archaeological age estimate is about 400 A.D., gave an average age of 302 ± 60 A.D. In the East City Wall area, a layer of mortar with 4th century pottery above and below gave a date (no. Tx-1940) of 115 ± 60 B.C. This particular sample may have been contaminated because the area in which it was taken is subjected to annual flooding. Another possibility is the redeposition of the mortar. Thus, the geologic circumstances that can affect mortar evidently need to be investigated. All the dates above were subsequently corrected for ¹⁴C half-life of 5730 years, dendrochronology, and were tentatively corrected for δ¹³C fractionation



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