Radiocarbon dating of the master Buvina’s door and choir benches from the Split Cathedral and wooden beams from the church of St. Donatus in Zadar

Određivanje starosti metodom radioaktivnog ugljika 14C je vrlo pogodna i točna metoda apsolutnog datiranja organskog materijala. Izotop 14C je kozmogeni izotop koji nastaje u gornjim slojevima atmosfere te nakon oksidacije kao CO2 zajedno s ostalim izotopima ugljika (12C, 13C) postaje dio prirodnog ciklusa ugljika. U svim živim organizmima uspostavljena je ravnoteža između radioaktivnog raspada 14C i njegova nadoknađivanja iz atmosfere. Nakon smrti prestaje nadoknađivanje dok se raspad nastavlja. Mjereći koncentraciju preostalog 14C u organskom materijalu možemo odrediti vrijeme proteklo od smrti organizma. Zbog vremena poluraspada izotopa 14C od 5730 godina, najveća starost koja se može odrediti ovom metodom iznosi oko 60 000 godina. Različiti organski materijali se mogu datirati metodom 14C (drvo, drveni ugljen, kosti, sjemenke, žito, papir, pergament, kosa, dr.). Drvo je vrlo pogodan materijal za datiranje budući da svaki god drveta koncentraciju 14C u atmosferi tijekom formiranja goda. Sve mjerne tehnike su destruktivne te se uzorak mora kemijskim metodama prevesti u odgovarajući materijal pogodan za mjerenje. Ukoliko se mjerenje provodi tehnikom akceleratorske masene spektrometrije (AMS), uzorak se pretvara u grafit. Prednost tehnike AMS u odnosu na radiometrijske tehnike su: manja potrebna masa uzorka i manja mjerna nesigurnost. Konvencijska 14C starost izražava se u godinama prije sadašnjosti (Before Present, BP) pri čemu je 0 BP = AD 1950. Kako bi se dobile kalendarske godine, potrebno je provesti dendrokronološku kalibraciju primjenom kalibracijskog programa (npr. OxCal) i korištenjem najnovijih kalibracijskih krivulja. Rezultat kalibracije su vremenski rasponi dani svaki sa svojom vjerojatnošću. Važno je napomenuti da ova metoda daje starost materijala, a ne vrijeme korištenja tog materijala (za gradnju ili izradu umjetničkih predmeta). Datiranjem drva dobije se dakle vrijeme kada je nastao određeni god drveta. Tijekom istraživanja Buvininih vratnica splitske katedrale metodom 14C određena je starost 5 drvenih fragmenata iz vratnica te 2 drvena fragmenta korskih klupa. Određena starost uzoraka Z-5720 i Z-5721 (vratnice) smješta te fragmente u razdoblja cal AD 1155 – 1210 i cal AD 1054 – 1191. Može se zaključiti da ta dva fragmenta predstavljaju dio originalnih vratnica iz godine 1214. Uzorci Z-5719 i Z-6037 su nešto mlađi (cal AD 1245 – 1272 i cal AD 1225 – 1266) te nisu mogli biti dio vratnica iz 1214. godine. Uzorak Z-5469 je značajno mlađi od ostalih, cal AD 1481 – 1650, te nije dio originalnih vratnica. Određena je starost dvaju drvenih fragmenata korskih klupa. Z-6605 datiran je u 13. stoljeće (cal AD 1266 – 1281), a Z-6038 u godinu cal AD 1955, pa se može zaključiti da je taj fragment ubačen tijekom restauracije ili popravka klupa u 20. stoljeću. U ovom izlaganju prikazat ćemo i rezultate određivanja starosti drvenih greda iz crkve sv. Donata u Zadru. Uzorci greda uzeti su 1970. godine tijekom restauracije crkve i datirani su u laboratoriju u Zagrebu kao uzorci Z-175, Z-177, Z-178, Z-179. Konvencijske starosti su rekalibrirane tijekom 1999.-2000. Dobiveni rezultat smješta sječu hrastovog drveta za gredu na kraj 8., odnosno početak 9. stoljeća, što potvrđuje hipotezu da je crkva sv. Donata izgrađena u prvoj polovini 9. stoljeća.Radiocarbon (14C) dating is a convenient and accurate method of absolute dating of organic materials. Radiocarbon is a cosmogenic isotope formed in the upper atmosphere and after oxidation to CO2 it becomes involved in the carbon cycle together with other carbon isotopes (12C, 13C). Equilibrium between radioactive 14C decay and its replenishment is established in all living organisms. After death the replenishment stops while the radioactive decay continues. By measuring the 14C remained in the organic material we can determine the time elapsed since the death. Due to the half-life of 5730 years, the maximal age that can be determined is about 60 000 years. Various organic materials can be 14C dated (charcoal, wood, bone, seeds and grains, paper, parchment, hair, etc.) and wood is a very convenient material since each of the tree rings reflects the 14C activity of the atmosphere in the year of its formation. Measurement techniques require conversion of a sample to a suitable measurement material. In case of accelerator mass spectrometry (AMS) technique, a sample has to be converted to graphite. Advantages of the AMS techniques over the radiometric techniques are lower mass of a sample required and lower uncertainty. A conventional 14C age is expressed in years Before Present, where 0 BP = AD 1950. Dendrochrological calibration has to be performed to obtain calendar ages by a calibration program such as the OxCal and most recent calibration curves. As a result of calibration, a calibrated age span or spans are obtained each with a certain probability. It is very important to stress that by the 14C method the age of a material is determined, and not the time when the material was used, e.g., for building or sculpturing. In other words, when the wood is dated, the result will be the time when the tree-ring was formed. During the study of the Master Buvina’s door of the Split Cathedral 14C method was used to date 5 wood samples from the door and 2 samples from the choir benches. Samples Z-5720 and Z-5721 from the door give the calibrated age spans cal AD 1155 – 1210 and cal AD 1054 – 1191, respectively. It may be concluded that these two wooden pieces belong to the original door from AD 1214. Samples Z-5719 and Z-6037 are somewhat younger (cal AD 1245 – 1272 and cal AD 1225 – 1266, respectively) indicating that these pieces could not have been built into the door in AD 1214. Sample Z-5469 is considerably younger that the others, it is dated to cal AD 1481 - 1650 (probability 68.2 %), and therefore it was not a part of the original door. Two samples from the choir benches were also dated. Z-6605 was dated to the second half of the 13th century (cal AD 1266 – 1281). Z-6038 was dated to cal AD 1955, so it may be concluded that this piece of wood was added to the bench during reconstruction/repair in the second half of the 20th century. Radiocarbon dating of wooden beams from the St. Donat church in Zadar will be also presented. Samples were taken in 1970 during the restoration of the church and dated in the Zagreb Radiocarbon Laboratory as samples Z-175, Z-177, Z-178, Z-179. The conventional ages were re-calibrated in 1999-2000 by applying development in radiocarbon calibrations. The obtained result placed the felling of the oak tree used for making the beam to the end of 8th – beginning of 9th century supporting the hypothesis of construction of the St. Donat church in the first half of the 9th century