Ramped pyroxidation radiocarbon dating of a preservative contaminated early medieval wooden bowl

Preservatives and consolidants make it difficult or impossible to obtain accurate radiocarbon dates on many organic artefacts from museums or archaeological collections. Discovered in an Irish bog in the 1960s, the Cuillard Bowl is a triangular wooden (Alder - Alnus glutinosa) dish of special interest on account of its unique geometry and style. However, the persistence of preservative (polyethylene glycol, PEG) following conventional pre-treatment methods produced problematic radiocarbon dates. A new ramped pyroxidation (RPO) setup at the 14CHRONO Centre was applied to pretreated material from the Cuillard bowl to investigate if a preservative-free CO2 fraction could be isolated, taking advantage of the release of CO2 from wood cellulose at a lower decomposition temperature than CO2 is released from PEG. Radiocarbon dates on low temperature fractions (280–340 °C) produced statistically identical radiocarbon ages that calibrate from the mid-6th to mid-7th centuries AD, in agreement with stylistic dating of the bowl. The validity of these results is further supported by models of the radiocarbon ages from theoretical thermal decomposition curves of PEG-contaminated wood. Higher temperature fractions (> 350 °C) were statistically different and older due to the removal of CO2 derived from the PEG preservative. RPO and infrared-spectroscopy (FTIR) results on non-pretreated Cuillard bowl material, early medieval alder of known age (dendro-dated), and modern alder were used to identify PEG as the preservative and to provide references for the thermal behaviour of both clean and heavily contaminated wood in the RPO system. This work demonstrates the development of a new technique for radiocarbon dating of organic artefacts that are contaminated by PEG, offering a new avenue for the investigation of museum or archaeological collections previously considered too difficult date.