Radiocarbon measurements of iron girders from Aachen cathedral

Radiocarbon dating of ancient iron and steel has been successfully demonstrated over the past decades. Yet, interpretation of the results could get complicated in view of possible contamination with exogeneous carbon from a) old wood used to produce coal, b) flux additives such as calcites, and c) cutting tools used in sampling. The latter contamination can be minimized to below common measurement uncertainties. Sample material from two iron girders of the Carolingian Palace Chapel as well as from two iron beams from the Gothic Annex of Aachen Cathedral became available for radiocarbon dating during renovation work. In addition, wood and charcoal from the basement and the roof of the palace chapel were radiocarbon dated, providing information regarding the age range of wooden material used during the construction as well as an age reference for the iron dates. Radiocarbon measurements from the extracted carbon of the Carolingian iron girders are in good agreement with the historical erection dates, i.e. between CE793–813. In contrast, radiocarbon measurements from the iron beams of the Gothic Annex are highly variable due to a) contamination of iron with exogeneous carbon from recent reconstructions, i.e. welding of modern iron with a fossil carbon signature, and b) because of heterogeneous iron forged together, probably typical for medieval iron made by blast furnace technique.

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Opracowanie rekordu ze środków MNiSW, umowa nr POPUL/SP/0154/2024/02 w ramach programu "Społeczna odpowiedzialność nauki II" - moduł: Popularyzacja nauki i promocja sportu (2025).