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Gothic cathedrals : an interdisciplinary team has shown that iron was used to reinforce stone from the construction phase

Using radiocarbon dating on metal found in Gothic cathedrals, an interdisciplinary team including the Laboratoire archéomatériaux et prévision de l'altération (CNRS/CEA), the Laboratoire de mesure du carbone 14 (CNRS/CEA/IRD/IRSN/French Ministry of Culture and Communication) and the team Histoire des pouvoirs, savoirs et sociétés of Université Paris 8, has shown, for the first time that iron was used to reinforce stone from the construction phase.

This study will be published in the January 2015 issue of the Journal of Archaeological Science. This innovative method could improve understanding of medieval buildings in Europe, such as the Sainte-Chapelle, as well as in Asia, such as the temples of Angkor.

Steel tie-rods between the flying buttresses of Beauvais cathedral. © P. Dillmann - CNRS
​Steel tie-rods between the flying buttresses of Beauvais cathedral.
© P. Dillmann - CNRS

Gothic architecture, which flourished from the middle of the twelfth century around Paris, included considerable amounts of iron or steel reinforcements, as shown by historical and archeological investigation. If certain architectural and technological evidence suggested that the metal was part of the initial design, the date of its assimilation was still debated. By combining their expertise (in archeology, history, materials science, chemistry), an interdisciplinary team proven that the metal reinforc​ements were integrated as a supplement to stone from the initial design phase.

Researchers achieved this result by measuring the quantity of trace amounts of 14C in the metal. In Europe, up until the Middle Ages, ore was smelt into metal in furnaces using charcoal, some of whose carbon was released and entrapped in the metal (in the form of iron carbide flakes).

This carbon can now be extracted from the metal, and the tree that provided the charcoal can be dated, thus making it possible to estimate the age of the metal. The method seems simple, but it had never before been reliably implemented, for ferrous archeological metals are highly complex materials, containing carbon from multiple sources. A carbon extraction method adapted to the material had to be developed with the Laboratoire de mesure du carbone 14

What also made this study a success was the expertise of metallographers from the Laboratoire archéomatériaux et prévision de l'altération, who in collaboration with archeologist and historian colleagues at the CNRS, have been studying the structure, manufacture and use of metals in Gothic cathedrals for the past ten years.

By cross-referencing radiocarbon dating with archeological evidence, the research team established a detailed chronology (with a margin of error of a few years) of the integration of metal elements in the cathedrals of Beauvais and Bourges. This absolute dating method opens the way toward a renewed understanding of medieval building yards. The research team will soon take samples from the Sainte-Chapelle, and is also focusing on the dating of temples and the iron trade in the Khmer Empire.



Consolidation or initial design ? Radiocarbon dating of ancient iron alloys sheds light on the reinforcements of French gothic cathedrals, Stéphanie Leroy, Maxime L'Héritier, Emmanuelle Delqué-Kolic, Jean-Pascal Dumoulin, Christophe Moreau, Philippe Dillmann. Journal of Archaeological Science vol. 53 (January 2015), pp. 190-201. DOI: 10.1016/j.jas.2014.10.016

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