Non-destructive magnetic and chemical characterization of granite column shafts traded in the Mediterranean area: the case of Piazza dei Miracoli in Pisa (Italy) and Basilica of Saint-Martin d’Ainay in Lyon (France).

Abstract

We present a scheme for non-destructive provenancing of granite shafts dating from the Roman to the medieval period using a combination of visual, magnetic, and chemical determinations. Our results on two monumental medieval complexes in Europe, in Pisa and Lyon, shows both oriental provenance, most likely spolia, and shaft from the quarries within the influence zone of the Pisa Republic (Elba, Corsica and Sardinia) that possibly correspond to shafts newly obtained in the quarries, particularly for the large diameter shafts of Ainay in Lyon (from Corsica) and Piazza dei Miracoli (from Elba and Sardinia). pXRF appears to be an efficient way to discriminate Corsican from Egyptian grey coarse grained granite provenance.

Systematic sourcing of granite shafts from Gallia Narbonensis and comparison with other western Mediterranean areas

“…Our definitive identification of shafts from Corsica in Die, using pXRF to solve the ambiguity with the Claudianus source, may help document the previously overlooked diffusion of this source outside Corsica-Sardinia (but see Clerbois et al, 2022). While Williams-Thorpe (2008) reported this source only in Rome, and a single shaft in Arezzo, our discovery of Corsican shafts in Die, as well as in Lyon and Pisa in another study (Rochette et al, 2021), suggests that this source was more widely exported than previously thought.…”

“…Long-distance transport of spolia or newly quarried shafts may have occurred after the Roman period. Further study is required, particularly on shafts used or reused in medieval constructions, as reported in the case of Romanesque churches in Pisa and Lyon (Rochette et al, 2021). This systematic study should also be extended to the rest of Gaul, as well as the entire western Mediterranean, where the published granite shaft corpus is rather incomplete.…”

“…When interpreting our corpus in terms of Roman architecture in Narbonensis, it is also important to bear in mind that shafts used in medieval monumental works may not have been spolia but extracted at the time from granite quarries. Medieval granite shaft production is demonstrated in Elba (Clerbois et al, 2022;Zecchini, 2018), and the numerous shafts observed in Pisa that measure over 1 m in diameter and were sourced from Elba were identified as 'likely" Pisan by Williams-Thorpe (2008; see also Rochette et al, 2021).…”