Determining the provenance of black limestone artifacts using petrography, isotopes and EPR techniques: the case of the monument of Bocco.

This paper investigates the provenance of the black limestone of the monument known as Bocco, named
after the king of Mauretania who presented Silla with this work as a sign of his submission to the power
of Rome. A multi-method approach, comprising petrographic observations, carbon and oxygen isotope
composition and electron paramagnetic resonance (EPR) spectra characteristics, was used for this
purpose. The monument is part of a reconstruction of a rectangular base (approx. 8 2 m) which is likely
to have served as the base for a bronze statuary group and is currently composed of six blocks of
limestone whose exterior is engraved with a fine frieze. The results of the different analyses were
compared with the data of a published database of the most important black limestone quarries
exploited during Roman times in the Mediterranean area. The petrographic and physico-chemical
parameters did not prove to be very effective in determining the provenance of the black limestone of
Bocco; the isotopes result even raised some uncertainty as to whether all six blocks came from the same
quarry, though this doubt was dispelled by the EPR data and, above all, by the evident petrographic
similarity between them. Differences in the isotopic composition of the various blocks may be due to an
in situ rock alteration process caused by interaction with an aqueous fluid. It was, however, impossible to
determine with certainty the origin of the material of the stone artifact using the database available; it is
possible that the black limestone of Bocco came from an ancient quarry that is likely to be located in
north western Africa but has not yet been uncovered. An alternative hypothesis, according to which the
limestone used for the monument of Bocco came from the ancient quarry of Ain el Ksir, one of the
Tunisian quarries present in the database, is also discussed.