Evolution of terra sigillata technology from Italy to Gaul through a multi-technique approach

To understand how the terra sigillata technology was transferred from Italy to Gaul, a large corpus of samples from various worskshops was studied by using standard laboratory techniques (Raman spectroscopy, SEM, electron microprobe analysis and colorimetry). Based on these results, a few representative samples were selected and investigated by synchrotron radiation at ALS on the 12.3.2 microdiffraction beamline. The beamline is very adapted to the crystallographic study of thin layers and allows us to determine precisely the mineral composition of the sigillata slips. The set of findings revealed significant differences between the Italic and Gallic slips, which suggest a modification in the manufacturing process. The transfer of the sigillata technique to Gaul is associated with an increase of the firing temperatures due to the use of more refractory clay. This change has no significant influence on their esthetic appearance (brilliance and color) but modify their mechanical properties, resulting in a stronger resistance for the Gallic slips. The implication of these results in archeological context is discussed. In particular, we will try to propose assumptions on the reason for the change in the manufacturing process.