Micro-Raman investigation of terra sigillata slips: a comparative study of central Italian and southern Gaul productions

Terra sigillata is certainly the most famous fine ware of the Roman period, and for this reason it has been a subject of interest for both the archeology and archeometry communities. X-ray diffraction studies showed systematic differences in the mineral compositions between central Italian and south Gaul productions. All the slips contain hematite; the Gallic ones embody also a significant content of corundum, while the Italian productions contain a great proportion of spinel. Raman investigation fails to reveal directly these differences because the spectra are dominated by the hematite peaks. However, we originally demonstrate that, due to the occurrence of an upshift of bands, the Raman spectrum of these sigillata slips can be associated with a substituted hematite. This premise has been confirmed by comparing the crystallization of hematite in four annealed clays to that of the ancient slips. The results suggested that the presence of the strong additional Raman band at around 680 cm(-1) present in both clays and ancient slips could be associated with the recrystallization of hematite, occurring above 750 degrees C. It could also be due to the magnetite content, but this hypothesis is not supported by X-ray diffraction data. In addition, the reduction seems to take place only above 1050 degrees C, as suggested by the unveiling of hercynite as well as the darkening of the clay samples. Raman scattering is shown to be very sensitive to small variations in clays composition and to firing temperature, and it has been successfully used as an in situ sensitive probe for discriminating between Italian and south Gallic productions. Copyright (C) 2010 John Wiley & Sons, Ltd.