Etude de la diffusion en Gaule d'une technique romaine d'élaboration de Sigillées à travers l'analyse microstructurale des surfaces décoratives (ou engobes)

Abstract : The terra sigillata ceramic is a decorated vessel with standardized forms characteristic of the Roman period and its manufacture was reserved to few specialized workshops. This pottery is recognizable not only by its decorations but by its colour and its brightness, attained by means of a red vitrified slip. This one was obtained by firing under oxidizing conditions an iron rich clay finely settled. This technique appeared in central Italy during the 1st century B.C. (Arezzo, Pisa) and it was spread across the Roman Empire during the Augustan period. From the beginning of our era, big centres of production of sigillata (La Graufesenque, Montans) were set up in the South of the Gaul, developing their own directory of shapes and decorations. Before it, these south-Gallic centres and other workshops with a localized production (Bram) produced in the South of The Gaul a non vitrified slip ceramic, reproducing the ancient forms of the italic sigillata. The microstructural analysis of the slip of these different productions have been performed in a wide range of scales (by means of complementary characterization techniques) brings us relevant information on the appearance context of the first workshops of south-Gallic sigillata. The main chemical reactions occurring in the formation of the slips have been identified thanks to a study of the behaviour in temperature of a selection of permo-triasiques clays (likely source of slips). From all these data, it is now possible to identify the conditions (nature and composition of the clay, the temperature and the atmosphere of firing) leading to specific types of slip microstructure and also to bring to light the resemblances and the characteristics of different productions. The spatial variation of the report Fe2 + / Fe3 + obtained by means of Fe-K Edge XANES in the K-edge of iron, allowed, example, to precisely identify the variations of atmosphere during the annealing, and to distinguish easily the true sigillata from red imitation of sigillata which always preserves the track of their firing in reducing atmosphere. The study of sigillata slip by spectroscopy Raman highlighted the crystalline defects of the hematite (main pigment) depending on conditions of elaboration (nature of the clay and the temperature of firing) and can be thus used as criterion to distinguish the different productions. These results show that the ceramic with non vitrified slip (sigillata imitation) corresponded well to a end product with a more traditional manufacture and in localized production, maybe aiming to fulfil a need connected to the deficit of vessels which existed during this period. The craftsmen adapted the composition of their slip according to kilns traditionally used (firing in reducing atmosphere) with the aim of obtaining a red slip, but have never tried to modify the mode of firing to make of the truth vitrified slip sigillata. The production of sigillata, fired in oxidizing atmosphere, suddenly appears in the years 20 AD, what suggests the Italian specialists’ intervention mastering perfectly this technique. This transfer came along with a modification of the process of elaboration which confers on slip a better mechanical resistance and also improves their optical properties, but requires a higher temperature and thus adding an additional cost. Nevertheless the study of the resources in clays of the various south-Gallic centres tends to demonstrate that it is not about an adaptation to local clays. It would seem that a difference in the preparation of the slip is responsible for this structural modification. The potters of La Graufesenque and Montans credibly adapted new processes (mode of firing) while preserving certain know-how to make native for the preparation of the clay. This modification thus results from a voluntary choice or in any case from a voluntarily preserved particular process, which leads an evolution of the product. This one becomes more functional at the same time that the aesthetics are preserved, maybe in answer to a more marked need in utilitarian vessels in this period. Even though we cannot really speak about a change of status of the sigillata, it is clear that this evolution follows in a parallel way, the variations of political and economic context (empire/Principate) which surrounds both considered productions (italic and south-Gallic).
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Yoanna Leon. Etude de la diffusion en Gaule d'une technique romaine d'élaboration de Sigillées à travers l'analyse microstructurale des surfaces décoratives (ou engobes) . Sciences de l'Homme et Société. Université Toulouse 3 Paul Sabatier (UT3 Paul Sabatier), 2010. Français. ⟨tel-01098597⟩

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