An archaeological and archaeometrical approach of ferrous semi-product : a diachronic qualitative typology (VIIth c. BC. - IInd c. A.D.)

From the raw bloom to the final product, depending on the nature of the latter, several semiproducts can be obtained along the chaîne opératoire. Some of them are directly transformed on the ironmaking site. Some other, on the contrary are marketed between several workshops, production centres or markets. On relation with the quantity of work to obtain them, or with the quality of the constituting material, they could have different values. For these reasons, understanding the way these semi-products were obtained and in which technico-economical context, but also the way they were comprehended by the ancient craftsmen and blacksmith is a fundamental aspect of ironmaking technical and economical history, whatever the period under interest is. The aim of the general overview presented here is to review, from the archaeological site to the laboratory, the different clues that allow to enlighten several aspects linked to the semi-product production and exchanges. Indeed, even if the archaeometric observations are scientific facts, they can never be considerate out of any archaeological and historical context. Thus, a constant come and go must be undertaken between the physico-chemical data and this context. For example, as far as the metal quality is concerned, added to the fact that numerous parameters will influence the mechanical behaviour of the material, it is important to know that it is not the only aspect that will determine the use of a given material. It is merely a more complex conjunction of its quality, the skill of the blacksmith, the availability of the metals in the technico-economical context under study, etc. Depending on the place in the chaîne opératoire, semi products can be more or less elaborated and of different natures: bloom slightly hammered after a first compaction treatment, ingot obtained after a more important hammering and cleansing, currency and socket bars, with a thinner shape closer to the form of the final artefact, etc. Semi-products can question several aspects of archaeological and historical studies. At the archaeological site level, it is of great interest to understand the spatial and technical organisation of the site, and sometimes of different production sites in the same area and period. It is also important to evaluate the nature of the forging sequences, the type of activities and production and the quantities. Another crucial question is the nature of the metal that is produced and marketed, linked to his mechanical behaviour, and the value it has in the different technicoeconomical contexts. To study this point it is necessary to try to evaluate it by identifying the parameters that mainly influence this quality and trying to define reproducible study methodologies in order to compare the different artefacts. Then, another important point is to study the semi-product diffusion and exchange on more or less long distances. To this purpose, crucial chemical data can be found in the slag inclusions (SI) embedded in the metallic artefact, that offers the possibility to identify homogeneous or heterogeneous corpuses but also, in specific cases, to enlighten the production region of the artefacts. Our research for 10 years concerns an important number of sites and artefacts covering periods from the 1st iron age to medieval times. Following the methodology of former works made in east of France and in the Swiss Jura, in the frame of several PhD works, we significantly completed our pluridisciplinary and diachronic investigations on the French territory for the following areas: Grande Limagne d'Auvergne, Nord du Bassin parisien, Narbonnaise (Languedoc-Roussillon, Provence-Alpes-Côte-d'Azur), Normandy, Pays de Bray, Pyrénées ariégeoises and an important part of central Burgundy. More than 600 ironmaking, sites, tens of historical buildings and several hundred artefacts were studied. These works allowed us to precisely define the internal characteristics of the materials in order to evaluate on the one hand the real nature of the forging activities on given sites, on the other hand the qualitative criteria that could help to identify some specificities and follow exchanges and technical breaks