06-2013, tome 110, 1, p. 105-119 - B. ARANDA, C. LE CARLIER DE VESLUD,  C. MARCIGNY, J.-C. LE BANNIER - Le dépôt de haches à douille de type armoricain  du Hallstatt D1-D2 de Trelly (Manche) Interprétations à partir des analyses chimiques élémentaires.

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06-2013, tome 110, 1, p. 105-119 - B. ARANDA, C. LE CARLIER DE VESLUD, C. MARCIGNY, J.-C. LE BANNIER - Le dépôt de haches à douille de type armoricain du Hallstatt D1-D2 de Trelly (Manche) Interprétations à partir des analyses chimiques élémentaires.

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Résumé : En 1998, le dépôt de Trelly (Manche) a été découvert à la suite de prospections électromagnétiques. Une rapide opération de sauvetage réalisée à la même époque a permis de constater la présence d'autres objets, la plupart remués par des labours récents. L'intérêt principal de ce petit dépôt, d'une trentaine d'objets à base cuivre, est l'association entre des haches à douille de type armoricain (types de Plurien, de Tréhou et surtout de Couville) et des bracelets typiques du Hallstatt D1-D2. Si le dépôt était bien connu grâce à la publication de quelques objets lors de l'exposition « Nos ancêtres les Gaulois » présentée à Nantes en 1999 à l'occasion du XXIIe colloque de l'AFEAF, aucune analyse n'en avait été menée. Dans le cadre d???un nouveau programme de recherche mené à l'université de Rennes 1-Beaulieu (UMR « CReAAH ») concernant les objets à base cuivre protohistoriques et leurs compositions élémentaires, des analyses chimiques ont été réalisées sur la plupart des objets du dépôt. Les résultats obtenus montrent que quelle que soit la taille des haches les teneurs en plomb et en cuivre sont variables et souvent très fortes. Les teneurs en étain sont faibles mais assez constantes. En réalisant deux analyses par hache, on démontre que cette variabilité de composition se retrouve au sein même des objets, conduisant à dire que les différences observées entre individus ne sont pas significatives. Cependant, la dispersion des compositions au sein de l'ensemble des objets du dépôt est continue, ne montrant pas de rupture, laissant penser que ces objets ont pu être fabriqués selon un même procédé technique. La comparaison entre les différents types d'objets montre que les bracelets sont seulement légèrement enrichis en étain. Les teneurs en éléments-traces pour l'ensemble des objets sont quant à elles très similaires, montrant ainsi l'homogénéité du dépôt. Ainsi, les analyses, en plus des travaux de terrain et de l'observation de la patine des objets, confirment une similitude de composition chimique. En conclusion, si elles ne confirment pas formellement le synchronisme entre les haches à douille et les bracelets du Hallstatt moyen/final, du moins apportent-elles un argument supplémentaire dans ce sens, et surtout elles ne le contredisent pas. Pour réellement confirmer cela, il faut continuer à analyser entièrement d'autres dépôts de la même région, en prenant aussi en compte des dépôts du Bronze final III.

 

Abstract: Discovered in 1998, the Trelly hoard (Manche, France) was discovered through electromagnetic surveys. A quick emergency dig revealed the presence of many artefacts, most of which had been displaced by recent ploughing. The main interest of this small hoard, consisting of about thirty copper-based artefacts, resides in an association of Armorican-type socketed axes with bracelets typical of the Hallstatt D1-D2 period. While this hoard was well-known due to the presentation of several artefacts at the exhibition "Nos ancêtres les Gaulois" held at Nantes in 1999, no analyses had been carried out and the artefacts remained unexploited. In the framework of a new research programme concerning prehistoric copper artefacts and their elementary composition, most of the artefacts from this hoard were the subject of chemical analyses. Many large hoards discovered in Brittany, Normandy and the Channel Islands contained only Armorican-type socketed axes. These deposits were said to belong to the final stage of the Bronze Age (Briard, 1965; Rivallain, 1971). New archaeological excavations in Brittany and in Normandy, such as Kergariou, Quimper, Finistère (Menez et al., 2005; Menez & Gomez de Soto, 2006; Gomez de Soto et al., 2009) confirm that this original phenomenon dates from the second period of the First Iron Age. The Trelly hoard also belongs to this period, because of the synchronism of Armorican-type socketed axes and bracelets dated from the middle???/???late Hallstatt period. This hoard is composed of socketed axes (one of the Tréhou type, one Plurien and eighteen of the Couville type, and two fragments of large axes), five bracelets and fragments of bracelets (one open bracelet decorated with transverse incisions, one open massive bracelet with a sub-rectangular section resembling a gear-wheel, a fragment of an open and massive boss bracelet with a plano-convex section, a fragment of a massive bracelet with a plano-convex section decorated with four transverse incisions, a closed and massive bracelet with a circular section), a fragment of a shank with a rectangular section, a fragment of an artefact made of grey metal composed of a shank with a sub-rectangular section ending in a conic boss, a circular ring with a ???bezel??? composed of two plates with joined central cupulae. The main artefacts of the Trelly hoard have been analysed to check their homogeneity. Similarly, the homogeneity of the artefacts??? composition was controlled by performing duplicate analyses whenever possible. Each sample consisted of 40 mg of metal. The samples were dissolved in 20 ml of 2/5 aqua regia in order to analyse them by inductively coupled plasma atomic emission spectrometry (Jobin-Yvon 138 Ultrace ICP-AES). The concentration of the metallic solution was 2 g.L-1. In order to determine the concentration of each metal studied by ICP-AES, a calibration range was defined between 0 and 2 g.L-1. Standards of the main elements (Cu, Pb, Sn) were prepared twice in order to have a calibration range close to the concentration of these elements in the samples (table 2). This procedure greatly improved the quality of the analyses. Five multi-element standards were prepared to analyse trace elements (As, Mn, Bi, Zn, Ag, Sb, Ni, Fe, Co; table 3). Calculated uncertainties are due to experimental errors when samples were weighed and dissolved. The values give an error of 1.6???% for tin and lead, and an error of 2.2???% for copper and trace elements. The analyses show that Trelly socketed axes have a high lead content, similar to that of Armorican-type socketed axes previously analysed. However, the Trelly socketed axes contained only a small amount of tin (table 4). The amounts of lead and copper present strong variations: between 42.5???% and 93.6???% for copper, and 4.4???% and 56.2???% for lead. However, most of the analyses show a copper content of between 55% and 80% and that for lead between 17% and 42???%. Tin contents are lower, ranging from 0.3 to 9???%, but most of the analyses lie between 0.3???% and 4???%. Trace element contents (As, Sb, Bi, Zn, Ag) are high, whereas nickel and cobalt contents are very low (table 5). On each axe, two analyses were systematically carried out in two locations: on the opening of the socket and on the cutting edge. These analyses revealed an important variability within each axe (fig. 5). These variations mainly concern copper and lead contents. The maximum differences reach 24???% for lead, 23???% for copper and 1???% for tin between the cutting edge and the socket opening. Variability between all the axes is less significant for the copper and lead contents than for that of tin. Differences in trace elements between the cutting edge and the socket of a given axe are clear (fig. 6). For the major elements (Pb, Cu, Sn; fig. 7), the boss bracelet, ???gear-wheel??? bracelet and ring contain little lead, unlike the main Trelly axes. The tin/copper ratio is not equal between the three artefacts and the axes because of a slight enrichment of tin in the bracelets and the ring. Nevertheless, it is difficult to distinguish the compositions of these three artefacts from those of the other artefacts in the hoard. Indeed, two Couville-type axes have the same chemical composition as the bracelets and the ring. The bracelets decorated with incisions have the same lead/copper ratio as the other artefacts, but their tin contents are higher than those for the Couville-type axes. The variability of trace elements is more important than that of the major elements. However, their mix is homogeneous between all the artefacts in the hoard. It could thus be seen as a signature of the hoard, if we make the assumption that these artefacts were made from the same copper, lead and tin raw materials. A significant difference of composition between the bracelets and the axes could indicate that these artefacts were not made from the same raw materials. Alternatively, another possible hypothesis is that the socketed axes could have been made earlier and re-employed during the Hallstatt D1-D2 period. Thus, even if the similarity of the chemical compositions of the Armorican-type socketed axes and those of the Hallstatt D1-D2 bracelets prove these artefacts are strictly contemporaneous, the results do not contradict the first hypothesis. The systematic analysis of several hoards like the Trelly one in the same geographical area will be interesting. It would be possible to see if a local or regional signature exists, suggesting the same metallurgical techniques and the same supply of raw materials. It is interesting to analyse a large quantity of samples in order to see the variability of trace elements. This information is important for investigating the homogeneity of hoards. If a strong heterogeneity is shown, it is possible to envisage the re-employment of metal cast during an earlier period. To confirm this hypothesis, not only should several hoards from the same metal horizon be studied, but also hoards from earlier horizons.