Garcia Vuelta et al 2012 SEM Congress

Preliminary archaeometric study of the metallic grave goods from a rich late Roman burial at Torrejón de Velasco (Madrid, Spain) Oscar García-Vuelta, Alicia Perea, Fabián Cuesta, Marc Gener, Ignacio Montero-Ruiz, Mercedes Murillo and Martina Renzi KEYWORDS archaeometallurgy, Late Roman jewellery, grave goods, Iberian peninsula, SEM-EDX Introduction Grave goods studied The site of Camino de Seseña (Torrejón de Velasco, 26 km south of Madrid) was excavated between September 2006 and February 2008.1 An Imperial Roman villa was documented as well as other structures, among which stands out an isolated Late Roman tomb ( fourth-fifth century AD), which is unique in the Iberian Peninsula in terms of its characteristics and funerary remains. The tomb consists of a rectangular pit (2.4 × 0.8 m, 1.5 m deep) without human remains. It contained a rich set of grave goods that included weapons, ritual and sumptuous objects of different materials (pottery, glass, bone and metal). This paper presents the preliminary study of the non-ferrous metallic materials using scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) carried out at the Laboratorio de Microscopía Electrónica y Microanálisis (Microlab) at the Centro de Ciencias Humanas y Sociales, Consejo Superior de Investigaciones Científicas (CCHS-CSIC) in Madrid. This study is part of an ongoing wider research project. The grave goods were found in two locations: at the bottom of the pit and in a lateral niche in the east wall of the tomb. The following objects were recovered from the bottom of the pit: a situla; two belt fittings with rivets; a brazier of copper-based alloy; three silver buckles and one in gold; and a possible ivory ring amulet (Figure 1). In the niche were two silver objects (a spoon and a bowl). Some types of objects, such as the buckles and the silver spoon are not commonly found in Roman Iberia and it is the paucity of archaeometric studies for this kind of material in the Iberian Peninsula that prompted this study. Figure 1 1: Silver spoon (TV5). 2: Silver belt buckle (TV9). 3: Gold belt buckle (TV11). 4: Sword amulet/pommel (TV8). (Images: O.G-Vuelta.) Methods The equipment used in this study was a variable pressure scanning electron microscope (VP-SEM) (Hitachi 3400N-II), provided with a secondary electron (SE) and a backscattered electron (BSE) detector. The EDX analysis was carried out Figure 2 Sword amulet/pommel TV8. Micrograph of the bone/ivory body and the analysed central silver ring (polished). SEM-SE image, 15 kV, X10. Right: object detail. (Image: O.G-Vuelta.) 145 HT-24-Garcia-Vuela-Poster.indd 145 19/07/2012 15:29 O S C A R G A R C Í A - V U E LTA , A L I C I A P E R E A , FA B I Á N C U E S TA , M A R C G E N E R , I G N A C I O M O N T E R O - R U I Z , M E R C E D E S M U R I L L O A N D M A R T I N A R E N Z I Table 1 Analytical results for the tomb objects. (Det: element identiied in the spectrum but not quantiied (< 1%); – : not detected.) Object Inventory number Ag Cu Au Zn Pb Sn Bowl 50393/4 93.3 6.6 Det – – – Spoon 50393/5 92.4 7.5 – – – – Sword pommel 50395/8 79.3 18.6 – 2.1 – – Large buckle (prong) 50395/9 95.6 4.4 – – Det – Large buckle (body) 50395–9 95.3 4.7 – – Det – Buckle (prong) 50395/10 96.3 3.7 – – – – Buckle (body) 50395/10 99 Det Det – – – Buckle (prong) 50395/11 1.5 – 98.5 – – – Buckle (body) 50395/11 1.7 – 98.3 – – – Buckle (body) 50395/15 95.7 4.3 – – – – Buckle (prong) 50395/17 90.3 8.2 – 1.6 – – Situla 50395/4 – 87 – – 7.2 6.1 Brazier 50395/3 – 73 – 21.1 3 2.8 Belt itting 50395/12 – 78.4 – 3.6 9.9 7.2 Belt itting (rivet) 50395/12 – 90.9 – – Det 9.1 Belt itting 50395/13 – 87.3 – 4.3 – 8.3 with a Bruker Quantax 200 Xflash 4010 detector (SDD) and a Bruker SVE III Xflash analytical signal processing unit. The analytical results were processed using Bruker Quantax Sprit v.2.1 software. Once the corrosion layer was mechanically removed from a small area (Figure 2), the objects were placed directly in the SEM chamber. Sampled metallic flakes were used to analyse objects that were too large for the chamber. Examination and analysis of the samples was done at high vacuum, using the SE detector. The results are given in weight % (Table 1), as the average of three analyses for each sample (300× window). Only elements over 1% were quantified. Those elements identified in the spectrum but not quantified are marked as ‘Det’ (< 1%). Figure 3 Plastic deformation working traces. Gold belt buckle TV11 (prong and body). SEM-SE image, 15 kV, X12. Results Copper-based alloys It is worth noting that each object was made using different alloys: a tin-bronze rivet, a leaded bronze situla, a brass brazier with high levels of lead and tin, and belt fittings of mixed alloys (gunmetal and leaded gunmetal). The rivet and the plates of each of the two belt fittings were also made from different alloys. The predominance of leaded bronze and mixed alloys seems to be characteristic of Late Roman metallurgy [1, 2]. Gold and silver alloys Figure 4 Wear traces at the inner face of the silver buckle TV9 body. SEM-SE image, 15 kV, ×10. High purity silver (>95%) is used predominantly in all silver objects, as is usual for Late Roman silver objects in the Iberian Peninsula studied thus far, although silver of less purity is also 146 HT-24-Garcia-Vuela-Poster.indd 146 19/07/2012 15:29 PRELIMINARY ARCHAEOMETRIC STUDY OF THE METALLIC GRAVE GOODS FROM A RICH LATE ROMAN BURIAL AT TORREJÓN DE VELASCO (MADRID, SPAIN) detected [3]. In addition, two items are noteworthy for being made of a silver-brass alloy, confirmed by the presence of zinc. The copper/zinc ratio varies between 84/16 and 90/10. Although silver-brass alloys were known since the second century AD in other areas of the Roman Empire [4], this is the earliest silver-brass alloy item currently identified in the Iberian Peninsula. Low levels of lead and gold have also been detected in some objects. The gold buckle (50395/11) is also notable because of the purity of its metal (>98% Au). Topographic examination of the surface of artefacts with SEM shows tool marks (Figure 3) and use wear (Figure 4) in both gold and silver buckles. Although the research regarding this grave is still in progress, and will include lead isotope analysis, the results obtained to date confirm the exceptional nature of this find, which provides new and important analytical data that augments what is currently available from the research of Late Roman metallurgy in the Iberian Peninsula. Acknowledgements This research was supported by the CONSOLIDER-INGENIO 2010 Program (CSD2007-00058) and the Comunidad Autónoma de Madrid (PADCAM Project S2007/HUM-543). Note 1. he archaeological reports of this excavation are still in progress by the directors, R. Flores and P. Sanabria. All the objects have been deposited at the Museo Arqueológico Regional de la Comunidad de Madrid (Alcalá de Henares, Madrid). 2. Montero Ruíz, I., ‘Estudios sobre metalurgia antigua en la provincia de Toledo: el proyecto Arqueometalurgia de la Península Ibérica’, in II Congreso de Arqueología de la provincia de Toledo. La Mancha occidental y la Mesa de Ocaña, vol. I, Diputación provincial de Toledo, Toledo (2001) 275–301. 3. Rovira, S., Blázquez, Mª L., Ballester, A. and González, F., ‘Estudio metalógráico’, in El disco de Teodosio, ed. M. Almagro Gorbea, Real Academia de la Historia, Madrid (2000) 139–150. 4. Mortimer, C., ‘Early use of brass in silver alloys’, Oxford Journal of Archaeology 5(2) (1986) 233–242. Authors’ addresses • Oscar García-Vuelta; Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain • Alicia Perea, Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain (alicia.perea@cchs.csic.es) • Fabián Cuesta; Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain • Marc Gener, Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain • Ignacio Montero-Ruiz, Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain • Mercedes Murillo, Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain • Martina Renzi, Arqueometal Research Group, Instituto de Historia, Centro de Ciencias Humanas y Sociales (CCHSCSIC), Madrid, Spain References 1. 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