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Early Iron Age gold buttons from South-Western Iberian Peninsula. Identification of a gold metallurgical workshop.

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Early Iron Age gold buttons from South-Western Iberian Peninsula. Identification of a gold metallurgical workshop.
  Early Iron Age gold buttons from South-Western IberianPeninsula. Identification of a gold metallurgical workshop  Estudio de botones de oro de la primera Edad del Hierro del Sudoeste de la Península Ibérica. Identificación de un taller metalúrgico de oro António M. Monge Soares  (*) Pedro Valério  (*) Rui J. C. Silva  (**) Luis Cerqueira Alves  (***) Maria de Fátima Araújo  (*) ABSTRACT Early Iron Age gold buttons from Castro dosRatinhos, Fortios and Outeiro da Cabeça were analysed by conventional EDXRF, Micro-PIXE, SEM-EDS andOptical Microscopy. EDXRF results point out to a rather homogeneous alloy composition throughout all the ana-lysed buttons. PIXE microanalyses show that all the but-ton components (disk, tab and peripheral grooved deco-rated rod) have the same alloy composition. PIXE andSEM-EDS microanalyses, supplemented with optical mi-croscopy characterization, show the absence of chemicalcomposition differences between distinct componentsand joining zones, suggesting that no solder had been ap- plied, i.e. that a partial melting/solid state diffusion pro-cess had been used for the welding of button components.Finally, the noticeable similar compositions together withthe use of the same welding process and the very similar artefact typologies suggest that those small gold treasurescould be interpreted as the result of the work of a singlemetallurgical workshop, probably located somewhere inthe South-Western Iberian Peninsula. RESUMEN  Botones de oro pertenecientes a la primera Edad del  Hierro, procedentes de Castro dos Ratinhos, Fortios eOuteiro da Cabeça (Portugal), fueron analizados por  EDXRF y Micro-PIXE. Los resultados de los análisis por  EDXRF mostraron una composición similar en todos losbotones, independientemente de su procedencia. Por otra parte, los microanálisis por PIXE permitieron verificar que los componentes soldados de cada botón (disco, pre- silla y cordón exterior) tienen la misma composición quí-mica. Además de eso, las áreas de soldadura fueron estu-diadas mediante Micro-PIXE, SEM-EDS y posterior análisis metalográfico por microscopia óptica de refle- xión. Estos análisis permitieron comprobar la ausenciade soldaduras en las zonas de unión de estos componen-tes, lo que nos permite concluir que debe haber tenido lu- gar un proceso de fusión parcial y de difusión en estado sólido para unir los componentes de estos botones. La gran semejanza en la composición química, junto a la presencia del mismo tipo de soldadura y tipologías simi-lares, nos sugiere que todas estas piezas fueron resultadodel trabajo de un mismo artesano-joyero, cuyo taller seencontraría localizado en el sudoeste de la Península Ibérica. Key words:  Gold alloy; Early Iron Age; South-Wes-tern Iberian Peninsula; Welding; Metallurgical work-shop; EDXRF; Micro-PIXE; SEM-EDS; Optical Mi-croscopy. Palabras clave:  Oro; Primera Edad del Hierro; Sud-oeste de la Península Ibérica; Soldadura; Taller de meta-lurgia; EDXRF; Micro-PIXE; SEM-EDS; MicroscopíaÓptica de Reflexión. TRABAJOS DE PREHISTORIA 67, N.º 2, julio-diciembre 2010, pp. 501-510, ISSN: 0082-5638doi: 10.3989/tp.2010.10053 (*) Environmental and Analytical Chemistry, Instituto Tec-nológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém,Portugal. Correos electrónicos:;;**) CENIMAT/I3N, Campus da FCT/UNL, 2829-516Monte da Caparica, Portugal. E-mail:***) Ion Beam Laboratory, Instituto Tecnológico e Nu-clear, Estrada Nacional 10, 2686-953 Sacavém, Portugal. Co-rreo electrónico: lcalves@itn.ptRecibido: 19-II-2010; aceptado: 26-IV-2010.  INTRODUCTION A set of seven gold buttons were recoveredduring recent archaeological excavations at Cas-tro dos Ratinhos (Fig. 1). This archaeological siteis an important proto-historic fortified settlementat the Guadiana Valley, located close to the Al-queva Dam. Since 2004, archaeological excava-tions revealed the presence and monumentality of a defensive system with several wall lines, slopesand a deep ditch (Silva and Berrocal-Rangel2005). The archaeological record has also showntwo main occupation phases: an older one datedfrom the Late Bronze Age, which is followed byan Early Iron Age occupation that ends still dur-ing the 8th century BC. The ruins of a large pal-ace-like building that stands up at the highest place of the settlement, a true acropolis, was un-covered during the 2006 field campaign. Theseven small gold buttons were recovered in that building during archaeological digging. The ar-chaeological record suggests that this gold trea-sure was hidden in some moment during the 8thcentury BC, when the large building in the acrop-olis was abandoned, probably due to violentevents. Close to the gold buttons a woven fabricimprint in a bit of clay was discovered suggestingthat the jewels were sewn to the tissue using thesmall tab in the reverse of each button (Berro-cal-Rangel and Silva 2007).Two other sets of gold buttons very similar from the typological point of view to those fromCastro dos Ratinhos were found some years agoat Fortios and Outeiro da Cabeça (Fig. 1) withoutany known or recorded archaeological context.In the early seventies of last century, sometens of gold buttons were found inside a ceramic pot during the ploughing of a land not far fromFortios (Portalegre). Some of them were acquired by the National Archaeological Museum (MNA),at Lisbon, while others enter private collections.In a preliminary study of eleven of these gold buttons, where their typology is described, thissmall gold treasure was wrongly ascribed to theChalcolithic Period (Ferreira 1974).At Outeiro da Cabeça (Torres Vedras), duringthe thirties of last century, in an arable land at a place called Casal das Passadeiras, other tens of gold buttons together with some gold jewels be-longing to a necklace plus ear-rings, bracelets andalso two possible ingots were discovered. Piecesof charcoal seen in the spot where the treasure wasrecovered led to the hypothesis that a goldsmithworkshop would be located at that place. Never-theless, no other kinds of remains were found andthe charcoal could be the result of a more recentforest fire. Due to the jewels typology the treasurewas ascribed to the Early Iron Age (Heleno 1935).Some of the buttons and also some of the other  jewels, with the exception of the bracelets whichfate is unknown, were acquired by the MNA andthe Torres Vedras Museum.Several analytical techniques, namely EnergyDispersive X-Ray Fluorescence spectrometry(EDXRF), Micro-Particle Induced X-Ray Emis-sion spectrometry (Micro-PIXE), Scanning Elec-tron Microscopy (SEM-EDS), and Optical Mi-croscopy (OM), were used to determine the alloycomposition and to investigate the welding pro-cess applied to join the button components. BUTTONS TYPOLOGY The gold buttons have remarkable similar typologies (Fig. 2). Each one is formed by a thin T. P., 67, N.º 2, julio-diciembre 2010, pp. 501-510, ISSN: 0082-5638doi: 10.3989/tp.2010.10053 502  António M. Monge Soares, Pedro Valério, Rui J. C. Silva, Luis Cerqueira Alves y Maria de Fátima Araújo Fig. 1. Location of the 3 treasure finds in the South-Wes-tern Iberian Peninsula.  flat circular body – a disk – with central sphericaldecoration in relief enclosed by several engravedcircles. On the obverse face of the button agrooved decorated rod is welded to the disk pe-riphery. The reverse of each button exhibits oneor two tabs that are made up by a small fragmentof the decorated rod that had been flattened,sometimes so extensively that removed all tracesof decoration. A simple visual examination of the buttons allows us to identify that the peripheraldecorated rod and the tab or tabs were welded onthe main disk.The dimension of the central decoration andnumber of circles can be used to subdivide thecollection of buttons from Castro dos Ratinhos(Ø    10 mm; wt    0.4 g) in two groups – buttonswith small central decoration and additional en-graved circles and buttons with larger central dec-oration and fewer circles. The same occurs for Outeiro da Cabeça. These jewels have a higher diameter (Ø    16 mm; wt    0.8 g) than those fromCastro dos Ratinhos and one or two of the circlesare formed by small points punched in relief in-stead of continuous smooth circles. The buttonsfrom Fortios exhibit the largest diameter (Ø    20mm; wt    1.4 g) and a decoration that only differsfrom the ones of Castro dos Ratinhos in thehigher number of concentric circles. METHODOLOGY AND RESULTS The study of diverse archaeological finds,such as it is the case of these gold buttons, mustrely mainly on non invasive techniques due totheir exceptional significance or unique charac-teristics and rarity. EDXRF and Micro-PIXE arenon invasive, multielemental and relatively fasttechniques, which have been frequently used inthe chemical characterization of archaeologicalmetallic materials, namely gold artefacts and sol-der alloys (Demortier 1991; Demortier   et al. 1999; Alves  et al.  2002; Bugoi  et al.  2003; Soares et al.  2004). Nevertheless, sometimes it is not possible to solve research problems only with anon invasive approach. It is the case concerningthe identification of the welding process used inthe joining of the buttons components. PIXE andSEM-EDS microanalyses, supplemented bymetallographic analyses carried out by OM, on avery small bit cut from a gold button alreadyfragmented allowed identifying safely the weld-ing process used. Alloy CompositionEDXRF EDXRF analyses were made in a Kevex 771spectrometer equipped with a rhodium X-raytube as the primary excitation source. Each arte-fact was irradiated during 300 s on the obverseand reverse faces, by the characteristic X-rays of a gadolinium secondary target (57 kV, tube vol-tage, and 2.0 mA, current intensity) and a silver secondary target (35 kV and 0.5 mA). The cha-racteristic X-rays emitted by the elements presentin the gold alloy were collimated at 90   and mea-sured in a Si(Li) detector with a resolution of 175eV at 5.9 keV (Mn-K   ).The quantification procedure uses experimen-tal calibration factors and relies on fundamental parameters to account for matrix effects (Tertianand Claisse 1982). The experimental factors werecalculated with the analysis of standard materials T. P., 67, N.º 2, julio-diciembre 2010, pp. 501-510, ISSN: 0082-5638doi: 10.3989/tp.2010.10053 Early Iron Age gold buttons from South-Western Iberian Peninsula. Identification of a gold...  503 Fig. 2. Buttons typology: a - Castro dos Ratinhos; b - Outeiro da Cabeça; c - Fortios.   – gold alloyed with silver (9.85 %) and gold al-loyed with copper (9.98 %). The accuracy of theEDXRF elemental quantification, calculatedthrough the analysis of gold-silver and gold-cop- per alloys, was found to be better than 1 % for gold, 5 % for silver and 10 % for copper (Araújo et al.  1993).The obverse and reverse faces of each buttonwere analyzed in order to identify eventualcompositional deviations due to different artefactgeometries (e.g. convex and concave surfaces),different components (main body, decorated rodand tab) and welded areas (connection of main body/rod and main body/tab).The elemental composition determined byEDXRF of the gold buttons from Castro dosRatinhos were already published (Valério  et al., 2008). Obverse and reverse faces of each button present almost identical gold, silver and copper contents, which also occurs with the buttons fromOuteiro da Cabeça and Fortios. The elementalcomposition of gold alloys is given as the averageof the obverse and reverse button results (Tab. 1).As at Outeiro da Cabeça the treasure find wascomposed not only by gold buttons but also byother jewels, namely a necklace, two ear-ringsand two possible ingots, these ingots (Fig. 3) andear-rings plus some of the necklace components(Fig. 4) were also analyzed by EDXRF in twodifferent areas. The average results are presentedin table 2. Micro-PIXE Due to the diameter of the incident beam, eachEDXRF analysis corresponds to the entire face of the button, compositional variations were further investigated using micro analyses. Micro-PIXEanalyses were performed with an Oxford Micro- beams type set-up, using a 2 MeV proton beamgenerated by a 2.5 MV Van de Graaf accelerator.The X-rays emitted by sample elements were col-lected by an 80 mm 2 Si(Li) detector placed at a backward angle of 45   and with 150 eV of resolu-tion. Beam currents of 100 pA were used for allspectra and the beam spatial resolution was kept T. P., 67, N.º 2, julio-diciembre 2010, pp. 501-510, ISSN: 0082-5638doi: 10.3989/tp.2010.10053 504  António M. Monge Soares, Pedro Valério, Rui J. C. Silva, Luis Cerqueira Alves y Maria de Fátima Araújo Button Ref. Au (%) * Ag (%) * Cu (%) * Castro dos Ratinhos CRAT06.1 83.2 15.0 1.7CRAT06.2 84.0 14.6 1.4CRAT06.3 83.2 15.1 1.7CRAT06.4 83.3 15.0 1.7CRAT06.5 83.4 14.8 1.8CRAT06.6 85.3 13.0 1.7CRAT06.7 82.8 15.5 1.8 Outeiro da Cabeça Au197 86.8 11.7 1.5Au198 84.6 13.7 1.8Au199 87.7 11.1 1.2Au200 87.2 12.1 0.7Au201 87.8 11.3 0.8Au202 85.8 12.9 1.4Au203 87.3 10.9 1.9Au204 84.6 13.7 1.7Au205 87.2 12.0 0.8Au593 86.2 12.3 1.41707 85.0 12.9 2.11711 86.7 12.4 0.91712 84.9 13.2 1.81713 88.0 11.0 1.01714 85.6 11.5 2.81715 86.1 12.8 1.01716 85.9 13.0 1.01717 86.7 12.6 0.71718 84.8 13.2 1.91719 87.4 11.2 1.2A 87.3 11.7 0.9B 86.5 12.7 0.8C 86.2 12.0 1.8 Fortios Au235 85.0 13.7 1.2Au237 84.9 13.8 1.3Au248 85.5 13.0 1.4Au252 85.5 13.1 1.3Au253 85.6 13.0 1.4Table 1. EDXRF results of gold buttons.* Mean value taking into account the obverse and thereverse button composition.Fig. 3. Ingots from Outeiro da Cabeça.  at 2-3   m. The data acquisition and beam scanwere controlled by the OMDAQ system andspectra analysis done with the GUPIX computer code (Alves  et al.  2000).Obverse and reverse faces of the buttons werescanned over several selected areas (2640    2640  m 2 ) to obtain the elemental distributions maps.Some points in these areas were chosen for quan-titative analysis. Particular attention was paid onthe reverse face of some buttons since from thereit would be possible to investigate possiblecompositional differences between disk, tab, pe-ripheral rod and welded areas.Results obtained from the gold buttons fromCastro dos Ratinhos are presented in table 3. Itmust be noted that as mentioned before tabs weremade from flattened fragments of the rods thatwere latter welded to disk periphery. Welding Process Micro-PIXE analyses of buttons welded areasdid not identify any significant compositional dif-ferences that could indicate the presence of a sol-der alloy (Fig. 5). Nevertheless, if the solder material is re-stricted to a small area between two attachedcomponents it could be possible that the solder material would not be reachable by this non inva-sive approach. As one of the buttons is reduced toa fragment (Fig. 6a,b), a small representativefragment from a welded area (disk/rod) was sam- pled. The fragment was mounted in epoxy resin, polished with SiC papers (P1000, P2500 andP4000) and finished with 1   m and 1/4   m dia-mond pastes. The mounted sample was etchedwith aqua regia and observed in a Leica DMI5000M optical microscope under bright field.The optical microscopy images of this cross sec-tion clearly identify both button components (rod T. P., 67, N.º 2, julio-diciembre 2010, pp. 501-510, ISSN: 0082-5638doi: 10.3989/tp.2010.10053 Early Iron Age gold buttons from South-Western Iberian Peninsula. Identification of a gold...  505 Fig. 4. Ear-rings (1 and 2) and necklace components (3 to6) from Outeiro da Cabeça. Jewel * Au(%)Ag(%)Cu(%) Ear-ring («Xorca») 1 83,0 14,0 2,86Ear-ring («Xorca») 2 87,6 11,1 1,20Hollow cylinder 3 85,0 12,2 2,80Ring 4 78,9 18,8 2,36Ring 6 78,7 19,4 1,90Coiled wire 5 80,7 18,2 1,11Ingot a 83,8 15,1 1,09Ingot Au206 89,4 10,2 0,37Table 2. EDXRF results of ingots and necklace compo-nents from Outeiro da Cabeça.* See Figs. 3 and 4. Button Com-ponent Au (%) Ag (%) Cu (%) CRAT06.1 disk 89.0 8.1 2.7tab 86.6 11.4 1.9CRAT06.2disk 89.3 9.4 1.3rod 89.4 9.2 1.3tab 90.0 8.5 1.4tab 88.2 10.0 1.7CRAT06.3 disk 88.3 9.8 1.6tab 89.8 8.8 1.3CRAT06.4 disk 87.0 11.0 1.8tab 86.5 9.7 3.5CRAT06.5 disk 89.4 9.1 1.4tab 88.4 9.1 2.3CRAT06.6 disk 88.0 9.4 2.0tab 89.0 8.2 2.6CRAT06.7 disk 86.1 12.1 1.5tab 87.0 11.2 1.6Table 3. Micro-PIXE results of gold buttons from Castrodos Ratinhos.
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