Archaeometalurgical Analysis and Archaeological Contextualization of a Bronze Spearhead from Gralheira (Muro Mountain Range, Baltar, Paredes, North Portugal)
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Arqueología en el valle del Duero. Del Paleolítico a la Edad Media. 6. ISBN: xxxxxxx, pp: xxxxxxxx ARCHAEOMETALURGICAL ANALYSIS AND ARCHAEOLOGICAL CONTEXTUALIZATION OF A BRONZE SPEARHEAD FROM GRALHEIRA (MURO MOUNTAIN RANGE, BALTAR, PAREDES, NORTH PORTUGAL) Maria Antónia D. Silva1 Fernando Castro2 Ana M. S. Bettencourt3 António Manuel S. P. Silva4 Alexandre Manteiga Brea5 RESUMO Este trabalho visa dar a conhecer o contexto de achado e a composição química de uma ponta de lança de alvado curto e de folha ligeiramente losângica com nervura central, inserível no Bronze Final e inédita. Esta foi encontrada na serra do Muro, freguesia de Baltar, concelho de Paredes, distrito do Porto. A serra do Muro corresponde a um monte com uma implan- tação orográ! ca dominante na região sobre os vales dos rios Ferreira e Sousa, este a" uente da margem norte da bacia do Douro, em área rica em recursos primários e secundários de estanho. No topo deste acidente geomorfológico foi edi! cado um povoado proto-histórico que ! gura na literatura arqueológica especialmente pelo grande perímetro e espessura das suas muralhas pétreas. Apesar da proximi- 1 Gabinete de Arqueologia da Câmara Municipal de Paredes, Portugal ([email protected]). 2 Laboratório de Análises Químicas da TecMinho; Departamento de Engenharia Mecânica da Universidade do Minho, Guimarães, Portugal ([email protected]). 3 Laboratório de Paisagens, Património e Território - Lab2PT; Departamento de História da Universidade do Minho, Braga, Portugal ([email protected]). 4 Bolseiro de Doutoramento da FCT na Univ. de Santiago de Compostela. Centro de Investigação Transdisci- plinar Cultura, Espaço e Memória – CITCEM-Univ. Porto ([email protected]). 5 Laboratório de Paisagens, Património e Território - Lab2PT/Universidade do Minho, Portugal ([email protected]). 210 dade destes dois contextos arqueológicos não há qualquer indicação precisa de que estejam vinculados, podendo esta peça constituir um depósito. Não sendo muito frequente o achado de pontas de lança no NO português, conhecem-se todavia alguns contextos, todos eles correspondentes a depósitos. É o caso da ponta de lança de Badim, de morfologia similar, que apareceu enterrada na base do monte cónico de Nossa Senhora da Graça, em Monção, e desvinculada do castro aí existente, localizado, aliás, em vertente oposta. Outros achados simi- lares associados a montes são os do Outeiro do Rego, em Lama Chã, Montalegre, e o da Quinta do Telhado, no Monte da Penha, em Guimarães, ambos associados a a" oramentos. Apesar das diversas pontas de lança conhecidas no NW português, apenas as duas de Vale Travesso, em Montalegre, foram alvo de análise de composição química (BOTTAINI 2012: 49-52), pelo que os resultados das análises arqueo- metalúrgicas obtidas por espectrometria de " uorescência de raios X da ponta de lança do Muro, contribuirão, certamente, para o conhecimento da metalurgia do Bronze Final do NO. Palavras-chave: NO. de Portugal; Bronze Final; Ponta de lança; Contexto; Com- posição química. 1. THE FIND AND ITS CONTEXT ponds to the “entrance” of the single natural passage between the valleys he artifact was found at the bo- of the rivers Sousa and Ferreira, both T ttom of the eastern slope of the located north and south of the Muro Muro mountain range, in a fallow ! eld mountain range, respectively (Figs. known as Loto or Veloto, in Gralhei- 1 and 2). According to the Geologi- ra, Baltar parish, Paredes municipality, cal Map of Portugal, scale 1:50.000, Porto district. $ e approximate geogra- 9-D (Pena! el), the area is dominated phical coordinates in degrees, minutes by porphyroid coarse-grained granites and seconds, in the WGS84 system with two-micas, but essentially bioti- are: 41° 11’59.65’’N; 8° 22’49.65’’O. tes, commonly found at the surface. WGS84, at an altitude of c. 350 m (see $ ey occur in veins of aplite-pegma- Figs. 1 and 2). tites (MEDEIROS et al., 1980). $ e $ e location of the ! nd constitutes municipality of Paredes is rich in pri- an open platform, of easy access, over- mary and secondary mineral resources, looking the Baltar stream, which " ows including tin, gold and silver, although into the River Sousa, a tributary on the these are not known in the vicinity of right bank of the Douro. It also corres- the ! nd spot. $ ese are found only at 211 X!!!!!!!!!!!! 2 km northwest of the Muro mountain 2. DESCRIPTION OF THE OBJECT range, in the tin mines of Rebordosa, and at 5 km to the south, in the gold $e artifact under study constitutes a and silver mines of “Covas de Castro- short socketed spearhead with a central mil” (DGE; COUTO 1993: !g. 3). midrib, present on both sides. With $ere is no evidence about other ar- an almost conical section, it is solid chaeological sites ascribed to the Bron- between 11.9 cm and 13.7 cm, from ze Age in the limit of this parish. $e which seems to be again hollow. $e only settlement that has been identi!ed base has two circular holes, both sym- to date is the Muro de Vandoma hill- metrical and aligned with the edges, fort, at the top of the Muro mountain designed to secure the spearhead, and range, whose speci!c occupations are the leaf is of slightly losangic shape. It poorly known (AZEVEDO 1898: 194, has been partially broken at the tip and SILVA 1986: 85, SILVA 2000: 100). It in the socket, therefore it’s not possible has been classi!ed as a Monument of to know its exact length. It currently Public Interest6. $e settlement was measures, 14.5 cm in height, 3.6 cm of acknowledged in the archaeological maximum width of the leaf and 2.5 cm literature due to the large perimeter of maximum width of the socket. $e and thickness of its stone walls (SIL- maximum thickness in the socket is 0.3 VA 1963, 1966), together with some cm, and at the tip (broken) is 0.6 cm. other particular features. Despite the Weights 82.5687 gr (Figs. 3 and 4). proximity of the two archaeological It presents a greenish patina resul- contexts, the settlement and the !nd ting of the actual state of corrosion, spot of the spearhead (which occurred however it has been recently under- at about 250 m to the east of the hill- going chemical stabilization, including fort walls), there is no clear indication a volumetric and chromatic reintegra- that both are connected, therefore, the tion process with a pigmented epoxy spearhead possibly constitutes a delibe- resin. It is curated by the family of the rate deposition. $e artefact was found heirs of Mr. José Vicente da Silva7, in in a property owned by José Vicente da Gralheira, Baltar, Paredes. Silva, during the opening of an access road to his thickets, in the late 1950s, however the circumstances of discovery are unknown. 6 Dec. nº 45/93. Diário da República. I Série. 280 (30- 7 Presently it is in the possession of Mr. Engineer 11-1993): 6698-6701. Manuel Cunha. 212 Fig. 1: Approximate location of the !nd in the Military Map of Portugal, esc. 1:25.000, pgs. 111 and 123 (IGE-1998-99). Fig. 2: Approximate area of the !nd spot in an orthophotomap (ETRS89-EN-TM06. CM Paredes). 213 X%%%%%%%%%%%% Fig. 3: Gralheira spearhead. Front and side views (Photos by M. A. Silva). 214 Fig. 4: Gralheira spearhead. (Drawing by Maria Antonia Silva). 215 X!!!!!!!!!!!! 3. CHEMICAL COMPOSITION structure of the alloy through a scanning AND MICROSTRUCTURE electron microscope focused on the inner zone of the fragment, in order to avoid !e determination of the chemical the surface corrosion layer, allowing for composition was made through quan- the identi#cation of the following ele- titative archaeometalurgical analysis ments, as illustrated in Figures 5 and 6: obtained by X-ray "uorescence spec- trometry, in the Chemical Analysis La- Z1 - Cu with 6.9% Sn; boratory of TecMinho (Department of Z2 - Cu2O; Mechanical Engineering, Minho Uni- Z3 - Cu “pure”; versity, Guimaraes) and through meta- Z4 - Cu with 30% Sn and llographic examination in a scanning Z5 - Cu2S electron microscope, in the CEMUP, Materials Center at Porto University. !e results of this analysis allowed !e sample, about 1x1 mm, was re- to consider that the spearhead corres- moved with a mechanical saw, at the ponds to a bronze of biphasic type (a distal end of the spear. !e chemical + į), with a predominance of Į phase. analysis focused on the cutting surfa- Given the tin content at this stage, it ce, using a suitable analytical program seems to us that the cooling after soli- for copper alloys, and calibrated by a di#cation was processed at high speed, set of certi#ed reference materials. For derived, for example, of air cooling. the purpose of observation by scan- !ere have been recognized the di- ning electron microscopy, the cutting vision in zones of the internal a pha- section was mounted in epoxy resin, se, which is normal in these materials followed by polishing of the silicon (SCOTT 1991). It is also evident the carbide with sandpaper, and #nally in presence of internal oxidized zones, diamond suspension grain of 1 mm. and of a smaller amount of sulphides. As can be seen in Table 1, the results !e presence of almost pure copper showed that the artefact was made in (Z3 zone), even if occurring in a rare a binary copper alloy with 88.71% co- form, it seems abnormal to us, althou- pper and 8.6% tin, being this a typical gh possibly motivated by an incomple- composition of the alloys used for the te dissolution between the two metals manufacture of metal objects (SCOTT Cu and Sn.