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Evidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho, Serpa, Portugal).

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Evidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho, Serpa, Portugal).
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  151 REVISTA PORTUGUESA DE Arqueologia.volume 8.número 1.2005,p.151-161  ABSTRACT The Late Bronze Age settlement of Passo Alto includes a defensive complexwith partial vitrification of the wall rampart. The mineralogical assemblage identified in the vitrified rock fragments includes neoformed phases and glass as a result of incongruent melt-ing of the srcinal micas of the local substrate (Palaeozoic micaschist). The temperaturesachieved were very high, probably exceeding 1100ºC. Mineral textures indicate fast coolingof the melt. Burning of a structure made of timber and rock fragments from the local sub-strate affected a small portion of the rampart adjacent to the probable main entrance, and isconsidered the most simple and coherent hypothesis to explain all the observed evidence. RESUMO O povoado do Bronze Final do Passo Alto possui um complexo defensivo, no qual sepode observar uma vitrificação parcial da muralha. Os minerais identificados nos fragmentosde rocha, que sofreram vitrificação, incluem vidros e fases de neoformação resultado de uma fusão incongruente das micas que fazem parte do substrato pétreo local (um micaxisto doPaleozóico). As temperaturas atingidas terão sido bastante altas, excedendo provavelmenteos 1100ºC. As texturas minerais indicam, por outro lado, um arrefecimento rápido do mate-rial fundido. A queima de uma estrutura feita de madeira e de fragmentos de rocha do subs-trato local, que constituiria a parte superior da muralha, terá afectado apenas uma pequena porção desta, na área adjacente à provável entrada principal do povoado. Esta hipótese deconstituição da muralha e do que lhe terá acontecido pode considerar-se como a explicaçãomais simples e coerente para a evidência arqueológica observada na área do complexo defen-sivo do Passo Alto. Evidence for wall vitrification at the Late Bronze Age settlementof Passo Alto (Vila Verde de Ficalho,Serpa,Portugal) ENRIQUE DÍAZ-MARTÍNEZ 1  ANTÓNIO M. M. SOARES 2 PETER KRESTEN 3 LIUDMILA GLAZOVSKAYA 4  1. Introduction  The settlement of Passo Alto is located in south-western Portugal near Vila Verde de Ficalho (Fig. 1).It extends atop a small hill near the confluence of tworivers (Fig. 2), and is formed by two loci 250 m apart(Fig. 3). The archaeological finds, namely pattern-burnished pottery (Fig. 4) collected during severalarchaeological surveys and excavations carried out atthe settlement, indicate that a Late Bronze Agechronology must be assigned to the human occupa-tion of this archaeological site (Parreira and Soares,1980; Soares, 2003, 2005).The good natural defenses of the settlement arecomplemented with a rampart along the easiest north-ern approach to the settlement (Fig. 3). Outside therampart, a broad band of chevaux-de-frise providesan additional line of defense around what most prob-ably was the main entrance-way. An archaeologicaltrench excavated near this entrance, perpendicular toand across the enclosure wall, allowed to characterizethe rampart (Fig. 5). Its base consists of a line of undressed superposed blocks of schist from the local substrate rock in the outer side (a), and a line of vertical schist slabs in the inner side (b), both limiting an earthen wall of compacted soilwith small stones (c). An earthen bank also seems to be placed against the inner side of the wall,i.e. against the vertical slabs. Enrique Díaz-Martínez,António M.M.Soares,Peter Kresten e Liudmila Glazovskaya 152 REVISTA PORTUGUESA DE Arqueologia.volume 8.número 1.2005,p.151-161Evidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho,Serpa,Portugal) Fig. 1 Location of the settlement of Passo Alto insouthwestern Portugal. Fig. 2  View of the settlement of Passo Alto from the northern access road. The arrow indicates the probable main entrancewhere the chevaux-de-frise and vitrified rocks are found.  Enrique Díaz-Martínez,António M.M.Soares,Peter Kresten e Liudmila GlazovskayaEvidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho,Serpa,Portugal) 153 REVISTA PORTUGUESA DE Arqueologia.volume 8.número 1.2005,p.151-161 Fig. 3 Survey of the settlement of Passo Alto: 1 - area with scattered pottery sherds; 2 - chevaux-de-frise; 3 - wall rampart.Distance between contour lines: 10 m. See text for further explanations. Fig. 4 Late Bronze Age pattern-burnished pottery shards from the settlement of Passo Alto.  Enrique Díaz-Martínez,António M.M.Soares,Peter Kresten e Liudmila Glazovskaya 154 REVISTA PORTUGUESA DE Arqueologia.volume 8.número 1.2005,p.151-161Evidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho,Serpa,Portugal) Fig. 5 Internal structure of the enclosure wall at the settlement of Passo Alto: a - line of undressed superposed blocks of schist(outer face); b - vertical schist slabs (inner face); c - compacted soil with small stones (base and inner part of the wall).  Blocks of schist covering the earthen wall and making part of the tumble (Fig. 5) suggestthat the upper part of the rampart wall was made of these blocks in an undressed and super-posed way. Rock fragments with evidence of partial melting as well as welding of clasts are abun-dant in a restricted area of the rampart zone near the possible main entrance. The first prelim-inary interpretation of these rock fragments assigned them to metallurgical processes (Soares,1988, 2003). However, more detailed analyses identified a lack of ore minerals or smelting slagat the site, and a large volume of rocks affected by partial melting, both characteristics hard toreconcile with typical small-scale Bronze Age metal smelting. Hence, an alternative interpreta-tion had to be considered, and this was partial vitrification of the enclosure wall near the prob-able main entrance. This new interpretation implies that, at least at the entrance, the defensivewall consisted of an upper part made of rock fragments within a timber framework, in a way similar to other Bronze and Iron Age vitrified hillforts known from northern and central Europe(Youngblood et al., 1978; Nisbet, 1982; Fredriksson et al., 1983; Kresten et al., 1993). The objec-tive of this article is to document the evidence which has led us to infer burning of a timber struc-ture and partial vitrification of the rampart as the most probable interpretation of the partially-melted rock fragments, and to discuss the results in the light of the latest evidence for protohistoric vitrification of fortresses in the region (Díaz-Martínez, 2004a, 2004b; Díaz-Martínez and Soares,2004). 2. Evidence for wall vitrification   A detailed study of the partially melted and welded fragments was undertaken in order toexplain the apparent incongruence of metallurgical processes resulting in such a large volume of these fragments. The study consisted of field description and hand-specimen study of the clasts(areal distribution, shape and size, surface features, etc.), petrological and mineralogical analysesunder optical polarizing microscope, identification of mineral phases with whole-rock powderX-ray diffraction (XRD), analysis of secondary and backscattered electron (BSE) images underelectron microscope, and chemical analysis with electron microprobe. Enrique Díaz-Martínez,António M.M.Soares,Peter Kresten e Liudmila GlazovskayaEvidence for wall vitrification at the Late Bronze Age settlement of Passo Alto (Vila Verde de Ficalho,Serpa,Portugal) 155 REVISTA PORTUGUESA DE Arqueologia.volume 8.número 1.2005,p.151-161 Fig. 6 Comparison of the local substrate rock at the settlement of Passo Alto (A), characterized by folded alternating bands of quartz (lighter) and micas (darker), and the vitrified rock (B), where the folding is preserved, but the micas have been meltedand transformed into dark glass with neoformed minerals. The 5-cm scale is valid for all samples.
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