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HIGH LEADED BRONZE IN THE LATE BRONZE AGE METALLURGY OF THE IBERIAN PENINSULA

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HIGH LEADED BRONZE IN THE LATE BRONZE AGE METALLURGY OF THE IBERIAN PENINSULA
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  HIGH LEADED BRONZE IN THE LATE BRONZE AGE METALLURGY OF THE IBERIANPENINSULAIgnacio Montero (1), Salvador Rovira (2), Germán Delibes (3), Julio Fernández-Manzano (3), MªDolores Fernández-Posse (4), José Ignacio Herrán (3), Concepción Martín (4) and Ruth Maicas (2)(1) Dpto. de Prehistoria, Instituto de Historia (CSIC). Serrano, 13. E-28001-Madrid (Spain)(2) Museo Arqueológico Nacional. Serrano, 13. E-28001. Madrid (Spain)(3) Dpto. de Prehistoria y Arqueología, Universidad de Valladolid. Avd. del Campus s/n. E-47001Valladolid (Spain)(4) Instituto del Patrimonio Histórico Español. El Greco, 4. E-28040. Madrid (Spain)ABSTRACTMetal is one of the most remarkable features in the Late Bronze Age. Metal findings as well asmetal deposits constitute a fundamental part of the archaeological record of this archaeologicalstage. The interpretation of the historical development produced during this period needs to consi-der the impact of metallurgy and define the function and use of metal (instrumental, ritual or socialfunction).High leaded bronze occurs for the first time in some types of objects, but it is not clear why thisalloy was mainly chosen in double-looped palstaves. In order to find an explanation, we must takeinto account three points: 1) The typology of high leaded bronze objects is related to their instru-mental function, 2) but not all the palstaves have been made of a leaded alloy and 3) this high lea-ded alloy apparently is not the best one for tasks where mechanical stress is required.Metallographic and X-ray studies of some items show the complexity of a general interpretation of this phenomenon, which has not yet been satisfactorily explained.KEYWORDSLate Bronze Age. Technology. EDXRF. Metallography. Leaded bronze. Alloy. Iberian Peninsula.PalstavesINTRODUCTIONTin bronze alloyed with lead to get leaded bronze is a step forward in the development of metal-lurgical technology. This alloy is the result of the mix of three different metals in order to get a newone with properties previously unknown. The proportion of each metal is the key to change such properties. It is generally accepted that the addition of lead lowers the melting point and increasesthe metal fluidity. Thus the casting is much better, which explains the use of this mix in all big cas-ting bronze statuary and complex ornaments, which do not require special mechanical features. Asmall amount of lead has no appreciable effect on the workability of the metal, but a high percen-tage of it makes the metal almost impossible to use if high mechanical stress is needed because thelarge areas of lead interspersed among the bronze form regions of weakness (1). This is due to the  low solubility of lead in copper, both in the liquid bath and in the solid state. Then, most of lead isnot properly alloyed but segregated causing heterogeneity of the alloy.Although some objects made of lead have been recorded in France, dated of Chalcolithic time, itwas in the Middle Bronze Age when the first leaded bronze items were produced. Notwithstanding,the use of lead in the alloy will be characteristic of the Late Bronze Age metallurgy and in someregions such as in the Iberian Peninsula it was alloyed mainly in the latest phase of this period(LBAIII) (2, 3).Late Bronze Age objects with high leaded alloys were discovered very early in the history of archaeometallurgical research. At the end of XIX th century and early XX th century, Pitre de Lisle duDréneuc and L. Marsille observed that some types of French Atlantic axes (socketed axes) had thiscomposition (4). Louis Siret (5) highlighted the presence of this alloy, which appears mainly in thedouble looped palstaves of the north and northwest of the Iberian Peninsula. Doubts about the metalfunctionality were discussed in these former works. Due to the fact that this type of alloy is not themost suitable for weapons and tools, alternative hypotheses such as its use as ingots, as currencyitems in premonetary societies or as votive objects were suggested to explain why such apparentlynon-utilitarian metals were made. These hypotheses are still supported nowadays and researchersfind arguments in favour and against them.However, the distribution pattern of high leaded alloy shows that this is not an exclusive pheno-menon of the Atlantic Bronze Age. For instance, Trampuz Orel´s papers (6, 7) show that high lea-ded bronze was also used in specific types of artefacts (shaft-hole axes and cast ingots) in Slovenia.In all the previous examples, alloys containing a high lead percentage are related to distinctiveobjects with determined shapes. These objects have been further sub-divided into several groups.Thus, in the French socketed axes, high lead percentages are detected especially in the Dahouet,Tréhou and Couville types (4). The Slovenian shaft-hole axes belong to different types, but all of the typical Italian forms were made of this alloy, that was used for cast ingots as well but not for  plano-convex ones. Regarding the Iberian Peninsula, double looped palstaves are the only itemsmade of leaded bronze.Another interesting feature is that binary Cu-Pb alloy prevails in those Slovenian objects, and whentin exists is meaured in very low percentages. Some deposits of Armorican socketed axes also havea Cu-Pb composition, such as those of Teilliéres and Saint Martin des Champs. At present, the binary Cu-Pb alloy is unknown in the Iberian palstaves, except for one axe in the British Museum,which very likely comes from Spain (1). In fact, the average of tin in the Iberian high leaded pals-taves is 9,2 %.The arguments to consider these axes to be votive or premonetary items, lacking any functionalityas inferred from their typology, are: use of a metal of an alloy deemed as non-functional, the elec-tion of a specific type of object (although within a range of morphological variants) and finally,most of these items were recovered from big hoards that give them an accumulative character. Thefact that some axes were made of almost pure lead further reinforces the non-functional hypothe-sis. Furthermore, in the case of some palstaves from the Iberian Peninsula, the presence of the cas-ting jet points out its unfinished state, and it is used as an argument of clear identification of itsalternative function. Nevertheless, high leaded bronze alloys are occasionally detected in other types of contemporaryobjects and sometimes in different contexts, what invalidates the premonetary hypothesis beapplied to them but makes more likely the votive hypothesis. They will be commented immedia-tely after.  Firstly, we must pay attention into Luzarches's hoard (Val d'Oise, France), dated in the LBAII andcomposed principally of single looped palstaves, fragments of leaf-shaped swords besides a frag-ment of spearhead. All the analysed items from this hoard, except two, are high leaded bronzes(>20% Pb) (8). In this case, it should be stressed, on the one hand, the use of a common alloy thatsupposedly is not biased according to the type of object, and on the other hand, that many itemswere broken. This looks like a foundry man's hoard.Another example comes from the Petit Villatte (Cher) hoard (5). A sword is the only item that exce-eds 20% Pb. The hoard is composed of several types of objects among which there is a socketedaxe containing only 3% Pb.Some exceptions from the British Isles must be mentioned. An arrowhead from Penard reaches40% Pb and 1.12% Sn, whereas the rest of pieces are tin-bronzes without lead. In the case of Guilsfield, the whole material is made of leaded bronze but only a sword exceeds 10% Pb, with avalue of 44.3% Pb and 5.9% Sn. In Peters, two socketed axes contain more than 20% Pb, althoughwe must also mention a sickle with 18% Pb. Finally, from Fengate Power Station and Flag Fen, thehigh leaded bronzes are a spearhead with 23% Pb, a sword with 27% Pb and another one in whicha rivet takes 28.7 % Pb, whereas the blade contains 7.6% Pb (9).In the Iberian Peninsula, a sickle from Castropol (Asturias) with 20.5% Pb and 10.4 % Sn (1) is theonly exception documented until now.The previous discussion has led us to analyse with more detail the high leaded bronzes in theIberian Peninsula in order to shed light on the plausible use of this kind of alloy.PALSTAVES FROM THE IBERIAN PENINSULAWe can distinguish both typologically and geographically two major groups: the axes from Galiciaand the north of Portugal and the ones from the Cantabrian seaboard and the Duero Basin (Fig. 1).The former group uses to display narrow and elongated shapes with parallel sides and short cuttingedges. The axes form the latter, more graceful in shape, exhibit longer, curved cutting edges.Fig. 1. Double looped palstaves from the Iberian Peninsula: A Asturias & Duero Bassin; BGalicia & Portugal. 1 Padilla de Abajo (Burgos); 2 Cármenes (León); 3 Samieira(Pontevedra); 4 Paredes de Coura (Minho).  The chronological framework of the Spanish palstaves is under controversy because of the lack of reliable archaeological contexts. For the more developed types, that usually were made of high lea-ded alloy, a chronology of the VIII th century BC can be assigned, lasting until the beginning of theIron Age (11).From the analytical point of view, the number of palstaves showing high lead content has beenincreased shortly since Siret´s paper (5). Unlike hoards from France and Slovenia, the bulk of thisgroup is formed usually by single finds or very small hoards. Only two big hoards have been analy-sed with enough detail. One is the São Martinho de Bougado (Porto, Portugal) hoard, having 36double looped palstaves. Thirteen have been analysed (5) with the following results: 9 are made of high leaded tin-bronze (averages 7.3% Sn and 43.5% Pb) and 4 are made of nearly pure lead. Theother is the Samieira (Pontevedra, Spain) hoard (1). Fifty five of the 170 double looped palstaveswere analysed by Atomic Absortion Spectrometry. The averages for this set are 8.8% Sn and 26.5%Pb. However, we have to point out that a small group of palstaves (9%) does not belong to the highleaded alloy we are dealing with because the lead content is lower than 15 %. An interesting fea-ture to take into account for functional explanation is revealed by radiography of some of theseintems: the images show casting failures in most of them, mainly big size gas bubbles. On the other hand, the four analysis from the Noalla hoard show ternary alloys but only one palstave containsmore than 20% Pb (12).Harrison et al. (1) published analyses of Spanish objects in the British Museum. The set contains 9 palstaves, 5 of them exhibiting the casting jet on the butt. Only one axe contains leess than 20%lead.Finally, 36 single looped and 43 double looped palstaves have been analysed by the  Proyecto de Arqueometalurgia de la Península Ibérica . Axes of the first type use to be made of tin-bronzewithout lead, except two cases the lead content of which is lower than 10% Pb. Diversity of tin andlead percentages is much higher in the second type: 40% of the axes were made of tin-bronze and60% of ternary alloy. Among these last, only 8 axes contain more than 15% Pb in the alloy. Four of them had been previously analysed by Siret and for the other four axes analysis have been donenow for the first time. Aditional, interesting information in order to evaluate the use of the alloycan be obtained from the palstaves from Cármenes and Pola de Laviana (Tab. 1). They were reco-vered within mining contexts, to all appearances inside a gallery. The find of axes inside mines inSpain and Portugal is not an exception (13), being clasified in some occasions as miner tools.However, given the lack of precise archaeological record the finds occurred, the alternative of ritualhoarding can not be discarded.Table 1: Analysis of double looped palstaves from mines. EDXRF analysis (wt. %). Analysis # Site Fe Ni Cu As Ag Sn Sb Pb PA3485Cármenes0.160.2465.3nd0.01017.9nd16.4PA6834Pola de Laviana0.160.1347.6nd0.0174.030.03748.1 Note: nd, not detectedPALSTAVES FROM VARA (LUGO)The set is formed by three double looped palstaves, one of them having a flat blade and the other two showing a rib in the middle of the blade (Fig. 2) (14). Two axes have been analysed by EDXRF,firstly in 1983 using a Kevex 7000 spectrometer (AA analyses in Tab. 2) and recently using a por-table Metorex X-MET920 spectrometer (PA analyses in Tab. 2). Despite the presumable heteroge-neity of this kind of alloys the results are close enough.  Table 2. Palstaves from Vara. EDXRF analysis (wt. %) Inventory # Analysis # Zone Cu Sn Pb Fe Ag Sb MAN35370PA10361BButt51.94.1643.80.030.0140.040MAN35370AA1268BButt44.17.0748.10.150.0720.052MAN35370PA10361ACutting edge67.16.1126.40.240.0170.044MAN35370AA1268ACutting edge55.78.1235.80.150.0090.068MAN35371PA10362AButt64.44.2731.00.050.0220.043MAN35371AA1275BButt67.74.1626.80.060.0070.046MAN35371PA10362BCutting edge74.63.6121.20.080.0170.041MAN35371AA1275ACutting edge70.54.5924.20.140.0090.045Regarding MAN35371, metallography in the loop shows a slow cooling, as-cast microstructure.Abundant segregated lead appears as dark-grey and black masses in the photograph (Fig. 3).Metallography in the cutting edge shows a slow cooling as-cast microstructure deformed by coldhammering. Segregated lead appears as dark-grey and black masses. Light-grey nodules are of cuprite and chalcosine. Series of Newman lines (parallel lines) affecting the metal grains suggestsome cold deformation probably due to the work of the axe for cutting a material slightly harder than the bronze the tool had been made (Fig. 4). Metallography few millimetres far from the cut-ting edge shows a slow cooling as-cast microstructure deformed by cold hammering. Mechanicaldistortion flattened both the segregated light-grey nodules of chalcosine and the black nodules of lead. Instead, the cuprite inclusions keep up its spherical shape (Fig. 5).Chaîne opératoire: After casting, the blade was selectively cold hammered in the cutting edge toharden it.Fig. 2. Palstaves from Vara (Lugo).
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