ABSTRACT BACKGROUND A microscopic study on ancient metals was established to identify chemical composition and microstructure of archaeological tin bronze objects. For this purpose, ten

small bronze pieces from Baba Jilan Iron Age site, western Iran, were selected and studied by optical microscopy (OM) and scanning electron microscopy–energy dispersive X-ray spectroscopy

(SEM–EDS) methods. RESULTS The results showed that all objects were made of variable tin containing binary copper-tin alloy. It may be related to the application of an uncontrolled procedure

to make bronze alloy. In some samples, lead and arsenic are detected as major elements, but they can be considered rather as impurities from the original ores. Different microstructural

bronze production in the Iron Age of Iran was to produce bronze alloy with different amounts of tin and without any third alloying element, using copper sulphidic or a mixture of copper

sulphidic and oxidic ores to smelting copper, and applying working and heat treatment to shape and manufacture thin sheet objects. SIMILAR CONTENT BEING VIEWED BY OTHERS CHEMICAL ANALYSIS

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2025 BACKGROUND The study of microstructure in archaeological metal objects is a remarkable and important subject to determine ancient manufacturing techniques in the ancient time. The use

of copper and its alloys to make different routine, religious and decorative objects has been common during the prehistoric period. For example, copper has been used in the Iranian Plateau

for 9000 years and it has been continued with production of bronze alloy for about 5000 years to make various artefacts [1, 2]. The long time and variety of production and usage of copper

and its alloys in the ancient world prove that it is necessary to investigate about different aspects of its metallurgy to identify its various features. Application of bronze alloy to

manufacture decorative and religious objects was usual during the Iron Age of Iran (ca 1500–550 BC) [1, 3, 4]. One of the outstanding archaeological findings from the Iron Age of Luristan

(western Iran) was different objects deposited in cemeteries and sites [5]. Many of discovered objects from these graves are produced with bronze alloy and are dated to first half of first

millennium BC [6, 7]. These so-called Luristan bronzes have been found often in excavated cemeteries (e.g., War Kabud and Bard-i Bal) and, in some cases, sanctuaries (e.g., Sangtarashan and

Surkh Dum) [8]. Few bronzes in Luristan region were found during archaeological excavations. Many bronze objects from Luristan in different museums were found during illegal excavations

carried out by local people from the late 1920s to now [7]. The technical and archaeometallurgical studies on Luristan bronzes have always been a great interest for archaeologists and

scientists. Despite the large scale of bronze objects related to Luristan, there exist a limited number of investigations about technology and metallurgy of bronze objects from this region

[8–12]. The archaeological site of Baba Jilan is an Iron Age graveyard that has been surveyed and excavated from 2006 to 2008. It has been located in the west of Lorestan (Luristan)

province, near Nurabad city. The graveyard is situated on the flank of the mountain. Eleven tombs, including five jar burials and six cist tombs were excavated in which different burial

goods were deposited, such as potteries, bronze and iron objects, stone arrowheads, and decorative objects and ornaments made of agate, shell and bone. The radiocarbon dating done on three

bone samples from this site showed a calibrated date of about 800 BC (2707 ± 28, 2784 ± 30 and 2693 ± 31 years). Of course results of dating on another sample showed a different calibrated

date of 1538 ± 32 years. Based on archaeological studies and analytical dating, the Baba Jilan site is a typical graveyard belonging to the Iron Age II period (first half of the first

millennium BC) with different objects including some bronze objects related to Luristan bronzes [13]. The aim of this research is to provide evidences about the use of tin bronze alloy to

produce some grave goods recently found in Baba Jilan site from the Iron Age II period. For this purpose, some small metallic pieces were selected and examined by microscopic and

microanalysis methods to characterize microstructural features of ancient bronze artefacts. The importance of this type of research is that only a few excavated bronze objects from Luristan

are analyzed till now [10–12], and results of analysis such artefacts can be used as a baseline to compare many looted bronze objects available in different museums for authentication and

provenance purposes. EXPERIMENTAL To study the microstructure and chemical composition of Baba Jilan bronze artefacts, ten metallic samples from archaeological excavations were selected for

experiments (Fig. 1). The samples are different in typology/function and include spout of vessel (BJ-01), shield head pin (BJ-03), small circular plate (BJ-07), small spring (BJ-09), small

bead (BJ-11), vessel’s body (BJ-13 and BJ-19), small bead from a necklace (BJ-16), small button (BJ-17) and bracelet (BJ-18). The selected samples were mounted in epoxy resin in room

temperature and ground by abrasive silicon carbide papers of 120, 240, 400, 800, 1200, 2000 and 3000 grade respectively. Then, the samples were polished with diamond paste from 3 to 0.5

microns. Alloy composition and phases were identified by SEM–EDS (semi-quantitative) on mounted samples in order to observe and analyse details of the microstructure. The SEM–EDS analyses

were performed in low vacuum using FE-SEM instrument model MIRA III manufactured by TESCAN Company, with a RONTEC backscattered electron detector (BSE) and an energy dispersive spectrometer

(EDS). In addition, microstructures were observed in mounted samples using a polarized light microscope model BK-POL/BKPOLR manufactured by Alltion Company before and after etching by ferric

chloride solution in ethanol [14]. RESULTS AND DISCUSSION ALLOY COMPOSITION SEM–EDS investigation was employed to determine alloy composition in Baba Jilan bronze objects in a

semi-quantitative manner. Table 1 shows the results of alloy composition in ten samples carried out by SEM–EDS method. According to Table 1 it is obvious that all ten samples are made of tin

bronze alloy. Also, it is clear that the main alloying elements of all samples are Cu and Sn. The percentage of Cu content varies from 82.3 up to 90.2 and the Sn 3.9 up to 14.6. Through

these analyses one can observe that the Sn contents show different values. Lead is detected in minor concentrations in samples, only in one case, it has been determined as major amount, 6.95

 wt%. Similarly, As, Zn. Fe and S have been detected as minor constituent, despite of a sample that arsenic has been measured in 2.87 wt%. Presence of the As and Pb amount of lower than two

wt% may explain that it has entered into the alloy composition as an impurity from copper ores [1, 15, 16]. Also, the Iron Age bronze objects from Iran usually contain minor or trace amounts

of lead and arsenic, suggesting its presence might characterize an impurity within the ores smelted [17, 18]. Generally, it is evident that eight samples are made of a binary copper-tin

alloy, and in two cases (BJ-16 and BJ-18) leaded bronze and arsenical bronze are used to manufacture metal objects. The variety of tin content proves that the objects have not been made by a

particular alloy proportion to reach a homogenous bronze composition such as adding a distinct amount of tin to copper and melting them, but an uncontrolled process is used to produce tin

bronze alloy, such as co-smelting, cementation, recycling or smelting copper-tin containing ores [3, 12, 19, 20]. The variability of tin content is commonplace in the study of the Iron Age

bronze objects of Iran, especially in Luristan bronzes. Results of analysis of different bronze objects from Luristan show that there was no specified procedure to control the alloy

composition in the manufactured bronzes in that time [8, 10, 12]. Nevertheless, each of the processes may have been used for the bronze production in these Iron Age bronze artefacts. On the

other hand, presence of considerable amount of lead and arsenic in one sample each state that either these samples are made deliberately to produce ternary alloys or these may not be added

as alloying elements deliberately but may be entered the alloy during ore smelting. Regarding to composition of previously analysed bronze objects from the Iron Age of Iran (especially

Luristan), the binary Cu–Sn alloy has been the commonplace event in copper alloys production and no deliberately ternary Cu–Sn–As or Cu–Sn–Pb alloys have been used to make metal objects [8,

10–12]. Nevertheless, these ternary bronzes are considerable due to their composition. MICROSTRUCTURE Mounted samples of all objects were examined under the optical and SEM microscope.

SEM–BSE and OM micrographs of bronze samples before etching show a metallic matrix with numerous scattered inclusions in it (Figs. 2, 3). Many of these inclusions appear as a grey-green

colour in OM observations (Fig. 3). These are elongated in the latitude of the cross section. On the other hand, some very fine, globular bright inclusions are visible in the matrix in

SEM–BSE micrographs (Fig. 2). To identify the chemical composition, some elongated and globular inclusions were analysed by SEM–EDS microanalysis (Fig. 4). The results showed that the

grey–green elongated inclusions are composed of copper and sulphur with a low content of iron. Tin and zinc are detected as major components in some inclusions, while they are determined in

others as minor elements together with arsenic. According to the EDS results, it is obvious that the dark elongated inclusions are copper sulphide compounds (Table 2). The smelting of

sulphidic or sulphidic/oxidic ores has been long done in ancient time to produce metallic copper [21, 22]. It should be noted that many copper ore deposits used in the ancient Iran were

weathered copper sulphide ores, consisting of copper sulphide and oxide compounds [4]. Due to smelting such ores, Cu–S inclusions are often observed in Iron Age bronze objects from Iran. In

fact, some copper sulphide didn’t transform to metallic copper during the smelting and will be evident as grey-green inclusions in the bronze microstructure. It should be noted that these

Cu–S inclusions are observed in many copper and bronze objects from the Bronze Age to the Iron Age of western and northern Iran [8, 12, 20, 23]. Presence of low amount of iron is most

probably due to the presence of iron in smelted copper ore, in the form of iron sulphides or use of an iron–copper sulphide such as chalcopyrite [22]. Further, any light inclusions of small

dimensions were also visible scattered in the bronze matrix in SEM–BSE micrographs. EDS analyses conducted on five white globules showed that they are Pb-rich metallic compounds containing

about 60–90 wt% of lead (Table 2). Theoretically, the chemical composition of these globules must be nearly a 100 % of Pb, but the presence of some copper (and tin) in composition these

inclusions might be due to influences from the surrounding bronze matrix in the EDS microanalysis. Based on the Cu–Pb binary system [14], lead is immiscible in copper and appears as fine or

large separated lead rich phases scattered in the microstructure of copper alloys. The number and size of these lead globules are strongly related to the amount of lead. For example, in

sample BJ-16, lead has detected more than 6 wt% (Table 1) and the number of white phases is much more than other samples in its SEM–BSE micrograph (Fig. 2b). Also, a metallographic sample of

each bronze object was taken and examined under the optical microscope. As noted above, optical microscopy (OM) micrographs of bronze samples before etching show an alpha solid solution

matrix with numerous scattered dark grey-green inclusions in the matrix that are elongated in the latitude of the cross section (Fig. 3). To reveal grains’ microstructure and the

manufacturing process, samples were etched in an alcoholic FeCl3 solution. After etching, the microstructure of the samples showed a typical grain structure consisting on deformed and

recrystallized grains of copper–tin solid solution with twinned and strain lines within some grains. Samples with straight twin lines also do not show strain lines (Fig. 5). This type of

microstructure is common in archaeological bronze objects. In fact, the metallographic features of these bronzes state that all objects are manufactured and shaped during a cycle of cold

working and heat treatment. This is apparent from equi-axed and recrystallized grains [14, 24]. Cold working on copper alloys leads to strain-hardening: the grains are deformed and become

increasingly flattened until they are too brittle to deform any further. At this point, the grains are fully work-hardened [14, 25, 26]. For further shaping by hammering, it requires to

anneal the metal in order to restore malleability. If more shaping with hammering is operated, then annealing may be needed again. Thus, many bronze objects are made by several cycles of

cold working and annealing to reach the final shape. The variable grain sizes imply (i) Different amount of deformation; (ii) Different annealing conditions and/or different numbers of

thermomechanical cycles [14]. Based on the results, the main metalworking processes to manufacture Iron Age bronzes from Baba Jilan produce binary Cu–Sn alloys. Some elements are detected as

major content in alloy composition such as Pb and As, but they may be high amount impurities that are entered to the alloy from used ores and can’t be considered as deliberate additions to

the Cu–Sn alloys. The microstructure of samples includes the matrix of alpha solid solution with copper sulphide inclusions and lead globules. These appear as segregated phases in the solid

solution, due to their low miscibility in molten copper. In some samples, the Cu–S inclusions are elongated due to mechanical operations used to shape these fine objects. Metallographic

study of these bronzes showed that cycles of mechanical operation and subsequent heat treatment have been applied to shape and transform original bronze ingots to final objects. This was

proved by the presence of worked and recrystallized grains of alpha solid solution in etched microstructure as well as elongated sulphidic inclusions scattered in the matrix. Further, grain

size is different in bronze samples; this implies a variety in degree of deformation used to shape the bronze objects by different times of the working-heating cycle. Thus, based on the

microstructure and composition of the studied bronzes, it reveals that the following simple process can be suggested for these bronze objects: * Bronze production with an uncontrolled

smelting/alloying process, the product may be bronze prill with high amount of other elements such as Pb or As. * Using bronze prill to make very small objects or melt several prills to

produce bronze ingots. * Using ingots for casting large sheets/pieces of bronze (optional) or cold working on sheets or pieces to shape the objects. * Annealing of work-hardened pieces to

return workability. * Continuous thermo-mechanical process to reach the final shape of the objects. CONCLUSION Microstructural study on some bronze objects discovered from the Baba Jilan

Iron Age site in western Iran was carried out by microscopy and microanalysis methods to identify their manufacturing processes. Results of analysis of ten objects showed that all were

manufactured with a variable composition of tin bronze alloy with some elements accidentally entered in the composition such as lead and arsenic. It proved that the bronze production may

have been performed by an uncontrolled production method, such as co-smelting or cementation. It has been observed in many bronze objects from the Iron Age of Iran, especially Luristan.

Other elements such as Pb and As are present in the composition that may accidentally derive from the original ores. Also, the microstructure of bronze objects shows the application of

thermo-mechanical operation, including cold working and subsequent annealing as a cyclic procedure to transform a bronze ingot/prill to some fine objects. Based on the metallographic

observations, bronze objects showed similar grain microstructures, including worked and annealed grains with twin lines. In fact, based on the results, the application of microscopic and

microanalysis methods can help to understand manufacturing techniques and characteristics of archaeological bronze artefacts. REFERENCES * Oudbashi O, Emami SM, Davami P. Bronze in

archaeology: a review of archaeometallurgy of bronze in ancient Iran. In: Collini L, editor. Copper alloys—early applications and current performance—enhancing processes. 2012. p. 147–74. *

Thornton CP. The emergence of complex metallurgy on the Iranian plateau: escaping the levantine paradigm. J World Prehistory. 2009;22:301–27. Article  Google Scholar  * Pigott VC. On the

importance of Iran in the study of prehistoric copper-base metallurgy. In: Stöllner T, Slotta R, Vatandoust A, editors. Persia’s ancient splendour, mining, handicraft and archaeology,

Deutsches Bergbau—Museum, Bochum. 2004. p. 28–43. * Pigott VC. The development of metal production in Iranian Plateau: an archaeometallurgical perspective. In: Pigott VC, editor. The

archaeometallurgy of Asian old world, University Museum Monograph 89. Philadelphia: University of Pennsylvania Museum; 1999. p. 73–106. * Overlaet B. The chronology of the Iron Age in the

Pusht-i Kuh, Luristan. Iran Antiq. 2005;XL:1–33. Google Scholar  * Overlaet B. Luristan metalwork in the Iron Age. In: Stöllner T, Slotta R, Vatandoust A, editors. Persia’s ancient

splendour, mining, handicraft and archaeology, Deutsches Bergbau—Museum, Bochum. 2004. p. 328–38. * Overlaet B. Luristan bronzes: I. The field research. In: Yarshater E, editor.

Encyclopaedia Iranica online, Originally published: November 15, 2006. Available at http://www.iranica.com/articles/luristan-bronzes-i-the-field-research. * Oudbashi O, Emami SM, Malekzadeh

M, Hassanpour A, Davami P. Archaeometallurgical studies on the bronze vessels from “Sangtarashan”, Luristan, W-Iran. Iran Antiq. 2013;XLVIII:147–74. Google Scholar  * Moorey PRS. An interim

report on some analyses of “Luristan bronzes”. Archaeometry. 1964;7:72–9. Article  Google Scholar  * Fleming SJ, Pigott VC, Swann CP, Nash SK, Haerinck E, Overlaet B. The archaeometallurgy

of war kabud, Western Iran. Iran Antiq. 2006;XLI:31–57. Article  Google Scholar  * Fleming SJ, Pigott VC, Swann CP, Nash SK. Bronze in luristan: preliminary analytical evidence from

copper/bronze artifacts excavated by the Belgian mission in Iran. Iran Antiq. 2005;XL:35–64. Article  Google Scholar  * Oudbashi O, Davami P. Metallography and microstructure interpretation

of some archaeological tin bronze vessels from Iran. Mater Charact. 2014;97:74–82. Article  Google Scholar  * Hasanpur A, Hashemi Z, Overlaet B. The Baba Jilan graveyard near Nurabad, Pish-i

Kuh Luristan—a preliminary report. Iran Antiq. 2015;L:171–212. Google Scholar  * Scott DA. Metallography and microstructure of ancient and historic metals. Los Angeles: Getty Conservation

Institute; 1991. Google Scholar  * Figueiredo E, Araújo MF, Silva RJC, Senna-Martinez JC. Corrosion of bronze alloy with some lead content: implications in the archaeometallurgical study of

late bronze age metal artefacts from “Fraga dos Corvos” (North Portugal). METAL 07, ICOM-CC Proceedings, Amsterdam; 2008. p. 61–6. * Figueiredo E, Valério P, Araújo MF, Senna-Martinez JC.

Micro-EDXRF surface analyses of a bronze spear head: lead content in metal and corrosion layer. Nucl Instrum Methods Phys Res Sect A. 2007;580:725–7. Article  Google Scholar  * Moorey PRS.

Archaeology and pre-achaemenid metalworking in Iran: a 15 year retrospective. Iran. 1982;20:81–101. Article  Google Scholar  * Thornton CP. Of brass and bronze in prehistoric southwest Asia.

In: La Niece S, Hook D, Craddock P, editors. Metals and mines: studies in archaeometallurgy. London: Archetype Publications; 2007. p. 123–35. Google Scholar  * Valério P, Monge Soares AM,

Silva RJC, Araújo MF, Rebelo P, Neto N, Santos R, Fontes T. Bronze production in southwestern Iberian Peninsula: the late bronze age metallurgical workshop from Entre Águas 5 (Portugal). J

Archaeol Sci. 2013;40:439–51. Article  Google Scholar  * Oudbashi O, Naseri R, Malekzadeh M. Technical studies on the bronze age metal artefacts from the graveyard of Deh Dumen,

South-Western Iran (third millennium BC). Archaeometry. 2015. doi:10.1111/arcm.12208. Google Scholar  * Rostoker W, Pigott VC, Dvorak JR. Direct reduction to copper metal by oxide-sulfide

metal interaction. Archeomaterials. 1989;3:69–87. Google Scholar  * Bachmann HG. The identification of slags from archaeological sites. Occasional publications 6. London: Institute of

Archaeology, UCL; 1982. * Oudbashi O, Hessari M. Iron Age tin bronze metallurgy at Marlik, Northern Iran: an analytical investigation. Archaeol Anthropol Sci. 2015.

doi:10.1007/s12520-015-0280-1. Google Scholar  * Caron RN, Barth RG, Tyler DE. Metallography and microstructures of copper and its alloys. In: ASM handbook, metallography and microstructures

9. Materials Park: ASM International; 2004. p. 775–88. * Nerantzis N. Shaping bronze by heat and hammer: an experimenatl reproduction of minoan copper alloy forming techniques. Mediterr

Archaeol Archaeom. 2012;12:237–47. Google Scholar  * Nerantzis N. Experimental simulation study of prehistoric bronze working: testing the effects of work-hardening on replicated alloys. In:

Hauptmann A, Modarressi-Tehrani D, editors. Archaeometallurgy in Europe III, Der Anschnitt, Beiheft 26. 2015. p. 329–35. Download references AUTHORS’ CONTRIBUTIONS OO performed the

analytical work and OM observations and prepared the manuscript. AH is the archaeologist that documented the excavated objects, helped to draft the manuscript and participated in the

research design. Both authors read and approved the final manuscript. ACKNOWLEDGEMENTS The authors are thankful to Behnam Rahmani and Nazanin Nikroo, SEM laboratory of RASF, Tehran for their

helps to carry out SEM–EDS analyses and Siamak Makhzani, IAU, Hamedan Branch, for editing the final text. The experimental and technical work presented in this paper has been carried out

within the framework of research project no. 939/10, financed by the Research Office of the Art University of Isfahan, Iran in 2015. COMPETING INTERESTS The authors declare that they have no

competing interests. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Faculty of Conservation, Art University of Isfahan, Isfahan, Iran Omid Oudbashi * Department of Archaeology, ICHTO of

Lorestan Province, Khorramabad, Iran Ata Hasanpour * Department of Archaeology, Science and Research Branch, Islamic Azad University, Tehran, Iran Ata Hasanpour Authors * Omid Oudbashi View

author publications You can also search for this author inPubMed Google Scholar * Ata Hasanpour View author publications You can also search for this author inPubMed Google Scholar

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Oudbashi, O., Hasanpour, A. Microscopic study on some Iron Age bronze objects from Western Iran. _Herit Sci_ 4, 8 (2016). https://doi.org/10.1186/s40494-016-0079-0 Download citation *

Received: 14 December 2015 * Accepted: 01 April 2016 * Published: 06 April 2016 * DOI: https://doi.org/10.1186/s40494-016-0079-0 SHARE THIS ARTICLE Anyone you share the following link with

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content-sharing initiative KEYWORDS * Archaeological bronzes * Luristan * Baba Jilan * Metallography * SEM–EDS * Annealing