A Study on Composition and Manufacturing Method of Bronze Pins from Dia Ardizi of Moorani Iron Age Site, Luristan

Abstract

Luristan, located in the west of Iran, is an important region in the field of metallurgy of the Iron Age in Iran, but comparing to its huge bronze finds or attributed to it, there is little information about the metallurgical technology in this area. Dia ardizi Moorani is one of the prehistorical sites which is excavated recently in southern Luristan. According to the various finds, such as a number of architectural structures, debris of the six grave stone, a burial, pottery, stone, bone, metal etc., during one excavation season, a settlement from chalcolithic to Iron Age has been observed in this area. A significant number of bronze objects are among the findings of this site. Therefore, to recognize the ancient metallurgical technology in the site, seven bronze pins belong to Iron Age were studied using laboratory method including metallography and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) in order to identify chemical composition and manufacturing techniques. The results of SEM-EDS showed that except for one sample made of arsenic copper alloy, all of the pins were made of tin bronze with different amount of tin which indicates the method of producing bronze alloy without control on tin amount similar to other areas of Iron Age in Iran. SEM-EDS analyses also revealed elements such as nickel, silver, arsenic and iron as minor elements in all samples which could be attributed to the impurities of the ore used in the metal extraction process. In addition, microstructural investigation clearly showed various sulfide inclusions and lead globules. Moreover, elongated inclusions can be an indication of significant hot working or cold working and annealing to achieve the final shape. Also, the micro structures of the pins revealed re-crystallized twin grain structures that indicate the use of multiple procedures, including hot-working or cold-working and annealing to manufacture these pins.\xa0 Keywords: Iron Age of Luristan, Dia Ardizi of Moorani, Bronze Alloy, Metallurgical, SEM-EDS. Introduction Study of metallic objects and archaeometallurgical materials discovered from archeological excavations is very important because of revealing the role and importance of metallurgy during the ancient time and understanding various aspects of this technology in different region and periods. Furthrmore, metals and metallurgical technology had very impressive influence on the human life in the ancient world. Therefore, archaeometallurgical investigations with the aim of providing information about mining, smelting, alloying and manufacturing technique were carried out on archaeological and historical metallic materials. Despite the importance of the Iranian Plateau as one of the origins of ancient metallurgy in Southwest Asia and Near East, it is noteworthy that a significant number of metallurgical evidences have not been studied so far in this region. Study on metal objects belong to different prehistoric sites in Iran have been applied to identify chemical composition and various metalworking processes in prehistoric period especially the Bronze Age and the Iron Age. Luristan is one of the important regions in the Iranian Plateau that is located in western Iran, and is very important in the field of metallurgy of copper alloys during the Iron Age (1500-550 BC), but it has not been studied widely in the field of archaeometallurgy in comparison to its significantly huge tin bronze objects that are find from the archaeological excavations in Luristan or are attributed to it. Therefore, there is little information about the metallurgical technology in the Luristan region. One of the recent archaeological activities in the Luristan region is the excavation of the prehistoric site of Dia \u200cArdizi of Moorani that was identified during modern works by a mechanical excavator in order to widening road accidentally in 2014. According to the architectural remains and various artefacts and cultural materials discovered from the excavations, including various architectural structures, remains of the six stone graves, different potteries, stone and bone objects, metal artefacts, etc., the settlements has been dated from the Chalcolithic to the Iron Age periods (4th to 1st millennium BC). Among the archaeological finds, a significant number of bronze objects are discovered from this site that they are not studied previously. The aim of this study is to perform an analytical study to characterize the manufacturing process applied to produce some metallic pins dated to the Iron Age. Materials and Methods Therefore, regarding to the aim of the study, seven metallic pins dated back to the Iron Age (firstmillennium BC) from the site were selected. For analytical study, a small fragment from each object was cut by jeweler’s saw and was mounted in epoxy resin and then was ground and polished. The prepared cross sections then were studied by analytical methods including metallography (OM) and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) in order to identify chemical composition of the metal/alloy and manufacturing technique used to shape the objects. Results and Discussion The results of SEM-EDS showed that except for one object made of arsenical copper alloy, all of the pins are made of tin bronze with variable amount of tin. This Sn-variable containing alloy indicates that the alloying process to obtain tin bronze alloy has been performed by ancient metalworkers without control on tin content by using some alloying processes such as smelting of copper and tin ores (co-smelting), smelting of metallic ores containing copper and tin compounds (complex Cu-Sn ores) or cementation as is observed in other tin bronze objects from the Iron Age in Iran. SEM-EDS analyses also revealed presence of other elements such as nickel, silver, arsenic and iron as minor elements in samples which could be attributed to the impurities from the metallic ores used in the metal smelting process. Presence of sulphide inclusions in the microstructure of samples also indicates the use of sulphidic copper ores (or mixture of sulphidic and oxidic copper ores) in the process of copper smelting to produce these objects. On the other hand, small lead globules are present in the SEM-BSE micrographs of the cross sections. Moreover, the elongated sulphidic inclusions can be an indication of undergoing heavy working (hammering) on the objects (pins) to achieve the final shape. Also, the etched microstructure of the pins indicated re-crystallized twin grains that proved the use of multiple thermos-mechanical procedures, including hot-working or cycles of cold-working and annealing to manufacture these pins. In addition, metallographic study showed some remnant coring superimposed on the re-crystallized grains in the microstructure of some pins. In this case, casting has been used to produce primary ingot or primary shape of the pins leading to coring that has not been removed completely by thermos-mechanical processes. In addition, slip lines were also observed in the microstructures of some samples which suggesting the use of cold-working as a final process. Conclusion An analytical study was undertaken on a series of pins excavated from Dia Ardizi of Moorani, a prehistoric site located in southern Luristan. The results showed application of Sn-variable tin bronze to produce these objects, a commonplace process observed during the Iron Age of Luristan. On the other hand, the objects are shaped by thermos-mechanical processes that has not been enough to remove all coring occurred during the casting of the tin bronze pieces, in some cases. Totally, the process of manufacturing the Iron Age pins from Dia Ardizi prehistoric site is similar with previous analytical works undertaken on the Iron Age copper-base objects from Iran, and in particular Luristan.