Formation, Development and Evolutions in Metallurgy of Copper and its Alloys during the Prehistoric Period of Iran: From Copper Usage to Evidences of Brass

The Iranian Plateau and its residents can be enumerated as one of the pioneers in progress of technology, science and knowledge in the ancient world. The development of metallurgy on the Iranian Plateau has been a topic of interest to both archaeologists and scientists for many years because of the remarkable history of the metallurgical activities in this region and concerned the wide variety of the technologies, compositions, innovations, etc. (Figure 1). Results of many investigations in metallurgy of ancient Iranian Plateau show usage of copper and its alloys in different periods of Iran history from prehistoric to Islamic era. In this paper, formation, evolutions and developments occurred in metallurgy of copper alloys have reviewed and in prehistoric period of Iran (7000-500 BC) based on archaeological and archaeometallurgical investigations. The results of analytical and archaeological studies state the first metal used in prehistory of Iranian Plateau has been copper. It is apparent that the Iranian Plateau has a significant history in the metallurgy of copper and its alloys in the prehistoric period. It has begun in the Neolithic period and during 4000 years has transformed from forming objects by hammering native copper to extensive smelting of oxidic and sulphidic ores in Chalcolithic era. The ancient metalworkers used native copper to manufacture small decorative objects, as it was observed in Ali Kosh Neolithic site (Figure 2). It was developed by shaping native copper to produce small functional objects such as objects discovered from late Neolithic sites (Figure 2). It was continued by melting native copper to cast objects. Furthermore, metallurgical processes were extended by smelting copper oxidic ores in crucibles during 5th and 4th millennium BC. In fact, the chalcolithic period (ca. 4500-3000 BC) is the period of emergence and development of smelting of oxidic and then sulphidic copper ores in small scale. There are numerous evidences of copper smelting in the chalcolithic archaeological sites such as Qabristan, Tal-i Iblis, etc. showing the crucible smelting technology in different regions of the Iranian Plateau (Figure 3). This technology was developed by large-scale smelting of copper ores, as it was discovered in Arisman, near Kashan. Of course, the metallurgical technologies during the late chalcolithic period (ca. 3500-3000 BC), is a mixture of crucible smelting and furnace smelting (Figure 4). It is worth noting that the main metallic composition in the Chalcolithic period is arsenical copper that may has been produced accidentally by smelting As-bearing copper ores leading to obtain metallic copper with significant amounts of arsenic (Figure 5). Nevertheless, some evidences of intentional arsenical copper production have been found during late Chalcolithic and early Bronze Age archaeological sites, such as Arisman. The third millennium BC was occurred with occurrence of a new alloy, tin bronze. Early evidences of this technology was observed in western Iran, Luristan at the beginning of the third millennium BC. Some tin bronze objects with significant amounts of tin were detected among copper and arsenical copper objects discovered from Early Bronze Age graveyards such as Kalleh Nisar and Bani Surmeh (Figure 6). Although, early evidences of tin bronze metallurgy have been occurred in the third millennium BC but this technology was limited for about 1000 years in western and south-western Iran. Results of analytical studies revealed that the main copper base metallurgy has been copper and arsenical copper in other regions of the Iranian Plateau during the third millennium BC. Tin bronze was emerged in central Iran during the middle and late Bronze Age (ca. 2500-1500 BC) such as evidences from Malyan (Fars). Therefore, no evidence of tin bronze has been observed in eastern Iran, even at the mid of the second millennium BC. Although tin bronze was occurred during the early Bronze Age and was spread during middle and late Bronze Age in western and central Iran, but it was the main copper-based alloy during the Iron Age of the Iranian Plateau (ca. 1500-550 BC). Results of analytical investigations states that tin bronze has been the main material in production of metallic objects at the whole of the Iranian Plateau. The ritual objects from Iron Age graveyards of western, northern and central Iran show application of tin bronze to produce these objects. The large scale tin bronze production is the Iron Age of Iran is an interesting aspect, as this alloy has been observed in different archaeological sites such as Hasanlu and Marlik (Figure 7). One of the important collections form this category are the Luristan Bronzes, the enigmatic and extraordinary metallic objects that were produced in high-quality craftsmanship and were placed in graves and sanctuaries as ritual objects (Figure 8). The results of chemical analysis on the Luristan Bronzes, as well as other tin bronze objects from Iran, shows that thy may have been produced by uncontrolled alloying methods. In fact, controlling tin content has not been an important case for ancient metalworkers during the Iron Age (and also the Bronze Age). Also, no correlation between tin content and object’s typology is visible in the tin bronze objects from prehistoric Iran, that is in contrast with accent cuneiform texts from Mesopotamia. It may show that tin bronze metallurgy in the Iranian Plateau may has not been in connection with the Mesopotamian technology. Although, evidences of copper-zinc objects have been observed among the other copper based artefacts from second and first millennium BC, but it can’t be stated that this material has been used as a deliberately produced metallurgical product. It is more probable that these limited examples of copper-zinc objects (probably brass) were produced by smelting Zn-bearing copper ores. Nevertheless, evidences of low-zinc objects from Tappeh Yahya (southern Iran) and Luristan shows occurrence of early brasses (probable accidentally) during the prehistoric Iran. Results of numerous analytical studies on the prehistoric copper base metallurgy during the last decades revealed interesting aspects of this technology from about 8000 BC. It has been started by using native copper and then developed by smelting oxidic and sulphidic copper in crucibles during the Neolithic and Chalcolithic periods. Large scale smelting sites also were occurred during the late chalcolithic and early Bronze Age. Furthermore, application of different copper alloys such as arsenical copper, tin bronze and brass from Chalcolithic to Iron Age are important developments in archaeometallurgy in Iran. Totally, process of formation and development of copper metallurgy in prehistoric Iran has been introduced and revised based on technical and archaeological finds belonging a period about 6500 years (Figure 9).