عنوان مقاله [English]
Majority of metallic objects discovered from the Iron Age graveyards of Iran are made of tin bronze alloy. In this paper, some bronze objects from two Iron Age graveyards including War Kabud of Luristan and Marlik of Gilan are studied by instrumental analyses. The aim of this study is to identify the alloy composition and microstructural characteristics in some Iron Age bronze objects. For this purpose, chemical and microscopic analyses were performed by using micro-PIXE and metallography methods. Results of chemical analysis revealed that copper-zinc alloy is used to produce one object while three objects are manufactured by tin bronze (Cu-Sn) alloy. In the bronze objects, tin content is variable showing that the alloying process may has been undertaken by an uncontrolled procedure. Zinc concentration is measured about 14 wt% showing probable application of brass alloy in manufacturing this object. Results of metallographic observations showed that all objects have been cast at first while some of them have been subjected to the thermos-mechanical operations. Only in the microstructure of one object, evidences of application of cycles of working and annealing alternatively are visible.
One of the most important periods in prehistoric Iran is the Iron Age. After the Chalcolithic and the Bronze Age periods, the Iron Age began in the middle of the second millennium BC and ended in the middle of the first millennium BC (ca. 1500-550 BC) (Muscarella, 2006; Peregrine, 2002; Danti, 2013; Overlaet, 2013; Vahdati, 2018). This important period in the history and archeology of Iran cover the period before the rise of the Achaemenid Empire. According to laboratory studies on metal objects made of copper alloys in Iran, the use of tin bronze in the production of various objects in the Iron Age has been commonplace in different parts of Iran and of large collections of bronze objects belonging to this period have been obtained during excavations. In fact, it can be mentioned that the western Iran (Luristan) and northern Iran are important areas in the field of metallurgy and the development of tin bronze alloy in the Iron Age (Oudbashi and Davami, 2014; Oudbashi and Hasanpour, 2018; Fleming et al; 2006). However, studies on bronze metallurgy in the Iron Age in these areas are limited to a few laboratory studies on some metal objects in areas such as War Kabud, Sangtarashan and Baba Jilan in Luristan, Marlik in Gilan and some other sites. Of course, it should be noted that the results of these studies have revealed aspects of the metallurgy of copper alloys, especially tin bronze in the Iron Age.
Accordingly, a laboratory study has been designed to develop metallurgical studies on the Iron Age metal objects using laboratory methods, as well as to compare the results with previous studies. According to studies, it is required to develop metallurgical studies using various methods of chemical and microstructural analysis to better identify the various aspects of metallurgy of bronze alloy in the Iron Age of Iran and also compare the results of new studies with previously published results. It is also necessary to develop the methodology of metallurgical studies and to use different laboratory methods to obtain appropriate and reliable results.
Research question: The main question of this research is what has been the method of manufacturing and production of copper alloy objects in the Iron Age based on the results of studied objects in Marlik and War Kabud?
Aim of the research: In fact, the aim of this study is to identify the composition of alloy and study manufacturing technology in a number of bronze objects obtained from two important cemeteries of the Iron Age of Iran, including Marlik in Gilan and War Kabud in Luristan.
Results and Discussion
The results of micro-PIXE analysis are presented in Table 1 to identify the composition of the alloy in four samples of studied objects belonging to the War Kabud and Marlik. The results indicated that the combination of the three samples, including the seal (Marlik), the engraved bracelet and the simple bangle (War Kabud), are a binary copper and tin (Cu-Sn) alloy and copper is the main constituent of the composition. only in bangle from War Kabud, zinc is main alloy element beside copper. The results of the analysis indicate that the composition of this sample includes 80.93% copper, 14% zinc and 2% lead. On the other hand, conducted comparison on four studied objects and the results of the analysis of other objects from two sites published earlier, indicate the widespread use of tin bronze alloy as the main compound in the metallurgy of copper alloys.
Figure 1 shows the scatter plot of copper and tin in the composition of different groups of objects in the Marlik and War Kabud areas, as well as four analyzed objects in this study. Based on the presented diagram, the following can be stated:
The amount of tin in the composition of the analyzed samples varies in two areas and its amount varies between less than one percent and about 18 percent.
The amount of tin is not related to the type and function of objects, and the amount of tin is very variable, for example in the vessels from War Kabud (red dots) or Marlik (yellow dots).
In two cases, the amount of tin is very low, which indicates the lack of use of tin bronze in these two objects.
The four objects analyzed in this study (blue-green and gray dots) are similar to other previously analyzed samples (except for one case containing very low tin content and high zinc content).
It is also worth noting that based on metallurgical studies, the studied objects were made using the casting method, and in one case the work and refraction cycles (thermomechanical operations) were used on the formation of the object after casting.
The study of production method and composition of alloys in a number of metal objects of the Iron Age of Iran (1500-550 BC) was performed using chemical and microscopic analysis methods. Measurement of the elemental composition using the micro-PIXE method showed that the combination of the three objects included the two-component copper and tin alloy (Cu-Sn), indicating the use of tin bronze alloy in the production of these objects.
Only in the combination, the significant amounts of observed zinc can indicate the production of brass alloy, although only a small number of objects produced from copper and zinc alloy have been observed in prehistoric Iran. Lack of significant amount of other metal elements can be due to their lack of significant amount in the composition of the primary ore. The microstructure of the studied samples was somewhat different and indicated the use of various production and shaping methods, including Casting in the form, Thermal operation, as well as hammering have been for constructing and shaping of these decorative objects. In general, it can be said that the production of copper alloy objects in the Iron Age of Iran has mostly included the use of tin bronze and the use of various methods of construction and shaping. In fact, metalworkers in the Iron Age tried to produce tin bronze alloys using uncontrolled methods and have used various metallurgical methods to produce and manufacture a variety of objects.