Document Type : Research Paper

Authors

• Vahid Porzarghan ¹
• Maryam Alizadeh ²
• Mehdi Razani ³

¹ PhD in Conservation, Cultural Histirical Properties, MCTH, Fars (Zarghan), Iran.

² Graduate, Faculty of Art and Architecture, University of Zabol, Zabol, Iran.

³ Associate Professor, Department of Conservation & Archaeometry, Faculty of Cultural Materials Conservation, Tabriz Islamic Art University, Tabriz, Iran (Corresponding Author).

Abstract

Kerman Province, the largest province in the country, is located in the southeast of the Central Iranian Plateau. This province encompasses numerous archaeological sites such as Jiroft, Yahya, Tepe Abrish, among others. To date, a significant number of metal artifacts have been recovered from these sites, most of which are housed in museums. This paper investigates two copper-based objects from the Herandi Museum in Kerman: Object 1 (registration number 11593), a vessel-form artifact, and Object 2 (registration number 11594), an artifact in the form of a mirror or a functional tool. The primary research questions addressed are: What are the manufacturing techniques, alloy composition, identification, and stability of the corrosion products on these bronze objects from the Herandi Museum, and what role do these factors play in the preservation of these two artifacts? To this end, analytical techniques including X-ray radiography, Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), and X-ray Diffraction (XRD) were employed. Based on the X-ray and SEM-EDX analyses, it was determined that these artifacts are composed of a copper-arsenic (Cu-As) alloy. Given the high percentage of arsenic, they are classified as arsenical bronzes, which exhibit good workability and toughness. Furthermore, SEM analysis indicates that these two objects were manufactured using cold-working and annealing techniques. Radiographic images reveal a sound metallic core in both artifacts. XRD analysis, conducted to identify the principal corrosion products (damage) on these objects, identified chloride, carbonate, and oxide corrosion products. pH measurements of the corrosion products, correlated with the Pourbaix diagram, indicate that the most significant corrosion products at pH levels of 4.5 and 5 are malachite, atacamite, and brochantite. These corrosion products were confirmed to be present on the artifacts through X-ray diffraction analysis.
Keywords: Archaeometallurgy, Arsenical Bronze, Copper Alloys, Corrosion Products, SEM-EDX, Kerman, Iran.
 
1. Introduction
Arsenical copper alloys represent one of the copper-based alloys of particular importance in the Early Bronze Age. Throughout the 4th millennium BCE and into the Late Bronze Age, arsenical copper alloys were produced and utilized across the Near East, with tin bronze completely replacing arsenical alloys around 1500 BCE (Lechtman H. et al., 1996).
Recent decades of metallurgical research on the Iranian Plateau demonstrate that copper-arsenic was not merely a ‘provisional’ or ‘inferior’ material, but rather a successful and widely used alloy with desirable mechanical properties and a distinct place in the economy and technology of the Chalcolithic and Bronze Age periods (Heskel, 1982; Thornton, 2009). Archaeological evidence points to a complex technological sequence in southeastern Iran. This sequence began with the sophisticated use of native copper at sites such as Tepe Yahya in the 5th millennium BCE (Thornton et al., 2002), gradually evolved towards the exploitation of natural copper-arsenic minerals like domeykite (Cu₃As) and algodonite (Cu₅₋₆As), as well as the smelting of copper from oxide ores in the 4th millennium BCE (with early crucible smelting evidence observed at Tal-e Iblis), and culminated in the widespread production of alloyed artifacts in the 3rd millennium BCE (Karlovsky & Potts, 2001). A potential, rare source of arsenic for the region is the Anarak-Talmessi polymetallic ore deposit in central Iran, which contains significant surface deposits of these arsenides (Smith, 1965; Heskel & Karlovsky, 1980). However, as Thornton (2009) emphasizes, direct archaeological or archaeometric evidence for prehistoric exploitation of these mines is still lacking, and this hypothesis requires confirmation through future field studies. The production and use of arsenical copper was not confined to a single region; evidence has been reported from numerous key sites in southeastern Iran, including Shahr-i Sokhta (Hauptmann et al, 2003), Shahdad (Vatandoust, 1999), Jiroft (Majidzadeh, 2003), and Espidej-Bazman (Sabouhi Sani, 2017; Meier et al, 2011; Pourzarghan et al, 2023). The status of this alloy type in the creation of Early Bronze Age artifacts reflects the functioning and behavior of a civilization capable of harnessing this specific metallurgical knowledge for the purposes of daily life. Its widespread distribution indicates a network for the exchange of technical knowledge and possibly raw materials across a vast area of the Iranian Plateau. Nevertheless, a detailed examination of the technological trajectory and distribution patterns of these alloys, particularly in relation to broader cultural interactions during the 3rd millennium BCE (such as Proto-Elamite influence and contacts with Central Asia), still requires integrated research (Thornton, 2009). In any case, the first raw material used for copper smelting was undoubtedly obtained from oxide ore deposits (Rapp Jr., 1998). As mentioned, arsenical alloys constitute a significant part of the highly important cultural artifacts of the Early Bronze Age and hold a special place within cultural heritage and archaeology. Most of these artifacts have been recovered from sites such as Tepe Sagzabad (Boscher, 2016; Mortazavi et al, 2011), Arisman (Thornton, 2010), Tepe Yahya (Thornton et al., 2002; Thornton and Karlovsky, 2004, 267; Piggot, 2004:30-34), Tepe Hissar &  Tepe Malyan (Thornton et al., 2009), and Tepe Miamantabad (Kashani et al., 2013) (Thornton and Lamberg-Karlovsky, 2004, 267). Iran is, in fact, one of the most ideal regions for the production of arsenical copper, involving the smelting of complex arsenic-sulfide ores (Thornton et al., 2009), as discussed in the aforementioned sites, and holds particular importance in archaeometallurgical investigations.
Given that the artifacts under study lack a specific provenance and are housed in the collection of the Herandi Museum in Kerman (Daei Parizi, 2016; Naghavi, 2010), they could potentially be among the artifacts recovered from archaeological sites in Kerman province. Information on manufacturing technology encompasses the materials and methods for evaluating their metallurgy in ancient times (Thornton, 2009). Most metal artifacts, having been buried for extended periods, have undergone corrosion (Scott, 2002; Chase, 1999). The artifacts under study exhibit corrosion with green and red surface layers, along with deposits of dust. In any case, these objects are examined as historical documents and from an aesthetic perspective .In this context, the collection of metal artifacts in the Herandi Museum of Kerman, which largely lacks precise provenance, holds particular importance as a research dataset. Preliminary stylistic and typological analysis reveals similarities between this collection and finds from major sites in southeastern Iran, such as Shahr-i Sokhta, Shahdad, and Jiroft. Therefore, a scientific study of a portion of this collection can contribute to a better understanding and identification of the technical capabilities, alloying preferences—aiming at stability in corrosion products as per the conducted analyses—and exchange networks in the region during the Late Bronze Age. The primary objective of this research is to identify the manufacturing technology of two metal artifacts with unknown provenance in the Herandi Museum of Kerman, focusing on the analysis of their metallurgical microstructure (metallography).
 
2. Introduction to the artifacts under study
This research focuses on two bronze artifacts from the Harandi Museum in Kerman, Iran. Artifact No. 1, with registration number 11593, is a plate with outward-folded edges, as shown in images (1a-c). This vessel is simple and devoid of decorations, with its surface covered by environmental and mineral deposits. It has also undergone significant corrosion. On the rim of this artifact, an inventory number is inscribed. Artifact No. 2, depicted in images (2a-b) with registration number 11594, consists of a handle and a base (support). Based on the examination and research conducted on these artifacts, it can be concluded that the two objects studied in this project are confiscated items and belong to the Harandi Garden Museum in Kerman. The artifacts likely date back to 2600–2400 BCE and serve as historical evidence from this region.
 
3. Materials and Methods
Analytical studies were conducted using optical microscopy. Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDX) was employed to identify the elemental composition and alloy structure of the artifacts. The SEM-EDX analysis was performed using a VEGAII TESCAN instrument manufactured in the Czech Republic. The EDS analysis was carried out using a Rontec Quantax/QX2 system from Germany at the Razi Metallurgical Research Center in Tehran, Iran. X-ray Diffraction (XRD) was utilized to determine the crystalline phases and identify the crystal structure of the samples. For this purpose, an X-ray diffractometer model PW1800 by PHILIPS (supplied by Bime Gostar Taban Co.) was used. Sample preparation involved creating a homogeneous powder from the study samples with a grain size of less than 45 microns, followed by drying at 60°C for 24 hours to remove moisture. The experimental parameters included a Bragg angle (2θ) range of 5° to 70°, a step size of 0.05°, and a time per step of 1 second. The X-ray source used was a copper tube (Cu-Kα) with a wavelength of 1.5406 Å. The obtained data were processed using the HighScore Plus software (version 2016) and matched with the ICDD database. Additionally, pH measurements of the corrosion products were conducted using a Metrohm 744 pH meter (manufactured in the USA) by ASTM D4972-1 standards at the Faculty of Conservation and Restoration in Isfahan.
 
4. Discussion
The radiography of Artifact No. 1 (Registration No. 11593) reveals a healthy metallic core, as indicated by the gray tonalities in the images. Similarly, the central portion of Artifact No. 2 (Registration No. 11594) also exhibits a well-preserved metallic core. According to Figure 1, environmental contaminants such as gypsum and calcite indicate the presence of a carbonate and sulfate-rich environment. Additionally, the presence of albite and muscovite suggests the existence of alkali feldspars in this environment. To identify and assess the stability of the corrosion products on these artifacts, the samples were subjected to pH testing. The pH of the corrosion products was measured to be between 4.5 and 5, indicating acidic conditions. These findings were further analyzed using Pourbaix diagrams, as illustrated in Figure 2. Based on the EDX analysis, it was determined that the artifacts in question contain a high percentage of arsenic. Artifact No. 1 contains 1.1% arsenic, while Artifact No. 2 contains 2.7% arsenic, classifying them as arsenical bronzes (see: Fig.1).
 Additionally, in Artifact No. 2, approximately 3% chlorine was detected at Point 2. The presence of sulfur at Points B and C in Image 7 indicates the formation of corrosion products such as brochantite and anilite. At Point D in Image 7, zinc was detected at 2.7%. Furthermore, 1.46% iron was identified as an impurity. Traces of zinc were observed in both artifacts. The presence of zinc in these artifacts suggests that smithsonite (zinc carbonate) may have been roasted to oxidize it, then heated in a crucible with metallic copper and charcoal to produce various copper alloys (Pollard & Heron, 1996). In Image 8 of Artifact No. 2, the compacted layers in its structure, arranged in parallel and oblique patterns, indicate that the manufacturing technique involved cold-working. Subsequently, annealing was employed to shape and form the vessel (Scott, 1991, p. 8).
 
5. Conclusion
The results of this laboratory study on two arsenical bronze artifacts from the Herandi Museum in Kerman provide new insights into the preservation status, material composition, and manufacturing technology of metallurgy in southeastern Iran during the late 3rd millennium BCE.Radiographic findings indicate that both objects possess a sound metallic core, with only Object 2 lacking a core at its edges.The copper-arsenic alloy composition, containing zinc—likely obtained through the co-smelting of arsenic- and zinc-rich oxide/carbonate ores (possibly from ores such as Smithsonite (ZnCO₃) and Sphalerite (ZnS))—aligns with findings from key sites such as Tepe Yahya and Shahdad. This reflects a technological mastery in the selection and preparation of complex raw materials, reinforcing the region’s potential role as a center for producing multi-component alloys. Metallographic examinations confirm a conventional manufacturing sequence involving cold working followed by annealing. This procedural pattern, employed to balance hardness and malleability, demonstrates the establishment and dissemination of an advanced “technological style” in the region. From a conservation perspective, the identification of active bronze disease, caused by the presence of chlorides and an acidic surface environment, serves as a serious warning regarding the vulnerability of these objects to fluctuations in ambient humidity. This finding clearly underscores the necessity of implementing preventive conservation interventions and maintaining precise control over display and storage conditions. Overall, this study demonstrates that even artifacts lacking a specific provenance can, through targeted laboratory methods, reveal valuable information about their technological lineage and material status. The present data serve as a foundation for linking this museum collection to the broader geography of metal production and consumption in Bronze Age Iran, while also highlighting the need to expand such studies to include more samples and employ advanced tracing techniques (such as lead isotope analysis) to more precisely map the networks of raw material and technology exchange in the future.

Keywords

• Archaeometallurgy
• Arsenical Bronze
• Copper Alloys
• Corrosion Products
• SEM-EDX
• Kerman
• Iran

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