Mendeley  

Zotero  

Kamali A A, Azizi Kharanaghi M H, Beheshti S I, Aarab A. (2025). Exploration of Mining and Slag Sites of Ancient Metal Smelting in Khatam County, Yazd Province. Parseh J Archaeol Stud. 9(32), 37-64. doi:10.22034/PJAS.832
URL: http://journal.richt.ir/mbp/article-1-832-en.html

Abstract
The slag sites under study are located in Khatam County, Yazd Province. In the archaeological surveys of Khatam County in 1400 AH, twelve metal smelting sites were identified through abundant metallic slag, and each of these sites was sampled. Petrographic analysis revealed that the predominant slag is iron, with only one instance of copper slag. The sites where metal smelting occurred, attributed to historical and Islamic periods based on pottery, exhibited evidence of iron smelting and its compounds in eleven samples. These samples contain metallic minerals such as wustite, marcasite, hematite, and magnetite. Marcasite and wustite minerals are related to smelting furnace processes and are products of mineral substances. It appears that in some mines in the region, magnetite and hematite are the predominant minerals, while in others, hematite is the predominant mineral, with a smaller amount of magnetite, which is evident in these primary minerals within the slag. Another sample related to copper slag exhibited small vesicular structures and limited copper ore minerals (chalcopyrite, digenite, and metallic copper) within the slag matrix. Alongside these primary minerals, there is a flow-like green glassy component indicating high furnace heat. The analytical results show that the MgO content in the samples is less than the amount of lime. Therefore, the limestone in this area is mainly ordinary limestone and not dolomite. Chemical analysis revealed that metal workers in this area were more successful at producing sponge iron.
Keywords: Archaeological Survey, Slag, Iron, Ancient Mining, Khatam.

Introduction
Iran has long been recognized as a center for mining and metal smelting. Archaeological evidence indicates that northern and central Iran are among the oldest centers of metallurgy in the world. The presence of rich mineral reserves in Iran, among other factors, has influenced the growth of mining and metalworking in this region (Momenzadeh, 2005). Due to the existence of various metal ores and advanced cultures in Iran, this area can be identified as one of the main hubs of technological innovation in the field of ancient mining and metalworking. Khatam County, located in the southern part of Yazd Province, holds particular significance in the realm of iron slag. One of the earliest efforts to produce steel worldwide took place in this region (Alipour et al., 2021). Considering the evidence of steel production in this area, it is essential (Alipour, 2017) to understand the role Khatam played in iron production during the Islamic and Sassanian periods. To investigate this matter, 12 sites in Khatam County were selected for studying iron slag. The main objective of this research includes petrographic and geochemical analysis of the slag to identify the type of extracted metal(s) and the extraction process and production of metal(s) at these sites. Additionally, the provision of necessary minerals for mining in this area is also under scrutiny. Historical and field research methods were employed for this study, involving the collection of data and archaeological investigations; field studies, such as topographic mapping, photography, identification of sites and metal smelting furnaces; and examination of samples using polarizing microscopes and XRF devices. This research has addressed primary inquiries related to the type of metals in slag, the mining process, and metal production at Khatam’s iron slag sites. Overall, Khatam County held significant importance in the production of metals during ancient and Islamic times. This region is recognized as one of the ancient mining and metalworking centers, and further research into the history and mining processes in this area could provide additional insights into the history of metalworking in Iran.

Discussion
Based on XRF chemical analyses of the slag, the results indicate that the majority of the mineral content in these slags consists of iron ore, with only one case showing the presence of copper. The CaO concentrations in these slags range from 3.59 to 28.41%, and an increase in CaO leads to the production of calcium-rich olivine. The type of slag (flow, permeable, massive, or furnace bottom) significantly impacts the results of chemical analysis and the ratio of oxides of the main elements (metallic oxides and silica). Additionally, the high amount of CaO facilitates the formation of a calcium-rich silicate phase. Petrographic microscopy studies confirm these findings, revealing observable olivine phases and primary silicate phases with metallic iron minerals such as magnetite and hematite. Due to the silica content, the addition of limestone to the smelting process increases the amount of duplex iron (Fe3O4). Consequently, silica stabilizes triplex iron oxide (hematite), while limestone stabilizes spinel iron oxide (magnetite). Moreover, microscopic examinations primarily reveal metallic minerals such as magnetite and metallic iron. Furthermore, sponge iron, like many other ancient civilizations in the region under study, was produced. The production of this type of iron requires less technical knowledge than other types of iron (Abbasnejad, 2009).
Surveying the region revealed that plants such as pistachios and wild almond produce high-quality charcoal. Since blacksmiths have no idea about using additional limestone in the furnace, the smelted slags were highly adhesive, leading to significant iron loss. The use of limestone in iron removal creates slags with fine properties that are easily separated from the iron (Abbasnejad, 2009). A good slag resulting from smelting should contain 30 to 40% limestone. Tests conducted on iron ore in this region show limestone percentages ranging from 3.59 to 28.41%. The slag analysis results also indicate a small amount of limestone, averaging approximately 11.38%. The deficiency of these two elements in slag, as they play crucial roles in reducing smelting heat and separating iron from slag, can indicate high iron levels and the inadequacy of slag (adhesiveness, viscosity, high density), resulting in low-quality sponge iron. The percentage of Fe2O3 ranges from 23.20 to 74.25%, and the percentage of Al2O3 ranges from 0.003 to 0.94%. The percentage of MgO in the tested slags is less than 0.003%. According to the mineral analysis, the most important iron minerals in this region include hematite (Fe2O3) and magnetite (Fe3O4). Due to technical flaws in these furnaces, sponge iron contains impurities such as silica, phosphorus, aluminum oxide, manganese oxide, and other metallic oxides, as confirmed by various tests conducted on ore and slag.

Conclusion
Eleven samples from the metal smelting site showed evidence of iron smelting and its compounds. In these samples, metallic ores such as wustite, marcasite, hematite, and magnetite are observed. Marcasite and wustite ores are related to smelting furnace processes and are mineral byproducts. It seems that in some mines in the region, magnetite and hematite are predominant, while in others, hematite is less prevalent, and magnetite dominates. Additionally, in the sample related to copper smelting slag, small and limited vesicles of copper ores (covellite, digenite, and metallic copper) are observed alongside a part of the green glassy matrix, indicating high furnace heat. This primary mineral evidence is observed in the slags. Considering the changes in the calcium oxide (CaO) concentration, it can be inferred that this substance was added during smelting operations to aid in smelting and reduce the temperature of the furnace materials. The microscopic results of some slags reveal primary minerals, mostly hematite and magnetite metallic ores, indicating a magmatic origin for the utilized minerals. The percentages of silica (SiO2), magnesium, and aluminum in these slags are relatively low. Analyses of these slags and iron stones from this region show that a deficiency of CaO and SiO2 leads to iron loss in the slag while increasing the iron content within it.
Based on this research, it is likely that iron ore was extracted from mines near the site and was subsequently transported to this location. Given the presence of iron mines at distances of 8, 10, and 15 kilometers from these sites, these mines are likely the source of these slags. Regarding the archaeology of the region, historical references indicate that the area held significance and prominence in various historical periods, particularly during historical and Islamic eras. However, due to insufficient information about the archaeology of the region and the lack of precise dating of these sites, accurate dating of these sites is unfeasible.