1 Introduction

Research on the economies of eastern Eurasian pastoralists focuses mostly on subsistence economy regardless of the discipline, be it archaeology or history. This is even more true when it comes to research on the Mongolian Plateau. Targeted studies on craft production in pastoralist communities and the steppe empires are rare (Brosseder and Miller 2011, 27–28; Reichert 2018, 2020, esp. 27–35; Ishtseren et al. 2020; Vodyasov et al. 2021), despite the fact that it is widely recognized as an important field of research. Iron metallurgy is no exception, despite huge advances over the past years (Ishtseren et al. 2020; Reichert 2020, 99–110; Vodyasov et al. 2021).

Viewed from a global perspective, iron metallurgy arrived late in eastern Eurasia, appearing at the end of the third century BCE, roughly one millennium after it initially spread from northern India (Turner 2020). The existence of iron metallurgy should not be confused with the incipient stage of iron use, when a few objects made of iron are found but no slags or production debris are known. This is true for both the Near East and Europe, as well as eastern Eurasia and Asia (Wagner 2008, 91–93; Bebermeier et al. 2016; Erb-Satullo 2019, esp. 564–566; Kašuba et al. 2019). The ways in which this new material and technology were adopted were, however, wildly diverse across Eurasia, as metal craftsmen also attempted independent approaches (Kašuba et al. 2019, 199).

Explanations given for the spread of iron technology, especially with regards to the Near East, have been discussed recently (Erb-Satullo 2019, 576–583). Erb-Satullo sees two main strands of arguments: the first group that focuses on material properties pursue the argument that iron, especially with carburization, is harder than bronze, which eventually led to the replacement of bronze. However, the synopsis of case studies in the Near East indicates that the evidence is ambiguous and suggests that “increasing hardness may have been the consequence of the widespread use of iron, rather than its initial impetus” (Erb-Satullo 2019, 579). The second group of explanations focuses on the organization and economics of iron production in relation to bronze. One of the widespread and often repeated, but also criticized hypothesis, is the suggestion that a tin shortage drove the rise of the iron industry (Snodgrass 1971; Waldbaum 1980).

Only with more abundant research on workshops and production debris over the past two decades, research can start investigating the link of the spread of iron and the social context and oganization of iron (and copper alloy) metallurgy and the cultural and political factors that enabled the spread (Erb-Satullo 2019, 583; 593).

Turner, who studied the spread of iron metallurgy through Afro-Eurasia identified several stages, with the first being the time “when iron becomes a material used for multiple object types (military and agricultural tool use, plus at least one of another used type such as construction, utilitarian or ornamental) with increasing frequency…” (Turner 2020, 23). He characterizes the second stage as the time period when “iron first almost completely replaces an older material for a critical object type …” (Turner 2020, 23). In our geographical area of interest, the Mongolian Plateau, these first two stages cannot be differentiated due to the low resolution of the current chronology. But unlike several other regions of Eurasia, the moment when this new material came into regular use can be identified clearly, and it coincides with the early **ongnu Empire from the second century BCE onward. At that time, a variety of objects were made from this new material: weaponry – especially arrowheads – knives, parts of horse gear, and agricultural tools. These objects are found abundantly in burials. At the same time also debris from local iron production, slags, and iron furnaces are attested. As is the case in other regions of the world, both the debris and increase in the numbers of iron artifacts indicate a new period “in the trajectory of iron adoption” (Erb-Satullo 2019, 567). The contemporaneous processes of empire formation and the iron metallurgy boom highlight the link between innovation and socio-economic conditions.

In the eastern Eurasian Steppe, metallurgy has been studied abundantly in the Minusinsk Basin, where not only a very developed Late Bronze Age metallurgical industry thrived but also a flourishing iron industry. Iakov Sunchugashev, who studied iron smelting sites in Tuva and the Minusinsk Basin for over three decades between the 1960s and late 1990s, asked the crucial question that remains valid for the region today: Since iron smelting had already been mastered during the Tagar period (c. 800–300/200 BCE), why was there only a surge in iron smelting activity during the 2nd to first centuries BCE? (Sunchugashev 1979, 22–23). Because of the abundance of metallurgical sites in the Minusinsk Basin this region has been viewed as the center, from where iron-smelting technology was adopted in Mongolia, before it was further developed locally (Sasada and Amartuvshin 2014, 1023; Sasada 2015, 50; Sasada and Ishtseren 2020, 12). In addition, while it has been noted that the iron’s total adoption in Eastern Eurasia is associated with the **ongnu polity, this process is dated to the late first century BCE and the early first century CE, thus only during the empire’s later phase (Amzarakov 2015a; Vodyasov et al. 2020, 2021).

A rapidly growing number of investigated **ongnu period iron smelting furnaces, together with a larg series of radiocarbon dates from the past decade in Mongolia beg for revisting the state of research on iron metallurgy in Mongolia and its neighboring regions. In this article, we take a closer look at the evidence, question the primacy of the Minusinsk Basin with respect to the spread of the iron technology, suggest that the spread of iron metallurgy in the eastern Eurasian Steppe was driven by the **ongnu polity from its very beginning. While it has been acknowledged that there are three different types of furnaces in the **ongnu Empire, among them an underground furnace with a tunnel construction, which reflects a distinct way of applying iron technology in eastern Eurasia. We suggest, based on the current data, that the processes that drove the rise of the iron industry in the **ongnu Empire also led to the invention of this new furnace type. Our discussion is used to better contextualize several furnaces published for the first time in this article from the Orkhon Valley in central Mongolia, which represent the oldest, most securely dated furnaces discovered so far.

2 Beyond the artifact: debris from iron production in the archaeological record

Before we investigate iron-smelting on the Mongolian Plateau and its neighboring regions, it is worth considering the kind of remains in the archaeological record that attest to iron production. Each phase in the iron production process creates its own unique debris (Fig. 1): the first step is ore extraction (prospecting and gathering ore). The second is smelting (converting ore to bloom, which creates smelting slags in the process), followed by primary smithing (converting bloom to bars/billets, which creates primary slags) and secondary smithing (commonly known as forging or blacksmithing; converting bars/billets into iron objects, which creates smithing slags and hammerscale in the process) (Arnoldussen and Brusgaard 2015, 115).

Fig. 1
figure 1

Schematic overview over the steps in the iron production process from ore extraction to secondary smithing (Arnoldussen and Brusgaard 2015, 116 Fig. 1)

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The most archaeologically visible and important sources of information on the metallurgical process are slags (Arnoldussen and Brusgaard 2015, 115; Hauptmann 2020, 199–201; Rijk 2007), which reflect both the smelting and processing methods (Fig. 1). Beyond general distinctions between flowing slags, furnace slags, and dome-shaped slags, slag typologies are often idealistic compared to real-world specimens that are not clear-cut. Numerous factors influence the appearance of slags, with slags from the same smelting or processing methods often turning out differently, but also those from different processing methods appearing similar. Above all, slags are often preserved fragmentarily, which makes a typology difficult (Rijk 2007, 114). However, single features of slags point to either smelting or primary smithing: dome-shaped slags are generally produced from iron processing, while the flowing structure is produced from smelting (Rijk 2007, 114–19). Additionally, analyses of the chemical composition and properties can support distinctions, and issues caused by slags’ heterogeneous structures can be mitigated by examining larger numbers of samples, since a single analysis is subject to too many uncertainties (Rijk 2007, 113–14).

Iron furnaces are less abundant in the archaeological record and vary considerably across the globe (Pleiner 2000, 141–94). When Pleiner wrote his overview, nothing was known about iron furnaces in the Mongolian Plateau, but Pleiner noted that in regions next to our area of interest – the Minusinsk Basin and Tuva – underground tunnel-type furnaces are characteristic. He points out that such furnaces are extremely well insulated and mechanically very stable, though the disadvantage was the limit “to which production capacity could be increased” (Pleiner 2000, 188–89). He hypothesizes that this well-insulated underground type was developed in areas with hard climatic conditions, although at the time he wrote the book, he could not trace its origin (Pleiner 2000, 189). He did not know about remains even further east at Ivolga in the Transbaikal region, where such an underground tunnel furnace from the **ongnu period between the late third century BCE and end of the first century CE has been known since the 1950s (Davydova 1956, 273–274 Fig. 7; Davydova 1995, pl. 176).

3 Before the **ongnu: iron smelting furnaces in Eastern Eurasia

It is quite clear that iron metallurgy in Mongolia was adopted from outside. **ongnu iron technology and furnace construction techniques were not, however, adopted from the regions of Kazakhstan and China.Footnote 1

Iron smelting is known much earlier in Kazakhstan than on the Mongolian Plateau, as attested by iron slags found in the metallurgical center of Kent at the settlement of Alat, which dates no later than the twelfth century BCE (Varfolomeev et al. 2016, 8) and represents the earliest experiments with iron production in central Kazakhstan (Varfolomeev et al. 2017). In the lower layer of Alat only copper smelting was attested, while in the upper layer four iron furnaces, several pits, and two buildings came to light (Žauymbaev 2013; Evdokimov and Zhauymbaev 2013; Varfolomeev et al. 2017). The excavated iron kilns were constructed differently than the ones known from Mongolia (Zhauymbaev and Evdokimov 2008; Evdokimov and Zhauymbaev 2013, 436). Besides furnaces for iron-smelting, kilns for roasting ore were also identified (Žauymbaev 2013, 437).

The widespread use of iron in China is known from the third and second centuries BCE (Wagner 2008, 112). Since the earliest iron artifacts are known from **njiang (Wagner 2008, 91–93), it was suggested that bloomery iron technology was transmitted to China from the northwest, the Fergana Valley, or the Eurasian Steppe (Wagner 2008, 97; Qian and Huang 2021, 4). However, from the Warring States period (457–221 BCE), blast furnaces were used in China to produce cast iron, whereas the bloomery smelting process did not catch on (Wagner 2008, 105–7; Qian and Huang 2021).

The Evidence from the Minusinsk Basin.

Sasada and Ishtseren point out that the **ongnu adopted iron metallurgy from the Minusinsk Basin, where the metal industry was already well developed in the Bronze Age, and this has been the subject of much research (Sunchugashev 1969, 1975, 1979, 1993). Copper smelting furnaces of the Tagar and Tes’ periods in the Minusinsk Basin are very different in overall construction to the iron smelting furnaces (Sunchugashev 1975, 87 Fig. 28; 94 Fig. 34). But with its long flourishing bronze industry, these experts in pyrotechnology had gathered experience in managing heat and airflow to furnaces for centuries prior to the **ongnu. By 1979, Iakov Sunchugashev had identified 34 sites from different periods associated with iron production in the Minusinsk Basin and excavated many of them (Sunchugashev 1979, 13 Fig. 1). However, little research has been conducted on this issue after him (Murakami 2013; Amzarakov 2015a, b; Tulush 2017).

The oldest iron smelting furnace in the Minusinsk Basin is found at Ulus Zimnik, and it is said to belong to the Tagar period because diagnostic Tagar pottery was found there (Sunchugashev 1993, 70). The kiln was truncated by a modern canal and not preserved completely (Fig. 2.1). From the image published (Sunchugashev 1993, 70 Fig. 63), it appears that this furnace was a rectangular pit measuring 60 × 30 cm and 40 cm deep. Similar pit furnaces were uncovered in Sarala (Fig. 2.2), and Sunchugashev dated the undecorated pottery to the late Tagar period (Sunchugashev 1993, 89–96), which he assigned to the third to second centuries BCE (Sunchugashev 1993, 91). Since his publication, the absolute dates and cultural attributions have changed: for example, it was still customary in the 1990s to assign the Tes’ phase to the Tagar culture (Parzinger 2006, esp. 621). Since neither Sunchugashev’s descriptions of the pottery nor his black and white drawings allow for a more refined chronological attribution, the sites could either be what is currently defined as late Tagar (fourth to second centuries BCE) or the subsequent Tes’ culture (second to first centuries BCE) (Amzarakov 2015b).

Fig. 2
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Iron kilns from the Minusinsk Basin. 1 Ulus Zimnik (Sunchugashev 1993, 70 Fig. 63), 2 Sarala (Sunchugashev 1993, 92–93 Fig. 88–89), 3 Tolcheya (Amzarakov 2015a, 105 Fig. 5), 4 Troshkino-Uius (Amzarakov 2014, 34–35, Figs. 7 and 8)

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Most of the furnaces that Sunchugashev investigated, however, were attributed to the Tashtyk period (ca. first to fourth centuries CE by current definitions) based on pottery unearthed at some of the sites (Sunchugashev 1979; 1993). The installations were regularly accompanied by finds of clay tuyères (Sunchugashev 1993). The dominant furnace type attributed to the Tashtyk phase is an underground furnace with an attached tunnel; however, its appearance varies considerably (Sunchugashev 1979, 30 Fig. 3; 34 Fig. 7; 40 Fig. 13). Whether this variation indicates changes over time or different kilns being used concurrently remains to be determined. In any case, it remains uncertain when such underground tunnel-type furnaces came into use, since we lack a decent series of modern radiocarbon dates. There are two modern excavations at iron smelting sites that provide insight also into the dates. In Tolcheya, a Russian-Japanese team has uncovered a central rectangular pit measuring 1.5 × 1.2 m surrounded by five iron smelting kilns (Amzarakov 2015a, b). The kilns were connected to the central pit by underground tunnels (Fig. 2.3). In one instance, a ceramic pipe was preserved, indicating that the bellows were operated from the central pit (Amzarakov 2015a, 96). A radiocarbon date places the use of this particular installation between 41 BCE and 56 CE (Amzarakov 2015a, 98).Footnote 2

At the site Troshkino-Uius, six furnaces of a different type were investigated (Murakami 2013; Amzarakov 2014). A rectangular- or oval-shaped furnace, of which a shallow pit remains, was connected to a larger roundish disposal pit (Fig. 2.4), but no underground tunnel was evident (Amzarakov 2014, 33–38 Figs. 4, 5, 6, 7, 8, 9, 10, and 11, 2015b, 43). Pottery sherds at Troshkino-Uius were attributed to the Tes’ phase and early Tashtyk culture. Radiocarbon dates for the furnace provide a time span between 61 and 216 CE.Footnote 3 On the surface, no sherds or burnt clay from the furnace wall were found and Murakami reconstructs this furnace type as one without a superstructure that looked like a chimney. Since the bottom of the furnace pit was not burnt at all, Murakami suggests that the tip of the tuyère was inserted into the furnace 20 to 30 cm above the pit bottom (Murakami 2013, 236). Such a construction would create a relatively weak reductive environment low in carbon, which was confirmed by analyses of the slags (Murakami 2013, 236).

Based on the excavations of these two sites, Amzarakov suggests that the types of furnaces with underground air ducts at Tolcheya and the later pit furnaces from Troshkino represent a technological advance at the turn of the era (Amzarakov 2015a, 98). If accurate, this development is similar to the changes to iron-smelting furnaces seen in Mongolia, as shown below. Overall, one has to bear in mind that there is so far no evidence for underground tunnel-furnaces prior to the Tes’ period in the Minusinsk Basin, parallel to the early ** down to the river. The trenches are located in squares HD 84/56–100 and HD 94/6–50. Four small technical iron-smelting installations (feature 1–4) were unearthed immediately below the surface. F 1, 3–4 were negatively excavated. We documented feature 2 only incompletely as it is located in the northwestern corner of the trench and extends beyond the trench’s borders.

7.2.1 Feature 1 (Fig. 16)

Furnace pit with lateral lower air duct on the terrace edge slo** to the southeast. Rectangular plan with rounded corners of max. 48 cm × 34 cm and a max. depth of 44 cm. Outside the pit the heat had burnt the surrounding soil up to 10 cm thick. The channel opened to the southeast with a length of 48 cm and a max width of 32 cm. The furnace pit and the channel are filled with two layers. On the bottom a max. 10 cm thick layer of gray-black burnt loose sand, the upper layer contains a fill of hard, brown to gray sand with fragments of slag, burnt clay (roof of the kiln?) and stones.

7.2.2 Feature 2 (Fig. 16)

Partly excavated iron furnace in the northwest corner of the trench. On level 1 immediately below the surface a rectangular pit with rounded corners that measures 1.05 m in length became visible. Width to northern edge of trench max. 36 cm. In the drawing of the northern profile, a pit of 1.2 m length is visible, extending further west beyond the trench. The depth of the pit measures max. 36 cm in the western part, while the eastern part is only 20 cm deep. The fill consists of two layers: the lower layer, up to 16 cm thick, consists of gray-black burnt loose sand with black streaks of ash and charcoal in its upper part. The upper layer consists of up to 23 cm thick of hard brown to gray sand and contains fragments of slag and burnt clay.

7.2.3 Feature 3 (Fig. 17)

After having cleared the top soil the outline of a elongated pit extending northwest-southeast became visible. The southeastern part is incompletely preserved because of the slo** terrain of the terrace. The pit measures 70 × 60 cm. The northern part of the pit has been excavated. Its northwestern and northern border is of reddish color and burnt up to 6 cm thick. The southern profile reveals a maximum depth of 22 cm. The fill of the pit consists of gray to dark brown sand, in its southeastern part, the lower part of the fill consists of a max. 8 cm thick layer of gray-black burnt sand. This feature represents the remains of a small furnace pit with a channel to the southeast. No findings, but 30 cm to the south pieces of slag were found.

7.2.4 Feature 4 (Fig. 17)

Ca. 1 m further south of feature 3 we excavated another small furnace pit with lateral duct on the terrace edge slo** to the southeast. The pit we found was rectangular with rounded corners and measures max. 52 cm × 42 cm and reaches a depth of 44 cm. The borders of the pit were red and the soil burnt up to 8 cm thick. The duct to the southeast had a length of 52 cm and a width of 38 cm. The furnace pit and the channel were filled with three layers. On the bottom a max. 6 cm thick layer of gray-brown sand (discoloration of the subsoil?), above it a max. 6 cm thick layer of gray-black burnt loose sand. And the upper fill consists of hard brown to gray sand which contained fragments of slag and burnt clay.