1. Introduction
Iron was produced in antiquity in most parts of the world, via a direct method—the bloomery process. Archaeological evidence, mainly as slag, for the use of the bloomery process, and secondary operations, has been identified in the southern Levant, Africa, and Europe from the late second millennium BCE to the mid/late second millennium AD [
1,
2,
3,
4,
5,
6,
7].
In the bloomery process, iron is obtained through the reduction of ores in a small furnace usually operated by bellows, and sometimes via natural draft, using charcoal as fuel and as a reducing agent. Easily accessible and iron-rich ores (with more than 55–60 wt.% of iron) were generally preferred due to the uneconomic loss of iron into slag during the process, resulting in low metal yield [
7] (p. 87), [
8] (p. 93), [
9] (pp. 44–45). A further crucial aspect of the process consists of the fact that the composition of non-metallic impurities (gangue) significantly affects the viscosity of slag, favoring or hindering the reduction process [
9] (p. 91–92), [
10,
11,
12].
Identification of ore resources that were exploited in the past, therefore, has a particular significance as it can contribute to the understanding of the socio-technological factors affecting development of craft-production in the past societies. This identification can be achieved through the study and comparison of chemical and mineralogical composition of archaeological slags and ores, as well as through experimental smelting of ores from local geological outcrops to determine feasibility of their exploitation in the bloomery process [
13,
14,
15,
16]
The assaying (i.e., small-scale experimentation to determine quality of an ore) has always been an important component of ancient technology, as it represented an easy and resource-efficient technique to predict and coordinate subsequent large-scale processing and smelting [
17]. For past societies, this was an advantage, given that properties of materials could be grasped only empirically and use of metallic resources was often associated with high resource and labor costs (e.g., extraction, transportation, and processing of the ore and fuel, cf. iron production in Africa [
18]). The assay furnaces described by Agricolla [
19] are a historical example of methods traditionally used for cupellation of small amounts of costly argentiferous ores to predict the outcome of large-scale cupellation performed in larger furnaces set to a greater consumption of raw materials.
Determining the iron content and the quality of iron ores, prior to large-scale bloomery smelting, thus, is highly advantageous. Significantly, despite the common occurrence of iron ores in nature, knowledge about the types of deposits exploited in the past often remains limited and poorly understood. In fact, for some regions of the Ancient Near East, for example, south-eastern Arabia and Cyprus, there is still a lack of evidence about indigenous iron ore exploitation, although certain types of iron-rich rocks (e.g., gossans) exist in these regions [
20,
21,
22,
23]. Whereas in other regions, such as the southern Levant, evidence of past ore exploitation is only known so far from the
Mugharet el-Wardeh deposit in the Ajloun, Jordan [
5,
24], although this is certainly not the only ore deposit in the region.
In this paper, we present the results of a series of iron smelting experiments, conducted in a muffle furnace using graphite crucibles. Fourteen ore samples from three geological regions of the southern Levant were used as a test case. Among these samples, only three (Ajloun, see below) represent ores that are known to have been exploited in antiquity. Bloom and slag produced in each experiment were studied in correlation with the ore chemical composition and mineralogy. Results of conducted analysis are used to reflect on the conditions needed to implement in order to allow smelting of these ores in a bloomery furnace, accounting for the inherited differences between the assaying system and the bloomery furnace. The overall purpose of this research methodology was to create the scientific foundation for a set of iron smelting experiments, conducted on a much larger scale using the more authentic shaft bloomery furnace (Field bloomery smelting experiments are ongoing and will be discussed in forthcoming publications).
4. Discussion
Results of the assaying method show that some of the utilized/tested ores produced well consolidated blooms. Although similar results may not be fully reproducible in a bloomery furnace, they allow reflections on some necessary parameters that if implemented, may permit successful smelting of these ores in a bloomery furnace. Due to the limited archaeological evidence for iron ore exploitation and the presence and availability of some quality ore deposits in the southern Levant, the results of this study are significant for understanding the emergence and development of early iron production in the region.
Prior to discussing the results of the crucible assaying experiments, it is important to highlight the similarities and the inherited differences of the method to bloomery smelting. In both methods, the final product is a bloom formed by coalescence of solid metal particles. The effect of coalescence is particularly evidenced from the WCB group (see below), in which metal forms a tightly interconnected network. Further, similarly to the ancient bloom, lies the fact that the metal is richly penetrated by slag.
A major difference between the two methods lies in the fact that consolidation of reduced metal is not assisted by gravity, nor by the convection of hot air. Moreover, slag is not separated from metal through tapping. These specifics affected different mechanism of iron and slag formation, resulting in different slag mineralogy. However, the non-evacuation of slag in the crucible assaying system can be paralleled with smelting in a bowl furnace operated without tapping [
35].
The differences in the slag mineral assemblages can be clearly inferred from the present study. Generally, the slag obtained in the assaying non-tapping system is predominated by hedenbergite-fayalite assemblages, whereas slag from the tapping bloomery system results in fast cooling and non-equilibrium conditions, under which fayalite-kirschsteinite solid solutions usually form [
36,
37] (p. 197), [
38] (pp: 68–76). Furthermore, slag of the present experiments, at least in the case of W
CB, show very low FeO contents (FeO = 27 wt.%), while typical bloomery slag has FeO 50–75 wt.% [
10]. This, along with a very high percentage of metal reduced from the ore in the experiments (i.e., the estimated yield of W
CB is c. > 70 mass %), suggests a highly reducing atmosphere, far surpassing that of a typical bloomery furnace, generally resulting in a much lower yield (avg. metal yield is 30–50 mass %, [
39]). However, it is possible that smelting with a lower charcoal to ore ratio would have potentially promoted lower reducing conditions and higher FeO contents in the slag, resulting in different amounts of reduced metal and different degrees of consolidation.
Another parameter strongly affecting the results of the assaying experiments is the limited amount of ore used. In each experiment, 20 g of ore were used, homogenized from an initial 40–80 g sample. Nevertheless, this cannot compensate for the natural variability within a given ore body and is, therefore, not sufficiently representative. In contrast, several kg of ore is usually used in a bloomery furnace, allowing for a better homogeneity of the ore composition.
Despite the abovementioned complications, the assaying experiments allowed to produce some conclusions regarding the feasibility of the tested ores for smelting in a bloomery shaft furnace. Out of the 14 ore samples incorporated in the study, six were smelted into well consolidated blooms (categorised as WCB), two were moderately consolidated (MCB), and six were poorly consolidated blooms (PCB). The PCB group was further divided, based on chemical and mineralogical composition, into two types; PCB-type 1 and PCB-type 2. Generally, ores utilized to produce these bloom types did not provide sufficient amount of specific lithophile elements in the gangue to enable the formation of fluid slag.
Ores that produced P
CB-type 1 blooms have low FeO contents in the range of 43–54 wt.%, high silica contents of 30–38 wt.%, and relatively low contents of CaO and Al
2O
3. The poor outcome of smelting such ores is evidenced by the low quantity of reduced iron metal and excess of quartz, indicating high viscosity of resultant slag. Bloomery smelting of these ores would likely have a similar outcome, converting all iron oxide into slag minerals (such as fayalite) and zero to low metal yield, as suggested by [
8] (p. 93).
However, altering the composition of the bloomery slag, by using the correct flux additives, such as calcium rich minerals, with some amounts of alumina and magnesium may potentially allow smelting of non-sufficiently rich iron ores through the formation of low viscosity slag characterized by low FeO contents. Such additives would bind with silica in the ore to form minerals which, similarly to fayalite, can have relatively low melting temperature and viscosity; such as kirschsteinite (CaFeSiO
4) and the melilite group. Although endmembers of the melilite group, akermanite (Ca
2MgSi
2O
7) and gehlenite (Ca
2Al
2O
7), have relatively high melting points (>1400 °C), the incorporation of iron into melilite significantly lowers its melting point and Fe-bearing melilites are reported as dominant minerals in some bloomery iron slags [
40,
41,
42,
43].
P
CB-type 1 group consists of hematite-rich sandstones from
Wadi Amram and
Timna in the Arabah Valley, both regions well famous for copper mining in antiquity [
44,
45], and one sample from the
Negev ores. Although only two samples of Arabah ores were tested, the possibility for bloomery exploitation of these ores seems rather unlikely. Nevertheless, the possibility of smelting these ores using mineral additives deserves further investigation.
Three ore samples resulted in blooms categorized as PCB-type 2, generally have too high CaO content (6–16 wt.%) versus too low SiO2 content (2 wt.%) and are characterized by a ratio of CaO/SiO2 > 3.7. In the cases of ore samples Ajloun-1 and Negev-9, the iron oxide content is rather moderate (<57 wt.%), while the ore Ajloun-2 is extremely iron rich FeO (86 wt.%).
Ores of such composition are difficult to smelt by the assaying method, as evidenced in the significant amount of unreacted iron ore in the produced bloom. These difficulties may be due to the high viscosity of the resultant slag. Bulk analyses of slag from P
CB-type 2 blooms plot in the high temperature domain (1600–2000 °C) of the ternary diagram FeO-CaO-SiO
2, thus corresponding to Ca
2SiO
4 and Ca
3SiO
5 (
Figure 8a). The presence of calico-olivine and srebrodolskite in such slag further supports this hypothesis, as these minerals are uncommon in most bloomery slags and instead are often found in modern slag formed under oxidizing conditions and temperatures often higher than 1300 °C [
46] (p. 3), [
47].
Among the ores resulted in P
CB-type 2 blooms are two samples of
Ajloun ores. These ores, as revealed in the comprehensive study of
Mugharet el-Wardeh deposit, are often characterized by high CaO contents (10–30 wt.%) and an extremely high CaO/SiO
2 ratio (mostly in the range of 3–10) [
24] (p. 48,
Table 1). Such chemical composition may be hard to smelt in a bloomery furnace. Indeed, archaeological evidence of vitreous slag with high SiO
2 (46 wt.%) and CaO (20 wt.%) comes from the site of Tell Hammeh, situated near the
Mugharet el-Wardeh deposit in the Ajloun area. Analysis of these slags indirectly supports difficulties for bloomery smelting of local ores and indicates that the ancient smelters attempted to improve the viscosity of the slag using fluxes or residues from gangue processing [
5].
Another possible way to improve slag viscosity would be to increase the amount of iron entering the slag, either by deliberately selecting highly enriched iron ores and low gangue content, or by running the furnace under moderately reducing conditions to promote formation of wustite and/or magnetite. Slags rich in wüstite and magnetite with high FeO content were recorded from various archaeological sites exploiting the Ajloun ores [
24] (pp: 51–54), Figures 67, 68, and 70 in Reference [
48].
The significance of low-viscosity slag for successful reduction and consolidation of bloom is highlighted by the analysis of the well-consolidated blooms (W
CB) obtained in the present experiments. All ores of the W
CB group originated from various locations within the northern Negev, Israel:
Wadi Nekarot, Wadi Nekarot-Evos, Eshet, and
Zavar. The chemical and mineralogical composition of these ores, characterized by FeO in the range of 68–80 wt.% and a CaO/SiO
2 ratio lower than 2, is “well-balanced” for the crucible assaying system. Smelting of these ores allowed the formation of a sufficient amount of fluid slag, incorporating, depending on the sample, phases such as fayalite, hedenbergite, melilite, kirschsteinite, Ba-phases, and Ca- and Ba-rich glass. The low viscosity of this slag is further supported from the ternary diagram, as analyses plot in the low-temperature domain of 1100–1300 °C (
Figure 8a). Hence, the fluid slag acts as liquid media favouring sintering of the solid metal particles, both in the assaying system and in the bloomery furnace [
9] (p. 94). However, the sintering of metallic iron is significantly accelerated in a bloomery shaft furnace, mainly due to the downward gravitational movement of the ore and charcoal along the shaft.
Indeed, recent smelting experiments conducted in a bloomery shaft furnace, using some of these ores (samples Negev 1, Negev 3, Negev 6 mixed with Negev 7) were extremely successful, proving the feasibility for use of these specific ores for ancient iron production [
49].
Moderately consolidated blooms, MCB (Negev-4 and Ajloun-3), resulted in significant amounts of iron oxide and only moderate amounts of poorly sintered metal particles. This is apparently due to the high content of 78 and 88 wt.% FeO and only low content of non-metallic elements. Smelting of such ores in a crucible limited the amount of fluid slag phase, therefore restraining the reduced iron particles from sintering into a bloom. In contrast, the smelting of such ores in a bloomery furnace under fairly modest temperatures and moderately reducing conditions, would have converted more iron oxide of the ore into wüstite, therefore contributing to the increase of volume of fluid slag and favouring the consolidation of reduced iron particles into a bloom.
Overall, the undertaken study allows to reconsider the definition of a “good ore”, in the context of direct iron reduction. Such ore is more than a rock with a significant amount of iron mineral, but rather a resource characterized by the right balance between a sufficient amount of iron mineral and lithophile elements.