By reviewing the matrix data of the Type A slag samples, there were obvious differences in the contents of silicate and ferrous oxides, while the bronze prills distribution and morphology was different from each sample. However, according to the laboratory work conducted for this study, the metal particles containing arsenic and copper could be detected in all the slag samples, while tin was absent in most of the Type A slag, and the distribution of other trace elements like silver and lead demonstrated no different patterns. There was no deliberate alloy portion control or process differences that could be identified, therefore, these differences between the slag matrices could be caused by the time of slag discharge during similar procedures or different isothermal or reducing conditions in the furnace. Parts of the slag matrix composition reached a reasonable viscosity and melted into the liquefied phase which yielded out the samples with streamline patterns, while others might be attached to the furnace lining and formed under non-ideal thermal conditions. The highly varied composition of metal prills in slag samples suggest that the smelting charge and furnace conditions were not carefully controlled and the product would have a relatively wide range of compositions.
Comparing the data of ore samples with the slag, the high arsenic content of the bronze prills might correspond well with the Fe–Cu–As ores found at the site. The mass ratio between arsenic and copper in ore could reach as high as 28%. These slags reflect a production process in which tin was barely involved. Interestingly, the published data of the LXC bronze artifacts from the Dadianzi cemetery site contained only tin bronze and pure copper with no trace of arsenic. Based on these data [11], it is traditionally argued that the LXC people did not use arsenical copper in contrast to the contemporary cultures in northwest China and the Eurasian Steppe [23, 24]. The investigation of the Habaqila site presented here has further enriched our understanding of the metallurgical activities of the LXC people. Considering the LXC’s relatively widespread geographical distribution and long chronological span, their metallurgical technology might be diversified and subject to change over time. Different technological choices between Dadianzi and Habiqila might be explained by the chronological gap (c. 3500 BP for Dadianzi) or varied metallurgical traditions within a single cultural complex.
Crucible/furnace fragments represent a tin metal smelting process based on rather pure cassiterite. The high tin metal and cassiterite contents indicate a rather rough single-stage smelting procedure [25], comparing with the modern two-step melting, in which a free-flowing silicate-based slag rich in tin oxide is first produced and later carbothermally reduced resulting in a full recovery of tin. However, with high-grade raw materials, the two-step method may not be necessary since the yield of slag would be very little, as would be the tin loss in the whole smelting process [26]. Similar smelting technology could be able to reach a temperature over 1200 °C near the tuyere with a properly set-up blowpipe [27] connected by the holes on the furnace, theoretically [21]; but in practice, such temperature and atmosphere controls require very accurate operations and rich experience [28]. Also, a proper furnace lining, high-quality charcoal, and a well-beneficiated charge are crucial for the success of such a one-step smelting operation [26].
The absence of Cu and As suggest rather pure cassiterite was used in this process to produce pure tin metal. Type B slag attached to the crucible/furnace fragments would need such pure cassiterite deposits as the raw material recovered in our survey, or there could be natural methods, such like alluvial or eluvial deposits, or artificial methods to separate and purify the paragenetic ore for the smelting process. The residual wolframite particles and the cassiterite structure provide a connection between the polymetallic ore and the cassiterite smelting, which served as the first field evidence of tin smelting in the Bronze Age north China. Tin can be added to copper as tin metal or tin oxide. The latter way is also called cementation and tin oxide would be reduced simultaneously after being mixed with molten copper. Though dozens of Bronze Age copper smelting and bronze casting sites have been identified in China [29], none of them revealed clear evidence of how tin was added to copper. The importance of this new find is not only to prove that the technology of tin smelting had been mastered by the LXC people but has also suggested two different lines of metal production at this site. The directly reduced arsenical copper can be employed for casting objects at the site but the smelted tin might be alloyed with copper at other sites for tin bronze production. The separation of these two production lines indicates a rather complex production organization in the metallurgical industry. Different types of alloys might play varied social and economic roles, and therefore be produced in separated contexts. To further develop this argument, more analytical data of the LXC artifacts, especially those made with arsenical copper, is necessary.
The morphology analysis of the ore samples has shown the arsenic and copper minerals were tangled together, indicating that the separation of these two minerals would be almost impossible, but the cassiterite with independent mineral structure could be separated by mechanical crushing with lithic tools or natural corrosion and sediment. As observed in the ore, the wolframite and cassiterite were conjoined quite closely and their specific gravities (around 7) were alike, causing wolframite residue after the separation procedures in slag. Similar smelting strategy was found in Iron Age sites of Africa, utilizing similar ore resources with both cassiterite and arsenic-rich minerals [30]. As a result, the lithic grinding mortar (anvil) and other grinding tools, such as hammers, chisels and grinding balls (Fig. 4) unearthed from the Yihewoment and Xiquegou site could be rather crucial for these ore mining and refining activities.
The driving force behind this chosen strategy could be the demand for production of tin bronze, but there has been hardly any archaeological evidence for the tin metal, as none of the ingots [31], mining or smelting workshop sites were discovered before now. In this region, there has been no evidence for a sustainable pure tin source based on the large-scale tin mine exploitation in central Asia has been found [32, 33]. And the geological conditions and stannic ore sediment around Habaqila site were also quite different from the discovered mining sites [34]. So, the reason for the ore separation and tin smelting could be a compromise choice for the lack of pure cassiterite. The whole array of technical material culture, such as the lithic tools utilized for mining and grinding, and for the panning and beneficiation of the ore [35] could be an adaptation for this kind of polymetallic vein which even extended to the hinterland of modern Mongolia where more abundant mineral veins can still be found nowadays [36]. With sophisticated skills of ore treatments, both tin and arsenic bronze techniques could be developed with polymetallic ore. Production of handheld tools and small ornaments to assist in the resource extraction and metallurgical process would have been a high priority for LXC peoples, especially if pure tin or other metal products formed the basis for their desirability as a partner in long-distance exchange. The entire metallurgical process was likely embedded in technical and social decision-making of the LXC people in this region, just like the Shang people pursued high-efficiency mass production and enacted a mature supply system for the production of ritual bronze [37].