AMP 05 July 2021

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 1 2 5 Early processing was done using crucibles containing crushed ores and charcoal, with forced airflow provided by bellows-powered blowpipes. Later on, crucible and hearth furnaces using forced air via tuyères provided more controlled conditions. Figure 2 is a schematic of a smelting furnace from the Near East Late Bronze Age (LBA: 1550‒1200 B.C). Besides initial metal production, such a furnace could be used to remelt additions of other copper metal before tapping into clay or stone molds to cast ingots or artifacts, for example, vessels, tools, and ornaments. Large “oxhide-shaped” copper ingots were widely used in Eurasia as trade items in the LBA[4], and these could be remelted with additions of tin or tin oxide (cassiterite), and possibly other alloying metals, to produce bronze ingots or cast artifacts including vessels, tools, weapons, and ornaments. Near East Early Bronze Age (EBA: 3300‒2100 B.C.) ingots were probably forged by cold-working rather than hot-working[5], andwith intermittent annealing, depending on the metalsmith’s experience with the materials and the required artifacts. This practice continued well into the Iron Age, beyond 1500 B.C. Hot-working would have gradually developed as an alternative, except for high-tin bronzes, because “hot shortness” (brittle cracking at high temperatures) would become increasingly likely with tin contents above 8 wt%[6]. ANCIENT BRONZES The history of ancient bronzes is complex, spanning a “classic” period of more than 2000 years in Eurasia (3300‒1200 B.C.). Many issues are still unresolved, despite extensive studies since the early 20th century. Perhaps the most important question is whether the presence of alloying elements in copper was always accidental or became intentional. Considering the threemain types of bronzes, antimony bronze, arsenical bronze, and tin bronze, the evidence of intentional alloying for tin bronzes is incontrovertible. However, deliberate alloying with antimony and arsenic can be questioned[5,7], since these elements were often present in copper-bearing ores. On the other hand, analysis of Early Bronze Age slags from Iran shows that speiss, an iron-arsenic alloy, was probably added to copper ore or during remelting to obtain arsenical bronzes[8,9]. Also, although digressing here from the Old World, there is convincing evidence that the Andes region arsenical bronzes containing 0.5‒2 wt% arsenic were intentionally produced from about 850 A.D. for cold-hammering into culturally desirable small implements and thin sheet materials[10]. Returning to Eurasia, two more important questions arise. Why did tin bronzes become the main type, largely replacing arsenical bronzes after 2500 B.C., and why did antimony bronzes almost disappear after 2000 B.C.[7]? Possible answers have been given, but there is no consensus. Firstly, antimony bronzes may have been supplanted because their lesser hardness, and hence lesser strength, made them unsuitable for tools or weapons. This could have resulted in a lack of demand and trade in favor of tin bronzes, though this is not (yet) known[7]. The more intriguing question is the predominance of tin bronzes over arsenical bronzes, beginning in the later EBA. There are three basic hypotheses: (i) tin bronzes were intentional alloys but arsenical bronzes were not; (ii) tin bronzes had superior mechanical properties; (iii) smelting arsenic-containing ores resulted in poisonous fumes that became recognized as a health hazard. The first hypothesis has been discussed already: intentional alloying to obtain Eurasian arsenical bronzes is a distinct possibility[8],, and the Andean region study reinforces this[10]. The second hypothesis is disfavored by an extensive study and comparison of the mechanical properties of arsenical and tin bronzes[10]. There remains the possibility that smelting arsenical ores was abandoned in Eurasia owing to health concerns. However, arsenical bronzes were still being produced in the LBA (Fig. 3), 1000 years after tin bronzes became predominant. The majority of Near East tin bronzes have tin contents less than about 12 wt%, typically ranging from 5‒10 wt% from about 3000 B.C.[4]. The earliest EBA alloys have lower tin contents, 1‒3 wt%; and there are occasional exceptions, the high-tin alloys already mentioned. Hence most of the materials and artifacts would have had homogeneous single-phase microstructures after working and annealing, very different from the inhomogeneous ascast structures (Fig. 4). ANCIENT SILVER ALLOYS Owing to native silver’s scarcity, there is limited evidence of its direct use for artifacts, a few of which have been dated to 4300‒4000 B.C.[11]. Silver was more abundant as a minor component in the ores of other metals, especially lead[12]. Beginning before 3000 B.C., lead obtained from smelting argentiferous lead ores was further processed by cupellation to extract the silver. This process became the primary source of ancient silver and silver artifacts, although some artifacts were obtained from direct smelting of silver ores[12]. Fig. 2 — Schematic copper smelting furnace, Crete, Late Bronze Age. Adapted from Tylecote[4].

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