The research focused on one of the most diagnostic classes of material that an ancient metalworking site can yield: slag, the stony waste product left behind when iron is produced or worked. Slag is far more than debris. Its mineralogy and chemical composition encode the temperature of the furnace, the type of ore and fuel used, and the fundamental nature of the smelting or melting process. Because different ironmaking traditions leave chemically distinct residues, slag can act as a fingerprint for reconstructing ancient technology even when the furnaces themselves have not survived.

The team applied three complementary techniques to the Zhujiazui assemblage. Macroscopic classification allowed researchers to sort slag fragments and iron artifacts into groups based on shape, texture, color, and other visible features, providing a first-pass typology of the material. Metallographic analysis, in which samples are polished, etched, and examined under a microscope, reveals the internal microstructure of the metal and slag, including phases that form only under specific thermal and chemical conditions. Finally, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, known as SEM-EDS, delivered precise elemental compositions at the microscopic scale, identifying the distribution of elements such as iron, silicon, calcium, and magnesium within individual slag phases.

The results pointed decisively in one direction. The analytical signature of the slags indicated operations within a cast iron technological system, rather than bloomery smelting or a fining process. This distinction is fundamental to the history of metallurgy. In bloomery smelting, the dominant early technology in much of the ancient world, iron ore is reduced in a solid state below the melting point of iron, producing a spongy mass of metallic iron mixed with slag that must be repeatedly hammered to expel impurities. Cast iron, by contrast, contains enough carbon to melt at a much lower temperature, allowing liquid iron to be tapped from a furnace and poured directly into molds. Ancient China was remarkable for developing and widely deploying cast iron technology centuries before it became common elsewhere, and the Zhujiazui evidence adds a late Tang data point to that long trajectory.

Equally telling was what the excavated area lacked. The researchers found no iron ore and no ore-processing debris, the materials that would be expected if smelting, the initial reduction of ore to metal, had taken place on site. Instead, the excavation yielded ceramic molds and cast iron artifacts. Taken together, the absence of ore and the presence of molds indicate that the excavated area was primarily used for the melting and casting of cast iron. In other words, Zhujiazui was not a place where rock was turned into metal for the first time, but a foundry where already-produced cast iron was reheated, liquefied, and shaped into finished objects. This division of labor, with smelting at ore sources and casting at separate workshops, reflects an organized industrial chain rather than a single self-contained production site.

The chemistry of the slags offered further clues about workshop practice. Some of the analyzed samples showed elevated levels of calcium oxide and magnesium oxide, which the authors suggest may indicate the use of dolomitic flux. Fluxes are added to furnaces to lower the melting point of gangue, the unwanted mineral components of the charge, and to promote the formation of fluid slag that can be easily separated from the metal. Dolomite, a calcium magnesium carbonate mineral, would have served this purpose effectively. Identifying a possible dolomitic flux through slag chemistry provides a rare glimpse into the deliberate choices made by Tang dynasty metallurgists, who understood through accumulated craft knowledge which additives improved furnace performance, even without a modern theory of silicate chemistry to guide them.

Fuel evidence came in a more direct and visually striking form. The slags preserved charcoal impressions, providing direct evidence that charcoal was the fuel used in the ironworking operations. Charcoal, produced by burning wood in low-oxygen conditions, was the standard metallurgical fuel of the premodern world because it burns hot and introduces relatively few contaminants into the charge. Impressions of charcoal fragments frozen into slag record the fuel in situ, anchoring the reconstruction of furnace conditions in physical evidence rather than inference alone. For a site of this period, the confirmation of charcoal fuel also connects the foundry to the broader environmental and logistical demands of large-scale iron production, which consumed substantial quantities of wood resources.

The chronological placement of the site is significant in its own right. The late eighth to early tenth century corresponds to the late Tang dynasty and its turbulent aftermath, a period when China’s economic center of gravity was shifting southward and when iron production supported agriculture, weaponry, coinage, and daily implements on an enormous scale. Ezhou’s location on the Yangtze River places it within a region long associated with metalworking, and the authors note that the site’s location and regional context raise the possibility that it participated in wider systems of metal supply and product circulation. A casting workshop positioned within such networks would have depended on pig iron delivered from smelting sites and would have distributed its cast products through riverine trade routes, embedding Zhujiazui in an economy of raw materials and finished goods that stretched beyond the immediate locality.

Methodologically, the study demonstrates the value of reassessing archaeological contexts alongside laboratory analysis. Macroscopic classification, metallography, and SEM-EDS each answer different questions, and only by integrating them with a careful re-reading of the excavation record could the team distinguish a casting workshop from a smelting site, a difference that surface finds alone might not reveal. The work was supported by the National Social Science Fund of China and the Shaanxi Provincial Natural Science foundation, and it was carried out with the assistance of the Hubei Provincial Institute of Cultural Relics and Archaeology, whose excavation made the assemblage available for study.

For the archaeology of Chinese metallurgy, Zhujiazui contributes a clearly characterized example of late Tang cast iron melting and casting, complete with evidence for flux use and charcoal fuel, and a persuasive argument that the site operated as one node in a larger industrial landscape. As analytical techniques continue to be applied to slag assemblages across China, studies of this kind are transforming fragmentary industrial debris into readable records of ancient technology, revealing not only how iron was made but how production was organized, supplied, and connected across the Tang world.

Subject of Research: Late Tang dynasty cast iron melting and casting technology at the Zhujiazui site in Ezhou, China

Article Title: Iron production at the Zhujiazui site of the late Tang dynasty in Ezhou, South China

Article References: Zhang, M., Gao, Y., Su, J., Ma, B., & Li, X. (2026). Iron production at the Zhujiazui site of the late Tang dynasty in Ezhou, South China. npj Heritage Science. https://doi.org/10.1038/s40494-026-03010-8

Image Credits: AI Generated

DOI: 10.1038/s40494-026-03010-8

Keywords: iron production, cast iron, Zhujiazui site, Tang dynasty, archaeometallurgy, slag analysis, SEM-EDS, metallography, dolomitic flux, charcoal fuel, Ezhou, ancient foundry

The study, published in the journal Archaeological and Anthropological Sciences, examined twelve copper-based artefacts, seven slag fragments and a single ore fragment, all recovered during excavation campaigns directed by E. O. Negahban in the 1970s and H. Tala’i in the 1990s. The researchers deployed an impressive analytical arsenal: optical microscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy to reveal microscopic structures, inductively coupled plasma optical emission spectroscopy to pin down elemental compositions, wavelength-dispersive X-ray fluorescence for the slags and ore, and lead isotope analysis by multi-collector mass spectrometry to trace the geographic fingerprints of the metal. Each technique interrogates a different aspect of the metallurgical story, from the chemistry of the alloy to the temperature of the furnace.

The compositional results delivered a surprise. Of the twelve artefacts, eight turned out to be unalloyed copper, while only four were tin bronzes, and three of those four dated to the Late Bronze Age. In the Iron Age I samples, just one object was a bronze. This pattern hints at a shift from tin bronze in the Late Bronze Age toward predominantly plain copper in the early first millennium BCE, although the researchers are careful to stress that a sample of twelve objects cannot support claims about a site-wide technological transition. Corrosion further complicates the picture: one bronze measured an implausible 28.6 percent tin, an artefact of intergranular corrosion and copper leaching over millennia of burial rather than an intentional recipe, meaning the true tin contents were likely below fifteen percent.

Under the microscope, the metal told an even richer story. Every artefact displayed a single-phase copper matrix studded with dark non-metallic inclusions of two distinct types. Rounded inclusions rich in copper and oxygen point to copper-oxide phases, while elongated inclusions composed of copper and sulphur, occasionally with traces of iron, indicate copper-sulphide residues trapped during smelting. These sulphide inclusions are particularly evocative, because they are consistent with the processing of sulphidic copper ores, or possibly the co-smelting of sulphidic and oxidic ores, a technique documented on the Iranian Plateau as early as the third millennium BCE. However, the authors caution that oxide inclusions can also form innocently during melting and casting, when dissolved oxygen segregates as the metal solidifies, so inclusions alone cannot identify the original ore type.

The etched microstructures revealed the hands of skilled smiths. All twelve artefacts show annealing twins, the microscopic signature of repeated cycles of heating and hammering after initial casting, and some preserve slip lines indicating a final cold-working pass without subsequent annealing. Finer grain sizes in certain objects betray more numerous thermo-mechanical cycles, suggesting that the number of working and annealing rounds varied from piece to piece depending on the desired form and finish. Together with tin-rich eutectoid phases and tiny lead-rich globules scattered through some samples, these features paint a picture of sophisticated workshop practice in which casting was only the first step in a deliberate sequence of shaping and heat treatment.

The slag fragments, the waste products of high-temperature chemistry, proved equally informative. Their glassy matrices are consistently fayalitic iron-silicate glasses, laced with magnetite crystals and small droplets of metallic copper that never managed to coalesce and settle into the ingot. By plotting the glass compositions onto ternary phase diagrams, the team estimated the apparent liquidus temperatures of the melts. One sample plots near the fayalite eutectic at roughly 1150 to 1250 degrees Celsius, a technologically realistic smelting range, while the others fall into fields corresponding to seemingly impossible temperatures of 1400 to 1650 degrees. The researchers resolve this paradox elegantly: the high-silica compositions reflect fractional crystallisation during cooling, not superheated furnaces. As fayalite crystals grew, the residual liquid became progressively enriched in silica and alumina, so the final frozen glass records the end point of solidification rather than the peak furnace temperature.

Perhaps the most tantalising findings concern the slags’ chemistry. Several samples contain barium oxide, and the copper prills trapped within the slags carry consistent traces of arsenic, up to 6.5 percent in one case, hinting at the smelting of arsenic-bearing copper ores. Notably, none of the copper prills contain any tin whatsoever. This is a crucial clue: the slags record the smelting of plain copper, yet tin bronzes exist among the artefacts. The most likely explanation is that alloying happened as a separate, later step, possibly through cementation, in which copper metal was mixed with tin or tin-rich material in a crucible, a process previously demonstrated at the Elamite site of Haft Tappeh in southwestern Iran. Alternatively, some bronze objects may have been manufactured elsewhere and imported to Sagzabad as finished goods, though the current data cannot distinguish between these scenarios.

Lead isotope analysis added a geographic dimension, and a puzzle. The twelve artefacts display markedly heterogeneous isotopic signatures, forming a broad linear trend that suggests the copper derived from multiple ore sources rather than a single mine. When compared against 158 published Iranian ore analyses and 154 archaeological metal analyses, the matches were only partial. A few artefacts overlap with the Veshnaveh deposit, the nearest known ancient copper mine, located about 250 kilometres southeast of Sagzabad and exploited from the early third through the second half of the second millennium BCE. Others align weakly with ores from the Kerman region in the southeast, the Lut Block in the east, or zinc-lead deposits in the Urumieh-Dokhtar zone and the Alborz Magmatic Belt, while several show strong isotopic affinities with artefacts from sites such as Sangtarashan, Baba Jilan and Tappeh Sarm. Most samples, however, match no known source cleanly.

The geochemical circumstantial evidence does lend some weight to a Veshnaveh connection. The Sagzabad slags resemble Veshnaveh ores in composition, and both the slags and the ores share a distinctive barium signature, with barite heterogeneously distributed in the deposit. The ore fragment analysed in the study, a piece of chalcopyrite surrounded by oxidised copper and iron minerals, is also geochemically compatible with Veshnaveh material. Still, the absence of isotope data for the slags and the ore fragment prevents the researchers from formally testing whether the smelting debris and the artefacts share a common origin, and the isotopic spread of the artefacts points toward a supply network drawing on both nearby and distant deposits, potentially through long-distance trade in ingots or finished metal.

Set in its regional context, Sagzabad fits neatly into a broader second-millennium BCE pattern across the Iranian Plateau, where unalloyed copper, arsenical copper and tin bronze coexisted in varying proportions. Arsenical copper dominates in the east, at sites such as Tappeh Yahya and Tappeh Hissar, while tin bronze prevails in the centre, north and west, reaching its apogee in the spectacular castings of Iron Age Luristan. The Sagzabad assemblage, with its mixture of plain copper and low-tin bronze, its evidence for sulphide-ore smelting, and its isotopically diverse metal supply, adds a crucial data point from a region that has long stood in the shadow of better-studied western and eastern Iranian metallurgical centres. The authors emphasise that these are preliminary results from a small and unevenly distributed sample, and that a larger assemblage, together with isotopic analysis of the slags and ores, will be needed to establish whether the observed patterns reflect genuine chronological change in alloying practice, the organisation of production, or the exchange networks that moved raw metal across the plateau. What is already clear, however, is that the smiths of Sagzabad commanded a sophisticated, multi-stage metallurgical technology, and that the copper flowing through their hands came from more corners of ancient Iran than anyone had suspected.

Subject of Research: Second-millennium BCE copper-based metallurgy and ore provenance at the Late Bronze Age and Iron Age site of Sagzabad, north-central Iran

Article Title: Archaeometallurgical evidence from Sagzabad in regional perspective: preliminary insights into second-millennium BCE copper-based metallurgy in North-Central Iran

Article References: Archaeometallurgical evidence from Sagzabad in regional perspective: preliminary insights into second-millennium BCE copper-based metallurgy in North-Central Iran. (n.d.). https://doi.org/10.1007/s12520-026-02570-x

Image Credits: AI Generated

DOI: 10.1007/s12520-026-02570-x

Keywords: archaeometallurgy, Sagzabad, Qazvin Plain, Iranian Plateau, Late Bronze Age, Iron Age, tin bronze, copper smelting, lead isotope analysis, slag analysis, Veshnaveh, ancient mining

The fragments came from pit H1573, part of a bronze workshop dating to the Early Spring and Autumn period at Guanzhuang, a walled site in Xingyang that has been under excavation for years. When the researchers fitted the pieces back together, they found something extraordinary: the character strokes on the same surface displayed both convex and concave profiles. That seemingly small detail is the smoking gun. It shows that the craftsman first carved recessed, concave characters into a leather-hard clay block and then pressed fine clay strips into those grooves, building up raised, convex characters on top. This two-step process, known in the literature as the modeling method or clay-strip technique, had long been hypothesized but never directly demonstrated from an excavated artifact.

The analytical work behind this conclusion is as important as the discovery itself. Because the mold fragments are precious and fragile, the team relied entirely on non-destructive methods. Super-depth-of-field microscopy allowed them to examine the three-dimensional profiles of individual strokes at high magnification, revealing the telltale combination of sunken grooves and raised ridges within single characters. Confocal laser scanning microscopy provided precise topographic maps of the surfaces, quantifying the depth and height of the features. Micro-computed tomography, or Micro-CT, peered inside the clay block without cutting it, imaging the internal structure of the inlaid strips and their relationship to the carved grooves beneath them. Together, these techniques built a virtual reconstruction of the craftsman’s actions more than two and a half thousand years ago.

To understand why this matters, one has to appreciate how Chinese ritual bronzes were made. Unlike the lost-wax casting common in the ancient Near East and Mediterranean, early Chinese founders used the piece-mold method: a clay model of the vessel was sculpted, molds were formed around it in sections, and the model was trimmed down to serve as the core around which the molten bronze would be poured. The decoration on the finished vessel was essentially the negative of what had been worked on the mold surfaces. Inscriptions, however, posed a special problem, because the raised characters that appear on many bronzes had to be created as recesses or reliefs on the appropriate mold or core surfaces, and the logical chain of steps was far from obvious.

Scholars proposed several competing theories over the twentieth century. Some argued that inscriptions were carved directly into the clay molds or cores with knives, working from the finished vessels backward. Others suggested that characters were formed separately and attached, either as pre-cut clay pieces glued onto the mold surface or as raised modeling of fresh clay. Japanese and Chinese researchers, including influential studies by Hayashi Michitaro and later synthetic work by scholars such as Li Feng, debated whether the long, flowing inscriptions of the Western Zhou period, with their distinctive raised outlines and occasional guide ridges, could plausibly have been carved directly or must have been built up by hand. Without surviving intermediates, the argument risked circling forever. As the new study emphasizes, reconstructing ancient craft processes ultimately demands direct archaeological evidence rather than inference from finished products alone.

The Guanzhuang block delivers exactly that evidence, and it comes with an architectural surprise: the reassembled block appears to be an independent insert, designed to be embedded into the core of the mold assembly rather than being an integral part of the core surface itself. This modularity aligns with a broader pattern in Chinese bronze production, where standardized modules and reusable components appear in decoration as well as text. It means that the founder could prepare the inscription separately, on a convenient flat block, and then set it into the core, a workflow that both simplified the work and explains some of the peculiarities seen on surviving vessels.

Crucially, the Guanzhuang find does not stand alone. It corroborates technological practices suggested by earlier discoveries at two other major foundry sites. At Xiaomintun in Anyang, the great Late Shang bronze-casting district associated with the capital at Yinxu, excavations uncovered mold fragments bearing evidence consistent with inscribed inserts. At Luoyang Beiyao, a Western Zhou foundry excavated in the 1970s, similar indications of clay-strip work were recorded. Taken together, the three finds trace a continuous technological tradition: the modeling method was in use from the Late Shang period, through the Western Zhou, and into the Eastern Zhou, surviving at least until the Early Spring and Autumn period represented at Guanzhuang. What had been a patchwork of hints now reads as a coherent, long-lived craft practice.

The implications reach beyond technology into the history of writing itself. Bronze inscriptions, or jinwen, are among the most important sources for early Chinese history, recording royal grants, lineages, military campaigns, and legal transactions. The famous Da Ke Ding, with its hundreds of characters, has been a touchstone in the debate precisely because its long, neat, raised inscription seemed hard to explain by direct carving. Studies arguing for the prevalence of the clay-strip technique in the mid to late Western Zhou now gain physical confirmation of the underlying method. Knowing how the characters were formed also informs how epigraphers interpret stroke morphology, joins, and occasional flaws in cast inscriptions, and it clarifies the division of labor within foundries, where a specialist might prepare text blocks that other workers integrated into complex mold assemblies.

The discovery also highlights the value of revisiting workshop contexts with modern instrumentation. Molds and cores are made of fired clay, materials that rarely survive intact and that are easy to overlook among the mass of kiln waste and debris at foundry sites. Reassembling fragmented mold blocks is painstaking work, and the convex-plus-concave signature at Guanzhuang might have been dismissed as damage or wear without microscopic and tomographic scrutiny. The study’s authors, including Xiping Hui, Qingzhu Wang, Sicong Ding, Xiangping Gao, Siran Liu, and Guohe Han, combined field archaeology with laboratory science, and their reconstruction of the pit’s contents, the block’s assembly, and the internal structure of the inlaid strips forms a mutually reinforcing body of evidence.

For the archaeology of ancient China, the Guanzhuang inscribed clay mold block resolves key uncertainties about how Shang and Zhou bronze inscriptions were cast, and it does so in the most persuasive way possible: with the craftsman’s own handiwork still preserved in the clay. It confirms the continuity of a sophisticated technique across roughly a millennium of bronze production, demonstrates the modular thinking of early Chinese founders, and underscores a methodological lesson that resonates far beyond this single pit.finished products can mislead; the processes that made them leave traces only in the workshop. Where those traces survive, modern imaging can read them, stroke by stroke, and turn century-old hypotheses into settled knowledge.

Subject of Research: The casting technique of inscriptions on Shang and Zhou dynasty bronze vessels, based on an inscribed clay mold block from Guanzhuang

Article Title: The casting of inscriptions on Shang and Zhou bronzes: new evidence from an inscribed clay mold block discovered at Guanzhuang

Article References: Hui, X., Wang, Q., Ding, S., Gao, X., Liu, S., & Han, G. (2026). The casting of inscriptions on Shang and Zhou bronzes: new evidence from an inscribed clay mold block discovered at Guanzhuang. Archaeological and Anthropological Sciences, 18(10), Article 204. https://doi.org/10.1007/s12520-026-02569-4

Image Credits: AI Generated

DOI: 10.1007/s12520-026-02569-4

Keywords: Guanzhuang, bronze inscriptions, Shang dynasty, Zhou dynasty, piece-mold casting, clay-strip technique, modeling method, Micro-CT, archaeometallurgy, Henan archaeology, Spring and Autumn period, jinwen