Micro-slag and “invisible” copper processing activities at a Middle-Shang period (14th-13th century BC) bronze casting workshop

Abstract Micro-slag artefacts from ancient bronze casting workshops were largely ignored in previous research despite their rich information potential. Current research demonstrates they could significantly enhance our understanding about past metallurgical activities but their identification requires careful in-situ analysis and a well-designed sampling strategy. Here we present an innovative methodology combining in-situ geochemical survey, wet-sieving of soil samples and detailed microscopic study, employed to investigate an important Middle-Shang site, Taijiasi, in the Huaihe River valley. The micro-slags from this site revealed that in addition to bronze alloying and casting, raw copper refining was also practiced. Material evidence for the refining process was not immediately visible in the archaeological excavation since most slag was mechanically crushed to retrieve any copper trapped in them, leaving only micro-slag fragments typically smaller than 3000 μm (3 mm). The fact that most micro-slag was recovered from one sector (H234) of a small building (F16) located on the same platform as the elites’ long houses suggests that mechanical processing of refining slag was conducted in a confined area and closely supervised. It might reflect people of this site valuing copper as a highly precious material and making all effort to recover copper otherwise lost in slag. This find will potentially shed new light on a range of important issues of Shang archaeology, including the regional variation of Shang metallurgical styles and the provenance of copper in the Shang period. This research also encourages researchers to look into archaeological soil samples with abnormally high copper content and understand the particles in them causing these high readings.