Recent research has deciphered an ancient recipe for bronze and shed new light on the complexity of the metal industry in China c.2,300 years ago.
The recipe in question is recorded in an ancient Chinese text known as the Kaogong ji, which is believed to be the world’s first encyclopaedia of technology, probably dating to c.300 BC. The Kaogong ji features six formulae for casting different types of bronze objects, and researchers have been attempting to interpret them for more than a century. The formulae are based on the combination of two components, referred to in the text as jin and xi, and it was originally thought that these terms referred to pure metals, probably copper and tin respectively. However, analysis of contemporary bronze coins has revealed that these objects do not contain the ratios that would be expected if this interpretation were correct. Significantly, the coins were found to contain high levels of lead as well, which would not be possible if the bronze was simply a combination of those two pure ingredients. Therefore, researchers claim, it follows that either jin was not pure copper, xi was not pure tin, or both.
The new study, led by Dr Ruiliang Liu from the British Museum and Professor A M Pollard from the University of Oxford, suggests an alternative theory: that jin and xi actually refer to alloys, rather than individual metals. This would mean that the bronze-casting process began not with pure metal ingots, but with pre-prepared mixtures of metals. The researchers have suggested that jin was a leaded tin-bronze alloy (copper:tin:lead = 85:15:5), while xi was a copper-lead alloy (copper:lead = 50:50). This hypothesis would explain how a product containing all three metals could have resulted from a formula that supposedly only had two ingredients.
This suggestion not only represents an important step in our understanding of ancient Chinese metallurgy and historical texts, but also indicates that the bronze-production process in ancient China was more complex than was originally thought, involving a whole extra step that was previously unknown. The research has been published in a paper in Antiquity (https://doi.org/10.15184/aqy.2022.81).