The oldest technical encyclopedia in the world, the Kaogong ji (Rites of Zhou), contains instructions for making bronze, but the two ingredients it lists – Jin and Xi – have puzzled historians. The identity of each, and what properties they offered, have been unexplained for centuries. Now, a paper in Antiquities answers those questions, revealing bronze making was a more complex operation in ancient China than previously understood.

By adding tin to copper, the ancients found they could make strong materials hard enough for many uses, and also able to be drawn into thin wires. It was a discovery that utterly changed the world. As so often happens after a great breakthrough, improvements followed, with the addition of elements such as nickel, zinc, or arsenic inducing specific properties desirable for certain purposes.

The Kaogong ji provides exceptional insight into Chinese technology during the late Eastern Zhou Dynasty (2,800-2,240 years ago) It describes how to make everything from swords to musical instruments, although often in quite vague terms. Bronze was so important to the era that the Kaogong ji’s authors provided not one but six formulae for making it for specified purposes.

Axes and bells should be four or five parts Jin to one part Li respectively, for example, while mirrors should contain equal mixtures of each. What they didn’t do was provide a list of definitions for what Jin and Li actually were. In modern China, Jin means gold, but no one thinks ancient daggers were three-quarters precious metal – at least not practical ones.

“These recipes were used in the largest bronze industry in Eurasia during this period,” the British Museum’s Dr Ruiliang Liu said in a statement seen by IFLScience, “Attempts to reconstruct these processes have been made for more than a hundred years, but have failed.”

Copper and tin are the most important components of bronze worldwide, but simply adding them in the ratio the Kaogong ji describes does not produce metal similar to that in surviving Chinese artifacts.

Liu and Am Pollard of Oxford University tested 235 “knife coins” dating to about 2,400 years old to reconstruct their composition. The coins are around 10 percent lead by weight – far too high to be a contaminant – yet the Kaogong ji makes no mention of a third metal. 

The authors conclude the coins were made in a two-stage process. First, two alloys – one of copper, tin and lead, the other of just copper and lead – were prepared. These were then brought together to produce the coins’ metal.

When looked at this way, Jin and Xi represent pre-prepared alloys, with the Kaogong Ji describing how to combine them, but skipping over the process of producing them in the first place.

At this stage, we can only speculate as to why the Kaogong Ji’s authors recorded only the second part of the process, but the paper notes the text appears to have been written for administrative supervisors, rather than artisans themselves.

Besides solving a long-standing mystery, this demonstrates how sophisticated the art of metal making was in China at the time, and opens up questions of why it was done this way.