With the help of some high-tech analytical instruments, an international team has found "unequivocal proof" of salt production by early societies that flourished in central China during the first millennium B.C. According to the researchers, the finding is China's oldest confirmed example of pottery-based salt production and gives archaeologists new methods for evaluating other salt-producing sites (Proc. Natl. Acad. Sci. USA 2005, 102, 12618).
To chemists, salt is simply an ionic crystalline solid. But to anthropologists, salt is the hallmark of a complex society. Because of salt's role as a dietary supplement and food preservative, anthropologists argue that salt production enables increased trade, population growth, and regional expansion and is therefore critical to the development of a complex society.
Until now, the ancient Chinese civilization that developed in what today is Zhong Xian County had perplexed anthropologists. Despite evidence of a complex society in this region, archeologists had been unable to find definitive evidence of salt production. At the archeological site known as Zhongba, a team led by Harvard University anthropologist Rowan Flad found the earthenware remains of a large-scale salt production facility.
Although the pottery was similar to that of other ancient salt-producing sites, the researchers wanted to prove that the vessels had indeed been used to make salt and not some other material such as fish sauce. Using X-ray fluorescence and X-ray diffraction, they were able to link trace compounds from residue on the salt-making tools to local brines that likely served as the salt's source, as well as to other ancient salt-production sites in China. The team also used scanning electron microscopy to identify traces of sodium chloride on the vessels' inner walls.
"Archaeologists frequently struggle to find adequate methods for exploring this ephemeral product in ancient contexts," Flad writes. "This work shows that new approaches to archaeological remains may bear fruit in the search for salt."