Multidisciplinary research, including residue analysis and experimental archaeology, suggests ancient producers repeatedly soaked plant ash in brine to absorb salt crystals before rinsing it with fresh brine to create a more concentrated solution for boiling.
"This technique is illustrated in the Yuan Dynasty (1271-1368) salt-making manual Aobo Tu (Illustrated Boiling of Seawater), and is still used in today's sea salt making," says Niu.
Among the debris, archaeologists found numerous fragments of round-bottomed pottery jars. Similar fragments recovered from waste deposits allowed researchers to reconstruct complete vessels and investigate their function.
Drawing on ethnographic evidence and simulation experiments, researchers believe the jars served as salt molds. Nearly crystallized brine was poured into them, where it hardened into blocks or ingots that were easier to store and transport.
Because such molds often spread through trade, archaeologists traced similar pottery vessels found elsewhere to map the distribution of Yushan salt.
"The results show that by the middle-to-late Warring States Period, Yushan had become an independent salt production center. Its salt mainly supplied the Wujiang River Basin. In contrast, salt from the Zhongba site — another major salt production center in the Yangtze River Basin — was distributed primarily along the Yangtze's main channel. The two major salt fields served distinct markets, representing an early form of China's regional salt distribution system," says Niu.
