A Secret to the Building of the Forbidden City Revealed

Study shows Chinese used ice paths to move heavy stones in the 15th and 16th centuries.

Based on historical research and experiments in mechanical engineering, an international study group has confirmed that Chinese workers in the 16th century CE used artificially created ice paths lubricated with liquid water to transport massive stones to the Forbidden City in Beijing. The method was used, despite the fact that the Chinese civilization had already developed wheeled vehicles capable of moving very heavy objects for 2,000 years.

Study lead author Jiang Li and colleagues first researched the historical literature on the topic, including a 500-year-old document that records the moving of the “Large Stone Carving”. Weighing about 123 tons, a team of men moved it during the deep winter of 1557 a distance of 70 km over 28 days to the Forbidden City using a sliding sledge over a path of ice repeatedly lubricated by water dug from succeeding wells along the path. They tested the technique using materials and conditions that duplicated similar activity, and what they found supported the plausibility of the historical record.

“We show that an ice lubrication technique of water-lubricated wood-on-ice sliding was used instead of the common ancient approaches, such as wood-on-wood sliding or the use of log rollers,” reported Jiang Li and colleagues. “The technique took full advantage of the natural properties of ice, such as sufficient hardness, flatness, and low friction with a water film. This ice-assisted movement is more efficient for such heavy-load and low-speed transportation necessary for the stones of the Forbidden City.”*

The authors determined that fewer than 50 men would have been sufficient to pull the sledge using this method, whereas pulling the same load over ground using the more commonly known ancient methods would have required a force of more than 1,500 men. Given the climatic conditions that existed in the Beijing region in the mid-16th century, the researchers concluded that a wood sledge over lubricated ice could have moved fast enough over a lubricating water film before the water froze to impede forward movement. 

The study report is published in full detail in the Proceedings of the National Academy of Sciences.


Cover Photo, Top Left: Workers likely slid massive stones, such as this 300-ton marble carving in front of the Hall of Supreme Harmony in the Forbidden City, Beijing, China, along artificial ice paths. Image courtesy of Chui Hu.

* Article #13-09319: “Ice lubrication for moving heavy stones to the Forbidden City in 15th- and 16th-century China,” by Jiang Li, Haosheng Chen, and Howard A. Stone.


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