Forbidden City
Ancient documents suggest rocks were transported to the Forbidden City on ice sledges (Reuters)

Ancient Chinese documents have revealed how Beijing's Forbidden City was built 600 years ago with thousands of peasants hauling huge stones from quarries 70 kilometres away.

Published in the Proceedings of the National Academy of Sciences, researchers from Harvard University examining the documents found these boulders were moved using wooden sledges along roads of ice.

Historically, it was commonly believed that huge stones were not transported by sledge in China because they had a well-developed wheel by around 1500BC.

However, ancient documents contain "brief remarks" that the Large Stone Carving in the Forbidden City was transported there on a wooden sledge along an artificial ice path.

"Lubrication plays a crucial role in reducing friction for transporting heavy objects, from moving a 60-ton statue in ancient Egypt to relocating a 15,000-ton building in modern society," the authors said.

Thomas Stone, one of the team members from Princeton University, told Nature: "You go to the Forbidden City and see these massive rocks, and you ask yourself: 'How in the world did they ever move this rock here?'"

Forbidden City
Wheeled objects could not have been used to transport the heavier rocks (wiki commons)

The ancient documents said that a stone slab weighing 112 tonnes was taken to Beijing over four weeks. Other rocks weighed even more.

Wheeled objects could not be used to transport heavy rocks as loads could not exceed 86 tonnes, so the Chinese had to come up with another method of transportation.

Through experiments and using the historical documents, the researchers found using an artificial road of ice to traverse stones on wooden sledges was more reliable and easier than other rudimentary methods used by other cultures.

"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," they said.

"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 transportation of the huge stones provides an early example of ice lubrication and complements current studies of the high-speed regime relevant to competitive ice sports.