Originally published on Green Building Elements.
By John Perlin
Special thanks to author John Perlin for this contribution about what is believed to be the world’s oldest solar device – a solar ignitor, or yang-sui . The material comes from Perlin’s recently published book, Let It Shine: The 6000-Year Story of Solar Energy.
During the sixth century BCE, Confucius wrote about the common use of curved mirrors shaped from shiny metal to concentrate the rays of the sun for making fire. These became known as yang-suis – translating to solar ignitors, or burning mirrors.
According to the great philosopher, upon waking up the eldest son would attach a solar ignitor to his belt as he dressed for the day. It was his duty to focus the solar rays onto kindling to start the family’s cooking fire.
According to another early text, the Zhouli, which describes rituals dating far back into Chinese antiquity, “The Directors of the Sun Fire have the duty of transferring with burning mirrors the brilliant flames of the sun to torches for sacrifice.”
Although scholars found over the years many ancient texts discussing solar ignitors, the discovery of an extant yang sui eluded them for centuries. Quite recently came the Eureka moment. Digging up a tomb that dated to about three thousand years ago, a team of archaeologists found in the hand of a skeleton a bowl-shaped metal object. While the inner side could have passed for a wok, the exterior trough had a handle in its center. That’s what caught the eye of the two archaeologist in charge of the dig, Lu Demming and Zhai Keyong. They immediately brought the relic back to the local museum and ordered its specialists to make a mold from the original and then cast a copy in bronze.
After polishing its curved surface to a high degree of reflectance, the inquisitive archaeologists focused sunlight onto a piece of tinder just as the eldest son would have done so many years past, and in seconds the combustible material burst into flames. “This verified without a doubt that the purpose of the artifact is to make fire,” Lu and Zhai later wrote, assured of having found the oldest solar device in the history of humanity.
Now that the world could see what a real yang-sui looked like, museums retrospectively identified 20 more previously unclassified objects as solar ignitors. Multiple molds for turning out yang suislater found at a Bronze Age foundry in Shanxi province, close to the first find, suggest a mass market once existed for them. In fact, yang suis were probably as ubiquitous in early China as are matches and lighters today. The yang sui “should be regarded as one of the great inventions of ancient Chinese history,” remarked its discoverers, impressed by the ability of their forefathers to figure out the complex optics for such optimal performance so early in time.
Author: John Perlin is author of four books: “A Golden Thread: 2500 Years of Solar Architecture and Technology;” “A Forest Journey: Wood and Civilization;” “From Space to Earth: The Story of Solar Electricity;” and his latest book, “Let It Shine: The 6000-Year Story of Solar Energy.” Harvard University Press Chose “A Forest Journey” as one of its “One-Hundred Great Books” published by the press, as well as a “Classic in Science and World History.” The Geographic Society and the Sierra Club chose the book as their “Publication of the Year.” “Power of the Sun” and “Sunrise” are two documentaries for which I did the screenplays. “Power of the Sun” was done in collaboration with two Nobel Laureates at University of California, Santa Barbara, where I am now a member of the Department of Physics.