Researchers at Princeton have created a flame so cool that it would be possible to run your hand through it without getting burned.

This may seem like a great magic trick, but cool flames are providing important insights into engine efficiency and pollution control. “We are developing a diagnostic process to optimize combustion at a very low temperature,” said Yiguang Ju, the Robert Porter Patterson Professor of Mechanical and Aerospace Engineering and the project’s lead researcher. “If we can better understand cool flames we can better understand the process by which future advanced combustion engines will work.”

Burning as low as 300 degrees Celsius, cool flames have fascinated scientists for hundreds of years. Sir Humphrey Davy first discovered the phenomenon in 1817, when he reported an unusual flame that he could run his fingers through without getting burned. That would not be possible with a standard visible fire, which burns between 1,200 and 1,700 degrees Celsius. But, Davy noted, the cool flame could change without warning into a standard flame when pressure or mixture conditions changed.

Since then, cool flames have been of great interest to scientists and engine designers. This unusual form of combustion causes knocking in gasoline engines and plays an important role in diesel operation. But because of their unusual slow-burning properties, cool flames have been extremely difficult to produce in a laboratory. In a series of experiments on the International Space Station in 2013, astronauts were astonished to learn that something like cool flames were formed after the extinction of a hot droplet flame in zero gravity.