Building the perfect fire can seem to be an art, but according to a new study, it's determined by the measure of its height to base. This explanation could shed light on how our distant ancestors learned to master fire, allowing ancient humans to spread around the globe.

As fires with a wide base grow in height, they become hotter. It's the opposite with narrow fires, which cool off as they rise in height. Researchers found that the hottest fires were those in which the length of the base was approximately equal to the height of the flame.

Throughout history, people have built fires that were roughly the same shape, and this new study explored why this commonality exists between people around the globe, and over time.

The Constructional Law, first postulated in 1996, suggests that flow systems – which include rivers and air currents –continually evolve into forms that provide easier access to the fluid being transported.

"The reason is that this shape is the most efficient for air and heat flow. Our success in building fires in turn made it possible for humans to migrate and spread across the globe heat flow from fire facilitates the movement and spreading of human mass on the globe, which is a direct prediction of the Constructal Law," said Adrian Bejan of Duke University.

Although this study is unique in carefully examining properties of fires at various height-to-base ratios, the conclusions won't surprise anyone skilled at building fires. As they have found, the pile of fuel allows access for air to help drive heat through convection, in order to drive temperatures higher.

"Our bonfires are shaped as cones and pyramids, as tall as they are wide at the base. They look the same in all sizes, from the firewood in the chimney, to the tree logs and wooden benches in the center of the university campus after the big game. They look the same as the pile of charcoal we make to grill meat. And now we know why," Bejan said.

Striving for flames that rise as tall as they are wide could be a valuable pointer for people who don't consider themselves skilled at building fires.

Future investigation into how the hottest fires may be achieved could involve studying how wind and the material used as fuel – as well as how it's packed – affects flames.

Research into how to build the perfect fire was published in the journal Nature Scientific Reports.

Photo: Rudi Schlatte | Flickr