Each May in some northern Greek villages revelers walk barefoot across a bed of burning wood coals as part of a three-day celebration in honor of Saint Constantine and Saint Helen.
"They believe that the power of Saint Constantine—the religious power—allows them to do it and that that is a miracle," said Loring Danforth, an anthropologist at Bates College in Lewiston, Maine.
The festival is just one of the many events around the world in which people walk across a fire pit without getting burned.
Danforth has extensively studied fire-walking rituals, including the event in northern Greece and the more recently established fire-walking movement in the U.S.
As interest in fire walking has grown, he said, scientists have attempted to demystify the phenomenon and tease apart the allure of the ritual. But no amount of debunking can take away from the empowerment a fire walker can feel, Danforth says.
David Willey is a physics instructor and an expert on the science of fire walking at the University of Pittsburgh in Pennsylvania. He said people are able to walk across a bed of burning coals because "wood is a lousy conductor."
"There're three ways heat can get transmitted: conduction, convection, and radiation," he said.
Conduction is the transfer of heat from one substance to another via direct contact. In convection heat is transferred through air or fluid circulation. In radiation it is transmitted as if spreading out in straight lines from a central source (think of the sun or a heat lamp).
Conduction is the main way heat is transmitted to a person's feet during a fire walk.
In fire walking, a person's feet, which Willey said are also poor conductors, touch ash-covered coals.
Since the fire walker is indeed walking, the time of contact between feet and coals is minimal—too quick for the coals to burn or char the feet, Willey said.
Today on the radio program Pulse of the Planet, Danforth explains the physics of fire walking by comparing it to putting your hand in an oven in which a cake is baking at 500 degrees Fahrenheit (260 degrees Celsius). (The National Science Foundation partially funded this article to serve as a complement to the Pulse of the Planet radio program.)
"If I open the stove and reach in with my hand and touch the air inside the stove, I don't get burned, because my hand is dense and heavy compared to the air, and so the air doesn't heat up my hand. ... ," Danforth says in the radio program.
Touching the metal inside the stove, however, would immediately lead to a burn, because metal, which is much denser than the air, is a good conductor of heat.
The cake, Danforth said, has the consistency of the coals in a well-prepared fire walk. Even though the cake is hot, it can be touched for brief periods without causing a burn.
State of Mind
Tolly Burkan is the founder of the Firewalking Institute of Research and Education in Twain Harte, California. He promotes himself as the creator of the U.S. fire-walking movement, which he says dates to 1977.
"I was the first person to come along and make it available to John Q. Public by offering fire-walking classes that anybody could attend," he said.
Burkan dismisses the idea that the low conductivity of coals is a reason that fire walking is possible. As evidence, he points to an incident in which members of his institute successfully walked repeatedly on a heated metal grill without getting burned.
According to Burkan, the basic physical principle behind fire walking is the same that allows an egg to boil in a paper cup when placed atop red-hot coals. The boiling water keeps the cup at 212 degrees Fahrenheit (100 degrees Celsius)—hundreds of degrees cooler than paper's burning point.
Burkan says that circulating blood likewise keeps the flesh on a fire walker's feet from reaching its burning point—as long as the walker is relaxed enough to allow strong blood flow and as long as the walker keeps walking.
"What controls [the ability to fire walk] is more than physics, it's your state of mind," Burkan said.
Willey, the Pittsburgh physicist, said such mind over matter theories have nothing to do with why fire walking is physically possible. He allows, though, that self-confidence is required to take that first step.
"You've got to believe you're going to be OK, otherwise you wouldn't do it," he said. "But what your mind-set is has got absolutely nothing to do with whether you're going to burn or not."
Danforth, the Bates College anthropologist, said that scientific explanations do not "debunk or diminish or invalidate the value of the ritual."
"[Fire walking] can have the power to affirm one's life. It can change lives, give confidence, all kinds of things," he said.