Come Firewalk With Me: The Physics of Hot Coals

Via io9, Jen-Luc Piquant learned that 21 very gullible people attempted to fire-walk as part of a four-day motivational seminar in San Jose last weekend run by Tony (?Unleash the power within!?) Robbins. They suffered second and third degree burns for their trouble. Instead of feeling empowered, a witness told the Associated Press, ?I just heard these screams of agony. It sounded like people were being tortured.?

Well, yes. A bed of piping hot coals does have a tendency to burn, as our friend David Saltzberg discovered back in 2007, when he attempted a firewalking stunt as part of UCLA physics department demonstration. Poor David ended up being escorted home with badly blistered feet, and spent the rest of the night soaking his very sore soles in cold water.

David isn?t the first scientifically minded sort to engage in firewalking: noted skeptic Michael Shermer has done it, as has Jearl Walker, a former columnist for Scientific American who has performed firewalking and other insane feats, memorably commenting, ?There is no classroom demonstration so riveting as one in which the teacher may die.? A physics professor in Pittsburgh named David Willey does it all the time, and has arguably done the most to spread the word about the underlying physics behind safe firewalking.

The earliest known reference to firewalking can be found in an Indian story dating to around 1200 BC, but firewalking shows up in cultures all over the world, spanning thousands of years. I?s often associated with religious rituals (eg, in certain Eastern Orthodox communities in Greece and Bulgaria), or done to demonstrate the mystical powers of, say, Indian fakirs. You?ll also find firewalking in Polynesia, among Japanese Taoists and Buddhists, and performed by certain bushmen in the Kalahari desert as part of their healing ceremonies.

In America, the practice is more crassly commercial. Sometime in the 1970s, an enterprising snake oil salesman motivational author named Tolly Burkan started offering evening firewalking courses to the public, selling it as a way of confronting one?s fears and asserting mind over matter. Think ?Fear to Power? instead of ?Will to Power.? He founded The Firewalking Institute for Research and Education. But he doesn?t claim that anything supernatural or paranormal is necessarily going on, which is smart, because the science behind firewalking has by now been pretty well documented.

A study performed in the mid-1930s by the University of London Council for Psychical Research concluded that the secret of the successful firewalk is as simple as the low thermal conductivity of the burning wood-turned-to-coal, an insulating layer of ash, and the short time of contact between the hot coals and the soles of the feet.

Infared photography shows that the foot stays cool during a firewalk. Credit: Willey/Kjernsmo study.

Back in 1998, Willey teamed up with teamed up with? Norwegian Physicist, Kjetil Kjernsmo of University of Oslo, to study the physics behind fire walking. They developed a computer model of a foot while a person fire walked, and then compared it to infrared imagery taken while volunteers in Seattle firewalked. Those images showed that the foot does, indeed, remain cool when the stunt is done correctly.? (Image.)

According to Willey: ?What I believe happens when one walks on fire is that on each step the foot absorbs relatively little heat from the embers that are cooled, because they are poor conductors, that do not have much internal energy to transmit as heat, and further that the layer of cooled charcoal between the foot and the rest of the hot embers insulates them from the coals.?

Psychologically, there also seem to be some benefits in terms of promoting group dynamics. Per Wikipedia:

?A scientific study conducted during a fire-walking ritual at the village of San Pedro Manrique, Spain, showed synchronized heart rate rhythms between performers of the firewalk and non-performing spectators. Notably, levels of synchronicity also depended on social proximity. This research suggests that there is a physiological foundation for collective religious rituals, through the alignment of emotional states, which strengthens group dynamics and forges a common identity amongst participants.?

But if firewalking is supposed to be so safe, and un-magical, and rooted in sound science, why did the 21 Robbins acolytes suffer second and third degree burns, and why did our friend David get blisters all over the soles of his feet? Well, like most scientific experiments, you have to set them up and perform them correctly to replicate successful results; there?s not much margin for error.

The most obvious explanation is that those who were burned lingered a bit too long in one place while walking over the bed of hot coals. (I, personally, would have bolted across. While wearing protective, flame-retardant shoes.)

Firewalking in Sri Lanka. Credit: Aidan Jones from Oxford, U.K. Creative Commons.

So maybe there was something not quite right with the set-up. It?s critical that the coals be allowed to burn down sufficiently so that they are at a relatively comfy 538 degrees Celsius or so (1000 degrees Fahrenheit), preferably with a thin layer of ash over them providing a bit of extra insulation.

This process also burns off any excess water content in the coals; any remaining water would increase both the heat capacity and thermal conductivity of the coals. It?s equally critical to make sure no bits of metal have found their way into the coals, because metal has very high thermal conductivity.

Perhaps David and the Robbins acolytes could have further lessened the risk of injury by dampening their feet beforehandm gaining some protective benefit from the so-called ?Leidenfrost? effect, in which a thin layer of sweat or water instantly forms an insulating boundary layer of steam when exposed to intense heat. However, per Willey, this probably isn?t a major factor.

For one thing, it carries an added risk of coals sticking to your feet as you walk ? increasing exposure time and therefore causing the soles to burn more than if you just crossed with dry feet. (Willey prefers firewalking with dry feet, and also places a water-soaked carpet remnant at the end of the walk for immediate cooling.)

I can?t speak for the Robbins acolytes, but David certainly learned something from the experience, per his email after I told him I was planning a blog post on firewalking: ?You can tell people that it really hurt, and no creams or sprays helped, not even 30% benzocaine. But ice water worked like a charm.? His advice for any aspiring firewalkers? ?It?s a good idea to hoard Vicodin in advance, which I neglected to do.? Heed his words, impressionable young people: David has suffered so you don?t have to.

References:

Kjernsmo, Kjetil. ?A Preliminary Empirical Study of Firewalking,? International Conference for Physics Students, Vienna 1997.

Konvalinka, I., et al. (2011) ?Synchronized arousal between performers and related spectators in a fire-walking ritual,? Proceedings of the National Academy of Sciences 108(20): 8514-8519.

Leidenfrost, Johann Gottlob. (1966) ??On the Fixation of Water in Diverse Fire,?? International Journal of Heat and Mass Transfer, Vol. 9, pages 1153?1166.

Leikind, Bernard J. and William J. McCarthy. ?An Investigation of Firewalking,?? in The Hundredth Monkey and Other Paradigms of the Paranormal, ed. Kendrick Frazier. Buffalo, N.Y.: Prometheus Books, 1991.

Price, Harry. ?A Report on Two Experimental Firewalks,? University of London Council for Psychical Investigation, Bulletin II, 1936.

Willey, David. ?The Physics Behind Four Amazing Demonstrations,? Skeptical Inquirer, November/December 1999.

Xygalatas, D., Konvalinka, I., Roepstorff, A., & Bulbulia, J. (2011)?Quantifying collective effervescence: Heart-rate dynamics at a fire-walking ritual,? Communicative & Integrative Biology 4(6): 735-738.

Adapted from a November 4, 2007 post on the old archived Cocktail Party Physics blog.

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