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A Straight Dope Classic from Cecil's storehouse of human knowledge
14-Jun-1991
Dear Cecil:
My friend Heather and her hubby just laid out some fairly big bucks to spend a weekend
learning how to maximize their total personhood and all that. Highlight of the weekend was
a scamper across hot coals. The idea was to impress you with the power of your mind--if
you kept thoughts of cool moss in mind as you walked, your feet wouldn't burn. The coals
were in a strip about a yard wide and ten feet long. Walkers, in bare feet, made it
through with few problems. Heather made it too, with no trouble and no sign of damage to
her feet.
Heather's (and my) question is this: she believes that, via mind over matter or hypnosis,
a person might lower his blood pressure or speed up his pulse or even will himself not to
feel pain. But whether she felt it or not, shouldn't the coals have burned her feet? After
all, if you throw a steak on the grill, it cooks regardless of the thoughts it happens to
be entertaining.
What's the deal? Does psychology triumph over physics? Is it a con- -aren't the coals hot?
Do sweaty feet protect you with a layer of sizzle? --Ed Dolnick, Chevy Chase, Maryland
Dear Ed:
Cecil initially feared he was going to have to risk cherished portions of his being for
the sake of the Teeming Millions (and what have you guys done for me lately?).
Fortunately, a quick check of the Straight Dope's vast data resources reveals that lots of
journalists have rushed in where more sensible people dared not tread. Firewalk therapy,
God help us, is widespread these days.
There's some disagreement on why firewalking works, but this much is clear: it does work.
There's no trickery involved, although modern firewalk entrepreneurs do take a few
precautions, about which more below. Mind over matter has nothing to do with it. Skeptics
have tried it with no preparation whatsoever, or (worse) while murmuring "hot rocks,
hot rocks." They got over just fine. There's not much margin for error, though.
Blisters are fairly common and a few people have been badly burned.
What protects the rest? Two phenomena get the credit:
(1) The sizzle effect, also known as the Leidenfrost effect: a thin layer of sweat
protects you. As Peter Garrison put it in a 1985 article on the subject in Omni
magazine, "a liquid exposed to intense heat will instantaneously form an insulating
boundary layer of steam." That's what enables you to snuff a candle flame painlessly
with your fingertips after you've moistened them with spit. The soles of the feet are well
supplied with sweat glands, and Lord knows I'd perspire while waiting to hike through
hell. In addition, sometimes you're directed to walk over a moist surface such as grass
before stepping out on the coals. (Another trick: don't replenish the coals; they quickly
cool.)
(2) Not enough heat. This one requires some thought. While glowing coals are plenty hot
(typically 600 degrees Celsius), they don't contain all that much heat--that is, thermal
energy. (To use a water analogy, you've got a lot of pressure but not much volume.) What's
more, the coals aren't charcoal but rather ordinary wood, which is a poor conductor. When
you walk on them, your feet absorb the surface layer of heat, but there's too little of it
to burn you. Fresh heat from within the coals makes it to the surface too slowly to do you
any damage.
But fire-walking in some parking lot is for wimps. If you're hard core there are more
challenging methods. I've just spoken to my bud Jearl Walker, the former Scientific
American columnist and, it turns out, the G. Gordon Liddy of physics.
As a classroom demo of the Leidenfrost effect, Jearl not only walked on hot coals (he gave
it up after getting badly burned once--he was so cool his feet didn't get sufficiently
damp), he also dips his bare hand in water and then plunges it momentarily into a vat of
molten lead, 700 degrees Celsius. Says Jearl, who's even done this on Johnny Carson,
"there is no classroom demonstration so riveting as one in which the teacher may
die." It'd definitely penetrate my ennui, I'll tell you. Just don't volunteer in
Jearl's class when he asks someone to give him a hand.
MORE ON FIREWALKING
Dear Cecil:
A friend of mine forwarded your column on firewalking to me. It was a good column, but as
you'll see from the enclosed journal article, which I co-authored, the sizzle effect is
not critical (and Jearl Walker agrees with me, but keeps forgetting). --Bernard J.
Leikind, Encinitas, California
Cecil Adams replies:
Bernie's article comes down foursquare behind the not-enough-heat theory and says in
addition that the firewalker's feet may be insulated by dirt, calluses or water. Bernie
discounts the Leidenfrost effect, noting that a firewalker with a pair of rope- sole
sandals gamboled through the coals without damage to the sandals. Since the ground was
dry, the day was cold, and sandals don't sweat, there was no moisture, which is the key to
the Leidenfrost effect. The sandals' survival therefore must be credited to something
else.
I am pleased to set the record straight, naturally, and hope I have not seriously retarded
the advance of science in the meantime. However, any time you've got a phenomenon that
lets you work in a Jearl-Walker-tempts-death story, as was the case with the Leidenfrost
effect, in my book you want to cut it a little slack.
--CECIL ADAMS
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