From: wsadjw@rw7.urc.tue.nl (Jan Willem Nienhuys)
Newsgroups: sci.skeptic
Subject: Re: fire walking
Date: 21 Jul 92 14:41:37 GMT
Reply-To: wsadjw@urc.tue.nl
Organization: Eindhoven University of Technology, The Netherlands

In article <1992Jul11.045821.16977@cco.caltech.edu> carl@SOL1.GPS.CALTECH.EDU writes:
>>>article in which the author claimed that the trick was just walk very 
>>>fast and everybody could do it without burning their feet. I am 
>>>wondering, does anybody know anything about this? ...
>>>
>For the Leidenfrost effect to be operative, you'd have to get the temperature
>of the perspiration above the boiling point of water.  Steam burns can be
>really nasty.  A more important consideration is the fact that the heat
>capacity of coals isn't very high.

Carl Lydick makes a good point here. 
The physiscs of firewalking is thought to be as follows:
When two solids of different temperatures come into contact (good
thermal contact, like two optically flat plates of metal pressed
together), the boundary layer assumes a "contact temperature", T0,
somewhere in between T1 (temperature of material 1) and T2 (temp. of
material 2).  The flux of heat accross the boundary layer depends then
only on T0 and T1.  The contact temperature stays the same.
This situation more or less applies to charcoal (or porous stone) and
feet.  T0 divides the interval from T1 to T2 into two subintervals,
and the ratio of the lengths of these two subintervals depends on the
materials only.
For both materails you compute
A = 1/sqrt (spec. density times spec. heat capacity times conductivity ).
Then the interval from T1 to T2 will be divided in a ratio A1:A2.
For example: feet are at 30 degrees (C) coals at 420 (C) (say).  Because
of the low density of charcoal and the low conductivity (heat capacity
per gram is not very low) the ratio A1:A2 comes out about 5:1.
(you will be surprised how difficult it is to find good estimates for
the relevant parameters in this case).
The interval of 390 is divided in the ratio 5:1, which means that the
contact temperature will be 95 C (just less than the temperature of
boiling water). Because the skin of the feet is rather thick, it takes
some time before the living tissue (about 1 mm inside) reaches the
temperature of 61 C, at which blistering and burning starts.

In some experiments a temp. of 150-200 C on a depth of 0.3 mm below the 
skin has been recorded.

Advice for firewalkers: use low density charcoal.  Remark above exposition
only valid for hot solids, so no flames please, especially no jets. 
Total contact time feet about 1.5 seconds per feet.  Coal bed should
be flattened (no rough pieces and thick lumps).  Avoid contact with 
sand or so below the coals.  Avoid anything that can make coals stick
to feet (traces of resins in the coal, sharp edges on the coals, even
sweaty feet), or that has different thermal properties (pieces of glass,
coins, nails, charred remains of plastic eating utensils and so on).

Warning:  experienced fire walking all say something different about
how to do it, and why it works.  This holds for "physics" explanations
as well as for "paranormal" explanations. I guess nobody dares experiment
much, once they have found a way of doing it. The above treatise on
contact temperature can be found in many engineer's books and manuals.

Maybe the fact that the rough surfaces of coals and feet do impede
thermal contact (so surface layer has a non-negligible heat resistance)
plays a role too.

JWN



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