AbstractStudents walked over glowing embers without coming for any harm (with one exception). The reason for it is: embers are bad heat - conductors.Our ExperimentOn our Christmas-party our students walked 4 m over glowing embers at temperatures of several hundred degrees without a special mental preparation and without coming to any harm with one exception: a journalist of the yellow press. (see fig. 1) We did it in order to prove that thermal physics is not something only for the brain, but also for the heart and the feet. The explanation for having just fun and no nasty burns in the case of many participants (but not all!) is that wood, a semiconductor, is also a poor conductor of heat. Twice of our firewalks were on television. This video can be lent.
Fig 1: The report of our Chrismas party in the yellow press (Bild,21.11.95, p. 3)
Explanations
We will give some explanations which are often told speaking about firewalk.
Wood is a poor heat conductor. Only a small amout of heat penetrates in the fect. Coal (wood) is a semiconductor, and therefore a bad heat conductor. Iron is a good conductor. Therefore, in winter times, you have the impression: iron feels colder than wood.
The time of contact is less than 1 sec. In such a short time only very few Jouls can pass over. (see fig. 2)
Sweat at the feet produces an isolating Leidenfrost steam layer
Blood transports heat as a cooling system. When the ambient temperature is high, circulation through peripheral blood vessels raises the temperature of the skin (reddening of the skin), K. Schmidt-Nielsen, Physiologische Funktion bei Tieren, Stuttgart, Fischer 1979.
Collegues in Berlin have implanted a temperature probe into a heel and have measured
that the temperature rises by 
by a firewalk. Blisters occur above 80oC. If feet are
aluminated (Alumn powder = Potassium aluminum sulphate KA1(SO4).(2H2O)
crystal water) before fire walking, then the temperature difference in the feet is
only: 
. Both results are
independent whether feet are wet or dry.
Fire walking as an introduction in thermodynamics
Questions lead to theories. "How to get a good shot ?" leads to
mechanics. "Why is firewalk possible ?" leads to thermodynamics.
An other question is: "What is the meaning of warm?".
"Warm" clothes are not hot but bad heat conductors.
What about the burned people?
After the experience of us and other 10 % of the firewalker have burns.
These feet should be washed with water. Put a little bit potassium permanganate in the
water to desinfect the wounds.Otto Gerber asserts "Burns are the most uncomfortable by -
products of fire walking. It seems as if the burns were caused by the psychological
expectation of being burnt:" ("Feuerlaufen" Otto Gerber)
The organizer of commercial fire walk make money out of the wounds, too by
pretending, that the places the burns indicate hidden problems (carious tooth, being
incontent with the goverment, with the stomac, with the wife, with the leaver or with the
collegues.)To continue our conductivity experiments we send our student Daniela
Fischer to Petrosawodsk, Russia, (degree of latitude 62O N, degree of longitude
34O E). On Janurary 30th, 1998, 23.20 it was so cold that her hand froze and
stuck to the car.In the car it was 3oC (38oF). Out doors it was -
47oC (- 53oF) cold. Her russian friend liberated her with a burnier.
The heat was conducted very fast out of her hand in the car, because metall of the car is
an excellent heat conductor. (In December 1941, the casualities of the Wehrmacht in Russia
by cold were much higher than by gunfire, because the German soldiers had steelnails in
sole-leather.)
If you fire walk the risks are
1. Embers must not stick to the feet - need dry feet
2. Ember path less than 6 m
3. Avoid metal pieces (nails) because heat conductivity of metal is 700
times that of wood. Good Luck !
Metaphysical hint for bad conductors
A main theme of Metaphysics is nothing. This concept was used by
Copernicus to prove the helio-centric system before classical physics was invented.
Copernicus means: Beyond the stars is nothing. Relatively to nothing the stars can not
move. Therefore the Earth moves.Let us try to use "nothing" for the problem of heat
conduction. Nothing has no conductivity. Empty space is nearly nothing. (Just a
possibility for radiation etc.) 1m3 air has only 1000 cm3 matter
with a lot of empty space between. Therefore air is a bad conductor. Clothes (like
pullover, stockings) are a lot of holes to keep air around the body.
Fig 2:
Josef Fourier (1768-1830) used temperature waves 1824-1826, to
determine the depth of water pipe-lines in Paris to 1,6 m.