Dr Karl › Dr Karl's Great Moments In Science

Why firewalking is a fraud

Firewalking is not as hot as it sounds. Just a little basic science will stop you getting burnt, says Dr Karl.

Last time I talked about firewalking, and how amazing it appears. After all, walking with your bare feet on a hot road at 44°C for 35 seconds can give you second-degree burns. So how do people walk on fire, and not get burnt?

My first point is that the name "firewalker" is not accurate. They don't walk on fire. In fact, most firewalkers don't even walk on naked hot coals. In general, they walk on hot coals that are covered by ash — but "ashwalker" doesn't sound as impressive as "firewalker".

My second point relates to the concept of 'heat conductivity". It sounds a bit complicated, but let's take it one step at a time. It relates to how quickly heat travels from one body to another.

Imagine that you have a cake cooking in a cake tin, inside an oven at 180°C. It has been baking for an hour, so everything in the oven is at 180°C. So the air, the cake, and the metal cake tin in which the cake rests are all at 180°C.

You open the oven and plunge your hand into the hot air at 180ºC. Your hand does not get burnt by the hot air. Weird!

Next, you gently, with your naked finger, touch the top of the cake, which is also at a temperature of 180°C. Once again your finger does not get burnt. More weirdness.

But if you touch the hot cake tin, you'll immediately get large blisters on your naked fingers. So you grab an oven mitt, and remove the hot tin and cake.

Why do the air, cake and metal cake tin all have different abilities to burn you?

Welcome back to our friend, heat conductivity.

Heat conductivity measures how rapidly heat energy can flow out of an object. 'Conductivity' and 'insulation' are opposites. If an object is a good conductor, it's a bad insulator — and vice versa.

Air is a bad conductor, and a good insulator. So the flesh of your hand can 'touch' the hot air in the hot oven and not get burnt.

The same goes for the cake. Even though the cake has a lot of heat energy stored in it, its poor conductivity stops the heat from getting into your hands. There is a lot of heat energy in the cake, but it can't flow quickly into your naked hand. Mind you, if you lay your hand on the hot cake for more than a few seconds, you'll get a nasty burn.

But the cake tin has both a high heat content and a high conductivity. Touch the tin for a second or two with your naked flesh, and you'll get burnt immediately.

But what about hot coals? They turn out to be very bad conductors of heat. A hot coal has a moderate amount of heat energy, but is incredibly bad at passing that heat energy to anything else.

Back in 1997, Kjetil Kjernsmo, a Norwegian scientist from the University of Oslo, investigated the phenomenon of firewalking with heat sensitive cameras. The first thing he found was that the temperature of the coal bed varied between 150°C and 700°C. 700°C is hot, but nowhere near the 1,200°C that some firewalkers claim to have measured.

The second thing that he found was that very little heat energy left the coals and entered the naked feet, during a firewalk. According to his heat camera, the naked feet didn't cool down the coals much at all — so the hot coals stayed hot, and the feet stayed cold, all thanks to the very low thermal conductivity of the coals. He measured that after a typical firewalk, the bare soles heated up by only 4°C.

Finally there is some physiology involved. The outer layer of human skin is dead. Even people who always wear shoes and have soft feet, have enough dead skin to provide good insulation. And as a final help, blood circulating through the feet is an excellent conductor, and helps to take the heat away.

It's not a higher state of spiritual awareness that protects you from blisters — it's basic high school thermodynamics.

So if somebody tries to take your money for firewalking, ask them to prove that they have something really special to offer. Ask them to walk on a hot steel plate — it might not be such a cake walk, after all!

^ to top