On Monday morning, Steve received a call from NBC Nightly News from a producer who wanted to learn more about the science of walking on fire.
People can actually walk on hot coals… but the trick is understanding how heat is transferred from the hot coal to the person’s feet. Prior to the walk, someone lights the fire well ahead of time to allow the wood to burn down to non-flaming coals. The stunt is usually held at night. If it were done during daylight, the bed of coals would look instead like a bed of ashes. There is always a layer of ash covering the coals. By doing it at night, the glowing red light is still visible through this layer of ash.
Here’s the basic science… people are able to walk across a bed of burning coals because wood is not really a good conductor of heat. Basically, there’re three ways heat can get transmitted… conduction, convection, and radiation.
Conduction is the transfer of heat from one substance to another via direct contact. In convection heat is transferred through air or fluid circulation. In radiation it is transmitted as if spreading out in straight lines from a central source (think of the sun or a heat lamp).
Conduction is the main way heat is transmitted to a person’s feet during a fire walk.
In fire walking, the participant’s feet actually touch the ash-covered coals. However, if you were to pick up one of these pure-carbon coals, you would notice that it is extremely light, and this lightweight carbon structure is a poor conductor of heat.
It takes a relatively long time for heat to transfer from the glowing coal to your skin. It’s also important to know that ash is a great insulator that rests between the hot coals and the person’s skin. So, the ash covered coals transfer their heat even more slowly because the ash acts as a layer of insulation.
Bottom line… walking on coals is dangerous regardless of the theoretical science. According to our friend, David Willey, professor of physics with the University of Pittsburgh, most fire-walks occur on coals that measure about 1,000 degrees Fahrenheit (550 degrees Celsius), but he once recorded someone walking on 1,800-degree (1,000 °C) coals.
Believe it or not, sweat or moisture on the person’s feet can also help to provide a layer of protection due to a principle called the Leidenfrost Effect. The phenomenon explains how a small amount of moisture can produce an insulating vapor layer which keeps that liquid from boiling very quickly. Most people have watched what happens when drops of water fall into a very hot skillet – the drops quickly dance on the surface for a few seconds before finally vaporizing. If the skillet’s temperature is at or above the Leidenfrost point, the water quickly skirts across the pan and actually takes longer to evaporate that it would in a skillet that is above the boiling temperature but below the Leidenfrost point. This protective vapor barrier is what allows science demonstrators to pour liquid nitrogen on their hands or to quickly dip a finger into a pot of molten lead. Both practices are bad ideas no matter how much you believe in or understand the scientific principle behind the stunt. Don’t do it… but you’ll never look at drops dancing around a hot surface the same way again!