Exactly How Does the Human Body Burn?

Linda Geddes, New Scientist: Until now, scientific knowledge about burned remains has been limited. Anything that wasn’t based on speculation has come either from post-hoc examination of burnt corpses — where the exact circumstances of the fire are usually unclear — or from the deliberate burning of pig corpses, which have key differences to humans. “There wasn’t much literature,” says Elayne Pope, a forensic scientist at the University of West Florida. “The science is young.”

Eight years ago, a medical institute in Memphis, Tennessee, agreed to provide Pope with some of its donated bodies and she began her unusual mission. To date she has made use of about 30 whole corpses and various additional body parts.

So what happens after they light the fire? “A human limb burns a little like a tree branch,” says John DeHaan, a fire investigator at Fire-Ex Forensics in Vallejo, California, who works with Pope. First, he says, the thin outer layers of skin fry and begin to peel off as the flames dance across their surface. Then, after around 5 minutes, the thicker dermal layer of skin shrinks and begins to split, allowing the underlying yellow fat to leak out.

“That’s when the fire gets most interesting,” says DeHaan. Body fat can make a good fuel source, but it needs material such as clothing or charred wood to act as a wick. Like that in a wax candle, a wick absorbs the fat and pulls it into the flame, where it is vapourised, so enabling it to burn.

Assuming there is sufficient wick material, the body can sustain its own fire for around 7 hours. During this time, the heat causes muscles to dry out and contract, making the limbs move and sometimes adopt characteristic postures (see illustrations). Bone takes longer to burn, so by the end the skeleton is usually laid bare like a charred anatomical model, coated in the greasy residue of burned flesh.