Cocktail Party Physics

Cocktail Party Physics

Physics With a Twist

Burn, Baby, Burn: Understanding the Wick Effect


Last month a BBC news story made the Internet rounds, with a somewhat sensational headline declaring the "first Irish case of death" by spontaneous human combustion (SHC). The badly burnt body of a 76-year-old man was found in his Galway home on December 22, 2010, lying on his back with his head close to an open fireplace.

There was no trace of accelerant, no evidence of foul play, and "forensic experts" concluded that the fire in the fireplace hadn't caused the blaze. Only the body, the ceiling immediately above it, and the floor underneath it showed any fire damage. What could the West Galway coroner do but reach a verdict of death by the "unexplained" phenomenon of SHC?

Color Jen-Luc Piquant tres skeptical about this ruling, a sentiment she shares with retired professor of pathology Mike Green, quoted in the BBC article as saying, "I go for the practical, the mundane explanation." As Green points out, the combustion is unlikely to be "spontaneous." Something set the body to burning, "but because the body is so badly destroyed, the source [of ignition] can't be found."

According to the BBC article, the coroner had "consulted medical textbooks and carried out [unspecified] other research in an attempt to find an explanation." But apparently he didn't consult the Oracle of Google, whereby he might have learned that the general consensus is that most cases of supposed SHC are due to something called "the wick effect," in which a body starts to burn, but the fat turns liquid, seeps into the clothing, and turns the body into a gruesome kind of human candle; it burns things in the immediate vicinity, but because it's a slow burn, nothing else is affected.

Heck, that coronoer could have just watched an episode of Bones called "The Foot in the Foreclosure" (I caught a rerun last night on TNT), in which Booth and Brennan investigate badly burned human remains found in a foreclosed house that was for sale and conclude the wick effect is to blame. Perhaps it's worth revisiting a post I wrote about SHC back in 2008, inspired by a rerun of the original forensic detective show, C.S.I.

In a subplot of "Face Lift," an elderly woman is found burned almost to ashes in her living room, dressed in what is left of her nightgown, save for her ankles and feet, which remained unburnt -- along with the rest of the room. The investigating CSIs assume there was an ignition source of some kind, most likely a cigarette, but Sarah Sidle finds herself suspecting it might be SHC, in part because she can't quite believe that anything else could reduce the body to that level of ash without burning down the entire building.

The human body isn't especially flammable, she reasons, and has high water content. Surely the fire would be doused rather quickly even if the body did manage to catch fire. That's why it takes flames of around 1600 degrees Fahrenheit over two hours or more to cremate human remains. A cigarette tip, in contrast, only burns at around 700 degrees Celsius. With the help of her colleague, Warrick, she performs an experiment with a dead pig in their headquarters parking lot, wrapped in an identical nightgown, with a single lit cigarette placed in the nightgown. The nightgown catches, and begins a slow, steady burn. Hours later, the pig, too, has been reduced to ash, save for its hoof-y extremities.

Sarah bows to the science and abandons her SHC theory, just as Grissom stops by and informs then that the phenomenon is known as the wick effect. (Yes, he'd known all along it wasn't SHC; he just wanted Sarah to do the experiment and see for herself. Science in action!)

The old woman's body fat served as a fuel source for the slow burn from the cigarette, with the nightgown serving as the wick. As the body burned, the melting fat seeped into the clothing, and the long chains of hydrocarbons that make up human fat provided the energy to consume the body -- locally, without damaging (much) the surroundings. Eventually the "candle" burns out.

Sarah and Warrick's impromptu experiment mimics a 1998 experiment conducted by Dr. John de Haan of the California Criminalistic Institute for the BBC TV science program QED. De Haan took a dead pig, wrapped it in a blanket and placed it in a furnished room, then set fire to the blanket with nothing but a match and a bit of gasoline. (Pig flesh is the closest to human flesh, so pigs are frequently used in these sorts of experiments.)

It took awhile for the pig body to catch fire -- Sarah was correct that the body isn't highly flammable - but once it caught, it burned at a high temperature and low flame, burning for several hours until de Haan extinguished the fire. The flesh and bones in the burned part of the body were reduced to ashes, but there was almost no damage to the rest of the room -- except for a melted TV set. De Haan reported that the heat from the burning body collected at the top of the room, making it hot enough to melt the appliance.

There has never been a definitively proven case of SHC, although -- as with any such mysterious phenomenon -- there are a handful of "true believers" out there, along with the usual skeptics. There have been some odd occurrences which initially seemed to point to SHC. For instance, in 1965, there was a case of an 85-year-old woman who died of a heart attack in her home, landing head-first in the hearth of an open coal fire. Both arms and her left leg were burned to ashes, but her right foot was intact. Internally, there was far less damage, enough for the autopsy to reveal that she had died from a heart attack, not from the burns.

Perhaps the most famous case is that of Helen Conway, an elderly woman, overweight, and an inveterate smoker who burned while sitting in an upholstered chair in her bedroom. The fire chief who responded believed it only took 21 minutes for the body to burn, convinced it was SHC.

The wick effect doesn't work that fast, of course, but others have speculated that the woman's body fat may have given rise to a much more intense fire, akin to a grease fire common to commercial kitchens. Apparently, while one of the firemen was searching for the victim's remains in the smoky bedroom, he stuck his hand in "something greasy" that turned out to be the remains. So who knows? The grease fire effect might be plausible.

De Haan himself encountered an interesting case in 1991, when two hikers near Medford, Oregon, found the still-burning body of a "well-nourished" (i.e., overweight) dead woman face down in the leaves. Cause of death was multiple stab wounds; apparently her killer had set fire to the body using barbecue starter fluid, hoping it would destroy the evidence. The woman's pelvis and spine were reduced to ash, as was most of the torso. Police caught the murderer, who confessed and said he'd set the body on fire 13 hours before the hikers discovered it. De Haan reasoned that the combination of "an immobile clothed body with a high fat-to-muscle ratio, accelerant, and artificial ignition" created perfect conditions for the wick effect -- hence the slow burn.

There's plenty of other documented cases of strangely burnt (or partially burnt) bodies, which is why belief in SHC prevails even today. Heck, Charles Dickens attributed the death of a heavy drinking character in Bleak House to SHC, because at the time, it was believed that heavy drinking could cause self-combustion. It was a moral thing, not founded in solid science, but Dickens drew on two actual cases he'd encountered in a collection of stories by Jonas Dupont published in 1763, under the title De Incendis Corporis Humani Spontaneis. The tales include one of a drunken German who supposedly self-ignited after drinking a great deal of brandy.

Alternative theories can sometimes be a bit, um, far-fetched. A man named John Heymer wrote a book called The Entrancing Flame in 1996, in which he advanced his hypothesis that SHC victims are loners who fall into a strange kind of trance that triggers a chain reaction of "mitochrondrial explosions" by "freeing hydrogen and oxygen within the body." That hypothesis might make sense if hydrogen and oxygen actually existed in gas form inside a mitochrondrial cell, but they don't -- and a good thing, too, otherwise the very act of inhaling could cause spontaneous ignition.

Even more far-fetched is the take of a man named Larry Arnold, who thinks that occasionally human cells get hit by a mysterious particle -- he calls it a "pyrotron" -- that causes a nuclear chain reaction inside the body. We give Arnold points for creativity and coming with a really cool moniker for his imaginary new particle. A "pyrotron" sounds really cool, much cooler than his alternative hypothesis that too much stress causes folks to burst into flame. Alas, it does not exist.

A slightly more plausible alternative explanation is that clothing in these cases catches fire because of a discharge of a large amount of accumulated static electricity. This is the pet theory of Robin Beach, founder of a scientific detective agency in Brooklyn, New York. (We like the idea of a scientific detective agency, in principle -- talk about a great TV series concept!)

One of Beach's early cases involved a young woman working at a factory plagued by as many as eight small fires every day -- caused by the fact that she retained more electric charge than the average person. Walking on carpets during dry winter weather can cause anyone to build up an electrostatic charge as high as 20,000 volts -- usually discharged the minute we touch a doorknob or other metal surface. Beach's take is that certain people retain even higher electrostatic charges and sometimes these cause give rise to small fires.

The problem with applying Beach's theory to SHC is that alleged SHC cases claim the victims are burned from within -- and no electrical discharge has been shown to cause anything remotely like that effect. Also, while the bodies are consumed, the surroundings are not in claimed SHC cases; a fire caused by electrostatic charge would cause damage to surrounding objects.

So I'll stick with the Wick Effect for the time being as my preferred rational explanation, even though the jury's still out on some of the stranger cases, where the Wick Effect really doesn't apply. I'm willing to bet scientists will figure it out one day. They won't convince the diehard True Believers, but perhaps the answer will supply an interesting plot line for a TV show of the future.

The views expressed are those of the author and are not necessarily those of Scientific American.

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