What causes Spontaneous Combustion

Spontaneous combustion has always had some mystical quality about it. How can an object suddenly (or slowly) burst into flames in the absence of any apparent ignition? During the Dark Ages it was associated with witchery or other satanical powers. Persons associated with such a phenomenon might soon find themselves under combustion in the form of an Inquisition auto da fe. Such superstitions have persisted into modern times. Witness the widespread (and totally unsubstantiated) claims of spontaneous human combustion.

But alas, science has solved this mystery and dispelled the magical myths. Combustion occurs when an object is raised to its kindling temperature, which is that temperature which will sustain combustion, in the presence of sufficient oxygen. This normally occurs when some form of ignition, such as a flame or spark, is applied to the combustible material. However, the material can also reach its kindling temperature by means of internal chemical or biological action, external heating, or, in the case of a volatile fuel mixture, by compression. When this happens spontaneous combustion occurs.

A common example of chemically induced combustion occurs with linseed oil soaked rags. As with all plant based drying oils, linseed oil heats by the chemical process of oxidation. When rags saturated with linseed oil are confined in a space which allows ample exposure to air for oxidation, but not enough air to dissipate the heat,the temperature of the rags will rise and can reach the ignition point of about 212 degrees F. At this point, they will spontaneously combust if sufficient oxygen is present.

A famous example of the foregoing was the fire that destroyed Jack London’s magnificent “Wolf House” in the forest near Sonoma, California around 1917. Jack suspected arson, owing to his leftist politics, and vowed to rebuild. Unfortunately, he died unexpectedly from uremic poisoning just past the age of 40, and only the ruins remain of this once extraordinary mansion. Eighty years after the conflagration, forensic experts hired by the state of California determined that linseed oil soaked rags left by the workmen caused this tragedy by spontaneous combustion.

Aerobic bacteria in combustible materials such as haystacks or compost piles cause the mass to heat through biological action. Again, there must be sufficient oxygen to support this process, but not enough to carry away the heat. One characteristic of these fires is that they can be apparently quenched only to erupt in flames sometime later. Indeed, moisture, as well as oxygen, is essential to the the entire process, which is why compost needs turning (aeration) and wetting for it to continue working.

Grain silos have been known to spontaneously explode when the dust-laden air inside reaches its ignition temperature through radiant heating by the suns rays on the silo walls. This threat can be ameliorated by keeping a moisture level inside the silo to prevent the grain dust from aerating, while preserving the grain quality and not promoting biological oxidation.

The most common and beneficial example of spontaneous ignition is the compressional heating which drives diesel-oil engines. Unlike gasoline engines, which rely on a spark to ignite the combustible fuel-air mixture, diesel engines continue to run after the starting ignition is removed. Compression of the air-fuel mixture in the cylinders raises the temperature to the ignition point, where it spontaneously ignites without a spark. This phenomenon is also sometimes observed in high compression gasoline engines, in which the engine may continue “dieseling” for a few cycles after the ignition is switched off.

This is a broad brush overview of spontaneous combustion based on scientific principles as I understand them. It is by no means exhaustive or rigorous, and I encourage my readers to explore this subject further on their own, hewing to the realm of science and eschewing the supernatural.

I would be be remiss, however, if I failed to relate a story pertaining to spontaneous combustion which borders on the paranormal. My friend Bruce Hering, the founder of Bruce Bread Bakery in Boonville, California, was awakened one morning around 3 am by some unnamed anxiety. After trying to shrug it off, he gave up and decided to go to the bakery. Pulling into the parking lot, he saw flames shooting across the bakery floor. He was able to extinguish them, but had he gone in at his normal time the building would have been incinerated. How to explain it? Beats me. Did something trigger an alarm in Bruce’s head? Was it just coincidence? A subconscious memory of having left something amiss?

I am an avowed skeptic and discourage unscientific explanations. Still, science has not encompassed “all things under heaven and earth”. Some things it hasn’t even dreamt of.