As with many of us, I have heard about spontaneous combustion, but have never "experienced" it first hand. There are stories about piles of damp hay igniting and oil-soaked rags bursting into flame. I am always fascinated by the heat generated by my compost pile. I have measured the internal temperature to be as high as 160 F, or 71 C.

After doing a bit of reading, it seems that oils from grains and plants are more prone to spontaneous combustion than petroleum oils. Linseed oil seems to be one of the worst/best candidates, depending on whether or not you want a fire.

The chemistry of spontaneous combustion is that of oxidation. An oil in its liquid form will oxidize, but there will be little heat buildup due to the limited surface area. If we spread out the oil, then there is a large exposure to oxygen, and more heat is generated. If we soak oil into a rag of some sort and confine it to some degree, we can end up with a magic (or tragic) combination which will ignite. There are various factors that do have to be properly balanced. Air needs to be able to get to the oil to enable oxidation. If there is too much air, then heat will be drawn away. If there is too little air, then there won't be enough oxidation, and the ignition temperature may not be reached. Having the whole rag/oil mass somewhat insulated also helps to retain/build heat. Lastly, there is a "critical mass" which must be established. A small piece of cloth will lose too much heat to its surroundings. One needs a large enough "pile" to make things work.

So much for theory, now let's try it out....

This was my first try.

A coffee tin with a few pieces of old T-shirt. Enough linseed oil was added to the rags to moisten them, but not so much that any oil would drip off.

A digital multimeter was used with a thermal probe to monitor temperature.

After three hours, the temperature of the rags had gone up to 178 C or 354 F.

Along the way, various fumes could be seem coming from the can. Sometimes it seemed like there was a bit of smoke.

From this point on, the temperature of the can decreased.

No fire. (Which was good since the coffee tin wasn't in the best location for too much heat!)

This is what the rags looked like after the experiment.

My thoughts after this first experiment: More rags for critical mass. More air for more oxidation. A safer location.

Experiment #2:

More Air.

	More rags for critical mass. A safer location. Digital download of temperature data.
	After about 4 hours, the temperature of the rags had reached 212 C or 415 F. The picture shows some fumes/smoke coming from the rags.
	More smoke. Temperature was 311 C or 593 F ! Note also the browning of the rags at the edge of the can.
	Now we're cookin. Total time since start: 5 hours. Temperature 370 C or 700 F !
	After about 7 hours, the temperature was leveling off at 431 C or 808 F ! I decided to lift a layer of the cotton to see what was going on underneath. It looks like the cotton had been slowly charring/burning for a while.
	With the inner layers exposed, a bit of wind blew the smoldering mass into flame.
	Which quickly grew in size.
	Performing this experiment in a safer location was clearly a good move!

Now for some science:

The graph above shows the temperature - time profile for the successful experiment. It took about 3 1/2 hours for the whole mass to start to heat significantly. Once it got going, the temperature rise was pretty dramatic. I don't know where the cotton actually started to burn. I was guessing that it would be clear from the graph - but it doesn't appear to be. Maybe this was due to the temperature probe being buried in layers of cloth that were not burning - and hence they acted as an insulator.

Some data points:

Linseed Oil

Boils at 343 C
Flash Point 222C
Ignition Temperature 343 C

Cotton

Autoignition Temp (Oily) 120 C
Glow Temperature 205 C
Flash Ignition 210 C
Self Ignition 407 C

It is interesting to compare the above numbers to the graph above. The only real conclusion from my experiment was that the cotton did ignite at some temperature below 431 C. From experiment #1, 178 C is not enough.

Conclusions

Yes, spontaneous combustion is real!

I now have a lot more respect for proper disposal of oily rags. (Soak them in water, and/or store them in an air-tight container.)

Yes, spontaneous combustion can be used to start a fire - but it does take time!

Further research:

When did the cotton actually start to glow?
What other types of oils can be used?
My gut feel tells me that even more air was needed.
A can is not a good insulator. Something like cardboard would be better for heat retention.
How much cotton is needed for "critical mass?"
How big a role does the outside air temperature play?

Be safe.

rob

Survival Fire Spontaneous Combustion

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