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Q & A: Why did ghee explode?

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Most recent answer: 10/05/2012
Q:
On a recent sunny day my neighbor was heating oil for use in butter lamps. It was ghee, a type of oil, quite thick at room temperature, made from butter with most of the fatty solids removed. She forgot about it and when I heard her scream I ran out to my veranda to see what was the matter. As I approached she exited her apartment carrying the small pot which was emitting a whitish cloud. Almost instantly as she entered the bright sunlight the fumes exploded in a burst of flame. Fortunately neither of us was burned and the flames extinguished quickly. My question is about the ignition source. Was it the increased oxygen outdoors causing spontaneous combustion or could it have been the effect of sunlight concentrated through the vapor droplets, as can also be accomplished with a magnifying glass? I think the answer depends on whether the oil was smoking or boiling, which I think must be the latter because the fumes were whitish in color not black.
- Steve (age 44)
India
A:
This is an interesting story. Your speculations sound very perceptive.  i don't know the answer, but am posting this in the hope that some reader may have helpful ideas.

As for the oxygen, I doubt that there would be a big difference between the overall oxygen level indoors and outdoors. However, just getting the pot in motion gets it away from a localized spot where perhaps the stove fire and the fumes from the ghee itself might have depleted the oxygen.

If the exploding ghee were not so messy and dangerous, one could devise several experiments to test your ideas.  For example, one could use two pots of ghee. One could be illuminated by a very bright bulb, the other by a similar bulb but with its light diffused by frosted glass, to avoid focusing. Would the light that can be focused consistently ignite the ghee sooner?

Likewise, one could test if blowing a slight breeze on the ghee sped up the ignition.

Mike W.

(published on 10/05/2012)

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