Why did a wet cloth catch fire when microwaved?

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Offline thedoc

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Why did a wet cloth catch fire when microwaved?
« on: 01/04/2012 09:32:01 »
a_cumming asked the Naked Scientists:
   Hi Naked Scientists! Your show is the best!

I put a wet soiled dish cloth in the microwave oven, as it was supposed to give it a less foul odour, killing some bacteria too. After a few minutes, the cloth started to smolder (eventually looking like the picture below), and the cloth had dried out. What is the science behind the cloth catching fire?

Thanks!

Andy from Winnipeg, Manitoba, Canada

PS. Yes, I know this was foolish, and plan not to do such things again.

   
What do you think?
« Last Edit: 01/04/2012 09:32:01 by _system »

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Offline syhprum

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Re: Why did a wet cloth catch fire when microwaved?
« Reply #1 on: 01/04/2012 20:44:15 »
If your dishcloth is so soiled that it has a foul oudur I think the first thing you should do is give it a good boiling in some water with detergent before microwave drying it,
Microwave cookers are not very suitable for drying thing as you have have found out as the wet parts absorb the radiation more than the dry parts with consequent scorching 
syhprum

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Offline Phractality

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Re: Why did a wet cloth catch fire when microwaved?
« Reply #2 on: 01/04/2012 21:31:01 »
Apparently, the cloth was not wet when it caught fire. A dry cloth usually does not absorb microwave radiation, depending on the kind of material. With nothing absorbing radiation, and the radiator continuing to radiate, the intensity of radiation inside the microwave would increase rapidly. Some microwave ovens have a sensor which shuts off the power to prevent current overload from burning out the coils when you turn it on with nothing inside.

To produce the temperature necessary to ignite the cloth, I'm guessing that there may have been some metal fibers woven into the cloth. Metal fibers in a microwave oven will conduct induced currents sufficient to vaporize the metal. That is why dishes with gold or silver trim often spark in the microwave.
Imagination is more important than knowledge. Einstein