[thedoc (OP)]

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?

[jon_bondy]

I have done something similar for years, but I only turn the microwave on for 20 seconds.  It works just fine, if you do not over do it.

[MikeS]

I speak from experience.  Don't dry your socks in the microwave.

[Airthumbs]

I don't understand I have dried my socks in the microwave, the idea of killing off some bacteria was a bonus also. I must admit that I did not run the waves for more then 30 seconds at a time. 

I suppose if your going to keep speeding up those molecules with microwaves eventually when the H2o goes it obviously gets a sniff of oxygen and hey presto, blazing undies!!! 

But if that was the case then anything dry should ignite inside a microwave, so is this just a textile thing? 

I just listened to the Podcast on this subject, so salt is the key hey.  So MikeS, would I be wrong in saying that your socks were not wet from washing but wet from sweat?  Haha!  Definitely sounds like student behavior. 

[RD]

is this just a textile thing?

beer bottles too ... http://boingboing.net/2009/11/21/saturday-morning-sci-6.html

[Phractality]

Dry cloth usually doesn't absorb microwaves, so after the water has all evaporated, the intensity of ratiation increases. Good microwave ovens shut off the power to the radiator when there is nothing absorbing the radiation. Cheap microwave ovens tend  to burn out the coil around the radiator because the current increases as radiation builds up.

Since dry fabric usually doesn't absorb microwaves, it doesn't get hot enough to ignite. However, some dish rags might contain metalic fibers, which heat up until they vaporize. When a metal fiber vaporizes, it can ignite the rag.

[Airthumbs]

I put a mug of tea into a microwave once that had a metallic coating, after about three seconds of intense crackling the glaze on the mug was covered in what looked like tiny black cracks.  I assume that these were caused by the electrical discharges across the surface.

I understood there were cold spots in microwaves so that if you put a load of ants in one they would seek these cold spots out.  However the newer designs have rotating discs, poor little ants!!  If ants can survive in cold spots then I suppose using the microwave as a bactericide is not such a good idea?