Aluminum Foil...please tell me!

[HAnn521]

Why does aluminum foil catch fire in a microwave (and quickly might I add) but it doesn't even get hot when it's in an oven ??

Can somebody tell me? I am always amused by this and never really understood why it works this way.

[Space]

Aluminium does get hot when it's in an oven.
It just cools nearly instantly when you get it out, because it's very thin.

And if you put it in your microwave, you'll be lucky if you only damage the aluminium foil... I suppose you know the risks...

[HAnn521]

So why does it get hot enough to cause serious fire/damage in a microwave, but doesn't even feel hot to touch out of an oven because it cools so fast?

Why can't it cool as fast or stay cool in a microwave as well...that's my question.

[snapdharlich]

It seems to be that the frequency that microwaves operate on have certain interesting qualities. The frequency will absorb water, fats and sugar but not plastic, glass etc. It's probably something to do with reflecting the waves and concentrating on the foil rather than penetrating therefore catching fire. Don't quote me it's only a guess at the second bit.

[Space]

according to the info that I found, it's not the heat that is the cause of the problem...
Metals react to microwaves by creating electric charges, which then discharge and make sparks...
and it's those discharges that can produce heat and damage.

[Hippy]

The waves of energy in the Microwave Oven are rapidly changing Electro-Magnetic waves. The metal foil is a conductor of electricity.

When the waves cut the conductor, like the moving magnetic field in your bicylcle dynamo, it produces electric current in the metal. This is shorted due to the shape and mass of the metal and it is this electrical current flowing that causes the foil to become hot enough to oxidise.

In an ordinary oven the heat conveyed to the foil is by convection of hot air inside the oven. The foil gets no where near as hot. Don't run away with the idea that the microwave energy acts on all things equally.

P.S. Don't do this as not only is a danger to humans but will cause damage to the magnetron that produces the wave energy by reflecting energy back and frying the clever cavities and coils.

[Clanad]

You may have not noticed that many "Microwaveable" foods are stored in aluminum containers and are placed directly into the oven when cooking.  There are some restraints... they shouldn't touch other metal parts and they should be fairly shallow... but this has more to do with cooking the food properly than any safety concerns.  The aluminum foil you've had experience with probably was wadded up or at least folded to where it had multiple sides resting on or touching each other.  The foil also probably had sharp corners and edges.  Each of these characteristics multiplies, in different ways, hazards in it's use.  If you lay a piece of foil flat in the oven, nothing will happen and it most likely will not heat up itself, since the action of heating anything in a microwave requires that the substance be able to absorb the microwaves.  Further to your experience with foil heated in a conventional oven... if you folded the sheet a few times or wadded it up, you can be guaranteed a severe burn when held in the bare hand, since it wouldn't readily radiate the heat as quickly as Space states...

[Hamish]

Just to add aluminium burns at 600C so you dont need to get it as hot as iron/steel to make it igite (OK iron wool would probably work but only because of the huge SA)

[Eddiecurrent]

The radily altering electromagnetic field in a microwave will cause voltages to be induced in the foil. These voltages set up what are called eddy currents these will cause the foil to heat up.