Aluminum foil in a microwave - when should you be worried?

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Aluminum foil in a microwave - when should you be worried?

Postby Random-person » Thu Jan 06, 2011 2:17 am UTC

I like kebab, and occasionally I buy kebab roll in bread which comes wrapped in aluminum foil. And occasionally I buy two so that I'll have another roll for later. Though cold kebab isn't very good, so I heat it up in a microwave oven. But if I take the wrap off before, the roll will usually fall apart, so I usually leave the aluminum foil, but this seems to be against common wisdom, with people nearby regularly panicking to stop the microwave, which forces me to stand guard if I want my food to ever get hot.

Now, from some experimentation I know that if you only put aluminum foil in the microwave and start it, it'll start flashing and parts of it will get burned. But if it's wrapped around kebab (eg. bread and meat), what's the worst thing that could happen?

As I don't care much for buying new microwaves and as I know that electrical appliances very well do explode when used incorrectly (I'm looking at you, old toaster who decided to explode without warning), explosions that could lead to microwave oven doors flying all over the place, I'd rather not put the testings to the extreme, but I know that two and a half minute of microwaving a kebab roll doesn't seem to make anything explode, at least not in the few times I've tried it yet.

Is there any way to calculate how much aluminum foil per food would be needed to make it go all sparkly? Which factors might there be that decide whether it'll go boom or not? It should be noted that the people I'm trying to calm down (so they won't turn off the microwave oven when I'm heating kebab!) are physics students with limited practical experience, but they are easily convinced with math talk.


Now, given what I know about how microwaves work, the reason aluminum foil gets hot is that it just like food will absorb microwaves, but unlike food it won't get evenly heated (eg. the microwaves will stop at the aluminium foil rather than going past it to the food at a higher rate than if it was in contact with the food directly). Is that part true? So, the food won't get heated (to any great extent) by the microwaves, but the microwaves heat up the aluminum foil, and the food is then heated via heat convection. If the foil is too thin I'm guessing that it'll quickly get so hot that it'll fuse with the food (which is what I have noticed) which is unwanted, so maybe I should wrap the roll with another layer or two of aluminum foil so that the foil won't get so hot that it'll fuse, but as there's more foil overall the food will be heated slightly faster. But then won't that increase the risk of sparkliness, wherever that comes from?

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Antimony-120 » Thu Jan 06, 2011 5:22 am UTC

Hoo boy, okay there's a couple things to explain.

One is that you can't have an electric field inside a conductor. You can force that with a really fast shifting field since nothing is a perfect conductor, but in most practical circumstances it holds. Now as you may know, a microwave works by oscillating electric fields that cause the water molecules in food to spin around rapidly creating friction (water molecules are polar, and try to line up with the electric field, which keeps changing direction). So when you wrap the food in a conductor the field no longer gets to the water molecules, meaning there's no heating. Some of the energy will be used to create all those sparks, and there will be some heating of the conductor, but it will be transferred to the food at a very low efficiency.

Now the sparks are unlikely to cause any major damage to the food, you will not get cancer (that's not even possible with microwaves), but they may do some damage to the foil, cause you to taste burning aluminum, or make a godawful smell. You also might burn the microwave a little.

Another common misconception is that a metal object will damage the magnetron, which is actually unlikely for most bits of metal you throw in there. What will damage the magnetron is if you try microwaving with nothing or very little food inside, in which case the magnetron may cook itself (the energy has to go somewhere).

the tl;dr: It's not a very efficient way of doing things, and it might cause a bad smell or superficial burns to the interior of the microwave, but it's not cancer inducing or dangerous.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby thoughtfully » Thu Jan 06, 2011 7:12 am UTC

It isn't all that unusual for some food items to come in "crispers" or other devices that have a metal foil bonded to cardboard or something to enhance the reheating process. You shouldn't see anything too untoward unless there are sharp points or ridges where the electric charge becomes concentrated. So you'll improve your chances by smoothing out the foil as much as is feasible.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby ++$_ » Thu Jan 06, 2011 7:59 am UTC

Antimony-120 wrote:Another common misconception is that a metal object will damage the magnetron, which is actually unlikely for most bits of metal you throw in there. What will damage the magnetron is if you try microwaving with nothing or very little food inside, in which case the magnetron may cook itself (the energy has to go somewhere).
Surely a piece of food completely wrapped in aluminum foil counts for this purpose, right? The foil should reflect most of the microwaves with negligible absorption, I think (although I could be wrong).

Of course, if your food is being successfully cooked, then that isn't what's happening.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Sandor » Thu Jan 06, 2011 2:20 pm UTC

The kebabs round here come in wrapped in aluminium, which are then put in a special paper bag. The one time I tried to put one in the microwave, it produced a fairly spectacular light show and set fire to the paper bag. And the result tasted metalic. I'm not doing that again.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Bhelliom » Thu Jan 06, 2011 2:44 pm UTC

I have never heard of wrapping food in foil to microwave it. That seems counter-productive. Perhaps wrap the kebab in wax paper or plastic wrap? That would allow the food to heat while still containing the roll.

Another thing I want to say is that some microwaves do allow the use of foil. I have a Magic Chef microwave that can also grill with an upper heating element, convection cook with a powerful fan in the back of the unit, and can also do all three at once, called Combi-cook for really stupid fast heating times. The manual also cheerfully suggests that one can use aluminum foil to shield part of the dish you don't want to over cook. It just says to make sure that the foil does not touch the sides of the microwave.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby Random-person » Thu Jan 06, 2011 6:25 pm UTC

Bhelliom wrote:I have never heard of wrapping food in foil to microwave it. That seems counter-productive. Perhaps wrap the kebab in wax paper or plastic wrap? That would allow the food to heat while still containing the roll.

Another thing I want to say is that some microwaves do allow the use of foil. I have a Magic Chef microwave that can also grill with an upper heating element, convection cook with a powerful fan in the back of the unit, and can also do all three at once, called Combi-cook for really stupid fast heating times. The manual also cheerfully suggests that one can use aluminum foil to shield part of the dish you don't want to over cook. It just says to make sure that the foil does not touch the sides of the microwave.

It comes wrapped from the kebab store: I could remove the wrapping, but then the roll would fall apart, making it really hard to eat. You want to remove the aluminum foil as you eat it.

Anyway, I've heard that what causes the sparks is uneven foil: if I wrap the roll in another layer of foil and try to make sure that it stays even, will this be enough to prevent sparks? I mean, it's still uneven under the outermost layer of foil, but the outermost layer is even...

Anyway, If I put the food on a plate, I'm guessing that even if it goes all sparkly, nothing would really happen, right? I just don't get how you are supposed to destroy the microwave oven just because it's all sparkly. I kind of need a good argument if I want to prevent people from running to the microwave to turn it off.

..I guess a tl;dr would be, WHAT causes a microwave to explode?

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Sagekilla » Thu Jan 06, 2011 7:13 pm UTC

Well, I would solve this problem by simply popping in a toaster oven if you had one.
There, the aluminum foil would work spectacularly and you'd get a nice crispy roll.

Of course, if you don't have a toaster oven then this is a moot point.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby Antimony-120 » Thu Jan 06, 2011 8:08 pm UTC

++$_ wrote:
Antimony-120 wrote:Another common misconception is that a metal object will damage the magnetron, which is actually unlikely for most bits of metal you throw in there. What will damage the magnetron is if you try microwaving with nothing or very little food inside, in which case the magnetron may cook itself (the energy has to go somewhere).
Surely a piece of food completely wrapped in aluminum foil counts for this purpose, right? The foil should reflect most of the microwaves with negligible absorption, I think (although I could be wrong).

Of course, if your food is being successfully cooked, then that isn't what's happening.


Random-person wrote:
Bhelliom wrote:I have never heard of wrapping food in foil to microwave it. That seems counter-productive. Perhaps wrap the kebab in wax paper or plastic wrap? That would allow the food to heat while still containing the roll.

Another thing I want to say is that some microwaves do allow the use of foil. I have a Magic Chef microwave that can also grill with an upper heating element, convection cook with a powerful fan in the back of the unit, and can also do all three at once, called Combi-cook for really stupid fast heating times. The manual also cheerfully suggests that one can use aluminum foil to shield part of the dish you don't want to over cook. It just says to make sure that the foil does not touch the sides of the microwave.

It comes wrapped from the kebab store: I could remove the wrapping, but then the roll would fall apart, making it really hard to eat. You want to remove the aluminum foil as you eat it.

Anyway, I've heard that what causes the sparks is uneven foil: if I wrap the roll in another layer of foil and try to make sure that it stays even, will this be enough to prevent sparks? I mean, it's still uneven under the outermost layer of foil, but the outermost layer is even...

Anyway, If I put the food on a plate, I'm guessing that even if it goes all sparkly, nothing would really happen, right? I just don't get how you are supposed to destroy the microwave oven just because it's all sparkly. I kind of need a good argument if I want to prevent people from running to the microwave to turn it off.

..I guess a tl;dr would be, WHAT causes a microwave to explode?


I'll answer both of these with the same point, magnetrons burn out (or explode) because in an empty mircrowave you get a standing wave of electricity. What that means is that every cycle the magnetron pumps energy into the field which just keeps sloshing it between the microwave cavity and the magnetron, getting bigger and bigger with each slosh (think of pumping your legs on a swing). Eventually the magnetron absorbs too much energy and starts to breakdown, at whichpoint it can have some thermal runaway, which in turn makes it absorb more energy. That is a technical way of saying "We done cooked it up and made it so it can't take heat no more". Basically you flash cook your own microwave in a sparky mess. If there was enough energy in the field at the time this can be quite violent. This is a bad thing.

The aluminum foil however will absorb energy. As the field oscillates large elecrtic currents will whip around on the surface, and the natural resistance of the foil will cause it to heat as it sparks. That is a very different method of cooking than the standard microwave cooking, but it is a method nonetheless. But do NOT cover the food entirely in smooth aluminum. That results in the problem ++$ mentioned, where the metal isn't absorbing any of the field and we done cooked up our microwave.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby Random-person » Thu Jan 06, 2011 9:18 pm UTC

Antimony-120 wrote:
++$_ wrote:
Antimony-120 wrote:Another common misconception is that a metal object will damage the magnetron, which is actually unlikely for most bits of metal you throw in there. What will damage the magnetron is if you try microwaving with nothing or very little food inside, in which case the magnetron may cook itself (the energy has to go somewhere).
Surely a piece of food completely wrapped in aluminum foil counts for this purpose, right? The foil should reflect most of the microwaves with negligible absorption, I think (although I could be wrong).

Of course, if your food is being successfully cooked, then that isn't what's happening.


Random-person wrote:
Bhelliom wrote:I have never heard of wrapping food in foil to microwave it. That seems counter-productive. Perhaps wrap the kebab in wax paper or plastic wrap? That would allow the food to heat while still containing the roll.

Another thing I want to say is that some microwaves do allow the use of foil. I have a Magic Chef microwave that can also grill with an upper heating element, convection cook with a powerful fan in the back of the unit, and can also do all three at once, called Combi-cook for really stupid fast heating times. The manual also cheerfully suggests that one can use aluminum foil to shield part of the dish you don't want to over cook. It just says to make sure that the foil does not touch the sides of the microwave.

It comes wrapped from the kebab store: I could remove the wrapping, but then the roll would fall apart, making it really hard to eat. You want to remove the aluminum foil as you eat it.

Anyway, I've heard that what causes the sparks is uneven foil: if I wrap the roll in another layer of foil and try to make sure that it stays even, will this be enough to prevent sparks? I mean, it's still uneven under the outermost layer of foil, but the outermost layer is even...

Anyway, If I put the food on a plate, I'm guessing that even if it goes all sparkly, nothing would really happen, right? I just don't get how you are supposed to destroy the microwave oven just because it's all sparkly. I kind of need a good argument if I want to prevent people from running to the microwave to turn it off.

..I guess a tl;dr would be, WHAT causes a microwave to explode?


I'll answer both of these with the same point, magnetrons burn out (or explode) because in an empty mircrowave you get a standing wave of electricity. What that means is that every cycle the magnetron pumps energy into the field which just keeps sloshing it between the microwave cavity and the magnetron, getting bigger and bigger with each slosh (think of pumping your legs on a swing). Eventually the magnetron absorbs too much energy and starts to breakdown, at whichpoint it can have some thermal runaway, which in turn makes it absorb more energy. That is a technical way of saying "We done cooked it up and made it so it can't take heat no more". Basically you flash cook your own microwave in a sparky mess. If there was enough energy in the field at the time this can be quite violent. This is a bad thing.

The aluminum foil however will absorb energy. As the field oscillates large elecrtic currents will whip around on the surface, and the natural resistance of the foil will cause it to heat as it sparks. That is a very different method of cooking than the standard microwave cooking, but it is a method nonetheless. But do NOT cover the food entirely in smooth aluminum. That results in the problem ++$ mentioned, where the metal isn't absorbing any of the field and we done cooked up our microwave.

Ah, thanks for the explanation! So, sparks is in a way a good thing? Eg. that means that the microwave probably isn't going to explode. So, even if there are sparks, everything should be fine?

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Clever-Username » Thu Jan 06, 2011 11:09 pm UTC

Do you have an oven? Easy solution to heat it up = throw it in your oven. This avoids the issue with your microwave. Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off. However I could go on a huge shpleel about what's already said but you've already read that and so it would be pointless to go over it again.

If a microwave is your ONLY solution to heating up the kebab then read what's already been said and be very careful in doing so. Otherwise, just turn on a damn oven and do it that way.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby torontoraptor » Fri Jan 07, 2011 3:59 am UTC

I once, very foolishly, decided to make kraft dinner in the microwave in a metal pot. Suprisingly, nothing happened until the handle touched the side of the microwave, which did cause a huge shower of sparks and a burn mark on the microwave. I'm guessing that the shower of sparks is caused by the electrical current on the outside of the pot discharging onto the side of the microwave?
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Re: Aluminum foil in a microwave - when should you be worrie

Postby Random-person » Fri Jan 07, 2011 5:06 am UTC

Clever-Username wrote:Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off.

WHY is it never a good thing? It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby ST47 » Fri Jan 07, 2011 6:50 am UTC

Sparks mean some sort of electrical discharge, which probably means something is going to get oxidized and that looks ugly. It can also probably damage the metal in the front door, which as we all know is the thing that protects you from being cooked.

In addition, covering your food entirely in foil is a bad idea because that creates a faraday cage around your dinner just like the outer case of the microwave, and so your food doesn't get cooked properly. Yes, a little bit of heat is generated by the electrical currents in the foil perhaps, but you can't possibly actually think that that's a good way to cook your food.

In addition again, sparks in a microwave are never a good thing because sparks in general are not a good thing. Are you suggesting the opposite, that it's just fine for your microwave to approximate the conditions inside a small lightning storm?

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Carnildo » Fri Jan 07, 2011 8:28 am UTC

Random-person wrote:
Clever-Username wrote:Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off.

WHY is it never a good thing? It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all.


A spark represents a high concentration of energy. High concentrations of energy can have a variety of effects, such as:

1) Melting aluminum
2) Burning aluminum
3) Burning food
4) Burning microwave oven.

Generally, these are considered bad things.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Clever-Username » Fri Jan 07, 2011 5:09 pm UTC

Random-person wrote:
Clever-Username wrote:Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off.

WHY is it never a good thing? It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all.


Are you kidding me? How can sparks BE a good thing. As already said in the above two posts, sparks are generally a discharge of highly concentrated electricity. This means SOMETHING, SOMEWHERE is going wrong. You NEVER want a discharge of electricity like that unless your ultimate goal is to blow something up or injure/kill someone.

Please, since you're being so arrogant here, tell me how sparks are a good thing. Give me one good reason that high energy discharge in an enclosed household environment such as a microwave, with potential bystanders, is a good situation to have.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Random-person » Fri Jan 07, 2011 7:07 pm UTC

Clever-Username wrote:
Random-person wrote:
Clever-Username wrote:Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off.

WHY is it never a good thing? It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all.


Are you kidding me? How can sparks BE a good thing. As already said in the above two posts, sparks are generally a discharge of highly concentrated electricity. This means SOMETHING, SOMEWHERE is going wrong. You NEVER want a discharge of electricity like that unless your ultimate goal is to blow something up or injure/kill someone.

Please, since you're being so arrogant here, tell me how sparks are a good thing. Give me one good reason that high energy discharge in an enclosed household environment such as a microwave, with potential bystanders, is a good situation to have.

I never claimed that sparks are a good thing!

It's just that most of what people think of microwave ovens are nothing but myths. You have to be able to explain your idea without becoming angered. I'm not in any way arrogant, if anything you are the arrogant person, telling me that "it's just what it is" rather than explaining your position. This is science, not religion.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Moose Hole » Fri Jan 07, 2011 7:44 pm UTC

And so it was that on the third minute the sparks flew, and they were sore afraid. The light of the kebab had created of them zombies, and many brains were eaten.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Dopefish » Fri Jan 07, 2011 7:53 pm UTC

I think the possibility being raised by the OP is that the sparks aren't necessarily harmful, and are more show than threat, in a similar sense to how some chemical reactions, while completely harmless, can vigourously bubble up at first. Or something like that anyway.

I wouldn't want to tempt fate though, even if apparently in your experiances it hasn't actually caused any harm, since sparks are pretty much concentrated energy as mentioned, and those inherently have potential for badness. I imagine if you literally set fire to the packaging you could under controlled conditions sucessfully heat your food to an acceptable level without anything particularly bad happening, however that doesn't mean you should do it.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Technical Ben » Fri Jan 07, 2011 10:20 pm UTC

So could you get poisoned by the aluminium this way?
Wiki says it's not really poisonous so I guess it's safe. I'd avoid it for the fire risk myself.
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Re: Aluminum foil in a microwave - when should you be worrie

Postby Clever-Username » Fri Jan 07, 2011 11:10 pm UTC

Random-person wrote:
Clever-Username wrote:
Random-person wrote:
Clever-Username wrote:Sparks in a microwave are never a good thing and people are in the right state of mind to run over and turn it off.

WHY is it never a good thing? It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all.


Are you kidding me? How can sparks BE a good thing. As already said in the above two posts, sparks are generally a discharge of highly concentrated electricity. This means SOMETHING, SOMEWHERE is going wrong. You NEVER want a discharge of electricity like that unless your ultimate goal is to blow something up or injure/kill someone.

Please, since you're being so arrogant here, tell me how sparks are a good thing. Give me one good reason that high energy discharge in an enclosed household environment such as a microwave, with potential bystanders, is a good situation to have.

I never claimed that sparks are a good thing!

It's just that most of what people think of microwave ovens are nothing but myths. You have to be able to explain your idea without becoming angered. I'm not in any way arrogant, if anything you are the arrogant person, telling me that "it's just what it is" rather than explaining your position. This is science, not religion.


So if you admit that sparks aren't a good thing then what's the big deal, why are you so bent on opposing the notion that sparks in a microwave are bad? There are dozens of documented cases of sparks being a precursor to a violent explosion, just check out the 'Microwave This?' channel on youtube.

While I admit a single ball of aluminum foil in the microwave will not make it explode, it still isn't a good thing to have in there, as violent discharges of electricity as produced with the arcing is a dangerous amount of high energy that's free to go as it pleases. There are precautions that can be made to prevent catastrophe, as already stated in several previous posts.

The primary point I'm trying to make is that sparks are NOT a good thing to have in an enclosed space, and no I'm not going to back up my arguement with 20 paragraphs of scientific proof. Fact is fact, high energy discharge in an enclosed envirnment MAY lead to catastrophic consequences, however in your case it isn't likely. My arguement is towards the general notion of it being a bad thing, not in specific your case of microwaving a kebab.

However, to directly address your situation, an oven will be much more efficient at heating up the kebab than using a microwave. Aluminum reflects electromagnetic radiation much like a mirror does, thus none of the energy is going towards heating up your food. Ovens don't operate using radiation, it's just a simple temperature gradient between the interior of the oven and the interior of your food. Guaranteeing safe heating instead of a potentially dangerous electrical storm.

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Re: Aluminum foil in a microwave - when should you be worrie

Postby Random-person » Sat Jan 08, 2011 1:20 am UTC

Clever-Username wrote:So if you admit that sparks aren't a good thing then what's the big deal, why are you so bent on opposing the notion that sparks in a microwave are bad?

Huh? When did I ever oppose it? I'm just applying skepticism to common notions. Most of what we think of the world is severely skewed, but by using science - knowledge that isn't just philosophical but empirical - we can come closer to reality.

There are tons of myths regarding microwave ovens, which is exactly why I want you to do explain your claims using reasoning and not just propagate what could very well be a myth. Notice that there are no articles on snopes consisting of nothing but "It's just how it is".

Be skeptical, not religious!

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Re: Aluminum foil in a microwave - when should you be worrie

Postby gmalivuk » Sat Jan 08, 2011 4:57 pm UTC

Random-person wrote:I'm not in any way arrogant, if anything you are the arrogant person, telling me that "it's just what it is" rather than explaining your position.
That is not what anyone did. And yes, you *are* being intensely arrogant, because you're assuming that if someone else believes a sparking microwave is bad, it must just be because they'd heard that somewhere, and not because they'd actually thought about it for a couple seconds on their own.

It's one thing to just ask, "And why are sparks bad?" That could be a quite effective way to lead someone else through reasoning about a problem. But it's quite another to add, "It seems like you are just repeating what someone else told you without having any practical knowledge of the subject at all." By adding that, you became an arrogant jerk, because now you're acting like you're the only one in this thread who applies skepticism to scientific claims.

Be skeptical, not religious!
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Re: Aluminum foil in a microwave - when should you be worrie

Postby meat.paste » Thu Jan 13, 2011 8:30 pm UTC

Where the aluminum sparks, you will very likely form aluminum oxide. Al2O3 is not great to breathe (http://www.sciencelab.com/xMSDS-Aluminum_oxide-9922858). So, probably not a good idea to keep your food wrapped in metal when using the microwave. Also, sparks in a microwave are indicative of something happening that the oven was not designed for.
Huh? What?

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Re: Aluminum foil in a microwave - when should you be worrie

Postby wbeaty » Wed Jan 19, 2011 7:49 am UTC

Common insurance claim for microwave kitchen fire: sparks inside unattended microwave oven set fire to paper also inside microwave oven. Bits of burning paper spew out the rear vent, land in trash can placed under microwave shelf. Big flames.

The electric arcs inside your oven aren't necessarily bad. Just recognize that they aren't "sparks," they are "kilowatt plasma torch." Your kitchen will fill with fumes of paint burned off the oven interior, plus some ozone and oxides of nitrogen. Some ovens have a plastic plate on the chamber ceiling, and if this gets charred by one of those plasma-torch flames, it will forever after spew smoke and flames whenever the oven is turned on. Carbon is a good absorber of microwaves. This can be cured by carving away every bit of the charred plastic. (Hint for obtaining microwave ovens for free: take them off the curb during neighborhood garbage night. Check for common failure mode: carbonized plastic inside. Carve off the black chunks, and microwave works fine again.)

Also: with a foil-wrapped kebab, does the foil get hot enough to cook the food? I'd think that the foil would form an electrical shield, so the meat stays cold. But thin foil may itself be a good enough absorber to become a heat source.
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