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I am a new, and newly sleep-deprived, dad. As such, I didn't think twice about taking a big, shiny, stainless steel spoon and sticking it right in a baby bottle sterilizer. ("I want this supplement to be scooped with a sterile instrument!")

Now, a bottle sterilizer consists of a large, sealed, plastic box into which you put a bunch of water. Put it in the microwave, and it's basically an autoclave. I sealed that sucker up and let 'er rip at 1300 watts. I then walked away and down a flight of stairs.

When I came back to my kitchen, everything was fine. The 3-minute microwave cycle was done, and my stainless steel spoon was apparently sterilized. Nothing was on fire, and the microwave seemed as happy as it had been. I, however, was mortified--and puzzled. I've read that only pointy surfaces cause problems in microwaves. But my spoons are fairly pointy, especially at the handle end:

My beautiful, pointy spoon

Right now, I am provisionally chalking this up to God. But I must admit that I don't find my explanation quite thorough enough. Does anyone have a more specific accounting?

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David Bruce Borenstein
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    Was the spoon completely surrounded by water? – Catija Mar 22 '17 at 20:57
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    Possible duplicate of [Why is some metal safe to use in a microwave, but others not?](http://cooking.stackexchange.com/questions/7830/why-is-some-metal-safe-to-use-in-a-microwave-but-others-not) – moscafj Mar 22 '17 at 21:00
  • oops just realized this was answered! – moscafj Mar 22 '17 at 21:01
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    Catija, the sterilizer is like a steamer: you put water in the bottom and the stuff sits in a basket above it. moscafj, thanks--I did cite that question and I think it's different but related. – David Bruce Borenstein Mar 22 '17 at 21:16
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    I usually reheat single bowls of soup or chili with a spoon stuck in it (puncturing a paper towel overtop if it looks like it might pop stuff over the inside of the oven). That's with a 1970s-era RadarRange. If the utensil is not near a wall and there is lots of microwave-absorbent food to go along with it there should not be an issue. Please don't invoke divine intervention when James Clerk Maxwell will suffice. – Spehro Pefhany Mar 23 '17 at 10:08
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    This is probably a better post for [Physics SE](http://physics.stackexchange.com/) since the correct answer is entirely related to the interaction of the microwaves with the water and the spoon. Microwaves will attenuate strongly in water (that's how it heats up) so the water offers a degree of protection to the spoon inside. The water will also act to dissipate and bleed any charge accumulation on the spoon surface and is otherwise a strong dielectric (so will polarize heavily to suppress surface potential on the metal). Physics SE could give you a much better answer. – J... Mar 23 '17 at 12:31
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    I find this question very difficult to answer. When you are warned to not do something because there is a risk of X happening (in this case fire) people don't mean "every time you do it, X will happen". When there is a random chance that something happens and it does not happen, it is not really possible to explain why - this is the definition of randomness. – rumtscho Mar 23 '17 at 17:14
  • Welcome to the Sleep-Deprived Dads' Club! Congrats on your kids and condolences on your sleep. – T. Sar Mar 23 '17 at 20:14
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    @rumtscho To be fair I am not sure this "random" as much as "something that would be understood by Physics users moreso than Seasoned Advice users". There's a fuzzy line between "not understood" and "random"! But of course there's a ton of variables we don't know as well, like the details of the design of this particular microwave, the detailed physical properties of the objects in it, etc. So, yeah. – Jason C Mar 23 '17 at 20:32
  • @rumtscho but the only thing that is random is the quantum world, and I don't think that the spoon not exploding would have been affected by that. There is no "random chance" that this will happen. – theonlygusti Mar 24 '17 at 22:55
  • @theonlygusti I disagree that the only thing random is the quantum world. Even with a narrow definition of randomness (I cannot predict it with any amount of information), determinism is an unsolved question. And if you accept a more pragmatic definition (I cannot predict it with the amount of information that I can reasonably gather) it is even more clear that we are talking about a chance of fire, not about a certainty of fire. – rumtscho Mar 24 '17 at 23:07
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    A note for the comments and flags of "should be on Physics": The SE network policy is to always respect the OP's choice of site. If a question is on topic on the site where it was asked, it does not get migrated, no matter how well it would fit on a different site. Cross posting is also not allowed. The only thing that might still happen is that the OP decides to self-delete the question here and then post anew somewhere else. – rumtscho Mar 24 '17 at 23:10
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    Time to consider buying a dish washer – Astor Florida Mar 27 '17 at 12:09

7 Answers7

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Metal has a lot of potential issues in the microwave (electric charge buildup + arcing, and microwave reflection). There are too many variables to make general statements like "such-and-such metal is safe" or "smooth objects are safe" with confidence, hence the sweeping guaranteed-safe blanket advice to not put metal in the microwave.

The reason it didn't cause issues for you is likely the fact that it was insulated in a plastic container (thus preventing arcing to the microwave walls / magnetron) and that you also had some water in there to absorb any excess reflected energy (and maybe electrical energy through physics that I don't understand, but don't quote me on that one).

In fact a cursory search just happened to turn up this paper regarding sterilization of metal objects in a microwave, which mentions:

Arcing back to the magnetron and damage to the microwave oven are prevented by placing a radar absorbent material within the oven and with proper insulation of the item to be sterilized

And you've got both of those things: A radar absorbent material (the water) and the insulation (a perfectly sealed plastic container).

The type and shape of your metal object is probably unrelated to the success of your accidental "experiment".

As an aside: It's worth mentioning that in the case where a microwave oven does catch fire, the NFPA mentions that 17% of those fires involved the housing/casing of the appliance itself as the first item ignited, where the danger is presumably excess energy reflected back to the appliance, rather than arcing (magnified by poor design or older microwaves that didn't have as good of an ability to absorb reflected energy). That's why, for example, the authors of the sterilization paper linked above included a radar-absorbent material, strange and unpredictable reflections off of metal can easily put the microwave under uneven energy loads it wasn't designed to handle (see also thermal runaway).

And of course, there's also issues with defective appliances or design flaws as well, e.g. that old GE spontaneous microwave fire lawsuit. While the causes of those fires aren't really related to the OP's situation, the point is making blanket statements about metals and such is further complicated by the potential for poorly designed or defective appliances.

Also, this answer isn't intended to say "insulation + water = always safe", it's just intended as an explanation for your specific experience. Even "safe" is flexible: A microwave fire doesn't necessarily lead to a kitchen fire or a burned down house or an injury, e.g. I suppose you could call a microwave fire hazard "safe" if you were standing there on the ready with a fire extinguisher and protective clothing.

Jason C
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    So it's safe to put metal in the microwave as long as I have a fire extinguisher handy. Got it – aebabis Mar 24 '17 at 16:45
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    @acbabis It's safe _and_ fun, but only if you don't really care about the microwave anymore (or the things you're sticking inside of it). I [made plasma](https://www.youtube.com/watch?v=G7lfzA7WzVI) using my old microwave, a glass, and a bit of aluminum foil. Fun to watch. Broke the glass, and the microwave. Didn't set fire to anything. – aroth Mar 25 '17 at 12:08
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    'hence the sweeping guaranteed-safe blanket advice to not put metal in the microwave.' This is plain wrong. It is ADVISED to put a spoon in a cup/glass if you heat liquids. Otherwise you risk high level burns when you take the cup out, because the water might explode. see here: http://physics.stackexchange.com/questions/234042/should-i-place-a-spoon-in-a-cup-of-liquid-before-heating-it-in-a-mircowave – Kami Kaze Mar 27 '17 at 09:23
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    @KamiKaze That advice exists all over the place, and everybody's heard it. It doesn't matter that it's not accurate advice, or that it's not the advice the manual gives, or that other sources give different advice: The point isn't that it's true (the opposite, actually), the point is only that it exists, it's prevalant (hence the OP's question, for example -- their expectation that the spoon would cause a problem had to come from somewhere), and that's why. It's also pretty minor in this answer. Btw, "This is plain wrong" reads unnecessarily aggressive, there are other ways to say that. :O – Jason C Mar 27 '17 at 09:34
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    @KamiKaze you can prevent superheating (the cause of "exploding") by using a wooden chopstick, or other non-metal utensil – Erica Mar 27 '17 at 11:25
  • Off-topicish: [Here](http://www.nfpa.org/~/media/files/news-and-research/fire-statistics/major-causes/oscooking.pdf?la=en) is a very thorough (and surprisingly interesting) detailed report on cooking-related fires in the US (includes microwave data among everything else), if anybody is curious. Also TIL electric ranges are apparently a higher risk of house fire than gas ranges, for reasons I can not begin to understand. – Jason C Mar 29 '17 at 02:14
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Spoons (most metal, in fact) are generally not a huge problem in the microwave. My microwave has metal parts...many do. Forks are sometimes a problem due to a build up of charge between the tines, which could result in sparks. As you note, shape can be a factor. The shape of spoons spreads the charge, the pointy edges of forks and narrow tines could allow a build up. It wasn't luck, but it probably should not be normal practice.

moscafj
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    I would be willing to bet that the fact that it was insulated in a plastic container and to some extent the water was the primary factor here, at least for preventing arcing to the microwave walls. For example, [the reason they insulate metal equipment when sterilizing it in a microwave](https://www.ncbi.nlm.nih.gov/pubmed/3884686) (with water serving as the microwave-absorbant material). Also "generally not a problem" is not entirely.... accurate. – Jason C Mar 23 '17 at 08:00
  • I also recall a case of a burger wrapper causing a microwave oven to be damaged, and drip plastic onto the food. Early ovens were more troublesome; modern ones are safer. – JDługosz Mar 23 '17 at 09:13
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    Shape may not be a factor as well. My brother once covered a plate with a pan's metallic cover, which was perfectly "round" - no spikes or whatsoever - . Well, the microwave was, after some seconds, VERY nasty to look inside, with sparkles and flashes happening. DO NOT EVER do that again! – Noldor130884 Mar 23 '17 at 10:57
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    The fact that the oven contains metal doesn't mean other metal is safe. The metal of the oven itself will be shaped and positioned so as to not do harm. The oven gets harmed from arcing or from reflecting too much energy back into the generator, not from metal per se. – Loren Pechtel Mar 25 '17 at 05:42
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There are actually two things worth considering with metal in microwaves.

The big danger is arcing and that happens with pointy things like forks and apparently grapes. It's also worth considering, being in a plastic box, there was nowhere for the spark to jump to. With a fork there's a small enough spark gap for current to jump. With a spoon, there is not.

However, metal also has a shielding effect, and the USDA even says it's safe to use small quantities of foil in a microwave. In this specific case, there wasn't really enough potential to cause a spark; the shielding just redirected microwaves elsewhere.

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Journeyman Geek
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    Grapes in a microwave are *amazing*. – Wayne Werner Mar 23 '17 at 20:41
  • I wouldn't mess around with putting foil in the microwave. When I was a kid, my friend and I blew out the light in his parent's microwave with a single twist tie like what comes with garbage bags. It was exciting. – JimmyJames Mar 24 '17 at 14:23
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    A twist tie has sharp points. If you don't get the physics, don't try this please. – Journeyman Geek Mar 24 '17 at 14:24
  • @JourneymanGeek I get the physics. It's just that even in the link you point to, it has 6 caveats and then says "If you see arcing (sparks), immediately remove the foil shielding." Your microwave may already be damaged. It's very hard to predict whether it will arc. Unless you are a physicist or engineer in a relevant field, it's not worth it unless you are prepared to pay for fixing your micro or getting a new one. – JimmyJames Mar 24 '17 at 14:27
  • In this case though, you have a smooth piece of metal with no points, away from anything metal, which is why it didn't spark. – Journeyman Geek Mar 24 '17 at 14:30
  • +1 Everyone should Read The Fine Manual more, my old microwave's owner's manual said that a little foil was good too, keeps the edges of thin food from turning into a hard crisp while the middle's still cold. PS sliced onions always put on a sparky light show too – Xen2050 Mar 25 '17 at 15:00
  • I'd like to avoid pretty light shows, since this would probably frighten my dear old mom. The best way to avoid those is to understand how it happens. – Journeyman Geek Mar 25 '17 at 15:02
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In order to reliably set your kitchen on fire, make sure there is enough fuel available, and that some of its mass is either brought to its autoignition temperature, OR vaporized and heated to its flash point, then ignited. While electrical arcs of any kind reach tremendous internal temperatures, they are notoriously bad at heat transfer to surrounding materials.

Something that doesn't belong in a microwave can be heated in more or less three ways:

  • By being a lossy material, absorbing radiation and heating up. Food gets heated that way.
  • By being an effective dipole antenna with a resistive load attached - very dependent on object geometry; a 2-3 inch strip of tinfoil is a good example.
  • By arcing
rackandboneman
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It didn't set your kitchen on fire because while it evidently arced, it was inside a metal box and it didn't have suitable/favorable conditions for igniting what little fuel it had available.

You have a plastic box on one hand, as fuel, and you have water, steam, a metal spoon and a metal microwave oven interior, all noncomustible.) So when the electricity stopped and the arcs stopped, flame was not sustained.

Personally, unless I'm destroying a CD/DVD and/or making "art" from it, by briefly (while watching) zapping it on top of a mug of water, I respect the "no metal in the microwave" rule - it's simpler.

Ecnerwal
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    I'm probably missing something, but why do you say it evidently arced? I can't find any indication of that. – Cascabel Mar 23 '17 at 03:05
  • I once microwaved an Arby's wrapper — against which there is a warning on the selfsame wrapper — and it burned holes in a few places. I don't remember where exactly were the other points of contact for the bolts, but the interior of the microwave shows no signs of scorching or similar. – can-ned_food Mar 23 '17 at 08:10
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    @Jefromi It's not the greatest picture, but I see what appears to be a blackened spoon (that's possibly only an artifact of the picture location, I guess), and at full-scale view I see pitting along the edges. – Ecnerwal Mar 23 '17 at 13:06
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    I see, could be. To me it just looked like a dark photo and a banged up old spoon. – Cascabel Mar 23 '17 at 14:07
  • There are plenty of fuel sources; interior paint, exterior shell, insulation on internal wiring, etc. Arcing is not necessary to start a fire, enough energy reflected back at the magnetron/walls and the subsequent heat build-up can be sufficient. Barring defective appliances, of course. I'm not saying any of this is likely, I just mean that "it was inside a metal box" isn't a great explanation. It's why you [don't want to run your microwave when it's empty](https://www.quora.com/Will-operating-a-microwave-oven-while-empty-damage-the-equipment) (well, old/cheap microwaves), for example. – Jason C Mar 23 '17 at 16:35
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    I've done the microwaving-a-CD thing a few times, and in my experience if you put any water in there with the CD, the water absorbs all the microwave energy and the CD is unaffected. – zwol Mar 23 '17 at 18:39
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    Sorry for bad photo. Spoon is not blackened or banged up; it came in that shape. It's possible that the edge is pitted. – David Bruce Borenstein Mar 24 '17 at 06:50
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I recently did an "experiment" with tin foil:

Putting just one piece into the microwave did basically nothing, but having two pieces together created sparks between them and burned them away.

Your spoon might just be too massive to reach enough temperature. Metal is usually a good thermal conductor. So a lot of the created thermal energy might just be distributed evenly inside the spoon. It takes a lot of energy to melt or burn iron. Its melting point is at 1811 K ​(1538 °C, ​2800 °F)

I doubt a common household microwave can emit enough energy in form of radiation to heat up the spoon to that level.

Even wood and plastic (some of the more common materials used in kitchens) need several hundred degrees to start burning. If we are talking about surfaces made of stone, then we basically have to create a volcano to melt it.

Long story short: a common microwave is just not powerful enough to burn a kitchen. With some manipulations you might be able to cook water outside of it, but that's all. (Not that I support that kind of experiment.)

Peter Mortensen
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lordmogul
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    It takes about 30 kJ to melt an entire spoon (a typical 25g stainless steel spoon), about half of which is bringing it up to its melting point. Magnetron efficiencies vary, but even at a low-end efficiency of 60%, a 1300-watt microwave would take 38 seconds to melt the entire spoon. So there was definitely enough power to melt the spoon. But the water probably absorbed almost all of the microwaves in this case. – Charles Mar 24 '17 at 14:27
  • Understand that the way radio waves of a certain wavelength interact with objects is extremely dependent on the shape and size of the object. Real life antennas don't come in odd shapes because they are modern art. – rackandboneman Mar 27 '17 at 12:19
  • ...And a 1300W heat source is plenty enough to set something on fire. A 13W soldering iron can. – rackandboneman Mar 27 '17 at 12:21
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Microwave ovens are made out of steel. There is no danger of putting any metal of any kind (steel, copper, aluminum, etc.) in a microwave PROVIDED IT IS NOT VERY THIN. Foil is much too thin, and even much thicker, but still thin metal like the trim on the glass lid of a rice cooker is too thin. When microwave EMR is directed at thin metals, the waves reflect back and forth quickly between the top and bottom surface, and the metal quickly both overheats and generates a strong static electric charge, creating a fire hazard. For most intents and purposes, the rule is 'no foil in the microwave,' not 'no metal.' However, as I said, I have seen the thin metals used as trim (but not bonded to a thicker layer of metal) cause trouble as well.

evan dean
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    Can you give some sources for your information? It looks pretty interesting, but your answer would be better with some references. I had not heard that thickness is the primary factor. – Megha Mar 26 '17 at 06:00
  • Thin metal will heat up like hell because a) there is little solid mass to absorb the heat, b) resistive heating if you get a dipole effect is intense, and both can lead to c) some of the metal vaporizing and building a plasma. *Reflection* can happen with thick metal just as well, and that is what can blow the magnetron if it happens at the wrong angle. The case is made of steel because reflection is exactly what you want - but in an engineered and controlled pattern.... static electricity, and intra-conductor reflections have not much to do with it. – rackandboneman Mar 27 '17 at 12:14
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    Had to downvote because it can be read as "you're safe with thick metal in a microwave every time" which is dangerous advice - especially in the framework of "you're safe if..." and not "there will be trouble if..." – rackandboneman Mar 27 '17 at 12:16