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:
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?
Best Answer
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:
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.