At this point, there ought to be an addendum to Internet Rule 34 that specifically addresses microwaved objects. Whether it’s a watermelon, dry ice, exploding hard boiled eggs, or yes, even a microwave, if it exists, it’s probably been microwaved.
Yet none of these objects have the beauty of a clump of micron-thick steel wool being microwaved in slow motion. As shown in a video posted Thursday by Steve Mould, the steel wool—each strand of which is just a fraction of the diameter of a human hair—turns into a glowing ember almost immediately after starting the microwave.
You’ve probably heard that it’s dangerous to put metal in a microwave, but that’s not strictly true. The danger—of sparking, mostly—comes from the shape of the metal, not the just the fact that a thing is metal. After all, the insides of microwaves are metal, too.
The reason for this has to do with the way electromagnetic radiation—microwaves, in this case—interacts with metal. As Mould explains in the video, when the microwaves interact with the free-moving electrons in a metal, it causes them to “bunch up.” Yet these electrons all carry the same negative charge, which results in a force opposite to the microwave repelling them from one another.
Importantly, this push-and-pull between the metal’s electrons and the microwaves happens at the surface of the metal and eventually reaches an equilibrium—assuming the electrons have room to expand as they are repelled from one another. If they don’t, that’s where you run into trouble.
Compare the video of the steel wool to another video Mould made where he microwaves a CD. Here, the CD can be seen splintering into jagged edges and sparks fly. This is because electrons get pushed up into these edges and corners and there’s a buildup of charge between these surfaces. The electrons are all repelling one another, but they have nowhere to go—that pent up energy is eventually released as sparks. No sharp edges, no problem.
In the case of the ultrafine steel wool, it’s super thin which means it can be heated up quickly. Since steel wool is mostly iron, this heat causes it to react with oxygen and produce iron oxide, a chemical reaction that itself generates heat and results in the cascading burning effect you see in the video. You don’t get the same sparking effect as with a CD because the electrons aren’t getting trapped and bursting out.
It’s a pretty cool effect, but if you’re worried about burning your house down, you can also just touch a 9 volt battery to steel wool and accomplish the same thing.