Sparking and scorch marks in microwave. What's gonig on here?

Last night, my wife put a single frozen Trader Joe’s turkey meatball in the microwave, in a small, open-topped glass container.

She said that, after a few seconds, she saw sparks in the microwave, and when we opened it up, there was a small scorch mark where the top of the door meets the ceiling of the microwave. Rubbing the mark caused some black to come off on my finger.

Is our microwave fucked? Is this dangerous? I’ve done some Googling, but haven’t found anyone with the same problem.

Edit: “going,” not “gonig”! :smack: Could a passing mod fix the typo please?

I’ve had this happen occassional with dense, frozen food. I am wildly speculating and will likely be corrected by an actual smart person, but I think really dense food acts something like a metal and causes the sparking you see with tin foil.

Ice is a poor absorber of microwaves, which may cause a buildup of microwaves to the point where arcing occurs between the door and case (this is why you should never run a microwave empty, also possibly one reason for a defrost setting, unless you had it set to defrost), although that shouldn’t happen unless there is poor contact between the door and case, which are made of metal since it reflects microwaves; metal itself isn’t dangerous in a microwave unless it has pointed edges, thus the microwavable containers which have metal rims, etc:

It’s possible that either the glass or the meatball allowed charge to build up to the point of arcing. It’s also possible that there is something wrong with the microwave. Try putting a cup of water in the microwave and running it for about 30 seconds. If there is no arcing then the microwave itself is probably fine. If you continue to have arcing then replace the microwave.

As for running a microwave empty, that’s a bit harsh on the magnetron (the thing that generates the radio waves). Back in the 70s it was very easy to break the magnetron doing this. These days magnetrons are a bit more rugged, but I still would advise against running the microwave completely empty (hence the cup of water above to test it).

You have to be a bit careful with metal in a microwave. Large pieces of metal can reflect the radio waves back into the magnetron (which again, is a bit harsh on it). The radio waves also induce eddy currents into the metal, causing it to heat up. If you have really thin bits of metal, the metal can actually heat up to the point where it burns or melts. Steel wool for example makes a pretty impressive display inside your microwave (again, magnetron warning, don’t do this it home, etc). A thicker piece of metal like a spoon will just get hot, possibly so hot that it will give you a nasty burn when you touch it. Charge will build up on pointy bits of metal, which can lead to arcing.

Does the glass have any kind of design on it? A glass in my collection has a gold ring that goes around the circumference near the edge and it must contain some kind metal because it causes sparks.

I tried that, and it worked just fine. No sparks or anything.

But, on looking closer at where the scorch-marks are, it appears that there is are a couple of small holes in the inside lining of the oven. The two marks, one on the door jamb, and one on the door itself, line up with one another when the door is closed.

I’ve taken pictures of them, with an inch-measure rule alongside for scale.

Door of microwave

Body of microwave

As you can see, it looks like there’s a little bit of bare metal showing in each area. Does that change anything?

Nope, just a plain glass container.

The damage looks pretty minor, and you aren’t getting any arcing now. I think your microwave is ok.

Many thanks! We’ll keep using it then. I appreciate your input, and everyone else’s.

always good to test for half a minute in a clean oven with an unpainted clear glass of water.

failure with sparks can be rapid with melting and burning plastic and paint.