I was melting squares Trader Joe’s Bittersweet Chocolate in the microwave oven last night, and there was a rather large electrical discharge within the chamber. There was no metal in the chocolate or bowl.
Why the arcing?
Not enough material in there to absorb the 'waves.
The level of the waves build up (standing waves) and created serious voltage.
You can put a cup of water in with a small bit to prevent the arcing.
Put the stuff on a saucer above the water and so some of the heat in the water will pass up to the target.
I routinely warm plates in the microwave oven (2 plates for 1 minute) with no arcing.
I think if the shapes are just right, you can cause arcing. (You can cause an arc between 2 grapes)
A lot of things that you wouldn’t expect will arc in a microwave. Grapes are pretty reliable. If you cut a grape almost in half, leaving just a tiny bit of skin holding the two halves together, and put that on a plate and microwave it, you will usually get an arc. Cut vegetables will sometimes arc, depending on the mineral content of the soil that they were grown in. Chocolate will sometimes arc as well. I’ve even heard of cat food arcing from someone who warmed up their pet’s food.
AaronX is right. The shape matters. Things with sharp edges and points build up more of a charge and are more likely to cause arcing. Hence, carrots grown in high mineral content soil are much more likely to arc when cut into small cubes than they are if left whole.
Using smaller amounts of chocolate (so all of the radio waves get absorbed into a smaller volume) will make arcing more likely. A higher power microwave will also be more likely to arc. You can try setting it on a lower power level, but often a lower power level means that it cycles the power on and off while it is running to average out to a low power level. While the radio waves are on they will still be blasting at full strength.
I would be a little leery of warming plates in a microwave. If there isn’t enough of a load inside the microwave to absorb the radio waves, a lot of the energy gets reflected back into the magnetron. Early microwaves had much more delicate magnetrons and could easily be burned out. Modern microwaves are a lot more rugged, but running a microwave empty, or close to it, is still very harsh on the microwave’s magnetron and can damage it. Sticking a cup of water in with the plates helps a lot.
By the way, if you want some more strange unexpected fun, pickles glow in the microwave.
Perhaps irrelevant, but on the topic of chocolate in the microwave and science, Measuring the Speed of Light with Chocolate and a Microwave Oven.
I cannot melt chocolate (at least bittersweet chocolate) in my microwave. A friend says he does it all the time. Different chocolate? Different microwave? The one time I tried it, some protective relay in the microwave functioned and the microwave appeared to be dead. But it was fine the next morning. Needless to say, I never tried that again.
What I find odd about this is the microwave oven was discovered by a guy working on radar in the war who discovered that a bar of chocolate in his shirt pocket had partly melted upon experiencing microwaves.
I believe that microwave ovens still have to pass the egg test: - YouTube
So long as the door stays closed - it passes.
Johnny, check the mica board in the oven. It’s at the side and it covers the thing that emits the microwaves. If it has a dark spot in the middle, it needs to be replaced.
The same thing happened to me a couple years ago. We took it to a repair place and they wanted to charge us a ton. The repairman liked us, and told us to go to Home Depot and get a sheet of mica.
Take the old piece out, trace the shape and cut out the new one. I’ll have to ask hubby what he used to cut it. We may even have a piece left. If I can find it you can have it.
Does that even work? I wouldn’t have thought plates would absorb enough microwave energy to get warm.
It depends on the plate. A lot of plates won’t absorb enough of the radio waves to get warm. Some will. It all depends on what material they are made out of. Some types of dishes, especially the glazed glass ceramic types, will heat up pretty quickly in a microwave, but since they won’t heat up evenly they will sometimes shatter from the thermal stresses.
Chances are if the dish is labeled “microwave safe” it won’t heat up much at all.
Avoid putting plates with metal in the microwave. Sometimes they’ll arc but often not simply because the metal band is too thick and doesn’t have any sharp edges or whatever. Eddy currents will be induced into the metal though causing it to heat up, which could cause the plate to crack or may burn you if you aren’t expecting it (the metal could easily end up a lot hotter than the plate).
I have a one-bowl brownie recipe I use baker’s chocolate squares for. It’s been a while, but I’m pretty sure I melted the chocolate with the butter and that seemed to mitigate the problem. I also used a large plastic bowl. I wonder if that helped?
I definitely don’t recommend putting a slice of cheese on a ceramic plate and then putting that in the microwave. Unless you want a plate split in two. This is why I don’t have youngsters visit. Well, just one of the reasons!
I did the same using a bowl and a Mars bar.
That was a great one.
The hell you say.
I tried it last night. It worked. And I’ve been warming my plates in the conventional oven all these years. I had no clue that microwaves would heat china.
Thanks for the awesome tip, Johnny L.A.
Bear in mind that these are plates I bought from Pic’N’Save (now BigLots!) for 99¢ each, and about 15 or 20 years ago one snapped in half when I put hot spaghetti on it. 
I expect them to break any time. (FWIW the larger, thicker, nicer-looking plates are also Pic’N’Save 99¢-ers.)
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