If you grew up in the 80s like I did, you probably encountered your first microwave at a young age.
And if there was one thing that was drilled into you with absolute certainty it’s this:
You can heat many things in a microwave…but metal DEFINITELY isn’t one of them.
What’s the reasoning behind this accepted wisdom though, and what happens when you put metal in a microwave?
This article covers everything you need to know about the subject.
It explains the science behind this rule, and also addresses some of the contradictions that might have occurred to you!
What Happens When You Put Metal In A Microwave?
The message about never putting metal in a microwave is very strong and very clear.
So clear, in fact, that you might imagine the end result will be fires, explosions, or something equally catastrophic.
In reality, what you’re likely to see is a lot of sparking and electrical arcing, depending on the shape and type of metal you put inside.
As a general rule, point things like forks will create arcing sparks. Interesting to look at, in other words, but if those sparks come into contact with flammable material you risk starting a fire.
A bigger risk comes from using smoother, rounder items that reflect the energy back into the walls of the appliance.
Why Does This Happen?
Given that your microwave itself is made of metal, you might be wondering why this is such a big deal.
Microwaves work by creating an electromagnetic field. This field then generates radiating microwaves.
The microwaves collide with the water molecules in your food, causing them to move around. This movement creates heat, and your food is warmed through as a result.
The problem with metal?
It’s much denser, and more rigidly solid than the food you’re heating.
When you reheat food, the water inside it converts to steam and the energy has somewhere to go.
That’s not the case with metal. The energy has nowhere to go, no way of being released, and the energy is simply reflected back into the walls of the microwave.
That energy is then redirected back towards the metal object, which then reflects it back to the microwave and…you get the picture.
Eventually the microwave itself becomes completely overheated, and likely permanently damaged as a result.
(*Interestingly, some products like Hot Pockets take advantage of this by including a very thin layer of flat foil. This attracts an intense amount of heat, speeding up the cooking process without risking any damage to the appliance.)
What About Aluminum Foil?
I’ve covered the subject of using aluminum foil in the microwave elsewhere on Viva Flavor.
This is one of those exceptions to the rule, but it’s important to use this material sparingly and in the correct way.
That means using only new, perfectly smooth aluminum foil. You should be able to smooth it out over the section of the food you want to protect, so that there are no sharp ridges.
Never cover the food entirely in aluminum foil! The food won’t heat up at all as the microwaves can’t penetrate the foil. You’ll just end up bouncing all that energy back into the walls.
If there are any sharp edges to the foil, the electrical current can use it to create a spark, igniting anything flammable nearby. That’s why it needs to be new foil, applied smoothly to a small area of the food.
The short answer? Although it’s technically safe to use aluminum foil in the microwave, I would recommend avoiding it unless you’ve really no other alternative.
Are Metal Containers, Plates, Mugs & Cups Microwave Safe?
The best rule of thumb when it comes to putting any container in the microwave is this:
Only put it in the microwave if it’s advertised as microwave-safe.
Round metal items are going to reflect microwaves back into the walls of the appliance.
Decorative metal elements risk creating sparks, which could themselves start a fire.
Container manufacturers don’t mess around when it comes to product safety. If they think it’s a good idea to put the product in a microwave, they’ll make that very clear on the product description.
For example, microwave-safe Tupperware is clearly marked as such. Certain Tervis mugs are also microwave-safe, as long as you heat them this way for a very limited amount of time.
My own take on all this is pretty simple.
If you want to use your microwave for a lot of different reheating jobs, invest in the right tools for the job.
Containers are pretty inexpensive things to add to your kitchenware collection.
Replacement microwaves? Not so much…
So How Does A Metal Rack In A Microwave Work?
If putting metal in a microwave is a no-no, how can certain microwaves have metal racks?
The simple answer is that the rack is carefully tuned for the oven it’s supplied with. It’s dense enough to absorb heat without bouncing it back into the walls of the microwave.
You’ll notice as well that these racks are carefully mounted to the sides of the appliance with plastic or rubber clips. This prevents arcs of energy flashing in the microwave.
If you’re using a metal rack in your microwave, make doubly sure you don’t add anything metal to the appliance.
Even if it doesn’t come into contact with the rack, if it’s close enough you might see sparks fly!
As with all things to do with kitchen appliances, your best bet is to always refer to the owner’s manual.
Now you know why you can’t – for the most part – put metal in a microwave!
If you are going to use aluminum foil, be very sparing with it.
Otherwise, make sure you refer to the owner’s manual before using any appliance in ways you’re unsure about.
Mark’s a lifelong food fanatic and spent ten years working as an entertainment journalist. He now combines his love of food, drink and writing as the founder and editor of Viva Flavor. Read more