On June 29th, 2017 we learned about

Frozen waffles and a flaming, melted plate teach us about toasters and microwaves

“Mommy! Daddy! There’s a fire!”

This is not how you want to start your day, but it wasn’t completely surprising when we heard it at 6:30 Tuesday morning. After all, just the night before we’d told our eight-year-old that if she wanted to cook a frozen waffle she was allowed to use the toaster. We told her where the waffles were and how long to cook them for. We reminded her to be careful with the hot toaster, and to get help if there was any sign of a fire. She was, therefore, just doing exactly as we told her.

The problem was what we hadn’t told her: Plastic plates can go in the microwave, but not the toaster.

The fire was contained, although at the coast of one Ikea plastic plate (dishwasher, microwave, but not toaster safe!) and the toaster itself. The plate’s flames were actually rather persistent, requiring some effort to be fully extinguished. Our daughter wasn’t actually too phased by all this— she just wanted to know why a plate could be used in one metal cooking box, but not the other?

Warmed from the inside by water

A plastic plate can survive your microwave because microwave ovens heat water, not plates. A magnetron behind the oven’s buttons generates microwaves that are then piped into the main compartment where your food is. Those microwaves are a lot like high-energy radio waves, but with shorter wavelengths. The wavelength is important, because it can affect what the microwaves will be blocked by, interact with, etc. In this case, the wavelengths are just below five inches long, and they’re kept contained by the metal shielding along the sides of the oven, bouncing around like light bouncing off a mirror.

When the microwaves hit your food, they primarily interact with water molecules your food contains. It will cause those molecules to vibrate in place, and some of that activity creates friction between molecules. As with other cases of friction in the world, that friction converts the initial source of energy into heat, and warming your food. In a way, the water molecules are doing the actual heating here, and so drier food (or plates) don’t get warmed very much, while water-filled items like pie filling can become scaldingly hot very quickly.

Cooking with heated air and coils

A toaster oven heats food through convection. Electric coils are pumped full of energy so that some of that energy can start heating the air in the oven. Warmer air rises, triggering a bit of circulation to eventually give you a very hot pocket of air in your metal box. All ovens heat food through convection heating, but some ovens (and even our replacement toaster) now bill themselves as “Convection Ovens.” These ovens include a fan to circulate hot air faster, making the warming process a bit faster, although sometimes drying out food more in the process.

Just as hot coils give away heat to the cooler air in the oven, hot air transfers heat into your cool food. Under ideal conditions, the temperatures would eventually equalize, and no heat would need to be transferred further. However, since the heat is being transferred from the outside in, thicker pieces of food need to wait for the outer layers to warm up before the insides warm up. The upside is that this “outside-in” heating can give you crunchy crusts and crispy skins in a way that a microwave will never do.

What pans go where?

Circling back to my daughter’s confusion over plates, plastic can go in a microwave, but only a few plastics can survive an oven. Because the plate doesn’t get heated as much as the food in a microwave, its temperature isn’t likely get hot enough to melt or ignite.  Metal is a big problem though, because it can reflect microwaves back at your magnetron and damage it, or build up an imbalanced electrical charge that results in dramatic sparking and arcing. That can damage the shielding of your microwave, allowing it to “leak” microwaves when you use it. Those microwaves can then harm body parts that have a hard time shedding heat, like your eyeballs.

In a convection oven, metal’s just fine. It can easily conduct heat from the air to your food, presumably without hitting its melting point. For aluminium, that’s 660° Fahrenheit, and 1200° Fahrenheit for a cast iron pan, both temperatures that would probably make a mess of your meal. Since common plastics like ABS can get squishy at 88° Fahrenheit, and ignite at 416° Fahrenheit, it’s makes sense that my daughter’s plate couldn’t handle the toaster. From here on out, hopefully the toaster will only have to deal with waffles.

Source: Microwave ovens by Chris Woodford, Explain That Stuff

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