There’s a lot of information out there about what microwaves do and don’t do. We all know we shouldn’t put aluminum foil in a microwave oven, but what about glass? Many microwave-safe containers are made of glass, after all, so how safe is it?
You can microwave glass as long as it’s not cold. Microwaves pass through glass without heating it. It doesn’t have any negative effect on your microwave oven or your food.
If you’re not sure about how you should use glass in your microwave, this article will clear up any doubts you have about it. Glass is an excellent material for microwave-safe containers, although there are a few caveats.
Why Microwaving Glass is Safe
As most people know, you aren’t supposed to microwave metals, but other materials are fine. Glass can be put in the microwave, just as paper and plastic. Why do microwaves react differently to metal?
Let’s get a couple of science facts out of the way first. Microwaves are in the electromagnetic spectrum, which is where visible light happens. However, microwaves are much lower on the spectrum, which means the frequency of the waves is larger.
Metal is extremely good at reflecting microwaves, making them bounce around against the oven’s walls. These bouncing microwaves can end up damaging the inside of the oven. Some brands put a metal coating on the inside of packages to heat them more evenly, but you should never put aluminum foil or any kind of metal in the microwave.
When it comes to glass, microwaves simply pass through it without heating it. The same happens with plastic and paper.
However, microwaves are absorbed by the water content of food. They make water molecules oscillate back and forth, which causes your food to heat up.
Even though microwaves can’t directly heat glass, glass containers will still heat up because they’re in contact with food, which is affected by microwaves.
Still, glass is an ideal material for putting food in microwaves. It’s no coincidence that you see so many microwave-safe containers made of glass.
There’s only one situation where you definitely shouldn’t microwave glass: when it’s cold.
Can You Microwave Cold Glass?
You shouldn’t microwave cold glass. The thermal shock may cause the glass to crack and break. Solids contract slightly when they’re cooled and expand when heated up. This is also true for glass, even though it’s unnoticeable.
When a cool glass container is heated up in the microwave, it’ll heat unevenly across its surface. This means different parts of the glass will heat up at different speeds.
Here’s where something called thermal shock happens. Some parts of the glass will expand sooner than others, and since glass isn’t elastic, these differences will cause cracks to appear.
That’s the reason you shouldn’t take a glass container straight out of the fridge or freezer and put it in the microwave. The microwaves won’t heat it, but the food inside it will. And as it heats up, the strain caused by thermal shock will likely break the container.
If you want to put a cold glass container in the microwave, wait until the glass has reached ambient temperature before doing so.
Some glass containers are specially made for microwave ovens. These containers may be able to withstand temperature changes without issue. Read the specifications of glass containers to know if it’s safe to heat them while they’re still cold.
How Long Can You Put Glass in the Microwave?
There isn’t a limit to how long you can microwave glass. Microwaves can’t produce temperatures high enough to melt glass. However, microwave ovens can overheat if you run them for too long.
Since microwaves pass through glass, the only thing heating a glass is the food it’s in contact with.
Don’t worry: no microwave will get any food or liquid to a high enough temperature to melt or damage glass in any way. The melting point of glass is 2600 to 2900°F (1426 to 1593 °C). Meanwhile, a microwave oven will rarely heat anything beyond 212°F (100°C).
If you leave something in the microwave for too long, your food and your microwave oven may take the hit, but any microwave-safe container won’t suffer any damage.
Microwaving food without stirring it for more than 5 minutes may cause it to be cooked unevenly. Microwave ovens have their own limits to how long they can run continuously, and they vary depending on the manufacturer.
Some cheap microwaves will stop working after around 10 minutes of continuous use, then boot up again after 5 minutes of rest. Higher-quality microwaves may be able to run without stopping for longer.