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Can You Microwave Glass Jars? (A Potential Hazard!)

We’ve all been through this. It’s late at night. You’re hungry. And, you’ve got some leftover food in a glass bowl or some soup in a mason jar and you want to heat it.

Rather than put the food into a known microwave-safe container, you’re now wondering, can you microwave glass bowls?

The answer is, it depends on the type of glass used. Generally, ordinary glass containers shouldn’t be placed in the microwave. Microwaving the wrong types of glass containers can be dangerous and can even cause an explosion. It’s hard to tell which glass containers are safe. Fortunately, there are a series of ways to find out.

If you’re curious, let’s dive into the details.

Is it safe to microwave glass?

You can find a microwave oven in almost every single home in America. It’s a powerful machine that combines electricity with waves (specifical microwaves) and almost instantly cooks your food from the inside out.

But there are certain things you just can’t put in a microwave. And the result of attempting to microwave anything reactive can become harmful.

Now, glass is a confusing option. There are different types of glass containers, some are microwave-safe while others are not.
In general, most modern glass containers that are microwave-safe are non-reactive and will have micro-waves passing through this material. These materials won’t conduct heat directly from the waves.

Now if you do happen to microwave the wrong kind of glass (a nonmicrowave-safe one), there’s a likely chance that it will heat up quite rapidly.

When it becomes too hot, then it could shatter or explode. Shattering is one thing, but an explosion of glass isn’t going to end well for anyone close by.

Even with the microwave door closed, the explosion could cause it to burst open.

There’s also the possibility that it will melt However, glass melts at approximately 1400 °C to 1600 °C. So, this is only if you’ve left it on for a very long time.

This video shows how a microwave can be used to melt glass.

And let’s not forget, if the food coming out of the microwave is piping hot, this could attribute to the glass breaking as well, which can cause serious burn injuries.

Why are some glass containers microwave safe and others not?

When a glass container is not safe for microwaves, it boils down the how it was made. These containers have tiny little air bubbles that could build up heat and pressure causing an explosion.
Microwave-safe glass is created in a process that gets rid of as many of those tiny little air bubbles as possible.

That’s what makes these two so different.

How to tell if a glass container or jar is microwave-safe

So, let’s talk about the many different ways you can figure out if a glass container is microwave-safe.

  • The microwave-safe sign
    This is usually a symbol that is found at the bottom of your glass container. When looking, you may either find wavey or squiggly lines which indicate the waves produced by a microwave.
    If you see that, then chances are, it’s safe. In many cases, it will also plainly say the words “Microwave-safe” somewhere on the container.
    If you can’t find this, try looking for it on the original box or manual.
    If that fails, contact the manufacturer.
  • The microwave-safe test
    In the case that it’s a really old container or you can’t get a hold of the manufacture, there’s a test you can perform.
    Without putting anything in the container, place it in the microwave for 1 to 2 minutes. When done, you’ll need to feel the container with your hand to see if it had conducted any heat. If it’s hot, then it’s not microwave-safe, but if it’s not, then it is microwave-safe.

Safety precautions when you heat food in mason jars

When using the microwave, it’s well-known that there are important general safety rules. But when you are microwaving glass, there are a few extra things you have to know.

  • The jar could be made with metal parts
    Almost everyone knows that you shouldn’t heat metal, and even if you didn’t know, your first experience with heating metal will scar you for life.
    However, some glass containers like mason jars, for example, can have hidden metal components laced all around the rim of the opening. You don’t want to microwave these kinds of jars because they create sparks.
    Again, you can check with the above subtopic to find out how to identify if a container is microwave-safe or not.
  • Never seal your glass containers when putting them in the microwave
    This is especially an important concept to understand and not knowing can cause major injury.
    When microwaving food in any kind of container, the food will react violently. This helps produce heat and even pressure.
    If you were to place food or water into a glass container, and seal it with a lid, the extreme heat and pressure produced during the heating process would built up and cause the glass to explode.
    This is because glass doesn’t flex and that rigid structure simply causes the glass to either shatter or explode due to the pressure pushing outward.
    This is exactly what happened in the video below. Notice how the person in the video attempts to microwave a sealed glass bottle of beer.
  • Always use protective gloves when handling containers coming out of a microwave
    Even though the glass container you’re using doesn’t conduct from the microwaves, the thermal heat from the food will make the container hot to the touch.
    It’s recommended that you wear protective gear. This can be anything from heat-proof gloves, hot pads, or even a regular towel in place between your hands and the container.
    As an extra piece of advice, invest in hot plate coasters so that the heat won’t damage your tabletops like it did mine.
  • Don’t microwave frozen food immediately in the microwave
    When microwaving food that has been frozen, the food will expand almost immediately and cause immense pressure.
    In rare causes, even if you didn’t have the lid on, the release in pressure can be so great that it can cause the glass to break anyway.
    When you’re faced with frozen food in a glass container, it needs to be defrosted.
    Some microwaves have a defrost button and what this does is a presetting that turns the microwave on and off in short time intervals allowing the food time to melt slowly and not too quickly like the regular process.

Rules for microwaving food in glass containers

There’s several ways you can microwave food in glass containers.

Microwaving glass v2 - Family Guide Central
Microwaving in glass containers
  • Check to see if the container is microwavable
    Check around the container, original box, or manual and look for a microwave-safe symbol (usually designated in squiggly lines, or having the image of a microwave). You can also look for any label you may find that could say “microwave-safe”.
  • Thaw or defrost, if frozen
    If there’s frozen food inside, make sure you thaw it in a water bath or set it under the microwave’s defrost setting. You can also manually microwave the frozen food in short intervals, however, I find it much easier just to use the defrost setting on your microwave because this is exactly what it does.
  • Remove the lid
    It doesn’t matter what type of container it really is. Don’t ever seal your container while microwaving it.
    The process of microwaving food or water involves building up a lot of heat and pressure. The unfortunate thing about glass is that it doesn’t bend or flex.
  • Inspect the container for metal components
    Some mason jars have a ring of metal around the opening. This immediately signifies that it’s not microwave safe and you should never microwave this glass container.
  • Stir the food in between microwave intervals
    It’s always a safe bet to stir your food in between microwaving intervals. This helps distribute the heat and helps even out the cooking processes. Microwave ovens don’t evenly cook food. I’ve written an article here about how engineers, who invented the microwave, had to develop an interesting work-around to help microwaves cook more evenly. It’s still being used today.

The bottom line

Glass containers and glass jars are incredibly rigid, meaning that they can’t simply flex or bent. Glass that is not microwave safe has these tiny little air pockets that will eventually expand while being microwaved and this can cause it to shatter.
However, there are several other reasons you should watch out for even if your glass container is microwavable such as not using the lid, avoid microwaving frozen food, etc.

It’s important to follow this guide because it can be very dangerous when not using a microwave correctly or placing the wrong things in the microwave.

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