March 31

Are Mason Jars Microwave-Safe? What about the Lids?

Written by: Michael

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You'll get a lot of websites telling you conflicting information about what's microwave safe and what's not.

So, how do you really know?

Well, we've done it for you. We've compiled our comprehensive research right here so that you don't have to look in a dozen places to find out what's safe to put in the microwave.

Whether you're using your mason jars to make a sourdough starter or to pickle red onion and cucumber, read on to understand when and how these jars can go in the microwave.

Can you Microwave Mason Jars?

tomato-sauce

In short, yes. At least for the newer models.

But...the metal lids can NEVER be safely put into the microwave!


Understand the Material

Mason jars are glass, which is not always microwave-safe. Some glass will shatter when exposed to high temperatures.

However, the manufacturers of mason jars are aware of the uses of which these nifty containers are often put – including that of storing, and then reheating leftovers.

There are two types of glass: tempered and annealed.

Most mason jars are made of tempered glass, which carries brand names like Pyrex. These are designed through processes that leave it with a less dense middle (in the walls of the glass) and allow it to withstand expansion at higher temperatures.

No matter the type of glass, never put it in the microwave directly after taking it out of the freezer!


Look for a label

Most mason jars should have a label that specifies that it's microwave-safe. If you see this, no worries!

(If you don't, better safe than sorry – don't microwave it.)


What about Mason Jar Lids?

mason-jars

Mason jar lids, on the other hand, are metal. Metal cannot go in the microwave!


Why not?

When metal goes in the microwave, the waves can't go through it. Instead, they bounce off it and get concentrated unevenly all around the oven, causing ionization (which sounds like loud snaps) and sparks that look like lightning.

These waves jump around and unevenly heat up the interior of the machine until it may get hot enough to start a fire.

It's not a good idea.


Are there Microwavable Mason Jar Lids?

There are not mason jar lids that are designated as microwave-safe. But even if there were, you wouldn't want them.


Why microwaving a closed mason jar is a bad idea

Here's what happens when you microwave your food: it heats up. More specifically, the air around it heats up. And this air expands.

In normal conditions, because the microwave isn't airtight, the expanded, heated air can escape.

But if you seal your mason jar while you're microwaving it, the air can't escape. Pressure will build until... boom!

You don't want that.


When is Microwaving Mason Jars Not  Okay?

mason-jars

Mason jars are almost all microwave-safe. Almost.


Don't microwave one that doesn't have the "microwave-safe" label

If there's no microwave-safe symbol, don't microwave it. It's just not worth the risk of the mess (and possible damage to your appliance or kitchen) if it explodes.


Don't use a mason jar to cook food

Furthermore, only use mason jars for reheating food, not cooking it. Mason jars don't hold food in a way in which the microwave can evenly heat it, meaning that some parts may not get cooked.

The problem? It will stay cold in patches, which you won't like. Also, bacteria can survive in the uncooked sections, and could get you sick.


Are all Mason Jar Companies Microwave-Safe?

mason-jars

All major manufacturers make mason jars that are microwave-safe.

However, if you're getting it from an off-brand company on Amazon that charges five cents a jar...then maybe not.

Look for the microwave-safe symbol to tell. All modern mason jars should have this.


"Microwave-Safe" Labels to Look For

There are many "microwave-safe" symbols, but they all mean the same thing. Look out for:

  • A microwave
  • Three horizontal wavy lines


These symbols mean that you're good to go!


Tips for Microwaving Mason Jars

galanz-microwave-oven

A mason jar can be great for heating up food – especially on low power settings, because the walls of the jar will contain heat and allow it to distribute throughout the food.

However, the glass itself may get hot! (Especially if it's colored glass)

So don't use your bare hands to take it out of the microwave. Use an oven mitt.


Are Mason Jars Oven-Safe?

oven

If they are made of tempered glass, they are designed to withstand heat. However, not all mason jars are made this way.

Furthermore, don't subject any glass (other than oven-safe glass designed for it) to temperatures above 350°F, or you'll risk it shattering.

This page has some good pointers for cooking in a mason jar, if you are looking to do this.


Conclusion

Are mason jars microwave-safe? Yes, most of the time – provided that you take off the lid.

However, if they don't explicitly have a microwave-safe symbol, don't try it. Better be safe with cold food than sorry with a burnt-down house.

Be smart.

Michael

Founder of Robust Kitchen 

P.S.

If you're curious about microwaving other things, check out my articles on when Ziploc bags are microwave-safe, and when you can microwave styrofoam.


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About the author

Michael spends his days eating, drinking and studying the fascinating world of food. He received his Bachelors Degree in Food Science and Technology at the University of California, Davis and spent much of his time at the school brewery. While school proved to be an invaluable experience, his true passion lies in exposing the hidden crannies of food for the cooking laymen.

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