Can You Microwave Mason Jars?

Mason jars are quite useful, as a permanent and sturdy container. You can fill them with anything and they’re good for display. But what if you want to heat up food while it’s in there…? Are mason jars microwave-safe?

The answer is that it depends, but usually no, most Mason jars are not safe for microwaves. A lot of folks might assume that it’s safe to heat up Mason jars, but it’s usually a bad idea.

This has to do with the way they’re constructed, and there are exceptions. Keep reading for a more in-depth explanation.

What Is A Mason Jar?

A Mason jar is made of relatively thick glass, though exactly how thick depends. These are named after their inventor, John Landis Mason, who first made them in 1858.

They were made to be canning jars, useful for storing not only food and beverages but also office supplies and loose household items. Quite versatile, Mason jars can be used as a substitute for coffee mugs or piggy banks.

A big part of the appeal is the fact that they can travel easily. They’re made to be sturdy and can handle a car ride just fine. But while the glass is thick and durable, it’s usually not tempered.

And if the glass is not tempered, that means it’s not capable of dealing with extreme heat. As a result, most Mason jars will most likely shatter if they’re left in an oven or microwave. 

What Is Tempered Glass? And Why Does It Matter?

Tempered glass, sometimes referred to as toughened glass, has gone through a process of controlled thermal or chemical treatments. This makes it stronger than average glass and conditioned to take more extreme conditions.

The tempering process means that the outer surface is in a state of compression, while the interior contains tension. It is quite intelligently made, as not only is it more durable, but in the event that it does break, it’s still safer than average glass.

That’s because tempered glass is made in such a way that it shatters into small particles, rather than breaking off into large sharp chunks. 

Obviously broken glass still has risks regardless, but tempered glass is definitely safer than the alternative. Less big pieces and less sharp, jagged edges is a good thing.

All of this together means that it’s much better to go with tempered glass than any other kind, if you intend to stick it in the microwave. Some Mason jars are tempered, but the standard traditional one is not.

Different Types Of Mason Jars

Even though Mason jars are named after a specific man, the name itself is not a brand name. As such there are many different brands of Mason jars out there, though the most famous one is likely Ball.

Ball Mason jars are a common sight at Walmart as well as Target or Home Depot. The glass used for Ball’s canning jars is not tempered, and so they are not appropriate for microwaves or ovens of any sort.

Some other common brands are Kerr and Bormilo Rocco. These, too, are not traditionally tempered and so it’s a pretty bad idea to put your standard mason jar in the microwave.

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Having said that, it does seem as though one can find generic tempered glass Mason jars if they do a simple web search for them.

How Can You Tell If A Glass Jar Is Microwave-Safe?

The simplest and easiest solution here is also the best one, in most circumstances. See, manufacturers already have you covered. 

If your jar is microwavable, odds are good that it will have “microwave safe” printed on a label on the bottom. If you don’t see this label, it’s probably safe to assume that it can’t withstand extreme temperatures.

Something to note is that even if you do have a Mason jar with tempered glass, their lids are almost never going to be safe. 

They’re usually made of metal and aluminum, which you should certainly be keeping away from your microwaves! Occasionally, they will also have aluminum lining across the glass lips.

Can You Test Out A Jar To See If It’s Tempered?

Yes, actually. It’s actually possible to see for yourself if glass is tempered, since it does have some visual distinctions from typical glass. 

If you happen to have polarized sunglasses, looking through that lens at the glass might reveal some curious patterns. 

Sometimes it looks like a grid while others see a very colorful array of diamonds. It will certainly look different than normal glass, at the very least! 

For thicker bowls and glassware, you could theoretically try out putting it by itself in the microwave and letting it heat for a minute or two. You can judge after that if any damage has been done.

But for something more thin, it’s best not to chance something like that, and that includes many Mason jars.

Other Labels On The Jar To Be Aware Of

As well as the “microwave safe” label, there’s other labels you might see. It’s actually possible that you’ll see the opposite such label, one indicating that the jar is not microwaveable. Of course you can immediately know what that means.

More often than not though, these labels won’t be there. It’s much more likely that there simply won’t be any labels at all. In that case, it’s best to just use these Mason jars for storage and canning and definitely not for cooking. 

Advice For Use Of Mason Jars

You’ll want to avoid extreme temperatures. That means avoiding taking it from hot water to a wet or cold counter. And when you take them into the fridge, you’ll want to put them on a towel or perhaps a heat-proof pad, to let it cool down first.

Incidentally if you’re looking to make use of a Mason jar as a substitute coffee mug or thermos for tea, you’ll want to get a cozy for it. After all, the glass will be nearly as hot as the beverage itself.

Mason jars are freezable, but you do want to be careful with this. The glass does tend to get a little bit more susceptible to cracking when the temperatures are frigid. That being the case, some added protection isn’t a bad idea.

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Wrap the jars in freezer-safe fabric or else keep something between the jar and whatever else you have in the freezer. That should help keep the Mason jars from clanking against their bunkmates!

How Long Can You Microwave A Mason Jar?

Okay, let’s say that now you’re aware of what a microwave-safe Mason jar is, and you have confirmed that you own one. So how long should a Mason jar be in there then?

Well, that depends on your needs of course. One big reason that people want to microwave these things is to sterilize them. The proper amount of time for something like that depends on the size of the jar you’re using.

As a rule of thumb, here’s times you can shoot for:

  • Small-sized Mason jars (capacity of less than 1 liter) – 1-2 minutes
  • Medium-sized Mason jars (capacity of around 1-4 liters) – 3-4 minutes
  • Large-sized Mason jars (over 4 liters) – 5 minutes

Now if you’re just looking to reheat a quick-heat item, that should only require around 1 to 2 minutes in the microwave with a Mason jar. Keep in mind, that glass heats up pretty quickly.

Those are the microwave times, but what else goes into properly sterilizing a jar?

How To Sterilize A Mason Jar In The Microwave

First things first, you’ll want to wash the jar out. Ideally using some baking soda here would be a good idea. Otherwise, warm soapy water will do for getting rid of any debris or dirt. 

From there, rinse the jar out thoroughly, until there’s no more soup or residue. Then, fill it up with water and place it in the microwave. Set it to heat for however long is appropriate, going by the times listed above.

We must reiterate though, make doubly sure that your jar is microwave-safe before you try any of this!

To summarize:

  • Wash the jar out with baking soda or soapy water
  • Rinse it thoroughly until there’s no soap or baking soda residue
  • Fill the jar with water
  • Place in the microwave, heat for 1-2 minutes if the jar is small, 3-4 minutes for a medium-sized jar and 5 minutes for a larger one.

In Conclusion…

By this point you should definitely get the picture. Some Mason jars are microwave-safe but it would seem that the vast majority of them are not. 

There’s a few ways to check and see if you’ve got a safe one, but the best way will always be to look at the bottom of the glass for a label. 

If there isn’t a label saying so, chances are good that the jar is not microwave safe and could shatter at extreme temperatures. And that would be a huge safety hazard. So make sure to check for that label and stay safe!