Mason jars are a must-have in every kitchen. They’re perfect for everything from canning to storing leftovers!
But can Mason jars go in the microwave? The answer is yes — with a few caveats. Newer Mason jars are almost always made from tempered glass that’s safe for the microwave. If the Mason jar is new, or if it has a microwave-safe symbol at the bottom, the jar is safe to handle the heat. We do not recommend microwaving older Mason jars.
Why is that the case? Keep reading to learn how to tell if a Mason jar is microwave-safe and how to use them in the microwave safely and effectively!
Microwave-Safe Glass Vs Regular Glass
Here’s a little physics lesson that you may remember from high school: glass expands and contracts when the temperature around it fluctuates.
That means when the glass is cooled or frozen, it physically shrinks. When it is heated, it will become larger.
When the cooling or heating process is too drastic or too sudden, this rapid expansion or contraction of the glass can actually cause it to crack.
Now, of course, engineers have figured out how to fix this issue. That’s why we now have such a thing called tempered glass (also known as safety glass).
Tempered glass or safety glass is manufactured through a process of extreme heating and rapid cooling.
This process makes the glass much harder than standard glass, and as a result, tempered glass is less likely to break when the temperature around it fluctuates.
Tempered glass can handle the heat of up to 1,100°F, whereas regular glass will crack when it meets consistent heat above roughly 350°F.
Heat-safe glass that can be used in the microwave and the oven is made from tempered glass, so you don’t have to worry about the glass breaking in the oven or the microwave.
However, tempered glass can also wear down over time. Even if you know the glass is oven-safe, if you spot any cracks in the glass, it’s best not to use it in the oven or the microwave anymore.
Can You Microwave Mason Jars?
Yes, as long as your Mason jar is made from microwave-safe glass.
Drastic changes in temperature can also damage any type of glass, including heat-safe glass. If you are heating up frozen food, for example, it’s best not to use a Mason jar.
When you heat frozen food, cold liquid can leak from the frozen food, and when it is heated, the massive change in the temperature can affect the glass’ stability, causing it to crack.
That’s why it’s best not to thaw frozen food in a Mason jar using a microwave.
In addition, the non-glass parts of the Mason jar are also not microwave-safe. Mason jars often come with a metal lid or a ring around the rim of the glass to help tighten the lid.
These parts will need to be removed if you would like to put your Mason jar in the microwave.
How To Tell If Your Mason Jar Is Microwave-Safe
It may not always be clear whether a Mason jar is microwave-safe. The distinction between heat-safe glass and regular glass is very subtle, and only experts will be able to tell the difference just by looking at the glass.
If you cannot tell whether your Mason jar is microwave-safe, here are a few signs that you can look for.
The first thing you need to look for is a microwave-safe symbol on the jar. The symbol may look a bit different depending on the manufacturer, but it usually has a few wavy lines (indicating the microwaves).
This symbol is often etched at the bottom of the Mason jar or printed inside the lid of the jar.
If you have bought the jar from a manufacturer, you can also find the product information on the manufacturer’s website, which will usually indicate whether the glass is microwave-safe.
If the Mason jar was a part of a product’s packaging (for example, a spaghetti sauce jar or a candle jar), you could also find information on the product’s labels.
Some products will indicate that the jar is “Not For Microwave Use,” which will tell you that the glass is not heat-safe.
The Jar’s Age
Mason jars were patented by John Landis Mason in 1958, and the first editions of Mason jars were made from regular glass to package canned foods, so there was really no need for the jars to be heat-safe.
However, as the jars became more popular for handling everything from canned foods to drinks and cakes, and even candles, manufacturers started making them using heat-safe glass.
If you have a Mason jar that is newly created, it’s likely that the jar is made from microwave-safe glass.
In the United States, the majority of Mason jars in circulation are made by the Ball corporation.
If you spot a Ball logo on your jar, you can tell the age of the jar using this Ball jars infographic. If the jar has a new Ball logo, it’s most likely microwave-safe.
If all else fails, you can do a small heat test to see if the Mason jar glass is microwave-safe.
First, fill your Mason jar halfway through with water, and then microwave it using the highest heat setting for a minute. After microwaving, take out the Mason jar and check the glass.
The glass should be cool enough to touch without discomfort, but your water should be properly heated. If this is the case, you can safely use the Mason jar to microwave everything else.
If the glass is too hot, then it’s not safe to use in the microwave. If you try to microwave it, your food and drinks may not be properly heated, and the glass may even crack or break after a few trips through the microwave.
How Long Can You Microwave a Mason Jar?
Even if you know you can microwave a Mason jar, the glass should still be safely handled so that it won’t crack and break.
Microwaving a Mason jar for too long may cause the glass to overheat, causing the glass to expand dramatically, and crack when it cools down.
That’s why you shouldn’t microwave Mason jars for too long. Small Mason jars (30 fluid oz or smaller) can only handle up to 2 minutes of repeated heat. Medium-sized jars (30 – 100 fluid oz) can be microwaved for up to 5 minutes.
If you need to microwave something for longer, we suggest you do so in several bursts, and mix or stir in between. This ensures that the glass doesn’t get too hot from being microwaved for too long, and your food will get heated evenly.
Can You Freeze Food In Mason Jars?
Similar to heating, freezing is also a tricky point for Mason jars. The glass will react poorly when the temperature around it fluctuates drastically, whether it’s too hot or too cold.
That’s why if you don’t pay attention, freezing food in Mason jars can cause the jar to break.
The first thing you need to know is that glass will shrink when it’s in the freezer, but water will actually expand when it’s frozen. In fact, water can expand up to 9% when it is frozen.
Since glass is not flexible, if you freeze any liquid using Mason jars, the container will contract while the liquid expands. The result? The glass won’t be able to handle the pressure, and it will crack when it comes out of the freezer.
That means you should not freeze liquids in Mason jars, only solid food.
When freezing Mason jars, you shouldn’t fill the content up to the brim of the jar. Since the jar will contract in the freezer, filling it too full will actually cause the content to overflow when the jar shrinks.
You should also avoid closing the lid too tightly when you freeze things in Mason jars.
Since the jar will shrink, the pressure inside the jar will increase if the lid is completely sealed, making it very difficult to remove the lid once it’s out of the freezer.
Instead, freeze the Mason jar first for a couple of hours and then close the lid once the jar is cold to avoid these issues.
If you need to defrost or reheat the content of a Mason jar, you should never do it in the microwave.
The journey from frozen to hot is a massive change in temperature, and if it happens too quickly (i.e., in the microwave), the glass will crack easily — even if it’s heat-safe.
Instead, you should let the Mason jar thaw out at room temperature for a few hours so that it gradually reaches room temperature. Then, you can safely microwave it as normal.
If you are in a hurry, you can also transfer the content of the Mason jar to another microwave-safe container and use that container to microwave your food to reheat it.