Can I Microwave Glass?

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Whether it’s your holiday left overs or just last nights dinner, anytime you’re reheating food in the microwave, it’s paramount you choose a microwave-safe container. In today’s post, I’ll share with you when glass is microwave-safe and when it isn’t, answering a question we’ve all asked ourselves at one time or another, can I microwave glass?

If your glass cookware or bakeware has a label indicating it’s microwavable, then it’s safe to heat it using this method. For glass kitchen items without that label, the glass can shatter when heated up in the microwave.

If you’re not sure where to find the microwave-safe label on your glass kitchenware, I’ll tell you in this article. I’ll also discuss how long you should microwave glass, whether microwaving cold glass is safe, and if Mason jars and Pyrex are microwavable. 

Is Glass Microwavable? 

As you slide your glass dish filled with day-old pasta into the microwave and close the door, a thought passes through your mind right before you start reheating. Can glass safely go in the microwave?

That depends on whether it’s labeled as microwaveable.

On the backs or bottoms of most glass cookware and bakeware, you’ll see a label. It will either read “Not for Use in the Microwave” or “Microwave Safe.” 

If your glass cookware isn’t made for microwaving, then you’ll need to find another way to reheat that pasta. For the latter label, you can nuke your leftovers in the micro without worrying about the integrity of the glass cookware.

There will be instances in which the glass item in question has no label. Does this automatically mean the cookware shouldn’t go in the microwave?

More than likely, yes, it does. 

For instance, if your glass bakeware is decorated with foil or metallic trim, then you should never put it in the microwave. The metal finish can begin to spark, which will then break the glass. A fire could also start. 

Colored cookware typically can’t handle the micro either. The manufacturer likely used a dye or additive to give the glass its color. Those materials can leach out into the micro if heated.

How to Test Unlabeled Glass Items for Microwave Safety

Let’s say you have an unlabeled piece of cookware or bakeware that’s uncolored and has no metallic trim. Rather than guessing whether it can go in the micro, you can test it.

The test involves a microwave-safe cup, some water, the microwave, and the glassware in question. 

Turn your microwave settings up to the highest degree. Fill the cup with water to the top. Then place your empty glassware in the micro with the full cup. 

Run the microwave for up to a minute. 

The glass of water will heat up in that time, but you’re more concerned with the glassware. If it’s warm or hot, that’s a sign it can’t go in the microwave. 

If the glassware stays cool, then you can safely put it in the micro. 

What Happens If You Microwave Glass That Isn’t Microwave-Safe?

Do you really have to tinker with your microwave settings and put a glass of water in there just to ensure whether your unlabeled glass cookware is microwave-safe? 

Yes, because the effects of microwaving glass can be severe. 

Almost all glassware has air bubbles. The higher-quality the piece is, the fewer the air bubbles, but they’ll be there, nonetheless. 

Upon being microwaved, those air bubbles can expand. 

In short cooking increments, nothing might happen. If you run your microwave for several minutes at a clip though, the air bubbles can grow to such a degree that the glass shatters. 

This won’t cause an explosion in your microwave, per se, but your new glass baking tray will be broken right down the middle (or it will be in several pieces). 

Plus, the food in the tray will have leaked all over the place, leaving you with quite a mess to clean up!  

Can You Microwave Cold Glass?

For freshness, you put a meal in glass cookware in the refrigerator overnight. Now, it’s time to heat it up, so you make a beeline to your microwave. 

Not so fast! Microwaving cold glass can be very dangerous, even if the glass is labeled as microwave-safe. 

Since glass conducts heat so effectively, the sudden transition from cold (or even ice-cold if your glassware was frozen) to hot is going to strain the glass. 

The result, as I’m sure you could guess, is cracked cookware. 

That’s the best-case scenario, by the way. In some instances, the glass cookware can even explode. This is especially likely to occur with synthetic glass. 

To spare your glasware, allow it to thaw to room temperature after coming out of the fridge or freezer. If you don’t see any fogging or condensation on the glass, then it should be fine to go into the microwave at that point. 

How Long Can You Put Glass in the Microwave? 

Your microwave has settings for a variety of food, everything from popcorn to potatoes. Yet glassware isn’t an appropriate choice for all your everyday microwaving. 

To maintain the condition your glassware, at most, microwave it for three minutes at a time. 

You can divide those 180 seconds however you want to, such as in 30-second bursts or 60-second intervals.  

If you need to microwave your food for longer than three minutes, I recommend using cookware other than glass. Now might also be a good time to turn on the oven if your food has a long cook time. 

Can You Microwave Glass Pyrex?

When it comes to glass cookware, Pyrex is one of the best-known brands. Pyrex was created in 1915. It’s made of borosilicate glass with a low rate of thermal expansion. 

Borosilicate glass can safely withstand temperatures up to 297 degrees Fahrenheit. Pyrex though is formulated for temps as high as 425 degrees. 

The average microwave temperature varies depending on its wattage and the selected power settings. That said, the standard temp range is between 210 and 300 degrees. Those temperatures are well under Pyrex’s temperature threshold, so you can safely microwave it.

You can even bake Pyrex glassware in the oven. If your Pyrex can spend time in the oven without shattering, then the microwave is small potatoes. 

Can You Microwave Glass Mason Jars?

Perhaps you’re a fan of Mason jars, which are used for everything from food storage to soap dispensers, small-item storage, salt and pepper shakers, pencil holders, and the list goes on and on. 

For those who use Mason jars for food storage and prep, it’s convenient to transfer the jar from your pantry right to the microwave. 

Still, you worry after reading everything to this point whether Mason jars are microwave-safe.

Some are and some aren’t. 

Mason jars, although they’re on every Pinterest board now, have been around for a long time. The original Mason jars were patented in the 1850s, which just goes to show how old they are.

I’m not saying you have a Mason jar in your kitchen from the 1850s, but if yours is old enough that it lacks a label, then it’s likely not able to go in the microwave. 

All Mason jars produced today are labeled as microwave-safe, which is good news for you.

That said, you should never ever microwave a Mason jar with its lid still on, and that’s for several reasons.

For starters, the lid is made of metal. As I alluded to earlier, metal and microwaves do not play nicely together. The lid will begin sparking in the microwave and can start a fire or even cause an explosion. 

The second reason not to microwave a Mason jar with its lid on is pressure. The Mason jar lid creates an airtight seal that allows pressure to accumulate in the jar as its contents heat up.

The pressure has nowhere to release since it’s not like the lid has holes or is vented in any way.

So what happens? The entire Mason jar can explode, shattering into tiny shards all over your microwave. Yes, this could happen even with jars that are labeled as microwave-safe.

When you open your microwave, you’d not only have to contend with the food mess sprayed everywhere but tiny shards of glass as well. Oh, and the Mason jar lid would be somewhere in there as well, burning hot to the touch.    

Thank you for sharing!

Catherine Cruzz

I first fell in love with all things kitchen when I was growing up and working alongside my father in Florida at our family's appliance service and installation company. Many years later, and thousands of miles away from family I was enjoying a wonderful experience at a culinary school in Pennsylvania. That’s when I realized that along with my passion for holidays and cooking, I was still just as interested in the appliances, kitchenware, and cookware that I grew up around.

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