How to Tell if Glass Is Microwave Safe (Guide + Safety Tips)

Microwave safety is one of those things you only ever hear about from someone else when they catch you going to heat up a brown paper bag, or maybe it’s something you consider right before you heat up your leftovers. 

You may be so spoiled by the commonness and convenience of the microwave that you may no longer even register its potential hazards.

Be forewarned that not all containers are microwave-safe, and even those that you thought were safe may not be at all.  Take glass, for example.

It’s widely assumed that glass can safely be put in the microwave, but it’s not that cut-and-dry.

It is easy to assume all glass is safe because the rotating plate in the microwave is made of glass. 

Generally speaking, you can put glass in the microwave. However, check the marking on the bottom of your glass dish to make sure that the dish is microwave-safe. Unfortunately, there are still glass dishes on the market that can shatter in the microwave.

Can You Put Glass In The Microwave?

It’s widely assumed that glass can safely be put in the microwave, but it’s not that cut-and-dry.

The truth is that it really depends on the glass. If it is regular glass, you risk the glass shattering. It needs to be microwave-safe glass. 

Luckily, whether or not the container can be microwaved is usually indicated on the object in question.

Are All Glass Cups Microwave-Safe?

Glass cups are not 100% microwave-safe at any given time, so they should always be verified for safety. You may need to individually inspect glass cups for a microwave-safety label. 

glass cup safe in microwave

Can You Put Cold Glass In The Microwave?

Regardless of its temperature, glass needs to be examined as to whether or not can be microwaved at all.  Cold glass may contain condensation. 

It’s recommended that this condensation be wiped away prior to heating in the microwave, even if the container is microwave-safe.

Can You Microwave Glass Jars?

In general, it’s safe to assume that you cannot microwave a glass jar, such as a mason jar, unless it specifically states that it is microwave-safe.

This indication usually appears in the form of a “safe for the microwave” logo on the bottom of the product.

Using Pyrex Glass In The Microwave

Pyrex, for example, is a popular type of glassware that can specifically be used for high heat situations related to cooking, baking, and reheating whether in the oven or the microwave. 

Interestingly enough, even though Pyrex is safe to be heated, it’s not recommended for processing through the dishwasher.

It is permitted, yes, but discouraged, as the dishwasher can eat away at the intently strong material.

Can You Put Frozen Glass In The Microwave?

A glass that has been previously frozen can be especially hazardous when thrown right in the microwave. 

Let previously frozen glass containers reach room temperature before putting them into the microwave.  Also, be sure to remove any condensation.

Is Borosilicate Glass Safe For The Microwave?

Borosilicate glass is a thin, durable glass often used for cookware as well as measuring cups.  

Since it is so resourceful when cooking, it is typically considered safe for the microwave.

Can You Put A Glass Candle In The Microwave?

You should never put a glass candle in the microwave.  Because it’s not a presumed action for the candle, the glass itself is likely not microwave-safe.  

Also, the reaction of the intense heat with the make-up of the wax has been known to have explosive results.

Can You Heat A Water Glass In The Microwave?

If the glass is made of an extremely durable type of glass, such as borosilicate glass, then the water glass can be heated in the microwave.

However, water itself can be hazardous when microwaved, which is why you should never boil water in there.

It can explode and cause serious injury, so heating water in the microwave is strongly discouraged.

Microwaving Glass Tupperware

Tupperware is mostly reserved for storing food so, even though it’s glass, you should never assume that it can safely be microwaved. 

Can You Put Glass In The Microwave For Ten Minutes?

If a glass is considered microwave-safe, then it is okay to be put into the microwave for the amount of time required to heat the food within the glass container.

If it is not microwave-safe, ten minutes in the microwave will likely cause the container to shatter.

How Can You Tell If Glass Is Microwave-Safe?

If glass is microwave-safe, the item will suggest such on it, usually on the label on the bottom or on the glass’ original packaging.

There is a common logo used to indicate microwave safety.

It’s a tiny image of an actual microwave with wavy lines. If this image appears on the container, it’s safe to assume that the product is microwave-safe. 

Alternatively, some containers will simply have some variation of verbiage advising against use in the microwave, such as “do not microwave” or “not intended for microwave use”.

How To Check If Glass Is Safe In The Microwave

You should never automatically assume that glass is safe in the microwave or to be warmed up at all for that matter.

If it is intended to be reheated, a label or special sign on the bottom should indicate such. If no label is clearly provided on the container itself, it helps to know that you can test it:

  • Set the microwave’s power setting on high.
  • Fill a confirmed microwave-safe bowl with a cup of water. Place both that bowl and the dish in question into the microwave.
  • Microwave both objects for a minute.
  • If the dish is hot to the touch following this process, it is not microwave-safe.
  • If the dish is cool and the cup of water is hot, the dish is microwave-safe.

If you’re unable to perform the safety test, assume it’s a no-go. Aside from potential injury and the ruining of a perfectly good glass dish and what was likely going to be a tasty meal, it’s no fun cleaning glass shards out of a microwave.

Why Are People Quick To Assume That You Can Microwave Glass

Lots of bakeware and cookware are made up of microwave-safe glass. They aren’t viewed by people as “microwave-safe glass”, but just glass.

Upon microwaving the safe glass, you won’t really encounter any issues. This creates comfort with the concept of heating food in glass containers. 

Glass dish in microwave
Not All Glass Dishes Are Safe In The Microwave

There’s a misconception of complete safety when doing so. As a result, you may wrongfully conclude that all glass is safe for the microwave.

Why Does Glass Break In The Microwave?

Glass is a heat conductor. Regular glass containers that are not considered microwave-safe have small air bubbles that originate during manufacturing. 

These tiny air bubbles then expand when heated, increasing the likelihood of shattering. 

This process is known as thermal shock.  Thermal shock is the result of a thermal gradient enabling certain parts of an item to expand in varying amounts.

The uncertain expansion level may result in breakage and explosion. 

Does It Matter If The Glass Has A Rim?

It does matter if the glass has a rim.  Metal trimming on glass is not microwave-safe.

Much in the way you cannot microwave those infamous red and white Chinese food take-out containers because of their metal handles, the metal on the glass can spark, causing the glass to break.  

Worse, a fire could occur. When looking for glass that you can use to reheat your food items, steer clear of those with trim.

Useful Tips For Microwaving Glass

Before you get all nervous around your microwave, rest assured that there are some additional precautions you can take beyond looking for the safety label:

  • Glass that has been previously frozen can be especially hazardous when thrown right in the microwave. Let previously-frozen glass containers reach room temperature before putting them into the microwave. 
  • This goes for tempered glass as well. Even with microwave-safe containers, it’s recommended that you wipe away condensation before putting the product into the microwave.
  • If you are unable to wait for thawing to occur, it’s recommended that you use a slight amount of power or the microwave’s defrosting option until all frost has subsided. 
  • Once this occurs, be sure to stir the contents before officially beginning the microwaving process. Since the use of the microwave can unevenly heat foods, this is to ensure that temperature is even throughout, in an effort to eliminate heat shock.

What Are The Risks Of Microwaving Glass?

Aside from compromising the shape of the product or breaking it all together, you could cause an actual explosion.

This could blow the door off the microwave and even seriously harm you or others. The risk isn’t worth the injury nor the broken microwave. 

Even if a non-microwave-safe glass looks to have survived the nuking process, it may explode in your hands once grabbed, causing serious injury.

Even if it doesn’t break, the non-microwave-safe glass will become so hot when heated, it could scald you.

Can I Put Glass In The Oven?

Some glasses are purposely made microwave-safe, and, therefore, heat-safe. They are typically manufactured specifically for cooking and storing purposes.

They do not contain the same air particles created during manufacturing, nor is there any metal trim to them. This makes them completely safe for the oven as well as the microwave.

Otherwise, your everyday-glass pans aren’t recommended for use in high- or direct-heat situations.

Is All Glass The Same?

There are truly more than six different types of glass but, for the sake of this concern, there is microwave-safe glass and regular glass. 

Regular glass is not intended for the microwave.  Also, colored glass could contain a dye that isn’t safe to be microwaved because of the dangerous impact it could have on the food.

There are certain ingredients within the dye that are changed once heated and could therefore become threatening to the meal being warmed, and therefore its consumer as well.

Tempered glass is an example of extra-strong glass that is non-toxic, safe for food storage, and microwaveable. 

However, even when a type of glass is considered microwaveable, it’s important to remember that all types of glass are still susceptible to thermal shock.

Why Does Heat Change Glass?

It’s no surprise that some glass is questionable when it comes to microwave safety. You’re taking an already extremely fragile material and subjecting it to several hundred wattages of intensity in an extremely short amount of time.

That’s a whole lot of pressure for an arguably thin piece of material. 

Often, glass that is microwave-safe will indicate such. There are also ways you can test to see for yourself if the glass is microwave-safe or not if need be.

If you’re not convinced that glass can be modified at high temperatures, just consider craftsmen who use the heating of glass to form vases, bowls, etc.

While this requires extremely hot temperatures in order to occur, the concept remains.  

Glass is considered molten between the temperatures of 900-1600 degrees Fahrenheit, and, while microwaves can send our food out piping hot, they cannot produce heat that intense in any amount of time. 

If you add boiling water to the non-microwave-safe glass, cracking is extremely likely as well.

Final Thoughts

Keep in mind that microwaves are an extremely efficient means of reheating or cooking foods in a relatively quick fashion.

That sort of convenience comes with a price and, here, it is high responsibility. 

To make things more complex, you are generally discouraged from waiting around the microwave too much in case something were to go wrong, like the time my friend’s spaghetti squash overheated and flew out of the microwave mid-session, breaking the microwave door in the process. 

Do not stand too close but, also, do not get so comfortable and confident with your microwave’s abilities that you underestimate its risks. 

Whether it’s exposure to cancer-causing toxins, potential explosions, or opportunities to be scalded, familiarize yourself with all material precautions and always err on the side of safety before putting any product— let alone glass— into the microwave.

You may be interested in our other post about can you put milk in the Keurig

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