Can You Put Glass in the Microwave?
Most glass is microwave-safe, but not all. Look for a "microwave-safe" label on the bottom. Avoid glass with metallic trim, thin or delicate glass, cold glass from the refrigerator, and any glass that's chipped or cracked.
Key Takeaways
- Glass itself doesn't absorb microwaves—it lets them pass through to heat the food.
- Vintage Pyrex (pre-1998) was made with borosilicate; newer Pyrex in the US uses soda-lime glass which is slightly less thermal-shock resistant.
- Mason jars are not designed for microwaves—the metal lids spark and the glass may crack.
Explanation
Glass itself doesn't absorb microwaves—it lets them pass through to heat the food. Tempered glass and borosilicate glass (like Pyrex) are designed to handle temperature changes and are generally safe. Regular soda-lime glass may crack from thermal shock.
The main concerns are thermal stress and metallic decorations. Cold glass going into a hot microwave, or hot glass touching a cold surface, can shatter. Any gold or silver trim will spark. Colored glass may contain metal oxides that heat up.
To test unmarked glass: microwave it empty alongside a cup of water for one minute. If the glass stays cool while the water heats, it's microwave-safe. If the glass gets hot, it's absorbing microwaves and shouldn't be used.
The two main types of glass used in kitchenware have different thermal properties. Borosilicate glass (European Pyrex, lab glassware, Bodum products) contains boron trioxide, which gives it a very low thermal expansion coefficient—it barely changes size when heated, making it highly resistant to cracking from temperature swings. Soda-lime glass (American Pyrex since 1998, most drinking glasses, mason jars) expands more with heat and is roughly 3 times more likely to crack from thermal shock. You can tell them apart by weight: borosilicate feels lighter for its size.
Microwave-safe labels are tested per FDA guidelines, which require that the container not heat above 212°F when microwaved with food for a specified time. Glass that fails this test may contain iron, manganese, or lead oxides that absorb microwave energy and heat the container itself rather than allowing the energy to pass through to the food. Antique and colored glass pieces are especially likely to have high metal oxide content and should never go in the microwave.
Things to Know
- Vintage Pyrex (pre-1998) was made with borosilicate; newer Pyrex in the US uses soda-lime glass which is slightly less thermal-shock resistant.
- Mason jars are not designed for microwaves—the metal lids spark and the glass may crack.
- Glass lids can be microwaved if they're labeled safe, but leave them slightly ajar to release steam.
- Don't microwave glass straight from the freezer—let it come closer to room temperature first.
- Tempered glass (like Duralex and some baking dishes) is heat-treated to be 4-5 times stronger than regular glass, but if it does break, it shatters into small rounded pieces rather than sharp shards.