Did You Know There Is an Actual Difference Between PYREX and pyrex?

It's not just in a name — the difference could affect your safety in the kitchen.

When you think of a staple kitchen brand synonymous with longevity and quality like Pyrex, you probably don't think of explosions. However, we recently saw a video on TikTok (of course) that was pretty surprising, and wanted to know more. Why are these dishes shattering? Is there a difference between "PYREX" and "pyrex" — and if so what is it? How do you make sure the one in the cabinet isn't going to leave shards of glass in your lasagna during baking? Let's dig into some answers.

What Is Pyrex?

First of all, let's start from the beginning. Three basic types of glassware are typically found in most home kitchens: soda-lime, tempered, and borosilicate. Borosilicate glass includes boron trioxide, which has a low thermal expansion. This suggests unlike normal glass, it won't break when exposed to major temperature shifts such as taking a dish from a fridge to an oven. This is thanks to boron trioxide, the element that makes glass resistant to major temperature changes. Pyrex is a sub-group of borosilicate.

Soda-lime glass is the most common glass type in kitchens since it's used for most drinkware from juice cups to jars. Untreated soda-lime glass is more susceptible to breaking from extreme temperature changes. This shock expands the glass at different rates, resulting in cracks and fissures.

Tempered glass is just soda-lime glass that's been heat-treated to make it more durable. During that heat-tempering process, the exterior of the glass is force-cooled so that it solidifies quickly, leaving the center to cool more slowly. As the inside cools, it pulls at the stiff, compressed outer layer, which puts the center of the glass in tension.

Two glass measuring cups, one with a pyrex (all lowercase) logo and the other with a PYREX (all uppercase) logo

Are "PYREX" and "pyrex" the Same?

Both trademarks were historically used interchangeably in the marketing of kitchenware products made up of both borosilicate and soda-lime glass. However, now Corning has licensed out the use of their PYREX (upper case lettering) and pyrex (lower case lettering) logos to other companies.

Lowercase pyrex is now mostly used for kitchenware sold in the United States, South America, and Asia. In Europe, Africa, and the Middle East, uppercase PYREX is still available.

So Which One Explodes?

Pyrex used to be made of the more heat-resistant borosilicate glass, which is more resistant to breakage when subjected to extreme shifts in temperature. Pyrex eventually switched to tempered glass most likely because boron is toxic and expensive to dispose of. Although tempered glass can better withstand thermal shock than regular soda-lime glass can, it's not as resilient as borosilicate. This is what causes the shattering reaction people are talking about. Watch out for those casseroles.

And What Does This Mean?

If the logo is in upper case lettering, PYREX, it's most likely made of borosilicate, and thus safer. The lowercase lettering is most likely made of soda-lime glass, so take extra care after any high-heat cooking. Most glassware products will include warnings so make sure to read about proper handling, cleaning, and storage.

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