Buzz
People often refer to the Wikipedia page for Pyrex as evidence that the all-caps brand is superior, but if you actually read the article, the situation is far more complicated. Corning Inc. registered the original U.S. trademark—PYREX in all caps—in 1915, when they were only making borosilicate glass. The lowercase trademark came in 1975, about 20 years after they started using tempered soda-lime glass in some of their products. This means there are PYREX-branded soda-lime items out there, a fact that's clearly stated in the Wikipedia article:
Both the PYREX (uppercase) and pyrex (lowercase, introduced in 1975) trademarks were used interchangeably in marketing kitchenware made from both borosilicate and soda-lime glass,
[1]
as well as various related accessories, for several decades.
The two trademarks, PYREX (uppercase) and pyrex (lowercase, from 1975 onward), were used together in advertising kitchen products made from both borosilicate and soda-lime glass,[2]
as well as other related items, over a span of many years.
When purchasing vintage PYREX made before 1975, you may encounter borosilicate glass, but it's just as likely to be tempered soda-lime glass. After 1975, anything produced could fall under various combinations of material and brand, as Corning, Inc. alternated between the two brands without specifying the composition of the items.
So, how do you determine if you have “genuine” Pyrex? A simple rule to follow is to assume it isn’t, especially in the United States. The original borosilicate Pyrex hasn’t been available on the U.S. market for over 30 years. Pyrex stopped using borosilicate glass in the 1950s and fully transitioned away from it by 1998 when World Kitchen (now Corelle Brands) acquired the rights to the PYREX and pyrex trademarks. Unless you find a vintage piece from a store that can prove it's from the 1940s or earlier, it’s unlikely to be made from borosilicate glass.
This doesn’t mean borosilicate cookware is unavailable—it simply can’t be found under the Pyrex brand. Reddit user Giant_Meteor_2024 mentions in a 2018 discussion on PYREX branding, laboratory glassware is often made of borosilicate glass: “If you don’t mind microwaving a 500ml beaker instead of a fancy PYREX dish, you can find them cheaply.” (Though a laboratory-grade casserole dish might be harder to locate.)
If you’ve come across some of those infamous exploding Pyrex tales, you might be cautious about using tempered soda-lime glass—and understandably so. However, it’s not inherently dangerous. It becomes hazardous only when exposed to sudden, extreme, localized temperature changes. For instance, in college, my friends and I once blew up a Pyrex pan of mac and cheese when we left it near—but not on—an electric coil burner. One of us turned on the burner for a kettle, then walked away. A few minutes later, a loud crash echoed through the house, and we found shattered Pyrex and mac and cheese all over the kitchen. The nearby burner’s heat caused too much of a temperature shock for the glass, leading it to explode.
This type of scenario is what you need to avoid when using soda-lime glass. It’s perfectly safe to transfer a Pyrex dish from the fridge or freezer into a preheated oven on a rack, but never place it directly on a preheated pizza stone, baking steel, or sheet pan. Similarly, putting a hot Pyrex dish on a cold or wet surface is risky, as is pouring boiling water into it. As long as you prevent extreme temperature shifts, it’s safe to use. It’s been 15 years since my encounter with exploding Pyrex, and I continue to use it daily.
