PYREX and pyrex Are Two Different Things—And One Might Shatter in the Oven

The original Pyrex.

In 1915, the Corning company invented Pyrex. The revolutionary material had its beginnings in the laboratory and was adopted for cooking due to its ability to stand up to heat in the oven—something pretty glass dishes had never been able to do.

The original was made of borosilicate glass, which as the kitchen pros at Allrecipies explain, contains boron trioxide, which makes it resistant to extreme temperature changes. This lightweight borosilicate glass was typically used for pots and pans. Unlike regular glass, it is thermal shock-resistant; it can go from freezer to oven without cracking.

The newer Pyrex.

Because boron is toxic and costly to dispose of, instead of the more heat- and breakage-resistant borosilicate glass, Pyrex turned to cheaper soda-lime glass. This is the glass most often used in everyday drinking glasses and storage jars.

Soda-lime glass, however, is very susceptible to shattering from thermal shock (think pouring a hot liquid into a cold jar). That's why Pyrex used tempered glass, or soda-lime glass that's been heat treated.

According to the New York Times, "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." This tension balances the glass's atoms so that it can withstand temperature fluctuations.

It's not clear when, exactly, the company made the switch, but the Times reports that they had at least begun introducing tempered glass in the 1950s. Then, in 1998, Corning licensed the brand to World Kitchen (now known as Corelle Brands), which is when it entirely moved to tempered glass, according to Gizmodo.

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What about the two brand names?

PYREX and pyrex were used interchangeably for borosilicate and soda-lime glass while both were being manufactured. Corning later licensed out the use of their PYREX and pyrex logos to other manufacturers, according to Allrecipes.

Now, pyrex kitchenware is mostly sold in the United States, South America, and Asia, while PYREX is still available in Europe, Africa, and the Middle East. The latter is more likely to be the borosilicate version.

So, which one might shatter in the oven?

Melissa Collins, food scientist and chef at Perfect Brew, tells Best Life that tempered soda-lime pyrex still can sometimes break from very high temperatures. "The reason it is more fragile than borosilicate is its contraction and expansion rates. It has more than double the expansion and contraction rates of other types of Pyrex."

The likelihood of your Pyrex dish shattering is still very minimal, however. "When compared to the millions of glass cookware items that are in use in consumers’ homes, the number of incidents is small and the risk is low," Patty Davis, deputy director of communications and press secretary for the Consumer Product Safety Commission (CPSC), told the Times.

It's also worth noting that at the time of publication, a search on the CPSC's safety recall page yielded no results for "Pyrex."

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How to keep glass cookware from shattering.

The best way to make sure the cookware you’re using is right for the job is to read the warnings that came with it, as well as any labels embossed or printed on the bottom (or check online). Of course, you'll only want to put glass in the oven if it's labeled "oven-safe."

Avoid drastic temperature shifts by letting dishes from the fridge or freezer come to room temperature before putting them in the oven (and vice versa). Then, Pyrex notes on their website that products are only pre-heated oven-safe. "While the glass is designed for temperatures typically used in baking, it can break when exposed to the direct heat element while the oven is preheating."

Other ways the product page says Pyrex can shatter from coming in direct contact with a heating element are by putting it in the broiler, on a stovetop (gas or electric), in a toaster oven, or on a grill.

If you're going to add liquid to a Pyrex dish—for example, you want to baste a chicken—only do so with liquid from the dish that is already hot. "Adding liquid to hot glass may compromise the product’s strength, potentially resulting in breakage."

If you're cooking something that may give off its own liquids (say, frozen vegetables), the company recommends adding a bit of liquid to cover the bottom of the dish before it goes in the oven. "This serves to minimize any potential sudden temperature change that may occur as the food releases liquid."

When your dish comes out of the oven, do not place it on a very cool surface. And finally, inspect the glass for any cracks before you use it.

Is this why vintage Pyrex is so sought-after?

It's part of the reason. Aside from the nostalgia factor and vintage colors and patterns, many cooks still swear by the original borosilicate Pyrex.

"As a cook, I find borosilicate Pyrex to be my go-to material for baking and slow-cooking dishes," says Michael Murdy, food scientist, chef, and founder of Robust Kitchen. "One of its standout features is the even heat distribution it provides, ensuring that my baked goods come out perfectly cooked and not overly crisp on the edges or bottoms. That's a common problem with other baking materials."