Not all casserole dishes are created equal. Pyrex is made of glass, which can experience thermal shock when it's exposed to sudden temperature swings. And that thermal shock can make the glass expand and contract, making the dish shatter or explode if the change in temperature is extreme enough.
It is important to note that Pyrex can shatter, it is simply more resistant to it than other ceramics. If it gets scratched or damaged in any way, throw it out. It cannot deal with high gradients in temperature, so you should always preheat your oven, etc.
As preposterous as it sounds, exploding Pyrex is not an urban legend. Under the right circumstances, the glassware will shatter dramatically or crack and split.
According to Pyrex's safety and usage instructions, “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.”
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.
This being said, you do need to follow one major rule when using Pyrex: Avoid extreme temperature changes. Shifts from one extreme to the other can cause thermal shock to the dish which can cause it to crack or shatter entirely.
"Heat expands-cold contracts. This is why sudden change of temperature breaks any ordinary glass. Pyrex contracts so slightly that sudden heat cannot break it." Image: From freezing to boiling without breaking.
Soda-lime glass is cheaper but doesn't have the same thermal shock-resistant properties of borosilicate glass. That's why so many people report their Pyrex exploding in the oven or due to extreme temperature changes (for example, placing a very hot dish on a colder surface).
The maker of Pyrex glassware and Instant Pot multicookers has filed for bankruptcy protection, with as much as $1bn (£790m) in liabilities. Instant Brands blames high interest rates and weak demand for its problems. The company, which has more than 2,400 workers, plans to keep operating while it shakes up the business.
All Pyrex is made of tempered glass, which is engineered to be tougher than regular glass. If it breaks, it's also designed to shatter into tiny, less-dangerous pieces, like a car windshield.
Pyrex, made of borosilicate glass, has a low coefficient of thermal expansion. This means it doesn't change size drastically with temperature shifts, preventing cracks when moved from hot to cold environments or vice versa.
Both the old and newer soda lime versions of Pyrex can break when dropped. However, the soda lime glass is said by the manufacturer to be less likely to break when dropped. Still, the old borosilicate glass is so very much stronger to rapid changes in temperature, so I always choose that instead.
The differences between Pyrex-branded glass products has also led to controversy regarding safety issues—in 2008, the U.S. Consumer Product Safety Commission reported it had received 66 complaints by users reporting that their Pyrex glassware had shattered over the prior ten years yet concluded that Pyrex glass ...
Not all casserole dishes are created equal. Pyrex is made of glass, which can experience thermal shock when it's exposed to sudden temperature swings. And that thermal shock can make the glass expand and contract, making the dish shatter or explode if the change in temperature is extreme enough.
Pyrex® has a very high resistance to attack from water, acids, salt solutions, halogens and organic solvents. Only hydrofluoric acid, hot concentrated phosphoric acid and strong alkaline solutions can corrode; making it ideally suitable for laboratory experiments.
Because borosilicate is the most durable when it comes to withstanding temperature changes, it is generally considered the safest choice for glass bakeware.
Pyrex is oven-safe up to 425 degrees. However, direct contact with heating elements can cause the glass to shatter or break.
The Bottom Line
Pyrex: Brand name for cookware brand. pyrex: Lowercase indication of containers that should not be heated in the oven or microwave. These are most likely made of soda-lime glass. PYREX: Uppercase label on cookware that is made of borosilicate glass and can be heated in a microwave or oven.
U.S. Pyrex and Anchor Hocking glass bakeware used to be made of borosilicate but no longer are. When glass bakeware does shatter, consumers report, it can break into sharp shards that go flying, raising the risks of injuries.
While Pyrex ovenware is a type of glass, it has been specially treated in the manufacturing process to withstand high temperatures, which makes it non-recyclable. Broken or chipped Pyrex should be disposed of carefully in the waste bin.
At its core, Pyrex is glassware and cookware that can resist chemicals, electricity, and heat. It is also shock-resistant and can withstand thermal expansion. As a result, it's an appropriate material to make cookware. Pyrex is also well-suited for lab tools, such as beakers, flasks, and test tubes.
Be sure to allow hot glassware to cool as provided above before washing, refrigerating or freezing. Oven must be preheated before inserting glassware. DO NOT use on or under a flame or other direct heat source, including on a stove top, under a broiler, on a grill or in a toaster oven.