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. I know because I’ve seen it happen three times. To be fair, only one was an explosion. The other two were merely breakages. Still, they were bummers. It’s really easy to poo-poo this explosion business until you have the misfortune to see it happen yourself, whereupon you become a crusader for the cause of Handling Pyrex With Care. Welcome to my crusade to vanquish fear by sharing facts.
While exploding Pyrex is far from an everyday event, it’s not a rare occurrence. It’s entirely preventable if you follow a few simple guidelines.
How Pyrex Is Different From Other Glass
Don’t blame Pyrex. It’s been a fixture in American kitchens since 1916, when the Corning, New York factory produced its first glass pie plate. When used correctly, it’ll soldier on for years.
Pyrex and other similar glassware are not like everyday glass jars and bottles. It’s tempered, meaning it’s been heated and cooled in a manner that makes it more durable under future temperature fluctuations. It’s less likely to break under an impact, and when it does, tempered glass breaks into diamond-like cubes instead of long, pointy shards, so it’s a little safer to deal with.
The Real Reason Pyrex Explodes
If you drop a metal loaf pan on the floor, it won’t break. That’s because metal has some flexibility. But all glass is brittle, and an impact like that might make it shatter. If you’ve ever left a canned beverage in the freezer, you may have seen the aluminum buckle out once the liquid inside it froze and expanded. But a full, tightly lidded glass jar in the freezer won’t warp; it cracks.
Similarly, straight out of the oven, a metal cookie sheet dunked into a sink of cool water will warp because of the sudden change in temperature. The cookie sheet doesn't crack, however; its shape is simply altered. But if you do the same thing to a hot piece of glassware, it could break because of what’s called thermal shock. Thermal shock is when an object goes through drastic changes in temperature quickly enough to fracture it. On a molecular level: heated material expands, while cooled material contracts. Brittle materials like glass are much more vulnerable to thermal shock, because they break when their molecules quickly expand under heat or contract under cold.
How to Prevent Exploding Pyrex
Relax, you can and should continue using your Pyrex. Just show it some love.
Avoid sudden changes in temperature. Don’t put frozen pies or casseroles made in Pyrex dishes straight from the freezer into a hot oven. Don’t pour boiling liquids into glass measuring cups, and let hot Pyrex dishes cool before you run water over them.
Store it reasonably. You may have heard that vintage Pyrex, which is made with sturdier borosilicate glass, is superior to the soda-lime glass pieces that the company began switching to in the 1950s. While it’s true that borosilicate glass is more resistant to thermal shock, it’s not immune to it. Also, if you are clumsy like me and frequently drop things or stack them carelessly during storage, the glass may get microcracks that you can’t see. These microcracks make even best-quality glassware vulnerable to thermal shock.
My Thrilling Stories of Exploding Pyrex
Don’t worry, I wasn’t going to leave before dishing the dirt. Here are my sordid firsthand accounts of exploding Pyrex from the kitchen battlefield.
The Cracked Pie Plate
As a teenager, I got really into baking bread, and I wanted to create a steamy environment in my oven to help a batch of baguettes bake up with a nice crust. I’d read that you can do this by pouring boiling water in a pan set in the bottom of the oven. I decided to put a glass pie dish on the bottom rack instead. When I poured boiling water into it, the pie plate cracked in half. This was not a disaster, but my mom sure was not happy about her broken pie dish.
The Exploding Casserole!
At a fully booked cooking class where I once worked, one of the assistants put a chicken in a 10x15-inch Pyrex dish and stuck it in the oven to roast. After about half an hour, there was a spectacular loud shattering. The dish had exploded in the oven! Luckily the oven contained the glass shards, but bits of glass were everywhere, as were chicken juices. What a sad mess, and the worst part was the loss of that amazing-smelling chicken. Our staff was very rough on things, and my theory is that over time people had knocked that 10x15 a little too hard one time too many. The oven was set at 450°F and perhaps the high heat expanded an existing microcrack.
The Shattered Cup
I go thrifting a lot, and my favorite glass measuring cup was vintage Pyrex. I needed exactly one cup of boiling water for a recipe and when I tipped the kettle into the glass, the cup cracked with a loud pop, breaking into four or five pieces. This was clearly a case of thermal shock, but also it’s possible a pesky microcrack was lurking unseen. Clearly I had not learned my lesson from that whole pie plate fiasco years before.
Like many “it’ll never happen to me” scenarios, it’s easy to think you can continue doing whatever you want with your Pyrex and it’ll never explode…until it does! All you need to do is avoid subjecting it to sudden changes in temperature and handle it with a modicum of care, and you’ll never have to join the anti-thermal shock crusade the hard way.