Shattering Pyrex glassware

Pyrex glass dishes have a reputation for being able to withstand extreme temperatures from the very hot to the very cold. Hence I was surprised when someone I know did something that many of us have done without incident. He took a hot Pyrex baking dish from the oven and laid it on top of the rods that surround the burners you find on gas top stoves that keep the pots slightly above the gas outlets for the flame. But this time, the dish shattered into fragments, sending shards in every direction. He was lucky that he did not suffer any cuts from the high-speed bits of glass.

It turns out that this can happen on occasion and this video shows what can happen when a drop of cold water is placed in a hot Pyrex container.

So why did this happen? Adam Clark Estes writes that the reason may be that the formula used in the US to make the glass has been changed from borosilicate to cheaper soda-lime and the new formula does not accommodate temperature changes nearly as well as the old one did. So when you bring the hot glass in contact with a colder surface, the different parts contract at substantially different rates, creating stresses that shatter the glass, something known as ‘thermal shock’.

The real roots of the current controversy were planted in the 1950s, when Pyrex began making cookware out of tempered soda-lime glass. Corning licensed the Pyrex brand to a company called World Kitchen—now known as Corelle Brands—in 1998, and by nearly all accounts, all Pyrex cookware sold in the United States after that year has been made of tempered soda-lime glass. This is where the controversy really heats up.

The vast majority of glass products are made of soda-lime glass: window panes, jars, bottles, all kinds of glass. Soda-lime glass is cheaper to make than borosilicate glass, which is undoubtedly why Pyrex started experimenting with it. However, borosilicate glass is not only harder, stronger, and more durable than soda-lime glass; it’s also more resilient to thermal shock. Thermal shock is what happens when a temperature change causes different parts of a material to expand at different rates, and the resultant stress can cause the material to crack. If the temperature change happens rapidly materials like glass can shatter or seem to explode. Resistance to thermal shock is part of why Pyrex became so popular for cookware; you could move a hot glass pan into a cool spot without worrying about it cracking or shattering. It’s also part of why laboratories prefer to use borosilicate glass rather than conventional soda-lime glass. Pyrex cookware currently sold in the United States goes through a thermal tempering process. In theory, this should strengthen the glass.

In practice, the difference between the performance of borosilicate glass and soda-lime glass is significant.

So perhaps appropriately, the shift from borosilicate to soda-lime glass was a big deal to Pyrex enthusiasts. Many believe that the new tempered soda-lime glass is more prone to exploding during temperature changes, a belief that is supported by some basic science.

This doesn’t mean you should throw away all of your Pyrex cookware. You should, however, follow the company’s safety and usage instructions, which you can find here. Here’s a key quote that long list of warnings: “Avoid Sudden Temperature Changes to your Glassware.” That means don’t pour cold water on a hot Pyrex pan. Don’t put a hot Pyrex pan on a cold marble countertop.

Class action lawsuits against Pyrex were initiated in 2018.

It turns out that in Europe, this glassware still uses borosilicate. What I would like to know is why the shift from borosilicate to lime-soda was done in the US and not in Europe.


  1. colinday says

    What I would like to know is why the shift from borosilicate to lime-soda was done in the US and not in Europe.

    Cost savings/More profit?

  2. says

    What I would like to know is why the shift from borosilicate to lime-soda was done in the US and not in Europe.

    “Because we could”

    I suspect borosilicate requires a higher temperature than lime-soda (which I believe acts as a flux) so it may cost substantially more to get it hot enough to melt.

  3. TGAP Dad says

    Not that I have any insider information, but I’ve been a corporate grunt for long enough to bet the shirt off my back that it was to lower costs. Lower cost of production, in the sociopathic mind of the corporate CEO automatically means higher profits, and by extension, stock price, which is the real goal in the end. Profits, after all, have to be shared with the shareholders, whereas stock price dramatically raises CEO compensation, and are taxed at a lower rate as capital gains. As for why the Europeans use borosilicate, I’d speculate that the Corning brand and manufacturing rights in Europe were sold to a lesser-known manufacturer, who made that decision independently. My second guess would be tighter regulations in the EU.

  4. says

    Perhaps it is the competition with the in Europe better known trade names Jenaer Glas and Duran that keep Pyrex from offering lesser quality there.

  5. says

    The US regularly goes the cheap route domestically because corporations can get away with things they can’t elsewhere (read: bribery and lobbying). Just look at Hershey’s (*ahem*) “chocolate” or coca cola and sugar versus corn syrup.

  6. Jazzlet says

    Products are certainly sold on being borosilicate glass in the UK, so consumers here know its something to look out for.

  7. says

    What I would like to know is why the shift from borosilicate to lime-soda was done in the US and not in Europe.

    Because legal informed management that the cost of settling damage suites would be less than the savings realized by the change.

  8. Ridana says

    The way the handle shatters in the video makes me wonder if the cup isn’t some sort of elaborate Prince Rupert’s drop. Now I’d kind of like to see them try to shatter a borosilicate Pyrex cup, to see if, when it finally does go, it shatters in the same way.

  9. avalus says

    I would guess regulations here in the EU, that heat resitant glass is actually heat resistant. I for one an quite in shock about ithe glass-substitution by pyrex. Oh well, all has to be sacrificed for extra profits. What do some hands/faces full of glas fragments.
    Let`s not forget, that even borosilikate glas can experience thermal shock, if there are manufacturing faults.

    @ Marcus: It does, with softening points of ~830°C vs ~700°C. To do glasswork with borosilicate, I need to use a oxygen-gas torch. For lime-soda glas, a air/gas camping burner is quite enough.

    @Ridana: I don`t think so. This looks like normal glass breaking from thermal shock. Borosilikate in my experience develops just a few cracks if it falls to thermal shock. (Chemist here, I had my fair share of these).

  10. Sharon Wood says

    My pyrex baking dish exploded last nigh after I took it out of a 350 degree oven, this is the second time this has happened to me, the first was I had my pyrex baking dish in the refrigerator for a couple days, then I found it sh as uttered on the shelf !! Not happy !!!! 😠 cut myself both time cleaning up the shattered glass, not to mention the tiny bn pieces I’ve stepped on later that shot out onto the kitchen floor !

  11. clearasmud says

    @avalus @Ridana: Prince Rupert’s drop /is/ tempered glass. Maybe what @Ridana is observing isn’t related to the tempering process, but it’s worth noting that in both cases tempering is the process that introduces tremendous tension within the glass and compression on its exterior, which is what gives it strength and what also causes explosive results when the strength /does/ compromise (the trapped energy has to go somewhere).

    Also note that “heat resistant” is not the same as “thermal shock resistant”. Slowly place a hammer on your pinky and there’s no problem. Drop it from a height of 5ft and it will hurt. In both cases the same weight is being applied to your pinky, but in the second case it’s being applied over a much shorter time.

    Similarly tempered glass is heat resistant, but not thermal shock resistant. If I had to, I’d probably personally still choose borosilicate to prevent accidentally inducing thermal shock (would prefer aluminum or cast iron over both), but if one is careful, tempered glass does just fine in the kitchen.

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