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.