Pyrex® Glassware: Is it safe to use?

Pyrex casserole dishesThe answer is complex—involving reports of flying glass and rebuttals calling “exploding” Pyrex® merely an urban legend.

Here’s the story: After hearing many frightening complaints about Pyrex® glassware that “exploded” during use, in 2008, had a Chicago TV investigative team research the matter to find out whether Pyrex® products were (in the words of “weapons of glass destruction.” The resulting TV report left the strong impression that Pyrex® glassware was dangerous, and the matter got a lot of attention. called the story “the second hottest urban legend on”


True, the original report did contain some errors (later corrected), but the fact remains that hundreds of users have claimed that their Pyrex® glassware suddenly broke apart, in some cases shooting out sharp glass shards that caused injuries. These occurrences weren’t actually explosions, which, are caused by pressure from within (as happens when a bomb or balloon bursts). However, they were still scary, dangerous events. These incidents tended to occur when a glass dish was moved from the oven and placed on a counter (especially a cold, wet one), but there was also at least one report of breakage while the dish was still in the oven.


Pyrex® glassware has been on the market since 1915, and complaints about flying glass fragments are relatively new. So what happened? Originally, Pyrex® was made by Corning with borosilicate glass. Later, Corning began making some Pyrex® with adequately tempered soda lime glass before selling the name and the technology to World Kitchen. Since  the 1980s, Pyrex glassware produced in the U.S. has been made with soda lime. But the switch to soda lime itself was not the cause of the problem, as we learned.


In seeking the answer to “Is Pyrex® Glassware Safe?”, contacted Dr. Sheldon Mostovoy, a metallurgist, Illinois Institute of Technology professor, and one of the scientists who tested the “new” (soda lime) Pyrex®. Here’s what he told us, “Borosilicate Pyrex® was much more heat resistant. The new Pyrex® was not adequately tempered. The tempering process is similar to the treatment auto glass gets to prevent shattering into sharp shards.” During Mostovoy’s experiments, he said, “The new Pyrex® shattered into sharp shards that were propelled many feet.”  Adequate tempering is extremely important because it 1) reduces the chances of sudden breakage due to thermal stress (from rapid temperature change) or mechanical stress (such as dropping the dish).  

Here’s what Mostovoy concluded: “I believe that, unless this issue of tempering the glass is corrected, there will be more people injured, even in cases where customers observe the company’s warnings.” Mostovoy recommends that World Kitchen either switch back to borosilicate glass or improve the tempering process on the soda lime glass.


You can play scientist and test your glassware yourself to see if it’s been properly tempered. Dr. Mostovoy says it’s easy. “I use a sheet of polarizing material but often just polarized sunglasses are enough. Take a Pyrex® dish, hold it up to the light, and look at it with polarizing plastic. If there are lots of colors, the dish has likely been properly tempered; if just a few colors show, it is likely poorly tempered.”


Does all this mean Pyrex® soda lime glassware is unsafe? World Kitchen denies this and has posted a page-long statement about the product’s safety record on the Pyrex® website. This statement does not respond to the concerns of scientists about inadequate tempering, nor does it indicate that any changes have been made in the manufacture of its products to make them more heat-resistant and less likely to “explode.”


Warnings about how not to handle Pyrex® come with the products and are on the Pyrex® website. Scientists working on the Pyrex® issue called the warnings inadequate because the print is too small and the contents too complicated, vague, and incomplete. Here are just SOME of the Pyrex® Safety and Usage Instructions:


• “NEVER use the product on top of a stove, under a broiler, in a toaster oven or place over oven vent or pilot light.”


• “Avoid severe hot to cold temperature changes.” (There are a number of specific warnings related to this general concept including not placing a hot Pyrex® dish on a cold, wet surface. In fact, the instructions probably don’t say so, but the safest way to handle a hot Pyrex® dish is to place DRY pot holders or a towel between the dish and the counter. Even a room temperature counter may provide sufficient temperature change to cause breakage.)


• “ALWAYS add a small amount of liquid to the vessel prior to baking foods that release liquids while cooking.” (What’s a small amount? Which foods release liquid? How much is sufficient?)


• “DO NOT use or repair any item that is chipped, cracked or scratched.” (Is the user supposed to scrutinize the dish with a magnifying glass before each use, looking for defects? Should I throw out the dish if it has even a small scratch? Incidentally, if you are nesting two or more Pyrex® dishes, they’re more likely to get scratched, but the instructions may not say so.)


If you can keep all of the company’s warnings in mind or are willing to post them in your kitchen, you will decrease the odds of having an explosive accident with Pyrex®. (But, Mostovoy reiterates, without proper tempering, “window glass” dishes can break even when all the cautions are observed.) Some of these warnings tell you what you’ve probably known for many years. Most people know that glass items can’t tolerate rapid temperature change. And who would try to broil a steak in a glass container?


If, after reading this far, you feel uneasy about using Pyrex® bakeware, consider the alternatives suggested by Barb Randall in WestLinn Tidings: “A quick tour of a local kitchen supply store showed many options, including foil, tin, metal…, ceramic…, and porcelain….”




Email exchanges with Dr. Sheldon Mostovoy, metallurgist, Illinois Institute of Technology and expert witness concerning Pyrex® glassware


Chicago CBS affiliate TV station, news reports by Pam Zekman: “The Trouble with Pyrex,” 2008. “Exploding Pyrex” Stats Articles 2009 “Exploding the exploding Pyrex rumor “Exploding Pyrex is an urban legend” “Setting the Record Straight: The Truth About Pyrex”


Links: “Exploding Pyrex” Stats Articles 2009 “Exploding the exploding Pyrex rumor “Exploding Pyrex is an urban legend”


Pyrex® “Setting the Record Straight: The Truth About Pyrex”

I once saw a glass bowl crack when it had cold water run over it (it had been holding hot fluid). Is that a similar issue as the pyrex?
Glass is likely to break when subjected to rapid and extreme changes in temperature. However, the new Pyrex glassware seems to be less heat- resistant than it should be since consumers reported that it broke when moved from the oven to the counter. But the bigger problem is that, when it breaks, small pieces of glass sometimes shoot out several feet and injure people.

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