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Exploding the exploding Pyrex rumor
Trevor Butterworth, October 14, 2009

Exploding Pyrex is the second hottest urban legend on – the hottest site on the Internet for debunking urban legends and rumors. Snopes says, it’s true – but then backtracks. Here’s what you need to know.

Update (October 15) has substantially revised their entry "Exploding Pyrex" based on STATS criticisms. is a giant of the Internet age, with 6.2 million visitors checking out the site each month to find out which of the latest wild rumors and wacky claims are true. It is, says Readers Digest, the “Internet's preeminent resource for verifying and debunking rumors,” which is a remarkable accolade and achievement, given that the site is a mom and pop shop, created, researched, and maintained by the husband and wife team of Barbara and Dave Mikkelson, who met through their amateur interest in folklore.

Snopes recently determined that exploding Pyrex is the second hottest urban legend circulating on the web and that the rumors were true – Pyrex was exploding. The site observed that Pyrex changed the kind of glass they used, stated the obvious (glass will shatter if exposed to extreme temperature shifts), and cited a letter from the manufacturer, World Kitchen, claiming that the number of complaints about exploding bakeware is low. But Snopes concluded that:

“Nonetheless, critics (including experts with doctoral degrees in materials science) have maintained that Pyrex brand glass bakeware is inadequately tempered and that the company’s product instructions are insufficient.”

And then during the course of October 14, “true” turned out to be less true than Snopes first claimed. The site switched its verdict to “mixture.”

The vacillation may have something to do with Snopes sourcing their analysis of exploding Pyrex to a notorious three-part local TV investigation by CBS-2 in Chicago in 2008. An examination of that “expose” by STATS at the time showed some glaring weaknesses in the station’s dogged attempt to prove that the nation’s kitchens were harboring weapons of glass destruction.

After scanning the Internet and finding 300 claims of Pyrex dishes “exploding” over the last five years – none of which appear to have been verified as actually involving Pyrex as opposed to other glass (that would require testing the glass) – and without any reliable evidence that the dishes hadn’t been used in ways the product labels warn against, CBS 2 wheeled out what Snopes calls “experts with doctoral degrees in materials science” to prove a danger. Except, they didn’t really prove anything.

The first expert was Professor Sheldon Mostovoy, PhD, of the Illinois Institute of Technology. He managed to get one dish to break after heating it to 450 degrees, filling it with sand to simulate food, and placing it on a wet granite counter. It cracked and sent a shard of glass flying for six to eight feet, which is something you would expect given the exposure of hot glass to cold water under the laws of physics. According to CBS, Mostovoy believed Pyrex was safe. It was the safety instructions that were inadequate because they were written in letters that were too small. [See sidebar]

CBS turned to another professor, Jack Mecholsky, Ph.D, of the University of Florida. He believed the warnings were too difficult to follow.

CBS went to the Consumer Product Safety Commission. It told the station that it didn’t believe that Pyrex is a “safety hazard.”

Changing glass – improved tensile strength.
One of the key claims about “exploding” Pyrex is that it all results from the manufacturer changing the type of glass used from boroscilicate to soda lime. Soda lime glass is cheaper and easier to make than borosilicate glass (which is still used in European-manufactured Pyrex), and, of course, that raises the specter of a manufacturer cutting corners for the sake of profit. But heat-treated soda lime glass has nearly double the mechanical strength of borosilicate glass.




Failure to follow these instructions can cause breakage resulting in injury or property damage:

• NEVER USE ON TOP OF STOVE, under a broiler, in a toaster oven, or place over oven vent or pilot light.

• AVOID SEVERE HOT TO COLD TEMPERATURE CHANGES and DO NOT add liquid to hot dish, place hot dish or glass cover in sink, immerse in water or place on cold or wet surfaces. Handle ALL hot ovenware and glass covers with dry potholders, including ware with Silicone gripping surfaces.

• DO NOT use in microwave to hold or support popcorn bags, microwave convenience foods with special browning wrappers, etc.

• DO NOT use to pop corn, caramelize sugar, or deep fat fry.

• DO NOT overheat oil or butter in microwave. Use minimumamount of cooking time.

• DO NOT use or repair any item that is chipped, cracked, or scratched… [etc]


Both soda lime and borosilicate glass can withstand pressures of roughly 6,500lbs per square inch before breaking; but heat-strengthening soda lime can add another six to seven thousand pounds per square inch of mechanical strength. This means that strengthened (sometimes call tempered) soda lime bakeware is less likely to break if you hit it or drop it.

And this is the most common way people injure themselves from glass bakeware. Look at the emergency room data. A query of the Consumer Product Safety Commission’s emergency room database of product related injuries (enter code 461), reveals that the most common injuries from glass bakeware are lacerations that occur after a dish has been dropped. Injuries from a thermal downshock “explosion” are much rarer. In fact, based on emergency room data for 2005, you had just a 1 in 3,706, 338 chance of sustaining a non-fatal injury from glass bakeware that didn’t shatter from mechanical breakage.

The downside is that soda lime isn’t able to withstand temperature changes in the way the old Pyrex (borosilicate glass) could. The coefficient of thermal expansion for borosilicate glass is 35 x 10 to the power of minus seven, inch per inch per degree centigrade, while that of heat-strengthened soda lime glass is 85 x 10 to the power of minus seven, inch per inch per degree centigrade. This means soda lime glass has three times the thermal expansion of borosilicate glass, which accounts for greater dynamic breaking when a hot soda lime dish encounters a cold, wet surface – which is why the manufacturer warns not to expose dishes to such temperature conditions.

The reality though is that if people are more likely to injure themselves by dropping a dish, the change to soda lime represents a greater net safety benefit.

Changing glass type to follow environmental regulations.
The other missing fact in the story  is that the switch to soda lime was required by environmental law. Last year, STATS spoke with Phil Ross, an independent consultant to the glass industry, whose clients have included World Kitchen, which makes Pyrex in the United States). He said the industry as a whole switched from borosilicate started in the 1980s for a variety of reasons, including the fact that soda lime was easier to melt and work with (fewer deformities in the glass). But one of the major reasons for moving to soda lime was environmental compliance: borosilicate glass produces far more emissions from a glass furnace , accounting, in part, for the presence of boric acid in the water and soil. And it was not economical for companies to install multi-million dollar filter systems.

Add to that, Ross says, the fact that furnaces have a longer lifespan if you use soda lime, and require less energy (15 to 20% lower than for borosilicate), and the economics for the switch were compelling. These aspects led the entire glass bakeware industry in the U.S. to switch to soda lime starting in the 1980s.

Which brings us to point out another myth circulating on the web: Pyrex glass bakeware is made in the U.S. and not in China. Perhaps the American flag on the box isn’t big enough.

Does glass explode?
Finally, this is a story driven by false terminology:  glass doesn’t actually explode; it just sounds like it does. It may sound like an explosion when it shatters (the sound is of atomic bonds being ripped apart), but there is no expulsion of gas, which is the signal for a true explosion. Glass can only be broken in tension, either by dropping it on a hard surface, hitting it with something, or subjecting it to dramatic changes in heat, which puts different areas of the glass in tension. So, for example, when a hot dish is placed on a cold surface or in cold water, the cooled surface shrinks in comparison to the glass inside, and the tension makes the glass break. Abrasions from, say, a chip or scoring a glass dish with a knife will exacerbate breakage.

If every one of those 300 claims about exploding Pyrex did indeed involve Pyrex, how many of these were scored or damaged or underwent rapid temperature shifts? We don’t know. It’s easy to assume that a product is at fault, but the reality is that people are often just as, or even more, fallible.


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