Urban myth debunker Snopes.com set the Internet alight with its pronouncement that Pyrex really does explode. And then it backtracked. STATS explains why Snopes got it wrong here.
I noted in an earlier post that STATS was looking into the environmental impact of making Pyrex with soda lime glass instead of boroscilicate glass. The switch has been blamed for a spate of “explosions” as Pyrex made with soda lime has a greater rate of thermal expansion than Pyrex made with borosilicate: if you put a hot dish on a cold, wet surface, it could shatter, warns the manufacturer.
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 (borosilicate is still used for laboratory glassware). Given the length of time soda lime has been used in bakeware multiplied by the number of times an individual piece has been used in a kitchen, the numbers cited by CBS and other local television news outlets for a trend in exploding dishes are tiny. If there was a substantial flaw in soda lime bakeware, or if heat tempering was insufficient, it’s reasonable to wonder why breakage isn’t more common. Literally, hundreds of millions of dishes are being used multiple times a week without apparent incident.
Does Pyrex “Explode” Because the Manufacturer Changed the Mix? CBS Chicago’s Epic Investigation ContinuesFebruary 28, 2008
It breaks down into the claim by CBS’s experts (one of whom is acting as an expert witness in a lawsuit against World Kitchen for a product other than Pyrex) that the glass Pyrex is now made from (heat-treated soda lime) isn’t able to withstand temperature changes in the way the old Pyrex (borosilicate glass) could.
True. 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 – and why the manufacturer warns not to expose dishes to such temperature conditions. CBS 2′s experts claim that U.S. made Pyrex isn’t “tempered” enough; World Kitchen says their product is.
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; on the other hand, manufacturing soda lime appears to be more environmentally friendly in terms of industrial emissions (we’re chasing down the data on that one).
But here’s what CBS 2 didn’t tell viewers: Heat-treated soda lime glass has nearly double the mechanical strength of borosilicate glass. 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.
As this is the most common way people injure themselves from glass bakeware, isn’t this aspect something which CBS should have mentioned? Think about the emergency room data. If you query the Consumer Product Safety Commission’s emergency room database of product related injuries (enter code 461), you’ll find 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 more rare. 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 injuries – two burns and a cut foot – reported by CBS 2 are unfortunate, and their cause alarming. The 300 reports of similar explosive accidents require much more systematic evaluation before they can count as evidence that something unusual is going on. In sum, we need to be guided by science and not just sympathy in risk evaluation. After all, we’re talking about glass; indestructibility and perfect safety can’t be the standards for measuring risk.
As with every product: read the warning label.
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 (that would require testing the glass), and without any reliable evidence that the dishes weren’t subjected to the kind of use the warning labels warn against, CBS 2 Chicago’s award-winning investigative reporter Pam Zekman turned to experts to find out what was going on in the second-part of an expose on how glass can… um… break.
The maker of Pyrex, World Kitchen, supplies test results to the station showing how the dishes can break if subjected to extreme temperature changes, but these aren’t dramatic enough, so CBS turns to Professor Sheldon Mostovoy, PhD, of the Illinois Institute of Technology to devise more tests to get the dishes to break. Finally, one dish, heated to 450 degrees, filled with sand to simulate food, and placed on a wet granite counter cracks, sending a shard of glass six to eight feet away. Which is what one would expect, given the laws of physics and the nature of glass.
A smoking gun? No. After all this, Zekman tells viewers that Mostovoy believes Pyrex is safe. It’s the safety instructions that are inadequate. Well, you can be the judge, as here’s the opening paragraph from the leaflet that accompanies Pyrex bakeware:
READ and SAVE
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.1
• 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.2
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
• DO NOT use to pop corn, caramelize sugar, or deep fat fry.
• DO NOT overheat oil or butter in microwave. Use minimum
amount of cooking time.
• DO NOT use or repair any item that is chipped, cracked, or
Professor Mostovoy thinks the lettering is too small. Another professor, Jack Mecholsky, Ph.D, of the University of Florida believes the warnings are too difficult to follow – even though hundreds of millions of Pyrex and other glass dishes are being used daily without catastrophic results. (One is tempted to say that if you can’t understand the warnings above, how can you possibly follow a recipe?).
Finally, the Consumer Product Safety Commission tells CBS Chicago that it doesn’t believe Pyrex is a “safety hazard.” But that’s still not good enough for Zekman: there will be a third segment running tonight (that’s Pyrex three nights running) to figure out what is going on.
Here’s what’s really going on: CBS Chicago is desperately trying to salvage some point to an investigation, which presumably ate up a lot of money but managed to turn up nothing more substantial than an opinion that the warning label should have larger lettering. This should be laughable; but even though CBS failed to verify any of the anecdotes about exploding Pyrex (see yesterday’s post), and essentially relies on self-reported incidents from other Internet sites (since when is this a credible method of reporting?), the fact that it keeps associating a product with a risk functions like bad advertising. When people hear the word Pyrex, they’ll think, “oh, doesn’t that explode?
And once that starts happening, how long can it be before people begin filing lawsuits claiming that they have been emotionally traumatized by the sound of exploding dishes?
Chicago’s CBS affiliate has discovered the laws of physics, and the shocking news is that…um… glass can break. Like, if you drop a dish made of glass it can… break (mechanical breakage), or if you put a very hot glass dish in cold water it can… break (thermal downshock).
In a segment blowing gale-force spin, CBS 2 tries to make out that a supposed spate of ‘exploding’ Pyrex dishes means that…um, glass bakeware isn’t safe. And if something isn’t perfectly safe, well, the question of where to apportion blame can’t be far behind.
“It may be the most popular glass bakeware in America,” said co-anchor Rob Johnson. “But tonight, some cooks are asking, ‘Is there a problem with Pyrex?” CBS investigative reporter Pam Zekman turned to one family for a shocking anecdote:
JULIA TUSO (Pyrex User’s Daughter): When it hitted (sic)
me in the hand I–I was crying. That’s why I said, ‘Mommy, I’m scared.’
ZEKMAN: Her mother had just taken a year-old Pyrex dish
out of a 350-degree oven. She put it on the oven door…
Ms. GINA TUSO (Pyrex User): .and when I turned back to
get a fork to flip the steak, I heard a loud explosion.
Mr. JOE TUSO (Pyrex User): There was glass scattered
everywhere, from 10 to 15 feet away from where it happened;
in three separate rooms.
ZEKMAN: Their daughter Julia was sitting across the room
at the kitchen table and was burned on her hand and neck by
a piece of flying glass.
First, glass doesn’t actually explode; it just sounds good for TV to say 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.
So what actually happened in the Tuso’s kitchen? The only thing we know for sure is that the laws of physics weren’t broken along with the dish. It had to have been dropped, or have some prior damage, or have been exposed to extreme temperature change or some measure of all three. These are the only ways glass can break. But there are some real problems with the story – beginning with cooking a steak in an oven at 350 degrees and needing to flip it. Who bakes steak? Were the Tusos actually broiling the steak? If so, this is something Pyrex warns you not to do with glass bakeware.
It’s also hard, given that the dish apparently shattered on the door of the oven, to understand how the force projected upwards across the room to where their child was sitting, injuring her neck. The position of impact and the food in the dish would have directed the force outwards and downwards. Odd too is the fact the the shard of glass, apparently, retained enough heat after traveling a considerable distance and possessed enough sticking power to cause a burn.
[update - after reviewing the video, the wall oven appears high enough not to require upward force.]
But the questionable aspects of this kitchen misfortune raised no questions; instead, to make something more significant out of this misfortune, CBS 2 trawled a bunch of consumer websites for corroborating events in order to create the idea of an explosive trend – and it found some 300 incidents over the past five years.
But wait: Did CBS 2 verify that each of these events involved Pyrex? Did it check out the conditions in which the breakage occurred? Or did the reporters simply assume that an anecdote on a consumer website must be about Pyrex and must be a case of explosive shattering? No systematic analysis is presented by Zekman, so one can only conclude that they skipped checking the details out.
CBS 2 also cited the Consumer Product Safety Commission’s (CPSC) list of 66 complaints about for the last 10 years. That’s 6.6 complaints per year for 369 million Pyrex glass products manufactured since 1998 – assuming that the complaints did actually refer to Pyrex and not a rival glass product (once again, no-one checked). Add that number to Pyrex dishes still being used from before 1998, the overall rate of usage (say times per week), and one is left wondering: by what rational measure is this a problem worthy of headline treatment?
Based on emergency room data for 2005, you had a 1 in 3,706, 338 chance in the United States of sustaining a non-fatal injury from glass bakeware that didn’t shatter from mechanical breakage (i.e., being dropped). And even that is a crude estimation – a more accurate picture of the risk would involve the chance calculated per times glassware was used in cooking during the year, or per hour of cooking usage. For 2006 all calculation is moot: there were no injuries recorded by emergency rooms participating in the CPSC database, so the risk, as best we know, was zero. All of which means, compared to other kitchen equipment, glass bakeware is pretty darn safe. Take blenders for example, based on 2006 data, you have 1 in 95,152 risk for a non-fatal injury. And they have moving blades.
Because CBS 2 can’t figure out a way of explaining how Pyrex glass could defy physics and break in some special way that presents a hitherto unknown risk, the segment resorts to trying to show that the safety instructions are confusing to people whose Pyrex bakeware shattered (well, they think it was Pyrex, again no-one has checked). But the instructions explicitly warn about temperature changes and other conditions that can cause breakage and the overwhelming majority of people seem to be able to use their glass bakeware without mishap. CBS 2 promised a second segment tonight showing the results of its safety tests.
But if glass bakeware is going to be a benchmark for risk, and a handful of anecdotes the measure of a problem, people should stay out of the kitchen altogether. That’s where knives cut, and water boils, and things get hot. And, in fact, to be super-duper safe, people should probably avoid watching local TV news too, and not because it can damage your sanity, but because vastly more people injure themselves from television sets in the U.S. each year than from glass bakeware or blenders – in fact, based on 2006 data, you had a 1 in 5,613 chance of incurring a non-fatal injury from lifting, moving, and, believe it or not, watching TV.