Pyrex Glass Isn't As Shatterproof As It Once Was, Report Finds

By Veronique Greenwood | September 13, 2012 3:39 pm

If you’ve ever poured hot water into a Pyrex glass dish and been shocked to see it fracture before your eyes, a new report may give you some insight into what’s going on. Pyrex glassware, which came out in 1915 and was long marketed as “icebox to oven” cookware that did not expand or compress when exposed to high heat or low temperatures, is no longer made of that hardy borosilicate glass. And the new stuff, scientists publishing in the American Ceramics Society Bulletin have found, doesn’t stand up well to some of the temperature changes involved in cooking.

Over the past century or so, many cooks have gotten used to the idea of glass cookware that’s heat- and cold-resistant. However, in the early 90s, Corning, the company that invented Pyrex, started using soda lime silicate glass instead of borosilicate (another manufacturer now owns the line). The switch was, its makers say, to boost the glassware’s ability to withstand being dropped.

Several materials scientists, though, have concluded that various lines of soda lime silicate cookware leave something to be desired. Just running the numbers on the physics of the glass, they found that the temperature change needed to break borosilicate glass is more than 300 degrees F, while soda lime silicate glass will shatter after a change of just 99 degrees F. Water boils at 212 degrees F, so you can see how pouring boiling water into a soda lime silicate measuring cup would produce explosively different results than pouring it into a borosilicate one. “Even at modest kitchen temperatures,” the scientists write, “there is a definite possibility of thermal shock fracture.”

Various lab tests by Consumer Reports and others support this conclusion—see the video above. And the team found that though the glassware is heat-tempered, as auto glass is, so it should fall into small cubes rather than shatter, the tempering process didn’t have the intended effects—the glass cookware still broke into long, sharp shards.

Scientists using Pyrex-brand glassware in the lab can breathe easy—that stuff is borosilicate still, the researchers say. But to our cooking readers: be careful in the kitchen.

CATEGORIZED UNDER: Physics & Math
  • http://sep.stanford.edu/sep/prof/ Jon Claerbout

    Pyrex was formerly made by Corning. The name was sold off to a company that went bankrupt. Somewhere I seem to have read the product is now made in China, but I can’t track this down right now.

    I have a bicycle beautifully adorned with the “John Deere” label. It’s not made by the reliable company you might hope.

  • Fern

    One more reason I will prize the old Pyrex baking dishes handed down from my grandmother.

  • Brian Too

    This seems like a very plausible failure mode for a dish. Just imagine accidentally burning a casserole. Lots of people would take the dish out of the oven and put it in the sink, either with or without water in the basin.

    Now factor in the consumer education angle. There are decades of experience with cooks around the world, using Pyrex. They build up expectations of what such cookware can and cannot take. It’s safe to say that Pyrex has a pretty good reputation for durability and heat resistance.

    This change in product composition tends to violate those consumer expectations. Of course if the soda lime silicate cookware really is more resistant to drop damage, that is not a bad thing either and might be worth the tradeoff for reduced heat shock resistance.

  • Simon

    Phew, this only applies to US Pyrex. The stuff we get over here in Europe is still the borosilicate glass.

  • Georg

    Corning, the company that invented Pyrex,

    Nonsense, born out of American bigmouthedness.
    Borosilicate glass was invented by Schott & Gen
    in Jena long before that date. I guess in 1905 thes
    “invented” the brand “Pyrex” , a very brave deed, indeed.

  • runa

    Yes, Georg, that´s true.
    BUT in the last years I encountered dishes sold as “Jenaer Glas” IN GERMANY that exploded just like seen in the film.
    Only ONE piece of my mother´s which survived without being broken in all the years behaves like I was used to.
    So this firm must have tampered with the formula without announcing it, too.

  • John Lerch

    And certain people say that free enterprise creates a better product.

  • Enrique Segre

    So who makes borosilicate kitchenware?

    Or do we have to start cooking in Pyrex labware?

  • Nate

    @Georg

    No where in the article does it say Corning invented borosilicate glass. Just a brand of dishes everyone is familiar with. The average consumer has no clue what it is made of, nor do they care. The brand, in this context, IS the important part to note, as its comparing older dishes against newer dishes.

  • Chuck

    Yes Georg, it does say that in 1915 they “invented” the brand “Pyrex.” Thats how it reads to anyone who speaks English. It doesn’t say they invented borosilicate glass.

  • Georg

    The real problem I see, is that a company with such a long tradition as Corning sells a brand name to “somewhere”. Don’t they fear that their reputation is damaged?

    Schott (“Jenaer”) stopped to produce bakeware in 2005, but they sold the brand name to Zwiesel, another well known German Glass producer. They and Sovirel (Saint Gobain, France) produce the old quality, any deviation would cause an uproar.

  • http://estherkimcotton.com Esther

    I had a Pyrex dish explode in my hands before! Scared the crap out of me, and I was surprised because it was always marketed as such a durable “icebox to oven” product. Apparently, not! “They don’t make ‘em like they used to” is a phrase that seems to be growing…

  • David_42

    This was noted at a Science Pub a few months ago. The presenter shattered a measuring cup with hot water. A cop in the audience said meth makers are being “forced” to steal lab equipment because consumer Pyrex isn’t safe.

  • Gizmonic Man

    Unfortunately, it’s the same old story–the product was changed because it was deemed necessary by World Kitchen management to make it cheaper. But in this case, cheaper isn’t better. Soda-lime glass is not Pyrex–it’s just glass, very close to what is in your home’s windows or a Coke bottle. Sure, it’s tempered, but that doesn’t seem to keep it from shattering due to thermal upshock or downshock, or to prevent it from breaking into dangerous shards, when it inevitably does.

    The justification offered by the company for changing the formulation of the glass is half-right—it’s cheaper. The other half–added resistance to breakage by dropping–sounds pretty fishy to me. Judging by the complaints, it sounds like there is more Pyrex breaking these days, not less.

    In my view, the brand name Pyrex means borosilicate glass, not Coke-bottle glass. It is a major disservice to consumers to sell cheaper, substandard stuff under the old and respected brand name of Pyrex. A classic case of short-term thinking and selling out the brand name, making money until the word (maybe) gets out that today’s Pyrex glass isn’t what the word used to mean. Sad, but not totally unexpected these days.

    I think a new name and slogan is justified, like “Not Really Pyrex–but it’s cheaper to replace!”

    Or maybe just “Lyrex”. Sort of says it all.

  • Paul A.

    So who still makes consumer kitchen glassware out of borosilicate?

  • BradS

    Regarding Georg’s somewhat combative comment about Corning inventing Pyrex.

    The article never said that Corning invented the entire family of low expansion borosilicate glass, which is the type of glass Pyrex was originally – and is sometimes still – made of. “Pyrex” is a brand name that was originally applied to one particular glass formulation used for borosilicate ovenware (although that brand name has since been expanded to cover a number of different types of heat resistant glasses).

    Corning most definitely invented – and patented – the original Pyrex glass formulation. While it was similar to the low expansion borosilicate glass invented by Otto Schott in 1893 (brand name “Duran”), the composition of the Corning glass was not the same.

    An analogy would be that Chevrolet invented the Corvette, which is both an automobile and a sports car. But Chevy never claimed to have invented either the automobile or the sports car. The same is true of Corning. They invented “Pyrex”. But nowhere in the article is it claimed they invented either glass or borosilicate glass.

    Calling people names based on one’s misreading of an article shows who has the problem of “bigmouthedness”.

  • http://www.ceramictechtoday.com Peter Wray

    @Jon Claerbout, Corning licensed the Pryex name to Borden, that did go thru some financial and ownership changes, but eventually morphed into World Kitchen. Also, it appears that the US-sold Pyrex is made in the US (World Kitchen has at least one plant in New York State). @Brian Too Says, yes, the expectations are a problem and, anecdotally, we also hear they are being compounded by countertop makers who market the heat resistance of their materials, e.g., granite or granite-like materials. @George Says, it does appear that Schott developed the borosilicate glass composition. Then both Schott and Corning started developing applications. The history of Pyrex seems to be that Corning was using borosilicate glass for a battery casing and a worker had the idea of taking a unit of the casing and seeing if his wife could cook with it. Corning gets credit, as far as I can tell, for being the first to market with borosilicate cookware.

  • Ravius

    My youngest son a few months ago burned some brownies in the oven (350 F) in a “Pyrex” baking dish. They started to smoke so he grabbed them out of the oven and ran the tap to get the hottest water then put the dish into the stream. The dish literally exploded in his hands throwing sharp shards of glass all over the kitchen and cutting his hands some. Thank goodness he looked away or he would have very easily lost his sight. I have used Pyrex for years, and have sometimes put hot pans into hot water without thinking before, and they have never had a problem. I was at a loss to explain to him what happened outside of contraction of one surface due to rapid cooling, etc. . . Now I understand, the new versions of Pyrex dishes are not the same. Needless to say, I had already went back to good old metal bake-ware, and I would advise anyone, especially those with children, to consider doing the same. I can attest to the violence in which the dishes can shatter.

  • Michael

    I doubt Corning worries about their reputation. They are far too busy with iPhone glass and fiber optics to worry about kitchen glassware. Do people even associate Pyrex with Corning anymore?

    Thanks for the alterate brands, Georg. I’ll look for those.

  • DeWitt

    All the more reason to shop second hand shops for Pyrex!

  • Signup

    @ Jon Claerbout:

    All Pyrex glassware are made in the USA by a US company (and still in business). (http://www.snopes.com/food/warnings/pyrex.asp)

    We are just as capable of making substandard products as the Chinese (assuming that that’s the case here.)

  • Mike

    I have a box floor fan made in China that has an Apple logo on the side. We can’t trust Chinese goods in general.

  • Mike

    Bought it at Walmart several years ago.

  • Roberta

    This would explain why my “Pyrex” baking dish exploded in the oven when I added boiling water to a partially-cooked squash. Frightening indeed, as sharp shards of glass went everywhere, and yes, it’s a good thing I did not have my face too close. It’s a shame, as I love my old Pyrex bakeware but will no longer be buying new Pyrex. Will have to go to metal for baking and plastic for measuring cups where temperature changes are involved. Where is the testing and consumer warnings from the company to consumers? I will be passing this info along to my friends.

  • Jory

    Yea…I specifically bought this shit under the impression they still used borosilicate and that it was still as shatter-resistant as the classic stuff. :
    Anything to make a buck damn it…

  • http://www.facebook.com/profile.php?id=754065664 Cathy Hinga

    Mine too. Shattered all over, tossing hot glass around the kitchen.

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