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August 31st, 2012

Hell’s kitchen: Thermal stress and glass cookware that shatters

Published on August 31st, 2012 | By: pwray@ceramics.org

A reconstructed soda lime silicate Pyrex glass bowl fractured by thermal shock. Credit: George Quinn.

Not all Pyrex glass cookware is made the same. Surprised? So was I when I first started looking into this story.

Corning devised and first manufactured the ubiquitous clear “Pyrex” cookware in the early part of the 20th century. In the beginning, it was made from low thermal expansion borosilicate glass, aka Corning 7740. Some borosilicate glass Pyrex (“original Pyrex”) products are still being made, such as Corning’s excellent line of laboratory ware (PDF) and Arc International’s glass cookware sold in Europe.

However, aside from its labware, Corning no longer makes Pyrex products and it has licensed the use of the Pyrex brand name for many years to World Kitchens LLC for sales of cookware in the US, and to the aforementioned Arc International.

But, here is where it gets confusing and problematic for consumers. World Kitchens’ Pyrex cookware is not made of the original Pyrex borosilicate glass composition. World Kitchens’ Pyrex-labeled cookware is composed of a soda lime silicate glass. (World Kitchen correctly notes that Corning, itself, changed the composition from original Pyrex to a soda lime glass before it got out of the business).

To make matters more complicated, the Anchor Hocking Glass Co. also makes clear glass cookware products using soda lime silicate glass, and has done so for decades. Apparently, Anchor Hocking first got into the market in the mid part of the 20th century by offering a something of a copycat product to compete with Corning.

Why am I explaining all this? Because borosilicate glass behaves very differently from soda lime glass, however, you don’t have to be a glass or materials scientist to know that. No less than the independent and reputable Consumer Union organization (publisher of Consumer Reports magazine and website) figured out there was a difference after it starting receiving reports of glass cookware suddenly shattering, including reports of dangerous, explosive-like failures and severe injuries when glass shards were sent flying several feet.

After learning of these anecdotal problems, Consumer Reports staff conducted a year-long investigation in which they tested various glass cookware in their labs, including those made by Anchor Hocking, World Kitchens and Arc International. They were able to show that under unprescribed, although not unreasonable circumstances (in the context of a typical kitchen, e.g., taken from a 450°F oven and put on a countertop where moisture is present), the soda lime cookware samples frequently were, in fact, prone to frequent failure. In contrast, the organization found that the borosilicate glassware is more resilient and did not shatter until removed from a 500°F oven. See the video below:

While Consumer Reports verified the existence of the shattering problem with the soda lime glass, it didn’t attempt to explain why the problem occurs. Leave that to R.C. Bradt and R.L Martens, who in the pages of a paper (“Shattering Glass Cookware”) in the September issue of ACerS’ Bulletin deftly lead readers through fairly simple materials science to make sense of these failures.

Bradt and Martens review concepts such as thermal stress, elastic modulus, thermal shock and temperature differentials and then apply them to the borosilicate and soda lime glasses.

Perhaps surprisingly, Bradt and Marten explain that their findings show that the glass failures (soda lime and borosilicate glass) have less to do with the maximum temperature of an oven and more to do with the total temperature change the cookware is subjected to. According to their calculations, soda lime glass cookware shatters more frequently because, in theory, it can resist fracture stress as long as the temperature differential is less than about 100°F. In contrast, borosilicate glassware can tolerate a differential of about 330°F. That, obviously, is a big difference. (Bradt and Marten acknowledge that time-dependent heat transfer conditions are another factor, as well as whether the cookware is pristine or contains nicks and scratches.)

While it is clear that borosilicate glass performs better, thermally speaking, than soda lime, World Kitchens and Anchor Hocking say their soda lime glassware is mechanically stronger than original Pyrex. Basically, their argument is that soda lime glass will not break as easily if it is dropped on a floor or countertop, or struck with a utensil.

In their defense, manufacturers of the soda lime cookware also say they use heat strengthening or thermal tempering to further increase its impact resistance and resistance to thermal stress fracture. However, Bradt and Marten also looked into this and found minimal, if any, evidence of thermal or heat strengthening in commercially bought soda lime silicate glassware.

Concerned? If so, you will want to read Bradt and Marten’s entire paper. The authors emphasize that consumers should read and follow the warnings contained in the glass cookware packaging. However, they sound a final note of caution, based on their research:

“[D]ocumented reports of incidents of dramatic shattering failures during what most kitchen cooks would consider normal use suggests that the margin of safety for avoiding thermal stress failures of soda lime silicate cookware is borderline. It does not appear to be adequate for all household cooking.”

Meanwhile, Consumer Union is urging the Consumer Product Safety Commission to delve deeper into the complaints about the soda lime silicate cookware.


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11 Responses to Hell’s kitchen: Thermal stress and glass cookware that shatters

  1. Jeanie Perkins says:

    Well, I’m no scientist. But I had an experience today that might be of interest. My 9 x 13 inch Anchor Hocking baking dish shattered to smithereens in an oven that was heating from zero to 450 degrees. I can’t imagine that the temperature change was sudden or dramatic. One would think that an oven wouldn’t be able to heat so quickly as to cause a drastic change in temp. The dish was room temperature when i put it in the oven. Luckily, our explosion happened within an enclosed oven. Very fortunate that no one was handling the pan or peeking in the oven at the time. The result would have bee serious injury. The dish didn’t just crack; it literally exploded, sending shards of glass in every direction. Literally an explosion. I am not a litigious person, but I feel I need to do something to try to change this very dangerous situation. I suppose I will file a complaint with the Consumer Protection Agency. Thanks for indulging me.

  2. Stephanie says:

    I work for a company that produces 3.3 borosilicate glass cookware. We sell our glass under the brand name Simax, and I truly believe it is the best, stongest, safest, and most hygienic cookware anyone can buy. With prices very comparable to that of Pyrex..

    I wish more people knew about the dangers of Pyrex, and how its thermal shock resistance in not NEARLY high enough for safe usage..

    If anyone is interested please check us out at http://www.simax.com to learn more about our borosilicate glass. We are starting to try and get distributors in North America right now.

  3. George Quinn says:

    Regarding the granite countertops: the Consumer Union video showed one single failure scenario. Anecdotal evidence indicates the fractures can occur under many diffeent scenarios, including even while inside an oven.
    I want to congratulate Professor Bradt and the American Ceramic Society for bringing this matter to the public’s attention.

  4. Darin Lindig says:

    I wonder if the perceived increase in glassware thermal shock failures has less to do with recent glass quality and more to do with an increased trend to install granite countertops. Everyone knew not to place a hot pan on a laminate countertop, but it is one of the touted benefits of granite.

  5. Emilio Spinosa says:

    Like Dr. Rabinovich, I was shocked when I read the Consumer Report investigation. Glass experts in the companies involved had to know that changing the composition from borosilicate to pyrex would result in catastrophic thermal shock failure. For companies to ignore that expertise and change the composiiton without properly notifying the consumer/user was unconscionable. The attempt at tempering was just window dressing. From my personal experience of 40+ years in the glass industry I am certain that the change was made solely to compensate for the lower profitability of original pyrex induced by increasing cost of B2O3 raw materials. Perhaps a comparison of European consumer protection laws to the same laws in the US would shed some light on why such a dangerous change was allowed to occur.

  6. Jim Barton says:

    I read the Consumer’s Report when it first came out, wrote to them, and suggested that they check the thermal expansion of each glass and determine the lithia value of each glass. Then they would probably see where the glasses are different and why the new glass shatters. Lithia lowers thermal expansion.

  7. Peter Wray says:

    As far as “measuring the thermal expansions plus chemical analysis of all glasses plus a specialist’s expertise” goes, that is where Bradt and Martens felt public discussion was sorely lacking, and they asked for our support in getting a fair and balanced report published.

    For consumers, another complicating factor is that a lot of “used” glassware is in circulation, i.e., handed down in families, purchased at yard sales, left behind at picnics, etc., So, they get no benefit from the warnings on the boxes, and even if it is labeled “Pyrex” there is no easy way to determine if it is the original composition or not.

  8. Dr. Eliezer M. Rabinovich says:

    Thank you for your response. I think that just measuring the thermal expansions plus chemical analysis of all glasses plus a specialist’s expertise would be sufficient for the legal action. I would say it is a CRIME to replace borosilicate (#7740) with soda-lime and still to claim that you can take it straight from a refrigerator to the oven. The prime guilt is with Corning that sold the brand name and permitted to use it with the soda-lime glass. However, I am sure Corning did not sell them the technology of this kitchen ware with the soda-lime composition.

  9. Peter Wray says:

    The reason it took a year for Consumer Reports to report on the matter is that, from what they told me, they had to focus on consumer reported experience and wade through a lot of anecdotal information, file freedom of information request with the Consumer Product Safety Commission, interview consumers, run their tests, etc. In addition, there are substantial legal issues that are involved with going to print about such things.

  10. Dr. Eliezer M. Rabinovich says:

    P.S. Also I do not believe that a heat treatment to increase the resistance of the soda-lime glass used for kitchen is possible. Such a treatment was used for automobile glass but it cannot work for kitchen ware.

  11. Dr. Eliezer M. Rabinovich says:

    As a glass scientist and engineer with a 50+ years of experience, I am shocked that somebody needs to spend a year to investigate the obvious thing. Years ago I was greatly surprised to find out that Corning permitted to use the Pyrex name for a soda-lime glass. The reason of the difference in the behavior is so simple and obvious that it could be predicted. Borosilicate glass has a linear coefficient of thermal expansion near 35×10^-7, while soda-lime – 80 to 90. This alone and only this alone makes the soda-lime glass to shatter at thermal shock while Pyrex – specially developed to withstand the thermal shock – will endure it. What a low level of the expertise we have now!

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