New paper addresses causes of shattering glass cookware; Margin of safety described as 'borderline'

Sep 12, 2012
A reconstructed fractured soda lime silicate Pyrex bowl. Arrows outline crack paths. Credit: George Quinn

A new paper appearing in the September 2012 edition of the Bulletin of The American Ceramic Society for the first time provides a scientific explanation of why some glass cookware sold in the United States is more susceptible than others to "explosive" shattering and the possibility of exposing consumers to injury from flying glass shards.

Clear baking dishes and pots are a staple in many households around the world and have been since they were first introduced in 1915 to consumers by the Corning Glass Works, which created the Pyrex brand name. The original Pyrex cookware was made of a specific, thermally strong composition known as borosilicate glass. The durability of this glass, originally advertised as an "oven to icebox" and "icebox to oven" product, was startling at first, but eventually became taken for granted by generations of consumers who often passed on the rugged vessels to family members and friends. In later years, Corning competitors, and eventually Corning, itself, substituted a different glass composition—soda lime glass—for the borosilicate Pyrex cookware.

Corning exited the Pyrex cookware manufacturing business in the late 1990s, and currently licenses the use of the Pyrex brand name to World Kitchen LLC for sale in the United States. The other main competitor in this field is Anchor Hocking Glass Co., which has made a similar looking soda lime silicate glass product for over 60 years. Both World Kitchen and Anchor Hocking now use the soda lime silicate glass instead of the original Pyrex borosilicate glass composition.

R.C. Bradt and R.L. Martens, the authors of "Shattering Glass Cookware," became interested in the topic after hearing anecdotal reports of glass cookware shattering and reading reports of cookware failure and related injuries in publications such as the January and October 2011 issues of . Consumer Reports and others documented that the explosion-like glassware failures seemed to be linked to rapid changes in temperature, such as when the cookware was removed from the oven and placed on a counter or dinner table. The publication also noted that virtually all of the reports of glassware failure involved vessels made of the soda lime silicate glass. In contrast, the magazine reports that there are no reports in Europe of explosive cookware failure, a region where nearly all of the products sold are composed of the authentic borosilicate glass (manufactured and marketed by a separate company, Arc International).

Bradt, a professor of materials engineering (emeritus) at the University of Alabama and Martens, manager of the university's Central Analytical Facility, sought to make sense of why the soda lime silicate glassware is prone to thermal stress failure. Bradt and Martens applied basic materials engineering concepts—such as thermal stress, elastic modulus, thermal shock and temperature differentials—to the borosilicate and soda lime glasses.

Their investigation confirmed the borosilicate glass would withstand a much larger rapid temperature change. According to their calculation and those of others, soda lime glass cookware shatters more frequently because, in theory, it can only resist fracture stress for temperature differentials less than about 55°C (99°F). In contrast, they estimate that the borosilicate glassware could tolerate a temperature differential of about 183°C (330°F), a three-fold difference.

Bradt and Marten do note that all manufacturers of glass cookware warn consumers against placing hot vessels directly on a countertop or a moisture-containing surface as a precaution against rapid temperature changes that could trigger the explosive-like shattering. Practically speaking, however, the authors also note that a typical kitchen might easily be an environment for glassware failure. "From the perspective of kitchen applications, a good calibration point is that of boiling water 100°C (212°F)," write Bradt and Martin. "None of [our] calculations suggest the soda lime silicate glass would be likely to survive a rapid exposure to boiling water."

Bradt and Martens also investigated manufacturers' claims that they use heat strengthening or thermal tempering processes to increase the soda lime silicate cookware's resistance to thermal stress fracture and, should breakage occur, encourage the formation of small glass "dice" instead of sharp glass shards. After using a variety of methods, including fracture and fringe pattern analysis, Bradt and Martens say they found some evidence of heat treatment, but warn the treatment "does not appear to be sufficient to increase substantially the thermal stress fracture resistance of the cookware, nor is it sufficient to create a desirable dicing fracture pattern for the glass cookware."

Bradt and Martens emphasize that consumers should read and follow the warnings contained in the glass cookware packaging. However, based on their research, they conclude by warning, "the margin of safety for avoiding failures of soda lime silicate cookware is borderline. It does not appear to be adequate for all household cooking."

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More information: The story, "Shattering Glass Cookware" is available online at americanceramicsociety.org/bul… _files/sept_12/#/35/

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User comments : 12

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chuckscherl
1 / 5 (4) Sep 12, 2012
Og goody!!! Yet another excuse for government regulation
ormondotvos
5 / 5 (5) Sep 12, 2012
Yeah, why should the government regulate the shattering of glass cookware advertised as "icebox to oven" after the manufacturer makes it much more dangerous by cheapening the glass?

Let the buyer beware!
jerryd
5 / 5 (4) Sep 12, 2012
Again a company saving a few cents and injuring many. This happened to me and only because I was quick I didn't get burned badly.

The old Pyrex was great stuff so I look in yard sales to find a set and spares as I won't use the new stuff anymore.It's also really easy to keep clean as little sticks to it.
VendicarD
5 / 5 (1) Sep 13, 2012
Another case of Inferior American made Products.

"Let the buyer beware!" - ormondotvos

I agree. Before a purchase, American consumers should pull out their chemical test kit and lab glassware and do a chemical analysis of the glassware they intend to purchase in order to verify that it is capable of withstanding the temperature changes they plan to expose it to.

Then later they can entertain their Martian house guests without fear of an interplanetary glass shard incident like the one that soured Human/Martian relations back in 1968.

Martians don't like exploding glassware.
scidog
not rated yet Sep 13, 2012
oh goody another reason to buy stuff from europe...
cdt
5 / 5 (2) Sep 13, 2012
Another step toward America becoming a true 3rd world country with a 1st world military, stemming from the mentality that a drop in quality is ok as long as the drop in manufacturing cost is greater. It's nice to live somewhere where that isn't the norm, and where exploding glass is something that you only hear about in foreign news reports.
Torbjorn_Larsson_OM
5 / 5 (1) Sep 13, 2012
Duly noticed, not put glass into boiling water (unless certain it is equivalent to chemistry glassware.

"Yet another excuse for government regulation".

Well, yes. It works - in Europe.

You *could* be reactive on an anecdotal basis instead of being proactive on a factual basis. But you condemn people, even informed consumers, needlessly to death and damage.

We have a minimum of government because it reduces conflict but also because it prevents hardship. It is the moral thing to do.
Vendicar Dickarian
2.3 / 5 (3) Sep 15, 2012
Another step toward America becoming a true 3rd world country with a 1st world military, stemming from the mentality that a drop in quality is ok as long as the drop in manufacturing cost is greater. It's nice to live somewhere where that isn't the norm, and where exploding glass is something that you only hear about in foreign news reports.


Yeah? Where would that be? Where's the mystical utopia you live in where manufacturers don't make production errors. Let me guess - you won't answer and I won't get a chance to rebut your ridiculous comments.
alfie_null
not rated yet Sep 16, 2012
No restriction on selling, but the FTC should require manufacturers to cease making any claims that the sodium glassware is suitable for use in the oven or stove. That would include pictures or anything else that might lead consumers to assume otherwise. Rather, there should be reasonably prominent warnings against such use.

There's something wrong with the system when consumers (in the U.S.) don't yet have a choice, close to two years after the defects of this type of glassware were publicized.

Understandably, manufacturers are reluctant to market safe but more expensive cookware when their competitors can market something that appears to be comparable, at a lower price. Labeling would help consumers distinguish.
Vendicar Dickarian
1 / 5 (3) Sep 22, 2012
Another step toward America becoming a true 3rd world country with a 1st world military, stemming from the mentality that a drop in quality is ok as long as the drop in manufacturing cost is greater. It's nice to live somewhere where that isn't the norm, and where exploding glass is something that you only hear about in foreign news reports.


Still no response. Because all this chickenshit can do is flaunt his rather obvious America-envy.
Vendicar Dickarian
1 / 5 (2) Sep 22, 2012
Ah, the robo-lite strikes again. Successfully voted on every single one of my posts. A "1" of course. Seriously dude, get a life.
cdt
5 / 5 (2) Sep 22, 2012
It's nice to live somewhere where that isn't the norm, and where exploding glass is something that you only hear about in foreign news reports.

Yeah? Where would that be? Where's the mystical utopia you live in where manufacturers don't make production errors. Let me guess - you won't answer and I won't get a chance to rebut your ridiculous comments.


Sorry, didn't notice your response until now. Answer: grew up mostly in the US until age 28, been living in Japan for about 20 years since then apart from a couple of extended trips back to the US. No envy whatsoever for people living in the US. Even the rich have to deal with poor quality and even poorer service -- money can't buy what doesn't exist, after all.

And please cut the chickenshit crap. It demeans you. I know you don't mind, but I believe the majority of us do.