Shattering glass cookware has been in the news recently, and has been reported in Consumer Reports, in television news features, and many consumer feedback websites. Now a paper published in the September 2012 edition of the Bulletin of The American Ceramic Society for the first time offers a scientific reason why certain glass cookware is more susceptible than others to explosive shattering and the likelihood of exposing users to injury from airborne glass shards.
Clear glass baking dishes and pots and have been a staple in households worldwide since they were first introduced in 1915 to consumers by the Corning Glass Works company, which built the Pyrex brand name. The original Pyrex cookware was made using a particular, thermally robust composition called borosilicate glass.
Interestingly, the name Pyrex came from the accidental discovery that the composition, which was an attempt to improve the heat resistance of glass battery jars, could be used to cook food. The story goes that one of the Corning lab scientists, Jesse Littleton, one day brought home the bottoms of a few of the borosilicate glass jars for his wife to use as pie tins. The original name Py-Right was a phonetic nod to those home baked pies.
The durability of this glass, originally advertised as an “Oven to Icebox” and “Icebox to Oven” solution, was astonishing at first, but in due course came to be taken for granted by generations of owners who often passed on the rugged vessels to family members and friends. In later years, Corning competitors, and finally Corning, itself, substituted a different glass composition-soda lime silicate glass-for the borosilicate Pyrex cookware.
The End of Borosilicate the Start of Exploding Pyrex?
Corning currently licenses the use of the Pyrex brand name to World Kitchen LLC for sale in the United States, having sold its Pyrex cookware manufacturing to Borden KKR in 1997. The other main competitor in this field is Anchor Hocking Glass Co., which has made a similar 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.
The authors of the study, titled “Shattering Glass Cookware,” R.C. Bradt and R.L. Martens, became interested in the topic after listening to 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.
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 were related to vessels made of the soda lime silicate glass. Was any borosilicate glass cookware involved? On the contrary, 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, which is manufactured and marketed by a separate company, Arc International.
Materials Engineering to the Rescue
Bradt, a professor emeritus of materials engineering at the University of Alabama, along with Martens, from the university’s Central Analytical Facility, hunted for reasons why the soda lime silicate glassware is subject to thermal stress failure. They used fundamental materials engineering concepts, including thermal stress, elastic modulus, thermal shock and temperature differentials to investigate the borosilicate and soda lime glasses.
What they found confirmed that the borosilicate glass could withstand a much larger rapid temperature change. According to their calculation and those of others, soda lime glass cookware shatters more frequently for the reason that, 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.
Manufacturers Cautions and Claims
The two do state that all manufacturers of glass cookware caution 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 exploding pyrex effect. But in practice, the authors also note, 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 looked into 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, if breakage occurs, to promote 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.”
Consumers should read and follow all warnings contained in the glass cookware packaging, Bradt and Martens highlight. However, based on their research, they finish with a warning 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.”
Pyrex bowl photograph courtesy of G. Quinn.