In this month's Bulletin of the American Ceramic Society, University of Alabama engineering professor R.C. Bradt and U of A staffer R.L. Martens describe how they used their scientific know-how to figure out a distressing kitchen problem: shattering glass cookware.
The two investigators' interest was piqued by hearing anecdotes about the phenomenon, often resulting in injuries, in publications like Consumer Reports. In one incident, an Illinois woman, her daughter and granddaughter suffered burns and cuts after their glass baking dish exploded when the opened an oven door to baste a ham.
The original Pyrex cookware developed by Corning Inc. in the early 20th century used borosilicate glass, which is made from silica and boron oxide. Because of the chemical properties of borosilicate glass, it is much more resistant to the effects of high temperatures, including expansion and stress, than other forms of glass. That hardiness is why borosilicate glass is the gold standard for laboratory glassware.
For cookware, the resilience of borosilicate glass meant that it could survive sudden changes in temperature without cracking - hence the original marketing of Pyrex as something that could be used "oven to icebox," and vice versa.
And while Corning still holds the rights to the Pyrex name, it has licensed the brand to other companies, which make glassware using a different kind of glass called soda lime silicate glass. Even Corning has switched to making its glass cookware with the soda lime glass. Consumer Reports noted that there are no reports of similar explosive failure in Europe, where nearly all the glass cookware is still made with borosilicate.
Bradt and Marten tested the two kinds of glass, and found that soda lime indeed is weaker - while borosilicate glassware can handle a temperature change of up to 330 degrees Fahrenheit, soda lime glass can only tolerate a temperature change of up to 99 degrees Fahrenheit.
While most manufacturers do warn against putting heated glass cookware directly onto a countertop or otherwise inducing a rapid temperature change, normal kitchen use can be enough to overwhelm soda lime glass, according to Bradt and Martin.
"From the perspective of kitchen applications, a good calibration point is that of boiling water 100°C (212°F)," Bradt and Martin wrote. "None of [our] calculations suggest the soda lime silicate glass would be likely to survive a rapid exposure to boiling water."
Many manufacturers claim that soda lime glass is treated with tempering techniques that both increase its resistance to thermal stress fracture and ensure that even if a glass dish does break, it shatters into small dice shapes rather than dangerous shards. Bradt and Martin found that there is some evidence of heat treatment in soda lime glass, but not enough to truly increase its resistance or create a less dangerous fracture pattern.
"The margin of safety for avoiding thermal stress failures of soda lime silicate cookware is borderline," they wrote. "It does not appear to be adequate for all household cooking."
SOURCE: Bradt et al. "Shattering glass cookware." American Ceramic Society Bulletin 91: 33-38, September 2012.