New paper addresses causes of shattering glass cookware

http://ceramics.org/uncategorized/new-paper-addresses-causes-of-shattering-glass-cookware

New paper addresses causes of shattering glass cookware; margin of safety described as ‘borderline’

September 11, 2012

WESTERVILLE, OH —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 glass 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 silicate 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. 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).

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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.

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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 thermal stress failures of soda lime silicate cookware is borderline. It does not appear to be adequate for all household cooking.”