If you’ve ever poured hot water into a Pyrex glass dish and been shocked to see it fracture before your eyes, a new report may give you some insight into what’s going on. Pyrex glassware, which came out in 1915 and was long marketed as “icebox to oven” cookware that did not expand or compress when exposed to high heat or low temperatures, is no longer made of that hardy borosilicate glass. And the new stuff, scientists publishing in the American Ceramics Society Bulletin have found, doesn’t stand up well to some of the temperature changes involved in cooking.
Over the past century or so, many cooks have gotten used to the idea of glass cookware that’s heat- and cold-resistant. However, in the early 90s, Corning, the company that invented Pyrex, started using soda lime silicate glass instead of borosilicate (another manufacturer now owns the line). The switch was, its makers say, to boost the glassware’s ability to withstand being dropped.
Several materials scientists, though, have concluded that various lines of soda lime silicate cookware leave something to be desired. Just running the numbers on the physics of the glass, they found that the temperature change needed to break borosilicate glass is more than 300 degrees F, while soda lime silicate glass will shatter after a change of just 99 degrees F. Water boils at 212 degrees F, so you can see how pouring boiling water into a soda lime silicate measuring cup would produce explosively different results than pouring it into a borosilicate one. “Even at modest kitchen temperatures,” the scientists write, “there is a definite possibility of thermal shock fracture.”
Various lab tests by Consumer Reports and others support this conclusion—see the video above. And the team found that though the glassware is heat-tempered, as auto glass is, so it should fall into small cubes rather than shatter, the tempering process didn’t have the intended effects—the glass cookware still broke into long, sharp shards.
Scientists using Pyrex-brand glassware in the lab can breathe easy—that stuff is borosilicate still, the researchers say. But to our cooking readers: be careful in the kitchen.