Pyrex, Soda Lime, Borosilicate, and the Environment

I noted in an earlier post that STATS was looking into the environmental impact of making Pyrex with soda lime glass instead of boroscilicate glass. The switch has been blamed for a spate of “explosions” as Pyrex made with soda lime has a greater rate of thermal expansion than Pyrex made with borosilicate: if you put a hot dish on a cold, wet surface, it could shatter, warns the manufacturer.

STATS spoke with Phil Ross, an independent consultant to the glass industry, whose clients have included World Kitchen, which makes Pyrex in the United States). He said the industry as a whole switched from borosilicate started in the 1980s for a variety of reasons, including the fact that soda lime was easier to melt and work with (fewer deformities in the glass). But one of the major reasons for moving to soda lime was environmental compliance: borosilicate glass produces far more emissions from a glass furnace , accounting, in part, for the presence of boric acid in the water and soil. And it was not economical for companies to install multi-million dollar filter systems.

Add to that, Ross says, the fact that furnaces have a longer lifespan if you use soda lime, and require less energy (15 to 20% lower than for borosilicate), and the economics for the switch were compelling.

These aspects led the entire glass bakeware industry in the U.S. to switch to soda lime starting in the 1980s (borosilicate is still used for laboratory glassware). Given the length of time soda lime has been used in bakeware multiplied by the number of times an individual piece has been used in a kitchen, the numbers cited by CBS and other local television news outlets for a trend in exploding dishes are tiny. If there was a substantial flaw in soda lime bakeware, or if heat tempering was insufficient, it’s reasonable to wonder why breakage isn’t more common. Literally, hundreds of millions of dishes are being used multiple times a week without apparent incident.