It breaks down into the claim by CBS’s experts (one of whom is acting as an expert witness in a lawsuit against World Kitchen for a product other than Pyrex) that the glass Pyrex is now made from (heat-treated soda lime) isn’t able to withstand temperature changes in the way the old Pyrex (borosilicate glass) could.
True. The coefficient of thermal expansion for borosilicate glass is 35 x 10 to the power of minus seven, inch per inch per degree centigrade, while that of heat-strengthened soda lime glass is 85 x 10 to the power of minus seven, inch per inch per degree centigrade. This means soda lime glass has three times the thermal expansion of borosilicate glass, which accounts for greater dynamic breaking when a hot soda lime dish encounters a cold, wet surface – and why the manufacturer warns not to expose dishes to such temperature conditions. CBS 2’s experts claim that U.S. made Pyrex isn’t “tempered” enough; World Kitchen says their product is.
Soda lime glass is cheaper and easier to make than borosilicate glass (which is still used in European-manufactured Pyrex), and, of course, that raises the specter of a manufacturer cutting corners for the sake of profit; on the other hand, manufacturing soda lime appears to be more environmentally friendly in terms of industrial emissions (we’re chasing down the data on that one).
But here’s what CBS 2 didn’t tell viewers: Heat-treated soda lime glass has nearly double the mechanical strength of borosilicate glass. Both soda lime and borosilicate glass can withstand pressures of roughly 6,500lbs per square inch before breaking; but heat-strengthening soda lime can add another six to seven thousand pounds per square inch of mechanical strength. This means that strengthened (sometimes call tempered) soda lime bakeware is less likely to break if you hit it or drop it.
As this is the most common way people injure themselves from glass bakeware, isn’t this aspect something which CBS should have mentioned? Think about the emergency room data. If you query the Consumer Product Safety Commission’s emergency room database of product related injuries (enter code 461), you’ll find that the most common injuries from glass bakeware are lacerations that occur after a dish has been dropped. Injuries from a thermal downshock “explosion” are much more rare. In fact, based on emergency room data for 2005, you had just a 1 in 3,706, 338 chance of sustaining a non-fatal injury from glass bakeware that didn’t shatter from mechanical breakage.
The injuries – two burns and a cut foot – reported by CBS 2 are unfortunate, and their cause alarming. The 300 reports of similar explosive accidents require much more systematic evaluation before they can count as evidence that something unusual is going on. In sum, we need to be guided by science and not just sympathy in risk evaluation. After all, we’re talking about glass; indestructibility and perfect safety can’t be the standards for measuring risk.
As with every product: read the warning label.