The linear elastic thermal stress,, on a glass is a measure of the stress the glass can withstand without shattering: The following equation provides a simple quantitative relationship between thermal shock and coefficient of thermal expansion. On the other hand, soda lime glass has a high coefficient of thermal expansion (9 x 10 -6 K -1), which means that it will undergo sizeable expansion upon heating and contraction upon cooling. Borosilicate glass has a low coefficient of thermal expansion (3 x 10 -6 K -1), which means that borosilicate glass does not undergo sizeable expansion upon heating or contraction upon cooling. The difference in heat shock resistance between borosilicate glass and soda lime glass can be quantified using some simple relationships. Now Pyrex is simply a brand name that has nothing to do with the chemistry of the glassware being sold. Imagine my surprise one day in the early 2000’s when I decided it would be safe to boil water in a Pyrex measuring cup directly on the stovetop.the measuring cup cracked, sending broken glass and hot water all over the stove and floor! In fact, reports of “exploding glassware” began to crop up as other folks began to unwittingly expose World Kitchens’ soda lime glass to extreme temperature differentials while working in the kitchen. World Kitchens made this change because soda lime glass is cheaper to manufacture and more resistant to breakage from mechanical stress than borosilicate glass. However, World Kitchens began to sell soda lime glass for kitchen use – also under the Pyrex name.

After the sale, Corning continued to sell borosilicate glass for laboratory use under the Pyrex name. sold rights to the Pyrex name to World Kitchens. This was indeed the case until the late 1990’s when Corning, Inc. I had always assumed that Pyrex glass was synonymous with borosilicate, thermal-shock resistant glass. This difference can be important! How many times have you rinsed a very hot beaker with room temperature water when working in the lab? I have done so many times, knowing that borosilicate lab glassware is capable of withstanding large differences in temperature without cracking.

The video below dramatically shows the effect of thermal shock in a measuring cup made of Pyrex soda lime glass. What’s the difference? Borosilicate glass is resistant to thermal shock, but soda lime glass is not. Did you know that Pyrex glassware used in chemistry labs is different than Pyrex glassware used in kitchens? Pyrex glass used in chemistry experiments is made of borosilicate glass, whereas the Pyrex used when baking is made of soda lime glass.