You might want to use a little extra caution the next time you pop a pyrex baking dish into the oven because Consumer Reports has noticed an uptick in the number of incidents involving exploding or unexpectedly shattering glass bakeware that have been reported recently. Not all of these incidents result in injury, but even if you don’t end up with a burn or a cut from a hot piece of glass, you could very well end up with an oven full of shards.
Pyrex and other glass bakeware is traditionally made of borosilicate, a type heat resistant glass that is fantastic for baking because it can easily withstand the high temperatures of the oven without breaking or cracking. This type of bakeware is even strong enough to hold up to sudden, sharp temperature changes. So why are some of them shattering unexpectedly when put into the oven or another high heat environment?
At some point, makers of glass bakeware, especially in the United States, switched from using borosilicate to using soda lime glass, which is less expensive to produce. Pyrex manufacturers in Europe still use borosilicate and any glass pans that you inherited from your parents or grandparents are made of borosilicate. Soda lime glass can more impact-resistant than borosilicate is, but it is not as resistant to heat or abrupt temperature changes as borosilicate is. Manufacturers World Kitchen and Anchor Hocking state that they put their soda lime glass through a thorough thermal tempering process to maximize its strength, but most of the reported incidents are from their products (not that surprising, since they produce about 75% of the glass bakeware in the US) and many of the incidents involved spontaneous breakage, rather than misuse or exposure to unusual conditions.
Consumer Reports put together a great video that shows the difference between borosilicate and soda lime glass. Glass bakeware is still safe to use, but if you have older pieces they are probably stronger and much less likely to shatter under normal use conditions.