Pyrex eventually switched to tempered glass most likely because boron is toxic and expensive to dispose of. Although tempered glass can better withstand thermal shock than regular soda-lime glass can, it's not as resilient as borosilicate. This is what causes the shattering reaction people are talking about.
Both the old and newer soda lime versions of Pyrex can break when dropped. However, the soda lime glass is said by the manufacturer to be less likely to break when dropped. Still, the old borosilicate glass is so very much stronger to rapid changes in temperature, so I always choose that instead.
The Bottom Line
Pyrex: Brand name for cookware brand. pyrex: Lowercase indication of containers that should not be heated in the oven or microwave. These are most likely made of soda-lime glass. PYREX: Uppercase label on cookware that is made of borosilicate glass and can be heated in a microwave or oven.
On a molecular level: heated material expands, while cooled material contracts. Brittle materials like glass are much more vulnerable to thermal shock, because they break when their molecules quickly expand under heat or contract under cold.
According to Pyrex's safety and usage instructions, “While the glass is designed for temperatures typically used in baking, it can break when exposed to the direct heat element while the oven is preheating.”
Borosilicate glass includes boron trioxide, which has a low thermal expansion. This suggests unlike normal glass, it won't break when exposed to major temperature shifts such as taking a dish from a fridge to an oven.
Thermal Stress:
When glass is exposed to a sudden temperature change, such as moving from a cold environment to a hot environment or vice versa, different parts of the glass will expand and contract at different rates. This creates stress points in the glass, which can cause it to crack or shatter.
While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subjected to rapid or uneven temperature variations.
Pyrex, made of borosilicate glass, has a low coefficient of thermal expansion. This means it doesn't change size drastically with temperature shifts, preventing cracks when moved from hot to cold environments or vice versa.
Tempered Soda Lime. The various formulations for Pyrex glass have been a source of controversy for many, as well as a source of consumer lawsuits over shattering bakeware.
In the U.S., modern Pyrex cookware is made from a type of tempered glass called soda lime glass, which isn't nearly as durable as the glass used in older pieces called borosilicate. Incredibly tough, borosilicate has one especially notable superpower: the ability to endure sudden shifts in temperature.
Pyrex products were made of borosilicate glass until the 1950s. At that point, they switched to manufacturing products with tempered soda lime instead. Today, every piece of bakeware made in the U.S. uses tempered soda lime, including the full line of Pyrex consumer glassware.
If you're interested in unbreakable glass that truly lives up to its name, polycarbonate panels are the type you want. Polycarbonate panels are significantly more difficult to break than both standard glass windows and laminated glass mentioned above.
Avoid Thermal Shock: Even though Pyrex is designed to handle heat, sudden temperature changes can cause it to crack or shatter. Avoid putting a cold Pyrex container directly into a hot microwave.
PYREX vs pyrex Construction Differences
Corning used borosilicate to produce all Pyrex products. However, the company that purchased the cookware products switched to soda-lime glass, adopting the name pyrex (spelled with all lowercase letters).
Borosilicate glass is more expensive to produce than ordinary glass due to the inclusion of boron trioxide, which adds to its thermal and chemical resistance. This increased production cost is often passed on to consumers, making borosilicate glass products more expensive.
Because borosilicate is the most durable when it comes to withstanding temperature changes, it is generally considered the safest choice for glass bakeware.
Microscopic flaws, scratches, and chips in the glass surface act as stress concentration points. Over time, especially if exposed to temperature changes or impacts, these weak spots give way and the glass breaks.
Place the pan on top of an unheated metal baking sheet before sliding it into the oven. This helps prevent abrupt temperature spikes that could damage the glass. This little trick smooths out any sudden spikes in temperature, keeping your glassware safe from shattering.
As we know, glass will contract and expand due to temperature changes, therefore, if there is not enough space within the frame for the glass to expand, it can cause the glass to shatter.
This being said, you do need to follow one major rule when using Pyrex: Avoid extreme temperature changes. Shifts from one extreme to the other can cause thermal shock to the dish which can cause it to crack or shatter entirely.
Don't pour cold water into a hot pan or put it under the broiler. Don't drop it, don't put it on a hot sheet pan etc. Should be just as safe as anything else like casserole dish.
This difference means that Borosilicate glass does not expand as much on heating, so it is less likely to break when heated. Pyrex is one particular blend of Borosilicate glass, with a particularly high heating tolerance.