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.
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.
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.
Pyrex is oven-safe up to 425 degrees. However, direct contact with heating elements can cause the glass to shatter or break. Pyrex is not oven-safe when used under the broiler or in a toaster 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.
"Heat expands-cold contracts. This is why sudden change of temperature breaks any ordinary glass. Pyrex contracts so slightly that sudden heat cannot break it." Image: From freezing to boiling without breaking.
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.
The intense temperature of an oven will cause the ordinary glass to crack. Different parts of a piece of bakeware can expand or contract at varying rates and cause kitchen and bakeware to lose its core structure and result in the dish shattering.
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.”
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.
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.
While Pyrex ovenware is a type of glass, it has been specially treated in the manufacturing process to withstand high temperatures, which makes it non-recyclable. Broken or chipped Pyrex should be disposed of carefully in the waste bin.
The maker of Pyrex glassware and Instant Pot multicookers has filed for bankruptcy protection, with as much as $1bn (£790m) in liabilities. Instant Brands blames high interest rates and weak demand for its problems. The company, which has more than 2,400 workers, plans to keep operating while it shakes up the business.
Because borosilicate is the most durable when it comes to withstanding temperature changes, it is generally considered the safest choice for glass bakeware.
Crazing generally occurs with age but there are other factors which cause immediate crazing which include: Temperature and humidity changes which causes the glaze to crack. It can be caused by moisture getting into the glaze and forcing cracks in the glaze.
If you put a hot glass pan on a wet or cold surface you are risking the glass shattering due to thermal shock aka a rapid change in temperature.
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.
Corning divested itself of the Corning Consumer Products Company (now known as Corelle Brands) in 1998 and production of consumer Pyrex products went with it. Its previous licensing of the name to Newell Cookware Europe remained in effect.
As long as you use it safely and appropriately, it will stand the test of time—and serve up plenty of good casseroles for years to come.
To know whether or not your glass cookware is oven safe, look on the bottom of the dish for a Pyrex symbol or other notation that the glass is safe to put into the oven. If you cannot find a definitive marking, consult the manufacturer.
Is Pyrex glassware susceptible to shattering? Shattering is relatively rare, but it can happen when glassware is exposed to sudden temperature changes (known as thermal shock), extremely high heat (over 425 degrees), or direct heat.
PYREX® WARRANTY
Corelle Brands LLC promises to replace any Pyrex glass product that breaks from oven heat, and any Pyrex non-glass accessory item with a manufacturing defect, within TWO YEARS from the date of purchase. The owner must follow the Safety and Usage Instructions provided with the product.