Pyrex is not resistant to thermal shock; therefore, there are instances where Pyrex glassware can explode when moved from a hot environment to a cold one and vice versa. In short, avoid exposing your Pyrex to extreme temperature changes at all times.
Pyrex is not thermal shock resistant, it can and does explode, the company did change its glass formula from borosilicate to soda lime, which is stronger.
Temperature Changes: Rapid temperature changes can cause glass to expand or contract quickly. If the jar was moved from a cold environment to a warmer one (or vice versa), this could create stress in the glass and lead to an explosion.
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.
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.
The next time you go to cook dinner in a Pyrex pan, take a look at the labeling. Cookware with lowercase logos are usually made of soda-lime glass and should not be heated up in the oven or microwave to minimize risk of them breaking.
Baking glassware such as Pyrex dishes in usable condition should be donated to local thrift stores or otherwise reused. If unusable they should be thrown away in the trash. They should not be added to curbside recycling. They are made of a different material than glass bottles and jars.
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.
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.
Spontaneous breakage of tempered glass is most commonly caused by chipped or nicked edges during installation, stress caused by binding in the frame, internal defects such as nickel sulfide inclusions, thermal stresses in the glass, and inadequate thickness to resist high wind loads.
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.
Temperature change may cause glass to expand or contract and break. In the U.S. and Canada several products — including a glass teapot — have been recalled over their inability to handle temperature changes.
Yes, Pyrex can go in the oven.
The effects of thermal shock are great and can cause the glass to break. Even pyrex (like Pyrex) can break when improperly poured boiling water. To prevent the glass from breaking when exposed to boiling water, sudden and drastic temperature changes should be avoided.
The differences between Pyrex-branded glass products has also led to controversy regarding safety issues—in 2008, the U.S. Consumer Product Safety Commission reported it had received 66 complaints by users reporting that their Pyrex glassware had shattered over the prior ten years yet concluded that Pyrex glass ...
I contacted Pyrex to ask if there's an easy way to tell what material your Pyrex dish is made from, and the company had a simple answer: Look at the logo. If it's all caps “PYREX,” it's vintage, made of borosilicate. If it's lower case “pyrex,” it's new, and it's soda lime.
Safe or toxic? A similarity between regular and borosilicate glass is that they are all completely non-toxic. You may have often observed that food and beverages from a glass container taste better than others. This is purely because ingredients cannot leach into the glass and do not absorb any stains or odours.
"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.
2 Year Limited Warranty on Pyrex® Glass Covers
Corelle Brands LLC promises to replace any PYREX cover that breaks from oven heat within 2 years from date of purchase.
Pyrex Goes in the Garbage
Pyrex cannot be recycled because it melts at a higher temperature than other container glass. Wrap up Pyrex and throw it in the trash.
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.”
PYREX vs pyrex Construction Differences
However, the company that purchased the cookware products switched to soda-lime glass, adopting the name pyrex (spelled with all lowercase letters). Corning continued to make its lab tools with borosilicate, dubbing these products to be PYREX (spelled with all uppercase letters).
The best way is to look at the bottom of the product. My vintage Pyrex, which is at least 40 years old, and definitely boro-silicate, reads “Not For Lab Or Stovetop Use - Corning N.Y. - U.S.A. - 516 - PYREX.” Newer products made of soda-lime glass do not read Corning N.Y., nor are they made by Corning.