"Avoid sudden temperature changes to glassware," the Pryex website reads. "DO NOT add liquid to hot glassware; place hot glassware on a wet or cool surface, directly on countertop or metal surface, or in sink; or handle hot glassware with wet cloth." (The all-caps emphasis is theirs.)
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.
Don't preheat the bakeware in the oven. Always preheat the oven first before placing the bakeware inside it. 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 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.
Yes, Pyrex can go in the oven.
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.
Most Pyrex is certified safe up to 425°F, but maintaining a slightly lower temperature reduces wear, mitigates the chance of shattering, and will ensure your dishes last for decades.
Glassware used for baking needs to be handled with care when it comes to changes in temperature. A glass pan tends to experience "thermal shock" - sudden, extreme shifts from very hot to very cold and back again. This shock can lead to cracks or even complete shattering of the glass.
According to the FTC, during the pandemic, Pyrex measuring cups sold on Amazon.com were advertised as made in the United States. In reality, some glass measuring cups were imported from China.
It's what made Pyrex famous. The glass part can withstand up to 752°F with a sudden temperature difference no greater than 248°F, per Pyrex's specifications for their standard borosilicate glass.
Because borosilicate is the most durable when it comes to withstanding temperature changes, it is generally considered the safest choice for glass bakeware.
This change was justified by stating that soda–lime glass has higher mechanical strength than borosilicate—making it more resistant to physical damage when dropped, which is believed to be the most common cause of breakage in glass bakeware. The glass is also cheaper to produce and more environmentally friendly.
"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.
"Pyrex was extremely popular during those years as people could add fun and function to their kitchen and dining spaces," Wright notes. The glass could withstand extreme temperature changes without shattering, which added to its practicality and made it ideal for baking and cooking, she adds.
"Avoid sudden temperature changes to glassware," the Pryex website reads. "DO NOT add liquid to hot glassware; place hot glassware on a wet or cool surface, directly on countertop or metal surface, or in sink; or handle hot glassware with wet cloth." (The all-caps emphasis is theirs.)
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.
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.
The lawsuit accused Pyrex manufacturer Corelle Brands LLC of switching the type of glass used to make its products. According to the complaint, Pyrex glassware was originally made of borosilicate glass, which is more heat tolerant. However, Corelle allegedly later started using less-expensive soda lime silicate glass.
Extreme Temperatures
While Pyrex cookware is better at going from extremes than plain glass, it is still not recommended. According to Cooks Illustrated, there are several ways to avoid exploding your Pyrex bakeware. One is to always make sure that the oven is preheated before placing your baking dish inside.
The most common cause of these is the use of stainless-steel machinery in the glass making and handling process. Small shavings of stainless steel containing nickel change structure over time and grow, creating internal stresses in the glass. When these stresses exceed the strength of the glass, breakage results.
Can Pyrex Go from Fridge to Oven? While a move from the fridge to a 350ºF oven likely won't cause any damage to your Pyrex dishes, I recommend erring on the side of caution. Avoid extreme temperature changes, and let your prepped enchiladas rest on the counter for 15 to 30 minutes before putting it in the oven.
DO NOT use or repair any glassware that is chipped, cracked or severely scratched. DO NOT drop or hit glassware against a hard object or strike utensils against it. Pyrex® Glassware can be used for cooking, baking, warming and reheating food in microwave ovens and preheated conventional or convection ovens.
Pyrex dishes are made of high-quality tempered glass. The glass used in Pyrex is specifically designed to be heat-resistant and durable, making it suitable for various cooking methods, including air frying.
A Pyrex 3-quart Utility Dish is ideal for roasting a turkey.
Borosilicate glass is a type of glass with the main glass-forming constituents' silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion (∼5 × 10−6/ °C at 20 °C) making them resistant to thermal shock more so than any other common glass.