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.
The brand switched its glassware formula in the 1950s, but has recently reintroduced borosilicate glass into some of its products, such as the Pyrex MealBox™ and Pyrex Hydration line.
If the logo is in upper case lettering, PYREX, it's most likely made of borosilicate, and thus safer. The lowercase lettering is most likely made of soda-lime glass, so take extra care after any high-heat cooking.
Here's where the branding comes in. All-caps PYREX, which is sold and made in France, is still made of borosilicate glass. But any Pyrex that's all lowercase — a recent change in branding — is soda-lime glass, which isn't designed to withstand drastic temperature changes.
All modern pyrex is tempered soda lime glass, which has good physical shock resistance but less than great thermal properties. Old school pyrex is borosilicate glass, which has outstanding thermal properties but is a bit less shatter resistant and is also more expensive to produce then tempered soda lime.
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.
Anchor Hocking has been manufacturing tempered soda-lime-silicate bakeware for nearly 40 years. Prior to that time, annealed borosilicate glass bakeware was the standard.
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.
If you're unsure which material your glass kitchenware is made from, you can try eyeballing it to make a rough estimation. Take a close look at the edge of your baking dish, container, or bowl and try to find a bluish-green tinge. If it's there, you've got soda-lime glass. If not, you've got borosilicate glass.
What Makes Pyrex Special. "Pyrex continues to hold its value because most of us associate it with memories of family and holiday gatherings and memories from their childhood," Wright says. "The vintage designs and patterns of Pyrex evoke a nostalgia, which adds to its value for collectors.
The brand, Pyrex, has historically owned the trademark for little-p pyrex, but its parent company, Corning, has since sold it. Lowercase "pyrex" is produced by other cookware companies that don't use the heat-resistant borosilicate glass the original company does (more on that below).
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).
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 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.
The Pyrex formula changed in 2020. This change was made to make Pyrex glass stronger and better able to withstand temperature changes. The newly formulated Pyrex combines classic trusted heat resistance with improved thermal shock resistance, making it a better choice for glass cookware.
So, if your cookware is spelled in lowercase, it's soda lime. If it is spelled in upper case then it is European and thus Borosilicate.
If it's all caps Pyrex, it's vintage, made of borosilicate. If it's lowercase Pyrex, it's new. And it's soda lime. You still have to be careful.
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.
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.
It is stronger than regular glass but it can still break or crack if it is dropped or hit hard. It is important to handle borosilicate glass with proper care to minimize the risk of breakage.
Borosilicate glass is sold under various trade names, including Borosil, Duran, Pyrex, Glassco, Supertek, Suprax, Simax, Bellco, Marinex (Brazil), BSA 60, BSC 51 (by NIPRO), Heatex, Endural, Schott, Refmex, Kimax, Gemstone Well, United Scientific, and MG (India).
It could be counterfeit glass! Soda lime and Borosilicate glass look very similar, but perform completely differently. Lately, there has been a rash of counterfeit glass introduced into the glass-gauge industry, some so-called companies even illegally using the corporate names of legitimate glass providers.
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.”