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.
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. (In Europe, Pyrex is still made of borosilicate.
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.
Borosilicate has a refractive index of 1.474, so the glass should "disappear" when submerged in a liquid with similar refractive index. Glycerin, turpentine, mineral oil have such an index.
Each Pyrex piece is assigned a model number, though some of the earliest sets of color ware pieces do not have them. Model numbers are typically found on the bottom of your glassware, though sometimes they are located elsewhere—look for them above the trademark on your dishes, and around handles and edges of lids.
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.
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.
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.
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).
And since it's so durable, you won't need to worry about your borosilicate bakeware cracking or exploding when transferring it to the oven from the fridge or vice versa. It's even safe to put in the microwave—and the dishwasher.
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.
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.
Whereas soda-lime glass has a greenish color borosilicate is very clear, it even can be used for high quality optics while soda-lime glass is seldom used only for low quality optics.
Optical Transparency: Borosilicate glass is highly transparent and has low dispersion. It ensures crystal clear clarity for a much longer time than regular glass. Mechanical Durability: Borosilicate glass is mechanically durable and can withstand high applied torque loads.
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.
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 follow the same guidelines above, vintage Pyrex is A-OK to go in the oven and can be used in oven temperatures up to 425ºF (and skip the broiler).
Borosilicate glass is the ultimate non-toxic choice. It's completely free from harmful chemicals like BPA, lead, or phthalates. No matter how hot or acidic your food gets, this glass stays clean and safe, giving you one less thing to worry 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.
Because borosilicate is the most durable when it comes to withstanding temperature changes, it is generally considered the safest choice for glass bakeware.
The Bottom Line
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.
However, it should be noted that borosilicate glass can still crack if subjected to sudden temperature changes or other stresses which are too extreme for it.
A more blue or green hue will indicate soda-lime glass, while borosilicate glass will be crystal clear, devoid of blue or green hues. In France PYREX® is STILL made of Borosilicate Glass. Blue or Green tones in the glass can indicate Soda-Lime Glass.