Glass cookware is the cleanest material story in the kitchen. No coatings, no leaching, no metal ions, no PFAS, no seasoning ritual. You can put it in the dishwasher, the freezer, and the oven — sometimes all in the same week — and the food-contact surface is the same inert silica it was on day one. That is the appeal, and it is real.
The trap is that "glass" is not one material. The Pyrex measuring cup in your cabinet, the Pyrex baking dish in your oven, and the Visions saucepan in your grandmother's hutch are three different glasses with three different thermal-shock profiles. They look identical. They behave very differently when something goes wrong. The story behind why is a 1998 ownership change at Corning that almost nobody outside the cookware industry noticed at the time, and that still drives most of the "exploding Pyrex" headlines a quarter-century later.
This is the explainer for what is actually in your glass dish, what changed, and what to look for if the dish you want is the dish your grandmother had.
The 1998 split that nobody told the consumer about
Corning Incorporated invented borosilicate glass in 1915 under the Pyrex trademark. For most of the twentieth century, "Pyrex" and "borosilicate" were synonymous in the American kitchen. The brand earned its reputation in part because borosilicate genuinely tolerates rapid temperature changes that ordinary glass cannot. You could pull a casserole from the freezer, put it in a 350°F oven, and the dish would survive. That was not marketing. It was chemistry.
In 1998, Corning sold its consumer-products division — including the US Pyrex business — to a separate company. The new manufacturer changed the formulation from borosilicate to tempered soda-lime. The trademark stayed. The glass did not. Pyrex (US) today confirms in its own FAQ that the bakeware sold under its name is manufactured from tempered soda-lime glass.
The European brand took a different path. The PYREX trademark in most of the world outside North America is held by International Cookware, which still manufactures borosilicate in France with a coefficient of linear thermal expansion of approximately 3.3 × 10⁻⁶/K. The PYREX you can buy in London or Paris is the glass the brand was famous for. The Pyrex you can buy in Cleveland is not.
Two trademarks, same five letters, different glass, different countries, different companies. The split is not a secret — both companies disclose the difference on their own websites — but the consumer-facing communication has been quiet enough that most American shoppers in 2026 still assume "Pyrex" means what it meant in 1985.
The chemistry, briefly
Glass is a non-crystalline solid built mostly from silicon dioxide (SiO₂). What you add to the silica decides what kind of glass you get.
- Soda-lime glass is silica plus sodium oxide (the soda) plus calcium oxide (the lime), with small amounts of magnesium and aluminum oxides. It is the cheapest, most-manufactured glass on Earth — windows, jars, tumblers, most ordinary bottles. The standard reference value for its linear coefficient of thermal expansion is on the order of 9 × 10⁻⁶/K.
- Borosilicate glass replaces a substantial fraction of the soda-and-lime fluxes with boron oxide (B₂O₃). The boron lowers the coefficient of thermal expansion to roughly 3 × 10⁻⁶/K — confirmed by NIST's reference standard for borosilicate, which sits in that range.
The ratio is the entire story. Soda-lime expands roughly three times as much as borosilicate over the same temperature change. When one part of a hot dish meets a cold surface — say, a wet granite counter — the cold part contracts faster than the hot part can follow. That mismatch creates internal stress. If the stress exceeds the glass's tensile strength, the dish fractures. With borosilicate, the expansion is small enough that the stress rarely gets there. With soda-lime, even tempered soda-lime, the stress can reach failure quickly. Tempering helps — it puts the surface into compression so cracks have to overcome that compression before propagating — but tempered soda-lime is still soda-lime, and when it fails, it tends to fail explosively into many small pieces rather than the few large shards a borosilicate fracture produces.
That is why pre-1998 Pyrex was a forgiving material and post-1998 Pyrex requires more care. The brand kept the name. The chemistry changed.
What to look for if you want the original glass
Borosilicate is still made. You just have to know where to find it.
- The word "borosilicate" explicit on the box, the product page, or the dish itself. Manufacturers who use borosilicate say so because it is the selling point. If a glass dish does not advertise borosilicate, assume tempered soda-lime.
- PYREX (capital letters) by International Cookware, manufactured in France. Their products carry borosilicate certification and the "Made in France" mark. This is the brand most easily mistaken for the American one — same letters, different company.
- Vintage US Pyrex from before 1998. Pre-split US Pyrex bakeware is borosilicate. The trademark on consumer goods was typically rendered "pyrex" (lowercase) through that era; the capitalized "PYREX" was used on laboratory glassware. Both eras of US-made Pyrex predating 1998 are borosilicate.
- Anchor Hocking — a US bakeware competitor that, like the post-1998 Pyrex, uses tempered soda-lime glass. Same considerations apply.
- Bormioli Rocco — Italian glassware manufacturer with multiple lines. Some are borosilicate, some are soda-lime. Check the specific product page.
- Laboratory borosilicate brands — Schott Duran, Simax, Kavalierglass — which are sometimes available as kitchenware and are borosilicate by construction.
If you are choosing between the two most common transparent cookware lines, our Visions vs Pyrex glass cookware comparison covers the material differences, use cases, and which glass belongs in which part of your kitchen.
Visions and CorningWare — the third material
Corning's other glass-cookware family is not glass at all in the sense that Pyrex is glass. It is glass-ceramic — a partially crystallized version of the same starting silica that has been heat-treated to grow microscopic crystals throughout the bulk material. The result has near-zero thermal expansion and tolerates direct stovetop use that would shatter ordinary glass.
Visions is the transparent line, sister to the opaque-white CorningWare. Both are glass-ceramic. They are rated for stovetop, oven, microwave, and freezer, and are the only mainstream "glass" cookware in the United States that can sit on a hot burner without thermal-shock failure. Production has been intermittent since the 1990s — the Visions name has come and gone across multiple license holders — and current availability is limited. Vintage Visions from the 1980s and early 1990s in good condition is sometimes prized in secondhand markets.
Glass-ceramic shares the inertness profile of borosilicate and soda-lime — no PFAS, no metals, no leaching — but its thermal-shock tolerance is on a different order of magnitude. It is the only mainstream consumer cookware glass that genuinely shrugs off the abuses (cold-to-hot transitions, direct flame, water on hot dish) that the user manuals of every other glass dish warn against.
Materials safety, briefly
All three materials — borosilicate, tempered soda-lime, and glass-ceramic — are inert under the food-contact conditions FDA recognizes. No PFAS. No fluoropolymer coatings. No metal-ion leaching. No seasoning that can break down. No coating that can wear off. From a chemical-exposure perspective, the differences across the three glasses are not the story. The story is mechanical — how the dish behaves the day it is asked to handle a thermal swing.
That is the trade. Glass is the cleanest food-contact material in the kitchen and the most fragile.
Thermal-shock guidance, regardless of which glass you have
The same handling rules apply across all three materials, with more margin for borosilicate and glass-ceramic and less for soda-lime. The rules are not specific to one brand — they are physics.
- Never put a hot dish on a cold or wet surface. Granite, marble, and stone counters at room temperature can pull enough heat from a hot dish fast enough to crack it. A dry trivet or a folded dish towel is the safe surface.
- Never add cold liquid to a hot dish, and never add hot liquid to a cold dish. The temperature differential at the contact line is what cracks glass.
- Never put a glass dish under a broiler unless the manufacturer explicitly rates it for broiler use. Most do not.
- Preheat the oven before putting the dish in. Loading a cold dish into a fully preheated oven is itself a temperature swing — borosilicate handles it, soda-lime sometimes does not.
- Do not use a glass dish on a stovetop unless it is rated for stovetop. Pyrex (US and EU) bakeware is not. Visions glass-ceramic is.
Soda-lime cookware is fully safe for the use case its manual documents — gradual oven baking with no extreme transitions. The shattering complaints documented in the Consumer Reports investigation almost always trace to a thermal-shock event the manual specifically warns against. The manual is correct. The mismatch is consumer memory, formed when "Pyrex" meant borosilicate and the dish forgave most of those abuses, against current reality, in which the same brand on the same shelf is a less forgiving glass.
Use-case recommendation
- For everyday oven baking with gradual temperature changes — soda-lime Pyrex (US) or Anchor Hocking is fine. Treat it as the manual specifies and it will outlast most of your other cookware.
- For households that want the original thermal-shock tolerance — PYREX (International Cookware, EU) or vintage US Pyrex from before 1998. Both are borosilicate.
- For stovetop glass cookware — Visions or CorningWare glass-ceramic. They are the only consumer glass cookware engineered for direct heat.
- For laboratory-grade durability — Schott Duran, Simax, or Kavalierglass kitchenware lines, which are borosilicate by construction.
The broader cookware decision — glass versus stainless versus cast iron versus ceramic — is a different question, and one we work through in our cast iron vs ceramic vs stainless explainer. Glass earns its place in the kitchen for the use cases the other materials cannot match: oven-to-fridge storage in the same vessel, microwave-safe reheating, and a food-contact surface that is the same on year ten as it was on day one. The trade is fragility. The way to live with it is to know which glass you have and to treat it like the material it actually is, not the material the brand name remembers being.