What Pyrex says about its own glass
There is a dish in your kitchen right now with the word Pyrex on the bottom. You think you know what it is made of. The company that makes it says, on its own website, that you are wrong.
Here is the sentence, from the official Pyrex site. "Pyrex glass bakeware is made of a soda lime glass composition. While Pyrex glass bakeware was originally made of borosilicate glass in 1915, in the U.S. it has been made consistently of heat-strengthened, soda lime glass for several decades." F25
They also say the new glass performs just as well. F25 In 2012, two materials scientists published the numbers. The old borosilicate glass survives a temperature swing of about three hundred and thirty degrees Fahrenheit. The new soda-lime glass survives about ninety-nine. F10
This is the story of how that swap happened and who made the call. At the end, how to tell in ten seconds which glass is sitting in your cupboard.
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1915: the lantern glass that moved into the kitchen
Picture Corning, New York, in 1908. F15 A glass company is trying to solve a problem that has nothing to do with kitchens. Railroad lanterns keep shattering. A hot glass globe meets cold rain, and it breaks.
Three men go to work on it: Eugene Sullivan, William Churchill and George Hollister. F15 They build a glass with boron in it, and they call it Nonex. F15 It does not care about the cold rain. It barely moves when the temperature changes.
On the eighteenth of September, 1916, Sullivan and Taylor file a patent for it, assigned to Corning Glass Works. F14 It is granted in May 1919, number one million three hundred four thousand six hundred twenty-three. F14 Read the patent and you find the kitchen already inside it. It describes a glass "especially adapted for fabrication into pressed or blown ware intended for culinary or laboratory uses," and it notes that the composition has already been "put into extensive commercial use." F14
Because by then somebody had taken it home. The commercial Pyrex bakeware line launched in 1915. F15 It was the same glass they made laboratory beakers from, and that was the whole point.
That is the promise your grandmother bought. Frozen dish straight into a hot oven. Hot dish onto a wet counter. It did not matter. The glass did not care, because borosilicate expands and contracts about a third as much as ordinary glass for every degree of temperature. F11 That is not a slogan. It is a measurement.
The swap: when borosilicate left the American line
Here's the crack. And it is not where almost everyone thinks it is.
Ask around and you will be told the ruin came in 1998, when the brand was sold. F17 It is a clean story. It is also wrong. Corning itself began moving its consumer Pyrex bakeware from borosilicate to tempered soda-lime glass in the late nineteen forties and fifties. F16 The sources disagree on the exact year; nobody disagrees about the decade. F16
That is forty to fifty years before the brand changed hands. The company that invented the glass is the company that stopped using it.
Why would you do that? Soda-lime melts at a lower temperature. It is cheaper to make and easier to form. The official site makes the argument out loud today: it says soda-lime "requires less energy" and "results in fewer harmful emissions" and produces a product "more recyclable than borosilicate glass." F25
And then, in the same breath, the sentence this whole video turns on. "Heat-strengthened soda lime glass provides the same high-quality performance as borosilicate glass." F25
Three hundred and thirty degrees, against ninety-nine. F10
Why soda-lime glass fails the way it does
Here is how the swap actually works, and why "tempered" is doing so much work in that sentence.
Every glass grows when you heat it. Borosilicate grows about three to four parts per million for each degree Kelvin. Soda-lime grows about nine to nine and a half. F11 Roughly three times as much movement, for the same heat.
Now put that dish in a four hundred and fifty degree oven and set it down on a counter with a little water on it. One face of the glass contracts fast. The face behind it has not caught up. The two halves of the same dish are now pulling against each other, and glass does not stretch. It goes.
Tempering is real, and it helps: you put the surface under compression so a crack has to fight its way out. But tempering has a second effect nobody advertises. When tempered glass finally fails, it does not crack. It releases all of that stored stress at once, and the dish comes apart into hundreds of pieces, with force.
And here is the tell, the thing that made me read the sentence twice. Anchor Hocking, the other big American maker, is open about using tempered soda-lime and not borosilicate. F8 Its own page says its glass has thermal shock resistance "equal to or greater than" borosilicate. F8 Read the rest of the sentence: they mean annealed borosilicate. F8 Untempered. The weakest version of the competitor that exists.
Nobody sells you annealed borosilicate cookware. The comparison is to a product that is not on the shelf.
Who owns Pyrex now — and who owns PYREX
So who owns the word on the bottom of your dish? This is where it gets strange, because there are now two Pyrexes, and they are made of different things.
On the nineteenth of March, 1998, Corning announced it was selling control of its consumer products company, the one that held Corelle, CorningWare, Pyrex, Revere Ware and Visions, to an affiliate of Borden. F17 The deal closed on the first of April and was valued at about six hundred and three million dollars. F17 Part of the agreement: the buyer had to stop using the Corning name within three years. F17 In 2000 it became World Kitchen. F17
But Corning kept the trademark, and still owns it worldwide. F18 It licenses the name out. In the United States, to Corelle Brands, for soda-lime bakeware, written in lowercase letters: p-y-r-e-x. F18 And in Europe, the Middle East and Africa, to a different company entirely, for cookware written in capitals: P-Y-R-E-X. F18
That European PYREX is made in Châteauroux, France, by International Cookware, and it is borosilicate. F1 The company says it runs the largest borosilicate furnace in the world and ships to a hundred and forty countries. F1
Same word. Same owner of the trademark. Two different materials, decided by which side of the Atlantic you are standing on. The capital letters are not a style choice. They are the recipe.
If you want the full ownership map, and the list of what is actually borosilicate today, it is on the site. One page. No email required.
What that difference looks like in a real kitchen
Now count what it cost. Not to the company. To the people who kept buying the name.
By 2008, the Consumer Product Safety Commission had logged sixty-six shattering complaints over the previous ten years, and concluded the risk was not unreasonable, because the number was small against the millions of dishes in American kitchens. F21 Those complaint counts then climbed: roughly two in 1999, to about a hundred and forty-four in 2011. F21 There has never been a recall of Pyrex-brand bakeware, and no deaths have been attributed to it. F21
Consumer Reports ran a year-long investigation, testing glass cookware from Anchor Hocking, World Kitchen and Arc International. F22 They found soda-lime cookware could shatter in ordinary conditions, such as coming out of a four hundred and fifty degree oven onto a counter with moisture on it. F22 By September 2011 the magazine had collected more than three hundred reports of American Pyrex and Anchor Hocking glass shattering. Sixty of them involved cuts or burns. F22
And then the part that tells you how much all this was worth to somebody. In 2012, two researchers, Richard Bradt and Richard Martens, published that paper in the American Ceramic Society Bulletin. F10 Their conclusion was that the margin of safety of soda-lime cookware against thermal shock is, in their word, borderline for normal household use. F23
World Kitchen sued. Not a competitor. It sued the American Ceramic Society and the authors, to suppress the paper. F23 A federal judge in the Northern District of Illinois ruled against World Kitchen on every count. F23
In June 2018, a separate consumer class action was filed against Corelle Brands, alleging it had failed to adequately warn buyers about the thermal breakage risk of its soda-lime Pyrex. F24 The company moved to dismiss it that December. F24
Picture the person this happens to. She has used Pyrex her whole life, and so did her mother, and nothing ever went wrong. [EX] She takes a dish out of the oven and sets it on the counter, the way she always has. The word on the bottom is the same word. The glass under it is not.
Instant Brands, Chapter 11, and the Charleroi plant
And what happened to the company that inherited the name? In 2017, Corelle Brands was bought by a private equity firm, Cornell Capital, and in 2019 it merged with the maker of the Instant Pot to become Instant Brands. F19 On the twelfth of June, 2023, Instant Brands filed for Chapter 11 bankruptcy. F19 It cited tighter credit, higher interest rates and a twenty-two percent drop in first-quarter sales, its seventh straight quarterly decline. F19 It listed over five hundred million dollars in both assets and liabilities. F19
In early 2024, another private equity firm, Centre Lane Partners, took both halves, and handed the Pyrex housewares business to Anchor Hocking. F20 Anchor Hocking: the company from earlier, the one comparing its glass to annealed borosilicate. F8
And then the detail that closes the circle. The official Pyrex site still says American Pyrex has been made in Charleroi, Pennsylvania for nearly sixty years. F25 That Charleroi glass factory, a hundred and thirty-two years old, was permanently closed in April 2025. F20
The page describing where your dish is made is describing a building that is shut.
How to tell in ten seconds which glass you own
So what do you actually do about it? Two things. Check what you own. Then know what to buy.
Start with your own cupboard, and it takes ten seconds. Turn the dish over. Collectors use one rule: capital letters, PYREX, in a circular logo, means the old Corning-era borosilicate. F13 Lowercase, pyrex, means the newer soda-lime glass. F13 Pieces from the nineteen forties and fifties carry PYREX in capitals inside a circle with the letters C G, for Corning Glass Works. F12 Mid-fifties pieces add MADE IN U.S.A. F12 The circular logo gave way to a straight-line format in the nineteen sixties. F12
Capitals in a circle: keep it, it is the good glass. Lowercase: it still works, just never take it from a hot oven to a cold or wet surface.
Now what to buy. If you want borosilicate, you have to go looking for it, because the American shelf mostly does not carry it. Four names that say so themselves.
One. OXO. Its own product page says OXO glass bakeware is thermal-shock-resistant borosilicate, and that it goes from freezer to oven without thawing. F26 The three-quart baking dish with a lid is listed at twenty-six dollars ninety-nine on their site. F26 Their glass bakeware runs roughly fifteen to seventy dollars depending on the size. F6
Two. Simax, made by Kavalierglass in the Czech Republic, at a glassworks founded in 1837. F3 They state Type 3.3 borosilicate, meeting the ISO 3585 standard. F3 Sold in the United States through Amazon, Walmart and Target, roughly twenty-five to forty-one dollars a dish. F3
Three. Borosil, from Mumbai, with an American arm, naming borosilicate directly in its product lines. F5 Roughly ten to twenty-five dollars a dish. F5
Four, and this is the strange one. European PYREX. The capital-letter version, borosilicate, made in France. F1 It reaches the United States through third-party importers, not through the American brand. F2 You identify it by the all-caps PYREX inside a border, and "Made in France" on the bottom. F2
A fifth, if you find it: Marinex, from Brazil, made by Nadir Figueiredo, whose Marinex Forno line the maker describes as the only borosilicate ovenware line on the Brazilian market. F7 There is no official American storefront, so in the United States it reaches you only through importers and third-party listings. F7 Good glass, awkward to buy.
There is a sixth name worth knowing for a different reason. DURAN, from DWK in Germany, publishes the number: a linear thermal expansion coefficient of three point three times ten to the minus six per Kelvin. F4 That is the original Corning class. F4 It is mostly laboratory glass, with little American kitchen presence, so treat it as the yardstick rather than the purchase. F4
And one warning, because the internet will tell you to just switch to enameled cast iron. It has the same weakness. Le Creuset's own care guide says never to plunge a hot pan into cold water, and to heat and cool gradually, because sudden swings craze and chip the enamel. F9 Different damage. Same physics.
Cost per year, not price on the shelf. A twenty-seven dollar borosilicate dish that you use for thirty years costs you about ninety cents a year. [EX] A twelve dollar soda-lime dish you replace every six years costs you two dollars a year. F26 [EX] The cheap dish costs more than twice as much, and it is the one that can come apart in your hands.
The full Lifetime List for glass bakeware, with every source in this video, is linked below. Buying through those links supports the channel and costs you nothing.
Where this leaves the dish in your cupboard
Somewhere tonight a dish is coming out of an oven, and the person holding it is certain it is the same dish their mother used. [EX] Same word on the bottom. Same shape. Same shelf in the same store.
It is not the same glass, and it has not been for about seventy years. That is not a scandal anyone hid. It is written on their own website. It is just that nobody ever told the people who kept buying it.
So go turn yours over. Capital letters in a circle, and you are holding a piece of the old promise.
If you do, tell us in the comments what you found, and how old it is. Capitals or lowercase, and what it has survived. This archive gets written by the people who own the things, not by the people who sell them.
And if you want the story of another American promise that got quietly smaller, the one about who really makes your tools, that video is on screen now. Tap the card, and I will see you there.