Before Pyrex, there was Nonex, which was developed by Corning in 1908 by Dr. Eugene Sullivan, who created this non-expanding borosilicate glass for use in lantern globes and battery jars. These items were developed before the idea of planned obsolescence, so when Pyrex came along in 1913, it opened up a whole new market for kitchenware. Pyrex was developed by Dr. Jesse Littleton, who once brought home a portion of a Nonex battery jar for his wife, Bessie to use for cooking. After redeveloping the glass formula to omit lead, and with Bessie's suggestion, Pyrex debuted with its very first, glass pie dish. Pie-rex, Py-rex, Pyra-ex.. the origins of its name seem a little murky.. but I was always led to believe the pie story. Side note, Jesse and Bessie Littleton were parents of the artist, Harvey Littleton who pioneered the American studio glass movement in the 60's.
In recent years, there has been talk about the difference between PYREX and pyrex. I have only heard of this because of this youtube short I saw a few months ago.
According to this article on LifeHacker, the notion of what is and what isn't authentically pyrex is oversimplified:
Everyone links to the Wikipedia article for Pyrex as proof that the all-caps brand is better, but when you actually read the text of the article, it’s not that simple. Corning Inc. filed the original U.S. trademark—PYREX in all caps—in 1915, at which time they were exclusively manufacturing borosilicate glass products. The company introduced the all-lowercase trademark in 1975, about 20 years after the brand had already started experimenting with tempered soda-lime glass in its products. (Newton, 2023)
All of my handmade glass is annealed soda-lime glass, and does not go through a tempering process. Tempering is an industrial process of heating and rapid cooling the soda-lime glass to produce a very strong and heat resistant glass body. The reason why tempered glass shatters into a million pieces (as in a car window) is because of the way the glass is held in tension on a molecular scale, which is a state that is created by rapid cooling. On the other hand, borosilicate glass includes boric oxide which makes for a glass that is resistant to extreme fluctuations in heat and has a high melting point of 3000°F.
As far as safety in the kitchen, an oven-ready, tempered pyrex dish is completely safe to use. If you look at the fine print on the bottom of your dish, it usually specifies "no broiler" and "no stovetop", because despite being shock resistant, tempered glass can only withstand heat fluctuations of up to 250°C (482°F). And the thing about glass is that it doesn't like uneven heating, where one part of the body is cold relative to another part of its body being hot. So you might shatter your glass if you're baking on a particularly cold day, and you place your hot casserole on a cold countertop. Comparatively, borosilicate glass is far more resilient to heat fluctuations, but that doesn't mean that you need to buy a pie dish from the 1920s. If you have a boro pie dish, you could theoretically take a frozen pie, put it straight into a hot oven, and then back in the freezer without shattering your pie dish. Although, you risk breaking the freezer's glass shelf if you're not careful. I always place hot items in the fridge or freezer with a mitt, trivet or a towel underneath... and you should too. But let's not forget that glass is glass, and even boro will break without proper care.
Sources:
Borosilicate glass: A short history. (n.d.). BoraxCorp. Retrieved February 11, 2024, from https://www.borax.com/news-events/january-2023/borosilicate-glass-history
Brumagen, R. (n.d.). The Chronicles: The Littletons. Retrieved February 11, 2024, from https://pyrex.cmog.org/content/littletons
Corning pyrex bakeware | industrial design history. (n.d.). Retrieved February 11, 2024, from http://www.industrialdesignhistory.com/node/137
Did you know there is an actual difference between pyrex and pyrex? (n.d.). Allrecipes. Retrieved February 11, 2024, from https://www.allrecipes.com/article/what-is-the-difference-between-two-pyrex-types/
Newton, A. A. (2023, February 27). That viral ‘pyrex’ brand hack is horseshit, folks. Lifehacker. https://lifehacker.com/that-viral-pyrex-brand-hack-is-horseshit-folks-1850157381
Reynolds, S. (2018, February 15). Material of the month: Borosilicate. Swift Glass. https://www.swiftglass.com/blog/borosilicate-material-focus
Vitello, P. (2014, January 4). Harvey k. Littleton, a pioneer in the art of glass, dies at 91. The New York Times. https://www.nytimes.com/2014/01/05/arts/design/harvey-k-littleton-pioneer-in-glassworks-dies-at-91.html