The short answer is that the change fr
om upper to lower case signified a re-branding of the trademark Pyrex® in the late 1970s but is not a conclusive way to determine, historically, what type of glass formulation the product is made from.
Pyrex® the Brand
Rather than being a type of glass, Pyrex® is a brand that represents multiple types of glasses and has been manufactured by several different companies. The brand was first established in 1915 by Corning Glass Works (eventually Corning, Inc.) In 1998, Corning, Inc. sold its consumer products division to Borden (later World Kitchen). In 2018, World Kitchen changed its name to Corelle Brands. The company merged with Instant Pot (later Instant Brands) in 2021 but after a Chapter 11 bankruptcy, Corelle Brands became a separate company once again. Corning has licensed the trademark to other companies, including International Cookware, based in France, where products are made from tempered borosilicate glass.
According to research done by Pyrex® collector Dianne Williams, over 150 different glass compositions have been used for Pyrex branded products since 1915, including different formulations of borosilicate, aluminosilicate, and heat- and air- tempered soda-lime glass.
Transparent Ovenware
At some point, Corning, Inc. began to experiment with creating transparent ovenware with tempered soda-lime glass. Likely this change began after Corning started producing its popular opalware in the 1940s, which is made from a soda-lime formulation.
Some manufacturing plants switched to tempered soda-lime formulations, while others continued to produce borosilicate Pyrex® products. According to Herb Dann, a designer at Corning, Inc. from 1961 until the 1990s, by the time the consumer products division sold, Corning had mostly switched to tempered soda lime glass for almost all of its tableware products. The exception, said Dann, in his 2014 interview with the Corning Museum of Glass, was the 13x9x2 pan, which he said Corning never manufactured with soda-lime glass. Corning's Charleroi plant was one plant that produced transparent Pyrex ovenware made from air-tempered soda-lime glass (Paul Topichak interview, 2014).
According to Dave Huber, a product liability specialist at Corning, even plants that used only soda-lime formulations were using a variety of batch recipes depending on the plant. Huber also notes, in his 2014 oral history, that Corning relied on a complex system of dots and dashes (known as manufacturing date codes) stamped on its products to identify what type of glass and when and where it was produced.
Most sources agree that tableware sold in the United States under the Pyrex® license is primarily tempered soda-lime today.
Borosilicate vs. Tempered Soda Lime
The various formulations for Pyrex glass have been a source of controversy, as well as a focus of several consumer lawsuits over shattering bakeware. As a result, the Consumer Product Safety Commission investigated this question and concluded that borosilicate glass is less likely to break under thermal shock, while tempered soda lime glass is less likely to break on impact. The New York Times explores this topic on its Wirecutter blog in the article, "Why we are not worried about Pyrex bakeware exploding."
For more about Pyrex history, including interviews with Corning employees who worked with Pyrex products, consult our Pyrex Research Guide.
Resources:
Dann, Herbert I et al. Oral History with Herb Dann, Session One, / 2014 September 25. Corning, N.Y: Corning Museum of Glass, 2014. Available at the Rakow Research Library, Corning Museum of Glass and on YouTube.
Dann, Herbert I, et al. Oral History with Herb Dann, Session Two, / 2014 October 8. Corning, N.Y: Corning Museum of Glass, 2014. Available at the Rakow Research Library, Corning Museum of Glass and on YouTube.
Huber, David et al. Oral History with David Huber, / 2014 May 23. Corning, N.Y.: Corning Museum of Glass, 2014. Available at the Rakow Research Library, Corning Museum of Glass.
Topichak, Paul et al. Oral History with Paul Topichak, / 2014 September 16. Corning, N.Y: Corning Museum of Glass, 2014. Available at the Rakow Research Library, Corning Museum of Glass and on YouTube.
Williams, Dianne. Unpublished manuscript on Pyrex. Dianne M. Williams Collection on Pyrex, 1907-2010. Available at the Rakow Research Library, Corning Museum of Glass.
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