PYREX and pyrex (the lettering)

How to read it

Pyrex is a brand, first used by Corning Glass Works in 1915, and the trademark still belongs to Corning Incorporated. It has covered many glasses: the Corning Museum of Glass’s library cites the collector Dianne Williams’s count of over 150 compositions sold under the name since 1915, among them borosilicate, aluminosilicate and heat- and air-tempered soda-lime glass. Corning sold its consumer products division in 1998, and Pyrex kitchenware has been made under licence since then: by World Kitchen, renamed Corelle Brands in 2018, in the Americas, Australia, New Zealand and Asia, and by International Cookware in Europe, the Middle East and Africa. Corning still makes PYREX laboratory glassware, spelled in capitals, which it describes as roughly 81% borosilicate.

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Capitals or lower case: two accounts

When America’s Test Kitchen asked the US Pyrex company how to tell which glass a dish is made of, the company’s answer was the logo: all-caps PYREX is vintage and borosilicate, lower-case pyrex is new and soda-lime. America’s Test Kitchen adds that this holds only for Pyrex in the United States, because Pyrex in Europe is still borosilicate.

The Corning Museum of Glass is more cautious. Its library says the change from upper to lower case was a rebranding of the trademark in the late 1970s, and is not a conclusive way to tell, historically, what glass a product is made from.

When soda-lime glass came in

Corning didn’t change over all at once. The museum’s library says Corning probably began making clear ovenware in tempered soda-lime glass after it started producing opalware in the 1940s, and that some plants switched while others went on making borosilicate. Herb Dann, a Corning designer from 1961 into the 1990s, told the museum in 2014 that by the time the consumer division was sold, Corning had mostly switched to tempered soda-lime for almost all of its tableware. The exception he named was the 13 x 9 x 2 inch pan, which he said Corning never made in soda-lime. Smithsonian magazine puts the start of the change around the 1950s. Most sources agree, the library says, that Pyrex tableware sold in the United States today is mainly tempered soda-lime.

The printed patterns are soda-lime too

The opalware that Corning printed with patterns such as Butterprint and Snowflake is itself a soda-lime glass, the museum’s library says. Capital letters on the base of one of those bowls don’t make it borosilicate.

What the safety commission found

Pyrex’s glass formulas have been a source of controversy and of several consumer lawsuits over shattering bakeware. The museum’s library reports that the Consumer Product Safety Commission looked into the question and concluded that borosilicate glass is less likely to break under thermal shock, and tempered soda-lime glass less likely to break on impact.

Who holds the Pyrex name where

Corning licenses the name by region. World Kitchen’s 2016 merger filing with the US Securities and Exchange Commission describes its licence as fully paid, royalty-free and perpetual, for North, Central and South America, Australia, New Zealand and Asia apart from the former Soviet Union and Mongolia, with the rights in Europe and the Middle East licensed to International Cookware. The US business later became part of Instant Brands, the maker of the Instant Pot. After Instant Brands filed for Chapter 11 bankruptcy in 2023, a US bankruptcy court approved its sale to the private equity firm Centre Lane Partners, and Corelle Brands became a separate company again.

International Cookware sells Pyrex across Europe, the Middle East and Africa under an exclusive licence from Corning Inc and Corning Limited, and makes its cookware mainly from borosilicate glass. The London firm Kartesia bought it in 2020. In 2022, after buying the French glassmaker Duralex, it took the name La Maison Française du Verre.