Why All Pyrex Is Not Created Equal: The History of Ordinary Things
The critical issue for glass is the ability to adjust to temperature changes. Glass expands at different rates resulting in tension, cracks, and shattering. Three types of glassware are typically found in American kitchens:
- Soda-lime glass, used for most drinkware from juice cups to jars, is more susceptible to breaking with temperature changes.
- Tempered glass is soda-lime glass that’s been heat-treated making it more durable.
- Borosilicate glass contains boron trioxide, the element that makes glass resistant to temperature changes. For example, it doesn’t break when taking a dish from a refrigerator to an oven. Pyrex is a sub-group of borosilicate glass.
In 1893, a German chemist added boronoxide to glass which increased the heat resistance. It was called Duran. In the US, 1909, Corning Glass Works developed Nonex initially for battery jars and later, for railroad lanterns. The colored glass in lanterns signaled train engineers (Stop/Go). Normal colored glass shattered in extreme temperature changes, resulting in multiple railway disasters. Nonex glass rarely broke.
Corning’s business peaked in the railroad industry and then waned, because there were few repeat sales. They created a new market for heat resistant glass introducing Pyrex for laboratory glassware and kitchenware. Initially, Corning produced twelve clear glass ovenware dishes under the brand name Pyrex. The pie plate, their first item, cooked faster and more thoroughly than metal pans. By 1915, the first Pyrex line had 12 pieces, including pie plates and a loaf pan. The 8-ounce liquid measuring cup came out 10 years later.
In 1936, Corning bought a glass factory in Charleroi, PA, which had the capability to produce tempered colored opal glass with the same heat-resistant properties as clear glass. Originally this white opalware was produced for military mess halls during WWII.
In 1945, Corning debuted “Primary Colors”, the first colored opal Pyrex. The set of 4 nesting bowls came in robin egg blue, Chinese red, jade green and canary yellow. Between 1945 and 1983, Corning Glass Works created more than 150 different patterns for bowls, casseroles, refrigerator dishes and more.
When Corning’s patent expired in 1936, other companies entered the market competing in composition, colors and styles. Soda glass was often added to products making them less heat resistant than the original pyrex. Trademark disputes resulted.
Both trademarks “PYREX” and “pyrex” were historically used in the marketing of kitchenware made up of both borosilicate and soda-lime glass. Pyrex eventually switched to tempered glass perhaps because boron is expensive to dispose of and is toxic. (The brand has recently reintroduced borosilicate glass into some products.)
Today, Corning has licensed the use of their PYREX (upper case lettering) and pyrex (lower case lettering) logos to other companies. If the logo is in upper case lettering, PYREX, it’s likely made of borosilicate, and is thus safer. Today upper-case PYREX is used for laboratory glass items. The lowercase lettering is likely made of soda-lime glass, so take extra care with any high-heat cooking. This pyrex is not the PYREX of the early days and the glass can shatter! Read the contents and warnings. Know your glassware.
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