Is There A Difference Between PYREX And Pyrex Labels On Cookware? Here's What We Know

With the brand's colorful shades, whimsical patterns, and baked-in nostalgia, vintage Pyrex is a darling of social media as a home decor accent. Yet if you're new to collecting it, you might mistakenly pick up incorrect knowledge online. When searching the thrift store for vintage pieces to boost your kitchen's aesthetic, you might see items marked PYREX (uppercase) or pyrex (lowercase). Some fans claim these labels indicate what type of glass it is. They say that the former is borosilicate, a glass made of silica, boron trioxide, soda ash, and alumina, while the latter is made of soda-lime, which features soda, lime, and silica. However, this isn't true. According to the Corning Museum of Glass, "[the mark] is not a conclusive way to determine, historically, what type of glass formulation the product is made from." Instead, the PYREX/pyrex labeling switch was part of a 1970s rebranding effort, and beginning in the 1940s, the company switched to manufacturing with tempered soda-lime glass. The brand has reportedly used over 150 different glass compositions since its inception, so pinpointing the exact composition for a piece is nearly impossible. 

Pyrex's origins trace back to 1915 with a heat-resistant glass called Nonex, which was a type of borosilicate glass used to make durable cookware in response to complaints about other bakeware shattering in the oven. The company then cleverly marketed Pyrex with catalogs of various must-have patterns and color options. Today, the love for this classic brand endures, but so does this persistent myth. Here's what to know about borosilicate versus soda-lime glass.

Corelle, which currently produces Pyrex products, says that both materials "meet our high-quality standards with over 100+ years of manufacturing experience" (per Corelle.com). They add that soda-lime glassware offers more impact resistance, while borosilicate is better with extreme temperature changes. On a chemistry level, however, some glass experts believe soda-lime glass is weaker, as it only has 69% silica contents compared to borosilicate's 80% composition. This, combined with past news reports of some customer's Pyrex brand products exploding from thermal shock, make some collectors repeat the PYREX/pyrex labeling myth. However, the capitalization of the letters does not indicate when the change in materials occurred. Further, these "exploding" products may have broken from the stress of already-present chips or cracks. 

Pyrex items are some of the most valuable thrift shop finds around for their collectibility as well as durability. Regardless of whether you want to use it as the focus of your kitchen shelf decor styling or simply as a planter, you obviously want a piece of long-lasting glassware. While you can't tell a piece's precise glass composition by label alone, you can look for other age indicators, which may give you a clue. Pieces made from 1915 to the 1940s reportedly used mainly borosilicate glass. In that case, you could possibly check in a collector guide or browse an old Pyrex catalog to determine the cookware's age. Overall, when thrifting Pyrex, look for quality pieces without chips or cracks. Then, as long as your piece is stored safely, you won't need to question its resilience.