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This Gizmodo article on Pyrex and the pros and cons of borosilicate soda-lime glass has all of things we've come to expect from the Gawker remnants, sharp writing, solid science and genuinely helpful advice. It also fits nicely with our thread about the radical and ubiquitous technological and scientific changes of the late 19th and early 20th centuries.Nowhere was this more true than with material science, where Bessemer steel was one of the primary drivers of innovation and aluminum went from being the world's most expensive metal to the stuff of housewares.  Borosilicate glass fit right in.

Pyrex made headlines recently, because its parent company made a big move. Corelle Brands, parent company of Pyrex among others, is planning to merge with Instant Brands, maker of the very popular Instant Pot. Terms of deal were not disclosed, and it’s unclear how the merger will affect any of the companies’ products. However, the news does bring to mind that decades-old controversy involving beloved glass pans, violent explosions, and some gnarly injuries. Pyrex is also the subject of a class action lawsuit in Illinois. In court filings, Pyrex’s parent company, Corelle Brands, insists that incidents of breakage result from customers improperly using their products. More on that case in a minute.

To understand the Pyrex controversy, you have to look at the reports of explosions within the context of the history of glass. Not the whole history of glass, of course, but rather a series of innovations that started with Otto Schott, a German scientist who invented a new type of glass in the late 1800s. This so-called borosilicate glass was not only heat resistant but also stood up to sudden temperature changes. Corning Glass Works developed its own recipe for borosilicate glass in 1908, and Corning employee Jesse Littleton discovered a new use for the material after his wife Bessie used a sawed-off borosilicate glass battery jar for baking. Seven years later, Pyrex cookware hit the American market. The company referred to its products as “fire-glass” in early ads.

These dates are important because Corning’s patent on the borosilicate glass used to make Pyrex pans expired in 1936. At that time, the company developed a new formula for aluminosilicate glass, which it used to create a line of frying pans called Pyrex Flameware. (This line was discontinued in 1979.) The real roots of the current controversy were planted in the 1950s, when Pyrex began making cookware out of tempered soda-lime glass. Corning licensed the Pyrex brand to a company called World Kitchen—now known as Corelle Brands—in 1998, and by nearly all accounts, all Pyrex cookware sold in the United States after that year has been made of tempered soda-lime glass. This is where the controversy really heats up.

 The vast majority of glass products are made of soda-lime glass: window panes, jars, bottles, all kinds of glass. Soda-lime glass is cheaper to make than borosilicate glass, which is undoubtedly why Pyrex started experimenting with it. However, borosilicate glass is not only harder, stronger, and more durable than soda-lime glass; it’s also more resilient to thermal shock. Thermal shock is what happens when a temperature change causes different parts of a material to expand at different rates, and the resultant stress can cause the material to crack. If the temperature change happens rapidly materials like glass can shatter or seem to explode. Resistance to thermal shock is part of why Pyrex became so popular for cookware; you could move a hot glass pan into a cool spot without worrying about it cracking or shattering. It’s also part of why laboratories prefer to use borosilicate glass rather than conventional soda-lime glass. Pyrex cookware currently sold in the United States goes through a thermal tempering process. In theory, this should strengthen the glass.

In practice, the difference between the performance of borosilicate glass and soda-lime glass is significant. When asked about the science behind the glass, Dr. John C. Mauro, a professor of engineering and materials science at Penn State, said in an email that the coefficient of thermal expansion (CTE) is the main parameter used to measure thermal shock resistance. A higher CTE number means the material is less resilient to thermal shock. For example, Corning Visions cookware, a descendent of Pyrex Flameware, is designed for stovetop use and has a CTE close to zero, Mauro explained. Borosilicate glass has a CTE of 3 or 4 parts per million per 1 Kelvin change (ppm/K). But soda-lime glass has a CTE of 9 to 9.5 ppm/K.