Make sure it is "Borosilicate glass". Corning sold the Pyrex brand to World Kitchen in 1998 and they started marketing Pyrex as pre-stressed soda-lime glass.
This from Popular Science:
"Pyrex, which originally was always borosilicate glass, solved this problem by adding boron to the silica (quartz), the main ingredient in all glass. Boron changes the atomic structure of glass so it stays roughly the same size regardless of its temperature. Little thermal expansion means little stress. Thus borosilicate glass withstands heat not because it’s stronger, but because it doesn’t need to be stronger."
"When World Kitchen took over the Pyrex brand, it started making more products out of pre-stressed soda-lime glass instead of borosilicate. With pre-stressed, or tempered, glass, the surface is under compression from forces inside the glass. It is stronger than borosilicate glass, but when it’s heated, it still expands as much as ordinary glass does. It doesn’t shatter immediately, because the expansion first acts only to release some of the built-in stress. But only up to a point."
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