Pyrex became known worldwide mainly for the high temperature glass. Originally, Pyrex was borosilicate glass with added boron to the silica (quartz), the main ingredient in all glass. But today, high-temperature glasses may not follow this successful recipe. Instead, prestressed soda-lime glass is used. The difference is that in the original 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 itís stronger, but because it doesnít need to be stronger. With pre-stressed, or tempered, glass, the surface is under compression from forces inside the glass. It is stronger than borosilicate glass, but when itís heated, it still expands as much as ordinary glass does. It doesnít shatter immediately, because the expansion first acts only to release some of the built-in stress. But only up to a point.
Watch this video to see what happens when this size change happens rapidly!!!