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Pyrex is created by adding Boron to the traditional glassmaker's "frit" of silicate Sand, soda, and ground lime. The boron gives borosilicate glass a reduced Thermal expansion coefficient (about one-third that of ordinary glass), making it more resistant to heat. Pyrex is also less dense than ordinary glass. Since Pyrex melts at a higher temperature than ordinary glass, some new techniques were required to bring Pyrex into industrial production. Borrowing from the Welding trade, new burners combining Oxygen with Natural gas were required.
Caltech's famous 200-inch telescope mirror at Mount Palomar was cast by Corning during 1934-36 out of Pyrex, which expands and contracts less than ordinary glass.