For decades, Pyrex was made of borosilicate glass, a special type of glass in which boron oxide is added to the mix. The added boron allows Pyrex to handle heat much better than typical glass, so Pyrex is commonly found in kitchens, laboratories, and in use with aquarium heaters (as the heaters are, necessarily, submerged in much cooler water).

GLASS COMPOSITION. Borosilicate (CORNING 7740) The most commonly used glass is borosilicate. Sutter Instrument offers only TYPE I-CLASS A borosilicate as described by ASTM Standard 3.1.2 *. This glass softens at 821 degrees Celsius and, as it is pulled, maintains its ratio of inside diameter to outside diameter over the total taper length. The coalescence filter elements are based on three-dimensional microfiber fleece made of coated borosilicate glass fibers, oleophobic and hydrophobic media. The MF UltraPleat and MFP filter elements are suitable for AG, SG, and HD filter housings. The P-MF filter elements are used in our P-EG and PG-EG housings.