Always a bestseller Pyrex® items are manufactured from thermal & chemical resistant Pyrex® borosilicate glass.Pyrex® heavy-duty conical flasks are designed to provide optimum mechanical strength under adverse conditions. These flasks have extra thick walls with carefully controlled uniformity and heavily beaded rims to resist chipping. They are able to resist fracture by a factor of up to 300% over standard glassware. The bases are specially shaped to provide efficient magnetic stirrer operation.