These Pyrex® round-bottom boiling flasks have short ring necks, take rubber stoppers, and are precisely tooled for uniform stopper fit. The reinforced rim increases mechanical strength. These flasks are used in applications requiring rigid stoppering and as wash bottles in routine lab washing. They are manufactured from No. 7740 borosilicate glass. Wall thicknesses are carefully controlled to provide the proper balance between thermal expansion and mechanical strength for greater shock resistance. The normal working temperature for Pyrex® boiling flasks is 230°C.