These PYREX® round bottom boiling flasks have short ring necks and are designed to be used with rubber stoppers. These ring neck boiling flasks 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. Their excellent chemical durability results in contamination-free results and long life under the toughest lab conditions. The normal working temperature for PYREX® boiling flasks is 230°C.