These PYREX® Round Bottom Long Neck Boiling Flasks have tooled mouths. These boiling flasks boast excellent chemical durability to eliminate contamination under proper use. Controlled wall thicknesses of these long neck boiling flasks help provide a balance between thermal expansion and mechanical strength for more resistance to shock. These PYREX® long neck boiling flasks are constructed of #7740 borosilicate glass and have an approximate upper working temperature for normal use of 230°C (446°F).