Flask made of heavy duty borosilicate glass. 250ml Capacity Long neck Stopper not included 250ml Flat bottom Florence boiling flask made of heavy duty borosilicate glass. Flask has a flat bottom. Long neck, Stopper not included. The flask is ideal for dealing with liquids at different temperatures due to its thick-walled design. It is easy to use in any lab with its flat bottom, narrow neck, and beaded rim. Eisco Labs Borosilicate glass has the following heat tolerances: Strain Point -515°C | Annealing Point 565°C | Softening Point 820°C.