Flask is heavy walled made of borosilicate glass. The opening is 2 inches inner diameter so use a #12 or #13 stopper. The barbed glass side arm accepts a vacuum hose. The 3.3 borosilicate is prefect for heating and cooling as it has a low coefficient of thermal expansion. The flask is typically used to create a vacuum and filter solutions. Flask also works as a Buchner flask for filtering solutions in chemistry, physics, and life science labs and educational settings.