All of our flasks are made from borosilicate type glass which is nice because they can be transferred directly from heat to ice and not crack like normal glass.,,Great Flask for yeast culturing.,,Uses a #6.5 stopper (see below),In the lab world, these flasks are known as "Student Grade". A great flask for making yeast starters! (Student grade is not the highest grade flasks and will have minor defects)