plastic should be nonexistent between the test tube and stopper brother . and the more fancy one tries to be with this , the more room for error . I do not even like the larger round bottom boroscilicate graduated flasks as some others try to incorporate , too much alcohal to operate , volume is too great for a good pressure to shoot the alcohal through the stem . I work in a hospital and work in the lab for extra hours . We use bunsons , flasks , test tubes , and if caught without all necessary PPE , THERES NO WARNING , you are terminated and escorted off the property by police escort . The lab has been moved out of the main hospital due to some idiot burned up aprox. 100k in equipment . He had no PPE , no extinguisher , and did not have the thermometers and pressure meters assigned to the heated flasks and cylinders . He also left a gallon of medi alcohal wide open near a burner .