Boro 3.3 or Borosilicate 3.3 is a type of glass having very low coefficient of thermal expansion Chemical Resistant PTFE Stopcocks regulate the flow of liquids or low pressure gases & serve many functions in the laboratory & has the distinct advantages over glass for use with high vacuum & being much easier to clean due to grease free operation. For liquid applications, it also means that you won't contaminate your reaction mixture with grease. PTFE keys are interchangeable and replaceable so that there is no need for “matching the pairs” during cleaning.