Upper section is unbreakable, transparent polycarbonate Chemical-resistant polypropylene lower section permits the use of incandescent crucibles (when conventional porcelain plate is substituted for supplied plastic plate) Molded groove in the flange holds the sealing neoprene O-ring Seal is unaffected by low temperature Holds vacuum of 74cm Hg below ambient pressure for 24 hours Order Info Retention valve takes 0.6cm I.D