Chemical glassware generally comes with ground joints, the opaque parts of the glassware. These joints do not form a hermetical seal whenever an inner (male) and outer (female) joint are connected. By applying a thin strip of grease (Silicon or Teflon based), you can form an airtight seal between the joints. This is especially useful in situations where the reaction is under a different atmosphere (like Nitrogen), under a (slight) vacuum, or contains highly volatile compounds.

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