Laboratory Glassware - Lubrication and Sealing

Lubrication and Sealing

A thin layer of grease is usually applied to the ground-glass surfaces to be connected, and the inner joint is inserted into the outer joint such that the ground-glass surfaces of each are next to each other to make the connection. The use of grease helps to provide a good seal and prevents the joint from seizing, allowing the parts to be disassembled easily.

Grease can be washed out of tapers by the flow of solvents past them. Reagents may react with the grease or it may leak from the tapers at higher temperatures. The latter is prone to occurring when the system is under vacuum. Grease leaking from tapers can, of course, contaminate an operation either passively or by actively reacting with something passing by. For these reasons, it is advisable to apply a light ring of grease at the fat end of the taper and not its tip, to keep the material away from insides of the glassware. If the grease smears over the entire taper surface on mating, too much is being used. Using greases specifically designed for this purpose is also a good idea, as these are often better at sealing under vacuum, thicker and so less likely to flow out of the taper, become fluidic at higher temperatures than Vaseline (a common substitute) and are more chemically inert than other substitutes.

Grease allows chemists to easily see when a taper is leaking, as bubbles can usually be seen flowing through the taper.

When contamination is a serious concern, PTFE (Teflon) sleeves and PTFE sealing rings can be used in between joints to fit them together instead of grease. PTFE tape can also be used, but requires a little care when winding onto the joint to ensure a good seal is produced.

Keck clips and other clamping methods can be used to hold glassware together.