Sciencemadness Discussion Board

Greasing joints

danton - 10-5-2025 at 11:49

Hi, I'm reading: Elementary laboratory experiments in organic chemistry, by Roger Adams et al and this caught my eye

"Greased joints should always be cleaned thoroughly when the apparatus is disassembled. Hydrocarbon greases are readily removed by acetone, carbon tetrachloride, and many other organic solvents. Except for special situations, the use of silicone greases is to be discouraged. Although these have the advantage over hydrocarbon greases of lower vapor pressure and lower solubility in most organic solvents, they present a formidable cleaning problem"

I recently purchased some Molykote vacuum grease (it's silicone). I haven't used it yet. Will cleaning the joints afterwards be a problem? What does everyone else use?

Thanks for any guidance.

Texium - 10-5-2025 at 15:27

Molykote is easily cleaned up with hexanes, heptanes, or any other mixture of hydrocarbon solvents on a paper towel. More polar solvents like acetone are not as effective and will leave residue behind.

danton - 11-5-2025 at 00:15

Quote: Originally posted by Texium  
Molykote is easily cleaned up with hexanes, heptanes, or any other mixture of hydrocarbon solvents on a paper towel. More polar solvents like acetone are not as effective and will leave residue behind.



Thanks for the feedback. What hydrocarbon based joint grease would you also suggest?

jackchem2001 - 11-5-2025 at 02:21

Here is my earlier post on silicone grease: https://www.sciencemadness.org/whisper/viewthread.php?tid=16...

I think some amount of grease contamination is inevitable no matter what so I wouldn't worry about fully removing it. However, for some experiments residual grease must be removed. If you want to completely remove silicone grease:
  1. Wipe off most of it with a paper towel
  2. Use a hydrocarbon solvent like naphtha. A little bit of solvent goes a very long way. I recommend filling up a beaker or jar and repeatedly dipping the joint into the solvent (magnetic stiring would probably help). This removes most grease, however the joints are still hydrophobic so clearly a layer of grease remains
  3. A base bath is needed to get rid of absolutely all grease. The joint does not need to be submerged for very long

I always use silicone grease so after an experiment I just wipe off the excess (this is needed because otherwise dust and debris will accumulate). I only do the other two steps when doing experiments with chlorine gas since a thick, sticky white residue forms when Cl2 reacts with silicone grease.

Note that you should try to avoid using excess grease since silicone grease can flow out of the joint a little bit (especially if the joint is warmed due to the reduction in the grease's viscosity).

[Edited on 11-5-2025 by jackchem2001]

danton - 11-5-2025 at 11:20

Quote: Originally posted by jackchem2001  
Here is my earlier post on silicone grease: https://www.sciencemadness.org/whisper/viewthread.php?tid=16...

I think some amount of grease contamination is inevitable no matter what so I wouldn't worry about fully removing it. However, for some experiments residual grease must be removed. If you want to completely remove silicone grease:
  1. Wipe off most of it with a paper towel
  2. Use a hydrocarbon solvent like naphtha. A little bit of solvent goes a very long way. I recommend filling up a beaker or jar and repeatedly dipping the joint into the solvent (magnetic stiring would probably help). This removes most grease, however the joints are still hydrophobic so clearly a layer of grease remains
  3. A base bath is needed to get rid of absolutely all grease. The joint does not need to be submerged for very long

I always use silicone grease so after an experiment I just wipe off the excess (this is needed because otherwise dust and debris will accumulate). I only do the other two steps when doing experiments with chlorine gas since a thick, sticky white residue forms when Cl2 reacts with silicone grease.

Note that you should try to avoid using excess grease since silicone grease can flow out of the joint a little bit (especially if the joint is warmed due to the reduction in the grease's viscosity).

[Edited on 11-5-2025 by jackchem2001]



Thanks for this. Very interesting and helpful.

Texium - 12-5-2025 at 09:43

Quote: Originally posted by danton  
Quote: Originally posted by Texium  
Molykote is easily cleaned up with hexanes, heptanes, or any other mixture of hydrocarbon solvents on a paper towel. More polar solvents like acetone are not as effective and will leave residue behind.



Thanks for the feedback. What hydrocarbon based joint grease would you also suggest?
I have no recommendation. I always use Molykote silicone grease and clean it up with hexanes on a paper towel prior to normal glassware cleaning. I've never had problem with grease contamination in my products. My NMR spectra don't even show traces of grease. Anyone observing grease contamination in their products is either being sloppy with their cleaning or using way too much grease.

Deathunter88 - 12-5-2025 at 10:11

I find the use of grease unnecessary for 99% of amateur chemistry experiments if you have decent quality glassware. The capillary action of your reagents does a good enough job sealing itself. I would only use it if you are working with highly highly toxic materials (cyanogen, HCN, CO ect.)

Texium - 12-5-2025 at 13:48

I've found it to be necessary for long refluxes, even with good quality glassware. Without the grease, solvent vapor will slowly leak out of the joint and your reaction will go dry. Otherwise I agree and don't usually use it.

Keras - 13-5-2025 at 12:12

I usually use teflon tape seal when I’m in need of very tight joints (high vacuum distillation, e.g.). It works well, resists to almost everything up to 250 °C and does not pose any cleaning problem, of course. I also use it to completely seal my screw-cap bottles of bromine, DCM or ether. Works well.