I have some glass flasks that have glass stoppers. It’s basically the typical glass stopper with the frosted tip which fits into a glass opening. Basically think of a typical glass volumetric flask. i attached a pic of my flasks with the stopper. they are not threaded connections. i believe they slight self lock. like i put the stopper in and push down and turn to get it tight.

So my question is, if I am travelling/moving theses flasks around in a situation where they may tip over or be upside down for some time with fluid inside them , is it better to have the glass to glass contact between the glass stopper and the glass opening fully dry? Or is it better to add some oil to the glass stopper?

If I do glass to glass and they both are dry, it’s usually pretty sealed however I believe I have always been told to use some type of oil as it helps it seal better but I tbink I am against using oil becusse I feel like the stopper has a higher chance of coming off or slightly moving causing a leak becusse of the slipperiness of the oil.

*the connection is not threaded, it is just flat like a typical glass volumetric flask”

I of course will be using parafilm/tape on top so I am just wondering which method is best so no liquid leaks out. There will be no gas build up, its just liquid like water or alcohol sometimes but i dont care if they slightly evaporate. i just dont want leaks

Basically I am looking for the best method to reduce leaks

Thanks enter image description here

  • $\begingroup$ I think this might open up a discussion based on taste and luck of the operator. I never used dry contact. Sealing wasn't the point but removal of the apparatus. I always used silicon based lubricant in minimal amount or Teflon bands coming in sizes fitting your joint. Teflon is also available in rolls like band aid, I use this for valves of plumbing style connections and apparatus but I think it can be used for glassware as well. $\endgroup$
    – Alchimista
    Aug 15, 2020 at 21:48
  • $\begingroup$ Recall that a flat bottom flask shouldn't be used for a distillation. Using grease on the stopper may end up as an impurity of the material you aim to stock in these flasks, too. The use of these flasks is restrained, though. Don't use them for alkaline liquids; once the surface between stop cock and flask is wet, lye (e.g. NaOH reacting with CO2 of air) builds a cake which may render opening the flasks difficult. If you store these flasks clean and empty for sometime, either keep the stop cocks separate from the flasks, or put a strip of paper (e.g., newspaper) between stop cock and flask. $\endgroup$
    – Buttonwood
    Aug 16, 2020 at 13:46

1 Answer 1


Sometimes dry glass-to-glass sealing of the stopper leads to seizing of the stopper and can become very hard to remove. Therefore vacuum grease is often applied lightly. However, if your material in the flask can dissolve the vacuum grease, the material can become contaminated by the vacuum grease. If your material is volatile, you will need the vacuum grease to ensure an airtight seal. Another method you can use is to use teflon tape between the stopper and the ground glass joint to make a seal that will not seize. But, vacuum grease will give you the best seal.


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