when using glass to glass stoppers, is it best to have them dry or use lubricant?
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.