I am dissolving steel sample (containing Fe, Si, and ...) in HCL acid in a 3-neck flask with N2 gas, condenser, and thermometer connected to it. I don't want O2 to get into the system and form SiO2, so I need it to be sealed temporarily. The necks are closed with stopper or, in case of condenser, ground glass joint. I'd rather not to use high vacuum grease since it contains SiO2. Is there anything else that I could use to cover the neck and make them as much airtight as possible? I appreciate your help.