Every technician will have their own method of removing stubborn stoppers from glassware, for example, a volumetric flask. Here are a few ideas; The usual safety considerations should be observed. Make sure that you are wearing appropriate ppe gear like heavy duty gloves. Glassware that shatters can cut or lacerate the skin. So make sure that you exercise a higher degree of care in terms of mounting or holding the glassware in your hand before attempting to remove any stuck stopper. Be aware of remedial measure in the event of any accidents due to cuts.
- If stoppers etc are stuck in glassware, spray a little WD40 into a small beaker then using a pipette put some on the joint and let it soak in. Usually that usually loosens stoppers.
- Alternatively, If it is the type of plastic stopper that volumetric flasks use try turning it the other way. When you put them in you give them a slight turn to lock them, turning them the other way unlocks them. I picked that tip up from someplace in the past.
- If you don't mind destroying your stopper then, if it a plastic stopper, you can drill it out as a last resort. A number of small holes joined up to give one big hole. Then use a pad saw to cut a slice out.
- If somebody has been stupid enough to place a glass stopper in a bottle of alkali then a few drops of dilute acid in place of WD40 might work.
- Run the stopper/flask under a hot tap for a few minutes, the heat might just expand the flask enough to loosen the stopper.
- Gently tap a glass stopper with another glass stopper, the shock sometimes loosens it.
- If the glass tap of a separating funnel or the plunger and shaft of a glass gas syringe becomes stuck together, place the dried item in a sealed plastic bag and place in the freezer for at least an hour. Let it be for a minute after removing it from the freezer, then the contraction of the cold glass should be just enough to allow you to gently free the tap or syringe. (It may be advisable to wear safety spec and gloves in case of breakage but I have not had one yet)