1.Rubber stopper fitted tightly in the flask. Flask neck might be slightly narrow for the stopper or the stopper is slightly too small for the flask.
2.Rubber stopper has hardened somewhat with age to the point where rubber attains a strong bond with glass.
3.Rubber stopper is stuck due to vacuum formed inside the flask.
1.Scrape around the edge of the flask with a scalpel to try to loosen the stopper to let a bit of air in.
2.Lever out the stopper with a mounted needle placed into the edge of the stopper at an angle.
3.Run the flask under a lot of running hot water.
4.Spray around the edge of the flask neck with lubricants such as WD40 or dish washing liquid.
5.Lubricate around the edge of the stopper with light oil or water.
6.Get a piece of thin wire such as paper clip and push down the side between the glass and stopper.
7.Place conical flask in a beaker of full water and create a water bath by heating the beaker. This should cause the expansion of air and water molecules and eventually build pressure in the flask to push the stopper out.
8.Pressurise the flask by gently heating on a hot plate. Ensure fit is not too old to avoid failure at the neck.
9.Gently push a mounted needle down the side of the flask to help break the seal. Do it at quarterly around the edge first and push towards the middle. Then you should be able to prise it out by pulling the mounted needle outwards.
10.Drilling the stopper: (image below and attached)
a.) Make sure that the flask is securely held in place, if by hand some welders, gauntlets or such are recommended to reduce the likelihood of cuts in the event of glass failure.
b.) Start with a small diameter drill and carefully drill a pilot hole through the stopper.
c.) In steps, open the hole using larger diameter drills being careful to not have a sharp drill bit bight too deeply into the rubber, as this may torque the flask too much.
e.) Once opened up enough the contents can be drained and the final part of the stopper removed, this may require cutting into sections with a scalpel or such, scrapping off the residue and remaining thin sections with a small, flat blade screwdriver or other implement.
11.A combination of all the solutions can also be used at the same time.
a). WD40, hot water bath and needle probe can be used together to remove the stopper.
b). Spraying the flask neck with a lubricant and then pushing a needle on the side between the glass and stopper.