The best solution to a frozen joint or cap is to prevent it. Always keep the closure clean, especially before closing. Glassware joints should be appropriately lubricated.
Solutions may be:
(1) simple and safe
(2) more complex and hazardous
Simple solutions:
(1) gently tapping all around the closure
(2) putting the closure under warm water for several minutes
(3) gently heating the closure
(4) letting the container soak in water or solvent
(5) placing the container in an ultrasonic cleaner
(6) using a covered wrench with minimal force
Risk factors for a frozen closure:
(1) caustic chemicals
(2) adhesive
(3) contents that have dried along the closure
The first 2 may difficult if not impossible to release.
If simple does not work, then try to drain the contents if possible.
If the material inside is hazardous then it may be safer to dispose of it before trying to open the frozen closure.
More hazardous methods:
(1) heating the closure to a higher temperature
(2) using more force with covered wrench
If a hazardous method is used then:
(1) wear maximum protection in case the container bursts or explodes
(2) attempt in an area where any breakage or leakage can be contained
