Sometimes conical ground glass joints can lock together, preventing the user from rotating them - this is known as freezing or locking. Ball and socket-type joints are much less susceptible since they have more degrees of rotation than conical joint. This may happen for a variety of reasons:
- Lack of lubrication between the two glass surfaces. If organic solvents come into contact with the joint, they can slowly dissolve the grease, leaving a dry glass-glass surface.
- Exposure to a strong base (hydroxide, phosphate, etc) may dissolve some of the SiO2 surface, generating silicic acid (H4SiO4 / Si(OH)4)
- Solids from reaction mixtures
- Allowing sealed vessels to cool, which creates a pressure difference across the joint
Frozen joints may be removed by working solvent into the joint while rocking the stopper, heating the outer joint, or cooling the inner stopper. The last two methods employ the property of thermal expansion to crease a small space between the two surfaces. There are also specialized glassblower tools to unfreeze the joint.
Read more about this topic: Ground Glass Joint
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