A few people have contacted me recently asking if their tube of glue was OK as it seemed to be very viscous or partially cured in the tube. In fact, this is not the case and it is not a problem.
When the glue is very cold is does get more viscous. I put the tube in a tea cup and pour boiling water on it and leave it for a few minutes to warm up. Then the glue becomes quite fluid again and is perfect to use right away.
Note that partially used tubes should be kept in the fridge or freezer, as it will make the glue go waxy and hard but prevent it from curing inside the tube. Then the glue will be in good condition to use again a few weeks or months later. (The glue is very sensitive to contact with water or water vapour which initiates the curing reaction.)
Often people use the neck of the tube to spread the glue around when they are making a repair, and this can cause the cap to get permanently fixed the neck of the tube. Just pour out some glue (put the cap back on right away) and use a spatula or knife to spread the glue around.