Tapier, a gifter from the german north sea coast recently published on the german forum these nice pictures of ice plugs which had formed in the tubes of his CB. He discovered them when he was pulling out the crystals for inspection.
What a cool surprise.
There is clearly to see a vortex-structure in the core of the ice plugs.
Above the crystals there had formed a funnel-like cavity which then continues in a thiner, somehow twisted cavity. Enjoy!