Riddling
During the ageing on lees, a layer of sediment builds up on the walls of the bottle. Once this stage has finished, the layer must be removed.
The final part of the ageing process, therefore, is that
the bottles are gradually rotated and tilted until they are almost vertical (riddling).
This leads to the layer of sediment being separated from the walls and gathering above the cap.
Only when all the sedimmetent has gathered above the cap, can the disgorgement phase be started, which is removing the crown cap along with the fermentation sediment.
Disgorgement
The vertical bottles are partially immersed in a freezing solution that creates a frozen plug in the neck of the bottle containing the sediment.
When the cap is removed, the pressure inside the bottle ejects the frozen plug, leaving the wine clean and free from sediment.