Several things can be done at this point, depending on the kind of final product required. First of all however, regardless of the final outcome, barrels are sampled to give an indication of the quality and character of the contents. This, together with market dynamics and consumer preference drives the final outcome.
A single cask bottling involves bottling one barrel individually. The quality of the whisky in this barrel is usually of such a high quality that it can be bottled as is and doesn’t require enhancement from other barrels. All other bottlings are a blend of various casks to achieve a uniform taste profile.
Let’s use a 10 year old as an example from here. So we’ve decided that we want to release a 10 year old because we know that consumers like a bit of age on their whisky and we have a lot of 10 year old in stock. The age statement must reflect the age of the youngest whisky in the batch, so we taste a few of our 10 year old barrels and decide that they need a bit extra flavour. So we get a few 12 year old barrels, some 18s and maybe even some older ones. We blend all of this together, play around with strength and finally we are very happy with the end result.
The next step is to break the whisky down to bottling strength because it is currently at cask strength, around 65%. This is not as simple as one would think because a chemical reaction (called flocking in whisky industry) occurs when water is added to alcohol. Some of the proteins and oils come out of suspension and turn the liquid opaque and so the whisky has to be cleared before it can be sold. If left long enough, the flocking will settle at the bottom of the vessel in which it is held, and the liquid will clear naturally. However, this process, known as non-chill filtration is costly and very timely. Chill-filtration was therefore invented to combat this problem. Chill filtration involves chilling the whisky to between -10 and 4 degrees Celsius and forcing the liquid through a fine filter. This method is criticised because it strips the whisky of a lot of its colour, flavour and viscosity and necessitates the use of spirit caramel (E150) to colour the liquid. Sometimes glycerine is added to improve mouthfeel due to oils being lost in the process. As you can imagine, non-chill filtered whiskies generally cost a bit more than chill filtered whiskies.
Finally, once the spirit is fully blended, at the correct strength and clear it is bottled for our immense enjoyment.