Whisky must be aged in oak barrels or casks for a minimum amount of time (three years for Scotch whisky). Most of the time, producers will decide to keep the spirit in one type of barrel the whole way through its maturation time. But sometimes, in order to impart a different flavour, producers will decide to ‘finish’ the now-mature spirit in a different type of cask for a much shorter length of time, perhaps weeks or months rather than years. This is what’s happened with ‘rum cask-finished’ or ‘oloroso sherry cask-finished’ expressions, for example.
Why bother? Casks are SUPER important for matured spirits, as this is where a huge proportion of flavour comes from. Casks aren’t airtight so can ‘breathe’ – oxygen enters the barrel through the wood and water can evaporate out. This process draws the liquid inside a barrel part-way into the wood itself. When a producer empties that cask, the liquid drawn into the oak stays there. Refill it, and when the ‘breathing’ process starts again, that new liquid is drawn in and will be influenced by the first liquid still held in the wood. That’s why spirits can take on some of the character of whatever was in the barrel previously!