Scotch, unlike wine, does not mature or age once it is bottled. It matures when placed in oak casks. The quality of the casks where the scotch was aged is very important. According to experts, the nature and quality of the casks, together with the quality of the barley, the making process can make up for 95% of the final quality of a malt whisky.
Naturally, wood, in the form of the casks, plays a very important role in the production of whisky. Only oak casks are used, as they are flexible yet solid enough to be constructed into the shape of a cask. Oak also adds very distinct character and flavor profile to the spirit as well. Only specific oak species are used for whisky casks: White American Oak, and European Oak.
In the process of making scotch, second-hand casks are used almost exclusively. A virgin oak wood cask, with its fresh "woody" elements" could potentially overpower the subtleties that we appreciate in a fine glass of scotch.
The second-hand cask used for the maturation of scotch, almost always, has been used before in the production of bourbon or sherry to absorb some of the harshness of the new wood.
But why bourbon? Well, that is because American law (bourbon is mostly produced in Kentucky) states that a bourbon cask can only be used once for the production of bourbon. The cask become of little use to the bourbon distillers, and the Scots can then pick it up for a nice price.
Bourbon casks, generally speaking, import a characteristic vanilla flavor to the whisky.
In olden times, when alcoholism was more socially-acceptable and rampant, sherry (fortified Spanish wine) used to be shipped to Britain from Spain in casks rather than bottles. It was expensive to ship the sherry casks back to Spain when the alcohol was drained from the cask, and so empty and discarded sherry casks were scattered all over the 19th century Scottish landscape. In what is quite possibly one of the world's earliest recycling initiatives, the Scots started gathering these second-hand sherry casks and re-used them for the maturation of their scotch.
Sherry casks are more expensive than bourbon casks, and account for only 7% of all casks imported for whisky maturation. In addition to transferring flavors from their former contents, sherry casks also lend maturing spirit a heavier body and a deep amber color. For this reason, single malt scotches that have been matured in sherry casks are prized by blenders and usually fetch a price premium.
These days, it is not uncommon to see both bourbon and sherry casks being used more than once. A sherry cask that has seen four or five fillings is not unusual. Naturally, with each filling, lesser and lesser of the wood's character is transferred to the whisky, which just goes to give scotch their uniqueness in the sense that one bottle can quite easily differ from the next bottle, based on the cask they were in.
Till the next time, bottoms up my friend.