The Whisky Barrel has become synonomous with the storing and ageing of a number of drinks, non more so than Whiskey. It has also become a bit of a garden fashion icon in the form of a barrel tub with increasing numbers being used as planters and decorative garden implements.
One of oak's most attractive properties of oak is that it is pliable. The wood is bent to form the traditional barrel, and it is that shape that provides the strength needed so that when a full barrel is dropped or rolled as often happens then not a drop of the valuable content is spilt.
In the US regulation says that Bourbon must be matured in new oak, but on the other side of the Atlantic precisely the opposite happens with barrels being filled on 3 or 4 occasions.
So if a whisky is being matured for 25 years, and that oak vessel is matured 3 or 4 times then you can see that this oak ages, and matures and takes on some of the character of the drink stored in it so that it ends up becoming one of the major contributors to the ultimate flavour of the whiskey.
Bourbon Whisky barrels are toasted or charred on the inside. This allows the whisky once stored for maturation to permeate the wood to allow it to influence the taste. It also allows the harsh spirits to evaporate.
An American barrel is offered at three levels of charring. Light, medium and alligator. It's called alligator because it's so burned that it looks like the skin of an alligator.
A sherry butt or bourbon barrel will give a considerable aroma and flavour to it's first fill of whisky. First fill sherry casks were used in the maturation of some great whiskies and is often seen on labels.
Whilst a second fill helps mature the whisky, any subsequent fills will add little other than maybe improving the overall balance of the flavours.
When a whisky is first placed in a barrel for ageing it has a fairly harsh "spirity" flavour. This will disipate with time as the spirit evaporates, but as time progresses the whisky barrel will warm up and cool down depending on the season, the natural characteristics of the area such as the local air / sea breezes, or the wood will contribute to the flavours. A sherry cask may add some nutty tastes or bourbon barrels may add a sweet caramel or vanilla hints to the spirit.
With time the Whisky slowly oxidises and it's this process that is believed to add complexity and intensity. Everything that spirit touches from the copper of the still to the wood of the barrel will add something special to the ultimate final flavours of the finished article.