Casks

The most used type of cask in Scottish whisky production are bourbon casks. Bourbon casks started being used after the sherry market in the UK started to fall and as a result it became more difficult and expensive to use sherry cask.

In the USA it is illegal to use casks more than once, this means there is an abundance of ex-bourbon casks. These casks are usually used 3 times for Scottish malt whisky before either being discarded or rejuvenated in a cooperage depending upon condition.

These cask are made of American white oak grown in Iowa, Illinois, Missouri and Arkansas. They are charred by flame to about 2mm deep where the wood sugars are caramelised. Charring allows more wood penetration and releases more flavours such as vanilla and in addition acts as a filter to remove undesirable compounds that affect aging
The process was invented by Rev Elijah Craig (Scots) who lived in Georgetown Kentucky in 1789.

Bourbon casks will impart flavours such as vanilla, honey and spice.

A typical cask

Charring the casks

Charring the casks - detail

1st fill casks: provides the biggest wood influence.
2nd fill casks: reduces the effect by around 50-70%
3rd fill casks: the wood influence is negligible and these are usually used for marrying different whiskies together.

By comparison Sherry casks are only lightly toasted and mature a whisky to a more mahogany colour, The flavours imparted can be fruit cake, resin, almond and walnut notes.