Cask : Cask is the general term for the containers of variable capacity in which spirit is stored during the maturation process. It is also sometimes referred to as “wood”.
Traditionally, whisky was matured in oak casks (Quercus) which had previously contained bourbon or sherry. For the bourbon casks, the American white oak is used (mainly Q. alba) and European Oak (mainly Q. robur and Q. sessilis) for the sherry casks. Surprisingly, it was only in the Scotch Whisky Act from 1988 that the definition of oak casks and the size of the cask, less than 700 litres, was introduced. Until then the use of other woods, e.g., chestnut, was allowed although seldom used.
In certain aspects the European oak differs from the American white oak. The Spanish casks are more porous and charred in order to make the staves more pliable. The American (bourbon) casks are roasted (“more burnt”). In addition, the wood used for the Spanish casks is left to dry considerably longer that the American casks,
Until the early 20th Century, whisky was matured in sherry casks as well as Port pipes and other wine casks. After the prohibition in the USA in 1933, a law prevented the producers of bourbon to use their casks more than once. As a result, after the World War II, there has been a massive importation of cheap ex-bourbon casks. In addition, the availability of Spanish oak was severely decreased when the European Economic Community prohibited the shipment in bulk. This resulted in making sherry casks about 5-times more expensive than bourbon casks.
Cask can be re-used as often as desired (except in the USA). However, after each refill, the influence of the wood on the spirit is getting less pronounced. The cask is sometimes rejuvenated in order to expand its life span.
The casks can be of different sizes. The ex-sherry casks are mainly butts (500 L), with smaller number of sherry hogsheads (250 L) and puncheons (558 L). The ex-bourbon casks are mainly barrels (200 L) and hogsheads (250 L, re-coopered barrels). Quarter casks (45-80 L) are also sometimes used and eventually a type of cask called bloodtub (20-32 L).
Cask Strength: Whisky bottled straight from one or several cask without any dilution with water to reduce the strength of alcohol to the standard 40-46% abv. Cask strength whiskies are usually not chill-filtered, but only with a particle filter to remove the wooden particles in suspension in the cask. Of note, "preferred strength, "best strength" and distillers might use other similar words but those terms are NOT synonym for "Cask Strength". "Natural Strength" is however an ambigous term, and is sometimes used as synonym for "Cask Strength".