Where do whisky casks come from?

A whisky cask is almost always made of oak, and for Scotch whisky that requirement is literally in the law. The Scotch Whisky Regulations 2009 prescribe maturation in oak casks. The European Regulation 2019/787 sticks to wooden casks, but in practice it is oak.

Oak has two properties that are rare in this combination: it is strong enough to hold liquid for years and at the same time porous enough to let air through. The trade magazine Scotch Whisky Magazine adds that oak is easy to cooper and releases substances into the drink when toasted. The Consejo Regulador of Jerez writes that cooperages and bodegas select American or European oak for their casks.

A cask for Scotch whisky may hold no more than 700 litres. The same maximum appears in European Regulation 2019/787, which applies to whisky from across the Union.

Why exactly 700 litres, the law does not explain. Scotch Whisky Magazine points out that the limit roughly coincides with the largest sherry cask size that was ever widely used. A cask of 500 litres, the bota from Jerez, stays well below that. The larger the cask, the less wood there is per litre of spirit, and the more slowly the maturation proceeds.

American law requires bourbon to be aged in new, charred oak, which means every barrel becomes available for another drink afterwards. Regulation 27 CFR 5,143 allows only new, charred oak barrels for bourbon, filled at no more than 125 proof.

The same requirement applies in the United States to rye, wheat and malt whisky. For corn whisky the wood may be used or uncharred, so even within the American market empty casks get a second life. Scottish law does not prohibit reuse, and the Scotch Whisky Association guidance of August 2019 lists bourbon in so many words among the permitted previous fills. This creates a steady flow of empty casks from the United States to Scotland.

Sherry casks come from Jerez in southern Spain, where the Consejo Regulador has issued certificates since 2015 for casks that have been seasoned with sherry. A bota there is not only a cask but also a measure: 500 litres, or thirty arrobas.

In England that size is called a butt. Historically, sherry was shipped to England in casks, and the Consejo Regulador describes how exports grew from about 7,000 casks a year around 1800 to some 30,000 casks in the 1850s. Today many thousands of casks are made in Jerez every year, destined for the maturation of whisky and other spirits. With the certificate, a buyer can check how long a sherry cask has been seasoned.

For Scotch, a cask may previously have held wine, fortified wine, beer or spirits, but no stone-fruit drink and nothing with added fruit or sweetener. The Scotch Whisky Association put that list on paper in August 2019.

Permitted are, among others, casks of bourbon and other whisky, grape brandy such as cognac and armagnac, rum, fortified wine such as sherry, madeira and port, still wine, and beer or ale. Excluded are casks of drinks from stone fruit, and casks that held anything to which fruit, flavouring or sweetener was added after fermentation or distillation. On top of that there is an open requirement: the matured drink must still have the colour, taste and aroma that belong to Scotch whisky. Maturation moreover happens only in Scotland, in a bonded warehouse or another permitted place, and for at least three years.