What does a sherry cask do to whisky?

A sherry cask is an oak cask that has held sherry, as a result of which the wood absorbs wine, colouring matter and wood sugars that it later gives off to whisky.

These are almost always casks from the Marco de Jerez in Andalusia. American white oak (Quercus alba) is mostly used for the cooperage, along with Spanish oak (Quercus pyrenaica) and French oak. The choice of wood helps determine how much tannin and colour the cask can give off.

The Consejo Regulador of Jerez states that a cask bearing the Sherry Cask mark must hold less than 1,000 litres and must never have held water. The certification is carried out in accordance with the standard UNE-EN ISO/IEC 17065.

Hardly any more. By far most casks sold as sherry casks are seasoned casks: casks deliberately soaked with sherry before going on to a distillery.

In the early 1980s the Consejo Regulador decided that sherry could only be bottled within the production area. That ended the flow of shipping casks that used to travel full of wine to Great Britain and be emptied there.

Since 2015, the Consejo has had its own guarantee for sherry casks. A cask must then have been seasoned for at least twelve months with wine that meets the requirements of the designation of origin. A cask from a running solera rarely becomes available, because a solera runs precisely on casks that remain in use.

Each type of sherry leaves something different behind, mainly because the wines differ in method of maturation, colour and sugar content.

Oloroso matures fully oxidatively, without a veil of yeast, and usually sits at 18 to 20 percent alcohol. Pedro Ximénez is a naturally sweet wine of 15 to 18 percent and chestnut brown to jet black in colour, so it leaves behind the most colour and residual sugar.

Fino matures biologically under a layer of flor and sits at around 15 percent, so the wood has little oxidative influence. Amontillado starts biologically and then continues oxidatively, at 17 to 20 percent. Palo cortado sits at 18 to 20 percent and, after a short biological phase, also matures oxidatively.

Sherry is a small wine category, and a cask must first contain wine for one to two years before it can go to a distillery.

That envinado takes up space in the bodega and wine that meets the requirements of the designation of origin. Bodegas and cooperages deliberately build up that stock of casks, but the lead time remains long.

Because demand from the whisky world is greater than supply, you increasingly see a short finish in sherry wood on labels instead of full maturation. Anyone who wants to know for certain what was in the cask should look at the mention of the wine type next to the words sherry cask.

A bourbon cask is always made of new, charred American oak and becomes available after one fill, which makes it widely available and gives it a different effect from a sherry cask.

US regulations require bourbon to be made from a mash of at least 51 percent corn, distilled to no more than 80 percent alcohol and put into a new charred oak container at no more than 62.5 percent. Because that cask may only be used once for bourbon, it then travels the world.

A sherry cask is therefore second-hand wood with wine in it, a bourbon cask is new wood with a charred layer. For Scotch the same framework applies in both cases: oak, at most 700 litres, and at least three years of maturation in Scotland.