Casks Decoded: What Bourbon, Sherry, Port and Mizunara Do to Whisky

Spirit leaves the still as clear as water. Everything that turns it amber, and a large share of what it tastes like, comes from the cask. That is why distillers talk about wood with the passion other men reserve for engines. Scotch law sets the frame: maturation in oak casks of no more than 700 litres, in Scotland, for at least three years. Inside that frame, the choice of cask is where personality is built, one barrel at a time.

Six casks worth knowing

• Ex-bourbon. American white oak barrels of around 200 litres, charred and first used for bourbon. The workhorse of Scotch: vanilla, coconut, honey and soft spice.
• Oloroso sherry. Larger European or American oak butts seasoned with dry, nutty oloroso. Expect dried fruit, walnut, clove and a deeper colour.
• Pedro Ximénez (PX). Seasoned with an intensely sweet sherry made from sun-dried grapes. Fig, date and molasses, often used as a finish.
• Port. Casks from the Douro's fortified wine. Red berries, plum, cocoa and sometimes a rosy tint.
• Mizunara. Japanese oak, slow-growing and porous, hard to cooper and prone to leaks. Prized for sandalwood, incense and coconut notes that emerge with long ageing.
• Virgin oak. New casks that have held nothing before, toasted or charred. Powerful vanilla, spice and tannin, so contact is usually short.

The still gives a whisky its skeleton. The cask gives it its accent.

First fill, refill and finishes

A cask is used more than once, and each fill takes less from the wood. A first-fill cask, used for Scotch for the first time after bourbon or sherry, gives bold colour and flavour. Refill casks are gentler and let the distillery's own character speak. Smaller casks mature spirit faster because more of it touches the wood. Many sherry casks used today are seasoned with wine in Spain specifically for whisky, rather than coming from old soleras.

• Full maturation: the whisky spent its whole life in one cask type.
• Finish, or double maturation: a final period, from months to years, in a second cask.
• Marrying: whiskies from different casks blended together before bottling.
• Single cask: one barrel, bottled on its own, often at cask strength.

Read the label like a blender

• Look for the cask words first: bourbon, oloroso, PX, port, Mizunara, virgin oak.
• Note whether it says matured or finished; the difference in intensity can be large.
• Colour can mislead: Scotch may contain plain caramel colouring, so taste before you judge.
• Compare an ex-bourbon and a sherry expression from the same distillery, and enjoy both in moderation.