Cask-strength – also known as barrel proof, batch proof or full proof – whisky is bottled pure and undiluted. As whisky matures in barrel, the alcohol strength slowly increases, with water evaporating during the ageing process – what is known as “the angel’s share”. When they’re bottled, most whiskies are watered down to make a consistent product and lower the alcohol level, producing a spirit that is more approachable at between 40 and 50%.
Cask-strength whiskies can carry a premium, often coming from a single cask, and containing only the liquid from the barrel, rather than any water. Depending on the alcohol strength (which varies between 50 and 70% depending on the age of the whisky in question), people tend to add a drop of water, or an ice cube, to cask-strength whisky to soften their fiery character. As water is added, the spirit releases different aromas and flavours, so is best done gradually and slowly.
The term isn’t legally controlled, so while the idea is that whiskies are totally undiluted (and most are), some may have a small amount of water added, but they remain higher-than-average strength spirits.