A slow conversation with oak
A cask is not a neutral container. Three things happen inside it at once. The spirit takes from the wood: colour, vanillin from the breakdown of lignin, coconut-scented oak lactones (especially from American white oak), tannins (especially from European oak), caramel notes from the toasted sugars of the wood. The char removes: like a charcoal filter, it absorbs harsh sulphur compounds left by the still. And the spirit slowly reacts with the air that seeps through the staves, its acids and alcohols forming new, fruitier esters.
Toast and char
To bend staves into a barrel the cooper heats them over a fire, which also toasts the inside. Bourbon barrels are then set alight for under a minute: a #1 char takes about 15 seconds, a #4 'alligator' char about 55, leaving a black crust cracked like reptile skin. Under the char sits a red layer of caramelised wood sugars that the spirit soaks into as the warehouse warms and cools.
New wood and old wood
Bourbon must go into new charred oak, so each barrel makes one bourbon. Most of the world's other whiskies, many rums and tequilas age in those same barrels, second-hand, where the wood gives more gently over many more years. Scotch distilleries also fill casks that once held sherry, port or wine, which bring dried fruit and darker colour. Size matters too: a 200-litre barrel has more wood for each litre of spirit than a 500-litre butt, so it works faster. The Scotch and EU rules cap whisky casks at 700 litres.
The angels' share
Wood breathes. Every year a little of the spirit evaporates through the staves: in cool, damp Scottish warehouses about 2% of the volume, in Kentucky's hot rickhouses roughly 3–5%, in the tropics as much as 6–12%. That is why a Caribbean rum or an Indian whisky can taste old at an age when a Scotch is barely begun, and why few tropical casks are left for 20 years: there is little left in them. Distillers call the loss the angels' share. In some warehouse districts the vapour feeds a black fungus that coats nearby walls and trees.
Stronger or weaker?
Alcohol and water both escape, but not at the same rate. Where the air is damp, water evaporates less, so the alcohol leaves faster and the strength slowly falls: a Scotch filled at about 63.5% may be in the high fifties after a decade or two. In the dry heat at the top of a tall American warehouse water leaves faster, and the strength can rise.
A note on the numbersThe rates here are typical values drawn from distillers' reports, not a model of any real warehouse. Two casks filled the same day and stacked side by side can come out quite different, which is why blenders taste casks one by one.