Most barrels—particularly those used for bourbon—are set on fire inside for anywhere from 15 seconds to a few minutes. The resulting interior char contributes additional color and toasty notes, and works like a charcoal filter, drawing impurities out of the spirit. It browns the wood's sugars, producing a caramelized flavor, and helps release vanillin from lignin (a cellulose-binding compound found in wood).
The smaller the barrel, the more spirit is exposed to wood; the whiskey takes on characteristics of the wood more quickly. In a larger barrel, whiskey ages more slowly and, some argue, more completely, thereby preventing woody qualities from overwhelming it. It matters, too, what part of the tree the staves come from. Those from the bottom half impart deeper color and richer caramel fllavors.
By federal law, bourbon must be aged in new, charred oak barrels. Scotch-style American single malt whiskeys are often aged in used bourbon barrels, which imbue the spirit with some of the character of the barrel's original contents. Some craft distillers finish their whiskeys in used wine or sherry casks, too, which add their own flavors to the finished spirit.
Over time, the porous barrels allow oxygen to penetrate and some of the whiskey to evaporate (known as the angel's share), concentrating flavors and potency. Warehousing of barrels is an art: Whiskey stored at the top of the building, where temperatures are highest, matures the fastest. Each distiller has its own method for rotating barrel's through the ware house for even aging.
Some say more than 60 percent of a whiskey's flavor comes from the barrel. The spirit draws color, as well as sugars from the wood. American white oak (shown here)—the wood often used for whiskey barrels—contains other flavor-imparting compounds: Vanillin lends a vanilla flavor; amyl acetate, a fruity one. It is also high in coconut-flavored lactones, as well as mouth-gripping tannins.