Barrel & Maturation

A Kentucky barrel gets only one job. Bourbon matures exclusively in brand-new oak barrels charred on the inside, and by law each barrel may only do this once. This rule is stricter for bourbon than for almost any other spirit in the world. Once the bourbon is bottled, many distilleries sell the empty barrel on, often to Scotland or Ireland, where it gets a second life as a used cask for Scotch or Irish whiskey.

The charring itself takes only seconds but burns deep into the inner layer of wood. It caramelizes the wood sugars and exposes a thin layer of charcoal through which the whiskey is later filtered. This is where vanilla, caramel, and the whiskey's characteristic amber color come from almost entirely.

The barrel is stored in a rickhouse, a multi-story warehouse made of wood, tin, or brick, of which hundreds stand between Bardstown, Frankfort, and Lawrenceburg. Depending on the floor, the whiskey matures at different speeds. Up under the roof it gets scorching hot in summer, while it stays cooler down below, which is why some distilleries move barrels between floors during maturation. Some distilleries also rotate the barrels so that each side gets equal sun exposure.

Each year, part of the whiskey evaporates through the wood, colloquially known as the angel's share. In Kentucky's hot summers and cold winters, this loss is greater than in temperate Scotland. In return, the whiskey also matures faster, picking up more barrel character in just a few years than a Scotch would gain in the same time.