A Cask is a barrel-shape container used for the production, storage, and service of cask-conditioned beer, widely called “real ale” in the UK. The shape of the cask, longer than it is wide and bulging along its length, evolved more than 2,000 years ago for ease of its manufacture out of wood. It is also a relatively easy vessel to handle manually because even large, heavy casks can be rolled on their sides.

Traditionally casks were made of wood, and these remained common until the mid-20th century. Today most casks are made from stainless steel, but they are also made from aluminum and plastic. Wood casks were strong but they were also heavy, hard to clean, and nearly impossible to sterilize. Unlike a modern keg, the cask has no dispense valves or interior tubes. Instead, the cask has a 2-in. round opening along the belly, through which the cask is filled. Along the rim of one of the cask’s circular faces is a smaller round opening through which beer is dispensed. After filling, a plastic or wooden stopper called a shive is driven into the large bunghole on the belly, and a smaller one called a keystone is driven into the tap hole.

The center of the keystone is deliberately weakened, allowing the tap to be driven through it at the pub with a mallet. The center of the shive also has a hole, which is thinly sealed—a hard wooden peg is driven through this. During dispense, air or unpressurized carbon dioxide will enter through this hole, which is also used for regulation of the beer’s condition (CO₂ level). See cellarmanship, art of. When set up for service, the cask is “stillaged” (set firmly in place), and the beer’s yeast, often aided by finings, will sediment out to the bottom belly of the cask, leaving the dispensed beer perfectly clear.

The grade of stainless steel used for casks is important so as to avoid corrosion. Plastic casks are often made of a laminate of materials and must be impervious to air and impart no flavor to the beer. Aluminum, which cannot be cleaned with alkaline cleaning solutions, is no longer used to make casks, although some aluminum casks remain in service in the UK.

Within the brewing trade there is a family of cask sizes, each with a different name and with volumes that are multiples of each other. The most common size is the firkin, which contains a volume of 9 Imperial (UK) gal, which adds up to 72 Imperial pints, a quarter of a UK beer barrel, or 40.9 l. Other sizes, now rarely seen, are the kilderkin at twice the size of the firkin, the barrel at four times the size, and the hogshead at six times the size.