You hear a lot about how whisky casks can affect the flavor of the spirit, but you do not hear a lot about how these barrels are made to ensure a great whisky. Coopering is an art that dates back as far as wine production—that is, thousands of years—and the profession continues to this day, with traditional methods augmented by modern technology. Here are a few things that every whisky lover should know about coopering.
Scotch whisky by definition must be matured in ex-bourbon or ex-sherry casks. These are made of oak from mature trees, which is defined as being 25 feet tall and 3 feet in diameter. This wood is sawed into planks that are turned into staves. Staves are planks that have been cut and curved so they can be combined into a round barrel. In order to bend the planks into staves, they are heated with a fire. In some whisk(e)y traditions, such as in bourbon making, the inside of the cask is also charred. The heating and/or charring of the casks adds flavor to the wood and also helps the liquor penetrate the wood.
A cooper—a cask maker, that is—then smoothes and finishes the staves so they can be joined tightly without leaks. No glue or nails are used; the barrel must be able to hold liquid with the help of metal hoops alone. A cask traditionally uses 32 staves for the body and 15 to seal the ends. No finishes, such as varnishes or paints, are used because these would interfere with the ability of air to penetrate the wood. It is very important that the spirits inside the casks be exposed to air during maturation. Last, a hole is bored into the side to allow the spirit to be checked and sampled as it matures. This obviously is kept closed through most of the maturation process.
Bourbon casks are charred, while sherry casks are not. However, there are other variations within the barrels used for whisky maturation. There are several different sizes, from a 4 or 5 gallon pin to a 108 gallon butt. The smaller the cask, the more the whisky will interact with the wood. There are a variety of shapes, although these are mainly due to the historical need to ship barrels on either sea or land. How they affect the end product depends on the unique aspects of that cask.
Often, whisky will be aged in a certain cask and then finished in another type. This is intended to create a more complex flavor by allowing the spirit to interact with other types of wood and spirits. Finishing usually involves just six months to a year, while the first part of the maturation will take three or more years. Sometimes a whisky that has obvious flaws will be finished in a way that partially removes them or at least compensates. Deciding how to finish a whisky is an expert matter requiring years and even decades of experience in whisky making.