How to make a whisky cask

Casks are vital to making whisky. The majority of a whisky's flavour, and all of its colour, comes from its time ageing in a wooden barrel, typically oak. But how are the casks, themselves, made? In this article, we'll take a look at how skilled coopers create these all-important tools of the whisky trade.

Choosing the oak

The first step is to select the right oak. Historically, oak would have been chosen for its properties. It's durable and hard-wearing. It has a tight grain and unique chemical (flavour) properties that work well with whisky. It would also have been plentiful, either from natural forests or from those planted to support shipbuilding. Over time, oak has become mandated, particularly in the US and Scotland.

The two main species of oak commonly used are Quercus alba and Quercus robur.

Quercus alba is also known as American white oak and is the go-to species in the US. It grows quickly and has a wide grain. A mainstay of the bourbon industry, it has high levels of vanillin, which creates the vanilla notes associated with bourbon.

Quercus robur is European oak and typically comes from France, Spain, and Hungary. It is slower growing than its American cousin and has a tighter grain. It's often used in sherry casks, and has higher tannins which create spicy notes in whisky.

Once straight, healthy, trees are selected, they are felled. The trees are cut into logs and then transport to the sawmills to be further broken down.

Milling and seasoning

At the sawmill, the oak logs are split or sawn into rough staves. These are the long, narrow strips of wood that will eventually form the sides of the barrel. Before they can be used, the staves are dried. Known as seasoning, this reduces the moisture content in the staves, reducing the risk of warping or cracking. Seasoning can be done naturally, by air-drying the staves outdoors for several months to two years. Or, it can be done more quickly by in a kiln.

Shaping the staves

Once seasoned, the staves are ready to be shaped. This is a precise process that requires a skilled "cooper" - a carpenter specialising in barrels. Each stave is planed and bevelled to ensure that it fits tightly with the other staves. This forms a liquid-tight seal without the need for nails or glue. The staves are also tapered so that the barrel can take on its characteristic curved shape. This shape helps to withstand the internal pressure once filled with whisky. It also makes the barrels easier to move around, particularly by hand.

Once the staves are ready, they can be arranged vertically in a temporary metal hoop. This is known as "raising a barrel". It can be quite a juggling act, and can take an apprentice cooper a long time to master.

Toasting and charring

Once the wood is prepared, the next step is toasting or charring. This involves exposing the inside of the cask to fire. This process breaks down the wood's lignin, releasing flavour compounds that can then benefit the whisky. How much you burn the cask influences the degree and style of flavours that will develop. A light toast will add subtle flavours of vanilla. A heavy char will create flavours of caramel, spice and smoke.

Assembling the cask

Once the staves are shaped and toasted, the cask can be assembled. The barrel is fitted with metal hoops, or bands, that hold the staves together tightly. The circular ends of the barrel, known as the heads, are inserted and secured. The barrel is filled with water to check for any leaks. A well made cask will hopefully be leak free, but any small leaks can be sealed using small reeds or shavings of wood. With the staves toasted and charred, the barrel is ready to be fully assembled.

Once tested and the water is emptied, the cask is then ready to be filled with whisky!

Whisky cask ownership with Spiritfilled

If you'd like to learn more about whisky casks, you might enjoy reading our free Whisky Cask Investment Guide. We help clients buy and sell whisky casks every day, and if you'd like to explore cask ownership, we'd be glad to help. You can arrange a free consultation with one of our cask experts here.