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How is a whisky cask made?

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When whisky spends years resting in casks it’s vital that the vessel is in optimum condition to do its job. Yet the construction of a quality cask is far more complex than simply bounding a few staves together, writes Becky Paskin.

A cooper constructing a cask
Sturdy vessel: Casks are designed to be able to store liquid for many years (Photo: Kelvin Cooperage)

It’s a credit to the cask’s genius engineering that its shape and construction hasn’t changed in hundreds of years. Used by breweries, wineries and distilleries the world over, the humble cask is responsible for slowly maturing some of our favourite beverages, adding sweetness and spice while mellowing the liquid at the same time.

Maturation is one of the most important processes in the creation of whisky, with many distillers believing the cask contributes as much as 60-70% of a whisky’s flavour. But a cask’s background, including how it has been made, can have a huge impact on its maturation efficiency.

Oak selection

Oak is the wood of choice for manufacturing whisky casks, and is a legal requirement for Scotch and Bourbon. Why oak? It’s strong, can be coopered into a cask, is liquid tight and releases flavours such as vanilla, caramel and nuts when toasted.

There are over 600 varieties of oak, each of which embodies characteristics that reflect its own terroir, although not all are suitable for maturing whisky. The most commonly used species are Quercus alba (American oak) and Quercus robur (European oak), although distillers are experimenting with numerous oaks grown around the world from Oregon garryana to Japanese mizunara.

Casks are constructed in cooperages, which will often work closely with sawmills, timber suppliers and forestry commissions to ensure the oak is sourced sustainably.

Air drying: Oak staves are often left out in the elements to season for several months (Photo: Kelvin Cooperage)

Seasoning the oak

The process of crafting a cask begins in the forest. Oak trees that grow tall and straight without low-hanging branches are harvested and sent to a sawmill where the heartwood is carved into staves. Each tree can provide enough wood to make around three barrels. The mill makes what is called a ‘quarter cut’, sawing the wood against the grain (for American oak) to prevent the oak from leaking when it is made into a cask.

When it is felled, a white oak tree contains around 60% water, so when the quarter-sawn lumber arrives at the cooperage, it must first be dried out.

A cooperage will usually air-dry the lumber for a period lasting a few months to a few years, depending on the oak species, its moisture content and its future use. This process not only allows the moisture to evaporate from the wood gradually, preventing any cracking or splitting, but also lets any tannins and other unwanted compounds in the wood break down.

Quarter cut: The heartwood is sawn in a way that makes the cask liquid tight

‘We like to let it age for 3-6 months out on our yard, let Mother Nature do its thing,’ says Michael Nelson, plant director at the Brown-Forman cooperage in Louisville. The site manufactures the majority of new American oak barrels destined to mature the likes of Jack Daniel’s, Woodford Reserve and Old Forester whiskeys. ‘The process is called seasoning,’ he says. ‘The wood sits out in the weather, gets rained on and snowed on in the winter time, which releases some of the tannins in the wood, but it’s also starting to dry the lumber.’

Oak destined to make wine casks on the other hand are seasoned outside for considerably longer, often up to 36 months.

The Brown-Forman cooperage then finishes drying the lumber in a kiln at around 150°F for anywhere between a few days to a few weeks to reach a moisture content of around 12.5%. ‘There’s a whole science behind drying wood and lumber,’ Nelson says. ‘If you dry it too fast it will cause splits and defects as the moisture tries to escape the cells of that wood, so it’s got to be dried slow.’

Not all oak is air seasoned though – some distillers choose to work with casks which have been kiln-dried only. Regardless of the seasoning approach, once the oak is dry it can be cut and shaped.

Shaping the cask

All the surfaces of the wood are first planed to create similar widths and to clean up the exterior. The longer staves will be used to form the body of the cask, while the shorter pieces will be used to create the heads, or ends.

Holes are drilled into the sides of the smaller pieces which are fitted with dowels and attached together to form a square of flat oak. The squares are cut into circles with a rounded edge, and will later be fitted to the cask to form its ends.

The long, planed pieces of oak are shaved into a trapezoid shape to create staves, which will neatly form a barrel shape when fitted together.

Barrel raising

The act of assembling, or raising a cask is a bit like solving a puzzle. The cooper uses an average of 31 staves per barrel, but with each one a different width, they must select the most appropriate sizes to form a leak-proof cask with no gaps. A very wide stave is selected at the beginning, in which a bung hole will be drilled following assembly. The cooper uses temporary steel hoops to secure the staves together.

To bend the staves into a cask shape without cracking or splitting the wood it is steamed, and bound by further temporary hoops.

Before the ends can be fitted, the interior of the cask must first be toasted and/or charred, depending on the requirements of the distillery, winery or brewery.

Puzzle pieces: Staves are arranged in a circle and bound by temporary hoops

Toasting and charring

Toasting and charring both involve heat-treating the oak in preparation for filling with new make spirit, although the processes do differ. ‘Toasting is the art of slowly heating the wood,’ says Andrea Wilson, Michters’ master of maturation. ‘Sometimes people might confuse that with charring which is the ignition of the interior of the wood; it goes up in flames.’

Toasting involves holding the cask over a heating element or flame for several minutes, depending on the depth of toast required, to break down wood compounds which extract specific characters into the whisky. For instance, toasting breaks down lignin which creates vanillin, the source of whisky’s vanilla flavours. ‘If you go to a really high toast temperature, you might get smoky or mocha-type characters, whereas lower temperatures may extract flavours that are more fruity or spicy,’ Wilson adds.

Charring involves setting the interior of the cask alight for a much shorter period, just a few seconds, to create a char layer that will react with the spirit and remove unwanted flavour compounds during maturation. While toasting goes deep into the wood, charring affects only the surface of the cask. The intensity of the char is graded, with a number four char – also known as an ‘alligator char’ because of its scaly appearance – being the most intense.

While it is a legal requirement for all barrels used in the maturation of Bourbon to be charred, they do not have to be toasted. On the flipside, the barrels used to mature wine and beer are simply toasted, and not usually charred.

Heat application: Casks are toasted and/or charred using direct flame or a heat element (Photo: Kelvin Cooperage)

Testing for leaks

Once the interior has been toasted and/or charred, the cask is fitted with its permanent steel hoops – the head hoops go around either end, the quarter hoops around the tapered lengths, and the bilge hoops either side of the belly. A groove, or croze, is carved at the end of the cask to fit the ends (which have also been toasted and or/charred). A bung hole is drilled in the widest stave and fitted with a cork or plastic bung.

The cask is then filled with a small amount of water and pressurised air to test for leaks. If the cask is deemed ready, it is shipped out to the distillery, brewery or winery ready for filling. If leaks are found, the cask is repaired with reed or cork wedges and spiles, although sometimes individual staves may have to be replaced. Of the 2,300 barrels made at the Brown-Forman cooperage each day, just 50 require repair.

Reusing casks

A cask’s lifespan depends on what it is used for. Those used by the Sherry industry in solera systems can often be decades old, while Bourbon producers, which can only use new American oak casks, only use theirs for a few years before selling them on to another industry.

The majority of casks used by Scotch distillers will have already been used to mature or store Bourbon, rum, Sherry, Port or wine, among other beverages. These are either shipped to Scotland whole, with a small amount of liquid retained inside to keep the cask wet, or broken down into staves for reassembly at a Scottish cooperage.

The first time one of these casks is used to mature Scotch it is known as a ‘first fill’. Subsequent uses are known as ‘refills’. Sometimes whisky is given an additional period of maturation in another cask which will impart different flavours. This process is called cask finishing, or secondary maturation.

After some time a cask’s ability to effectively mature whisky diminishes as the toasted oak’s sugars are depleted and the char layer exhausted. However, these casks can be rejuvenated through a process called STR – shave, toast, rechar. Back at the cooperage the cask’s spent inner layer is shaved off, before it is toasted, recharred and filled again. Obviously there are only so many times this process can be repeated before the cask is deemed ‘knackered’ and retired from use.

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