Barrels are a particular size and shape of wooden cask. There are numerous other types of cask and the term ‘barrel’ is often wrongly used as a catch-all alternative generic for cask. Usually made of white oak, casks have been used since Roman times as a means of storing and transporting goods - everything from nails to whiskey.

The development of cardboard boxes, shipping containers, steel/plastic drums and palletisation have rendered the cask obsolete as a shipping container but the ability for an oak cask to improve the flavour of wine, beer and spirits is more recognised and used today than ever before .

Casks are hollow cylindrical containers, traditionally constructed from wooden staves bound by wooden or galvanized iron hoops. Bulging at the middle, their shape allows them to be easily rolled and turned by one person, even when fully filled. The rounded construction, both widthwise and lengthwise, also makes casks incredibly strong allowing them to be stacked. As every schoolboy knows, castles have rounded rather than square edges to their walls because a rounded construction is stronger.

Over some 2,000 years, white oak has become the timber of choice in which to mature wine, spirits and even beer. White oak contributes luscious flavours yet is relatively neutral compared to pine and other woods. When quarter sawn, white oak becomes water liquid tight - unlike red oak and many other woods. White also has a cell structure ideally suited be worked to make a cask.

The oak used to make the casks is split and sawn into blanks that are stacked and left to weather, exposed to the elements for at least six months and in many cases three years, depending on the wine maker or distiller's specifications. The weathering bleaches and washes out bitter tannins in the oak and allows the development of vanillin.

Steam and toasting over gentle flames helps shape the staves and also converts some of the starches in the wood into sugars. Charring or burning of the inside of the barrel caramelises these sugars and forms a layer of charred and cracked oak which helps impart flavour and colour to any spirit stored in the cask. Distillers can choose the degree of charring from one (light char) to four (deep char) depending on how they want the cask to affect their spirit.

Casks types & sizes

As I mentioned above, most people tend to call wooden casks barrels but a 'barrel' is actually a particular size and shape of cask. A barrel is a cask but not all casks are barrels. The American whiskey industry's use of the 180 to 200 litre barrel has made this the prevalent shape and size of cask in use today.

The dominance of the US standard barrel is helped by rules regarding the production of bourbon whiskey which only permit the use of new charred white oak barrels. Consequently whisky and rum makers the world over use second-hand casks sourced from America's bourbon producers. However, there are numerous other types of cask. Due to being hand-made, the size of each cask varies slightly and the following list of cask types indicates their average capacity.

English Tun

982 litres / 259 US gallons / 216 imp gallons

The English tun is a standard imperial measure equal to 216 imperial gallons. A tun is twice the size of a butt and equal to six brewery barrels.

Gorda

Capacity: 700 litres / 185 US gallons / 154 imp gallons

These huge casks made from American oak are traditional to the American whiskey industry but are not often used for maturation in the Scotch whisky industry as their capacity is too close to the maximum permitted cask size of 700 litres. However, the large capacity of Gorda casks make them useful for marrying of different whiskies to produce vatted whisky.

Madeira Drum

Capacity: 650 litres / 172 US gallons / 143 imp gallons

Made from very thick staves of European oak, as the name suggests, these short, fat, dumpy casks are used in the Madeira wine industry. Madeira seasoned drums are sometimes used to age spirits, particularly for finishing some whiskies.

Port Pipe

Capacity: 650 litres / 172 US gallons / 143 imp gallons

Made from thick staves of European oak, the name is both descriptive of these casks being used in the port wine industry and their long narrow shape. Port seasoned pipes are often used for finishing Scotch whisky.

Machine Puncheon

Capacity: 500 litres / 132 US gallons / 110 imp gallons

There are two styles of puncheon cask and this short fat cask made from thick American oak staves chiefly used in the rum industry is the most common. Ex-rum puncheons are sometimes used to finish sherry.

Sherry Shape Puncheon

Capacity: 500 litres / 132 US gallons / 110 imp gallons

Shorter and squat-shaped, this second type of puncheon is made with thinner staves of Spanish oak and is used in the sherry industry. Sherry seasoned puncheon casks are used in the whisky industry.

Sherry Butt

478-500 litres / 126-132 US gallons / 105-110 imp gallons

These long slender casks made from thick European oak staves are the most common type of sherry cask. Sherry butts are widely used in the whisky industry, to the extent that an industry has grown in Spain to make these casks and season them with sherry style wine according to the specification of Scotch whisky distillers - usually oloroso sherry for 3 years. The wine held in such new oak is not suitable for sherry making so the wine used to season these casks is usually distilled into Spanish brandy. Hence, strictly speaking, whiskies aged in such casks should not be described as being "aged in sherry butts" as these butts never produced sherry.

In the British brewing industry a butt is officially 108 imperial gallons.

Barrique (Cognac type)

Capacity: 300 litres / 79 US gallons / 66 imp gallons

Commonly used throughout the wine and cognac industries, and consequently also as second-hand casks by the Scotch whisky industry. They are best described as butt-shaped hogsheads.

Barrique (Bordeaux type)

Capacity: 225 litres / 59 US gallon / 49 imp gallon

Commonly used throughout the wine industry, this is the size favoured by producers in Bordeaux. They are best described as butt-shaped hogsheads.

Hogshead (re-purposed barrels)

Capacity: 225 to 250 litres / 59-66 US gallons / 49-54 imp gallons

It is common practice in the Scotch whisky industry for five ex-bourbon standard barrels to be broken down into staves and then reassembled with new ends to make four larger 250 litre casks known as hogsheads (or affectionately 'hoggies'). Scotch whisky distillers favour hogshead casks over standard barrels as the larger casks enable more whisky to be stored in the same warehouse space. Many also say that whisky matures better in the slightly larger casks. The name hogshead originally derives from a 15th century English term 'hogges hede', which referred to a unit of measurement equivalent to 63 gallons (considerably larger than a modern day hogshead which is officially 54 imperial gallons).

British Brewery Barrel

Capacity: 164 litres / 43 US gallons / 36 imperial gallons

A standard British brewing industry measure and barrel size. One Brewer's Barrel equals 288 pints.

American Standard Barrel (ASB)

Capacity: 180 to 200 litres/ 50-53 US gallons / 40-44 imp gallons

Made of American white oak (and very occasionally European oak) the American Standard Barrel is the most common type and size of cask in use today. They are usually first used by the American whiskey industry to age bourbon before being sold to whisky and rum producers around the world, particularly Scottish whisky distillers. They are often broken into staves to reduce shipping costs and reassembled in Scotland as hogsheads.

Many consider American Standard Barrels to be the optimum size for maturing whiskey delivering the perfect liquid to cask surface area ratio. In a Scottish distillery the American bourbon casks are easily distinguished from European sherry casks as their ends have a small indent when compared to the large indent of sherry casks.

kilderkin

Capacity approx: 82 litres / 22 US gallons / 18 imp gallons

A kilderkin is half the size of a British Brewery Barrel.

Quarter Cask

Capacity approx: 50 litres/ 13 US gallons / 11 imp gallons

Made to a quarter of the size of an American Standard Barrel but to the same proportions. The smaller size provides a much greater surface to liquid ratio, meaning that spirits aged in such casks mature more quickly. However, great things come to those that wait and spirits aged in larger casks tend to be better.

Blood Tub

Capacity approx: 40 litres / 11 US gallons / 9 imp gallons

These small casks traditionally have an elongated oval shape, originally to enable them to be carried on horseback.

Firkin

Capacity approx: 41 litres / 11 US gallons / 9 imp gallons

A firkin is quarter the size of a British Brewery Barrel and the name originates from the Middle Dutch vierdekijn meaning 'fourth'. This size of cask holds 72 pints and is popular in the British brewing industry where it is widely used for cask ale. Scottish apprentice coppers traditionally make a firkin sized cask at the end of their apprenticeship as proof that they have honed their cask making skills.

Pin

Capacity approx: 20.5 litres / 5 US gallons / 4.5 imp gallons

A British pin is equal to half a firkin. This size has become popular with home brewers who use plastic bag-in-box containers of this size known as polypins. A pin holds 36 pints.

Minipin

Capacity approx: 10.25 litres / 2.5 US gallons / 2.25 imp gallons

Minipins tend to be plastic bag-in-box containers used to serve traditional British ale in people's homes. These are filled by decanting from a firkin or other larger cask and hold 18 pints.

Barracoon/barrack

Capacity approx: 4 litres/ 1 US gallon / 0.9 imp gallons

HDPE drum

Capacity approx: 250 litres / 65 US gallons / 55 imp gallons

HDPE (high density polyethylene) a material designated "food safe" and is what cutting boards for food preparation are made from. HDPE is also used to make the large blue drums which have replaced wooden casks for many uses in the drinks industry. These inert containers are used to ship neutral spirit to boutique gin and vodka distillers and then often to send their finished product to third party bottlers.

Cask making is a long and costly process

If you thought waiting for whiskey to mature was a long term investment then compare it to the economics of growing trees to make casks. A small acorn may grow into a big tree but to reach the maturity required for a cooper to make it into a cask requires some 75 years, and very few acorns survive to grow into trees.

Oak trees don't start dropping acorns until they reach the age of 20. Then many of the acorns that fall from the parent tree will be eaten by squirrels, or succumb to frost, soil erosion, or other perils and predators. Seedlings that sprout from acorns that have dropped and remained directly under the parent tree will wither from a lack of sunlight and nutrients. Of the few trees that do survive into maturity, many will be unsuitable for coppering into casks due to broken limbs, branches that shoot off low down the trunk, knots, insect infestation, or simply not being straight enough. Spread out or broken tree bark is a sign of an unhealthy oak and if such a tree is made into casks then those casks are likely to leek due to microscopic holes.

In the 1950s, foresters tended to fell 90 to 95 year old oak trees for cask making (coopering) but studies have shown that at such an advanced age, oak trees no longer impart as much flavour as those that are 20 years younger. Older trees have passed their photosynthetic prime, and like old men, lack the energy and charisma of their younger brethren.

Even then, after 75 years, tall mature healthy oaks maybe felled to be made into furniture rather than casks. When the housing crash hit the States in the late 2000s, the coopering industry was one of the few to benefit as the market for quality furniture dropped, so did the demand and price of oak.

Cask construction

The tree trunk must be cut into planks in such a way that the wood's grain, medullary rays which transport nutrition between the tunk's core and the outer parts of the tree, run perpendicular to the plank so when made into staves the wood acts as a semi-permeable seal. If the medullary rays ran from the inside to the outside of the cask it would leak.