Maturing Scotch and Japanese whisky is kept in oak casks – sometimes called barrels, hogsheads or butts.

 

Casks are sourced mainly from the US and Spain, where they have previously been used to age whiskey and sherry. While Scotch and Japanese whisky rules allow repeated use of casks for maturation, the law for bourbon is such that they can only use the cask once. This normally ensures a good onward supply of casks for the Scotch and Japanese whisky trades. Securing a supply of them is part of the distillers’ challenge, and getting long use out of them is important too. 

 

Most casks come in various sizes from about 200 to 650 litres. 200 litres (barrel) and 250 (hogshead) are the most common sizes. A hogshead is essentially a barrel made from the staves of a bourbon cask with new oak ends. A butt is the standard size cask used for maturing sherry. As the interaction between wood and spirit is integral to the maturation process, smaller casks tend to mature Scotch and Japanese whisky quicker. By contrast, large casks such as butts, puncheons or port pipes usually require a longer maturation process, often of 15-20 years or more.

 

The types of casks most commonly used for maturing Scotch and Japanese whisky are as follows:

1. Barrel (approx. 190-200 litres | ~120-140 LPA): Also known as the ASB (American standard barrel), or bourbon barrel, due to its role as the principal size of cask used in the American bourbon whiskey industry. Barrels used for maturing bourbon are required by American law to be made from American white oak which has been charred prior to usage. As these casks cannot be re-used to make bourbon, they often experience a second life maturing Scotch and Japanese whisky

2. Hogshead (approx. 225-250 litres | ~142-175 LPA): After barrels, hogsheads are the second-most common type of cask used in maturing Scotch and Japanese whisky. Hogsheads are generally also made from American white oak, and indeed are often built from staves originally taken from bourbon barrels. However, the larger size of hogsheads makes them better suited for a slightly longer period of maturation. Hogsheads used to mature Scotch whisky may previously have held other beverages and spirits, with sherry hogsheads the most common of these

3. Butt (approx. 475-500 litres | ~302-350 LPA): Butts are the most commonly used type of cask in the sherry industry, and thus, apart from those having previously held bourbon, the type of cask most commonly utilised for maturing Scotch and Japanese whisky. They are traditionally made from Spanish oak, although a significant amount of butts are also made from American white oak.

4. Quarter cask (approx. 45-50 litres | ~29-35 LPA): A quarter cask is a quarter of the size of the ASB, but with the same dimensions. As quarter casks have a significantly higher ratio of wood-to-liquid than most standard casks, they tend to accelerate the maturation process. However, this method of maturation can have mixed results, and quarter casks are therefore most effective with more robust spirits, or when used to ‘finish’ a whisky.

5. Barrique (approx. 250-300 litres | ~159-210 LPA): A barrique is a slightly larger hogshead with the long shape of a butt, which is common throughout the wine industry (and, with slightly smaller dimensions, in Cognac too). Barriques are usually constructed from French oak, although some may also be made from American white oak. Although barriques were historically uncommon in the maturation of Scotch and Japanese whisky, recent decades have seen an increasing amount of single malts and blends experimenting with these casks.

6. Puncheon (approx. 450-500 litres| ~286-350 LPA): After butts, puncheons are the second-most common type of casks used to mature sherry. Dumpier than a butt, these are generally made with Spanish oak staves. Machine puncheons are made from American white oak and generally used in the rum industry. While machine puncheons are still rarely used for maturing Scotch and Japanese whisky, they have become more common in recent years.

7. Port pipe (approx. 550-650 litres | ~350-455 LPA): Port pipes are the industry standard cask for maturing port wine. Port pipes are long and similar in proportion to sherry butts, although their width is close to an ASB. Port pipes are generally only used to ‘finish’ Scotch and Japanese whiskies for a final few years.

8. Madeira drum (approx. 600-650 litres | ~381-455 LPA): Like sherry butts and port pipes, Madeira drums are the industry standard for maturing Madeira wine. However, Madeira drums are significantly squatter than these counterparts, and are built from thick French oak staves. Madeira drums are relatively uncommon for maturing Scotch and Japanese whisky and are generally used to ‘finish’ aged stocks.

 

Casks can be described as ‘first fill’ or 'refill'. An American whiskey cask or barrel that is being used to mature Scotch and Japanese for the first time is referred to as 'first fill'. It becomes a ‘refill’ cask when used for a second or subsequent time. ‘First fill’ casks are more active in the maturation process of Scotch, imparting stronger flavors to the whisky from the oak and the previous contents of the barrel. ‘Refill’ barrels, by contrast, are usually less active in maturing Scotch, allowing the spirit to dominate the maturation process. The value of the casks within the maturation process can be seen in the costs for empty whisky barrels.

 

It is the job of a good cooper to maintain the casks well to extend their useful life and value, and to ensure they continue to mature the whisky to the right quality standards.

 

The casks are stacked either three high in traditional ‘dunnage’ warehouses or more commonly now in modern palletised warehouses. The key is to allow lots of cool, damp, air to circulate.

 

All casks are porous, allowing the spirit to evaporate which is necessary for maturation.  A small number of casks may leak, and lose more whisky than they should in the first year. In modern warehouses leaky casks are usually left where they are, because moving them around to sort out a leaky one costs almost as much as a cask of newly made spirit is worth, and risks damaging others in the process. 

Losses from evaporation and leakage amount to around 2% per year, with an extra 3% lost on filling as spirit is absorbed by the wood. This is known as ‘in-drink’.

Good casks, well cared for, can last for up to 50 years or longer.