The Production and Preparation of Whisky Casks
This article describes the different types of casks and how they are manufactured or reconditioned after a longer period of use.
When Whisky comes from the still as a colourless and clear liquid with an immature aroma, the final process of maturation begins. The maturation process, unlike the production process, takes most of the time. An ageing period of three years is the legal minimum in Great Britain after which a distillate can only be called Whisky. These three years are of course only the beginning for a Single Malt Whisky. The top products of the Malt distilleries reach their peak only after 15 to 25 years. During its slow maturation over many years, the complex aromas of the young Whisky become smoother.
The wooden casks are made of oak wood. Casks of pinewood are not suitable for maturing, as the resin embedded in the wood prevents the Whisky from breathing. The oak wood is cut into staves, which are then bent into their rounded shape at a high temperature. These bent staves are then 'bound' together with metal hoops. It is important that the hoops are adjusted accurately so the casks are watertight. The casks used must also have a capacity of less than 700 litres. In general, three different types of casks are used:
- The first is the Sherry Butt or Puncheon. Both have a capacity of around 500 litres, but the Puncheon is shorter and wider than the Butt. They are made of both European and American oak. In Spain, the Puncheon is also produced with a capacity of 600 litres.
- The second type of cask is the Bourbon Barrel. Bourbon matures in charred American oak casks (barrels). These have a maximum capacity of 60 American gallons (= 227 litres); however, the casks usually only hold 200 litres. You can see how these Barrels are made by the example of the Brown Forman Cooperage.
- The third type of cask is the Hogshead. There are two types of Hogshead. The Sherry Hogshead, made from old Sherry casks and the American Standard Barrel, which has been enlarged to a Hogshead. The latter is produced by enlarging a Barrel with additional staves to a volume of 250 litres.
During the maturation of Whisky, slow chemical reactions take place between alcohol and wood. These reactions increase the amount of esters and aldehydes in the Whisky. In addition to the Sherry or Bourbon residues in the inner side of the cask, the alcohol also extracts tannins, vanillin and caramel from the wood. During the maturation process, the intimate contact of the Whisky with the wood of the cask wall is of crucial importance. The volume of Whisky in a Hogshead (250 liters) is only 75% of the cask wall area due to the larger volume of the cask, than a smaller Barrel (158 liters). With a Sherry Butt (500 litres) it is even only about 50% of the surface. Distilleries with a high demand (for example Glenfiddich or Glenmorangie) therefore usually use Barrels and Hogsheads, as the Whisky reaches its desired degree of maturity faster in these types of casks.
If the cask has been used several times in a row, its ability to mature Whisky is reduced as the wood is exhausted. Such casks can, however, be reconditioned to a limited extent by removing the old inner charcoal layer and re-charring the inside. This process releases the vanillin and caramel taste in the wood once again. In the past, a sweet, boiled, sherry-like liquid made from grapes (paxarette) was even pressed into the cask wall with pressure to give the Whisky a Sherry-like character. However, this method has fallen into disrepute and has not been used for some time.
If you add the cost of a Bourbon cask to the price of the Whisky that matured in that cask, you get a cost of about 5 cents per 0.7 litre bottle of 10 year old Single Malt Whisky (assuming triple cask use).
During the maturation process, the cask expands in the warm summer months, whereas it contracts again in the cold winter months, ‘breathing’ the surrounding air. This is particularly evident on the Isle of Islay, where the sea air gives a Whisky its character. Normally Whisky loses between 0.5% and 2% alcohol content per year during this maturation period. The Scots call this the 'Angel's Share', as it evaporates irretrievably and rises to the sky.
However, there are exceptions to this principle. Maturation in a dry, hot environment leads to increased evaporation of the Whisky’s proportion of water. Under the roof of new, high warehouses, the temperature can become so hot that the alcohol content of the final Whisky rises to 66 - 67% during maturation. This increase in alcohol content leads to different reactions with the wood and the production of different aromas. Most Scottish distilleries fill their Whisky into casks with 63.5% alcohol content, as this is a reasonable compromise between short maturation time and loss of alcohol through evaporation.
For a few years now, oak casks, which were previously used for the ageing of other products such as portwine, wine or brandy, have also been used for the ageing of Whisky. This results in unique Whiskies that draw additional flavours from the product previously aged. Important representatives of these types of maturing are: