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Production and Repair of Whisky Casks

Scottish Single Malt Whisky receives its unique taste from a multitude of production and maturation factors. The following list shows the four most important factors of influence on the taste of the whisky.

• Peatyness of the malt
• Geometric form and the number of pot stills (2 o. 3)
• Cask types used for maturation
• Duration of the maturation as well as the maturation place (Microclimate)

In this story is about the different cask types, their production and their possible repair after a longer use.

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The Casks

The whisky production takes place in a comparatively short period of time. From the first soaking of the barley until the end of distillation usually no more than 2 weeks pass. Out of this time the malting of the barley takes the largest portion.

Speyside Cooperage - Entrance

After the whisky leaves the pot stills, still colorless and as a clear liquid with an unripe flavour, the final process of maturation begins. In contrast to the production, maturation takes the most time. A maturation time of three years is the legal minimum in Great Britain. From then on a whisky may actually call itself whisky. These three years are obviously only the beginning for a Single Malt Whisky. The top-grade products of Malt distilleries only reach their height after 15 to 25 years. During its slow maturation over many years, the complex flavours of the young whisky becomes softer.

Speyside Cooperage - Cask Treatment

The wooden casks consist of oakwood. Casks from resinous wood (softwood) are not suitable for maturation, since the resin between the wood fibres prevents the breathing of the whisky. The used casks also have to have a capacity of less than 700 litres. Three different cask types are generally used:

• First is the Sherry Butt or Puncheon. Both have a capacity of 500 litres, but the Puncheon is shorter and thicker than the Butt. They are made from European oak as well as from American oak. In Spain the Puncheon is also manufactured with 600 litres content.
• The second type of casks is the Bourbon barrel. Bourbon matures in toasted American oak casks (barrels). These have a maximum capacity of 60 American gallons (1 Gal. = 3.78 litres); however the barrels usually only contain 200 litres. You may see the production of these American Standard Barrels in the following story about Jim Beam.
• The third type of cask is the Hogshead. There are two types of it. The Sherry Hogshead made from old sherry casks and the to the Hogshead increased barrel. The latter is made by increasing an American Standard Barrel with additional staves to a volume of 250 litres.

Speyside Cooperage - Wood Processing

During maturation of the whisky chemical reactions take place between the alcohol and the wood. These reactions increase the amount of esters and aldehytes in the whisky. Beside the Sherry or Bourbon remainders in the barrel wall, the alcohol also extracts tannine and vanillin from the wood. During maturation it thus depends on the close contact of the whisky with the wood of the inner cask wall. Due to the larger volume in a Hogshead (250 litres) there is only 75% of the cask wall per whisky as for smaller barrels (158 litres). For a Sherry Butt (500 litres) it is only 50% of the surface. Distilleries with a high output (e.g. Glenfiddich or Glenmorangie) therefore usually use barrels and Hoghsheads, since the whisky matures faster in these smaller cask types.

Speyside Cooperage - Assembly Machine

If the cask was used several times, its ability to mature whisky is reduced, since the wood is sucked out. These casks however can in a limited way be rejuvenated by removing the old charred layer and burning out on the inside again. This procedure sets free the vanillin and toffee tastes in the wood. In the past a sweet, boiled, sherrytype liquid from grapes (Paxarette) was pressed with high pressure into the cask walls in order to give to the whisky a sherrytype character. These procedures however fell into discredit and havn’t been practiced again for quite some time now.

If you calculate the costs of a Bourbon barrel for the price of the whisky which matures in this cask, you have costs of approx. 5 cent per 0.7 litre of a bottle of 10 years old Single Malt Whisky - (three times use assumed).

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During maturation the cask expands in the warm summer months and pulls together again during the cold winter and ‘breathes’ the surrounding air. This becomes particularly clear on Islay, where the sea-air delivers its character to the whisky. Usually the Whisky loses between 0,5% and 2% alcohol content per year during this maturation time. The Scots call this portion the Angels Share since it evaporates irrecoverably and ascends into the sky.

There are also exceptions of this principle. A maturation in a dry and hot environments leads to an intensified evaporation of the water content of the whisky. Closely under the roof of a new and high warehouse it can become so hot that the finished whisky rises in alcohol strength up to 66 - 67% during maturation. This rise of the alcohol content leads to different reactions with the wood and the production of different flavours. Most Scottish distilleries fill their whisky with 63,5% alcohol content into casks, since this means a reasonable compromise between a short maturation time and loss of alcohol by evaporation.

Speyside Cooperage - Cask Testing Area

Starting only few years ago oak barrels which were used before for maturation of other products such as Portwine, wine or Brandy are also used for maturation of whisky. This leads to unique whiskies, which contain additional flavours from the products stored in the casks before. Important representatives of these kinds of maturation types are:

• Glenmorangie: Portwood Finish, Madeira Finish
• Balvenie: Double Wood, Port Wood
• Classic Malts: Distiller's Edition
• Bowmore: Dusk, Dawn

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