The different types of casks used in the Scotch Whisky industry vary in size, shape and previous use. All of these aspects can have a profound effect on the character of the whisky maturing in the cask and the rate at which it matures. Recent years have seen an increase in the use of all manner of different casks to mature or finish whisky, which has created some very interesting whiskies and an even greater variety of options.

The shape and size of a cask has a significant influence on the maturation of whisky, because it will dictate the ratio of the wood coming into contact with the spirit, e.g. a smaller cask will have a higher ratio of wood contact to spirit than a larger cask. Therefore, the wood will interact with the spirit more rapidly or slowly, depending on the size of the cask. This not only influences the rate at which the whisky matures, but also the final characteristics of the whisky.

The infographic and table below outline the typical features of the most frequently used types of casks, although these can vary between cooperages and whisky producers.
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Scotch Whisky, unlike American Bourbon Whisky, is very rarely matured in virgin oak casks. This means that nearly always the casks will have contained another alcoholic drink prior to maturing whisky. This is done for a number of reasons, not least as using virgin oak would result in a very strong wood influence, which is not typically associated with the more delicate, smooth style of Scotch Whisky. In addition, the previous liquid held in the cask can also impart some very desirable characteristics and add variety to the industry. By far the most common casks used in the Scotch Whisky industry are Bourbon Whisky and Sherry casks (typically known as ASBs and Butts), but other types of cask are being used increasingly frequently to either mature or finish whisky, with some very interesting results (e.g. wine barriques and cognac casks). The table below offers an overview of the influence that some of the most common predecessor liquids can have on the final character of a whisky.

In addition to the species of wood, the size of the cask and the previous liquid held in it, there are many other factors that influence cask maturation and the character of a whisky greatly. Outlined below is an overview of just some of those other factors.

AGE

Perhaps one of the most widely recognised and well advertised influences on a whisky’s character is its age, meaning the length of time it has spent maturing in the cask. This is widely recognised for a very good reason – it’s very important. By law, to be classed as Single Malt Scotch Whisky the spirit must have matured in an oak cask for a minimum of three years, but the longer a whisky spends maturing in a cask, the more significant the influence on the whisky.

Older age does not necessarily equal better whisky, however. Maturation of whisky in casks is a fine balancing act. If a whisky is matured in the cask for too long, the wood flavours can overpower the character of the spirit; but if matured for too short a time, the spirit will not have mellowed sufficiently. Depending on the type of cask and the character of the spirit inside it, this time can vary hugely. However, generally speaking, Single Malt Scotch Whiskies that have aged well over a longer length of time tend to be more complex and mellow than whiskies that have aged over a shorter length of time – these tend to be a bit more vigorous and punchy.

NUMBER OF CASK REFILLS

Another significant factor affecting the maturation of whisky is the number of times a cask has been filled. It is very rare within the Scotch Whisky industry to use virgin oak casks, as new wood has a very powerful impact on the spirit. A more subtle influence from the wood is generally preferred for Scotch Whisky, so casks that have been used to mature other alcoholic drinks first are the most common, e.g. Bourbon Whisky and Oloroso Sherry casks. However, the casks are not simply used once more and then discarded; they can be refilled with young spirit multiple times before the wood becomes exhausted. Each time a cask is refilled, it can have a different effect on the whisky maturing inside and each whisky producer will have a preferred number of cask refills to achieve a desired effect.

ENVIRONMENT

The environment a cask is kept in can affect the pace of maturation, the alcoholic content, rate of evaporation of liquid from the cask (known as the Angels’ Share) and even the flavour and aroma of a whisky. For example, the hotter the climate, the faster the whisky will mature, due to more aggressive evaporation and interaction with the wood. Or, for example, if a whisky is matured in a warehouse near the coast, it may pick up some marine-like, briney characteristics from the atmosphere.

The kind of warehouse in which a cask is stored (is also significant. There are three main categories: Dunnage; Racked; and Palletised. Dunnage warehouses are the most traditional in style and generally have earthen floors and stone walls. Typically casks are stacked no more than three high and the average temperature is cooler and more stable than the other styles of warehouse. This causes the casks to ‘breathe’ more slowly and produces different results in the final character of the whisky. Racked warehouses are built in more modern materials, such as concrete blocks. Casks are stacked much higher than in Dunnage warehouses (up to eight high) and are much more practical for maturing large numbers of cask. Due to the construction and layout of these warehouses, casks can experience a more varied range of environmental changes than in Dunnage warehouses.  In Palletised warehouses the casks are stacked closely together on end, as opposed to on their side in the other warehouse. As the casks are stored closely together, this can influence airflow between the casks, again impacting on maturation.