Sherry casks can be acquired on the open market from brokers and cooperages. However, distilleries generally have agreements with sherry bodegas in Jerez, ensuring an 'a la carte' supply of casks.
An initial choice is whether to use European Oak, which is predominantly Spanish from northern regions such as Galicia; or American Oak imported from the USA in plank form and assembled in cooperages in Jerez. Casks from either type of oak are a hogshead (capacity 250 litres), with the next size up a butt or puncheon, which both have a capacity of around 500 litres, though a puncheon is shorter and rounder.
"The length of a stave determines the type of cask it is used for, which maximises efficiency. A stave around 1.3-1.4 metres long is used to make a butt. However, if a stave has any defects, such as splitting, and has to be cut shorter, then a 1.2 metre stave is used to make a puncheon. If more of the stave needs to be cut off, it's used to make a hogshead," says Stuart MacPherson, Macallan's master of wood.
The next stage is to apply a flame to toast (but not ignite) the interior of each cask, creating a toasted layer about 2-3mm deep. Oak contains a range of constituents, including sugars, which are broken down by the heat of toasting, and this results in a range of flavour compounds being created (including vanilla notes, for example).
Casks are subsequently filled with sherry in order to 'extract' the overt spiciness and woodiness that is inevitable with a new oak cask. This stage entails two options. A 'seasoned cask' is filled with a particular style of sherry for a specific ageing period. Alternatively, an 'ex-bodega cask' means it has been through a solera system.
The solera is the traditional system used to age sherry, comprising rows of casks stacked on top of each other. The first row (i.e. ground level) is known as the solera, and contains the oldest sherry. The row above (the first criadera) contains sherry one stage younger. The row above that (the second criadera) contains sherry a stage younger than the first criadera, and so on, up to the top row of casks containing the youngest sherry.
Periodically some sherry is drawn from the solera casks, perhaps a third of the contents or less, in order to be bottled. The solera casks are then topped up by taking an equal amount of sherry from casks in the first criadera, replenished in turn by some sherry drawn from the second criadera, and so on. Casks in the final criadera are topped up with the youngest sherry.
Meanwhile, the influence of sherry casks on malt whisky varies according to the origin of the oak.
"American Oak typically gives the same range of flavours as European Oak, but not as intense, though American Oak gives a higher level of vanilla notes, while European Oak adds greater richness. There is also a slower interaction between American Oak and the maturing spirit compared to European Oak. Which type of oak to use all depends on the character of the new-make spirit. American Oak is better for ageing a lighter style of new-make spirit, as the cask won't dominate as much, and more of the original 'distillery character' shows in the mature malt whisky compared to using European Oak, whereas European Oak is better for ageing a richer style of new-make spirit," says Stuart Urquhart, associate director - whisky supply, Gordon & MacPhail.
Another consideration is that a hogshead has a more immediate and more intense impact than a butt or puncheon, as a smaller cask size means a larger surface area contact with the oak in relation to the volume of whisky. But what about the difference between a butt and a puncheon, can this have any significance? "The influence of a puncheon can be marginally different to a butt, however, there are so many variables with casks that it's difficult to say exactly what this difference can be," says Gordon Motion, Edrington's master whisky maker. Additionally, a first fill cask has a more intense influence than a second or third fill, as each time a cask is filled its influence diminishes. This means the original 'distillery character' remains more visible in subsequent fills.
Another consideration is the sherry the cask originally held (which is drained from the cask prior to departing Jerez). However, on arrival in Scotland the staves of a butt, for example, could still contain around 10-20 litres of what is called 'wood extractive liquid' rather than sherry, as it includes various compounds extracted from the cask. Traditionally this liquid was considered to have a significant influence, though whether the oak is European or American is now seen as the primary driver. "Residual liquid in the staves has a negligible impact on the mature malt whisky, it's about the oak," adds Stuart Harvey.