It was entirely due to practicality that sherry casks became a staple choice for maturing malts. Sherry was originally shipped in casks from Jerez to the United Kingdom, principally the ports of Bristol and Leith. As the UK was a major sherry market, with bottling undertaken locally, the empty casks that remained took up residence in aging warehouses across Scotland.However, the supply of casks began to diminish during the 1980s, when sherry bodegas moved to bottling at source in Jerez. With bottles rather than casks arriving in the UK, distillers have been heading for Jerez to negotiate long-term supply contracts.Unlike bourbon barrels which are continually released from the inventory, as they can only be used once, bodegas use their casks long-term and typically only release ‘ancient’ specimens which are too exhausted to benefit malts. Consequently, casks for malt distillers are commissioned to order.Depending on the source of the oak in Spain, differences in ‘terroir’ can result in varying characteristics. Staves are air-dried for 18 months to two years, with casks subsequently toasted to a lesser degree than bourbon barrels, though sufficiently to caramelise wood sugars and mobilise flavour compounds. As the source of the oak and the way it’s toasted can vary significantly, distillers tend to develop long-term relationships with the same cooperage.Seasoning casks is vital to ‘flush out’ undesirable elements from the oak, such as overt spiciness. This can be undertaken on-site at certain cooperages, or at a bodega, either by filling casks with sherry (typically dry olorosso) for a specified period, or creating a solera system.As the distinctive method used to age sherry, the solera system comprises rows of casks stacked on top of each other. The first row, known as the ‘solera,’ contains the oldest sherry. The row above (‘the first criadera’) contains sherry which is 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 transferring some sherry from the second criadera, and so on. Casks in the final criadera are topped up with the youngest sherry. Maintaining consistency by judging the amount of sherry drawn off, and the length of time between transfers of sherry from one criadera to another, entails considerable skill.“We have our own solera system operated by Gonzalez Byass,” says Whyte & Mackay’s Richard Paterson. “Casks spend at least two years within the solera system, and are seasoned with old oloroso sherry, aged 15-30 years. The key to this is that if a single malt is being aged for 15-30 years, you must have the same muscle in the sherry, it must be complimentary. The 30 year old Dalmore for example was finished in a cask seasoned with a 30 year old Matusalem sherry.” Casks seasoned using a solera system will exert an individual influence, compared to casks filled with a single type of sherry for the duration.However, quantifying that difference is hardly straightforward.Seasoned casks are kept fresh en route to Scotland by giving them four to five litres of ‘transport sherry’ (emptied of course prior to filling with new make spirit in Scotland). The principal type of sherry cask is a butt (500 litre capacity), currently around £420, with a hogshead (250-305 litre capacity) at around £250. This compares to a bourbon barrel (180-205 litres) at £35-40. Moreover, the price of sherry casks is continually rising, reflecting the Spanish RPI (Retail Price Index).Although a hogshead is half the size of a butt, an equivalent reduction doesn’t apply to the price. This reflects various practicalities, as it effectively takes the same amount of time to produce and season both types of cask, with hogsheads entailing a relatively higher wastage of oak.In terms of flavour, sherry casks typically contribute rich fruit, including raisins, prunes, dates, figs and apricots; fruitcake, fortified wine, almond and walnut notes; spices such as nutmeg, ginger and cloves, not to mention Christmas pudding, creme caramel, chocolate, and a (positive) sulphurous note, all delivered within a rich, drying sweetness.The flavour profile is also influenced by the ‘fill’ (the number of times the cask has been filled with spirit). A second fill can contribute around 50 per cent of the influence of a first fill, with a third fill (when relevant) dropping to around 35 per cent or less. However, successive fills don’t simply deliver the same flavour profile in a progressively milder format. As the depletion of extractives varies in different fills, the manner in which a cask matures a malt, and how the original distillery character is influenced, changes with each fill.“The second fill is less intense than the first, and maybe a few notes would drop off, such as sulphur. Dates, raisins and spices would be a lot lower than the first fill, with no apricot, but the nuttiness would remain. Tannin levels would be down from a first to a second fill, resulting in lighter mouthfeel, the balance will still be there but the complexity is less,” says David Boyd, deputy director, inventory and blending, Chivas Bros.“Most sherry cask maturation is at least eight years, so by the time you get to a third fill it’s probably 20 years, so most wood-derived flavours have been shipped out, and you get a holding vessel with a very limited influence.” In addition to which type of fill is used, the duration of each fill is another vital factor. “If the first fill was 15-20 years the second fill will be pretty average, though if the first fill was eight years the second fill will still be pretty good. A recipe of first fill and second fill can give you the best of both worlds,” adds David Boyd.It’s a case of imparting an appropriate influence, depending on the house style of the new make spirit, and level of sherry character required.A lighter, unpeated malt, for example, may principally be aged in second fill casks, to prevent sherry domination, with some distillers using first fill casks to age malts destined for blended Scotch.“I would usually only use a first fill for a special finish, not the main maturation as it’s too dominant, with a second fill used for main maturation,” says Richard Paterson.The level of wood extractive liquid within the staves of the cask, which is not simply residue sherry, as it also incorporates woodderived compounds, could total around five-10 litres in a butt. Traditionally this residue was considered an important element, though whether the oak is European or American is now established as the primary driver, with wood extractive liquid a supplementary influence.Released during the first few years, and so only having a significant impact in a first fill, the nature of this wood extractive liquid is also influenced by the style of sherry used to season the cask. Meanwhile, the level of ‘saturation’ within the staves remains consistent in subsequent fills in terms of volume, but shows different characteristics in each fill.Another issue is comparing the influence of a butt to a hogshead, as the smaller the cask the larger the surface area contact with the wood in relation to the volume of whisky. This accelerates the rate of maturity, which raises the question of whether it’s also a different maturation influence (rather than just being faster). However, the consensus is that the source of the oak, and how it’s been treated, is a more significant influence than differences in cask size.“There’s a more immediate impact of flavours from a hoggie than a butt,’ says David Boyd. “But I don’t think the difference between a hoggie and a butt significantly impacts on a product.” A far more pragmatic aspect of the hogshead versus butt debate is the question of handling, with a butt amounting to serious weight. Adunnage warehouse usually means an entirely manual operation, while even in racked warehouses where fork lift trucks operate, butts can still require at least some manual handling.With The Manual Handling Operations Regulations 1992 laying down the law, The Scotch Whisky Association also works closely with the Health & Safety Executive in developing guidance, issuing SWA Manual Handling Guidelines in the year 2000. Under the guidelines, all casks should be handled to a minimum. Companies are recommended to provide suitable training in manual handling techniques, regularly rotate staff to allow rest from repetitive movement, and use mechanical aids such as lift trucks, together with safety footwear, safety helmets and gloves.An estimated 18,000 sherry casks enter the industry annually, which certainly represents a lot of handling, but numerically this is modest compared to the 300,000-400,000 bourbon barrels arriving each year.Nevertheless, despite bourbon barrels dominating most cask inventories, many malts actually include a sherry influence.