Although bourbon barrels dominate most cask inventories, it’s ironic that most malts also include the influence of sherry casks, with only a certain number of malts, or individual expressions, aged exclusively in bourbon barrels. This includes Laphroaig, Glemorangie 10 year old, Ardbeg 10 year old, The Glenlivet 12 year old First Fill, The Balvenie Single Barrel 15 year old, Talisker 20 year old and Port Ellen 1979.Bourbon barrels became more widely used during the Spanish civil war, 1936-9, and World War II, when sourcing sherry casks (a traditional favourite) became harder.Around 300,000 bourbon barrels are acquired annually by the Scotch whisky industry, though this total fluctuates and can exceed 400,000 barrels in peak years. Amere 18,000 sherry casks arrive in Scotland annually, with a hogshead (250-305 litre capacity) around £250, while a butt (500 litre capacity) is around £420.Three grades of bourbon barrels (180-200 litre capacity) are available. ‘Select’ barrels, around £35-40 each, are selected after the bourbon has been dumped, but inspected prior to filling in Scotland, of which around 95 per cent are typically fillable, with only a minority requiring repairs.‘Distillery run’ barrels, around £25-30 each, are shipped from the dump trough without any selection procedure. Around 60 per cent are typically ready for filling, with 40 per cent requiring repairs. However, it’s hardly a fixed percentage and could be 40 per cent fillable and 60 per cent requiring repairs.‘Cull’ barrels are expected to need repairs before filling, though the degree of repair is unknown. But with instances of up to 30 per cent of a container not requiring repairs, there can be pleasant surprises. Prices fluctuate around $18 a barrel, with price and availability depending on the ‘planter market,’ and how many barrels are bought by retailers to sell in garden centres.American oak barrels are typically fashioned from 40 to 100 year old trees harvested in Missouri, Kentucky and Tennessee, with the soil type also influencing how the oak grows. Glenmorangie’s inventory, for example, includes oak from the Ozark Mountains in Missouri, where a combination of low rainfall, shady conditions and poor soil fosters slow growth.“The oak tree has two distinct spurts of growth each year, early wood in Spring and late wood during the rest of the year. Slow growth gives more early wood, resulting in a more porous oak with creamy, buttery, sweet vanilla, silky and syrupy flavours.Glenmorangie Artisan Cask is the first expression to be entirely matured in these special casks,” says Glenmorangie’s Dr Bill Lumsden.Most bourbon barrels are fashioned from oak which is dried in a matter of weeks using a kiln, compared to air-drying for a couple of years. Air-drying breaks down tannins in the oak more than kiln-drying, reducing astringency, while increasing vanilla levels and establishing more nuances of the classic range of flavours.However, one school of thought is that differences between kiln-dried and air-dried oak may only be influential when aging bourbon, but not when re-used to age Scotch.Toasting and charring barrels also establishes the flavour profile which the spirit extracts.Barrels are initially heated in order to bend the staves, with continued heat toasting the surface, while applying a naked flame to the interior creates a layer of char typically two to four mm deep. Wood sugars are partially caramelised in the underlying two to three mm, releasing flavour compounds such as vanilla (the majority of the stave, around 20 mm, remains unaffected).While there are various degrees of charring, there aren’t standard terms to define them.Some distillers stipulate a ‘light’ or ‘heavy’ char, while others use a scale of one to four.Anumber one char is typically described as burnt toast, with a number four popularly termed an ‘alligator’ char as that’s what it resembles. Even the same term may not mean exactly the same at different cooperages.Aheavier char breaks up the surface of the oak more than a lighter char, enabling the spirit to access the underlying toasted layer more readily, from which various flavour compounds are extracted. This includes vanilla, while tannins help promote body, balance and structure.Consequently a lighter char, and so less access, promotes greater sweetness, honey and body, while heavier charring provides more direct access to the toasted layer, and typically results in more vanilla, creme caramel, toastiness and oakiness.The char also acts as a ‘filter,’ partially absorbing immature elements and sulphur character (variously manifested as struck match, rubbery or meaty notes) in the spirit, with a heavier char having a greater absorption capacity.As sulphur character masks other characteristics such as floral and ester notes, reducing sulphur character enables these floral and ester notes to show through, which can significantly change the flavour profile.Consequently, while a heavier char provides greater access to the toasted layer, and a higher level of absorption, which can benefit a heavier spirit, a lighter spirit requiring less ‘absorption’ may become dominated by oakiness.Similarly, whether to use a first, second or third fill depends on the nature of the spirit and level of oak influence desired. Each time a cask is filled its degree of influence on the spirit diminishes. A second fill contributes around 50 to 60 per cent of the influence of a first fill, with a third fill dropping to around 25 to 30 per cent of a first fill.In terms of the flavour profile, a first fill leads with vanilla and associated flavours such as caramel, créme brulee, fudge, cream soda and oakiness, followed by coconut, almonds and hazelnuts, with spices such as cinnamon typically developing thereafter.Asecond fill leads with a reduced level of vanilla flavours, and a marked decrease in coconut, nutty nuances, which complement rather than dominate the spirit.Athird fill shows even more subtle vanilla, and as second and third fills have less extractives to contribute, ‘distillery character’ shows through more clearly.“At Diageo we like to produce malts that are distinctive but differentiated. For that reason, for the Classic Malts Selection we opt to pick from refill casks that allow the individuality of the distillery character to be the dominant partner, as opposed to the wood flavours that certain casks can impart,” says Maureen Robinson, Diageo’s master of blending.Each successive fill doesn’t simply deliver the same flavour profile in a progressively milder format, as the balance of maturation influences also changes. Oxidation, for example, shows more in a second fill than a first , which means fruity, estery notes. The rate of reactions within the spirit also diminish, which means a different rate of development. But leaving spirit for longer in second and third fill barrels won’t give the same result as a shorter period in a first fill.The char’s ability to absorb sulphur compounds also diminishes, not to mention physically breaking down within the cask.When the spirit is emptied, some char also leaves the barrel, which means less char available during the second fill, which is also less active.The level of ‘residual bourbon’ within the staves, technically a wood extractive liquid as it contains various oak-derived flavour compounds, can amount to 75 cl, or more.However, this only has a meaningful impact in a first fill barrel.Consequently, for some distilleries a first fill means a risk of ‘bourbon’ notes being prominent or dominant, though there’s no risk of this with a second fill.The level of residual liquid also depends on how fresh the barrel is, and practicalities such as whether it’s been shipped with a bung in place, or not.A barrel may, have stood in the open for several weeks, even up to a year in the case of ‘cull’ barrels, before being shipped, which progressively reduces the level of residual liquid. Meanwhile, the fastest door to door service is around 21 days.As different fills offer varying influences, it’s a case of selecting the most appropriate influence for particular styles of malt.“A first fill cask can be more oaky and give more tannin, as well as giving a slightly drier, slightly sharp note, while a good refill cask can give more fruity, honeyed sweetness, so a refill cask can show The Balvenie character better,” says Wm Grant’s David Stewart.Iain McCallum, Morrison Bowmore’s blender, adds, “My personal favourite are refill bourbon barrels, everything is there at a level that works well and balances together, nothing stands out on its own.“Enough has been taken out by aging the bourbon and the first fill to smooth and polish the spirit.” Another factor is that bourbon has become so much more specialised during the past 10- 15 years, which includes longer aging.Barrels used for bourbon’s minimum two year aging period impart different influences, compared to a barrel used for several years.This is because longer aging removes more extractives from the barrel.“I think there’s still too much life in the oak of a barrel used to age bourbon for two years, and if the spirit becomes really woody at a young age you can get a sharpness in the spirit. We want the wood to enhance the flavour, and I think older bourbon barrels do this better,” says Iain McCallum.