Casks Casks Casks

  • 10 mins

Here is a rundown of the different variables that are in play when it comes to maturing the spirit in wooden casks.

How many times has the cask been used?

Historically, casks in Scotland have been used multiple times. It’s a factor which has huge bearing up on the balance of influence between wood and distillate. The skill of the whisky-maker lies in identifying these influences and managing them in an overall marriage of different casks of different ages and levels of wood activity. In Scotland, here’s how you would define the filling order of casks.

Virgin fill: This is more of a recent phenomena in Scotland that has arisen from experimentation amongst modern day distillers.  Historically, the vast majority of Scotch whisky casks would have held some sort of previous contents - usually a wine in Europe, or a Whiskey in North America. Virgin oak has usually undergone a varying degree of air or kiln seasoning and some kind of heat treatment (see below). 

Seasoning helps to properly dry out oak staves and open the pores of the wood which allows the spirit to gain access deeper into the wood, where it can thus dissolve more sugars and colour compounds. Air-seasoning over many months is the superior version of this process as it allows for a degree of wood tissue decomposition which creates more interesting compounds for whisky to access. 

Virgin casks of any oak type are still rare in Scotland and can have a swift and powerful impact on even the most robust of distillates, as with our Octomore .4 editions.  Adding swift colour, sweetness and intense notes of spice, liquorice and vanilla - often producing characteristics reminiscent of American bourbon. At Bruichladdich Distillery, we use only a few parcels of virgin oak casks to create options for recipes.

First fill: Casks that have previously held sherry, port, wine, or another type of whisky or spirit are shipped to Scotland. Sometimes they are broken down into component staves which are then re-coopered into different cask sizes by Scottish coopers. These are first fill casks, in that they have held alcoholic contents prior but they are being filled now for the first time with Scottish new make spirit. These are widely prevalent in today’s whisky industry as distillers seek to create richer, often sweeter, Scotch whiskies and have more control over maturation. Here at Bruichladdich we use a predominance of first fill American oak ex-bourbon barrels. This type of cask matures whisky at a good, consistent pace; contributes sweetness, creaminess, and spice flavours; while also leaving more room for the distillate to express its character than you get with virgin oak.

Second Fill: By this stage most casks have given a reasonable proportion of their natural wood sugars and colour compounds. As a result their whiskies are lighter, mature at a more stately pace and can therefore be more dominated by flavours of cereals, fruits and natural distillate character. It is often observed that second fill casks are better at preserving distillery character over longer periods of maturation. Although, the level of activity of these casks is still very much dictated by the duration of its first fill. We use varying proportions of second fill casks in our recipes at Bruichladdich Distillery, often to add balance and complexity to a final whisky.

Refill: Historically, casks that were filled more than twice would have been referred to as ‘plain oak’. These casks still have trace amounts of sugars and compounds, but they are far more neutral in character than the more active earlier fills. Refill barrels and hogsheads of this age depend greatly on the duration of their earlier fills and are often used to mature whisky for many years as they take an extremely long time to exert dominance over a distillate. Some of Bruichladdich’s oldest releases over the years have depended on long periods of maturation in refill hogsheads.

What has the cask previously held?

The prior use of a cask can have a huge impact on the character of the spirit eventually filled into it.

Bourbon & American Whiskey: Bourbon and American whiskey: America’s distillers are required by law to use virgin oak. When combined - as many frequently are - with hot climate maturation, this creates an extremely powerful and assertive style of whiskey. One dominated by the influence of the oak and highly concentrated in style. Often, they display intense flavours of spice, liquorice, dried fruits and vanilla. This residual character contributes to the signature effect of first fill ex-bourbon barrels on Scotch whisky. The slightly less active oak along with a different type of distillate and the cooler climate maturation conditions of Scotland combine to produce whiskies that are often filled with the natural sweetness of barley but bolstered by the creamy vanilla, coconut and soft spice tones of the casks and their residual American spirit.

Sherry: The British have long been the world’s leading consumers of sherry. This has had the fortunate historic side-effect of making sherry casks abundant throughout these isles and of ready availability to the fledgling whisky industry of the 19th century. Casks - historically often made of American oak but also Spanish - would be used to mature and also to transport sherry from Spain. These casks, once disgorged here in the UK, often in merchant ports like Bristol, would then be transported to Scotland and used for the maturation of whisky.

Today many of these ‘old style’ sherry casks are extremely hard to come by as sherry is no longer legally allowed to be bottled outside Spain and UK consumption levels have dropped significantly. So, many producers commission specially seasoned casks in Spain or operate their own seasoning bodegas. Here at Bruichladdich Distillery, we still source parcels of traditional sherry casks from our partner in Jerez, which have been used for many years to mature top quality drinking sherries. Sweeter styles of long-aged sherry such as Pedero Ximenez or Cream Sherry will impart dense, dark fruits and syrupy sweetness. Whereas drier Oloroso or Amontillado can give a more leathery, earthy and rancio (musty) character to a whisky. Casks which have contained paler, dry sherries, such as Manzanilla or Fino, commonly impart lighter colours and nutty, salty, and exotic fruit flavours.

Wines: A large number of ex-wine casks are deployed in contemporary whisky maturation. The primary factors of influence here tend to be sweet or dry, and red grapes or white. Sweeter white wines such as Sauternes can add complex layers of orchard fruits, honey and elegant sweetness. Wheres a drier red grape wine cask - like an ex-Bordeaux or Burgundy - will often add punchier notes of spice, red fruit and tannin. Then again, sweet red wine casks like Port “pipes” can add concentrated, sweet, jammy red fruit characteristics.

These types of cask often have residue of the previous contents absorbed into the wood, which can be dissolved by the stronger spirit and imbue the developing whiskies with additional flavours and colours. That's why we're so proud about the quality of the wineries whose casks we carry.

The activity level of a wine cask is also greatly dependent on its fill number. A fresh, first fill ex-red wine barrique can be a swift acting and powerful cask, whereas that same cask on its second fill could be a perfect vessel for a longer, slower full- term maturation. We use a wide variety of these such casks at Bruichladdich to broaden the range of options and flavours for our whisky recipes.

Heat treatment?

Almost all casks are subject to varying degrees of heat treatment at cooperages. This consists of charring, toasting or usually a subtle combination of the two. These processes are more associated with ex-Bourbon casks than with historic sherry casks. Although, nowadays distillers often use heat treated French oak casks from coopers and wineries across Europe, or specially commissioned European oak sherry casks which have received varying degrees of heat treatment.

Charring: This is the exposure of the inner surface of the staves to a direct flame. It creates a layer of charcoal which acts as a carbon filter. This is essential during the initial years of maturation as it effectively purifies a distillate, drawing out sharper or less pleasant sulphur compounds and congeners which might otherwise develop into off-notes. It’s part of the ‘extractive’ process during maturation and one of the key reasons why oak is such an integral material in maturing Scotch whisky. There are varying levels of charring, usually graded from 1 - 5. The lowest being very light, is achieved with around 15 seconds of direct flame, whereas a 5 would need above 55 seconds. Charring also caramelises wood sugars, chiefly hemicellulose, which when dissolved into spirit add colour and many of the sweeter flavours associated with more active oak. This type of charring is more common with American oak ex-bourbon barrels.

Toasting: : Different to charring, toasting is more about deeper heat treatment of wood - although some incidental toasting occurs when wood is charred too. This is again about caramelising sugars and opening up the pores to make deeper parts of the oak staves accessible to the spirit. It’s often done in combination with charring to varying degrees and tends to accentuate spiciness, sweetness and colour in a maturing whisky.

Types of Oak

European / French / Spanish oak - ‘Quercus Sessiliflora’ and ‘Quercus Robur’: These oak varieties originate from all over Europe, from France and Spain to Hungary and Slovenia. These genus are more loosely grained and thus more porous. This affects how casks can be constructed and makes it a more costly material to work with. However, it also affects the characteristics it brings to Scotch whisky: often giving deeper tannic structure, softer spiciness and darker colours. These wood types tend to be more highly prized by ‘old world’ winemakers and many ex-wine casks and some ex-sherry casks are constructed from these oak types.

American Oak - ‘Quercus Alba’: American oak is tighter grained and therefore easier to work with - it stands heat treatment better and maintains its watertight properties more easily. These qualities make it a cheaper material to work with, one of the reasons why it is more prevalent in today’s whisky industry. However, it also delivers equally fine and valued characteristics to maturing whisky. Higher in substances like vanillin, it provides greater sweetness, vanilla notes, creaminess and notes of exotic fruit. All American bourbon casks are made of Quercus Alba and many sherry casks also.

Other factors influencing flavour at Bruichladdich

Time in cask: this is undoubtedly the most intrinsic factor in determining cask influence. The longer any spirit rests in cask, the greater the influence in terms of extractive and additive maturation that cask will have. At Bruichladdich, we monitor different cask types carefully and our recipes often include younger whiskies from more active oak, while the older ones will often have spent longer in less active casks. This helps to build balance and complexity in a final whisky.

Storage conditions: Many external factors can influence an individual cask and the profile of maturation it gives to its contents. Depending on the location in the warehouse it might be warmer or cooler most of a year round. The top of a rack warehouse is notably higher in annual average temperature than a stow in a dunnage warehouse. Similarly, different warehouses offer different humidity levels, all of which can affect how quickly and in what proportion of water and alcohol a casks’s contents evaporate. Some casks from the upper levels of a rack warehouse can increase in strength as they age, whereas the vast majority of casks at lower levels or in dunnage warehouses will lose content and alcohol at a more predictable and steady rate.

Head space: As casks mature they lose content (commonly referred to as the Angel's Share), naturally increasing the ‘head space’ and the resulting air to spirit surface ratio. This is where the extremely complex process of ‘interactive maturation’ comes into play, or in other words the ‘breathing’ of the cask allows liquid out and air in. That air can influence the spirit and help to break down alcohol and compounds revealing new flavours incrementally over the years. The whiskies from lower activity refill casks, left to mature for many years, often show deep complexity and enhanced fruit characteristics as a result of this process.

You Might Also Like

Discover the latest stories and news from Bruichladdich Distillery