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Caskology - Maturing Whisky in Oak Casks - History & Practice

Updated: Mar 5


whisky barrel warehouse

Image by Medium


Caskology isn't a word you'll find in the dictionary. However, we're going to define it as part of our understanding of casks in maturing whisky and other aged spirits.


So, what does caskology mean? We believe it's a term that describes the knowledge of extractive maturation, the importance of wood quality, and the craftsmanship that goes into making casks. Simply put, caskology is about understanding cask maturation as a complex sum of variables. If that's too fancy, we could just call it cask ageing.


In this article, we will focus on the seven subcategories that make it easier to understand the complexity of caskology in whisky production. Although the idea behind the maturation of spirits is quite simple (take a spirit, put it into a cask, wait, and drink), many factors affect the final product. Why should we care? Because around 70-80% of the taste of whisky comes from the cask.



Here are our 7 category picks:

  1. History of Casks

  2. Cask Types

  3. Cask Making

  4. Cask Sourcing

  5. Cask Seasoning & Cask Finishes

  6. Extractive Maturation

  7. Cask Recycling & Reuse

We are unable to go into extreme detail on all categories as this is an article, not a book.



 
  1. History of Casks

Mesopotamia wine cask tablet

Image by Sherry Wine


Casks have a long history, dating back to at least 350 BC when Armenian wine was transported to Babylon in Mesopotamia using palm-oak casks. By this time, cask-shaped containers had already been developed that were watertight, stackable, and easy to transport. Even earlier, around 2,600 BC, cask-like wooden containers filled with grains were discovered in a burial site. It is believed that the art of constructing hooped barrels was kept secret by the Celts, who passed it down through generations as a skill. Later, the Romans and Greeks also adopted the use of casks.


For almost 2,000 years, casks were the go-to method for transporting all kinds of goods, from nails to gunpowder, due to their durability and movability. Rolling a barrel onto a ship was much easier than carrying small amounts of goods at a time.


old sherry wine warehouse

Image by Sherry Wine


Until recently, the concept of using casks to enhance the flavour of alcoholic beverages was new. Prior to that, alcohol was kept and fermented in clay pots or bronze vessels. It is believed that the technique of ageing in barrels was discovered by accident while transporting wines from one place to another. Another theory suggests that sailors realised that their rations tasted better on their way back from a long sea voyage than when they left. Regardless of how it was discovered, the practice of maturing alcohol in casks quickly became widespread. Today, many liquids and even some foods spend time in casks, including Tabasco hot sauce, salted fish, wine, and spirits.



 

2. Cask Types


American and Scottish whiskies must be aged in oak barrels. Oak became the norm due to its softness, which made it easier to bend into the desired shape. The first known casks were made from palm trees, but oak emerged as the preferred option after multiple attempts. Although other trees can also be used to make barrels, they are not commonly used in ageing whisky due to regulations. For instance, Canadian whiskies can be aged in any 'small' barrel of less than 700 litres with no specification of wood.


Historically, people would use any type of wood to make their barrels for convenience, especially when waterproofing was not an issue. However, modern wood management practices have made the use of oak barrels standard in whisky ageing.


oak cask pores

Image by The Wood Database


As much as the softness of the wood used for making barrels matters, it also comes down to the pores of the wood. Oak has fine, small pores that make it more resistant to leaks and evaporation. American white oak, used for maturing bourbon, has fewer tannins than European oak, which is excellent for maturing wines.


Some Irish whiskies have been finished in different woods, including acacia, cherry, mulberry, and chestnut. But what about Sherry or Port casks? When another wine or spirit has previously been matured in the same cask, the barrel can be labelled as such. This doesn't mean that such a thing as wood is coming from a tree called a sherry or port. Usually, the distinction between 'casks' and 'wood casks' will give you a hint.


Probably the most exclusive type of cask is made using Japanese Mizunara Oak. Only a few casks are made yearly due to the difficulty of making them from a tree that doesn't grow straight and has high moisture content and larger pores than other types of oak, making it more prone to leaks. Many whiskies spend only a few months in these casks because labelling a whisky with Mizunara Cask guarantees a quick sale. The Mizunara oak tree takes around 200 years to grow before it can be cut, and the wood is usually sold at auctions where a barrel's price can rise to around £6,000. That's twelve times the price of an average ex-sherry cask. Retired casks are sometimes treated like royalty, with ceremonious festive send-offs.

Mizunara oak casks at Bowmore distillery

Image by Wine Enthusiast Magazine


There are many different sizes of casks that can be used to mature wines and spirits. The most commonly used barrel for ageing spirits like whisky is the American Oak cask due to its availability.


The smaller the barrel, the greater the contact between the spirit and the wood, which results in faster maturation. For instance, in an Octave, whisky matures much faster than in a butt due to the larger wood surface area that has contact with the liquid. In some cases, the time for maturation can be reduced from several years to just a few months. However, according to the Scottish whisky-making regulations, the spirit must be aged for at least 3 years in oak to be called whisky. Before that, it was simply a spirit. Smaller casks are sometimes used for secondary maturation or display purposes in shops and distilleries.


whisky cask sizes

Image by Elixir Distillers



 

3. Cask Making


The craft of cooperage and wood management is a complex and fascinating subject that could fill an entire book. However, in this article, we will cover some of the key points.


Traditionally, many Scottish distilleries used to have their cooperage on-site, but today, only a few directly employ coopers. At the turn of the millennium, only 250 skilled coopers were in Scotland. However, thanks to active apprenticeships, that number has since risen to 300 in recent years.


In America, the Associated Cooperage Industries of America (ACIA) lists around 30 cooperages as members of the association. The cooperage industries in the two countries differ in many ways. For example, American whiskies are mainly matured in virgin oak casks, while Scottish whisky is matured in refill casks that are re-coopered from shipped staves.


parts of a cask barrel

Image by Barrel & Garden


To create a good quality product, the rough staves used for making barrels must be left to air-season outside for approximately 60-90 days. Trying to make a good product from freshly cut wood is impossible in the wood-related industry. During the drying process, which is also known as equilibration, the moisture content is reduced. If the staves are not properly dried, unequal shrinkage can occur, which can negatively affect the barrel when the liquid is already inside. In Scotland, the seasoning of the staves is a by-product of storing the barrels outside in the sea air. Some argue that the casks can pick up minute traces of the surroundings, but this claim is debatable.


Stave seasoning in US versus Cask seasoning in Scotland

Image by ISC Barrels/ Islay Cottages



Crafting watertight barrels is a highly skilled and intricate process. Some liken it to solving a puzzle. For instance, a hogshead barrel, which holds approximately 230-250 litres of liquid, typically consists of 31-33 staves that are roughly 98cm/39" long and 3cm/1.25" deep, with varying widths. Hogsheads are a Scottish tradition created by combining the staves of five ex-bourbon casks that can hold roughly 200 litres each with new heads to create larger casks. The staves are held together by six steel hoops and 12 rivets.


Building a whisky barrel whiskey cask

Image by Science Direct


Assembling a barrel for whisky involves a process called toasting or charring. This process helps to increase the surface area of the wood, making it easier for the spirit to penetrate the surface. It also breaks down the wood's structure so that it can be bent to shape.


Toasting is a crucial step in the barrel-making process, allowing the flavours to absorb into the whisky. The level of toasting affects the flavour profile, with gentle toasting creating nutty, light vanilla and sweet notes, while heavy toasting gives more caramel notes. On the other hand, charring is mostly used in bourbon production as a secondary barrel toast. It increases the levels of vanillin and cacao-like flavours. With different levels of charring, you can even create an 'alligator skin' deep char, where the burned wood also works as a filter for unwanted impurities of the base spirit.


whisky cask whiskey barrel char levels

Image by Flaviar/ Difford's Guide/ Distillery Trail



 

4. Cask Sourcing


As with many other industries, the spirit industry also recycles and resells parts used in production. Casks are no exception. In America, bourbon whiskey must be matured in charred virgin white oak casks for a minimum of 2 years, so barrel-makers focus on creating new casks. The new barrels are responsible for most of the flavour in the first three years, and many whiskies are not matured beyond that point. Once the casks have been used, they are sold to other whiskey makers around the world, such as Scottish and Japanese distilleries.


broken down whisky barrel cask

Image by Midwest Barrel Co.


Distilleries in Scotland could make their casks from scratch, but one of the reasons they don't is that it is much more expensive to use virgin oak casks than to reuse them. To make transportation easier and take up less cargo space, barrels used in bourbon production are usually broken down and shipped to distilleries worldwide. Once they arrive at their destination, skilled coopers re-cooper the barrels.


re-coopered whisky casks barrels

Image by Celtic Timber


In contrast to America, Scotland doesn't have regulations on using only virgin oak for whisky production. They usually employ previously used casks, though they can use new ones if they choose. These casks are also reused multiple times until they are no longer useful in whisky maturation. This way, whisky distilleries get their money's worth.


Another type of 'second-hand' barrel that's commonly used to mature whisky is sherry casks, which come from Spain and are highly sought-after. They give the whisky a bolder and sweeter flavour, and their deep colour comes from the decades of maturation of the sherries they once held. Ex-sherry casks are about 5 times more expensive than ex-bourbon casks, which could also be reflected in the final product's price.


Sometimes, casks are used to 'marry' whiskies together. The 500-litre sherry butts are perfect for this purpose.


sherry butt compared to a whisky cask barrel

Image by Jerez-Xeres-Sherry - Sherry Butt vs Whisky Barrel



 

5. Cask Seasoning & Cask Finishes


There are various methods to season a cask, excluding the option of simply leaving it outside for some time. However, some people have been known to take shortcuts when it comes to seasoning Spanish fortified wine casks by introducing sherry to the casks for only a brief period. This results in a weaker sherry cask whisky compared to those done properly.


It is believed that sherry casks were initially used due to an abundance of empty casks on Scottish docksides in the 1900s when sherry was a popular beverage in Britain. However, as British consumption of sherry declined towards the end of the century, the casks became highly valued for their flavour-infusing properties. This led to the creation of Paxarette, or Pax for short.

Paxarette whisky cask paint

Image by Eye For Spirits - Paxarette


A man named WP Lawrie devised a solution to the shortage of ex-sherry casks by using an additive from the winemaking industry to rejuvenate tired sherry casks. Paxarette, the additive, is made from fermented grape juice and concentrated Pedro Ximenez grape must. It quickly gained popularity in the whisky industry, which desperately needed sherry casks. However, the Scottish Whisky Association banned its use in the production of whisky in 1990, rendering it illegal. Prior to the ban, Paxarette was widely used, so if you had a sherry cask whisky before 1990, chances are it was infused with Paxarette.


stacked up whisky casks barrels

Image by Mixology Magazine


Whisky can be matured entirely in ex-bourbon or sherry oak casks, but sometimes, a secondary maturation process is applied to the whisky in a different type of cask. This is called a cask finish. Whiskies can be finished in a variety of casks, including rum, ex-Islay (for a smoky finish), Sauternes, Madeira, port, red and white wine, Tokaij, and even cider. Each finish imparts a unique flavour and character to the whisky. Some argue that finishes are added to whiskies to improve the flavour after the first maturation process. It's worth noting that it is illegal to add any additional flavours to the whisky after maturation, but colouring agents (documented in E150) can be added to enhance the colour.



 

7. Extractive Maturation


In the whisky industry, the process of maturation is known as extractive maturation because it perfectly describes what happens when whisky is left in a cask for several years. In Scottish whisky, the spirit must be matured for a minimum of three years to be considered whisky, but many whiskies are aged for decades. During this time, the whisky will extract various things such as vanillin, tannins, colour, and esters from the oak in which it is stored.


different compounds whisky gets from a cask barrel

Image by Compound Interest


One of the most significant factors to consider when it comes to whisky maturation is the process of evaporation, also known as Angel's Share. The wood quality and the cask's storage location can greatly impact the level of evaporation that occurs. During the maturation process, each barrel will lose some of its contents to the air, which is referred to as 'stolen by angels'. The evaporation process can also have other effects, such as decreasing the ABV in colder climates and increasing it in hotter ones. Each cask in a warehouse has its own unique rate of maturation, which can vary greatly depending on where it is stored and the size of the cask.


Laphroaig distillery barrel cask warehouse

Image by PJP


Angel's Share plays a vital role during the maturation process of whisky. It refers to the evaporation of whisky from the cask, which reduces the quantity and concentrates the flavours and colours of the remaining whisky. Over the years, the smoky character fades in heavily peated whiskies, and the longer it matures in the cask, the less smoke can be detected in the final product.


The quality of the wood used for making the cask and its ability to last decades determine the success of the maturation process. The coopers play a crucial role in this by ensuring that the whisky gets everything it needs from the cask. Although it's impossible to foresee everything about the maturation of whisky, it's a part of its attraction.


The casks are laid on their sides inside the warehouse, and every cask has a hole on one side to get the whisky in and out. These holes are also used by master blenders to monitor and choose which whiskies are ready to be blended and bottled.


seeing whisky ageing in barrels casks angel's share see-through cask

Image by whisky.com



 

7. Cask Recycling & Reuse


In the production of bourbon, the casks are only used once. This is due to regulations that stem from the fact that virgin oak casks release the majority of their compounds within the first three years. After the casks have been used, they lose the prefix 'virgin' when they are sold and shipped for the maturation of other whiskies all over the world. While many whisky producers use the casks multiple times, the first time they fill the cask is called the 1st fill, followed by the 2nd fill and ultimately the Refill.

hand holding a sponge

Image by The Iron Maiden


The cask plays a vital role in contributing to the taste and colour of whisky, acting almost like a sponge. Just like squeezing a sponge releases water, the cask imparts compounds to the whisky. However, with every use, the cask loses its ability to contribute to the whisky's flavour and colour. Even a well-made cask has a limited lifespan, and after decades of use, the wood becomes 'tired' and can no longer mature the spirit. Despite this, some whiskies are still stored in these casks, even when the oak is 'dead'.


For example, one of the oldest whisky released to date has been aged for 80 years, but it is unclear how much of that time was spent actively maturing in the cask.


Glenlivet 80 year old Gordon & MacPhail

Image by whisky.com


The used casks can be rejuvenated in some cases. This involves removing the charred wood and re-charring the staves to open the pores again. However, this process can only be done once or twice due to the thinning of the stave's 3cm depth with every rejuvenation.


When a cask is no longer useful for the whisky industry, it can be repurposed for various other uses. People have been known to use them as garden elements, furniture, and even to make guitars. Some distilleries give away used barrels for free, so you only need to pick them up.


guitars made of used whisky barrels casks

Image by Bourbon Barrel Guitar Company



 

Thank you for reading! Stay tuned to The Whisky Ardvark with #whiskyardvark #thewhiskyardvark

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