Not all whiskey is created equal, and that’s certainly the case here at Athrú. We pride ourselves on making some of the best whiskey in Ireland, and in our mission to make every drop delicious, we always go the extra step from the very start, to the exquisite finish.

And, the finish is what we’re here to talk about, because we don’t just distill our single malt whiskey, pop it in a barrel and then bottle it. We know it’s worth the wait, so we take our spirit andfinish it in unusual or special casks to bring out those refined aromas, flavours and textures.

It’s a method used to create some of the best Scotch whisky, and it’s one we’re proud to use here at Athrú - our single malt Irish whiskeys all benefit from this extra step in the maturation process, and it’s what gives them their unique flavour profiles and decadent hues.

Of course, once it’s distilled our spirit is perfectly sippable, but, if you love whiskey as much as we love whiskey, then you’ll be willing to wait a little longer to get your drams on an expression with added character. 

Wood finishing, or secondary maturation may only be a relatively ‘new’ way of making whiskey, having emerged over the past two decades. But, it’s an exciting extra step in the process which yields some fantastic results on the profile of the whiskey, giving it a lot of its character and uniqueness, and it’s those touches we’re proud to add to every bottle of Athrú.

What is wood finishing?

In a nutshell, it’s the process whereby a distiller transfers their whiskey from one type of cask to another. Also known as secondary maturation, this adds complexity to the whiskey, changing its colour and flavour profile, depending on the type of cask used.

Irish whiskey has to be aged in barrels for three years to be officially be classed as a whiskey - and these barrels can be new, or, previously used to house other liquids. Ex-bourbon casks are most commonly used in whiskey production. They’re made from American oak, and during its time in the barrel, the liquid inside takes on some of the flavours and aromas from the wood - for example, ex-bourbon casks will typically have an essence of vanilla, and this characteristic will find its way into the new make whiskey inside, so that by the time the liquid has matured and can be classed as whiskey, it’s developed some of these flavours and characteristics.

Some distillers choose to just barrel-age their whiskey in one type of cask, but wood finishing sees this process taken a step further; the distiller will then select another wooden cask, which has previously contained a different liquid to the one used in the first maturation stage. These can be from a number of different liquids - ex-sherry casks are incredibly popular, but distillers are constantly experimenting and innovating; wine casks, beer barrels and even barrels formerly used for tequila have all been used in whiskey wood finishing.

The origins of wood finishing

While wood finishing, or secondary maturation has become much more popular in recent years, the practice is one that’s steeped in tradition. In fact, Hazelwood Demense, home to our Lough Gill distillery used to import entire casks of sherry and port which were shipped from Spain and Portugal. Within Hazelwood House, there are records which show these purchases being made by the Wynne family as far back as the 1700s.

Once finished with, these casks were not cast aside - instead, they were recycled, and used to store Poitin. This colourless spirit then took on some of the flavours of the previous fortified wines - while this home-distilled spirit may not have had the characteristics of today’s Irish whiskey, it seems the foundations for double maturation may have been laid long before the method was officially recognised.

Wood finishing as it’s known today was first seen in Scottish distilleries and became a popular method of Scotch whisky distilling in the 1990s. Now, it’s a technique used all around the world to create unique, flavoursome whiskeys.

The effects of wood finishing on whiskey

Yes, we could make a delicious single malt in an ex-bourbon cask and bottle it, but wood finishing gives us the chance to add additional levels of depth and flavour. 60-70% of the final whiskey profile comes from the type of cask used and the length of maturation period, which means that everything from the aromas that fill your nose when you breathe in your expression to the flavours on the palette and even the colour of the whiskey have been influenced by the cask.

Typically, the first cask and second casks have complementary flavour profiles - for example, ex-bourbon and ex-sherry casks are often used for the first and second maturation processes, because the sweeter vanilla and toffee notes from the bourbon harmonise with the sherry’s rich notes of dried fruit and spices. 

You may be wondering why distillers don’t use these casks for first maturation, and instead save them for wood finishing. Generally, previous liquids such as wine, sherry or rum would have too much impact on the whisky were the barrels to be used for the new make spirit, so instead they’re saved for the second maturation, where they impart enhancing flavours.

The time whiskey spends being wood finished can vary - anything from several months to several years is commonplace, and the length of time impacts the final nose, palette and colour of the whiskey.

Why does cask size matter?

When it comes to wood finishing and the flavour of the finished whiskey, the size of the cask used for second maturation is very important. You may notice that a lot of the choices for second maturation casks are larger than the bourbon barrels used to age the whiskey. This isn’t fluke - there’s some science behind it. Larger barrels and casks have lower surface areas compared to their volume, so they will have less of an influence on the whiskey than a smaller barrel or cask. This makes wine and sherry casks ideal containers for wood finishing - they’re able to subtly impart flavours to enhance the overall finish, without overpowering the whiskey.

Timing is also an important factor; whiskey matures faster in smaller casks, which is ideal for the initial aging process in an ex-bourbon cask, but isn’t so essential for wood finishing where it may only mature for a few months rather than a number of years.

First-fill casks vs Refill casks

Neither first-fill or refill casks are casks that are being used for the first time in whiskey production. Instead, they’re being used for the first time with the new liquid inside, but in both instances these casks have had a previous life, maturing other whiskeys, sherries or spirits.

First fill: In the world of cask filling, first-fill, or first use casks, don’t actually mean they’re being used for the first time - this term refers to a cask that has been previously used to age something other than whiskey, such as sherry, port, or other spirits or wines. This cask is now being used to age a whiskey for the first time, hence the term ‘first fill’. First-fill casks impart the most prominent characteristics of the previous liquid on to the whisky, while each subsequent fill will receive progressively less influence.

Refill: Whereas first-fill casks are casks which have previously housed other liquids, refill casks have formerly been used to age whiskey, and are then refilled again with whiskey. While the characteristics of the previous liquid inside the cask might be more similar to the new whiskey going in, this process still sees the cask having an impact on the liquid inside; sweet, smoky, fruity or spicy notes can still be transferred to the next whiskey to be matured in the cask.

The different casks used in wood finishing

Wondering what the difference is between the casks used for wood finishing? Most of it comes down to their sizes, but just as importantly, the flavours and characteristics of the previous liquid inside, and the effect that will have on the whiskey while it’s in there.

 PX - A Pedro Ximenez or PX cask is one that was previously used to mature Pedro Ximenez - a dark, sweet sherry hailing from Southern Spain. Whiskey which is wood finished in a PX cask will take on flavours of dried fruits and raisins, whilst likely adopting some of the rich colour from the sherry too.

Oloroso - Oloroso sherry is drier than PX, so despite there being some similarities in the two variants of fortified wine, a whiskey aged in an Oloroso cask will be very different to one which has matured in a PX cask. The Oloroso is nutty and oily in character, with a dry, smooth finish, and whiskeys aged in Oloroso casks will benefit from fig and raisin notes with a walnut-like nuttiness. 

Bourbon - Bourbon casks are a popular choice for whiskey production, but it’s not just the former liquid that impacts the whiskey during second maturation. Bourbon casks are made from American white oak, which are then toasted and charred before being used for the first time. These flavours from the wood come through in the whiskey, along with characteristics from the bourbon itself, which will give the whiskey vanilla, butterscotch and caramel notes.

Virgin oak - Virgin oak casks are usually used in the production of bourbon, and are toasted and charred before use. However, they can also be used for wood finishing, and in this case they go through the same toasting and charring processes used in bourbon production, but haven’t housed any bourbon. This makes them a bigger investment for distilleries to purchase, as these casks are being used for the first time during the wood finishing stage of distillation. The new wood, combined with the toasting and charring give the whiskey a distinct woody flavour and aroma. The colour of the whiskey is also impacted significantly by the wood - bourbon gets much of its deep colour from the new, charred barrel and the same is true for whiskeys wood finished in virgin oak - a deeper colour profile can be expected, along with depth of flavour too.

European oak vs American oak

It’s not just the former liquid that has an effect on the wood finishing process; the type of wood used also has a major influence on the outcome of the final product. Typically, two types of oak are used for whiskey barrels - American oak and European oak - but what’s the difference?

American oak: American oak is the oak generally used for the barrels of whiskeys distilled in America and Canada. This wood is quite dense, and contains compounds which impart a sweet, vanilla taste on the whiskey inside. Many of the typical characteristics in bourbon; mellow vanilla and caramel notes are created by the influence of the oak.

European oak: European oak is less dense than American oak, and tends to be more expensive - this is because the trees grow more slowly than those used for American oak. Price aside, there’s also a big difference in the influence this oak has on the whiskey. This wood contains gallic acid, which, when combined with the whiskey, gives it a spicy flavour and a bitterness - decidedly different notes to those which are created by American oak.

The effect of charring on whiskey

Irish whiskey usually begins its life in ex-bourbon barrels, and the reason there’s so many of these barrels available worldwide, is because bourbon is only allowed to be made in new barrels. This means once the distiller is finished with them, they’re available to be used (and re-used) by distilleries across the world.

However, it’s also a requirement that all bourbon and Tennessee whisky barrels be charred - this act is done during the coopering process, and consists of burning the inside of a barrel - the amount of which will then impact the flavour and characteristics of the whiskey. Some coopers choose to char for just a few seconds, some as many as three minutes - and these levels of charring are crucial in determining the effect on the whiskey.

So, what is the effect? Well, charring converts the wood sugars into the vanilla and caramel flavours so prevalent in bourbon, and the intensity of the charring determines the intensity of these flavours. The carbon in the charred wood also acts as a filter, eliminating sulphur compounds in the spirit.

Barrels can be charred more than once, which is why during the wood finishing process, they can be re-charred. This will help maintain the effectiveness of the barrel both from a maturing and flavour perspective.

Cask costs

Whiskey distillers are constantly striving to get creative with their spirits, finishing them in curious casks from specialist producers across the globe. While ex-bourbon casks are fairly easy to come by, sourcing those specialist sherry casks can be a little tricker, and those purchased from boutique producers can set a distillery back hundreds or even thousands of euros. Some of the world’s most expensive whiskeys are finished in casks which previously contained sherries first distilled almost 100 years ago. 

When it comes to finishing whiskeys in rare and exquisite casks, we know a thing or two. Our own Athrú trilogy series of whiskeys have been expertly finished in rare casks, from our Annacoona, finished in a combination of Pedro Ximenez and Oloroso Sherry casks, to our  Knocknarea - a 14 year old single malt whiskey finished in the finest Oloroso Sherry casks. Then, there’s our Keshcorran - a 14 year old single malt whiskey finished in Hungarian Tokaji casks.