The Wood in Whisky

Every once in a while at the liquor store, I get stumped each time I come across whiskies that are tagged with sometimes mind numbing phrases such as being Solera Vatted, or matured in Triple Casks, PX or Select Casks.  And I’m sure other novice drinkers like me do find themselves in the same predicament too?

And then began my journey…

As of late, several distilleries have been increasingly exploring this territory of varied styles of maturation, so much so that some of them employ a ‘Wood Master’ to oversee this amalgamation. Both Wood Master and his/her team would be responsible for the health of the barrels and replace or repair those that were either broken or leaking. Additionally, they would need to have an acute knowledge of the interaction between wood and whisky which also allows these distilleries to better place themselves considering the ever so dynamic external environment.  And why wouldn’t these distilleries do so, especially when 60% of the flavor comes from the time interacting with the wood and the residues left by its previous inhabitants.

So first let’s understand that the preferred wood of choice is Oak and legally is the only choice when it comes to aging Scotch whisky. Its tight grain structure, tough and yet malleable characteristics (when heated) make it a perfect choice when it comes to making wooden casks.

Now having said that, cutting the wood, drying it and then shaping it into staves doesn’t quite cut it for whisky maturation; for this we would need to ‘Toast’ and ‘Char’ the wood so as to breathe life. This heat treatment converts the cellulose to wood sugar and lignin to vanillin; while the charring caramelizes this sugar and creates a layer of charcoal which not only mellows the young spirit but also removes any kind of impurities.


Next is the kind of Oak used, which varies from place to place and therefore exhibits different physical and chemical characteristics, some desirable while others not. The most predominantly used Oaks are:

  1. American Oak | Vanilla, caramel, honey, butterscotch, spice
  2. European Oak, mostly those from France and Spain | Dried fruit, spice, nuttiness, chocolate
  3. Mizunara / Japanese Oak, the most recent amongst the lot. Although more prone to leakage, it is considered to be quite flavorsome with a relatively high content of vanillin. Casks made of this particular oak are used in the latter part of the maturation process for shorter durations owing to its overly porous characteristics. | Vanilla, floral, fruit, spice

cask sizes

Now add to this the different shapes and sizes which lend that additional personality to the whisky and you’ve now got a variety to choose from. Dimensions could be of the traditional 200 liter American Standard Barrels (ASB) that are the most widely used in the whisky industry. Then there is also the lesser but still common 250 liter cannibalized version of the ASB known as the Hogshead; I use the word cannibalized, since 4 ASBs are used to construct 3 such barrels.

Distilleries also make use of smaller sized barrels like that of the 50 liter Quarter casks. The increased surface contact between the wood and spirit allows faster maturation and imparts that added dimension of flavoring which distilleries take advantage of when using these for cask finishing. Other casks such as Cognac (~350), Barrique ‘French wine’ (~225 liters), Madeira and Port Pipes (650) are also used for finishing the final product.

Now if you were slapped across with the term, ‘Solera Vatted’, you could almost surely guarantee that the whisky was finished in 650 liter Solera Butts. Although the Solera process is used in Sherry maturation, be it Fino, Oloroso , or Pedro Ximenez (PX), do not confuse this with the term ‘aged in sherry casks’. *Interesting huh?! Quite contrary to what I thought initially*

The Solera process, better known as fractional blending works on a layered array of casks with the oldest set placed at the bottom and younger aged wines stacked above and so forth. At the time of bottling, a certain volume is taken out from the lower most stacks, and is then subsequently filled by the barrels above. This way the younger wines are blended with the older more mature Sherries giving a more consistent character and flavor profile every time a portion is extracted. This entire process might involve barrels several decades old, which makes getting hold on one even more difficult. And is also why they are more commonly used for finishing the whisky and not as a means to completely mature it.

With the lowered demand for Sherry and the shortage of Sherry casks (read as Solera), distilleries now take efforts in manufacturing their own barrels, post which they are seasoned with sherry for several months to even years giving it time to soak up those signature flavors. Once done, these barrels are then drained and sent to the distilleries for aging whisky.

So unless explicitly mentioned, whiskies aged in sherry casks would not necessarily be Solera but rather Butts that are purposefully filled with sherry so as to impart that signature sherry note to the amateur spirits that are poured into the barrel.

Deciphering a few of the most common descriptors printed across whisky labels(or at least how I comprehend them…):

Select Oak/Cask: The whiskies in the bottle are married from a host of ‘select’ casks; means pretty much nothing.

PX : Cask finished using sherry butts that previously contained Pedro Ximenez; a desert wine with a very high content of sugar.

QC : Finished in Quarter casks

QA : Quercus Alba, aka American Oak

Solera Vatted: The whiskies are married in Solera Butts.

Rum Finish: yup…just like the name suggests, finished in Rum casks.

Triple Wood/Cask : Whiskies that are matured in three different casks, which could be a combination of Sherry Oloroso, Quarter, Re-used Bourbon/Sherry casks or First fill Bourbon Casks.

Hope the contents of this week’s blog were informative (or at least a wee bit) and not as boring as my wife had thought!  🙂



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