Casks of whisky, maturing in a warehouse
The cooper at work

Any spirit younger than
three years isn't whisky
according to the SWA.
And their word is law.

So, WOOD plays a crucial role in the development of a malt
whisky. First of all there's the species of OAK that is used by
the COOPER to make the casks. Only oak casks are used.
Not only because it's a legal requirement; oak is flexible and
solid at the same time while it adds very distinct elements to
the spirit inside the cask as well. Just like all French cognacs
have been matured exclusively in Limousin oak casks, only a
few oak species are used for whisky casks; White American
oak ( Quercus alba) and European oak (Quercus robur).

Interestingly enough, the fact that it does mature in oak casks was
discovered only by accident. In the old days wooden casks were used
merely as vessels for the freshly distilled spirit. A lot of the spirit was
consumed before it ever got the chance to evolve into anything that
smelled or tasted like the whisky we drink today. Nevertheless, here
and there some casks were left alone long enough to age in peace.
During those months or even years the rough spirit became whisky.

I can only imagine the drunken delight of the lucky Scotsman who discovered the
secret of maturation in an oak cask. Scotsmen like to gossip (at least that's what I've
been told by other Scotsmen). It wasn't long before the news of the wonders of wood
became known to more people and after a while most customers developed a clear
preference for the matured stuff. Nowadays the wood works its magic for at least 3
years before a spirit is bottled as a Scotch whisky. Younger spirits can't be legally sold
as Scotch whisky, but I've actually sampled a few young spirits that are just stunning.


Did I say that whisky does not
mature after being bottled? Well,
it actually does - but very slowly.
More information on the process
of oxidation can be found in the
Advanced Beginner's Guide PDF.


Main Segments

However, not only the type of wood of the cask is important. Factors like the size and shape of the cask
can have their own influence on the whisky inside as well. However, I'll get into that topic later on...

So, now the whisky is distilled - can you drink it yet? Not quite.
Well, you CAN, but it won't be much fun. Just like a fine wine, a malt
whisky is shaped by many different influences; the type of water at
the distillery, the shape and size of the pot stills, the climate within
the warehouse, etc. But unlike a fine wine (or even a not so fine one),
a single malt whisky hardly ages or changes, after it has been bottled.

Macallan 15 Fine Oak whisky

Ex-Bourbon Casks  versus Ex-Sherry Casks

The standard ex-bourbon cask holds some 200 liters of whisky.
It is usually called a BARREL and has been used only once for
the maturation of bourbon in the US before it was taken apart
and shipped to Scotland. There are many different brands like
Jack Daniels, Jim Beam, George Dickel, Maker's Mark, Old Crow,
Wild Turkey & Four Roses, but all these bourbons are actually
produced at just a handful of distilleries. The precise heritage
of the bourbon casks is considered a topic of relatively minor
importance. That seems rather odd when you look at all the
fuss that is made about the trouble that some blenders go
through to secure just the right sherry casks or wine casks
for their compositions. Bourbon barrels are usually heavily
charred on the inside. The carbon from the burnt layer not
only acts like a filter (removing certain nasty elements from
the spirit), it also helps to transfer vanilla and woody notes
to the bourbon. In its 'second life' in Scotland a cask  keeps
influencing the contents. However, the effect that the wood
has on the whisky inside a cask slowly diminishes over time.
This is especially true when a cask is used 'more than once'.
The casks that Americans use (just once) for their bourbons
are made from the oak trees that grow in America - which is
another species (Quercus alba) from European Quercus Robur.
Most American oak casks are sourced from Kentucky, Missouri
and Tennessee. As I mentioned earlier, the size of the casks is
much more standardised (+ 200 liter) than that of sherry casks.

Sherry casks are usually larger than bourbon barrels and come
 in several different sizes. A HOGSHEAD holds 250 liters, while
 a BUTT is twice that size with 500 liters. Distilleries sometimes
use other sizes (like the 450 liter PUNCHEON ) as well, but the
vast majority of the casks are still barrels, hogsheads & butts.
Just like the discovery of the significant benefits of maturation
 in oak casks, the magic of sherry was uncovered by accident.
 Scotland relied on oak trees from the forests of England for
 a long time, but at some point distilleries needed to find an
 alternative source for their casks. Sherry (fortified Spanish
 wine) was once very popular on the British isles. Actually,
 it still is -  the UK alone takes care of a whopping 29% of
 the world's sherry consumption, with Holland coming in a
 close second with 27%. The sherry bodegas use Quercus
for their casks - i.e. European Oak. There are many
different 'types' of sherry, which means there are different
 types of sherry casks as well. To name just a few of them;
 Fino, Oloroso, Manzanilla, Amontillado, Pedro Ximénez, etc.
 Are you dizzy yet? No?  Good - then you might like to learn
 that  'Pedro Ximénez' is also the name of the grape species
 used for sherry. In fact, only two species of grape are used
 for the production of sherry; the other one being 'Palomino'.
Compared to the 1970's, sherry consumption in Holland and
the UK has dropped off dramatically. As a results, production
of sherry dropped as well - and therefor availability of casks.

Cask sizes

Anyway, sherry used to be shipped from Spain to England by the cask. Alcoholism was running rampant in those days, so empty and discarded
sherry casks were littering the 19th century Scottish landscape very much
like empty beer cans and bottles are doing today. In one of the world's
first recycling initiatives the Scots started picking up those second hand
sherry casks to use them again for the maturation of their whiskies.
And then use them again & again & again - waste not want not...
These days, both bourbon and sherry casks are used more than once.
A sherry cask that has seen four or five fillings is no exception.
When a cask has held and aged whisky for the very first time
it's called a FIRST-FILL cask, after a second filling with fresh
whisky it's a SECOND-FILL cask, etc. With each filling less
and less character is transferred to the whisky. Depending
on the pedigree of the cask it will become 'exhausted' after
a couple of fillings. You can find more information about the
rejuvenation of casks in the Advanced Beginner's Guide.

When it comes to casks, SIZE matters as well. In a smaller cask, the surface of the wood interacts with a (relatively) small
volume of whisky. This results in a relatively fast maturation compared to larger casks. A 500 liter sherry butt has to 'convert'
much more spirit than a 'small' 200 liter bourbon barrel, while its surface isn't significantly larger. But then again the effects of
bourbon casks are different from those of sherry casks. And with the spreading of finishing, even more different types of casks
(wine, rum, etc.) appeared on the scene. And it's not just what you've got - it's how you use it that's probably most important.


There are various wood effects.
Sherry casks impart winey and
raisin notes to a malt, and port
casks even more so; bourbon casks
tend to produce soft, sweeter malts
with vanilla tones and fresh wood.


Some people feel that STORAGE
is a important factor in the life of
a cask of whisky as well. Issues
like the type of warehouse, the
location of the warehouse and
even the location of a cask within
the warehouse) play a role in the
maturation process. That goes for
humidity and temperature as well.

Barrels in whisky warehouse

Not everybody is convinced of the relevance of storage conditions, though.
It's hard to distinguish fact from fiction and observation from imagination when
it comes to malt whiskies. When you know a distillery is located on the shore
it's easy to find coastal elements like salt and fish in the whisky they produce.
But a chemist would often have a hard time finding the corresponding chemicals.

Whisky casks

Every cask BREATHES while it matures. The wood of the casks expands during the heat of summer and contracts during the cold of winter - not unlike the private parts of any kilt wearing Scotsman who's minding his sheep, I would imagine. As a result of evaporation the spirit will annually lose up to 2.5% alcohol while it matures.

The part of the maturing spirit that vanishes between
casking and bottling is called the ANGEL'S SHARE.
But some angels are thirstier than others, it seems.
The Saint Magdalene 1979/1998 in the UD Rare Malts
series still had an alcohol percentage of 63.8% when
they bottled it after 19 years, but other malts are in
danger of dropping below the legal minimum of 40%
in their mid-20's. I will take a closer look at the illegal
activities of angels in the Advanced Beginner's Guide.

Recommended Reading about whisky

Whisky Classified (David Wishart)
Peat, Smoke & Spirit  (Andrew Jefford)
Malt Whisky Companion (Michael Jackson)
The Making of Scotch Whisky
  (Moss & Hume)
Advanced Beginner's Guide  (Johannes v/d Heuvel)

TIMING is probably the most important element in the malt whisky equation.
Deciding when the whisky in the cask is ready to be bottled is not an easy task.
That's why I devoted an entire chapter of this guide to the topic of bottling - so please click on to chapter six of the Beginner's Guide...

Previous ChapterA Beginner's Guide to malt whisky - OverviewNext ChapterAdvanced Beginner's Guide on WhiskyA Beginner's Guide to malt whisky - OverviewAdvanced Beginner's Guide on Whisky
Brora 22 years olt whisky
Whisky barrel charring

Most of the single malt whiskies that were not matured in bourbon casks have been matured in (more
expensive) sherry casks, mostly from France and Spain. After the finishing fad hit Scotland a few years
ago we've also seen single malts that were 'finished' (or even matured completely) in relatively exotic
casks that used to hold port, madeira, cognac, Cuban rum and even various wines like Chardonnay,
Bordeaux, Sauternes, Fontalloro, Pomerol and Yquem. You can find more information about those
'freaky' casks in the Advanced Beginner's Guide . On this page I'll focus on the 2 main wood types.

Sherry is a wine that's
produced near Jerez de
la Frontera in Andalucia,
deep in Southern Spain.

Almost every cask that's used for the maturation of malt
whisky has been used before in the production of other
beverages - BOURBON or SHERRY. About 90% of all the
Scotch malt whisky ends up in casks that have contained
bourbon before they were shipped from the USA. That
makes sense, because USA law dictates that a bourbon
cask can be used only once for the production of bourbon.

The HISTORY of an individual cask plays an important role in the maturation process.
Completely 'fresh' oak casks have actually been used in the past, but the current wisdom is that the 'woody' elements
from a virgin cask would overpower the subtleties we appreciate in a single malt whisky. That means that even casks
that are described as 'fresh oak casks' on the label of a bottle have usually contained another liquid that would have
absorbed some of the harshness of the new wood. That other liquid is usually bourbon.

After being used once, a cask is useless for the bourbon distiller.
That means that the Scots can pick it up that cask for a friendly price - and we all know
that the Scots are almost as thrifty as the Dutch. So, over the past few decades, more
and more ex-bourbon casks were used for the maturation of Scotch whisky.
This change in wood management (among other things) has caused a
noticeable shift in the flavour profile of Scotch whisky in recent years.

a wall of whisky casks


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A Beginner's Guide, Chapter 5 - Maturation


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