Sherry, a fortified Spanish wine, was very popular on the British isles at one time.
In fact, it still is. Even today the UK takes care of 29% of the world's sherry
consumption with Holland coming in a close second with 27%.
There are different 'types' of sherry which means there are different types
of sherry casks; Fino, Manzanilla, Oloroso, Amontillado, Pedro Ximénez, etc.
That last bit can be a bit confusing because it's also the name of one of the
two species of grape used for sherry, the other one being Palomino.
(More info and statistics on sherry can be found at www.sherry.org)

So, wood plays a crucial role in the development of a 'modern' single malt.
First of all, there's the species of OAK used by the cooper to produce the casks.
Only oak casks are used. Not only because it's a legal requirement; oak is flexible and solid at the same time and it adds very distinct elements to the spirit inside. Just like all French cognacs have been matured exclusively in Limousin oak casks, only a few species of oak are used for whisky casks - white American oak and European oak (mostly from France and Spain).
But not only the type of wood is important.

But unlike a fine wine, a malt whisky does not age or change once in the bottle. Interestingly enough, the fact that it does mature in oak casks was discovered only by accident. In the old days wooden casks were used as mere vessels for the freshly distilled spirit. A lot of the spirit was consumed before it ever got the chance to evolve into anything remotely like the whisky we drink today. Nevertheless, here and there some casks were left alone long enough to let the wood work its magic.
I can only imagine the drunken delight of the Scotsman who discovered the secret...

Just like a fine wine, a single malt whisky is shaped by a lot of different influences; the type of water at the distillery, the size and shape of the pot stills, etc.

TIMING is probably the single most crucial element in the single malt equasion.
Deciding when the whisky in the cask is ready to be bottled is no easy task. Whisky has to be stored in oak casks for a minimum of three years. This minimum is set by law, but usually the malt gets a chance to develop much longer. Different malt whiskies reach their peak at different ages. A lot of single malts leave their distillery of birth at the age of 10 or 12 years, but some are stored for 50 years or more.

Some people would argue that STORAGE is another important factor in the life of a cask of whisky.
There's the type of warehouse, the location of the warehouse and even the location of the cask within the warehouse. Not everybody is convinced of the importance of these factors, though. It's hard to distinguish fact from fiction and observation from imagination when it comes to single malts. 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. (I'll get back to this 'label effect' later on.)

The HISTORY of a cask plays an even bigger part in the process.
'Fresh' oak casks were used in the past but the current wisdom is
that the 'woody' elements can overpower a subtle single malt.
Most casks that are used for maturing whisky have been used before
in the production of BOURBON or SHERRY. After the finishing fad hit
Scotland a few years ago we've also seen single malts that were 'finished'
in more exotic casks after a period of maturation in sherry or bourbon casks.
We've seen wine, madeira, port and even rum casks But such fancy finishes
are rarities. About 90% of all the malt whisky in Scotland is still matured in
bourbon casks. Most other single malts has been aged in sherry 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,
very much like the private parts of any kilt wearing Scotsman I imagine.
As a result of evaporation the spirit will annually lose up to 2.5% alcohol
while it matures. The part of the spirit that vanishes between casking
and bottling is called the ANGELS SHARE.

A bourbon cask is usually called a BARREL and holds 55 gallons (200 litres) of whisky.
The barrels have been used (only once) for the maturation of bourbon in the USA before they were shipped to Scotland.
There are many bourbon brands but they are all produced at just a handful of distilleries. The precise heritage of bourbon casks seems to be considered a topic of minor importance but when it comes to sherry casks more distinctions are made. Bourbon barrels are usually heavily charred on the inside. Not only does the carbon act like a filter, removing certain undesirable elements from the spirit, it also helps to transfer vanilla and woody notes to the bourbon. In its 'second life' in Scotland the cask keeps influencing the contents, although the effect diminishes over time. This is especially true when a cask is used more than once.

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, resulting in a relatively fast maturation compared to larger casks. A 500 litre sherry butt has to 'convert' much more spirit than a 200 litre bourbon barrel while the surface isn't significantly larger. But then again the effects of bourbon casks are different from those of sherry casks.

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 is used to hold and mature whisky for the first time it's called a FIRST-FILL cask, after the second filling it's a SECOND-FILL cask, etc.

Sherry casks are larger than bourbon barrels and come in several different sizes.
A HOGSHEAD holds 66 gallons (250 litres) while a BUTT is twice that size at 132 gallons (500 litres). Distilleries use other sizes like the 450 litre PUNCHEON as well but the majority of the casks are barrels, hogsheads and butts. Much like the discovery of the benefits of maturation in oak casks the blessings of sherry were uncovered
by accident. Scotland relied on oak from the English forests for a long time but
at some point the distilleries needed to find an alternative source for their casks.

Anyway, sherry used to be shipped from Spain to England by the cask.
Alcoholism was running rampant in those days, so empty sherry casks were littering the 19th century English 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 whisky. And then use them again and again....

A cask has only so much 'character'.
With each filling less and less of it is transferred to the whisky.
Depending on the pedigree of the cask it will become 'tired' and eventually 'exhausted' after a couple of fillings. These casks are usually referred to as PLAIN WOOD. Sometimes casks are 'rejuvenated' by charring them again and/or filling them under high pressure with sherry or paxerette (boiled and concentrated sherry). That doesn't sound very traditional, now does it?

www.sherry.org
www.interlog.com/~contech/article.html
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Some angels are thirstier than others, it seems.
The Saint Magdalene 1979 in the United Distillers Rare Malts series still had an alcohol percentage of 63.8% when they bottled it after 19 years, but some other malts are in danger of dropping below the legal minimum of 40% in their mid-20's.

So now you probably think you know what's so special about single malts, eh?
You don't know half of it, mate! Click on for the next chapter of the mAlmanac, dealing with the bottling of single malts.