So, wood plays a crucial role in the development of a malt whisky.
First of all, there's the species of wood (usually oak) which is used by
the cooper to make the casks.

As if being discarded, broken apart and shipped off to Scotland isn’t bad enough for those poor
old second-hand casks, they are often ‘toasted’, charred or outright burnt from the inside out
after being re-assembled. (Sometimes like a Frankensteinian mix of parts from various casks...)

Only a few oak species are used for whisky casks.
The whiskey industry and coopers use many different names for various kinds wood,
but on the ‘lumber’ level there are only two significant species; White American oak
(Quercus alba) and European oak (Quercus robur). When it comes to the ‘primary’
maturation, it’s mostly these types of casks - which are not ‘fresh’, by the way.

Oak is flexible and solid at the same time, while it adds very distinct elements
to the spirit inside the cask as well. That is why, for example, every French
cognac has to be matured exclusively in ‘Limousin’ oak casks.

Virtually all casks are second-hand and have contained another brew
or spirit before - usually American bourbon whiskey or Spanish sherry.
The fact that the casks are used didn’t just appeal to the thrifty nature
of the Scots. They also found out that the previous contents of a cask
shaped the maturing whisky inside it in various ways. (I’ll delve a little
deeper into the “bourbon vs sherry” issues in the Advanced Guide.)

The vast majority of the casks that are used in Scotland are second-hand bourbon barrels.
That’s partly because those casks are dirt cheap. And because several ‘interested parties’ and
lobby groups used the end of American prohibition in the 1930s to “help” introduce a new law
that required American bourbon & rye whiskey to be matured only in NEW, charred oak barrels.
Those interested parties included the lumber industry and the cooper’s union of barrel makers.
And that’s how capitalist and socialist forces joined hands to bring us today’s ‘bourbony’ Scotch...

Is wood an ingredient of whisky?

Traditionally, only oak casks are
used for maturing Scotch whisky,
but on rare occasions there have
been casks made out of walnut,
chestnut and mahogany as well.
For now, I’ll just look at the oak...

So, now the whiskey is distilled - can you drink it yet? Not quite.
Well you CAN, but it won't be much fun. Very much like a fine wine,
a malt whiskey is formed by a multitude of factors and influences.
The character of the spirit is shaped by the type of the water at the
distillery, the shape and size of the pot stills, the type and size of
the cask, the micro-climate within the warehouse, etcetera.

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 laying around long enough to mature.

I can only imagine the drunken delight of the
lucky Scotsman who accidentally discovered
the great effects of oak on whisky maturation.

But unlike a fine wine (and even a not so fine
one, I imagine), a malt whisky matures further
only VERY slowly after it has been bottled. (*)

Oak cask maturation

Interestingly enough, the fact that whisky matures in oak casks was discovered by accident.
In the olden days, wooden barrels were used merely as vessels for the freshly distilled spirit.

It wasn't long before the word about wood got around.
Most customers developed a preference for the matured stuff, so during the 19th century
the number of whisky warehouses in Scotland grew along with the number of distilleries.
These days, British law and the the Scotch Whisky Association dictate that distilled spirit has
to mature in wood for at least three years before it can legally be bottled as a Scotch whisky.
Younger spirits may not be labeled as ‘Scotch whisky’ - but they are sometimes bottled.

What about ‘double matured’ or ‘finished’ whiskey?

The ‘ABG’ has chapters with more information on wood, casks and ‘batches’.

Before we get to ‘double matured’ whiskey, I first have to shatter a popular illusion.
Even single malt Scotch whisky is often ‘blended’ to some extent. Especially when it
comes to the more popular brands like Glenfiddich or Macallan, the ‘bottlings’ are
produced in ‘batches’ of thousands of bottles - all with the same contents and label.

Because you can only draw a few hundred bottles of whisky from the average cask
(more on that later), several casks have to be used for each batch. The chapter on
whisky bottles and bottling delves deeper into this issue, but for now all you need to
remember is the fact that all contents of a bottle of single malt Scotch whisky comes
from a single distillery - but not neccessarily all from the same cask. If that’s what
you want, you’ll need to order a ‘single cask whisky’ - but I wouldn’t, if I were you.

The Scots are crafty, and the fact that all of that single cask whisky was poured into
a few hundred (or less) bottles doesn’t mean that all whisky also evolved in that cask.

Why (cask) size DOES matter

The main point is that you shouldn’t get too
distracted by the wood varieties that the label
specifies - other kinds of wood may have been
involved as well. Just judge a ‘double matured’
whisky as you would any other - and perhaps
buy a few spare bottles if you do like a batch.

So, the fact that a whisky was ‘finished’ or
‘double matured’ or even ‘triple matured’ could
mean a lot of things - there is no legislation at
all that defines what these classifications mean.

Have you seen the movie ‘The Incredible Shrinking Man’?
It may surprise you to know that it lasts much longer than it needed to...

The ‘fiction’ part of this movie from 1957 eclipses the ‘science’ part on
several occasions, and in reality the protagonist would have died soon.

However, it’s hard to
say what would bring
him to his (swift) end.
Probably the cold..

You see, in thermodynamics the relation between volume and
surface area is a pretty big deal. Larger organisms and objects
(like mammoths or copper pot stills) have a fairly small surface
area compared to their mass and volume. This means that they
take a lot of time and/or energy to heat up and cool down.

By contrast, smaller organisms like the shrew and the shrunken
man lose their body heat very quickly. The shrew’s metabolism
has adapted, so it keeps eating constantly. Shrunken men can’t...

The same principles apply to casks of maturing whisky.
The shapes and sizes of casks vary greatly. Larger
wine and sherry casks tends to have a relatively low
surface area compared to their volume. So, If all other
things were equal, those large casks should exert less
influence on the spirit than (smaller) bourbon barrels.

However, not all other
things are exactly equal.
Quite the contrary, in fact!

We’ve now reached the end of chapter 5, so we need to move on.
Just remember that this Beginner’s Guide to Whisky only skims the surface and that
there is more to learn about the mysterious maturation process of single malt whisky.
The Advanced Beginner’s Guide deals with that...

Meanwhile, please don’t hesitate to let me know if you think that I haven’t got my facts
straight or if a certain statement or explanation is unclear. I’ve had to simplify some
issues and avoid some other topics, but if you were an absolute beginner when you
started reading, this should no longer be the case. Next topic: bottling the whisky.