The distillation process is unique for each distillery using pot stills. (Distilleries using Lomond stills - there are very few of them left now - can produce several types of whisky.)
This means that all the whiskies produced by a certain distillery are treated on the same way, with the same malt, the same stills on the same way by the same people... So, why can they be so different from each other? The answer to this question is in the aging process, the casks used, the nature of the warehouse, the taste of the air (it seems that a whisky aged in casks stored in warehouses close to the sea have a different taste from a whisky aged on some other place). Glenmorangie Cellar 13 is a good example of that phenomenon.
If the surrounding air has a (little) influence on the taste of whisky, one must realize that many distilleries bring their casks to some central place near Edinburgh for their aging. It it not clear to me if the whiskies aged that way are marketed as single malt or if they will be used in blends.
In other words, the influence of the air on the taste of whisky; myth or reality?
There is one thing for sure however, and that is that the role of quality of the barley, the making process, and the nature and quality of the casks where it was aged is very important. According to some specialists, this could be good for 95% of the final quality of a malt whisky.
To have the right to bear the name of whisky, a grain spirit (malted or not) must be aged at least for 3 years in a oak cask. Unlike Cognac which is stored in new casks, the Scottish always use second hand casks.
The kinds of casks
The oak casks are classified by capacity, and the following casks exist:
A gallon is 4.546 litres The capacity of the casks is approximated.
The information about the capacities of the various casks comes from the Campbeltown museum.
The picture has been taken in the yard of Old Pulteney. Casks on the foreground are "sherry butts"
The Scotch whisky industry uses mainly 3 kinds of casks:
the "barrel" : ±190 litres
the "hogshead" :± 250 litres
the"butt" : ± 500 litres
The shape of the casks is mainly due to historic reasons, related to storage problems on ships. Sherry was carried on Spanish gallions, and the slender shape of the butts was the best for storing on this kind of ships, while the Portuguese Port was stored in a more bulbous cask, which was easier to carry on Portuguese merchant ships.
Often whisky is aged for a while in bourbon casks, and finishes his aging period in some kind of other cask, in order to give is some new fragrances, before bottling. Generally it stays for 6 to 12 months in another kind of cask. This explains the "wood finish" mention on some bottling's. For instance, the 18 yo Glenmorangie finishes its maturation in next casks, which is rather uncommon in Scotland.
A whisky cask is always a second hand cask. It generally contained bourbon (american whiskey made from corn - (maize). Sherry is also very popular in the whisky industry. Other casks are used too, like Port, Madeira and more rarely Claret (French red wine) or rum, etc... Glenmorangie is specialized in "wood finishes" and some of them are very expensive, probably because of the rarity of the casks.
However, there is a question about this wood finishes. If the aim is to give some new and pleasant fragrances to the whisky, everybody knows (at least in the whisky industry circles) that this method is used sometimes to hide some distillation errors. Often, the casks are warmed up before transferring the whisky, in order to accelerate the fragrance transfer. Such practices are not acceptable, because the consumer has no way to know about this.
A quick mental calculation ca make you feel dizzy. There are about 100 active distilleries all over Scotland. The average production of each of them is between 1.200.000 and 2.000.000 litres a year. To deserve the "Scotch label", whisky must stay at least 3 years on the Scottish territory in oak casks. Assuming that the annual production is about 150.000.000 litres, the absolute minimum of whisky stored in Scotland is 450.000.000 litres This only to guarantee the legal right to be called Scotch whisky. This is without taking in account the huge quantity of whiskies which are aging for 10 to 30 years...
On the other hand, the casks used for storing whisky are never new casks. It is thus very important to maintain the casks in good state. Some distilleries have their own cooperages (like Balvenie or Bruichladdich for instance), but most of them prefer outsourcing this to specialized companies. There are lots of cooperages in Scotland, and the most famous of them (because it is a first class tourist attraction) is the Speyside Cooperage, situated half way between the Glenfiddich distillery and the centre of Dufftown. This cooperage has about 300.000 casks in stock. All of them need reconditioning. There are about 20.000.000 cask all over Scotland. A cask can be (re)used for a maximum of about 60 years.
The angels share
The advantage of oak for maturing alcohol is that it is not airtight. It lets surrounding air enter the cask (which explains the salted taste of a whisky aging near the sea), but is also lets evaporate the whisky it contains. It is generally admitted that between 1 en 2% a year evaporates this way. Evaporation can affect water contained in the cask, but also the alcohol itself, resulting in a diminution of the alcohol percentage. That is called "the angels share". However, this percentage is theoretical, because this could result in a strange situation, as old whiskies (30 years and more) would lose their right to be called whisky. Indeed, assuming a whisky has about 70% of alcohol when it leaves the spirit still, and loses about 1% of alcohol a year a 30 years old whisky would just have a percentage of 40%, which is the lowest limit for a whisky.
The angels share is indeed the part of alcohol which escapes to excise rights. Excise rights are calculated on the amount of alcohol coming out of the still (and not on the amount of water). As this amount is diminishing over the years, it would not be fair to tax the marketed whisky based on the alcohol percentage it had when it was distilled...
The nature of the warehouse is also very important. A damp cellar or a dry cellar will influence the evaporation of the spirit differently. In a dry cellar (with a concrete floor), water will evaporate mainly, letting a dryer whisky with a higher alcoholic percentage. In a damp warehouse (beaten-earth floor) the alcohol will evaporate, letting a rounder whisky, with a smoother taste.