English: The black walls of the Blair Athol Distillery in Pitlochry, Scotland.

The guide explained to us that the black color on the walls of the distillery was actually some kind of fungus, Baudoinia compniacensis, which feeds off the alcoholic vapors from the oak casks inside the buildings. Every year as much as 2% of the whisky evaporates through the wood so that after 12 years, the normal aging time for the Blair Athol Single Malts, only about 75% or three quarters of the original whisky remains in the cask! The rest, as they say, has been shared with the angels.

However, a good thing about this evaporation, even if it reduces the strength of the whisky in the cask, is that it's the most volatile alcohols that evaporate first, those that slash & burn. So the Angel's Share is really not such a bad thing.