One of the terms you’ll often hear in whisky distilleries is “the angels’ share”. When whisky is slowly maturing in its cask, a small amount of whisky evaporates through the wood and into the atmosphere. Each year, roughly 2% of the liquid leaves the cask this way, so over the years we’ve come to think of this as a sacrifice to the heavens. By giving the angels their share, we ensure the whisky is the best it can possibly be when it’s bottled. Why this happens and what we can do about it is just one of the more complex jobs for our Master Distiller to contend with.
There are a number of factors that affect just how much whisky is lost to the heavens; here are just a few of them.
Generally speaking, spirits tend to evaporate more quickly while they’re young, and then slow down as they age. So, at the very beginning of a whisky’s time in the cask, the angels’ share will be right at the top of the spectrum – around 3.5% to 4% every year. As it ages, the whisky will keep evaporating (it will never stop), but it will do so at much lower rates. By the end of its maturation period, a 20-year-old spirit might lose 40% of its volume.
This is all to do with where the liquid meets the wood. So, whiskies stored in smaller casks will evaporate more, because they just have that much more liquid-to-wood contact. In our experience, casks that hold fewer than 50 litres can show really remarkable losses, which also leads to a faster maturation.
A really good way to illustrate the impact of climate on evaporation is by looking at the bourbon warehouses in Kentucky, the south-eastern US state. During a Kentucky summer, the top floor of a warehouse might hit 50-60C, while the bottom floor may only be 20C. This makes a huge difference to the speed of maturation and evaporation; the casks on the warmer floors will lose liquid very quickly. Kentucky is humid, too, which means water leaves the cask first (rather than alcohol), strengthening the spirit. That’s in contrast to what happens here in our maritime-influenced Speyside climate, where it’s the alcohol that leaves the spirit first.
Airflow has an impact. In bigger warehouses, casks are often stored on racks or pallets, which means they’re raised slightly from the ground. Air circulates around them, leading to greater evaporation. At The Glenlivet, we have a traditional (dunnage) warehouse, with a gravel floor and only a small number of casks. This helps us to hold on to liquid as best we can. These are just some of the factors that affect the angels’ share. This is something that requires constant and careful attention from our experts at the distillery.