The Angel’s Share is a complicated feature of Scotch whisky maturation. It is also a most amusing film by Ken Loach which explores the shenanigans around a mythical cask of Malt Mill.
But we digress. For Scotch to be legally called Scotch, it must be matured in oak barrels, in Scotland, for at least three years. Oak barrels are permeable however, and they absorb a significant amount of liquid over time.
As much as 5% of the volume of the new spirit we put into a cask will be quickly absorbed into the thirsty wood when it is initially filled – but it does not stop there. A smaller proportion, up to 2%, will go right through the grain each year and out into the atmosphere to be lost forever (unless you are an angel). It is therefore not uncommon for a cask that is fifteen years old to be half empty by the time it is bottled.
A number of physical factors can influence the rate at which the angels help themselves. The skill of the cooper who made the cask will obviously have an effect. Also, oak is a natural substance that varies, and so there will be differences between casks, even if they were made at the same time by the same cooper from materials from the same source. The skill and attention of the warehouse man also comes into play over the years. Casks that are turned regularly will not develop the tiny cracks which form over time in those staves which are not in direct contact with the liquid.
Even these variables are by no means the end of the story however. New make spirit is, in simple terms, around 70% pure alcohol and 30% water. The rate at which the two are drawn through the wood and out into the arms of the angels varies according to the humidity levels in which the casks are stored. Casks stored in humid conditions lose a greater proportion of alcohol than those stored at lower humidity levels. Conditions of low humidity draws more water through the staves – leaving a higher proportion of alcohol in the cask. So the rate of loss is not consistent. It varies according to geographical location.
There are other factors too. Temperature plays a part, as do other variables such as salinity. This why Islay maturation is of fundamental importance to Bruichladdich. It will be apparent that the unique cool, humid, maritime conditions in which our precious spirit spends those long years maturing plays a major part in shaping the character of our whiskies. We refuse to compromise on this most important issue. If we cannot mature our spirit on Islay we will stop distilling. It is as simple as that. There can be no compromise.