What it is
A cask in a Scottish bonded warehouse loses a little of its contents every year as the spirit evaporates through the oak. The usual rule of thumb is about two per cent of the volume a year. The loss is called the angels’ share.
The figure is not exact. It varies with the cask, the warehouse and where the cask sits in it, so the volume is measured rather than assumed.
Why it happens
Oak is porous. A cask is sealed, but alcohol and water still pass slowly through the staves. Some of the spirit works on the wood, which is where much of a whisky’s colour and character comes from, and some leaves the cask altogether.
In Scotland’s cool, damp warehouses alcohol tends to leave a little faster than water, so a cask’s strength eases by roughly half a per cent a year as well as its volume falling.
What it means for a cask owner
The volume and strength are measured when the cask is regauged, and your statement shows the figures from the last regauge.
Storage and insurance are charged per cask, not per litre. They are included for the first twelve months, then a flat annual fee, typically £40 to £80, quoted in writing before you commit.
No duty is due while the cask stays in bond. It becomes payable at bottling, and by whoever bottles it.
Capital at risk. What a cask sells for depends on the market at the time. Values can fall as well as rise, and some casks take longer to sell than expected.