Walk into a Speyside warehouse on a still summer morning and you can smell whisky. The air is warm and slightly sweet, and the walls are tinted that particular shade of dark grey that warehousekeepers call "warehouse fungus" but which is technically a benign mould called Baudoinia compniacensis. The fungus eats the alcohol vapour. The vapour is the angels’ share.
What it is
Every cask of whisky in a Scottish bonded warehouse loses, on average, around two per cent of its volume every year. The figure is not exact — it varies with cask type, warehouse climate, position on the rack, time of year — but two per cent is the rule-of-thumb most warehousekeepers use.
Over ten years, a 200-litre hogshead loses roughly forty litres. Over twenty years, around eighty. By thirty years, the cask has lost roughly half its original volume.
It is called the angels’ share because it is gone. Whatever the angels do with it, you cannot get it back.
Why it happens
Oak is a porous wood. The cask is sealed but not sealed completely; alcohol vapour and water vapour move through the staves continuously. Some of the spirit interacts with the wood (this is what makes Scotch taste like Scotch). Some of it leaves the cask through the staves and goes into the atmosphere of the warehouse.
What evaporates faster — alcohol or water — depends on the climate. In a cool, damp Scottish warehouse, alcohol leaves slightly faster than water, so the ABV of the cask drops by roughly half a percentage point a year. In a hot, dry climate (think Kentucky bourbon warehouses), water leaves faster, and the ABV goes up. This is why the same age statement on a Scotch and a bourbon mean quite different things.
Why it matters to a cask owner
Three things follow from the angels’ share, and they all matter to anyone holding a cask.
One: stock that has aged is rarer. A 30-year-old cask is not just "older"; it is also physically smaller. Each year of ageing reduces the available stock of older whisky in the world. New stock cannot be created retroactively. This is one of the reasons old whisky tends to command a price premium that is not strictly proportional to age.
Two: cask owners do not pay for evaporation. Storage and insurance fees at our warehouses are flat per cask, not per litre. As your cask gets smaller, the per-litre cost of storage rises, but the absolute cost stays the same. Your statement shows the current volume on every reading.
Three: there is a sweet spot. Hold a cask too long and the angels eventually take everything. The traditional cut-off in Scotch is around fifty years — not because the spirit has gone bad, but because there is not enough of it left to bottle profitably. Most casks that get sold get sold somewhere between eight and twenty-five years of age.
Things to know before you buy a cask
The Scotch Whisky Association estimates that roughly the equivalent of 20 million bottles of Scotch evaporates every year. That is the angels’ share, summed across the whole industry. Quite a generous tip.
If you want to read more about what you actually own when you own a cask, our whisky casks page walks through the paperwork and the warehouse setup. If you want to talk to a real person about what kind of cask might suit you, drop us a line.