For rum and the same is true for Scotch, Brandy, Cognac, Wine and Whisky, the “Angel’s Share” is the amount of distilled spirits lost to evaporation from the barrel or cask into the air as the rum ages.
The amount the Angel’s take each year varies widely based on barrel size, time, environmental conditions and even the location in the warehouse. Warmer is the climate and more the evaporation is stronger.
In temperate climates, where cognac and whiskey are generally produced, average annual evaporation is about 2%.
Under the influence of the Caribbean tropical climate, average evaporation is 3 to 4 times higher, between 6 and 8% per year. This allows to measure the truly exceptional character of rums claiming up to 30 years of age!
After two years of ageing, the amount of rum remaining in casks (84%) is equivalent to that remaining after 8 years in Scotland.
After 8 years, more than half of the rum has volatilized in angel’s share, it takes 30 years to have the same result with a single malt.
After 12 years, only 35% of the original distillates remain, 28% after 15 years and only 8% after 30 years.
The style of warehouse and the location in the warehouse also greatly affects the Angel’s Share and many other characteristics of the final product. The atmosphere at the top of a rackhouse is very hot and dry, allowing the water molecules to escape much faster than the alcohol molecules thus driving the proof up in those barrels and concentrating the flavors inside the barrel. The atmosphere at the bottom of the rackhouse is moist and cool. Higher humidity will result in more alcohol evaporating than water.
Low humidity and higher temperatures will draw more water through the staves resulting in higher alcohol content.