When a spirit is aged in wooden barrels, the wood absorbs some of the more unpleasant aspects of the resting produce and, in return, imbues the liquid with rich flavours and golden hues from the oak. Over time, much of the liquor inside will evaporate into the atmosphere as it breathes through the wood layers. As a consequence there will be an annual loss of about 2% of the total volume. This evaporation is further exacerbated in warmer, drier climates and the loss can be much higher. The overall alcohol content increases as more water evaporates. This loss has been nick-named as the angel’s share because the liquid evaporates into the heavens.
The “angel’s share” helps with the maturation and smoothness of the liquid, as it reduces a high-proof moonshine into a clean-finished alcoholic beverage.