Alcohol strength (Scotch whisky)

The distilled spirit starts life at over 90% alcohol but is reduced to 63-68% for optimal maturation in oak casks. Over time the strength will slowly reduce as the liquid evaporates and matures. De-mineralised water is added to arrive at the eventual desired bottling strength.

Once bottled the whisky does not continue to mature or lose strength. 

Scotch whisky must be bottled at a minimum alcohol strength of 40% and this is the standard strength for most blended whiskies. Single malts are frequently bottled at higher strengths, and often at 'cask strength' which signfies that the whisky has not been reduced with water at the end of the maturation period and prior to bottling.