The Making of a bourbon barrel. 

Side note -  The barrels themselves, even after burning/charring, aren’t waterproof, but they are water resistent. In addition to adding smoky bourbony flavors the charring helps the wood absorb some of the liquid being stored in it, causing the wood to swell, which does make the barrel waterproof. However, they are not airtight. A whisky or bourbon barrel can lose 8-10% of total volume in the first year, and up to 60% in an 18 year bourbon. This is from a combination of evaporative loss and loss to the barrel. Evaporative loss is often referred to as “The Angel’s Share” and affects the alcohol content of the liquid in the barrel, because water evaporates out leaving alcohol behind, while the distillate lost to the barrel is often called “The Devil’s Cut”, and generally does not affect the alcohol content of the distillate, as both alcohol and water soak into the wood.