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.
One of those strange-but-true things that I learnt in a tiny distillery in Scotland is this:
Due to long-established legislation in the US to protect its timber industry, American bourbon distillers are only allowed to use each barrel once. This is insanely wasteful, and in Scotland and Ireland, whiskey barrels traditionally are reused. Reused barrels contribute to a depth of flavour for later batches.
But as a result, you’ll find ex-bourbon barrels from the US all over the world, because they’re sold off comparatively cheaply. Which is why you might find small distillers in Scotland ageing their whiskey in old bourbon barrels, or gin distillers in Australia doing likewise. I’m pretty sure I’ve come across wine makers using old bourbon barrels for some specific vintages, although if pressed I absolutely cannot remember who it was, nor what wine.