If it's made to hold water in a living history setting, it's probably paraffin lined.
A properly swelled cask or barrel will hold the alcohol fine for short periods.
As pointed out a charred cask or barrel is normally used to age some alcoholic beverages. The charring acts like activated charcoal, and draws out certain impurities, plus does remove a bit of the strength of the alcohol. Certain styles of distilled spirits are aged in specific casks that have previously held a type of wine. So some whiskeys are aged in used, red wine casks, some in used sherry casks, etc. Others are in straight, charred wood casks. Not only is the cask itself changing the distilled spirit, but the previous contents seeping out of the wood is as well.
Cheaper distilled spirits save aging time by being run through an activated charcoal filter, then being placed in a cask to conform to legal requirements.
Now I wrote "short period of time". Well if it's paraffin lined, it probably will sit indefinitely. IF it's a charred cask, and some folks use these at living history events for water, as they tend to make the local water taste a bit better. So if you use one for alcohol, then you will lose some of the alcohol over time due to evaporation through the wood. We're talking if you stored a gallon of rum for a year to use at the same event next year you probably would have like 7/8 of a gallon when you opened the cask. This is what is called the "angel's share".
A small charred cask, whether it's full of alcohol or water, needs to be stored on its side and rotated each month about 1/4 turn, so that the top most stave does not dry out. As was written before, once swelled, it should not be left to dry out, ever. Remember too that if it is a charred cask, if you return it to water use after having stored some spirit..., it will have a faint flavor of the previous spirit that was once stored within it.
LD
