We visited Buffalo Trace distillery, the oldest continuingly-operating distillery in America (yes, they were allowed to continue production during Prohibition). Bourbon whiskey must be more than 50% corn but can contain other combinations of grain, thus their various blends and vintages. The photo with the the 4 transparent barrels shows what happens over time - all Bourbon is aged at least 5 years, but due to evaporation and natural barrel-leakage, the oldest stuff, at 18-20 years of maturity, has less than a third of the original volume left in the barrel.