By Harold McGee
There are two ways for distillers to separate the vapor into undesirable heads, somewhat desirable tails, and the desirable main run of alcohol. The original way, and the way that is still used for many of the finest liquors, is separation in a simple pot still by time. It can take 12 hours or more for a batch of beer or wine to be heated close to the boil and then distilled. The very volatile head vapors come off first, followed by the main alcohol-rich run, and then the less volatile fusel-oil tails. So the distiller can divert the initial vapors, collect the desirable main run in a different container, and then divert the late vapors again. In practice, distillers repeat the pot distillation, the first pass giving spirits with 20–30% alcohol, and the second 50–70%.