During first distillation, the pot still (or "wash still") is filled about two-thirds full of a fermented liquid (or wash) with an alcohol content of about 7-12%. In the case of whiskey distillation, the liquid used is a beer, while in the case of brandy production, it is a base wine. The pot still is then heated so that the liquid boils.
The liquid being distilled is a mixture of mainly water and alcohol, along with smaller amounts of other by-products of fermentation (called congeners), such as aldehydes and esters. Alcohol (ethanol) has a normal boiling point of 78.4 °C (173.12 °F), compared with pure water, which boils at 100 °C (212 °F). As alcohol has a lower boiling point, it is more volatile and evaporates at a higher rate than water. Therefore, the concentration of alcohol in the vapour phase above the liquid, is higher than in the liquid itself.
During distillation, this vapour travels up the swan neck at the top of the pot still and down the lyne arm, after which it travels through the condenser, where is cooled to yield a distillate with a higher concentration of alcohol than the original liquid. This distillate, called "low wines" has a concentration of about 25–35% alcohol by volume.
These low wines can be further distilled a second time in a pot still to yield a distillate with a higher concentration of alcohol. In the case of many Irish whiskeys, the spirit is further distilled a third time. However, Cognac and most single malt scotch whiskys are only distilled twice.