How does distillation work?
By exploiting the fact that alcohol boils at a lower temperature than water — around 78°C against 100°C — so heating a fermented liquid produces a vapour richer in alcohol, which is then condensed back to liquid.
The crucial correction. Distillation does not create alcohol; fermentation does. Every spirit begins as a fermented liquid — a beer-like wash for whisky, wine for brandy, fermented molasses for rum, fermented agave for tequila. Distillation only concentrates what fermentation produced, and separates it from water and from unwanted compounds.
Why it is not a clean separation. Alcohol and water form a mixture that cannot be fully separated by simple distillation — beyond about 95% alcohol they boil together as a constant-composition mixture, which is why higher purity requires other methods.
The cuts, which are where the craft lies. The distillate does not come off uniformly:
The heads, or foreshots — volatile compounds including methanol and acetone, discarded.
The hearts — the desirable middle fraction, collected.
The tails — heavier compounds producing oily, unpleasant characteristics, discarded or redistilled.
Deciding where to make the cuts is the distiller's central judgement, and it determines the character of the spirit far more than any equipment.
Pot versus column stills. A pot still works in batches, is less efficient at separating, and therefore retains more flavour compounds — which is why it is used for malt whisky, cognac and many rums. A column still operates continuously and separates far more thoroughly, producing a cleaner, higher-strength spirit — used for grain whisky, vodka and neutral spirit.
What happens afterwards is frequently more important than the distillation. Whisky, brandy and aged rum spend years in wood, where the spirit extracts compounds from the barrel, oxidises slowly, and loses volume to evaporation — the angel's share. Gin is neutral spirit flavoured with botanicals, usually by redistillation.
On methanol. It is a genuine hazard in unregulated production, which is why the heads are discarded and why commercial distillation is regulated.