Concentration by steam
Alcohol boils before water, so heating the wash lets the distiller catch its vapours and condense them into something stronger. Malt whisky is typically distilled twice in copper pot stills (Ireland often three times); bourbon and grain whisky run through efficient column stills. Copper is not just tradition — it strips harsh sulphur notes from the vapour.
Shape is destiny: tall stills give light, delicate spirit (Glenmorangie); short, squat stills give rich, oily spirit (Macallan, Lagavulin).