Scotch whisky is typically distilled twice in copper pot stills. The primary distillation is carried out in a wash still, and produces alcohol of around 25% ABV from a 9% ABV wort. Copper pot stills are preferred because of better heat conduction ability and malleability. These pot stills also enhance catalytic processes and neutralise some of the sulphur present in the brew.
The stills are heated directly by coal, peat, gas or indirectly, by the steam coils. In the wash still, a rummager is used to prevent charring and excessive temperature fluctuations. The first distillation takes around 5-8 hours and is deemed complete when the distillate comes under 1% ABV. A lot of factors are considered during this process including charge, temperature, shape, and size of the still. The final residue left behind is termed as pot ale. The wash distillation is then mixed with the feints and the foreshots from earlier spirit distillations.
The second distillation is usually a fractionated one. The volatile compounds are boiled, known as heads or foreshots. The next process involves a spirit cut. The residue left in the last part is termed feints. The remaining liquid is the spent lees, unusable for the most part.
To put it simply, the distillation is a process of separation. However, the copper and the heat influence the flavor of the whisky. And thus, a fine Scotch is born.