The wash is filled into the first copper pot still, called wash still, and is heated from below and from the inside respectively. Today mainly hot steam is used for heating. Using an external gas flame has become rare. In the first case, hot steam is lead through specially shaped heating tubes inside the pot still, thereby heating the wash. At 78° C the alcohol starts to evaporate before the water does. The alcohol steam rises in the tapered tube.
Over the neck and the lyne arm the steam is led into a condenser where the alcohol steam is liquefied again. The water mostly remains in the pot still. All single malt whisky distilleries work with at least two series-connected pot stills. The first one, the wash still, distils the wash to 20% to 25% of alcohol. The resulting liquid is called “low wines”. The low wines are then transferred into the second pot still, called low wines still or spirit still, where they are distilled to an alcohol content of 65% to 70%. In the Scottish lowlands a lot of distilleries used to use a third still. This third pot still produced even purer alcohol at more than 75%. Today there are only few distilleries left in the Lowlands (Ailsa Bay, Auchentoshan, Bladnoch, Daftmill and Glenkinchie), and only Auchentoshan still has three pot stills.
Important! Keep in mind that pure alcohol tastes only like alcohol. A single malt whisky gets its taste from the heavier oils and fats and the lighter esters and other flavour carriers from the wash. The further you distil a whisky, the more it will lose its individual character.
During distillation, the unique shape of the pot stills is the main contributing factor to the taste of a whisky. A long and slim shape produces soft, pure alcohol (e.g. Glenmorangie), while a short, squat shape produces strong, intense flavours (e.g. Lagavulin). The intensity of the heating is also important for the taste. If you heat too strongly, many accompanying substances and fusel oils will get into the whisky, which will surely not be as smooth as if it had been distilled slowly. Typically the distillation process in the spirit still takes up between 4 and 8 hours.
The wash stills usually have a capacity of 20,000 to 30,000 litres, while the spirit stills can only contain 10,000 to 20,000 litres of the higher concentrated low wines.
The pot stills must be replaced after 15 to 25 years, when the wall thickness of the copper has decreased to 4 to 5 mm. The stillman makes sure that the shape of the still is not changed because this would lead to a change in taste, too.
The story goes that some stillmen even replicate dents and bumps in the new pot stills, but that’s just a fairytale.The outlets of the stills are sealed by the government so no thirsty Scot can get his hands on untaxed spirit. In order to assess the quality of the low wines and the spirit anyway, the pipes are run through a case usually made from glass and polished brass, the spirit and sample safe. The stillman checks the quality and runs the spirit back into the still or into the spirit receiver using valves and levers.
All this is done only by visual inspection and with measuring instruments. A stillman cannot taste the spirit! In the sample compartment of the safe he can measure the temperature and take samples in order to measure the density of the spirit (and its alcohol content) with hydrometers. The stillman’s most important task is to cut off the middle cut properly. At this stage it is decided whether the batch is going to be just good or excellent. The foreshots take about 30 minutes to run through. The middle cut is then extracted for about 3 hours. The last runnings of the distillation, called feints, are led back into the spirit still. They contain higher concentrations of propanol, isopropanol and fusel oils.
The foreshots might contain the highly volatile and poisonous methanol, which can lead to blindness or even death if consumed excessively. Modern yeast strains are grown not to produce any methanol at all. That’s why the separation of the foreshots is just a matter of taste today. The feints contain the fusel oils responsible for headaches. Since the feints are cut off rather early, people usually don’t get headaches after enjoying single malt whisky.