After fermentation the wash gets distilled and traditionally in Scotland, whisky gets distilled twice. Double distillation results in purer alcohol and less impurities in the final spirit. Distilleries have two separate vessels, called pot stills, made from copper. Copper is a great heat conductor and in addition purifies and “cleans” the liquid during the distillation process.
The liquid inside the stills will be heated up to at least 78.37°C. At this temperature alcohol evaporates and rises up to the top of the stills where it gets cooled down and condenses into liquid form again. The longer the still gets fired and the hotter it is, the more other components that impart certain flavours are evaporating as well.

This is the basic process of distillation, but there is a lot more to it. We will break the whole process down a bit more for you and explain each step in more detail.

Two copper stills at Glenfarclas: Spirit still (left) and wash still (right)

First distillation

The wash gets transferred into the first copper vessel, the wash still. Traditionally the still was heated by a fire underneath it but nowadays distilleries use mostly hot steam. It provides better control over the heat and keeps temperature more stable.
Now distillers want the alcohol in the wash to evaporate. To achieve that the wash gets heated up to the boiling point of alcohol (78.37°C) or above. It is important to keep the temperature below 100°C so water will not evaporate as well, as it doesn’t carry any flavours or other benefits for the desired spirit.
The wash boils up against the copper which binds sulphurous compounds that can form during fermentation. The more copper contact, the less sulphur in the final spirit.
With the alcohol, other flavour compounds with a lower boiling temperature than alcohol will rise up during the distillation process. Lighter chemicals such as esters for fruity flavours that formed during fermentation are one example of them.

The first distillation takes roughly 4-7 hours, it varies from distillery to distillery. If distilleries want a lighter spirits they also want to take only the lighter flavour compounds through the distillation. If a heavier spirit is desired, then distilleries can distill longer and at a higher temperature to carry through heavier flavour compounds.

When the vapours rise up in the still and reach the neck of the still (lyne arms) they cool down and condense into liquid again. To speed up condensation the vapours are led through condensers in the lyne arms. Some distilleries also use so called worm tubs, which are essentially copper pipes that spiral downwards through cold water to cool down the vapours inside the pipes.

The liquid now has roughly 20-25% ABV and is considered a low wine. The low wines are collected in the low wine receiver and are waiting for the next distillation.

The solid barley leftovers at the bottom of the wash still, the pot ale, are of no use for distillers anymore and are removed from the still. Distilleries usually sell their pot ale to local farmers as fertilizer.

Worm tubs at the Dalwhinnie distillery. The copper pipes spiral through the two big wooden tanks filled with cold water and cool the vapours down after distillation.

Second Distillation

Now the low wines are filled into the spirit still. Often this still is a bit smaller as it doesn’t need to hold as much liquid as the wash still.
The second distillation takes longer (up to 8 hours or longer) than the first one to more carefully separate alcohol and desired flavours from the undesired ones.
When the spirit reaches its boiling point and the first vapours are rising up, they condense and the spirit is led through the spirit safe. Here the still man monitors the density of the spirit to determine the alcohol content. According to that he decides when to make the cuts to seperate the flowing spirits into three parts:

Foreshots/heads: they contain poisonous compounds that should not get into the whisky, some of which can make you go blind. They are led back into the low wines receiver for the next distillation to get more flavours out of them next time.
Middle cut/hearts: This is our future whisky. The spirit, called new make, is collected in the spirit receiver. It is clear in colour when it comes out of the still and will get its recognisable golden to dark brown colour later. The new make is the base of every whisky and contains roughly 65-70% alcohol.
Feints/tails: The “leftovers” of the second distillation. They are also directed back into the low wine receiver with the foreshots to be re-distilled next time.

After distillation is finished we have the new make spirit at a strength of 65-70% ABV. It is now ready to be filled into casks for the final step of every whisky production: Maturation.

The spirit safe (Glenfarclas distillery)

More details of different factors to be considered during distillation are to come. We will have a look at the importance of still shape, the differences between worm tubs and condensers and what flavours are affected by it.
Finally we will dive into different types of distillation such as coffey still distillation (mostly used for grain whisky production) and the pros and cons of triple distillation.

Stay tuned!