The Distillation in the Pot Still

In the first part of our series on Scottish pot stills we dealt comprehensively with their geometrical shape and their production.

This article deals with operating the stills. Of course different distilleries operate their stills differently. Some heat fast and then distil slowly, others heat and distil fast. Not only the geometrical shape of a pot still but especially the distillation process affects the taste of the new make spirit.

The stillhouse of the Glenfiddich distillery
Glenfiddich - Still house

1. The Principle of Distillation

The following chart shows the principal setup of a Scottish malt whisky distillery with two pot stills. Distilleries with three pot stills and triple distillation are extended by one step correspondingly. Many large distilleries have four, six or more pot stills, which aren't operated in series but in parallel.

The connection of the pot stills can be even more complex, if for example the first distillate from several wash stills or from several production cycles is led into a single spirit still. A ratio of 3:2 or 4:3 of wash stills to spirit stills is also common. You can also triple distil with two pot stills by distilling the final product of the second distillation again in the emptied spirit still.

The chart shows a simple distillery with a wash still and a spirit still.

a functional chart of the distillation at a Scottish pot stills
Functional chart of a pot still distillery

The principle of distillation was already known to the ancient Egyptians. Different evaporation points allow for the separation of substances by heating. The substances that evaporate first at low temperatures may be collected and separated from the rest. But the Egyptians used distillation only for producing perfume. Only in the middle ages Celtic monks discovered the production of whisky - the water of life.

Through alcoholic fermentation the wash (beer) contains approximately 8% to 10% alcohol (ethanol = ethyl alcohol). The alcoholic strength is determined by the yeast used and the duration of the fermentation.

When heating the wash, the substances with a lower boiling point than water evaporate with rising temperatures. The wash can't be heated further than up to the evaporation point of the lowest-boiling substance. All heat energy is absorbed by the substance that changes its aggregate state (from liquid to vaporous), and the liquid can't be heated further. Not until all the substances have evaporated at this specific boiling point can the remaining liquid be heated further.

At standard air pressure (14.69 psi, 1013 hPa) ethanol evaporates at 173,3° F (78.5°C).

2. Wash Still

The Royal Lochnagar wash still
Royal Lochnagar - Wash still

The wash still has a simple task: It is used for the first distillation of the wash, or in plain English: the beer. The capacity of the stills and the wash backs is usually coordinated. 4000 US. Gal. (15,000 L) to 8000 US. Gal. (30,000 L) are most common.

When hot steam is led into the heating cylinders, the wash still starts to heat the wash. Through the heat movement (convection) inside the still the wash is turned. The wash rises along the warm areas of the cylinders and sinks back along the cooler areas.

After some 30 minutes it gets interesting: Above the heating cylinders the liquid starts to boil, and light substances (predominantly flavour-carrying esters) rise into the air above the liquid level.

The constant supply of gaseous substances leads to a slight overpressure in the still, and the gases rise into the neck of the still. But they don't get far. The wall of the still is still too cold, and the evaporated substances condense at the wall. As time goes by more and more droplets accumulate at the wall and form bigger drops that flow back into the pot.

With rising energy input the whole wash starts to boil at a temperature of approximately 173°F (78°C). The whole still heats up, and the liquid surface becomes turbulent. The high heat input generates mighty bubbles, and the liquid foams and splashes up to the upper part of the neck.

the watchglass at the wash still of the Macallan distillery
Macallan - Inspection window of a wash still

That's why wash stills have small windows in the neck, through which the bubbling wash can be watched. For if the boiling temperature of the wash is too high, liquid can get into the condenser via the lyne arm. This wouldn't be so bad if the wash didn't contain solid parts of the barley grains, which clog the thin pipes of the condensers.

Therefore the stillman must be watchful. Distilleries that don't have the time for watching the boiling put soap into the wash, which destroys the surface tension of the wash and prevents it from boiling over. Since the soap liquefies at 122-140°F (50-60°C) and only boils at temperatures far exceeding 212°F (100°C), no soap molecules can get into the distillate.

The first distillation in the wash stills takes approximately 4 to 7 hours. The wash still has a temperature of approximately 173°F (78°C), the evaporation point of ethanol. The whole heat input is used for the evaporation of the alcohol.

The distillation usually ends after 4 hours. Except for 1% abv, all alcohol has evaporated and has been collected in the low wines receiver.

The spirit receiver at the Mannochmore distillery
Mannochmore low wines & spirit receiver

However, the low wines receiver doesn't contain only alcohol but also all substances with a lower boiling point than alcohol, as well as some substances with a higher boiling point. They have been torn out of the molecule groups by the bubbling liquid and have been pulled into the low wines receiver together with the light alcohol molecules. Among these molecules is also plenty of water, which forms an azeotrope with the alcohol. After the first distillation the low wines typically have an alcohol content of 20% to 25%.

After distillation the pot ale (also called spent wash) remains in the wash still. It has a residual alcohol content of approximately 1%. However, not only alcohol but also valuable proteins and minerals from the barley grains remain in the pot ale. That's why after emptying the still the pot ale is concentrated through evaporation and sold as high-quality animal feed.

the Cencentration system at Glenfarclas
Glenfarclas - Concentration system for pot ale

Since the large pot stills only have a wall thickness of a few millimetres (ca.3/16") they are very sensitive to overpressure and negative pressure. The worst-case scenario is therefore the collapse of a still caused by negative pressure. When the distillation has been stopped, the pot ale is drained and the pot still cools down, negative pressure is created inside. If it becomes too high the pot still implodes with a loud bang. Since this happened more than once in the past, every pot still now has an automated pressure relief valve that keeps the pressure balance with the environment. For filling and draining the stills, there's another vent valve, which is usually operated simultaneously with the pumps.

Fettercairn - Automated pressure relief valve (top) and manual vent valve (bottom)
Fettercairn - Automated pressure relief valve (top) and manual vent valve (bottom)

The table below shows the distillation balance of a wash still distillation.

 

Wash

Low Wines

Spent Wash

Liter/gallon(US) Total

30.000/7925

11.212/2960

18.748/4952

Vol. % alkocol

10%

25%

1%

Liter/gallon(US) alkocol

3.000/793

2.813/743

187/49

The figures in the distillation balance show that the reduction of the water volume from the wash to the low wines significantly reduces the second distillation volume for the spirit still.

In summary, the sole purpose of the first distillation is to reduce the liquid volume by 1/3 and to remove the solid parts of the grains that are still in the wash.

This first, heavy distillation usually takes only four hours and thus affects the taste of the spirit only little. The extreme heating of the wash and the bubbling liquid are the reason why many heavy fusel oils also get into the low wines beside alcohol and water.

3. The Spirit Still

Royal Lochnagar - Spirit still
Royal Lochnagar - Spirit still

The second distillation in the smaller spirit stills is carried out much more carefully and slowly. It typically takes approximately 8 hours. Since this takes double as long as the first distillation, often the result of two wash still distillations is collected in the low wines receiver and filled into the spirit still as a whole.

Only few distilleries have such concentration systems because they are expensive. The town of Rothes in Speyside operates such a system for all nearby distilleries. In order not to lose the residual heat energy in the pot ale during transport (it still has a temperature of approximately 158°F, 70°C), the wash still is usually drained via a heat exchanger, which returns the pot ale’s heat energy to a future distillation process.

The pot ale reciever with with the heat exchanger
Dallas Dhu - Pot ale receiver (spent wash tank) & heat exchanger

As described in the first part of this article, the spirit still has the bigger influence on the taste of the new make spirit. The second distillation is carried out much more carefully so the alcohol and the flavour substances can be separated more effectively from the water. That's why the spirit stills don't need windows to watch the boiling low wines.

The distillation process itself is the same as in the wash still. The low wines are heated, and before the boiling point of alcohol is reached the more volatile foreshots evaporate first. These foreshots are predominantly light compounds such as ethyl acetate and volatile esters, which have a pungent aroma.

Functional chart of a pot still distillery

4. The Spirit Safe

Since the aggressive foreshots are unwanted in the new make spirit, they are redirected in the spirit safe and not led into the spirit receiver. The functional chart from above is shown again to illustrate the function of the spirit safe.

This spirit safe has a long history and a special function. Under British law all pot stills and pipework must be padlocked. So the stillman cannot taste the spirit. Then how is he supposed to know when the foreshots have run through and the desired middle cut has started?

The spirit safe with measurement tools at the Glenfarclas distillery
Glenfarclas - Spirit safe with hydrometers and thermometers

The first thing experience teaches a stillman is the time needed to heat the still until the middle cut appears. Since thousands of gallons/litres must be heated to more than 158°F (70°C), it takes some time until the first spirit runs through the spirit safe. Then the foreshots run for about 20 minutes. In order to determine the right moment to switch the spirit flow so the middle cut can be collected, the spirit safe contains several glass boxes in which the spirit can be collected and instruments start to swim. If you measure the density of the spirit with a hydrometer you can determine its alcohol content with a chart. While the foreshots are running, the alcohol content of the spirit sinks from approximately 85% to 75%. 

The switches in the spirit safe at the Glenfarclas distillery
Glenfarclas - Spirit safe with switches for the spirit flow

Yet this is not the only instrument that must be monitored. The density of the liquid depends heavily on its temperature. So the temperature is also measured in order to rectify the density. With density and temperature measured, the stillman can then read the alcohol content off a chart hanging next to the spirit safe.


What happens to the foreshots? They aren't poured away but led back into the low wines receiver. However, the foreshots aren't enriched by this constant reflux. This is where the real magic of distillation happens. The aggressive foreshots are transformed into enjoyable aromatic substances through catalytic reactions with the copper of the spirit still. This is a continuous process, and the quantity of foreshots remains constant in the spirit still.


After the foreshots have reached the low wines receiver, the stillman changes the flow direction in the spirit safe and leads the middle cut into the spirit receiver. If he does it too early the spirit tastes too aggressive. If he does it too late it doesn't matter, since the liquid that is led into the low wine receiver is again used in the next spirit distillation. Thus no alcohol is wasted.

The cask filling station at the Aberfeldy distillery
Aberfeldy - Spirit receiver and filling of casks

The distillation of the middle cut must be carried out slowly and carefully. If the spirit still is heated too much, the reflux of condensing substances with a higher boiling point at the wall of the still is prevented. Therefore fusel oils can pass the lyne arm and get into the spirit receiver.

While the middle cut is being collected, which takes approximately three hours, the alcohol content falls from 75% to 60%. But even after switching at 60% abv, the distillation continues. The fusel oils (faints) that appear now are led back into the low wines receiver where they are again catalytically transformed by the copper during the next distillation run. The distillation of the faints goes on for a long time and is only halted when a residual alcohol content of 1% is reached so no valuable alcohol is lost with the liquid remaining in the spirit still (called spent lees).

You don't often get the chance to view a low wines receiver from the inside. It contains a milky grey-white mix of alcohol and water on which the thin, oily layer of faints swims.

The distillation process is now complete. The distillation balance for the second distillation in the chart below shows the emerging quantity of new make spirit. In our example of 7925 gal. us. (30.000 L) of wash with 10% abv, the resulting quantity of 734 gal. us. (2.780 litres) of alcohol means a yield of 92.6%.

 

Low Wines

raw whisky

Spent Lees

Faints & Foreshots

Liter/gallons total

11.252/2972

4.117/1088

3.376/892

3.759/993

Vol. % alcohol

25%

67,5%

1%

 

Liter/gallons Alcohol

2.813/743

2.279/602

34/9

 

Particularly interesting in the balance are the faints and foreshots that are led back for redistillation. They increase the amount of liquid of the low wines as well as their alcohol content. Since they are a transit item appearing in every new distillation, they are neglected in the balance. This initially confusing fact is the reason why the alcohol content of the low wines is alternately stated between 20% and 27% in technical publications or when mentioned in a distillery. For the balance in our example we simply assumed an average content of 25% in the first distillation run and 67.5% in the second run. In our example the spent lees amount to 30% of the volume of the first distillate. This is, like all the figures in the balances, just an educated guess.

If you want to know more about the production of pot stills follow this link.