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The Scottish Pot Stills

Core pieces of every distillery

Glendronach - Still house by night

Who is not fascinated by these wonderful copper coloured gems of every distillery? Whoever once entered a still house, freezing from the icy winds of the Highlands, will never forget this warm welcoming place joined by alcohol saturated air.

But how are these bulgy round and mechanically tricky copper, sometimes even gold shining, distillation cauldrons made? Even experts still recognize a lot of mysteries. Nearly no pot still resembles another. Yet, many similar details can be found on most of them. Those are mostly technical attributes which are common to all or at least to most Scottish pot stills.

Richard Forsyth with Theresia Lüning

My thanks go to Richard Forsyth from the similarly named coppersmith company in the Scottish town of Rothes. With a lot of analytic expertise he explained to me the fundamental design criterias of Scottish malt whisky pot stills. The Forsyths Company has its roots in the design and manufacturing of copper pot stills. Today the company stands for the production and maintenance of approximately 50% of all working pot stills in Scotland. However only an experienced staff of 12 cares for the pot stills. The major part of the workforce today is employed in the production and maintenance of equipment in the oil and pharmaceutical industry worldwide.

Heating of Pot Stills

In the 1970s most pot still were fired with coal. Today the indirect firing with hot steam is implemented nearly everywhere. First a big water boiler with oil or natural gas as fuel is heated. Then the hot steam passes through insulated pipes in a closed heating system inside the pot still. The overheated steam loses its heat to the liquid inside the stills and the vapour condenses back to water. This water is pumped back into the boiler and is again reheated for circulation.

Circulation of an indirectly heated pot still

Only Glenfiddich, Glenfarclas and the wash stills of Macallan are still not heated by steam. They are heated in the old fashioned way with direct fire from beneath. However they no longer use coal like in the old times. Instead they feed the fire with easily manageable natural gas. Since the hot gas flames hit the copper of the still bottom directly, you need a special tool inside the pot called the rummager to avoid any scorching of solid particles at the bottom. The solid parts during the first distillation come from the peel of the barley corn. They make up 6 - 7% of the mass of the wash.

Glenfarclas - Rummager inside a directly fired wash still

Macallan - Gas firing of pot stills

The lower part of the pot still

Each pot still consists of an upper and lower part. The lower shell is predominantly responsible for the firing. The upper part defines the taste and characteristic of the distilled raw whisky. Looking at the lower part of the still, one realizes it is nothing else but a big round cauldron with a special bottom. If the still is heated from the outside (called directly), the bottom has to be curved - domed - upwards, so that the gas fire burns stably in the middle of the bottom (see picture of Glenfaclas above).

Forsyths - veteran pot still

The gas fired lower part of a pot still has to have a thickness of at least 16mm, so that the aggressive flames from the outside and the scraping rummager from the inside do not reduce the wall thickness (below the allowed minimum) too fast. The coned shaped side walls have to be 10mm thick as well, because the outside of the copper is heated in this fire flue up to 650 degrees Celsius.

Glenfiddich - Pot stills with rummager drive
(grey coloured electric motor in the foreground)

Forsyths - New bevel gear made from brass for a rummager drive
(see also picture of Glenfarclas above)

The pictures above show the internals of directly fired wash stills. The bevel gear is fixed inside the pot with the help of three, about 120 degrees shifted reinforcing plates with brass bolts on the outside and three cantilevers made from gun metal or brass on the inside.

An electric motor outside the still drives the rummager with a sealed shaft at about one rotation per minute. The rummager itself is made up of gun metal or brass as well and is draped with a webbed net of copper rings called chain. They scratch off the permanently sticking film of solid particles from the bottom and side walls. Not only the bottom shows usage with the time. The constant abrasion of the copper rings results in a replacement of the chains after two to three years of constant use.

Forsyths - Rummager chain
(see also picture of Glenfarclas above)

An indirectly heated pot still by steam looks completely different inside. The bottom may be shaped slightly conical towards the centre, so that the remains of the distillation (pot ale) may easily flow out into the pipe. Simple curved pipes with the old domed bottom were used in the beginning for the first indirect heating systems. They were bend like normal immersion heaters with the pipes passing close to the bottom and walls.

Linkwood - Indirect heating of a pot still

Nevertheless the solid particles from the barley corn still stick to the pipes. The cleaning of the pipes used to be a tedious and hard work, which reduced the possible productive working hours of a pot still significantly. The solution to this problem was found in special formed heating cylinders, like the ones shown on the following pictures.

Glenlossie + Linkwood - Indirect heating with heating cylinders

Several of these hollow cylinders are placed inside a pot still. The cylinders stand vertically on their open ends. In this arrangement the wash may easily enter from below and leave the cylinders at the top heated. The cylinders themselves have hollow walls. The hot steam enters from the top and leaves as condensed water at the bottom. Small baffles between the thin walls of the cylinders lead the steam into a homogeneous flow and heat the cylinders with a uniform temperature distribution over the circumference.

The steam is channeled through pipes at the top of the cylinders. The condense water collection is provided by a ring pipe. The outfeed of the water and pot ale can be followed easily below the pot stills of Longmorn on the picture below.

Longmorn - Pot ale and condense water pipes

Still the solid particles from the wash stick to the hot parts of the heating cylinders. Spray or rinsing nozzles for a cleaning liquid are therefore placed over the upper ends of the heating cylinders (see pictures of Glenlossie + Linkwood). When a pot still is completely emptied a cleaning liquid is sprayed over the cylinders which are then slightly warmed to let the chemicals attack the solids. After some time you can clean the cylinders with fresh water and wash the remains of the chemicals into a waste tank. All cleaning liquids from the stills are collected and send back to the producer for recycling.

The thermal load and the mechanical wear of an indirectly heated pot still is much less than those of a directly fired still. Therefore the wall thickness of the bottom and the side walls of these stills is with 6mm by far thinner.

Upper part of the pot stills

If a connoisseur mentions the form of a pot still he normally means the special form of the upper shell of the still. The detailed curvature is responsible for the evaporation of the spirit vapours, the flow of the liquid and as well the condensing situation. It is not the upper part alone, that decides about the character and quality of the raw whisky. The form and the angle of the pipe to the condenser, the so called lyne arm, is of great importance.

In general we distinguish four different upper pot still forms:

Picture 1: normal pot still (Drumguish)

The pot still on picture 1 stands for the basic pot still. Four elementary regions can be detected in the upper shell. The first is the spherical lid A which covers the pot on the upper side. The conical and tall neck C stands on top of the lid and is connected with an intermediate piece B. The lyne arm E is joined to the neck with a complicated and three dimensional bend D.

Picture 2: short pot still (Lagavulin)

Alcohol vapours and aromatic compounds rise in the neck of the pot still during distillation, condense again at the cool wall of the neck and flow back into the vessel. More and more lighter substances reach the top of the still with rising temperatures and finally flow through the lyne arm into the condenser. The taller and slimmer a pot still neck is, the better the substances divide by differences in temperature over the height and the purer the ethanol will be in the end. However, purer does not always mean better. Lagavulin produces an intense and heavy malt whisky, because the pot stills are very short in comparison to their widths. This kind of pot still is unable to divide the substances very well.

Picture 3: tall pot stills (Glenmorangie)

In contrast the pot stills of Glenmorangie are very tall and slim. The result is a whisky that is very mild and smooth. The heavier and more oily flavour substances remain in the pot and the pot ale during distillation.

Picture 4: constricted pot still (Glenkinchie)

The effects of tallness may also be achieved with a calmed vapour column inside the still neck. You have to separate the vapours from the heavily boiling and moving surface of the liquid by adding a constriction just upon the lid of the pot. The vapours will still be able to pass through into the neck, but the vapour column calms down without direct contact to the surface.

Picture 5: Pot stills with boil-balls or reflux bowls (Strathmill)

The separation of heavier and lighter substances in the vapour may also be achieved with a bulge in the lower part of the neck. This bulge is most often a bowl as the picture of the stills of Strathmill show. The additional surface increases the heat disposal to the outside and increases the reflux of condensed droplets into the boiling liquid. The remaining height of the neck therefore works better than without these balls.

A closer look at the stills of Glenmorangie shows that mere height is combined with a constriction and a boil-ball to achieve the best possible separation of the alcohol.

The wall thickness of the upper parts of the stills is significantly smaller than the one of the lower part. The difficult curved shapes require less material. Most wall thickness are between 3 and 4mm. Wash stills have mostly 4mm - spirit stills often less than 3mm. Most of the wear in the upper parts arises in the bend and the adjacent lyne arm where the hot alcoholic vapours are most aggressive. They steadily pull copper molecules out of the surface of the copper sheets.

There are a lot of complicated shapes needed for the manufacturing of the pot stills. The client does not ask the coppersmith if he is able to make it. He just orders the same shape as his predecessor did decades ago.

Forsyths - welding of formed copper sheets and
flame cutting of blank copper sheets (background)

The raw material is always sheet metal from 99,85% pure copper after British Standard BS2570C106 in varying widths. Approximately 80% of the copper is made from recycled copper of the electric industry and old pot stills.

Forsyths - lid of a pot still in basic form for Strathisla

After the production of the basic forms of circles, segments etc. from the blank sheets, the basic forms are then bent and formed into three dimensional shapes with automated hammers like in the old times. Today the joints are welded. In former times soldering or riveting provided a solid connection between the single parts of the stills. Today gas-shielded welding is best suited for joining purposes.

Forsyths - welding seams
top untreated
bottom hammered

Copper is very weak in its raw state. You may easily change its form into a three dimensional shape like part of bowls, ellipsoids or free form surfaces by just hammering. The hammering also provides an additional effect. The irregular surface of a welding seam can be flattened as you see on the picture above.

Forsyths - Lid after hammering

Finally the complete surface is hammered again to enforce the outer parts of the surface. The effect of hardening a surface if you hammer on it in a cold state is technically called cold hardening. After this final hardening the surface is ground and polished. As the final step you apply clear protective paint to the outside.

Prepared in this way pot stills last for at least 25 years of constant use. However the constant abrasion of the rummager on the inside combined with aggressive liquids and vapours leads to a steady reduction in wall thickness. The biggest wear, as stated above, is seen inside the lower part of the wash still due to solid particles in the wash. Also high is the wear in the most upper parts of the spirit stills due to aggressive vapours. Since the wall thickness of the upper part of the stills is thinner, you have to replace most upper parts of the spirit stills already after 10 to 15 years. It is a good estimate if a distiller looks for a replacement when the remaining copper thickness has reached 50% of the original figure. Otherwise the worst possible accident, the collapse of a still, may occur.

Alas, at the end of this article we have to clean up a fairy tale of the pot still business. It is often told, that pot stills are exactly rebuilt with every buckle they got during the decades of use, so that the taste of the malt will never change over the years. These statements are just wrong and impose a mystique on malt whiskies which has never been present. Nobody will wilfully damage a 50’000 EUR expensive and brand new pot still and put any risk into the operating life. No matter what whisky will flow out of this still afterwards.

 

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letzte Änderung: 20. Januar 2011