The Scottish Pot Stills
The Centrepieces of Every Distillery
Who isn't fascinated by these wonderful copper-coloured gems of every distillery? If you ever entered a still house, freezing from the windy Scottish Highlands, you will never forget the welcoming feeling of copper warmth and steaming alcoholic cosiness.
But how are the pot stills made? These copper-to-golden distillation cauldrons with their nice curves and mechanical details that puzzle the technically interested? Nearly no pot still resembles another, and yet some details are always the same. So there must be some technical basics that most pot stills have in common.
I would like to thank Richard Forsyth of the coppersmith company of the same name in the Scottish town of Rothes. He explained the basic design criteria of Scotch malt whisky pot stills to me with a lot of expertise. The Forsyths company's origins lie in pot still manufacturing, and today it is responsible for the maintenance of half of all pot stills in Scotland. However, only 12 experienced employees work in the pot still business. Most of the employees work in the production and maintenance of petrochemical and pharmaceutical equipment.
Heating of Pot Stills
In the 1970s most pot stills were still fired with coal. Today indirect heating with hot steam is widely used. A big water boiler fuelled with oil or natural gas is heated, and the hot steam is led through insulated pipes in a closed heating system inside the pot still. The overheated steam gives off heat to the liquid inside the stills, and the steam condenses back to water. This water is pumped back into the boiler and is reheated in the circuit.
Only Glenfiddich, Glenfarclas and the wash stills of Macallan are still not heated by steam but in the old fashioned way with direct fire from beneath. However, the traditional coal has now been replaced with more easily manageable natural gas. Since the hot gas flames hit the copper directly from below, you need a special tool inside the still called the rummager to avoid any scorching of solid particles at the bottom. During the first distillation there are still about 6 - 7% solid parts from the grains in the wash.
The Lower Part of a Pot Still
Each pot still consists of an upper and a lower part. While the lower part is designed according to the technical specifics of firing, the shape of the upper part determines the taste and the character of the new make spirit. The lower part of the pot still is basically a big round cauldron with a special bottom. If the still is heated from the outside (directly), the bottom has to be domed (curved upwards), so the gas fire burns stably in the middle (see picture of Glenfarclas above).
The gas-fired lower part of a pot still has to have a thickness of at least 5/8" (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 cone-shaped side walls have to be 3/8" (10mm) thick as well, because the outside of the copper is heated up to 1200°F (650°C) in this fire flue.
The pictures above show the fixtures of directly fired wash stills. The bevel gear is fixed inside the pot on three cantilevers made from gunmetal or brass, with the help of brass bolts and three reinforcing plates, which are offset by 120 degrees, on the outside. An electric motor outside the still drives the rummager with a sealed shaft at about one rotation per minute. The rummager itself is made of gunmetal or brass as well and is draped with a chain of interwoven copper rings. Both the bottom and the chain are subject to abrasion, and the chain must be replaced after 2 or 3 years of continuous operation.
A pot still heated indirectly, with steam, looks completely different on the 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 serpent-shaped pipes were used for the first indirect heating systems. They ran close to the bottom and the walls in order to retain the heating effect from outside and below, like in the directly heated stills.
Still the solid particles from the barley corn stuck to the pipes. Cleaning the pipes was a tedious and exhausting task that reduced the possible productive working hours of a pot still significantly. The solution to this problem was found in specially shaped heating cylinders as shown in the following pictures.
Several of these hollow cylinders are placed inside the pot, standing upright. That way the wash can enter from below and leave heated at the top. The cylinders are double-walled so the hot steam enters the walls from above and runs down as condensed water. Small baffles between the thin walls of the cylinders lead the steam into a homogeneous flow in order to guarantee a constant heat emission.
The steam is channelled through pipes at the top of the cylinders. Ring pipes collect the condensed water. You can see the exhaust pipes for pot ale and condensed water below the pot stills of Longmorn.
But also in indirect heating, solid particles still stick to the hottest parts of the heating cylinders. That's why spray nozzles for a cleaning liquid are installed above the heating cylinders (see pictures of Glenlossie + Linkwood). When a pot still is completely emptied a cleaning liquid is sprayed on the cylinders, which are then slightly heated. After some time of exposure the stills are rinsed with water. All cleaning liquid is collected and sent back to the producer for recycling.
Since the thermal load and the mechanical wear of an indirectly heated pot still are much less than those of a directly fired still, the bottom and the side walls only have to have a thickness of 1/4" (6mm).
Upper part of a pot still
When the shape of a pot still is mentioned, what's meant is usually the special design of the upper part. The detailed design affects the evaporation, the flow and the condensation of the liquid. However, not only the upper part but also the shape and the angle of the pipe leading to the condenser, the lyne arm, decide about the character and quality of the new make spirit.
There are four basic upper pot still types:
The still in the picture above can be seen as the prototype of every pot still. The upper part is made up of four basic areas. The first is the spherical lid A, which covers the pot on the upper side. The conical and tall neck C is attached to the lid via the intermediate connection B. The lyne arm E is connected to the neck by the bend D.
During distillation, alcohol vapours and aromatic compounds rise in the neck of the still, condense again at the cool wall of the neck and flow back into the pot. With rising temperatures, the lightest particles are the first to reach the condenser via the lyne arm. The taller and slimmer a pot still neck is, the better the substances, which all have different boiling points, are separated, and the purer the alcohol will be in the end. Lagavulin produces an intense, strong whisky, because the pot stills are very short in relation to their width, which means that the substances aren't separated so easily (see picture above).
In contrast, the pot stills of Glenmorangie are tall and slim. The result is a very smooth and mild whisky. The tall stills cause the heavier, oily flavour substances to remain in the pot during distillation.
The effects of a tall neck may also be achieved by calming the vapour column inside the neck. You have to separate the vapours from the heavily boiling and moving surface of the liquid by adding a constriction just above the lid of the pot. The spirit still of Glenkinchie is a good example.
The separation of heavier and lighter substances may also be achieved with a bulge in the lower part of the neck. This bulge is most often a bowl (see the picture of the Strathmill stills). The additional surface increases the heat emission to the outside and the reflux of condensed droplets into the pot. This way the remaining height of the still can be completely used for separating the lighter substances. A closer look at the stills of Glenmorangie shows that height is combined with a constriction and a boil ball to achieve the best possible separation.
The wall thickness of the upper parts is considerably lower than that of the lower parts. This makes producing the bent shapes easier. Most pot stills have a wall thickness of 1/8" (3mm) to 3/16" (4mm). Wash stills tend to have a wall thickness of 3/16" (4mm), spirit stills mostly have 1/8" (3mm). In the upper part of the still, the bend and the lyne arm are subject to the heaviest abrasive wear. Here the hot alcoholic vapours are most aggressive. They steadily pull copper molecules out of the surface.
However exotic the shape of a still might be, the coppersmith must manufacture the bent shapes according to the client’s instructions.
The raw material is always sheet metal made from 99.85% pure copper after British Standard BS2570C106 in varying widths. About 80% of the copper is composed of recycled material from the electronic industry and from old pot stills.
After the basic shapes of circles, segments etc. have been cut from the blank sheets, they are bent into three-dimensional shapes with automated hammers like in the old times. In former times the still parts were joined by soldering or rivetting. Today gas-shielded welding is best suited for joining purposes.
Copper is very soft in its raw state and can easily be brought into shape by hammering. Thus simple cylinders are made into bowl segments, ellipsoids or free-form surfaces according to the client's specifications. Hammering also serves another purpose: The irregular surface of a welding seam can be flattened as you can see in the picture above.
The complete surface is then hammered again to harden the outer parts of the soft copper in cold condition. Grinding and polishing provides for the shiny copper surface we all know. Finally clear protective paint is applied on the outside.
Thusly prepared the pot stills last for approximately 25 years. However, the constant copper abrasion by the rummager on the inside and the aggressive liquids lead to a steady reduction in wall thickness. As mentioned above, the pot of the wash still is subject to the heaviest abrasive wear due to the solid particles in the wash. The wear in the upper part of the spirit still is also heavy due to the aggressive alcoholic vapours. Since the wall thickness of spirit stills is lower, the upper parts must be replaced already after 10 to 15 years. It is recommended to replace a pot still when the wall thickness has been reduced to 50%. Otherwise the worst-case scenario could unfold, and the pot still could collapse.
Oh, and by the way: At the end of this article we have to do away with a fairy-tale. The story is often told that dented pot stills are exactly rebuilt with every dent and bump in order to keep the taste of a whisky absolutely constant over the years. That's just an uncalled-for mystification of malt whisky production with no roots in reality. Nobody will wilfully damage a new pot still that cost 70,000$ (50.000€) and risk reducing its operating life. No matter what kind of whisky will come out of it.
If you are interested in pot still then maybe you would like to read more about the distillation of whisky in the pot stills. Link