Construction of a Scottish Pot Still

As late as the 1970s, most stills were still fired with coal. Today, the indirect heating method with superheated steam has become established almost everywhere. A large water boiler is operated with oil or natural gas as fuel and superheated steam is fed through insulated pipes into a closed heating system inside the stills. The superheated steam releases its heat to the liquid in the stills and the steam condenses back into water. This water is collected and pumped back into the boiler and reheated in the circuit.

Only Glenfiddich, Glenfarclas and the Macallan wash stills are not fired with superheated steam but directly in the old way. Unlike in the past, however, natural gas, which is easier to handle, is used today instead of coal. Since the hot gas flames hit the copper directly from below, wash stills need a special device on the inside - called a rummager - to prevent the solid particles from burning on the bottom. When the first firing takes place, there are still around 6 to 7% solid particles from the cereal grains in the wash.

Each still consists of an upper and a lower shell. While the lower part must be predominantly aligned with the technical features of the still, the shape of the upper part determines the taste and the expression of the distilled raw whisky. Let us first look at the lower part of the stills. In principle, it is nothing more than a large round copper pot with a special bottom. If the still is heated from the outside (directly), the bottom must be curved upwards on the inside so that the gas fire burns stably in the middle.

The wall thickness of a gas-fired boiler must be a strong 16 mm so that the aggressive flames on the outside and the scraping rum bearing on the inside do not erode the wall thickness too quickly to the permissible minimum. The conical side walls must still be 10 mm thick, as the outer wall in this flame area, the fire flue, gets hot up to 650 degrees Celsius.

The pictures above show the installations in direct-fired wash stills. With three reinforcing plates offset by 120 degrees on the outer shell and brass bolts, a bevel gear in the centre of the stills is fixed to three supports made of bronze or brass. An electric motor running on the outside, whose shaft with a sealed bearing extends into the interior of the still, drives the rummager inside at about one revolution per minute via the bevel gear.

The bronze or brass rummager is fitted with a chain band made of interwoven copper rings that rub off the coating that is constantly caking on the floor. This wears out not only the floor but also the chain band. After about 2 to 3 years of continuous operation, this belt is worn out and must be replaced.

An indirectly steam-heated still, on the other hand, looks quite different inside. The bottom can be slightly tapered downwards to facilitate the draining of the residues of the distillation (pot ale). The first indirect heaters used very simple pipes that were bent in a spiral to form a huge immersion boiling coil. The pipes ran fairly close to the wall of the stills. The aim was to obtain the same heating effect from outside or below as with the directly fired systems.

But the solid remains of the grain husks also baked to these heating coils. Cleaning the heating pipes was a laborious and exhausting task that significantly reduced the possible operating hours of the stills. The solution to this problem was found in specially shaped heating cylinders, as shown in the two pictures below.

Several of these heating cylinders, which are hollow on the inside, are arranged inside the still. The cylinders stand vertically with their openings on the top and bottom. This allows the wash to enter from below and flow out heated at the top. The walls of the heating cylinders are double-walled, through which the hot steam enters from above and drains off as cooled condensed water downwards. Baffles for the steam flow are inserted between the thin walls of the cylinders to ensure uniform heat release over the entire cylinder wall.

The steam is supplied via the ring line above the cylinders. When collecting the condensate, ring-shaped collecting pipes inside have proved their worth. The drains for pot ale and condensate can be easily seen under the stills of Longmorn.

When a whisky connoisseur mentions the shape of a still, they are usually referring to the specific design of the top. The detailed shape is responsible for the evaporation, flow and condensation conditions. But it is not just about the top part alone. The shape and inclination of the transfer arm to the condenser, the so-called Lyne arm, also determines the type and quality of the raw whisky.

Basically, a distinction is made between the following four types of head.

The still in the picture above can be regarded as the archetype of every still. Four basic areas can be recognised in the upper part. First, there is the spherical lid (A), which covers the top of the still. It is followed by a conical neck (C), which is connected to the lid by an intermediate piece(B). The Lyne arm (E) is connected to the top via a spatially complex curved piece (D).

During distillation, the alcoholic vapours and aromatics rise in the still neck and condense on the copper wall cooled by the ambient air and flow back into the boiler. As the temperature rises, the lightest components first make it through the elbow into the Lyne arm and finally into the condenser.

The higher and slimmer a still is, the better the components that evaporate at different temperatures separate and the purer the alcohol becomes at the elbow to the Lyne arm of the pot still. Lagavulin produces a very intense, strong whisky because the stills, as seen in the picture above, are very squat and the flavour components do not separate so easily. Glenmorangie's stills, on the other hand, are tall and slender. The whisky turns out very soft and mild due to the very good separation over the height. The heavier, oily flavours remain in the still during distillation.

The same effect as high altitude can also be achieved by calming the steam column in the upper part of the still. One must fluidically separate the gas column from the boiling and strongly moving surface of the liquid. This is done by a strong constriction. This can be seen very clearly in the Spirit Still from Glenkinchie.

The separation of heavy and lighter components can also be achieved via a bulge, which is usually spherical, between the still cover and neck. The additional surface increases the heat emission to the environment and thus promotes the reflux of the condensed droplets into the boiler. Thus, the given height of the still is completely reserved for the separation of the lighter vapour components. A close look at Glenmorangie's stills shows that height, reflux spheres and constriction have been combined to achieve the greatest possible separation.

The wall thicknesses of the upper parts are significantly less than those of the lower parts. This makes it easier to produce the curved shapes. Most of the wall thicknesses of the pot stills are between 3mm and 4mm. Wash stills are usually 4mm thick. Spirit stills are more likely to be 3mm. The greatest wear on the upper part occurs in the elbow and Lyne arm. This is where the hot alcoholic vapours are most aggressive. They constantly tear copper molecules out of the surface.

No matter how the special shape of a still turns out. The coppersmith must produce the specified curved shapes in a reliable manner.

The starting material is always the sheet of 99.85% pure copper according to British Standard BS2570C106 in various thicknesses. About 80% of the copper consists of recycled material, which, in addition to waste from the electrical industry, also comes from discarded stills.

After the basic shapes of circles, segments etc. have been cut from the copper, these sheets are bent into cones with machine-driven hammers as in the old days and welded at the joints. In the past, rivets were used or the joints were soldered. Today, gas-shielded welding has become the best and most convenient method of joining.

The copper is still very soft in its raw state and can be easily shaped into spatial form with hammers. In this way, spherical sections, ellipsoids or free-form surfaces are created from simple cylinders according to the client's wishes. But hammering also serves other purposes. The irregular surface of the welds is smoothed, as can be seen in the left part of the picture above.

Finally, the entire surface is hammered again to solidify the edge layer of the soft copper when cold. Finally, the finished form is ground and polished to create the familiar shimmering copper surface. Finally, a coat of clear protective lacquer is applied to the outside of the mould.