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.

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 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.

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.

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 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.

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.