Back to Basics: How a pot still works

Back to Basics: How a pot still works

Matt takes us through the basic principles and details of pot still distillation

Production | 31 Jan 2023 | By Matt Strickland

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When it comes to producing great single malt whisky, it can be easy to get lost in the weeds of wood policy, the mysteries of malt, and the flavours of finishing. It’s easy to take for granted that if it weren’t for the humble pot still, the world would be left with nothing more than a sharply dry “beer” barely fit for consumption.

The pot still is where the alchemy of distillation happens. Despite often feeling magical it’s not all boozy rabbits being pulled from copper hats. Distillation is an art for sure, but there is an elegant underlying science that causes the metaphorical cogs to turn in synchrony. With that in mind, it’s worth spending a few moments discussing how a pot still actually takes beer from point A to its more spiritual point B.

Distillation is the physical process of separating compounds based on differences in volatility. For the lay person, “volatility” in this case simply refers to a compound’s boiling point. Water boils at 100°C and pure ethanol has a boiling point of 78.4°C. So, a mixture of water and ethanol in some proportion has a boiling point somewhere in between 78.4°C and 100°C.

It is important to understand that boiling a mixture of ethanol and water is not an “all of one” kind of scenario. Heat up a one-to-one mix of the two liquids and the vapour will not simply be composed of 100 per cent ethanol. Instead, the resulting steam is still a mixture of the two molecules but with a higher concentration of ethanol than the original liquid mixture. This is because ethanol is more “volatile” than water. If these vapours are then cooled back down to a liquid again, the “distillate” will have a higher concentration of ethanol than the original liquid.

That’s how a pot still works: heat a liquid with some alcohol in it, allow the concentrated alcoholic vapours to boil off, direct those vapours to an area where they can be cooled, and condense them into a separate vessel. And despite all our flowery, Burns-inspired prose on the voluminous flavours of whisky, the compounds responsible for all those beautiful aromas make up an incredibly small fraction of our total mixture. Whisky for all its sensorial depths really is just a mixture of ethanol and water.

The technology of the pot still has changed very little over the past few centuries. Yes, there are pot stills with doodads and gizmos galore, but the fundamental structures that make them work are all essentially the same. In order to distill beer into whisky in a pot still, four things are required: a heat source; a vessel to hold the beer that can be heated; a space to direct the spiritous vapours away from the original beer; and an apparatus that can condense those vapours through cooling back into liquid.

Pot stills used to be primarily heated with direct flame underneath the pot. While that still happens on occasion, it really isn’t too common anymore outside of France’s famed Cognac industry. Most pot stills for whisky are heated via steam being pumped through jackets surrounding the pot or coils encircling the inside. As the steam pipe work transfers heat to our roughly 8% ABV beer inside the pot, things will eventually begin to boil. At around 94°C alcohol-enriched vapours will evolve and begin to rise above the beer. The neck and lyne arm allow those vapours to be directed away from the boiling liquid below.

The height and shape of the neck contribute greatly to how some of the other flavour and aroma compounds evolve from the beer. Taller necks such as those found in Glenmorangie’s famous stills provide a large energetic hurdle for the enriched vapours, so much so that some of the vapour in these stills prematurely condenses back into liquid and drips back into the boiling beer below. The lighter, more volatile compounds continue on their merry way while the heavier compounds are “refluxed” back into the pot, producing a more delicate distillate in the end.
Similarly, the lyne arm can be angled upwards in an ascending fashion towards the condenser which also causes internal reflux to occur. A downward-pointing lyne arm does not produce much reflux and usually makes for a heavier-flavoured spirit.

Once the vapours make it to the condenser they are cooled back into liquid from the surrounding pipework. All the while the vapours are interacting chemically with the surrounding copper, removing unwanted sulphurous aromas and catalysing aromatic fruity esters. Shell and tube condensers, the most common condenser found these days, provide for more copper contact and thus produce a cleaner and fruitier spirit. The old worm tubs of yore, seen in some famous distilleries such as Dalwhinnie, do not provide the same amount of copper interaction and result in a meatier-tasting spirit.

Most pot stills (in the world of single malts, at least) require two distillation passes to produce a new-make spirit suitable for casking. The first takes an 8% ABV beer to a 25% ABV “low wine”. The second pass concentrates the low wine to something closer to 70% ABV. The second pass is also where the distiller will “cut” the spirit. Cuts are where the distiller makes a choice as to which part of the spirit to keep and which to dispose of or recycle in some way. The first distillate that comes off the still is often high in solvent aromas, esters, and aldehydes. This distillate is usually not the best-tasting stuff so the distiller will wait until the flavour and aromas hit the correct point and “cut” from the solvent-scented “heads” to the high-quality “hearts”. After a few hours of hearts coming off the still, the flavours and aromas will begin to steadily turn a bit bitter, and even sour smelling. The distiller will make another cut to the “tails” which can often be recycled in the next distillation run to collect more alcohol.

If this all sounds like it might be a bit too simple from the cold reality, that’s because it is – but not by as much as most people think. Part of the beauty of the pot still is in its simplicity. It just works. Add as many new-fangled gadgets as the heart desires, but at the end of the day the humble pot still will always be the workhorse behind amazing malt whisky.
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