The shape and size of a pot still help to determine the new-make spirit character, so it seems logical that each distillery would have pot stills with identical shapes, in order to promote the same characteristics. Some distilleries do have identical, or very similarly shaped pot stills, while others have greater variety. So, what's behind this, and what influence can it have on the resulting spirit?
The line-up of still shapes is typically 'inherited' from the past, but could result from practicality rather than design. For example, buying 'second-hand' stills from distilleries that closed was more cost-effective than new stills, though hardly likely to provide a perfect match (whether adding additional stills, or starting a new distillery). The late 19th Century also saw greater experimentation with still shapes, including designs by Charles Doig (renowned for pagoda roofs). Inevitably, distillers learned from experience.
"Even in the early 1900s some distilleries were replacing stills with like for like, in terms of the size and shape. They knew from experience what style of spirit they were getting from a particular still shape, and wanted to continue this," says Stuart Robertson, manager at Whyte and Mackay's The Dalmore Distillery.
Scientific research into the influence of still shape advanced significantly from the 1980s, and provides a certain (though not complete) understanding. Nevertheless, this enables new distilleries to select still shapes that promote the desired style of new-make spirit, ranging from elegant to fuller-bodied, and all points in between. "When clients commission Forsyths to design their distillery discussing the still shape could take half an hour, or it could take all day. The boil pot (base of the still) is essentially a container for the liquid being distilled, and doesn't influence the resulting distillate. It's the reflux area in the neck and head of the still that influences spirit character," says Richard Forsyth, chairman, Forsyths, which provides a design, installation and maintenance service.
Reflux is the process which helps determine the proportion of lighter and richer flavour compounds in the resulting spirit. Taller, broader necks typically promote greater reflux, which means a higher proportion of lighter flavour compounds in the spirit. Shorter, narrower necks have lower levels of reflux, promoting fuller-bodied spirit. The dimensions are significant because the temperature reduces progressively along the neck, and the longer and wider the neck the greater the reduction. Temperature matters because richer flavour compounds within the vapours have a higher boiling point than lighter flavour compounds, and so require higher temperatures to remain in a vapour form (enabling them to reach the condenser). Consequently, the cooler temperatures of a longer, broader neck result in a greater proportion of richer flavour compounds condensing back into liquid, on the side of the neck, and returning to the boil pot. Meanwhile, the temperature (even in longer, broader necks) is sufficient for lighter flavour compounds to remain in vapour form and reach the condenser. Distilleries with uniform wash stills (which conduct the first distillation) and spirit stills (second distillation) include The Glenlivet, Aberlour, Glen Elgin and Glenrothes.
"At The Glenlivet we can check the new-make spirit from each of the seven spirit stills to ensure they are all consistently producing the same floral, fruity house style. We know these are the traditional still shapes, as they've been very well documented since 1823," says Alan Winchester, The Glenlivet master distiller at Chivas Brothers. But is some variety among wash stills and spirit stills necessarily significant?
"Balmenach has three wash and three spirit stills, with very minor differences between all of them, some have slightly longer necks, and slight differences in the shape of the shoulders. But they're essentially creating the same character, and all the new-make spirit goes into one spirit safe where they mix together," says Derek Sinclair, distilleries general manager, Inver House Distillers.
Another test-case is uniform shapes but varying sizes.
"The Dalmore's wash stills are all the same shape, like an old-fashioned oil lamp, but we have two small and two large, the spirit stills are also the same shape as each other, bulbous pots with thin necks, again, two small and two large. This is the result of scaling up the original stills in order to double production capacity in the 1960s. Larger and smaller stills create distillate with the same character, as we adjust the charge (i.e. amount of liquid being distilled) to create equal conditions within the still," says Stuart Robertson.
Another question is whether wash and spirit stills are equally influential?
"Many characteristics in the low wines don't come from the first distillation, they're derived from the malted barley and fermentation, which we want to retain and refine in the second distillation. So, for us the shape of the spirit still is more influential than the shape of the wash still," says Allan Logan, production director at Bruichladdich Distillery. Meanwhile, even if still shapes vary, this is a vital part of producing the distillery's house style.
"At Mortlach all the stills are different shapes and various sizes, this gives different nuances which combine to create the distinctive new-make spirit character," says Douglas Murray, process technology manager, Diageo.
The influence of still shape is also determined by the distillation rate, which varies among distilleries. Applying heat gently to the boil pot produces a slower rate. Vapours rise more gradually and the density of vapours in the neck is lower, which means the neck remains relatively cooler. This increases reflux and promotes a higher proportion of lighter flavour compounds in the spirit. Applying heat more rapidly increases the distillation rate. The vapours rise faster, leading to a greater density of vapours, and relatively hotter temperatures in the neck. This means less reflux, which increases the proportion of richer flavour compounds in the spirit. However, it's usual for distilleries to have varying rates for the wash and spirit stills.
"At Balblair and Pulteney the wash and spirit stills are the same shape, but the distillation rate in the spirit stills is slower. This slower rate maximises on the shape of the spirit still, giving more reflux, and increasing the proportion of elegant, fruity notes in the new-make spirit," says Derek Sinclair.
In addition to the distillation rate, the still shape must also be considered alongside other influential factors, including the length and angle of the lye pipe, also known as the lyne arm, and whether the condenser is shell and tube or a traditional worm. So, where does that leave us ?
"The still shape is not on its own driving the flavour, it's manipulating what we put into it, and fine tuning the complexity of the resulting new-make spirit," says Douglas Murray.
Meanwhile, no one's taking any chances with their stills.
"All our stills are uniform onion shapes with a long neck, and when we need to replace a still, it's accurately measured and the size and shape carefully replicated, as we don't want to risk any changes to the new-make spirit character," says Alasdair Anderson, distillery manager, Glenrothes.