Although much of the flavour and colour of whisky arises during the long interaction between spirit and wood during maturation, the underlying character hugely depends on the nature of the still and condenser used to distil the spirit. Each distillery in Scotland has its own unique set of pot stills, and the size, shape, duration of distillation, and many other factors can profoundly affect the character of a single malt. This is the art and science of distillation.
The Magic of Copper
Although each pot still in Scotland is unique, one thing they all share is copper. This metal is the perfect choice for several reasons: it's malleable, long-lasting (a still can be used for 40-50 years), and an excellent conductor of heat. But copper's most important property is chemical, not physical.
Copper's pitted surface catches and reacts with various unwanted compounds in the vapour to help purify the distillate and remove undesirable sulphur compounds. Cyanides, carbonates, acids, and sulphurs are all returned to the wash. The greater the level of contact with copper, the lighter and cleaner the spirit tends to be—making still design crucial to whisky character.
The Art of Separation
Before distillation, the wash is a complex mixture of water, alcohol, and various flavour compounds known as 'congeners'. The art and science of distillation lies in carefully controlling the process so that only desired congeners are retained in the distillate—though exactly which compounds are sought will depend on the characteristics each distillery wishes to achieve.
During distillation, compounds separate based on their boiling points. Lighter, more volatile compounds evaporate first, followed by ethanol and desirable flavour compounds, and finally heavier, oily substances. The distiller's challenge is to capture the desirable middle portion—the "heart"—while discarding the less pleasant components at the beginning and end.
The Spirit Safe and Making Cuts
If you visit a distillery, you'll commonly see a spirit safe—a brass and glass chamber containing two bowls to catch condensed vapour. This Victorian-era invention was originally introduced to prevent distillery workers from illicitly sampling the spirit, but it's now an essential tool for monitoring distillation.
The length of fermentation varies between distilleries and can last anywhere from 50 to 120 hours. Longer fermentations are said to produce more complex flavours, particularly through a process known as late lactic fermentation. The choice of yeast also varies, with distiller's yeast producing more alcohol and brewer's yeast developing more aromatic flavours. It's common for Scotch whisky distilleries to use a mix of yeast strains to achieve their desired character.
After the heads have passed through, the distiller will direct the 'heart of the run' into the second bowl—this is the precious liquid destined for cask maturation. Finally, the 'feints' or 'tails' start to come through. These contain heavier, oily, less volatile compounds that offer bitter flavours, so the distiller switches the flow back into the first bowl for redistillation.
Capturing a small proportion of the congeners from the foreshots and feints will contribute to the character of the final whisky, so a key part of the distiller's art is knowing precisely when to 'cut' in and out of the heart of the run. Traditionally, this would have been done by hand using experience and intuition, but modern distilleries are supported by automated systems. Yet experience and a good nose remain crucial.
Factors That Shape Character
A whole host of factors affect the balance and character of the distillate that goes into the barrels: the number of times the spirit is distilled, the speed and temperature of distillation, the duration of each run, and when the distiller chooses to make their cuts. Each decision influences which flavour compounds make it into the final spirit.
The size and shape of the still and the manner in which the vapour is condensed also play crucial roles. Glenkinchie's large pot stills and traditional worm tub condensers lessen copper contact and result in a heavier, oilier character. The stills at Ardbeg, meanwhile, are short and squat, making it easier for vapour containing heavier components to reach the top of the still, enter the lyne arm, and pass through into the spirit safe.
Reflux and Still Design
In contrast, the stills at Glenmorangie have the tallest necks in Scotland. This height increases the amount of reflux—where some of the vapour condenses on the copper walls and falls back into the pot to be redistilled. With increased reflux, only the lightest components make their way into the distillate, creating a delicate, elegant spirit.
An upward-facing lyne arm (the pipe leading from the still to the condenser) or a boil bulb will also increase the amount of reflux and promote a lighter spirit. Conversely, a downward-facing lyne arm encourages heavier vapours to pass through, creating a fuller-bodied character. These seemingly small design choices have profound effects on flavour.
Worm Tubs vs Shell-and-Tube Condensers
The method of condensation also significantly impacts whisky character. Traditional worm tubs—coiled copper pipes immersed in cold water—offer less copper contact and faster cooling, which allows heavier, meatier compounds to remain in the distillate. Distilleries like Springbank and Mortlach still use worm tubs, contributing to their distinctively robust character.
Modern shell-and-tube condensers, by contrast, maximize copper contact through hundreds of small copper tubes, creating cleaner, lighter spirits. The choice between these condensing methods is another way distilleries craft their signature style.
The Legacy of Illicit Distilling
All modern distilleries trace their lineage back to the traditional copper pot stills and worm tub condensers used to produce illicit whiskies before the 1823 Excise Act. These illegal spirits were celebrated for their superior quality, possibly because the small, portable stills needed to evade tax collectors inadvertently maximised copper contact. The use of small barrels for the same reason—easy concealment and transport—may have also contributed to quality improvements.
Developments through the nineteenth century and up to the present day—not least the invention of the continuous column still in the 1830s—have transformed the spirits industry in terms of scale, efficiency, and quality control. Yet with each distillery taking its own path when building upon this common heritage, there is nothing 'standard' about malt whisky. The diversity of stills and condensers across Scotland creates an extraordinary range of flavours and styles.
Visiting Distilleries
The huge diversity of stills and condensers is best appreciated by visiting distilleries in person. Most provide guided tours offering fascinating insights into how particular design elements contribute to the character of their whiskies. Seeing the gleaming copper stills in person—from Glenmorangie's towering necks to Edradour's tiny pot stills—brings the science and art of distillation vividly to life.
Further Reading
- Managing Microbes, Ensuring Quality and Valorising Waste Dr Annie E. Hill
- Difford's Guide to Single Malt Scotch Whisky Difford's Guide