The Evolution of the Pot Still in Scotland by Iain Russell, Part 2

The calculation of Excise Duty based on still capacity was discontinued in the UK at the beginning of the 19th century. This ended the fashion for wide, shallow rapidly-worked stills at the great Lowland whisky distilleries in Scotland, and there was a return to the use of traditional bulbous pot stills with capacities ranging from just a few hundred gallons up to 24,000. The phenomenal growth of the whisky industry in the 19th century, and especially of the great blending houses, required huge volumes of malt whisky. Soaring demand encouraged pot still distillery owners to explore new ways to manufacture their spirit more efficiently and safely, while ensuring consistency of character and flavour. Many features introduced in the 1800s remain in widespread use today.

Illustration from Alfred Barnard, The Whisky Distileries of the United Kingdom (1887), showing the direct-fired stills at Bowmore Distillery. 

Some of the most pressing challenges facing pot the distillers related to direct firing – the traditional method of heating the still over a coal fire. This practice required significant investment in the construction of furnaces and flues, and there were irksome disruptions and significant labour costs involved in charging the furnace; in stoking the fire and damping it down when required to increase or reduce the heat; in clearing out ashes, and in cleaning surfaces contaminated by dust and soot deposits. In addition, a direct-fired pot still required an extra-thick copper bottom to withstand the direct application of intensely hot and potentially damaging flames to the lower portion of the pot. Any means of reducing the amount of copper required for a still could significantly reduce the cost of its manufacture.  

A painting of the stillhouse at Ardbeg, c.1900, showing the direct-fired wash and spirit stills.  The Exciseman sits by the spirit safes on the left. Courtesy of The Museum of Islay Life.

It was also a challenging task to maintaining the temperature of a coal-fired furnace at the required level at each stage of the distillation process, while the careless application of intense naked flame could have a detrimental effect on the character of the distillate. If the copper bottom of the pot was overheated, the spent yeast cells and other vegetable solids contained in the wash were scalded and imparted unpleasant burnt flavours to the liquid. Overheating could result in frothing, sending foam slopping over the lyne arm into the worm and contaminating the condensed spirit. Chain rummagers had become widely adopted in the later 18th century to help prevent scalding and suppress frothing, but they were known to damage the copper bottom. And the addition of soap or other anti-foaming agents to the still’s contents could impart unpleasant flavours to the distillate. For these and other reasons, there was certainly a keen appetite for an alternative method of warming the stills.

Steam heating provided an attractive alternative. The first examples were adopted in the USA and Caribbean and involved fitting steam jackets to the outside of the still. The journalist Alfred Barnard, in The Whisky Distilleries of the United Kingdom (1887), mentions a jacket installed at Bankier Distillery in the 1880s but most of the Scottish distillers who began to adopt steam heating in the later 19th century preferred to install heating coils or pipes inside the stills, using a simple control cock to increase or shut off steam to adjust the temperature of the wash or low wines within. While Barnard mentions the adoption of steam heating at less than a dozen distilleries in the mid-1880s, they were becoming increasingly popular by the end of the century, and steam-heated stills are common in distilleries today. However, there was a more conservative and contrary view…

Despite offering greater cleanliness, efficiency and control, steam heating was not universally popular. Barnard noted that although direct firing ‘may not appear the most scientific mode either of boiling or condensing the spirit, the all-but unanimous opinion among Scotch distillers seems still to be in favour of this old process.’ By increasing control over the distillation process, steam heating permitted the production of a spirit with less pronounced vegetal and burnt flavours making their way to the spirit receiver. For many distillers, however, the presence of ‘acceptable’ impurities gave the distillate a desirable depth of flavour and character. A writer in The Distillers Magazine explained that ‘‘It is very correctly understood by pot still distillers that a distinctive flavour attaches to the distillates by reason of furnace heating; the cooked odour and flavour are generally looked upon as agreeable rather than otherwise.’ And so many distilleries continued to use furnaces in their distilleries, with some compromising by using steam to heat only the wash or spirit still, and a few installing steam coils to permit heating by direct firing but switching to steam at the beginning of the process or later when the de-alcoholised wash was most concentrated. Today, many distilleries have retained or re-adopted direct firing, although oil or gas burners are used in place of coal fires to provide cleaner, more controlled and efficient temperature control.

Nineteenth century distillers gained a greater understanding of the beneficial effects of copper contact with alcoholic vapour: copper acts as a catalyst to remove the more volatile and undesirable sulphur compounds and to promote the creation of esters which provide fruity and floral flavours in the spirit. Many installed stills with longer necks than had been traditional, permitting more prolonged contact with copper surfaces as the alcohol vapour rose to the lyne arm. Longer necks also increased reflux – whereby some of the distillate condensed and fell back into the liquid below, to be redistilled. Later in the century, distilleries such as Glen Grant and Ardbeg amplified this process by installing purifiers - water-cooled chambers connected to the lyne arm, in which still more vapour was condensed and returned to the liquid warming in the pot, by way of a connecting pipe.   

Purifier on the spirit still, Ardbeg Distillery.

Perhaps the next significant innovation in pot still design did not actually relate to the pot itself, but to the condensers. Traditionally, distillate rose from the stills via the lyne arm into a coiled copper tube, or ‘worm,’ which was encased in a large vessel -the worm tub - containing cold water. The vapour cooled and condensed in the worm and was then collected in the spirit receiver. During the later 19th century, however, distillers began experimenting with what were known as shell and tube condensers. These alternatives consisted of a large number – as many as 200 - slender copper tubes contained within a large vessel, or shell. Cold water is pumped through the tubes: when the alcoholic vapour from the still is introduced to the shell, it comes into contact with the cold copper surfaces and condenses, before passing on to the spirit receiver (there are exceptions, in which the vapour is conveyed through the tubes). Like the use of taller stills and purifiers, the greater contact with copper in the shell and tube condenser helps create a lighter, fruitier and ‘cleaner’ spirit, while the more traditional worms tend to produce a ‘heavier’- tasting spirit with a more pronounced savoury and vegetal character.    

The Glenmorangie Distillery was rebuilt in 1886-87 and equipped with two new tall stills. This cut-away drawing shows the internal steam coils as well as the shell and tube condensers. Courtesy of The Glenmorangie Company.

Barnard mentions only a handful of distilleries equipped with shell and tube condensers by the time of his tour in the 1880s and, as with the use of steam heating, there were many distillers who preferred to use traditional worms and worm tub arrangements as well as squat stills, favouring the flavours and character that they help to create. Distilleries such as Glenmorangie, re-equipped with very tall steam-heated stills and with shell and tube condensers when it was rebuilt in 1886-87, were known to produce a much lighter and more ‘fruity’ spirit than their more traditional contemporaries, and often retain that distinctive character today.

A shell and tube condenser at McMillan Coppersmiths before final assembly, revealing the hundreds of copper tubes that cool spirit vapour back into liquid. Photo by Gary Smith for EWA.

Finally, another significant introduction to the stillhouse in the 19th century remains one of its most prominent features today: the spirit safe. This magnificent brass and glass locked cabinet contained instruments including a thermometer and hydrometer. The stillman worked at the safe to monitor the distillate coming off the stills; to calculate when to cut off the unwanted foreshots and feints to be redistilled, and to collect only the ‘middle cut’ – the part of the distillate which met the distillery’s chosen profile – to send to the spirit receiver for filling into oak casks for maturation.

The spirit safe at GlenAllachie distillery during a visit in October 2025. Photo by Gary Smith for EWA.

Spirit safes became an integral feature of Scottish stillhouses, not least because they enabled the Excise officer attached to the distillery to monitor the volume and alcoholic strength of the spirit for calculating duty payments. In many distilleries the safes have been replaced by automated digital measurement systems, but they are retained because they have become part of the traditional ‘furniture’ of the stillhouse.

By the early 20th century, Scotland’s malt whisky distillers had become far more knowledgeable about the scientific processes involved in distilling, and the majority had invested in more advanced and efficient stills and other equipment to enable them to produce a more consistent and distinctive whisky for their customers. However, tried and tested methods survived alongside technological innovations, permitting a range of Scotch whisky styles highly prized by the whisky blenders who had come to dominate the Scotch whisky market. The continued production of a wide range of malt whisky styles was to prove vital in propelling the rise of Scotch to become one of the world’s most popular spirits.

Iain Russell is an acclaimed whisky historian and author, recognised for his expertise in the heritage and traditions of Scotch whisky.

See also: The Evolution of the Pot Still in Scotland by Iain Russell, Part 1