Pot still shape and reflux
Two pot stills fed the same low wines can make different spirits. Much of the difference comes from shape, through reflux. Here is how reflux works and how to read it in a still you are buying or running.
Reflux is vapour that falls back
When a pot still boils, vapour rises through the neck. Some of it touches the cooler copper walls, condenses and runs back down into the pot. That returning liquid is reflux. It is distilled again, so only the lightest, most volatile vapour makes it over the top and into the lyne arm on the first try.
More reflux gives a lighter, cleaner, more delicate spirit, because heavier compounds keep falling back. Less reflux lets heavier, oilier compounds through, which gives a fuller, richer, more robust spirit. Shape is the main way a pot still designer sets the amount of reflux.
Neck height and width
A tall neck gives vapour more distance and more copper surface to cool against before it reaches the lyne arm. More of it condenses and falls back, so tall stills make lighter spirit. A short, wide neck gives vapour a quick, easy route over, with less reflux and a heavier spirit.
Width matters too. A narrow neck speeds vapour up and can carry heavier compounds over. A wide neck slows vapour and gives more time for condensation. Designers balance height and width together, which is why still shapes vary so much from one distillery to the next.
Boil balls, lanterns and other features
Several classic features add reflux without simply making the still taller:
- Boil ball. A bulge between pot and neck. Vapour expands, slows and cools. More of it condenses and falls back.
- Lantern or constriction. A pinched waist in the neck that does a similar job.
- Purifier. A small condenser on the lyne arm that returns some liquid to the still, used by some distilleries for a lighter spirit.
Each one is a fixed design choice. Once the still is built, these features shape every run it ever makes.
The lyne arm finishes the job
The lyne arm, the pipe that carries vapour from the top of the neck to the condenser, adds its own effect. An arm that rises towards the condenser sends condensed liquid back into the still, adding reflux. An arm that slopes down carries everything forward, reducing it. Length and diameter matter as well. Our guide to lyne arm angle and spirit character goes into the detail.
How the stillman changes reflux day to day
Shape fixes the baseline. Running practice moves it. A slow, gentle distillation produces less vapour pressure, so vapour lingers in the neck and more of it refluxes. A hard, fast run pushes vapour over quickly with less reflux. How full the still is charged changes the space for vapour and its contact with copper. Condenser water temperature can matter too.
That is why distilleries guard their heating rates as closely as their cut points. A new stillman who runs hot to finish early will change the spirit even though the still is the same. Our guide to what the wash still and spirit still each do explains where each still's run fits.
Copper contact goes with reflux
More reflux means vapour and liquid spend longer touching copper. Copper reacts with sulphur compounds and removes them from the spirit. So tall stills with plenty of reflux tend to give spirit that is both lighter and lower in sulphur. Short stills with little reflux keep more sulphur along with the heavier compounds, which some distilleries want for a meaty, robust character. See copper contact and spirit character.
Choosing a still for a craft distillery
For a new Indian craft distillery, the still is often built to order by a coppersmith, so shape is a real choice rather than a given. Decide the spirit style first: light and fruity, or rich and heavy. Then talk to the fabricator about neck height, boil ball, lyne arm angle and condenser type together. A light spirit design and a heavy spirit design can cost much the same. Changing your mind after the copper is shaped costs far more.
Matching still design to spirit style is a decision that needs a clear grasp of how every part interacts, which is exactly what a structured course can give you before you sign an order.
A first step before you buy
Write a three line description of the new make you want to make, in tasting words. Hand it to every still maker you speak to and ask each one to explain which features of their design deliver it. If an answer only talks about capacity, keep looking.
Common questions
What is reflux in a pot still?
Reflux is vapour that condenses on the still's walls and runs back into the pot to be distilled again. More reflux gives a lighter spirit.
Do tall stills make lighter whisky?
Generally, yes. A tall neck gives more reflux and more copper contact, so heavier compounds fall back and the spirit is lighter.
What does a boil ball do?
It is a bulge between the pot and the neck that slows and cools vapour, so more of it condenses and returns to the still.
Can the stillman change reflux without changing the still?
Yes. Running more slowly, changing how full the still is and adjusting condenser conditions all change how much reflux happens.
