Vapour temperature probe: A probe thermometer placed in the lyne arm gives real-time distillate ABV estimates and helps identify fraction boundaries. Position the probe tip in the vapour path, not in the liquid.
How Distillation Works
Distillation separates ethanol from water by exploiting the difference in their boiling points. Ethanol boils at 78.37°C at sea level, while water boils at 100°C. When a fermented wash is heated in a still, ethanol and other volatile compounds vaporize first and rise toward the top of the still. That vapor is then cooled in a condenser, where it returns to liquid and is collected.
The liquid collected in a single distillation pass is not pure ethanol. It is a mixture of ethanol, water, and varying amounts of other volatile compounds called congeners. Congeners include esters, aldehydes, fusel alcohols, and organic acids. Some congeners contribute positively to flavor, others negatively. The job of the distiller is to manage which compounds end up in the final product and at what concentrations.
Both pot stills and reflux stills use this same process. Where they differ is in how much separation they achieve in a single pass.
How a Pot Still Works
A pot still is the simpler of the two designs. It consists of a pot (the boiler where the wash is heated), a swan neck (the curved top section where vapor collects), a lyne arm (the pipe leading from the swan neck to the condenser), and a condenser (where the vapor is cooled back to liquid).
In a pot still, vapor rises from the heated wash, travels through the swan neck and lyne arm, and condenses in the condenser. There is no mechanism inside the still designed to cycle vapor back repeatedly through the liquid. Each molecule of vapor that reaches the condenser makes that journey only once.
Because of this, pot stills produce a relatively low-ABV distillate per pass, and they carry a significant proportion of the wash's congeners through to the output. This congener carryover is what gives pot still spirits their flavor and body.
The lyne arm angle
The angle of the lyne arm has a measurable effect on the character of the output. A lyne arm angled upward causes some of the heavier, slower-moving vapor components to condense before reaching the condenser and drip back into the pot. This is called natural reflux and it produces a lighter, cleaner spirit. A lyne arm angled downward reduces this natural reflux and allows more congeners through, producing a heavier and more characterful distillate. Many Scotch whisky distilleries use this to deliberately influence spirit character.
How a Reflux Still Works
A reflux still adds a vertical column between the pot and the condenser. This column is packed with material that increases surface area, typically copper mesh, copper scrubbing pads, or purpose-made packing media such as stainless steel structured packing. Some reflux stills use perforated plates or bubble cap plates instead of packing.
As vapor rises through the packed column, it cools slightly against the packing material and partially condenses. The condensed liquid trickles back down through the packing while fresh vapor continues to rise. The vapor rising past falling liquid picks up heat and more ethanol evaporates from the liquid. This repeated cycle of condensation and re-vaporization is called reflux.
Each cycle of reflux in the column is equivalent to an additional distillation pass. A column with more packing, or one that returns a higher proportion of condensate back to the column rather than collecting it, achieves more theoretical distillation stages and produces a purer, higher-ABV output.
ABV Output Comparison
The most practical difference between the two still types is the ABV of the output.
| Run type | Still type | Typical output ABV |
|---|---|---|
| Wash run (stripping run) | Pot still | 20 to 40% ABV |
| Spirit run | Pot still | 60 to 75% ABV |
| Single pass from wash | Reflux still | 85 to 95% ABV |
These are typical ranges. Actual output ABV depends on the starting ABV of the wash, the still design, how the run is managed, and for pot stills, what cut points are used. The output ABV also falls continuously throughout a run as the ethanol in the pot is depleted.
Use your still thermometer reading to estimate the ABV of distillate coming off the still in real time.
Congener Retention and Flavor
The two still types differ significantly in how many congeners they carry through to the distillate. This is the most important consideration when choosing a still for a particular spirit style.
- Retains more esters, fusel alcohols, and flavor compounds
- Output has more body and complexity
- Requires multiple runs to reach bottling strength
- Cuts are critical for managing congener balance
- Traditional for whisky, brandy, rum, cognac
- Strips most congeners, producing a neutral spirit
- Output is clean and relatively flavor-free
- Can reach target ABV in a single pass
- Reflux ratio can be adjusted to allow some flavor through
- Standard for vodka, neutral grain spirit, gin base
The congener profile of pot still output is one reason pot still spirits are described as having more character than reflux spirits. Congeners are not inherently bad. Esters produced during fermentation contribute fruity and floral notes. Some fusel alcohols, in moderation and after proper aging, contribute body and warmth. The skill in pot still distilling lies in making cuts that remove the congeners with negative flavors (acetaldehyde in the heads, harsh fusel oils in the tails) while preserving those that contribute positively.
The Role of Copper
Copper plays an active chemical role in distillation, regardless of still type. During fermentation, yeast produces sulfur compounds including hydrogen sulfide and dimethyl sulfide. These compounds have strongly unpleasant aromas at even low concentrations. When vapor contacts copper surfaces inside the still, copper reacts with these sulfur compounds to form copper sulfate and other salts that remain in the still rather than passing through to the distillate.
Both pot stills and reflux stills benefit from copper contact. The amount of copper surface area available, and the time the vapor spends in contact with it, affects how effectively sulfur compounds are removed. Stainless steel stills can be used but typically require copper packing or a copper column section to achieve adequate sulfur reduction.
Which Still for Which Spirit
The choice of still type should match the spirit you want to produce. The following table summarizes the conventional approach for common spirit categories.
| Spirit | Conventional still choice | Reason |
|---|---|---|
| Whisky / Whiskey | Pot still | Congeners from the grain contribute character that defines the style |
| Brandy / Cognac | Pot still | Fruit esters and congeners are core to the flavor profile |
| Rum (heavy / funky) | Pot still | High-ester profile requires congener retention |
| Rum (light) | Reflux still or column still | Cleaner, lighter profile requires more neutral output |
| Vodka | Reflux still | Neutral character requires maximum congener removal |
| Gin base | Reflux still | Neutral base spirit allows botanicals to define the flavor |
Two Runs vs One Run
A pot still typically requires two runs to produce a finished spirit at a usable strength. The first run, called the stripping run or wash run, passes the fermented wash through the still quickly and without precise cuts, concentrating the alcohol into low wines at around 20 to 40% ABV. The second run, called the spirit run, passes those low wines through again slowly and with careful cuts to produce the final spirit at around 60 to 75% ABV.
A reflux still can produce spirit at a similar or higher ABV in a single pass directly from the fermented wash. This is one of the practical advantages of a reflux still for home distillers who want to minimize the number of still runs per batch.
The tradeoff is that the single-pass reflux output will be considerably more neutral than a double-run pot still output. For spirits where neutrality is the goal, this is exactly what you want. For spirits where character from the base wash matters, the pot still approach preserves that character through the extra run.
Frequently Asked Questions
Understand your still inside out. The Brewer and Distiller's Handbook explains still design, reflux ratios, and distillation theory in depth — the reference to keep next to your still.