The focus of distillation is usually how the size and shape of the pot still influences the character of the resulting spirit. The condensers are usually a secondary consideration, although the type used (traditional worm tubs or more modern shell and tube condensers) also influences spirit character. But the lye pipe, as it's generally known, though technically the lyne arm, is rarely mentioned. This conveys alcohol vapours from the pot still to the condenser, which seems entirely pragmatic. However, being made of copper, the lye pipe helps to influence spirit character, and whether the lye pipe extends upwards, downwards, or is horizontal, also has an effect.
The significance of copper is that various reactions occur when alcohol vapours are in contact with it (the full extent of these reactions is still being researched). For example, when acids and alcohol within the alcohol vapours meet on the copper surface, complex interactions between them form esters (fruity notes). Ester creation is greater in the still, which provides a far larger surface area of copper, but additional esters can form in the lye pipe.
The longer the lye pipe, the greater the degree of contact with copper. However, this also depends on the distillation rate, which is determined by the degree of heat applied to the stills: the greater the heat the faster the rate of distillation.
"We distil very slowly to allow the vapours to travel slowly and have the maximum copper contact. If we distilled faster there would be less copper contact and the spirit would be much richer," says Allan Logan, Production Director, Bruichladdich.
Copper also absorbs sulphur com-pounds from the vapours. Essentially formed during fermentation, sulphur compounds include meaty, rubbery, vegetal notes. These are present in tiny quantities, though being very assertive they 'mask' lighter characteristics. Consequently, lowering the level of sulphur compounds 'reveals' lighter notes such as fruit, which changes the spirit character significantly. The neck of the still and condenser absorb the greatest level of sulphur compounds (having a far larger surface area of copper) though the lye pipe contributes to this. But to what extent?
"Within a few months, a lye pipe acquires an oily, waxy coating from the vapours passing through it, which probably reduces the level of interaction between the vapours and the copper, reducing the absorption of sulphur compounds as well as the production of new esters. The level of deposits in the lye pipe is higher than in the neck of the still, and there's no easy means of cleaning the lye pipe, whereas a pot still can easily be cleaned, for example by opening the door of the still and using a hose," says Andy Cant, Senior Site Manager, Cardhu Group, Diageo.
Meanwhile, the angle of the lye pipe helps to influence the proportion of lighter and richer flavour compounds in the resulting spirit.
Lighter flavour compounds, such as certain esters (fruity notes) are physically lighter, and evaporate from the charge (liquid being distilled) at lower temperatures. As distillation continues and the temperature rises, richer flavour compounds such as some cereal notes, which are physically heavier, begin to evaporate.
As vapours ascend the neck of the still and continue along the lye pipe they experience progressively cooler temperatures, and the longer the neck and lye pipe the cooler the temperature becomes. This results in some richer compounds condensing in the neck of the still and the lye pipe, which means the vapours reaching the condenser contain a higher proportion of lighter flavour compounds. Meanwhile, what happens to condensate in the lye pipe depends on the angle. "If the lye pipe is at an upward angle, condensate containing richer flavour compounds trickles back down the pipe and returns to the pot still, which means a higher percentage of lighter flavour compounds continuing to the condenser, promoting a more elegant spirit character.
If the lye pipe is at a downward angle, condensate trickles forward into the condensers, which means a higher percentage of richer flavour compounds continuing to the condenser, and consequently a richer spirit. A horizontal lye pipe is effectively a half-way house. The influence the angle of the lye pipe makes is arguably slight, but it's nevertheless important," says Alan Winchester, The Glenlivet Master Distiller at Chivas Brothers.
"The lye pipe is typically around 3m in length, though can be longer, and usually 4mm thick. As distillation gradually erodes the copper and reduces the thickness, this is measured annually. Once the thickness is below 2mm, it's time to replace the lye pipe. In the wash still (where the first distillation takes place) the alcohol vapours are very volatile and the lye pipe wears faster, and needs to be replaced in about 8 - 12 years. The spirit still (in which the second distillation takes place) has far less volatile vapours, and the lye pipe could last 30 years. Replacing a lye pipe usually takes a couple of days. Virtually all lye pipes are flanged, which means they are attached to the neck of the still and the condenser using bolts and joints, which is far more practical than welding," says Richard Forsyth, Chairman, Forsyths, which produces, installs and maintains distillery equipment.