Pot stills are made of copper, which offers significant advantages. Being highly malleable, copper complies readily with the shapes of pot stills stipulated by distilleries, however idiosyncratic. Copper also offers excellent thermal conduction properties, enabling the stills to be heated very efficiently (saving energy).
The standard method of heating the stills, and consequently the charge (i.e. liquid being distilled) is to use steam. This is conducted through copper or stainless steel coils within the pot (i.e. base of the still). An alternative to coils is using stainless steel cylinders known as 'kettles', also termed 'pans' and 'percolators'. Steam is conveyed to each kettle along small pipes that branch off a central coil, with 4-5 kettles the usual total. Whichever option is used it's vital that the heating elements remain 'submerged' beneath the surface of the charge.
Adjusting the flow of steam only means opening or closing a valve, equivalent to turning the volume up or down, all from the convenience of a control panel. Consequently, steam provides the gentlest, most uniform build-up of heat in the still, and the temperature can be adjusted more immediately than using the traditional heating method known as direct firing (i.e. burning a naked flame using gas beneath the base of the still).
The design of the stills varies enormously among distilleries and is another key factor. This is because the size and shape of the pot, and the neck extending above it, influence the level of reflux experienced by the vapours rising from the charge. Reflux is a complex process that determines the proportion of lighter and richer flavour compounds within the resulting spirit. Slimmer stills with taller necks typically promote greater levels of reflux, which means a higher proportion of lighter flavour compounds in the spirit. Meanwhile, rounder stills with shorter necks, have a lower level of reflux, promoting a fuller-bodied spirit. The significance of a longer or shorter neck is that the temperature reduces progressively along the neck, and the longer the neck the greater the reduction. Temperature matters because richer flavour compounds within the vapours have a higher boiling point than lighter flavour compounds, and so require higher temperatures to remain in a vapour form (enabling them to continue to the condenser, where vapours condense back into liquid). Consequently, the cooler temperatures of a longer neck result in a greater proportion of richer flavour compounds condensing back into liquid, on the side of the neck, and trickling back down into the pot. Meanwhile, the temperature (even with a longer neck) is sufficient for lighter flavour compounds to remain in a vapour form, and continue to the condenser. However, the level of reflux is also influenced by the rate of distillation, which is decided by each distillery. Feeding steam more gradually into the coils heats the stills more gently, giving a slower distillation rate. This also means a relatively cooler temperature in the neck, increasing the level of reflux and promoting a lighter spirit. Correspondingly, applying greater heat increases the distillation rate and the temperature in the neck, reducing the level of reflux and promoting a fuller-bodied spirit. Furthermore, the rate of distillation determines the degree of contact between the vapours and the copper surface of the neck. The slower the distillation rate the lower the density of vapours ascending the neck, which results in greater contact between the vapours and copper.
The more copper contact there is, the greater the reduction in the level of sulphur compounds, which are 'trapped' by the matrix surface of the copper. Sulphur compounds include rubbery, meaty, vegetal notes. And although only present in tiny quantities, measured in parts per billion, they are assertive and 'mask' other characteristics.
The level of reflux can be increased by adding 'accessories' to the still, such as a boil bowl. This is a bulbous section, fitted between the pot and the neck, which can vary in shape from mildly to acutely convex (ie. broad). Vapours rising from the pot expand into this larger area. There is a drop in temperature which causes richer flavour compounds to condense and return to the pot. The broader the boil bowl, the greater the level of reflux.
An alternative approach to a similar result is fitting a 'lamp glass' between the pot and neck. This feature is also known as a 'pinched waist,' because it looks as though a corset has been tightened around this part of the still. The vapours are initially concentrated within the smaller, relatively hotter area of the 'pinched waist,' before reaching the wider, relatively cooler area above, where the change of temperature causes richer flavour compounds to condense.