Why does every distillery have a different still shape?

Reflux is vapour that cools on its way up, condenses and falls back into the pot, where it evaporates again; the more often that happens, the lighter the distillate. With every round the heaviest and oiliest compounds stay behind in the pot. What leaves at the top of the neck is then finer in structure.

If there is hardly any reflux, those heavy oils pass over to the condenser. The distillate becomes full, oily and sometimes nutty. The shape of the still is the tool with which a distillery fixes that ratio, run after run and year after year. Two distilleries that use the same barley and the same water end up with a different distillate if they use a different still.

Coppersmiths work with three main shapes: the ordinary pear shape, the lantern shape with a widening in the neck, and the boil ball with a bulb between pot and neck. In the trade these shapes are called ogee, lamp glass and boil ball, and all three are finished off with a swan neck. The pear shape is the simplest: the vapour goes straight into the neck without a detour.

The lantern shape and the boil ball add a wider and cooler section, where vapour condenses and falls back sooner. The larger that intermediate section, the more reflux and the lighter the distillate. On the Brink in Bathmen there are 3,898 whiskies on the shelf, of which 2,783 from Scotland and 1,957 with a region stated, from 190 different brands. Put two bottles from the same region side by side, and you will notice that the region alone does not explain why one is light and the other oily.

Height, neck length and the angle of the lyne arm together determine how long the vapour stays in contact with copper, and thus how many heavy compounds make it to the condenser. Tall, slender stills such as those of Glenmorangie yield a mild, light distillate, because the heaviest compounds do not reach the top. Low, squat stills such as those of Lagavulin give a more powerful and intense distillate.

The lyne arm, the tube from the neck to the condenser, extends that contact further or breaks it off. If the arm slopes upward, vapour condenses on the inside of the tube and drips back into the still; the steeper the angle, the more reflux and the lighter the distillate. If the arm slopes downward, the heavy oils instead flow along towards the condenser and the distillate becomes heavier and oilier. Some distilleries build in an S-bend or a purifier to steer that effect.

Clean copper in the still and in the lyne arm removes volatile sulphur compounds, dimethyl trisulphide foremost, so that the distillate does not smell sulphury or rubbery. Copper therefore does more than conduct heat: it intervenes in the chemistry of the vapour and helps with the formation of esters.

Scottish stills are made of copper with a purity of 99.85 percent. At the bottom the wall is up to sixteen millimetres thick to withstand the heat of fire-heating, at the top three to four millimetres, so that the shape can be bent. The metal wears from the inside in the process, and that wear is not a defect but the proof that the copper takes part. Distilleries that once tried stainless steel immediately got a sulphurous smell in the distillate that had nothing to do with the cut points.

Shape, size and material of a new still are reproduced as accurately as possible, but trade journals and technical sources call copying every dent and repair spot folklore rather than technology. A still lasts on average about twenty-five years, the head of a spirit still sometimes only ten to fifteen years. When replacing, a distillery has the new still drawn after the old one, because the factors that determine flavour lie in shape, size and material.

Shape alone does not explain everything. The speed of distilling counts too: gentle, slow heating gives more reflux and a lighter distillate, running hard gives less reflux and a heavier distillate. How full the still is charged and how the vapour is condensed also play a part; a worm tub produces a heavier and more sulphury distillate than an ordinary shell-and-tube condenser. The still shape sets the framework, the operation fills it in.