Details of a copper pot still at the Glenfarclas Distillery (Speyside, Scotland)
The spirit stills at Glenfarclas are some of the largest I have ever seen in Scotland! With 21,200 litres they are larger than the wash stills in many distilleries.
The round structure in the centre is the boil ball. It causes less volatile substances (i.e. those boiling at a higher temperature) in the rising vapours to condense before they can raise through the neck. The result will be a lighter type of spirit.
Note the blue pipe leading back into the still at the extreme right edge of the image. This is the reflux pipe coming in from the purifier (not shown). The effect of this is the same as with the boil ball: to cause even more reflux. The vapours that make it up the neck of the still and over into the lyne arm (the horizontal pipe going through the wall) go through a crude condenser (the purifier) where an additional part of the vapours are condensed. Thus, many of the higher-boiling parts of the vapours are simply flowing back into the still and are distilled over and over again. So only the lightest substances make it into the new-make spirit.
Neither boil ball nor purifier are mandatory parts of a still, but if a distillery wants to create a light and floral spirit, they are two of the ways to achieve it.
Last not least, have a look at the valve with the black handle. This is an anti-collapse valve, a vital part of every copper pot still.
As the name suggests, it prevents the still from collapsing when the residual liquid is drained from the still after the distillation is completed. It is of vital importance to open this valve before the spent lees (or the pot ale in a wash still) are allowed to run off. Should the still man forget to open this valve, the resulting under pressure would cause the still to fold up like a plastic bottle that has the air sucked out.
You see, there are a lot of things to pay attention to before the spirit finds its way into the cask for maturation…