The still with the wash—alcoholic liquid resembling beer—is heated directly or indirectly. The boiling point of alcohol is lower than that of water and the alcohol fumes rise up the neck of the still and enter a condenser (either a worm tub or a shell and tube). The fumes condense into an oily fluid containing approximately 22% abv.
This liquid is pumped to a second, smaller vessel, called the spirit still, where the process is repeated while the condensed alcoholic liquid is separated into foreshots (heads), middle cut, and feints (tails). Only the liquid from the middle cut is suitable for making whisky.
A spirit still is used during the second round of distillation, in which the abv is increased again. It is filled with the distillate (low wines) from the wash still. When heated and condensed, the liquid passes through a spirit safe, where foreshots, middle cut, and feints are separated.
This is an edited feature provided by SMWS ambassador Hans Offringa from his comprehensive A Field Guide to Whisky: An expert compendium to take your passion and knowledge to the next level which you can purchase by clicking on the link.