Unlike Malt Whisky, Grain Whisky is distilled in a continuous operation in a Patent Still. This is sometimes known as the Coffey Still, after Aeneas Coffey, who developed it in 1831.
Steam is fed into the base of the analyser and hot wash into the top. As the two meet on the surface of the perforated plates, the wash boils and a mixture of alcohol vapours and uncondensed steam rises to the top of the column. The spent wash runs down and is led off from the base.
The hot vapours enter the rectifier at the base and as they rise through the chambers they partially condense on the sections of a long coil through which wash is flowing. The spirit vapour condenses at the top of the rectifier and is run off through a water-cooled condenser to the spirit safe and on to the spirit receiver. Once the spirit begins to be collected it runs continuously until the end of distillation.
Because of the rectifying element present in this process the distillate is generally lighter in aroma than most Malt Whiskies. It consequently has a milder character and requires less time to mature.
What is the worm?
The worm and its surrounding bath of cold running water, or worm-tub, form together the condenser unit of the Pot Still process of manufacture. The worm itself is a coiled copper tube of decreasing diameter attached by the lyne arm to the head of the Pot Still and kept continuously cold by running water. In it the vapours from the still condense. Fed by the still, it in turn feeds the receiving vessel with the condensed distillate.
The worm is being replaced gradually by the more modern tubular condenser.
What are low wines?
This is the name given to the product of the first distillation in the Pot Still process of manufacture. It is the distillate derived from the wash and contains all the alcohol and secondary constituents and some water. It forms the raw material of the second distillation, which is carried out in the Spirit Still. The feints and foreshots are added to the low wines when the Spirit Still is charged.
What is pot ale?
Pot ale, alternatively burnt ale, is the liquor left in the Wash Still after the first distillation in the Pot Still process. It is the residue of the wash after the extraction by distillation of the low wines.
What are foreshots?
Foreshots is the term applied to the first fraction of the distillate received during the distillation of the low wines in the Spirit Still used in the Pot Still process of manufacture. They form the first raw runnings of this second distillation and their collection is terminated by the judgement of the stillman. The following fraction of the distillate is the potable spirit. The foreshots are returned to the still, together with the feints.
What are feints?
Feints is the name given to the third fraction of the distillate received from the second distillation in the Pot Still process. They form the undesirable last runnings of the distillation. As noted above, they are returned with the foreshots to the Spirit Still when it is recharged with low wines.
The term is also applied to the first and last runnings from the Patent Still, in which process they are returned to the wash for re-distillation.
The feints and foreshots from the last distillation of the season are kept for adding to the first low wines of the succeeding season.
What are spent lees?
Spent lees are the residue in the Spirit Still after the distillation of the foreshots, potable spirits, and feints. They are usually treated and run to waste.