The cooled worts are injected with yeast and the fermented in a further tank or tanks called 'washbacks'. Thirty-six hours or thereabouts of sometimes violent fermentation produces a weakly alcoholic (10° or thereabouts) clear liquid called 'wash', which will now be distilled.

Fermentation can be extremely vigourous, producing a 'head' several feet deep. Washbacks are often mechanically agitated to knock back the head or it would overflow.

The wash is allowed to ferment to a beer-like liquid containing about 5% alcohol.