The corn mash is first soaked in water and brought to the boil. The mixture is then cooled and rye is added to flavour the mash and increase its starch content. The mixture is cooled again before barley malt is added. To ferment the whole mash, yeast is added along with some of a previously fermented mash that still contains live yeast (spent mash). This process is called sour mash. It creates the foundation for the consistent character of the end product. After the fermentation and distillation processes, the whiskey is filtered drop by drop through a layer of maple charcoal, which gives it its characteristic, fully aromatic taste. This filtration process, known as charcoal mellowing, lasts 4 weeks. This is a refinement phase that does not take place during the production of bourbon whiskeys.