The clear distillate which emerges from the condensers after distillation is dripped at a rate of 3 litres (eight tenths of a gallon) per minute through one of 72 large white oak vats filled with 10 feet (3 metres) of densely packed hard sugar maple charcoal. It takes 4 to 6 days for the spirit to trickle through. The process, which relies purely on gravity rather than being pressurised, is known as the Lincoln County Process – so named because ‘charcoal mellowing’ was developed in the county, whose borders used to include Lynchburg.
The charcoal is made from hard sugar maples (Acer saccharum) and tall trees growing on high ground are favoured. These are split into planks about 4 foot (1.2m) long and approximately 2 inches (5cm) square, then stacked in pyres and burned to produce charcoal. Each pyre is built from 343 planks with four pyres burnt together, each pyre tilted so they collapse into each other. They are set alight using 70% alc./vol. (140 proof) whiskey and allowed to burn for two hours before being extinguished by dowsing with water. The resulting lumps charcoal are ground down to a uniform size using a grinder with steel rollers and then held in stainless steel hoppers ready for use. It takes four such burnings to make enough charcoal to put in one charcoal mellowing tank.
A panel of tasters decide when to change the charcoal and each charcoal batch lasts up to six months. When it’s changed the charcoal is flushed through with water to wash out the impregnated whiskey which is retrieved and recycled. The spent charcoal is compressed into barbecue brackets and smoking pellets.
What’s the point of charcoal mellowing? To quote Jeff Arnett, “the reason bourbon was put into a barrel was because it has a little bit of a bitter edge when its distilled and when people taste whiskey that has flown through charcoal mellowing, they're amazed at how that bitterness has been removed, the charcoal has a tendency to absorb that into itself. And it can accomplish in days what the barrel takes a couple of years to accomplish. So we see charcoal mellowing as giving a two year head start for Jack Daniels, getting rid of its bitter graininess and better allowing it to take on the identity of the barrel that it's going to enter.”