For him, that meant filtering his whiskey drop by drop through a three-meter-thick layer of charcoal from the sugar maple. Seven generations later, the whiskey is still filtered the same way Jack did back then. The reason: Filtering gives it a distinctive mildness that has come to be expected from Jack Daniel's. Charcoal mellowing makes Jack Daniel's what it is: not a bourbon, but a Tennessee whiskey. This process refines the full flavor of the whiskey, even before it is fully aged in barrels, of Jack Daniel's own manufacture. A laborious process that requires special attention and makes the production of the whiskey a little more expensive. However, any other process would not have met with Jack's approval.