When the wood on the inside of the oak barrels begins to burn, several things happen on a microscopic level that play important roles in the whiskey aging process. The char works four compounds within the wood: lactones, tannins, lignin, and hemicellulose. Each of these compounds bring a combination of sugars, wood, toffee, and vanilla notes to the finished spirit (via Rabbit Hole Distillery). The four compounds perform different functions and are either heightened (hemicellulose) or toned down (tannins) depending on the level of charring.
According to VinePair, char levels for barrels bound for bourbon-dom are categorized using a numbered scale, with one through four being the most common. Each level coincides with the amount of time the barrels are burned for. Number One is a fifteen-second char, Two is thirty, Three is thirty-five, and Four, called the alligator char for it’s scaly appearance, burns for fifty-five seconds. Most barrels are not charred for more than a minute, though some cooperages, at the request of distillers, developed a Number Seven char which burns for three and a half minutes. The greater the char, the more impact the wood will have on the bourbon’s color and flavor. The higher char levels open the wood up and allow the spirit to interact with a wide surface area of both charred and uncharred parts of the barrel, leading to a more complex flavor.