Here's the wiki on fatlighter.
"Coniferous tree sap is a viscous liquid, that contains terpene, a volatile compound which is a type of hydrocarbon. Over time the evaporation of the terpene changes the state of the sap; it slowly gets thicker until it hardens into resin. New fatwood leaks the sticky sap, while in aged fatwood the sap has hardened and is no longer sticky. At every stage of the aging process, fatwood will burn readily, even when wet."