Wood fires do not burn cleanly. When wood burns, it releases a host of contaminants that coat the inside of the chimney. Creosote, an oily, black substance, is one byproduct of wood burning fires. Creosote forms as a result of gases that have not fully combusted, and it condenses on the inside of the chimney and remains until removed. Burning green or unseasoned wood hastens creosote build-up.